NIST NCSTAR1-3D

Federal Building and Fire Safety Investigation of the

World Trade Center Disaster

Mechanical Properties of Structural

Steels

William E. Luecke

J. David McColskey

Christopher N. McCowan

Stephen W. Banovic^

Richard J. Fields

Timothy Foecke

Thomas A. Siewert

Frank W. Gayle

National Institute of Standards and Technology • Technology Adminisiralion • U S. DeparlmenI of Comm

NISTNCSTAR1-3D

Federal Building and Fire Safety Investigation of the

World Trade Center Disaster

Mechanical Properties of Structural

Steels

William E. Luecke

J. David McColskey

Christopher N. McCowan

Stephen W. Banovic

Richard J. Fields*

Timothy Foecke

Thomas A. Siewert

Frank W. Gayle

Materials Science and Engineering Laboratory

National Institute of Standards and Technology

*Retired

September 2005

U.S. Department of Commerce

Carlos M. Gutierrez, Secretary

Technology Administration Michelle O'Neill, Acting Under Secretary for Technology

National Institute of Standards and Technology

William Jeffrey, Director
Disclaimer No. 1

Certain commercial entities, equipment, products, or materials are identified in this document in order to describe a procedure or concept adequately or to trace the history of the procedures and practices used. Such identification is not intended to imply recommendation, endorsement, or implication that the entities, products, materials, or equipment are necessarily the best available for the purpose. Nor does such identification imply a finding of fault or negligence by the National Institute of Standards and Technology.

Disclaimer No. 2 The policy of NIST is to use the International System of Units (metric units) in all publications. In this document,

however, units are presented in metric units or the inch-pound system, whichever is prevalent in the discipline.

Disclaimer No. 3

Pursuant to section 7 of the National Construction Safety Team Act, the NIST Director has determined that certain evidence received by NIST in the course of this Investigation is "voluntarily provided safety-related information" that is "not directly related to the building failure being investigated" and that "disclosure of that information would inhibit the voluntary provision of that type of information" (15 USC 7306c).

In addition, a substantial portion of the evidence collected by NIST in the course of the Investigation has been

provided to NIST under nondisclosure agreements.

Disclaimer No. 4

NIST takes no position as to whether the design or construction of a WTC building was compliant with any code

since, due to the destruction of the WTC buildings, NIST could not verify the actual (or as-built) construction, the properties and condition of the materials used, or changes to the original construction made over the life of the

buildings. In addition, NIST could not verify the interpretations of codes used by applicable authorities in determining

compliance when implementing building codes. Where an Investigation report states whether a system was

designed or installed as required by a code provision, NIST has documentary or anecdotal evidence indicating

whether the requirement was met, or NIST has independently conducted tests or analyses indicating whether the requirement was met.

Use in Legal Proceedings

No part of any report resulting from a NIST investigation into a structural failure or from an investigation under the National Construction Safety Team Act may be used in any suit or action for damages arising out of any matter mentioned in such report (15 USC 281a; as amended by P.L. 107-231).

National Institute of Standards and Technology National Construction Safety Team Act Report 1-3D

Natl. Inst. Stand. Technol. Natl. Constr. Sfty. Tm. Act Rpt. 1-3D, 322 pages (September 2005)

CODEN: NSPUE2

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Mail: Stop SSOP, Washington, DC 20402-0001
Abstract

This report provides five types of mechanical properties for steels from the World Trade Center (WTC):

elastic, room-temperature tensile, room-temperature high strain rate, impact, and elevated-temperature

tensile. Specimens of 29 different steels representing the 12 identified strength levels in the building as built were characterized. Elastic properties include modulus, E, and Poisson's ratio, v, for temperatures up

to 900 °C. The expression for E{T) for T < 723 °C is based on measurements of WTC perimeter column

steels. Behavior for T > 723 °C is estimated from literature data. Room temperature tensile properties

include yield and tensile strength and total elongation for samples of all grades of steel used in the towers. The report provides model stress-strain curves for each type of steel, estimated from the measured stress- strain curves, surv iving mill test reports, and historically expected values. With a few exceptions, the

recovered steels, bolts, and welds met the specifications they were supplied to. In a few cases, the measured yield strengths of recovered steels were slightly lower than specified, probably because of a combination of mechanical damage, natural variability, and differences in testing methodology. Highstrain-rate properties for selected perimeter and core column steels include yield and tensile strength, total elongation and strain rate sensitivity for rates up to 400 s"'. Measured properties were consistent with

literamre reports on other structural steels. Impact properties were evaluated with Charpy testing.

Properties for perimeter and core column steels were consistent with other structural steels of the era. The

impact toughness at room temperature of nearly all WTC steels tested exceeded 15 ft lbf at room

temperamre. Elevated-temperature stress-strain curves were collected for selected perimeter and core column and truss steels. The report presents a methodology for estimating high-temperature stress-strain curves for the steels not characterized based on room-temperature behavior and behavior of other

structural steels from the literature. The measured elevated-temperature stress-strain behavior of WTC

steels is consistent with other structural steels from that era. For the truss steels, the report presents a complete constitutive law for creep deformation based on experimental measurements. For the steels not

characterized, the report presents a methodology for estimating the creep deformation law.

Keywords: Creep, high strain rate, high temperature, impact, modulus, tensile strength, yield strength.

World Trade Center.

NISTNCSTAR 1-3D, WTC Investigation ill
Abstract

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IV NISTNCSTAR 1-3D, WTC Investigation
Table of Contents

Abstract iii

List of Figures ix

List of Tables xv

List of Acronyms and Abbreviations xvii

Preface xix

Acknowledgments xxix

Executive Summary xxxi

Chapter 1 Introduction 1

1 . 1 Overview of Report 1

1.1.1 Elastic Properties (Chapter 2) 1

1 . 1 .2 Room Temperature Tensile Properties (Chapter 3) 1

1.1.3 High-Strain-Rate Properties (Chapter 4) 3

1 . 1 .4 Impact Properties (Chapter 5) 3

1.1.5 Ele\ ated-Temperature Properties (Chapter 6) 3

1 .2 Description of the Major Building Components 4

1.2.1 Perimeter Columns 4

1.2.2 Core Columns 6

1 .2.3 Flooring System 6

1 .3 Specimen Nomenclature 7

1 .4 Symbols and Abbreviations 8

Chapter 2 Elastic Properties 11

2.1 Introduction 1

1

2.2 Experimental Procedure 11

2.3 Elastic Properties (E, v, G) for 0<T<723 °C 1

1

2.4 Elastic Properties (E, v, G) for T>910 °C 13

2.5 Elastic Properties (E, v, G) for 723 °C<T<910 °C 13

2.6 Uncertainties 14

2.7 References 14

NIST NCSTAR 1-3D. WTC Investigation v
Table of Contents

Chapters

Room-Temperature Tensile Properties 19

3.1 Introduction 19

3.2 Test Procedures 19

3.2.1 Steel 19

3.2.2 Bolts 20

3.2.3 Welds 26

3.3 Results 28

3.3.1 Steel 28

3.3.2 Bolts 28

3.3.3 Welds 28

3.4 Comparison with Engineering Specifications 33

3.4.1 Steel 33

3.4.2 Bolts 60

3.4.3 Welds 60

3.5 Recommended Values 63

3.5.1 Steel 63

3.5.2 Bolts 67

3.5.3 Welds 74

3.6 Summary 75

3.7 References 75

3.7. 1 References Available from Publicly Available Sources 75

3.7.2 References Available from Nonpublic Sources 76

Chapter 4

High-Strain-Rate Properties 79

4.1 Introduction 79

4.2 Test Procedures 79

4.2.1 High Strain-Rate Tension Tests 79

4.2.2 Analysis of High-Strain-Rate Tension Test Data 8

1

4.2.3 Kolsky Bar Tests 82

4.2.4 Quasi-Static Compression Tests 84

4.3 Results 84

4.3.1 High Strain-Rate Tension Tests 84

4.3.2 High Strain-Rate Kolsky Bar Tests 86

4.3.3 Quasi-Static Compression Tests 88

vi NISTNCSTAR 1-3D, WTC Investigation
Table of Contents

4.4 Discussion 90

4.4. 1 Calculation of Strain-Rate Sensitivity for Tension Tests 91

4.4.2 Calculation of Strain-Rate Sensitivity for Kolsky Tests 92

4.5 High-Strain-Rate Data Provided to the Investigation 94

4.6 Comparison with Literature Data 95

4.7 Summary 97

4.8 References 99

Chapter 5 Impact Properties 103

5.1 Introduction 103

5.2 Procedures 103

5.3 Results 105

5.3.1 Perimeter Columns 105

5.3.2 HAZ Materials from Perimeter Columns 106

5.3.3 Core Columns 106

5.3.4 Trusses 107

5.3.5 Truss Seats 107

5.3.6 Bolts 107

5.4 Discussion 107

5.4.1 Perimeter Columns 107

5.4.2 HAZ Materials from Perimeter Columns 109

5.4.3 Core Colunms 109

5.4.4 Trusses 109

5.4.5 Truss Seats 110

5.4.6 Expected Values of Impact Toughness 1 10

5.5 Summary 1 1

1

5.6 References 1 1

1

Chapter 6 Elevated Temperature Properties 129

6.1 Introduction 129

6.2 Test procedures 129

6.2.1 Tensile Tests 129

6.2.2 Creep Tests 130

6.3 Results 130

NISTNCSTAR 1-3D. WTC Investigation vii
Table of Contents

6.3.1 Tensile Tests 130

6.3.2 Creep Tests 130

6.4 Recommended values for steels 134

6.4. 1 A Universal Curve for Elevated-Temperature Tensile Properties 1 34

6.4.2 Analysis of Tensile Data 136

6.4.3 Estimating Elevated-Temperature Stress-Strain Curves 137

6.4.4 Analysis of Creep Data 149

6.4.5 Recommended Values for Bolts 155

6.5 Summary 157

6.6 References 158

Chapter?

Summary and Findings 161

7.1 Summary 161

7.2 Findings 162

Appendix A

Data Tables and Supplemental Figures 163

Appendix B

Effects of Deformation of Wide-Flange Core Columns on Measured Yield Strength 253

Appendix C

Provisional Analysis of High-Rate Data 263

Appendix D

Deformation of Steels Used in WTC 7 273

Appendix E

Specimen Geometry Effects on High-Rate Tensile Properties 279

vni NIST NCSTAR 1-3D, WTC Investigation
List of Figures

Figure P-1 . The eight projects in the federal building and fire safety investigation of the WTC

disaster xxi

Figure 1-1. Cross section of a perimeter column; sections with and without spandrels 4

Figure 1-2. Characteristic perimeter column panel illustrating the various components.

Designations in parentheses refer to the specimen nomenclature of Table 1-1 5

Figure 1-3. Typical welded box columns and rolled wide-flange shapes used for core columns

between the 83rd and 86th floors 6

Figure 1^. Schematic diagram of a floor truss 7

Figure 2-1. Young's modulus as a function of temperature 15

Figure 2-2. Young's modulus, EiT). and shear modulus, G(T) for 0°C < T< 725°C. Young's

modulus was measured on the WTC steels summarized in Table 2-1. Solid line is Eq. 2-2. Shear modulus, G, calculated from and v via Eq. 2-4 16

Figure 2-3. Poisson's ratio (v) as a function of temperature. The sohd line is the fit of a 4th order

polynomial (Eq. 2-3) for 0 °C < r< 725 °C 16

Figure 2—4. Fractional error in representing the Young's modulus data for the three specimens of

perimeter column steel (Table 2-1) using Eq. 2-2 17

Figure 3-1. Flat tensile specimen typically used for standard room-temperature quasi-static

tensile tests 21

Figure 3-2. Flat tensile specimen typically used for elevated-temperature tensile tests 2

1

Figure 3-3. Flat tensile specimen used for room and elevated-temperamre tensile tests 22

Figure 3^. Flat tensile test specimen used for some creep tests 22

Figure 3-5. Flat tensile test specimen used for some creep and elevated-temperature tests 23

Figure 3-6. Round tensile specimen used for room-temperature and elevated-temperamre tensile

tests ; 23

Figure 3-7. Round tensile specimen used for room-temperature tensile testing 24

Figure 3-8. Flat tensile specimen typically used for tensile testing of all-weld metal specimens 24

Figure 3-9. Heat affected zone tensile test specimen with flange/web weld intact. The flange is the specimen portion that is in tension 25

Figure 3-10. Heat affected zone tensile specimen with weld and web machined flush to the flange

surface. The flange is the specimen portion that is in tension 25

Figure 3-1 1 . Notched tensile specimens 26

Figure 3-12. The resistance weld shear strength test (a) before loading, (b) after failure 27

NISTNCSTAR 1-3D, WTC Investigation ix
List of Figures

Figure 3-13. Examples of stress-strain curves for perimeter column, core column, and truss steels. In most cases, the strains do not represent failure 29

Figure 3-14. Stress-strain curves for notched round bar tests using the specimens in Fig. 3-11 30

Figure 3-15. Load-displacement curves from four tensile tests of A 325 bolts, and the average

curve 3

1

Figure 3-16. Schematic diagram of the various definitions of yield behavior in mechanical testing

of steel.. 35

Figure 3-17. Methodology for identifying recovered steels 37

Figure 3-18. Ratio of measured yield strength to specified minimum yield strength for all longitudinal tests of perimeter column steels 39

Figure 3-19. Ratio of measured yield strength to specified minimum yield strength for all longitudinal tests of core column steels 50

Figure 3-20. Yield behavior in tests of webs of four wide-flange core columns specified as F,=36ksi 52

Figure 3-21. Section of C-88c that is the source of the specimens 56

Figure 3-22. Ratio of measured yield strength to specified minimum yield strength for all longitudinal tests of truss steels 59

Figure 3-23. Ratio of measured yield strength to specified minimum yield strength for all longitudinal tests of truss seat steels 60

Figure 3-24. Representative tensile stress-strain behavior for perimeter column steels from

flanges, outer webs, and spandrels (plates 1, 2, and 4) 69

Figure 3-25. Representative tensile stress-strain behavior for perimeter column steels from inner

webs (plate 3) 69

Figure 3-26. Representative tensile stress-strain behavior for selected core column steels 70

Figure 3-27. Representative tensile stress-strain behavior for truss steels 70

Figure 3-28. Representation of deformation that occurs in exposed bolt threads (left) and in bolt

threads coupled with nut threads (right) 71

Figure 3-29. Tensile strength change of A 325 bolts with exposed thread length 72

Figure 3-30. Displacement near failure of A 325 bolts as a ftinction of number of threads exposed 73

Figure 3-31. Data for load-displacement of A 325 bolts from Fig. 3-15 corrected for initial elastic slope from literature 74

Figure 4-1 . Specimen used for high-strain-rate tension tests 79

Figure 4-2. Schematic diagram of the slack adaptor apparatus 80

Figure 4-3. Schematic of the procedure for estimating the tensile yield strength when ringing in the load signal precludes reliable visual estimation 82

Figure 4-4. Schematic diagram of Kolsky bar apparatus 82

Figure 4-5. Oscillograph record of an incident pulse that is partially transmitted to the output bar

and partially reflected in the input or incident bar 83

X NISTNCSTAR 1-3D, WTC Investigation
List of Figures

Figure 4-6. Examples of tensile high-rate stress-strain curves for F, = 50 ksi perimeter column

steel M26-C1B1-RP ; 87

Figure 4-7. Example stress-strain and strain rate-strain curves for Kolsky tests 87

Figure 4-8. Quasi-static compression stress-strain curves for the tests summarized in Table 4-4 89

Figure 4-9. Strain rate sensitivity of yield and tensile strength as a function of specified minimum

yield strength 92

Figure 4-10. Flow stress as a function of strain rate for Kolsky tests 93

Figure 4-11. Flow stress as a function of strain rate evaluated at different strains for Kolsky tests on the A 325 bolt 94

Figure 4-12. Comparison of strain rate sensitivities of NIST WTC steels to values for structural

steels from the literature 96

Figure 4-13. Total elongation, El„ as a function of strain rate for high-strength, perimeter column

steels and high-strength steels in the literature 98

Figure 4-14. Total elongation, El„ as a function of strain rate for low-strength, core column steels and low-strength steels in the literature 99

Figure 5-1 . An example transition curve 118

Figure 5-2. Charpy impact specimen geometries and orientations with respect to the plate rolling

direction 118

Figure 5-3. Longitudinal and transverse Charpy impact data of samples from the flange and

adjacent HAZ of perimeter column N8-C1M1, the flange and adjacent HAZ of

perimeter column C 1 0-C 1 M 1 119

Figure 5-4. Longitudinal and transverse Charpy impact data for the spandrel associated with

perimeter column N8 and the web of wide-flange core column C-80 120

Figure 5-5. Transverse Charpy impact data from samples from perimeter column truss seats M4,

N13, and N8, and from floor truss components 121

Figure 5-6. Longitudinal Charpy impact data for A 325 bolts 122

Figure 5-7. Summary plot of the dependence of absorbed energy on test temperature for all perimeter and core column steels. The absorbed energy values of the sub-size

specimens have been corrected using Eq. 5-2 to compare them to data from full-size (10 mm by 10 mm) specimens 123

Figure 5-8. Summary plot of the dependence of absorbed energy on test temperature for all truss component and truss seat steels. The absorbed energy values of the sub-size

specimens have been corrected using Eq. 5-2 to compare them to data from frill-size (10 mm by 10 mm) specimens 124

Figure 5-9. Strength-toughness relationships for several types of structural steels from the WTC

construction era, after Irvine (1969) 125

Figure 5-10. Scanning electron micrographs of the fracture surface of a Charpy V-notch

longitudinal specimen orientation from an N8-C1M1 perimeter column (WTC 1-142-

97-100). (a) ductile dimples (oval feamres) and general surface morphology, (b) low

magnification view of large and small ductile dimples on the fracture surface, (c) higher magnification view 125

NISTNCSTAR 1-3D, WTC Investigation xi
List of Figures

Figure 5-11. Scanning electron micrographs of the fracture surface of an N8-C1M1 perimeter

column sample showing ductile tearing features that form due to fracture initiation and growth at elongated inclusions and pearlite on planes parallel to the rolling plane.

The "ductile dimples" in this case are linear features with a peak-valley morphology 126

Figure 5-12. Perspective view of the fracture surface of sample N8-C1M1 showing the long peakvalley features characteristic of the fracture surface for transversely oriented impact

specimens. The green line indicates the topography of the fracture surface 126

Figure 5-13. A gray-scale image (a) and compositional maps from fracture surface of an N8-

ClMl perimeter column Charpy V-notch specimen. The relative concentrations of

(b) iron, (c) manganese, and (d) sulfur. The surface of the "ductile dimple" is littered with the remnants of manganese sulfide inclusions 127

Figure 5-14. The fracture surface of a perimeter truss seat, N13-C3B1 that was tested at room

temperature shows cleavage facets, which indicate a brittle fracture mode 127

Figure 6-1. Elevated-temperature stress-strain curves. Specimen N8-C1B1 A-FL is from a Fy 60 ksi perimeter column flange plate from WTC 1 column 142 between floors 97-100. Annotations refer to individual test specimen numbers 131

Figure 6-2. Creep curves of A 242 truss steel from specimen C-132 at 650 °C. Dashed lines represent the fit from Eq. 6-14 using the parameters in Eqs. 6-16, 6-17, and 6-18.

Experimental curves are graphically truncated at e = 0.05 132

Figure 6-3. Creep curves of A 242 truss steel from specimen C-132 at 600 °C. Dashed lines represent the fit from Eq. 6-14 using the parameters in Eqs. 6-16, 6-17, and 6-18.

Experimental curves are graphically truncated at s = 0.05 132

Figure 6-4. Creep curves of A 242 truss steel from specimen C-132 at 500 °C. Dashed lines represent the fit from Eq. 6-14 using the parameters in Eqs. 6-16, 6-17, and 6-18.

Experimental curves are graphically truncated at e = 0.05 133

Figure 6-5. Creep curves of A 242 truss steel from specimen C-132 at 400 °C. Solid lines represent measured creep strain. Dashed lines represent the fit from Eq. 6-14 using

the parameters in Eqs. 6-16, 6-17, and 6-18. Experimental curves are graphically

truncated at s = 0.05 133

Figure 6-6. Ratio,/ of room- to high-temperature yield strength (F,.) for all steels characterized.

The spread of data at room temperature exists because for a given steel, the

individual tests are normalized to the mean room temperature yield strength. The

solid line is the expression, Eq. 6-1, developed using literature data on structural

steels, which are denoted by the smaller symbols 135

Figure 6-7. Ratio of room- to high-temperature tensile strength (TS) for the steels in Table 6-1.

The spread of data at room temperature exists because for a given steel, the

individual tests are normalized to the mean room temperature yield strength. The

solid line is the expression developed for literature data on structural steels, Eq. 6-2,

denoted by the smaller symbols 135

Figure 6-8. K(T), Eq. 6-5, for the A 36 steel of Harmathy (1970), used to model the behavior of

steel withF, = 36 ksi 140

Figure 6-9. n(T), 6-6, for the A 36 steel of Harmathy (1970) used to model the behavior of steel with =36ksi 140

xii NIST NCSTAR 1 -3D, WTC Investigation
List of Figures

Figure 6-10. Predictions for the model (dashed lines) for A 36 steel (F, = 36 ksi nominal) overlaid on the original data used to generate the model (solid lines). The model (Eqs. 6-4, 6-

5, and 6-6 and Table 6—4) makes essentially identical predictions for 0°C < T < 300 °C, so only one line is plotted. Note that the model should not be used

for strains in the elastic region (s- < 0.003). but the curves are shown in this region.

Instead, elastic lines of the appropriate modulus should be used 141

Figure 6-1 1 . K(T). Eq. 6-5,for the A 242 Laclede steel used to model the behavior of steel with

F,>36ksi 143

Figure 6-12. n(T), Eq. 6-6,for the A 242 Laclede steel, used to model the behavior of steel with

Fy > 36 ksi 143

Figtire 6-13. Predictions for the model (dashed hues) for steel with F,. > 36 ksi overlaid on the

original data used to generate the model (solid lines). Note that the model should not be used for strains in the elastic region {e~~ < 0.003), but the curves are shown in this region. Instead, elastic lines of the appropriate modulus should be used 144

Figure 6-14. Simulated elevated temperature stress-strain curves for the Laclede A 242 truss steel. The small-strain behavior is modeled using the appropriate Young's modulus, while

the large-strain behavior comes from Eq. 6^ 145

Figure 6-15. Example of predicted stress-strain curves for F, = 60 ksi perimeter columns

calculated using Eq. 6-4 146

Figure 6-16. Yield point calculated from intersection of appropriate Young's modulus and Eq. 6-4

compared with the expression for the decrease in yield strength for structural steels in general Eq. 6-1. The correspondence is within the uncertainty of either expression 148

Figure 6-17. Example stress-strain curves for F, = 36 ksi WF core columns calculated using

Eq. 6-4 '. 148

Figure 6-1 8. Prediction of the function CfT), Eq. 6-16, from strain rate data for A 242 truss steels 152

Figure 6-19. Variation of the parameter^ with temperature. Solid line is Eq. 6-17 153

Figure 6-20. Comparison of high-temperature yield, F, , and tensile, TS, strength for bolt steels and

the expression for structural steels in general, Eqs. 6-1 and 6-2. Solid symbols are

bolt steels. Open symbols are 'Tire-resistant" bolt steels. Expression for bolt steels is the dashed line 156

NiSTNCSTAR 1-3D. WTC Investigation xiii
List of Figures

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xiv NISTNCSTAR 1-3D, WTC Investigation
List of Tables

Table P-1 . Federal building and fire safety investigation of the WTC disaster xx

Table P-2. Public meetings and briefings of the WTC Investigation xxiii

Table 1-1 . Specimen nomenclature for perimeter column specimens 9

Table 1-2. Specimen nomenclature for core box and wide-flange shapes, trusses, and all other

specimens 10

Table 1-3. Mechanical testing definitions used in this report 10

Table 2-1 . Specimen data for Young's modulus (E) determination 15

Table 3-1 . Results of tensile tests on bolts 3

1

Table 3-2. Room-temperature weld properties as measured 32

Table 3-3. Results of transverse tensile tests on welds from specimen N-8 (WTC 1, column 142,

floors 97-100, specified F, = 60 ksi) 32

Table 3—4. Fillet weld sizes for various plate thicknesses in the core box columns 32

Table 3-5. Summary of mechanical properties and chemical compositions for steels from

low-strength perimeter columns 40

Table 3-6. Summary of mechanical properties and chemical compositions for steels from highstrength perimeter columns 43

Table 3-7. Summary of mechanical properties, chemical compositions, and relevant ASTM and

Yawata specifications for steels from high-strength perimeter columns 45

Table 3-8. Summary of mechanical properties and chemical compositions for steels from core column wide-flange shapes 49

Table 3-9. Summary of mechanical properties and chemical compositions for steels from core box

columns 51

Table 3-10. Common truss component dimensions and standards 57

Table 3-11. Summary of mechanical properties and chemical compositions, and specifications

for truss steels tested 58

Table 3-12. Summary of mechanical properties, chemical compositions for truss seat

steels tested 61

Table 3-13. Estimated static yield strengths and work-hardening parameters, Eq. 3-5, for perimeter

column steels 66

Table 3-14. Estimated static yield strengths and work-hardening parameters, Eq. 3-5, for core column

and truss steels 68

Table 3-15. Room-temperature weld metal properties as designed 74

NIST NCSTAR 1-3D. WTC Investigation xv
List of Tables

Table 4-1 . Summary of specimens and results for high strain-rate tests of perimeter columns 85

Table 4-2. Suinmary of specimens and results for high strain-rate tests of core columns 86

Table 4-3. Summary of Kolsky bar tests 88

Table 4-4. Summary of quasi-static compression tests 89

Table 4-5. Summary of stress-strain rate data plotted in Fig. 4-10 93

Table 4-6. Comparison of strain rate sensitivity measured in tension and compression 94

Table 4-7. Literature data for strain rate sensitivities of structural steels 95

Table 5-1 . Common sub-size to full-size upper shelf energy correction factors 115

Table 5-2. Summary of Charpy data 116

Table 5-3. Historical data on Charpy impact toughness of structural steel 117

Table 6-1 . Specimens and locations for high-temperature tensile tests with fiill stress-strain data. ... 13

1

Table 6-2. Values for the parameters in the strength reduction equations (Eqs. 6-1 and 6-2) 136

Table 6-3. Property data for the A 36 steel reported in Harmathy (1970) 139

Table 6-4. Individual Kj and n, used for steels with F, = 36 ksi 139

Table 6-5. Values of the parameters of Eqs. 6-5 and 6-6 for steels with F, = 36 ksi 141

Table 6-6. Property data for the A 242 Laclede truss steel tested as part of the Investigation 142

Table 6-7. Individual Ki and used for steels with F,. > 36 ksi 142

Table 6-8. Values of the parameters of Eqs. 6-5 and 6-6 for steels with F, > 36 ksi 144

Table 6-9. Scaling parameters (Eq. 6-4) for all WTC steels 147

Table 6-10. Stress-temperature-strain rate data used to evaluate the parameters of Eq. 6-16 151

Table 6-11. Sources of bolt data 155

Table 6-12. Values for the parameters in the strength reduction equations (Eqs. 6-1 and 6-2) for use with bolts 157

XVI NISTNCSTAR 1-3D, WTC Investigation
List of Acronyms and Abbreviations

Acronyms

AISC American Institute of Steel Construction

AISI American Iron and Steel Institute ASTM ASTM International

AWS American Welding Society

BPS Building Performance Study

CVN Charpy V-notch

FATT fracture appearance transition temperature

FEMA Federal Emergency Management Agency

HAZ heat-affected zone HSR high-rate style

HSLA high-strength, low-alloy

JIS Japan Industrial Standard

LERA Leslie E. Robertson Associates

LRFD load and resistance factor design

METT mid-energy transition temperature

NIST National Institute of Standards and Technology

PANYNJ Port Authority of New York and New Jersey

PC&F Pacific Car and Foundry

PONYA Port of New York Authority

SEAoNY Structural Engineers Association of New York

SHCR Skilling, Helle, Christiansen, & Robertson

SMA shielded metal arc SRS strain rate sensitivity

use United States Code

WF wide-flange (a type of structural steel shape now usually called a W-shape)

WTC World Trade Center

WTC 1 World Trade Center 1 (North Tower)

WTC 2 World Trade Center 2 (South Tower)

NISTNCSTAR 1-3D. WTC Investigation XVll
List of Acronyms and Abbreviations

WTC7 World Trade Center 7

Abbreviations

°C degrees Celsius

°F degrees Fahrenheit

|im micrometer

ft foot

Fy yield strength

gal gallon

GPa gigapascal; 1x10^ N/m"

h hour

in. inch

L liter

lb pound

Ibf pound force

kip a force equal to 1 000 pounds

ksi 1 ,000 pounds per square inch

m meter

min minute

mm millimeter

Mn magnesium

min minute

MPa megapascal; 1x10^

s second

xvni NISTNCSTAR 1-3D, WTC Investigation
Preface

Genesis of This Investigation

Immediately following the terrorist attack on the World Trade Center (WTC) on September 11, 2001, the

Federal Emergency Management Agency (FEMA) and the American Society of Civil Engineers began

planning a building performance study of the disaster. The week of October 7, as soon as the rescue and

search efforts ceased, the Building Performance Study Team went to the site and began its assessment. This was to be a brief effort, as the study team consisted of experts who largely volunteered their time

away from their other professional commitments. The Building Performance Study Team issued its report in May 2002, fulfilling its goal "to determine probable failure mechanisms and to identify areas of

future investigation that could lead to practical measures for improving the damage resistance of buildings

against such unforeseen events."

On August 21, 2002, with funding from the U.S. Congress through FEMA, the National Institute of

Standards and Technology (NIST) announced its building and fire safety investigation of the WTC

disaster. On October 1, 2002, the National Construction Safety Team Act (Public Law 107-231), was

signed into law . The NIST WTC Investigation was conducted under the authority of the National

Construction Safety Team Act.

The goals of the investigation of the WTC disaster were:

• To in\ estigate the building construction, the materials used, and the technical conditions that contributed to the outcome of the WTC disaster.

• To serx e as the basis for: - Improvements in the way buildings are designed, constructed, maintained, and used; - Improved tools and guidance for industry and safety officials; - Recommended revisions to current codes, standards, and practices; and - Improved public safety.

The specific objectives were:

1. Determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of the

aircraft and why and how WTC 7 collapsed;

2. Determine why the injuries and fatalities were so high or low depending on location, including

all technical aspects of fire protection, occupant behavior, evacuation, and emergency

response;

3. Determine what procedures and practices were used in the design, construction, operation, and

maintenance of WTC 1, 2, and 7; and

4. Identify, as specifically as possible, areas in current building and fire codes, standards, and

practices that warrant revision.

NISTNCSTAR 1-3D, WTC Investigation XIX
Preface

NIST is a nonregulatory agency of the U.S. Department of Commerce's Technology Administration. The

purpose of NIST investigations is to improve the safety and structural integrity of buildings in the United

States, and the focus is on fact finding. NIST investigative teams are authorized to assess building

performance and emergency response and evacuation procedures in the wake of any building failure that has resulted in substantial loss of life or that posed significant potential of substantial loss of life. NIST

does not have the statutory authority to make findings of fault nor negligence by individuals or

organizations. Further, no part of any report resulting from a NIST investigation into a building failure or from an investigation under the National Construction Safety Team Act may be used in any suit or action

for damages arising out of any matter mentioned in such report (15 USC 281a, as amended by Public

Law 107-231).

Organization of the Investigation

The National Construction Safety Team for this Investigation, appointed by the then NIST Director, Dr. Arden L. Bement, Jr., was led by Dr. S. Shyam Sunder. Dr. William L. Grosshandler served as Associate Lead Investigator, Mr. Stephen A. Cauffman served as Program Manager for Administration,

and Mr. Harold E. Nelson served on the team as a private sector expert. The Investigation included eight

interdependent projects whose leaders comprised the remainder of the team. A detailed description of

each of these eight projects is available at http://wtc.nist.gov. The purpose of each project is summarized

in Table P-1 , and the key interdependencies among the projects are illustrated in Fig. P-1

.

Table P-1. Federal building and fire safety investigation of the WTC disaster.

Technical Area and Project Leader Project Purpose

Analysis of Building and Fire Codes and

Practices; Project Leaders: Dr. H. S. Lew

and Mr. Richard W. Bukowski

Document and analyze the code provisions, procedures, and

practices used in the design, construction, operation, and

maintenance of the structural, passive fire protection, and

emergency access and evacuation systems of WTC 1, 2, and 7. Baseline Structural Performance and

Aircraft Impact Damage Analysis; Project

Leader: Dr. Fahim H. Sadek

Analyze the baseline perfonnance of WTC 1 and WTC 2 under

design, service, and abnormal loads, and aircraft impact damage on

the structural, fire protection, and egress systems.

Mechanical and Metallurgical Analysis of

Structural Steel; Project Leader: Dr. Frank

W. Gayle

Determine and analyze the mechanical and metallurgical properties

and quality of steel, weldments, and connections from steel recovered from WTC 1. 2, and 7. Investigation of Active Fire Protection

Systems; Project Leader: Dr. David

D. Evans; Dr. William Grosshandler

Investigate the performance of the active fire protection systems in WTC 1,2, and 7 and their role in fire control, emergency response, and fate of occupants and responders.

Reconstruction of Thermal and Tenability

Enviromnent; Project Leader: Dr. Richard

G. Gann

Reconstruct the time-evolving temperature, thermal environment,

and smoke movement in WTC 1,2, and 7 for use in evaluating the

structural performance of the buildings and behavior and fate of

occupants and responders.

Structural Fire Response and Collapse

Analysis; Project Leaders: Dr. John

L. Gross and Dr. Therese P. McAllister

Analyze the response of the WTC towers to fires with and without

aircraft damage, the response of WTC 7 in fires, the performance

of composite steel-trussed floor systems, and determine the most

probable structural collapse sequence for WTC 1. 2, and 7. Occupant Behavior, Egress, and Emergency

Communications; Project Leader: Mr. Jason

D. Averill

Analyze the behavior and fate of occupants and responders, both

those who survived and those who did not, and the performance of

the evacuation system.

Emergency Response Technologies and

Guidelines; Project Leader: Mr. J. Randall

Lawson

Document the activities of the emergency responders from the time

of the terrorist attacks on WTC 1 and WTC 2 until the collapse of

WTC 7, including practices followed and technologies used.

XX NISTNCSTAR 1-3D, WTC Investigation
Preface

NIST WTC Investigation Projects

Nisr

Figure P-1. The eight projects in the federal building and fire safety

investigation of the WTC disaster.

National Construction Safety Team Advisory Committee

The NIST Director also established an advisory committee as mandated under the National Construction

Safety Team Act. The initial members of the committee were appointed following a public solicitation. These were:

• Paul Fitzgerald. Executive Vice President (retired) FM Global, National Construction Safety

Team Advisory Committee Chair

• John Barsom, President, Barsom Consulting, Ltd.

• John Bryan, Professor Emeritus, University of Maryland

• David Collins, President, The Preview Group, Inc.

• Glenn Corbett, Professor, John Jay College of Criminal Justice

• Philip DiNenno, President, Hughes Associates, Inc.

NISTNCSTAR 1-3D. WTC Investigation XXI
Preface

• Robert Hanson, Professor Emeritus, University of Michigan

• Charles Thornton, Co-Chairman and Managing Principal, The Thomton-Tomasetti Group,

Inc.

• Kathleen Tiemey, Director, Natural Hazards Research and Applications Information Center,

University of Colorado at Boulder

• Forman Williams, Director, Center for Energy Research, University of California at San

Diego

This National Construction Safety Team Advisory Committee provided technical advice during the

Investigation and commentary on drafts of the Investigation reports prior to their public release. NIST

has benefited from the work of many people in the preparation of these reports, including the National

Construction Safety Team Advisory Committee. The content of the reports and recommendations,

however, are solely the responsibility of NIST.

Public Outreach

During the course of this Investigation, NIST held pubhc briefings and meetings (listed in Table P-2) to

solicit input from the public, present preliminary findings, and obtain comments on the direction and

progress of the Investigation from the public and the Advisory Committee.

NIST maintained a publicly accessible Web site during this Investigation at http://wtc.nist.gov. The site contained extensive information on the background and progress of the Investigation.

NIST's WTC Public-Private Response Plan

The collapse of the WTC buildings has led to broad reexamination of how tall buildings are designed,

constructed, maintained, and used, especially with regard to major events such as fires, natural disasters,

and terrorist attacks. Reflecting the enhanced interest in effecting necessary change, NIST, with support

from Congress and the Administration, has put in place a program, the goal of which is to develop and

implement the standards, technology, and practices needed for cost-effective improvements to the safety

and security of buildings and building occupants, including evacuation, emergency response procedures,

and threat mitigation.

The strategy to meet this goal is a three-part NIST-led public-private response program that includes:

• A federal building and fire safety investigation to study the most probable factors that contributed to post-aircraft impact collapse of the WTC towers and the 47-story WTC 7

building, and the associated evacuation and emergency response experience.

• A research and development (R&D) program to (a) facilitate the implementation of

recommendations resulting from the WTC Investigation, and (b) provide the technical basis

for cost-effective improvements to national building and fire codes, standards, and practices

that enhance the safety of buildings, their occupants, and emergency responders.

xxn NISTNCSTAR 1-3D, WTC Investigation
Preface

Table P-2. Public meetings and briefings of the WTC Investigation.

Date Location Principal Agenda

June 24. 2002 New York Cit>'. NY Public meeting: Public coinments on the Draft Plan for the pending WTC Investigation.

August 21. 2002 Gaithersburg. MD Media briefing announcing the formal start of the Investigation.

December 9. 2002 Washington. DC Media briefing on release of the Public Update and NIST request

for photographs and videos. April 8, 2003 New York City, NY Joint public forum with Columbia University on first-person interviews. April 29-30. 2003 Gaithersburg. MD NCST Advisor},' Committee meeting on plan for and progress on WTC Investigation with a public comment session. Ma\ 7. 2003 New York City. NY Media briefing on release of May 2003 Progress Report.

August 26-27. 2003 Gaithersburg. MD NCST Advisor)' Committee meeting on status of the WTC

investigation with a public comment session. September 17.2003 New York City. NY Media and public briefing on initiation of first-person data

collection projects.

December 2-3. 2003 Gaithersburg, MD NCST Advisory Committee meeting on status and initial results and release of the Public Update with a public comment session. Februar\' 12. 2004 New York Cit>'. NY Public meeting on progress and preliminary findings with public

comments on issues to be considered in formulating final recommendations.

June 18. 2004 New York City. NY Media/public briefing on release of June 2004 Progress Report. June 22-23, 2004 Gaithersburg, MD NCST Advisory Committee meeting on the status of and

preliminary findings from the WTC Investigation with a public

comment session. August 24. 2004 Northbrook. IL Public viewing of standard fire resistance test of WTC floor system at Underwriters Laboratories, Inc. October 19-20. 2004 Gaithersburg, MD NCST Advisory Committee meeting on status and near complete

set of preliminary findings with a public comment session. November 22. 2004 Gaithersburg. MD NCST Advisoiy Committee discussion on draft annual report to Congress, a public comment session, and a closed session to discuss pre-draft recommendations for WTC Investigation.

April 5, 2005 New York City, NY Media and public briefing on release of the probable collapse

sequence for the WTC towers and draft reports for the projects on

codes and practices, evacuation, and emergency response. June 23. 2005 New York City, NY Media and public briefing on release of all draft reports for the WTC towers and draft recommendations for public comment.

September 12-13. 2005Gaithersburg. MD NCST Advisory Committee meeting on disposition of public

comments and update to draft reports for the WTC towers. September 13-15. 2005

Gaithersburg, MD WTC Technical Conference for stakeholders and technical community for dissemination of findings and recommendations

and opportunity for public to make technical comments.

• A dissemination and technical assistance program (DTAP) to (a) engage leaders of the

construction and building community in ensuring timely adoption and widespread use of

proposed changes to practices, standards, and codes resulting from the WTC Investigation

and the R&D program, and (b) provide practical guidance and tools to better prepare facility owners, contractors, architects, engineers, emergency responders, and regulatory authorities

to respond to future disasters. The desired outcomes are to make buildings, occupants, and first responders safer in future disaster events.

NISTNCSTAR 1-3D. WTC Investigation xxiii
Preface

National Construction Safety Team Reports on the WTC Investigation

A final report on the collapse of the WTC towers is being issued as NIST NCSTAR 1 . A companion

report on the collapse of WTC 7 is being issued as NIST NCSTAR 1 A. The present report is one of a set that provides more detailed documentation of the Investigation findings and the means by which these

technical results were achieved. As such, it is part of the archival record of this Investigation. The titles of the full set of Investigation publications are:

NIST (National Institute of Standards and Technology). 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Final Report on the Collapse ofthe World Trade

Center Towers. NIST NCSTAR 1. Gaithersburg, MD, September.

NIST (National Institute of Standards and Technology ). 2006. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Final Report on the Collapse of World Trade Center 7. NIST NCSTAR 1 A. Gaithersburg, MD. . ,

Lew, H. S., R. W. Bukowski, and N. J. Carino. 2005. Federal Building and Fire Safety Investigation of

the World Trade Center Disaster: Design, Construction, and Maintenance ofStructural and Life Safety

Systems. NIST NCSTAR 1-1. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Fanella, D. A., A. T. Derecho, and S. K. Ghosh. 2005. Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Design and Construction ofStructural Systems.

NIST NCSTAR 1-1 A. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Ghosh, S. K., and X. Liang. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Comparison ofBuilding Code Structural Requirements. NIST

NCSTAR 1-lB. National Institute of Standards and Technology. Gaithersburg, MD, September.

Fanella, D. A., A. T. Derecho, and S. K. Ghosh. 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Maintenance and Modifications to Structural

Systems. NIST NCSTAR 1-lC. National Institute of Standards and Technology. Gaithersburg,

MD, September.

Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Fire Protection and Life Safety Provisions Applied to the Design and

Consti'uction of World Trade Center 1, 2, and 7 and Post-Construction Provisions Applied after Occupancy. NIST NCSTAR 1 -ID. National Institute of Standards and Technology. Gaithersburg,

MD, September.

Razza, J. C, and R. A. Grill. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Comparison of Codes, Standards, and Practices in Use at the Time ofthe

Design and Construction of World Trade Center I, 2, and 7. NIST NCSTAR 1-lE. National

Institute of Standards and Teclinology. Gaithersburg, MD, September.

Grill, R. A., D. A. Johnson, and D. A. Fanella. 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Comparison ofthe 1968 and Current (2003) New

XXIV NIST NCSTAR 1-3D, WTC Investigation
Preface

York City Building Code Provisions. NIST NCSTAR 1-lF. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Grill. R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Amendments to the Fire Protection and Life Safety Provisions ofthe New

York City Building Code by Local Laws Adopted While World Trade Center 1, 2, and 7 Were in Use. NIST NCSTAR 1-lG. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Post-Construction Modifications to Fire Protection and Life Safety Systems

of World Trade Center 1 and 2. NIST NCSTAR 1-lH. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Grill, R. A.. D. A. Johnson, and D. A. Fanella. 2005. Federal Building and Fire Safety Investigation

ofthe World Trade Center Disaster: Post-Construction Modifications to Fire Protection, Life

Safety, and Structural Systems of World Trade Center 7. NIST NCSTAR l-ll. National Institute of

Standards and Technology. Gaithersburg, MD, September.

Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Design, Installation, and Operation ofFuel System for Emergency Power in World Trade Center 7. NIST NCSTAR 1-1 J. National Institute of Standards and Technology.

Gaithersburg, MD, September.

Sadek, F. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center Disaster:

Baseline Structural Performance and Aircraft Impact Damage Analysis ofthe World Trade Center

Towers. NIST NCSTAR 1-2. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Faschan, W. J., and R. B. Garlock. 2005. Federal Building and Fire Safety^ Investigation ofthe World Trade Center Disaster: Reference Structural Models and Baseline Performance Analysis of

the World Trade Center Towers. NIST NCSTAR 1-2 A. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Kirkpatrick, S. W., R. T. Bocchieri, F. Sadek, R. A. MacNeill, S. Holmes, B. D. Peterson,

R. W. Cilke, C. Navarro. 2005. Federal Building and Fire Safety Investigation ofthe World Trade

Center Disaster: Analysis ofAircraft Impacts into the World Trade Center Towers, NIST

NCSTAR 1-2B. National Institute of Standards and Technology. Gaithersburg, MD, September.

Gayle, F. W., R. J. Fields. W. E. Luecke, S. W. Banovic, T. Foecke, C. N. McCowan, T. A. Siewert, and

J. D. McColskey. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: Mechanical and Metallurgical Analysis ofStructural Steel. NIST NCSTAR 1-3. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Luecke, W. E., T. A. Siewert, and F. W. Gayle. 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Contemporaneous Structural Steel

Specifications. NIST Special Publication 1-3 A. National Institute of Standards and Technology.

Gaithersburg, MD, September.

NIST NCSTAR 1-3D, WTC Investigation XXV
Preface

Banovic, S. W. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: Steel Inventoiy and Identification. NISTNCSTAR 1-3B. National Institute of Standards

and Technology. Gaithersburg, MD, September.

Banovic, S. W., and T. Foecke. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Damage and Failure Modes ofStructural Steel Components. NIST

NCSTAR 1-3C. National Institute of Standards and Technology. Gaithersburg, MD, September.

Luecke, W. E., J. D. McColskey, C. N. McCowan. S. W. Banovic, R. J. Fields, T. Foecke,

T. A. Siewert, and F. W. Gayle. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Mechanical Properties ofStructural Steels. NIST NCSTAR 1-3D.

National Institute of Standards and Technology. Gaithersburg, MD, September.

Banovic, S. W., C. N. McCowan, and W. E. Luecke. 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Physical Properties ofStructural Steels. NIST

NCSTAR 1-3E. National Institute of Standards and Technology. Gaithersburg, MD, September.

Evans, D. D., R. D. Peacock, E. D. Kuligowski, W. S. Dols, and W. L. Grosshandler. 2005. Federal

Building and Fire Safety Investigation ofthe World Trade Center Disaster: Active Fire Protection

Systems. NIST NCSTAR 1-4. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Kuligowski, E. D., D. D. Evans, and R. D. Peacock. 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Post-Construction Fires Prior to September II, 2001. NIST NCSTAR 1-4A. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Hopkins, M., J. Schoenrock, and E. Budnick. 2005. Federal Building and Fire Safety Investigation

ofthe World Trade Center Disaster: Fire Suppression Systems. NIST NCSTAR 1-4B. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Keough, R. J., and R. A. Grill. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Fire Alarm Systems. NIST NCSTAR 1-4C. National Institute of Standards

and Technology. Gaithersburg, MD, September.

Ferreira, M. J., and S. M. Strege. 2005. Federal Building and Fire Safety' Investigation ofthe

World Trade Center Disaster: Smoke Management Systems. NIST NCSTAR 1-4D. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Gann, R. G., A. Hamins, K. B. McGrattan, G. W. Mulholland, H. E. Nelson, T. J. Ohlemiller,

W. M. Pitts, and K. R. Prasad. 2005. Federal Building and Fire Safety Investigation ofthe World Trade

Center Disaster: Reconstruction ofthe Fires in the World Trade Center Towers. NIST NCSTAR 1-5. National Institute of Standards and Technology. Gaithersburg, MD, September.

Pitts, W. M., K. M. Butler, and V. Junker. 2005. Federal Building and Fire Safety Investigation of

the World Trade Center Disaster: Visual Evidence, Damage Estimates, and Timeline Analysis.

NIST NCSTAR 1-5 A. National Institute of Standards and Technology. Gaithersburg, MD,

September.

XXVI NISTNCSTAR 1-3D, WTC Investigation
Preface

Hamins. A., A. Maranghides. K. B. McGrattan, E. Johnsson, T. J. Ohlemiller, M. Donnelly,

J. Yang. G. MulhoUand, K. R. Prasad, S. Kukuck, R. Anleitner and T. McAllister. 2005. Federal

Building and Fire Safety Investigation ofthe World Trade Center Disaster: Experiments and

Modeling ofStinctural Steel Elements Exposed to Fire. NIST NCSTAR 1-5B. National Institute of

Standards and Technology. Gaithersburg, MD, September.

Ohlemiller, T. J., G. W. Mulholland, A. Maranghides, J. J. Filliben, and R. G. Gann. 2005. Federal

Building and Fire Safety- Investigation ofthe World Trade Center Disaster: Fire Tests ofSingle

Office Workstations. NIST NCSTAR 1-5C. National Institute of Standards and Technology.

Gaithersburg, MD, September.

Gann, R. G., M. A. Riley, J. M. Repp, A. S. Whittaker, A. M. Reinhom, and P. A. Hough. 2005.

Federal Building and Fire Safety Investigation ofthe World Trade Center Disaster: Reaction of

Ceiling Tile Systems to Shocks. NIST NCSTAR I-5D. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Hamins. A.. A. Maranghides, K. B. McGrattan, T. J. Ohlemiller, and R. Anleitner. 2005. Federal

Building and Fire Safety Investigation ofthe World Trade Center Disaster: Experiments and

Modeling ofMultiple Workstations Burning in a Compartment. NIST NCSTAR 1-5E. National

Institute of Standards and Technology. Gaithersburg, MD, September.

McGrattan, K. B., C. Bouldin, and G. Forney. 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Computer Simulation ofthe Fires in the World

Trade Center Towers. NIST NCSTAR 1-5F. National Institute of Standards and Technology.

Gaithersburg, MD, September.

Prasad, K. R.. and H. R. Baum. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Fire Structure Interface and Thermal Response ofthe World Trade Center

Towers. NIST NCSTAR 1-5G. National Institute of Standards and Technology. Gaithersburg,

MD, September.

Gross, J. L., and T. McAllister. 2005. Federal Building and Fire Safety Investigation ofthe World Trade

Center Disaster: Structural Fire Response and Probable Collapse Sequence ofthe World Trade Center

Towers. NIST NCSTAR 1 -6. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Carino, N. J., M. A. Stames, J. L. Gross, J. C. Yang, S. Kukuck, K. R. Prasad, and R. W. Bukowski.

2005. Federal Building, and Fire Safety Investigation ofthe World Trade Center Disaster: Passive

Fire Protection. NIST NCSTAR 1-6A. National Institute of Standards and Technology.

Gaithersburg, MD, September.

Gross, J., F. Hervey, M. Izydorek, J. Mammoser, and J. Treadway. 2005. Federal Building and

Fire Safety Investigation ofthe World Trade Center Disaster: Fire Resistance Tests ofFloor Truss

Systems. NIST NCSTAR 1-6B. National Institute of Standards and Technology. Gaithersburg,

MD, September.

Zarghamee, M. S., S. Bolourchi, D. W. Eggers, O. O. Erbay, F. W. Kan, Y. Kitane, A. A. Liepins,

M. Mudlock, W. I. Naguib, R. P. Ojdrovic, A. T. Sarawit, P. R Barrett, J. L. Gross, and

NIST NCSTAR 1-3D, WTC Investigation xxvn
Preface

T. P. McAllister. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: Component, Connection, and Subsystem Structural Analysis. NIST NCSTAR 1-6C.

National Institute of Standards and Technology. Gaithersburg, MD, September.

Zarghamee, M. S., Y. Kitane, O. O. Erbay, T. P. McAllister, and J. L. Gross. 2005. Federal

Building and Fire Safety Investigation ofthe World Trade Center Disaster: Global Structural

Analysis ofthe Response ofthe World Trade Center Towers to Impact Damage and Fire. NIST

NCSTAR 1-6D. National Institute of Standards and Technology. Gaithersburg, MD, September.

McAlHster, T., R. W. Bukowski, R. G. Gann, J. L. Gross, K. B. McGrattan, H. E. Nelson, L. Phan,

W. M. Pitts, K. R. Prasad, F. Sadek. 2006. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Structural Fire Response and Probable Collapse Sequence of World Trade

Center 7. (Provisional). NIST NCSTAR 1-6E. National Institute of Standards and Technology.

Gaithersburg, MD.

Gilsanz, R., V. Arbitrio, C. Anders, D. Chlebus, K. Ezzeldin, W. Guo, P. Moloney, A. Montalva,

J. Oh, K. Rubenacker. 2006. Federal Building and Fire Safety Investigation ofthe World Trade

Center Disaster: Structural Analysis ofthe Response of World Trade Center 7 to Debris Damage

and Fire. (Provisional). NIST NCSTAR 1-6F. National Institute of Standards and Technology.

Gaithersburg, MD.

Kim, W. 2006. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: Analysis ofSeptember 11, 2001, Seismogram Data. (Provisional). NIST NCSTAR 1-6G.

National Institute of Standards and Technology. Gaithersburg, MD.

Nelson, K. 2006. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: The Con Ed Substation in World Trade Center 7. (Provisional). NIST NCSTAR I-6H.

National Institute of Standards and Technology. Gaithersburg, MD.

Averill, J. D., D. S. Mileti, R. D. Peacock, E. D. Kuligowski, N. Groner, G. Proulx, P. A. Reneke, and

H. E. Nelson. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center Disaster:

Occupant Behavior, Egress, and Emergency Communication. NIST NCSTAR 1-7. National Institute of

Standards and Technology. Gaithersburg, MD, September.

Fahy, R., and G. Proulx. 2005. Federal Building and Fire Safety Investigation of the World Trade

Center Disaster: Analysis ofPublished Accounts ofthe World Trade Center Evacuation. NIST

NCSTAR 1-7A. National Institute of Standards and Technology. Gaithersburg, MD, September.

Zmud, J. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: Technical Documentation for Sun'ey Administi-ation. NIST NCSTAR 1-7B. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Lawson, J. R., and R. L. Vettori. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: The Emergency Response Operations. NIST NCSTAR 1-8. National Institute of

Standards and Technology. Gaithersburg, MD, September.

xxvni NIST NCSTAR 1-3D, WTC Investigation
Acknowledgments

Many people contributed valuable skills, facilities and information that made this report possible. We

thank:

Professor George Krauss (University Emeritus Professor, Colorado School of Mines) and Dr. Elliot Brown (EB Consulting. Golden, CO) for technical discussions regarding the metallurgy of the steels in the World Trade Center,

Dr Hugh MacGillivray (Professor, Imperial College, London, UK) for technical discussions regarding

high strain rate testing.

Professor David Matlock (Colorado School of Mines) for use of the high-rate tensile testing machine,

Tessa Dvorak and Brian Goudy (National Institute of Standards and Technology [NIST]) for data

reduction,

Lonn Rodine (NIST) for specimen and drawing preparation,

Ray Santoyo (NIST) and Mark ladicola (NIST) for conducting many of the room temperature tensile

tests.

Donald Hame (NIST ) for conducting some of the creep tests,

Richard Rhorer and Michael Kennedy (NIST) and Debasis Basek (University of Tennessee) for conducting the high-rate Kolsky bar tests.

Dr. T. Weerasooriya and Mr. P. Moy (Impact Physics and High Rate Laboratory, Weapons and Materials

Directorate, Army Research Laboratory, Aberdeen, MD) for performing initial Kolsky bar tests and

assisting in calibrating the NIST Kolsky bar facility,

Richard Ricker and David Dayan (NIST) for conducting the tests to determine the elastic modulus as a function of temperature,

Kazushige Tokuno (Nippon Steel USA) for supplying historical information on Yawata steel properties.

Former employees of Laclede Steel (St. Louis, MO) including David McGee and Larry Hutchison for locating Laclede records for NIST investigators during a visit to the Laclede Steel Mill.

NIST NCSTAR 1-3D, WTC Investigation XXIX
Acknowledgments

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XXX NISTNCSTAR 1-3D, WTC Investigation
Executive Summary

National Institute of Standards and Technology (NIST) had several objectives in characterizing the

recovered World Trade Center (WTC) steels. One was to characterize the mechanical properties of steels recovered from the impact and fire zone as a function of temperature to provide data for fire modeling,

and deformation rate to provide data for impact modeling. The second was to determine whether the

mechanical properties of the steels, bolts and welds were consistent with both the specifications that they were delivered to. and the expected properties of structural steels from the construction era.

The experimentally determined mechanical property data can be divided into five groups:

• Elastic properties as a function of temperamre were determined from recovered perimeter

column steels.

• Room temperature yield and tensile strength, and total elongation were determined from

specimens of recovered steel of all grades from the fire and impact zones. Complete stressstrain cur\'es are reported for 29 different steels with 12 yield strength levels. Loaddisplacement curves were measured from recovered A 325 bolts. Stress-strain curves and

strengths of several different perimeter column and truss weld geometries are reported.

• Yield and tensile strengths and total elongations of selected perimeter and core column steels were determined as a function strain rate at rates up to 400 s"'. Strain-rate sensitivities and

complete stress-strain curv es for these steels are reported. Several steels were characterized at higher rates using Kolsky bar tests.

• Impact properties as a function of temperature were determined using Charpy tests for selected

perimeter column, core column, truss, and truss-seat steels.

• Elevated-temperature yield and tensile strength, and total elongation were determined on

selected specimens of recovered perimeter and core column, truss, and truss seat steels. Complete stress-strain curves for temperatures up to 650 °C are reported for all steels characterized. Creep deformation as a function of temperature and stress was determined for the truss chord steel.

By combining these measured properties with historical averages from the literature, and in some cases recovered mill test reports, NIST developed values for properties of the important grades of steel including

• Elastic modulus and Poisson's ratio as a function of temperature,

• Model room-temperature stress-strain curves for all WTC steel grades from the fire and

impact zones corrected for dynamic effects,

• Room-temperature load-displacement curves for A 325 bolts.

NISTNCSTAR 1-3D, WTC Investigation XXXI
Executive Summary

• Yield and tensile strength for weld metals,

• Strain rate sensitivity of steel for high strain rate properties,

• Elevated-temperature yield and tensile strength and complete stress-strain behavior for all WTC steel grades from the fire and impact zones,

• Elevated-temperature load-displacement curves for A 325 bolts,

• Creep response for all WTC steel grades from the fire and impact zones.

This report makes several findings concerning the steel used in the WTC:

• Steels, bolts, and welds generally have properties that are consistent with the specifications to which they were supplied.

• Infrequently, the measured yield strengths are lower than called for by the appropriate

specifications. The measured yield strengths of 2 of 24 perimeter column samples are lower

than the specified minimum. This probably occurred from a combination of the natural

variability of steel properties, and small differences between the mill and NIST testing

protocols. The measured yield strengths of 2 of 8 core column samples are lower than the

specified minimum. In this case, mechanical damage that occurred in the collapse and

subsequent recovery removed the expected yield point behavior and lead to the low values.

• The average measured yield strength of the steels from the perimeter columns exceeds the

specified minimum values by about 10 percent, which is consistent with historically expected

values for steel plates.

• The measured yield strengths of the F, = 36 ksi wide-flange core columns are lower than

expected from historical measurements of other structural steels.

• The strain rate sensitivities of the yield and tensile strengths of perimeter and core columns are similar to other structural steels from the WTC construction era.

• The impact properties of the steels, evaluated by Charpy testing, are similar to structural steels from the WTC era. The ductile-to-brittle transition temperatures of the perimeter and core column steels are at or below room temperature.

• The behavior of the yield and tensile strengths of WTC steels with temperature is similar to that of other structural steels from the WTC construction era.

XXXll NISTNCSTAR 1-3D, WTC Investigation
Chapter 1 Introduction

1 .1 OVERVIEW OF REPORT

National Institute of Standards and Technology (NIST) had three objectives in characterizing the

mechanical properties of the structural steels, bolts and welds recovered from the World Trade Center

(WTC) site. The first was to compare the measured properties of the steels with the requirements of the

specifications that they were purchased under. The second was to determine whether their properties were

consistent with the properties and quality of other structural steel from the WTC construction era. The

third was to determine the constitutive behavior of the steels for input into finite element models of the

response of the building components to the airplane impact and to the high temperatures produced by the

subsequent fires. In support of these three goals, the Investigation characterized the elastic properties at elevated temperatures, the room-temperature tensile behavior, the high-strain-rate behavior, the impact

behavior using Charpy tests, and the elevated-temperature tensile and creep behavior.

This report comprises five chapters that summarize the results of investigations into the mechanical

properties of the WTC steels. Chapter 2 covers the temperature dependence of the elastic properties.

Chapter 3 co\ ers the room-temperature, quasi-static stress-strain behavior. Chapter 4 details investigations into the properties of the steel at high strain-rate, which are relevant for modeling the

airplane impact. Chapter 5 summarizes the results of Charpy tests to characterize the impact properties of

the steels. Chapter 6 addresses the elevated-temperature deformation behavior of the steel, which is relevant for modeling the response of the steel to the fires. Within each chapter, where appropriate,

individual sections address the relation of the measured properties to the standards and specifications used

in the WTC construction. Each chapter also compares the measured properties to literature data from the

WTC construction era as a means to establish whether the properties of the steel were anomalous or

ordinary. Finally, separate sections explain the methods by which experimental results, literature and

construction data, and theoretical models were combined to produce constitutive laws for input to the

finite element models.

1.1.1 Elastic Properties (Chapter 2)

After examining reported literature values of the change in elastic properties with temperature, the

Investigation characterized specimens of perimeter column steels. The scatter in the literature data is large

and probably results from differences in test technique. Analysis of the experimental data generated in the Investigation produced expressions for Young's modulus, shear modulus, and Poisson's ratio for temperatures up to 700 °C. For higher temperatures, the Investigation produced a methodology for estimating the modulus.

1.1.2 Room Temperature Tensile Properties (Chapter 3)

Metallurgically and mechanically, only the room-temperature stress-strain behavior detailed in Chapter 3

is relevant to the standards and specifications that the steel was to meet. In support of the first objective,

NIST NCSTAR 1 -3D, WTC Investigation 1
Chapter 1

NIST characterized the room-temperature tensile stress-strain behavior of several hundred tensile

specimens from the perimeter columns, core columns, trusses, and truss seats. The specimens represent

samples of all major strength levels of steel from all the fabricators who supplied steel for the floors in the

impact and fire zones of the towers. In addition, NIST characterized the load-displacement behavior of

recovered bolts and the strength of selected welds and weld assemblies.

In comparing the properties of the recovered steels to their intended specifications, NIST used chemical

analyses, room-temperature tensile tests, and archival information. A number of experimental difficulties complicated the positive identification of the exact specifications used for a given steel. Despite these

limitations, the Investigation made several findings regarding the properties of the recovered steel in

relation to the original specifications.

The yield and tensile strengths of the perimeter columns, with only a few exceptions, are consistent with

the strength requirements of the original specifications. The number of slightly under-strength plates and

the amount by which they fall short is consistent with expected values for the average strength and

coefficient of variation of yield strengths of plate steels from the WTC construction era. The ratio of

measured yield strength to specified yield strength is also consistent with literature estimates from the

WTC construction era.

Unlike the perimeter columns, the NIST-measured yield strengths of several of the wide-flange core columns recovered from the core were less than specified. It is likely that these low values arose from

damage that removed the yield point behavior, which could easily lead to NIST-measured strengths up to

3 ksi below the value in the mill test report. The average measured yield strengths and yield points for the

wide-flange specimens whose yield points were larger than the specified minimum are still several ksi

less than the expected value predicted from the literature of the WTC construction era, however.

Many of the components in the floor trusses were fabricated from high-strength, low-alloy (HSLA) steels, even when they were only required to meet the A 36 minimum requirements. The strengths of recovered

bolts are consistent with the ASTM A 325 specification, but are much higher than expected based on

reports from the contemporaneous literature. The limited number of tests on welds indicates that their

strengths are consistent with the expected values from the welding procedures used.

To provide constitutive data for finite element models of the Investigation, the third objective for the

room-temperature tensile testing program, NIST produced representative stress-strain curves for each of

the many grades of steel in the towers. For the perimeter column and truss steels, the yield strengths were

corrected for testing-rate effects and, where possible, experimental data were combined with surviving

WTC mill test reports to produce better estimates of the characteristic strength. NIST-measured stress- strain curves from specimens taken from recovered components, modeled using the Voce work-hardening

law, were used to describe the plastic deformation of each steel grade. For the core columns, either plates or wide-flange shapes, the yield strength was assumed to be the historical average from the literature of

the WTC construction era, corrected for dynamic effects. NIST-measured stress-strain curves from

specimens recovered from core columns, also modeled using Voce work hardening, supplied the plastic

behavior. For the truss steels, based on chemistry and mechanical characterization, the angles were assumed to be a high-strength low-alloy steel regardless of specification. Experimental stress-strain data were used to estimate the yield and work-hardening behavior. For the A 325 bolts, NIST supphed loaddisplacement curves measured on recovered bolts. For the perimeter and core column welds, NIST

2 NISTNCSTAR 1-3D, WTC Investigation
Introduction

estimated mechanical properties by combining data from archival materials and tests on welds in recox ered components.

1.1.3 High-Strain-Rate Properties (Chapter 4)

Because the strength of steel depends on the rate at which it is deformed, it is necessary to quantify this relation for the steels in the impact zone. Failing to properly account for the increased strength with

deformation rate could lead to an incorrect estimates of the damage caused by the airplane impact. In support of this goal, the Investigation employed two types of high-strain-rate tests. For rates, 50 s~' <s < 500 s~' , tests used a serv'ohydrauhc tensile test machine and special tensile specimens. For

higher rates, tests employed a Kolsky bar test apparatus in which the test specimen is a right circular

cylinder loaded in compression.

Like other structural steels, the strength of the WTC steels increase with increasing strain rate. The strain rate sensitivity of the WTC steels was evaluated for eight perimeter column plates, four wide-flange core columns, and one plate from a core box column. The measured strain rate sensitivities are of the same

magnitude as other structural steels reported in the literature and decrease with increasing yield strength.

1.1.4 Impact Properties (Chapter 5)

Charpy impact tests are a type of dynamic fracture test that probes the ability of steels to absorb energy

before fracturing. As such, they are particularly relevant to the airplane impact. In the Charpy test, the

energy used to break a notched specimen is measured as a function of temperature.

None of the ASTM International (ASTM) specifications for steels used in the WTC, then or current, put

limits on the impact properties, but the measured impact properties are similar to those of other structural

steels of the WTC construction era. All the perimeter column steels have large upper shelf energies and

transition temperatures well below 0 °F. The transition temperatures of the wide-flange specimen and the

truss rods and angles are near room temperature. The transition temperatures of the truss-seat steels are above room temperature, and the absorbed energy of these steels is low even at room temperature,

indicating a propensity for brittle failure.

1.1.5 Elevated-Temperature Properties (Chapter 6)

The high-temperature testing program had two thrusts. One was to characterize the elevated-temperature

stress-strain behavior of the steels most likely to have been affected by the post-impact fires. The second was to characterize the creep, or time-dependent deformation, behavior of the steels from the floor

trusses. In each of these two areas, in addition to the experimental characterization, NIST developed

methodologies to predict the behavior of untested steels.

For the elevated-temperature stress-strain behavior, the methodology recognized that the yield and tensile strengths of structural steels, normalized to their room-temperamre values, follow a master curve with

temperature. A modified form of this master strength curve, developed using literature data on bolt steels,

describes the more rapid strength degradation of the bolts with temperature.

NISTNCSTAR 1-3D, WTC Investigation 3
Chapter 1

To produce elevated-temperature stress-strain curves of WTC steel, NIST developed a methodology to account for the change in the work-hardening behavior using literature data for structural steels scaled by

the ratio of room-temperature tensile strengths.

NIST also characterized the creep deformation behavior of the floor truss steels. To estimate the creep

properties of untested steels, NIST developed a methodology that used either existing literature data or the

Investigation-generated floor truss creep data after scaling by the ratios of room-temperature tensile

strength.

1.2 DESCRIPTION OF THE MAJOR BUILDING COMPONENTS

This section suminarizes the major structural elements of the WTC buildings in the impact and fire areas. (NIST NCSTAR 1-3A') contains a more detailed description of the structure of the building.

The WTC towers had a frame-tube construction consisting of closely spaced perimeter columns with a rectangular core. The buildings had a square footprint, 207 ft 2 in. on a side with chamfered comers. From

the 9th to the 107th floors, the perimeter columns were closely spaced, built-up box columns. The core of

the building was approximately 87 ft by 137 ft and connected to the perimeter columns by a floor panel

system that provided column-free office space.

1.2.1 Perimeter Columns

Each building face consisted of

59 columns spaced at 40 in. The

coluiTuis were fabricated by welding

four plates to form an approximately

14 in. square section. Fig. 1-1. The

perimeter columns were joined by

horizontal spandrel plates to form

panels, which were typically three

stories (36 ft) tall and three columns

wide. Fig. 1-2. Heavy end, or "butt"

plates 1.375 in. to 3 in. thick were

welded to the top and bottom of each

column. The columns were also bolted

to the adjacent columns using ASTM

A 325 bolts except for the heaviest butt

plates, which used ASTM A 490 bohs.

Other than at the mechanical floors, the

panels were staggered vertically so that only one third of them were spliced in any

one story. Adjacent spandrels were bolted

together using splice plates.

14.0 in. /

Outer Web (Plate 2)

Flange

(Plate 1) 13.5 in. Section at individual column

15.75 in inner Web (Plate 3)

Outside of building

40.0 in.

Splice Plates

Spandrel Plate (Plate 4)

Inside of building

Section at spandrel

Figure 1-1. Cross section of a perimeter column;

sections with and without spandrels.

This reference is to one of the companion documents from this Investigation. A hst of these documents appears in the Preface

to this report.

4 NIST NCSTAR 1-3D, WTC Investigation
Introduction

The structural steel documents specified 14 grades of steel for the plates of the perimeter columns, with

minimum yield strengths of (36. 42. 45, 46, 50, 55, 60, 65, 70, 75, 80, 85, 90, and 100) ksi.

Source: Unknown. Photo enhanced by NIST. Figure 1-2. Characteristic perimeter column panel illustrating the various components.

Designations in parentheses refer to the specimen nomenclature of Table 1-1.

The side plates of a column are termed the flanges, Fig. 1-1, and the inner and outer plates are termed the

webs. In an individual column, the flanges were always at least as thick as the webs. Although the yield

strengths of the webs and flanges of an individual column were identical, the yield strengths of the

columns within a three-story, three-column panel could differ.

As the elevation in the building increased, the thickness of the plates in the columns decreased, but the

plates were always at least 0.25 in. thick. About one half of the recovered perimeter columns in the NIST

inventory are type 120, in which the flange and web plates are both 0.25 in. thick. In the WTC 1 fire and

impact zone, floors 92-100, columns of this type make up about 2/3 of the total columns

Twelve grades of steel were specified for the spandrels, with the same strength levels as the columns but

with a maximum F, ^ 85 ksi. In a panel, the yield strength of the spandrel plate was generally lower than

that of the webs and flanges, and its thickness was greater than the adjacent inner web plate. Where the

spandrels crossed the columns there was no inner web plate.

NISTNCSTAR 1-3D, WTC Investigation 5
Chapter 1

1.2.2 Core Columns

Core columns were of two types: welded box columns and rolled wide flange (WF) shapes. Fig. 1-3

shows some examples of the shapes of core columns. In the lower floors, the core columns were primarily

very large box columns, as large as 12 in.x52 in. with plates up to 7 in. thick. In the upper floors, the

columns were primarily rolled wide-flange shapes. Like the perimeter columns, the core columns were

typically spliced at three-story intervals. Core box columns were specified with F, = 36 ksi or Fy = 42 ksi. Core wide-flange columns were specified to be one of four grades, but were primarily F, = 36 ksi and

Fy = 42 ksi steel.

I 22.0 in. J ^

I i J I

, . 1.31 in. t

14.0 in.

1 1 Column 504 t 22.0 in 15.0 In. Column

701

X- 1 \ s 1 1

Columns 1001 1008

501 508 Columns 607 906 14WF219

iD.64 in.

17.89 in. Note: To scale.

15.825 in. 10.00 in.

Figure 1-3. Typical welded box columns and rolled wide-flange shapes used for core columns between the 83rd and 86th floors.

1.2.3 Flooring System

Other than in a few special mechanical floors, the floor outside the core was supported by a twodimensional network of 29 in. deep floor trusses. Figure 1-4 illustrates the major components of an

individual floor truss. The most common floor trusses were nominally 60 ft or 36 ft long. Although there were dozens of variants, the top chord was usually fabricated from two sections of 2 in. x 1.5 in.xO.25 in. thick angle specified to conform to ASTM A 242 with F, = 50 ksi. In the 60 ft trusses, the lower chord was usually fabricated from two slightly larger angles, 3 in.x2 in.xO.37 in. thick specified to conform to A 36. The 36 ft trusses generally used a 2 in.xl.5 in.x0.25 in. thick angle for the lower chord as well. The

truss web was a continuous round bar, which was usually D= 1.09 in. for the 60 ft trusses and

D = 0.92 in. otD = 0.98 in. for the 36 ft trusses.

6 NISTNCSTAR 1-3D, WTC Investigation
Introduction

Gusset f^ate

Core Column

Figure 1-4. Schematic diagram of a floor truss.

Before erection, the individual trusses were assembled into modules typically 20 ft wide. Each module

contained two sets of doubled trusses in the interior and a single truss along each edge. Smaller bridging

trusses ran perpendicular to the main trusses at 13 ft 4 in. spacing. The bridging trusses were usually

fabricated from 1.5 in. x 1.25 in. x 0.23 in. angles and D = 0.75 in. round webs.

At the core, the floor trusses were bolted to seats on channels that ran continuously along the core columns. At the perimeter columns, the floor trusses were bolted and welded to seats mounted on the spandrels at every other column. Fig. 1-2. Diagonal bracing straps welded to gusset plates secured the

installed floor truss modules to the perimeter columns. Fig. 1-2.

1.3 SPECIMEN NOMENCLATURE

The specimen nomenclature of the Investigation captures the vital information about the location of the

individual specimens within the larger component. Because the recovered sections arrived at NIST

unidentified, and it often required weeks or months to establish the original location in the building, the nomenclature does not capture details about the original location. The NIST report "Steel Inventory and

Identification within Buildings" (NIST NCSTAR 1-3B) contains tables that map the specimen

designation to its location in the building and specified dimensions and strength. Furthermore, because of

the significant damage, it was not possible to identify all the recovered sections.

NIST NCSTAR 1-3D, WTC Investigation 1
Chapter 1

Table 1-1 describes the naming convention for the perimeter column samples with an example.

Figure 1-2 illustrates the naming convention for the perimeter column and truss components graphically.

The example specimen name, N8-C1T2-IW-L2, describes the second specimen in a series of longitudinal

tension specimens from the inner web of a perimeter column panel from WTC 1 column line 143 near top

of the panel at floor 100. This panel is from the north face of the tower, just above and to the east of the

impact zone. In this nomenclature, the column numbering within a given panel is reversed from the

scheme used in the structural engineering drawings.

Table 1-2 describes the naming convention for all other specimens using an example from a core column.

The example specimen name, C65-F1, describes the first sample taken from the flange of the core wideflange column from WTC 1 on column line 904 somewhere between floors 86-89. This column is below

and on the opposite side of the building from the impact zone. The sample that the example describes

supplied the tension specimens used for characterizing the tensile behavior of the core columns.

1.4 SYMBOLS AND ABBREVIATIONS

The preface to this report contains tables of acronyms and units used in this report. Table 1-3 defines

several symbols and ternis relating to mechanical testing.

NISTNCSTAR 1-3D, WTC Investigation
Introduction

Table 1-1. Specimen nomenclature for perimeter column specimens.

Example N8-C1T2-1W-L:

Sub

Element

Possible

Values Description

N8 Reco\ ered item tag (letter denotes the recycling yard where specimen originated^

CI Describes column # in panel''

CI Column 1 : column on the left, looking into the building

C2 Column 2: center column

C3 Column 3: column on the right looking into the building

T Describes specimen location in column

T Near top

M Near middle

B Near bottom

-» 1, 2. 3 etc., chronological by specimen cut from this larger section IW Describes plate location within column or other component within assembly

IW Inner w eb' LF Left flange (looking into building); sometimes abbreviated as FL

RF Right flange (looking into building); sometimes abbreviated as FR

OW Outer w eb

S Can refer to either spandrels or truss seats SP Can refer to either spandrels or splice plates

Step Truss seat EP End or butt plate of column

DBS Diagonal bracing strap that ties floor truss to column

G Gusset plate that attaches diagonal bracing strap to perimeter column panel

Tab Plate welded to perimeter column panel and truss L Describes orientation with respect to rolling direction'' L Longitudinal

T Transverse

2 1.2,3... Specimen number w ithin series of orientations

a. The strucniral engineenng drau ings identify the perimeter column panels by the column number of the center column

and the floors at the splices. For example, WTC 1 142 97-100 is tower I, column line 142 floors 97-100. In the Investigation notation, this is recovered panel N8. The structural engineering drawings number the columns looking from

outside the building.

b. CI corresponds to column line 143; C2 corresponds to column line 142; C3 corresponds to column line 141 in the example above.

c. The structural engineering plans also number the four plates in each perimeter column: RF and LF correspond to plate 1, OW corresponds to plate 2, IW corresponds to plate 3. and S or SP corresponds to plate 4.

d. For spandrel plates, the orientation designation refers to the orientation with respect to the vertical axis of the building.

Metallographic analysis later revealed that the rolling direction of the spandrel plates is along their long direction. Therefore all spandrel specimens designated with an "L" are oriented transverse to the rolling direction.

NIST NCSTAR 1-3D, WTC Investigation 9
Chapter 1

Table 1-2. Specimen nomenclature for core box and wide-flange shapes, trusses, and al other specimens.

Example C65-F1

SUD

Element

Possible

Values Description

C65 Recovered item tag (denotes the recycling yard where the specimen was recovered)

F Describes location within recovered item

F Flange (for WF shapes or wider plate for box columns)

FL Flange (for WF shapes or wider plate for box columns)

W Web (for WF shapes or narrower plate for box columns)

WEB Web (for WF shapes or narrower plate for box columns)

BA Bottom chord bulb angle (for trusses)

TA Top chord bulb angle (for trusses)

TR Round bar (for truss webs)

LR Large diameter round bar (for truss webs)

MR Medium diameter round bar (for truss webs)

SR Small diameter round bar (for truss webs)

TS Core truss seat Weld Taken from a weld in the component

Describes orientation with respect to rolling direction (used for tensile specnnens)

L Longitudinal

T Transverse

1 1, 2, 3... Specimen number within series of orientations

Table 1-3. Mechanical testing definitions used in this report.

Yield strength

Generally used as the specified minimum yield strength or yield point depending on the steel standard in use. (AISC usage)

ROA reduction of area (ASTM usage Standard E 6) TS Tensile Strength

The maximum engineering stress in a tensile test (ASTM E 6 usage)

Often referred to in other literature as ultimate tensile strength, UTS. or by the symbol 5„. YP Yield point

The first maximum in the engineering stress-engineering strain curve, associated with discontinuous

yielding. Also called upper yield strength (ASTM E 6 usage).

This report uses the symbol YP to denote a measured value of the yield point as opposed to its specified

minimum value. YS Yield strength

The engineering stress at which it is considered that plastic elongation has commenced. This document

uses the 0.2 % offset stress. (ASTM E 6 usage)

This report uses the symbol YS to denote a measured value of the yield strength as opposed to its specified minimum value.

EI, Total Elongation

The elongation (of a tensile specimen) determined after fracture by realigning and fitting together of the broken ends of the specimen. (ASTM E 6 usage)

10 NISTNCSTAR 1-3D, WTC Investigation
Chapter 2

Elastic Properties

2.1 INTRODUCTION

This section repons the expressions for the temperature dependence of the elastic properties of steel supplied to other NIST Investigation Team members. Expressions include Young's modulus £, Poisson's

ratio V, and shear modulus G. World Trade Center (WTC) structural steels can be assumed to be

elastically isotropic, so the shear modulus is related to E and v:

2(1 + K)

Because structural steels contain relatively low fractions of alloying elements, there is no significant

difference in modulus between structural steel variants.

From room temperature up to about 720 °C the primary metal phase in iron-carbon alloys such as

structural steel is ferrite, usually denoted as a-phase. Above this temperature steel undergoes a phase

transformation to a mixture of a-phase and y-gamma phase, or austenite. At still higher temperatures,

910 °C in pure iron, but lower in most structural steels, the phase transformation to all austenite is complete. In the two-phase region, the fraction of a-phase and y-gamma phase, and therefore modulus,

depends strongly on carbon content and temperature. This phase change introduces some difficulty in representing the change in modulus with temperature.

2.2 EXPERIMENTAL PROCEDURE

Three specimens of WTC steels, summarized in Table 2-1, were characterized in a thermomechanical

analyzer (Model DMA 2980, TA Instruments) over the temperature range -140 °C < T<600 °C.

Rectangular specimens nominally 50 mmxlO mmxl mm were loaded in three-point bending. The

thermomechanical analyzer oscillated the specimens at 1.0 hz at a constant amplitude of 15 ^m.

Measurements were conducted in two stages. The first was at temperatures, -140 °C < r< 50 °C. The

second was in the range 50 °C < r< 600 °C. In both cases, modulus determinations were made at 5 °C

intervals while the furnace temperature increased at 5 °C/min. During the high-temperature

characterization the specimen chamber was filled with an inert gas to suppress oxidation.

2.3 ELASTIC PROPERTIES (E, v, G) FOR 0°C < T < 723 °C

The literature is rich with determinations of the change of Young's modulus, E, with temperature for

r<723 °C. Figure 2-1 shows some examples. The wide variation in the values is an artifact of the

different measurement techniques. The data sets with the highest values were obtained using ultrasonic

techniques. These data represent adiabatic determinations, and are not appropriate for estimating elevatedtemperature modulus in structures. The data sets with the lowest values were probably determined from

NISTNCSTAR 1-3D. WTC Investigation 11
Chapter 2

the loading portion of high-temperature tensile tests, and therefore include inelastic specimen

deformation. These determinations are also inappropriate.

Figure 2-1 also plots Young's modulus, E, determined for the three specimens of perimeter columns,

summarized in Tab. 2-1, determined using the thermomechanical analyzer.

A third-order polynomial was sufficient to represent the data for 0 °C < r<600 °C for the three steels summarized in Table 2-1 and plotted in Fig. 2-2.

EiT) = e^^ + ej + ej^' + e^T^

where

eo= 206.0 GPa

e,= -0.04326 GPa/°C

62= -3.502xlO-^GPa/(°C)-

-6.592x10-^ GPa/(°C)-

The constant term, eo, was fixed as the average of the three steels at 0 °C and was not part of the

polynomial fit. The room-temperature value for E for the fit lies within the range of the highly regarded

data that Galambos (1978) recommends.

Figure 2-3 summarizes the relatively scarce literature data for Poisson's ratio, v, as a function of

temperature. The data for iron (Dever 1972) and mild steel (Clark 1953 cited in Cooke 1988) do not differ

significantly. The solid line is the fit of a fourth-order polynomial to all the data for 0 °C < T < 725 °C:

v{T) = n^^ + riyT + n^T' + n^T^ + n^T'^

where ^

770= 0.28737362

/?,= 2.5302417x10"- (°C)"' 2-3

n2= 2.6333384x10"^ (°C)"-

773= -9.9419588x10 " {°C)-^

1.2617779x10"'-' (°C)"^

for T in the range 0 °C < T < 725 °C. The room-temperature value, (v = 0.288), is 4 percent smaller than

the commonly used value v = 0.3, which has been rounded to one significant digit.

The shear modulus, G, was determined from E and v, using Eq. 2-1 . To generate the curve shown in Fig. 2-2, values for the shear modulus as a function of temperature, G (T), were calculated at discrete

points from Eq. 2-1 using the expressions for E (T) and v (7), Eqs. 2-2 and 2-3. The resulting data were

fit using a fifth-order polynomial.

G{T) = g, +g,T + gj' +g,T'+gX +gsT'

where

go= 80.005922 GPa

gi= -0.018303811 GPa/°C 2-4

g2= -1.5650288x10"^ GPa/(°C)'

g3= -1.5160921x10 ' GPa/(°C)-'

g4= -1.6242911x10"" GPa/CX)"*

g5= 7.7277543x10"'-' GPa/(°C)^

12 NISTNCSTAR 1-3D, WTC Investigation
Elastic Properties

for Tin the range 0 < 7 < 725 °C. The room-temperature value of the shear modulus, G, calculated this way differs by 3.5 percent from the actual measurements cited in Galambos (1978) and the Metals

Handbook (ASM 1978), but it is consistent with the values for E and v from Equations 2-2 and 2-3.

In using the expressions for modulus at elevated temperature, it is important to recognize that the flow

stress of structural steel above the a-y phase transformation may be only 1 0 percent or less of the roomtemperature value. Calculations that use the high-temperature value of the modulus and do not account for the very reduced strength may be inaccurate.

2.4 ELASTIC PROPERTIES (E, v, G) FOR r> 910 °C

The literature data for elastic modulus of y-Fe, the high-temperature phase, is sparse. Koster (1948)

provided data for a function for calculating the modulus, Ey(T), of austenite (y-phase) in the range

910 °C < T < 1000 °C, which is also plotted in Fig. 2-1.

yi= 216.0 GPa 2-5

Y2= 26.85 °C

73= 4.7x10"' °C'

Data for the Poisson's ratio for austenite are not available. The shear modulus, G, can be calculated from

Eq. 2-1 after a suitable estimate of Poisson's ratio is made.

2.5 ELASTIC PROPERTIES (E, v, G) FOR 723 °C < T< 910 °C

In principle, it is possible to do a rule-of-mixtures calculation to evaluate the modulus as a function of

temperature in the two-phase field. Many assumptions are necessary to make the problem tractable, and

even then, each steel, because of its composition, would have a different value for the composite modulus.

Some of these assumptions include

• The extrapolation of Eq. 2-5 correctly describes the modulus of austenite in the two-phase

field.

• The extrapolation of Eq. 2-2 correctly describes the modulus of ferrite in the two-phase field.

• All alloying elements other than carbon have no effect on the positions of the phase

boundaries.

• The phase change is infinitely fast so that the time-dependence can be ignored. This may be a good approximation on heating, but it is much less appropriate for cooling.

The uncertainties that these assumptions introduce probably exceed the added fidelity that a rule-of- mixmres calculation might provide. Instead of the complicated rule-of-mixtures calculation, a much

simpler method for approximating the elastic modulus in the two-phase region, Ea.y is to interpolate

between the two endpoints:

NISTNCSTAR 1-3D. WTC Investigation 13
Chapter 2

E^U,)-E^{A,)

A, -A,

2-6

where yi, = 723 °C and = 910 °C.

2.6 UNCERTAINTIES

The expression for Young's modulus, Eq. 2-2, represents the experimental data obtained at NIST

(Table 2-1) to within 2 percent for all cases, Fig. 2-4

AISC. 1973. Manual ofSteel Construction. American Institute of Steel Construction. New York p. 6-11.

ASME. 2004. Boiler and Pressure Vessel Code. Section II Materials, Part D Properties (Customary),

Subpart 2 Physical Properties Tables, Table TM-1. ASME International. New York. p. 696

Clark, C.L. 1953. High-temperature Alloys. Pitman, New York.

Cooke, G.M.E. 1 988. An Introduction to the Mechanical Properties of Structural Steel at Elevated

Temperatures. Fire Safety Journal. 13 45-54.

Dever, D.J. 1972. Temperature Dependence of the Elastic Constants in a-Iron Single Crystals:

Relationship to Spin Order and Diffusion Anomalies. J. Appl. Phys. 43 (8). 3293-3301.

Fields, R.J. T. Weerasooriya, and M. F. Ashby. 1 980. Fracture-Mechanisms in Pure Iron, Two Austenitic

Stainless Steels, and One Ferritic Steel. Met. Trans. A. UA 333-347.

Galambos T.V. and M. K. Ravindra. 1978. Properties of Steel for Use in LRFD, J. Struct. Div. ASCE. 104

(9). 1459-1468.

Koster, W. 1948. Die Temperaturabhangigkeit des Elastizitatsmoduls reiner Metalle, Z. Metallkd. 39(1).

Uddin T. and C. G. Culver. 1975. Effects of Elevated Temperature on Structural Members. J. Struct. Div.

ASCE. 101 (7). 1531-1549.

2.7 REFERENCES

1-9.

14 NISTNCSTAR 1-3D, WTC Investigation
Elastic Properties

Table 2-1. Specimen data for Young's modulus (E) determination .

Specimen

Tower Column

Line and Floor

Range

Specified Yield

Point

F,(ksi)

Location in Column

N9-C2B-FR-1 WTC 1 Column 154

Floors 101-104

55 Right flange

C68-C3T1-EP-2 Unknown butt

plate from

perimeter column

50 Unknown

S9-C3T-FL-1 WTC 1 Column 133

Floors 97-100

55 Left flange

200

150

Qo

UJ

100

1 NIST

2 Tall 3 Stanzak

4 Brockenbrough

5 ASME [C]<0.3 %

6 ASME [C]>0.3 %

7 Clark

8 De-er

9 Koster austenite

1 0 Koster ferrite

T 1

I

1 I I

I

I I I E(T)= Gq + e^T + GjT^ + e^T^ = +206 GPa

= -4.326e-02 GPaAC

e, = -3.502e-05 GPa^C^

-6.592e-08 GPa/'C^

9^.

200 400 600 800 1000

Sources: Clark 1953; Cooke 1988; Koster 1948; Dever 1972; Uddin 1975: ASME 2004. Figure 2-1. Young's modulus as a function of temperature.

NISTNCSTAR 1-3D, WTC Investigation 15
Chapter 2

200

180

re Q.

CDm 160

140

120

0 200 400 600 800

T,°C

10:33:13 - Friday November 05, 2004

Figure 2-2. Young's modulus, E(T), and shear modulus, G(T) for 0°C < T< 725°C. Young's

modulus was measured on the WTC steels summarized in Table 2-1. Solid line is Eq. 2-2. Shear modulus, G, calculated from E and v via Eq. 2-4.

0.320

0.315

0.310

0.305

0.300

0.295

0.290

0.285

0 200 400 600 800

T, °C

10:33:16 - Friday November 05, 2004

Source: Dever 1972; Clark 1953; Cooke 1988. Figure 2-3. Poisson's ratio (v) as a function of temperature. The solid line is the fit of a

4th order polynomial (Eq. 2-3) for 0 °C < r< 725 °C.

16 NISTNCSTAR 1-3D, WTC Investigation
Elastic Properties

Figure 2-4. Fractional error in representing the Young's modulus data for the three

specimens of perimeter column steel (Table 2-1) using Eq. 2-2.

NISTNCSTAR 1-3D. WTC Investigation 17
Chapter 2

This page intentionally left blank.

18 NISTNCSTAR 1-3D, WTC Investigation
Chapter 3

Room-Temperature Tensile Properties

3.1 introduction

The National Institute of Standards and Technology (NIST) had three goals in characterizing the tensile properties of the steels, bolts and welds. The first was to compare the measured properties of the steels with the specifications they were purchased under. The second was to determine whether their properties were consistent with the properties and quality of other structural steel from the World Trade Center

(WTC) construction era. The third was to determine the constitutive behavior for input into the finite element models of the Investigation, which modeled the response of the building components to the

airplane impact and to the high temperatures produced by the subsequent fires.

To satisfy the first and objectives, NIST tested several hundred specimens taken from columns, trusses, and truss seats that represent all the major strength levels and most of the grades of steel in the fire and

impact zones of the two towers.

To satisfy the third objective, NIST produced values yield strengths and representative stress-strain curves

for each of the many grades of steel in the towers and supplied these to other Investigation Team

members. The representative tensile stress-strain curves were corrected for dynamic effects. The curves drew on experimental data as well as surviving WTC mill test reports and literature data. The Voce

work-hardening law (Voce 1948) described behavior of the representative curves. The parameters for the

Voce law originated from the tensile behavior of recovered WTC steels.

3.2 TEST procedures

3.2.1 Steel

Standard tensile tests were generally conducted according to ASTM International (ASTM) E 8-01,

"Standard Test Methods for Tension Testing of Metallic Materials" using a closed-loop, servo-hydrauhc

or a screw-driven, electro-mechanical test machine. In most cases, yield strength determination was made

at a running crosshead rate that produced post-yield strain rates between 0.001 min"' and 0.005 min"'. In some tests the extension rate was increased to produce a plastic strain rate of 0.05 min"' for tensile

strength determination. In other cases, the entire test was conducted at a constant extension rate. Several

tests were conducted at a slightly higher extension rate 0.0325 mm/s, which produced plastic strain rates

that conform to ASTM A 370-67, the test method used by steel mills to qualify steel.

Much of the recovered steel was damaged in the collapse of the towers or subsequent handling during

clearing of the WTC site. Specimens were selected from the least deformed regions. Even so, some of the

resulting stress-strain curves of the low-strength steels do not exhibit a yield point. In such cases, yield

strength was determined by the 0.2 percent offset method as specified in ASTM E 8-01.

The geometrical and thickness differences in the various components did not allow for a single, standard

specimen geometry for the tensile testing. Whenever possible, specimens were made using the full

NISTNCSTAR 1-3D, WTC Investigation 19
Chapter 3

thickness of the material, according to ASTM E 8. For plates less than 0.75 in. thick, the specimens were

a mixture of the standard flat and round geometries. For plates greater than 0.75 in. thick, the specimens were generally the ASTM standard 0.500 in. diameter round geometry. Truss rods were machined into

round tensile specimens while truss angles were machined into subsize flat tensile specimens. Truss seats and core columns were machined into rounds. Figures 3-1 to 3-11 show the various tensile specimen

configurations.

Specimen strain was measured using class B2 extensometers that conformed to ASTM E 83, "Standard

Practice for Verification and Classification of Extensometer System." In most cases, the operator

removed the extensometer prior to specimen failure. Total elongation was calculated by measuring gauge

marks on the specimen prior to testing and again following fracture. Reductions of area for the round

specimens were measured by comparing the original cross-sectional area to the area of the minimum

cross section at the fracture location. Reductions of area for the rectangular and square cross section

specimens were calculated based on the parabolic shape of the fracture surface, according to note 42 of

ASTM E 8-01. V

The notch sensitivity of two types of core column steels was evaluated using notched round tensile

specimens. Fig. 3-1 1, taken from the quarter depth of plates. All three specimens have identical minor

diameters of 0.25 in., but each has a different notch radius. Specimens were instrumented using an axial

extensometer mounted on the major diameter and a diametral extensometer mounted on the notch root. Specimens were loaded at 0.00033 in./s, which produced an initial, elastic stressing rate of approximately

0.6 ksi/s.

3.2.2 Bolts

Tensile tests for determining the room-temperature load-displacement relation on the A 325 bolts generally followed ASTM Recommended Practice F 606, "Test Methods for Determining the Mechanical

Properties of Externally and hitemally Threaded Fasteners, Washers, and Rivets." Although many

columns were recovered, very few contained undamaged fasteners suitable for mechanical testing. In all, only one A 490 and seven A 325 bolts were suitable for testing. Of all the perimeter columns recovered,

only one, the unidentified C-68, had any associated A 325 bolts from the endplates. These seven bolts were the only A 325 bolts characterized. The single A 490 bolt tested came from a "bow-tie" section from

floor 7. A cormnercial metallurgical testing firm tested three A 325 bolts and the A 490 bolt. To provide

load-displacement data and assess the magnitude of strain rate effects, NIST tested four A 325 bolts: two

bolts at each of two displacement rates. The rates chosen were very slow (0.0007 in/s crosshead

displacement) and as fast as the machine would go (2.0 in/s). For these tests, two threads were exposed

rather than the four required by ASTM F 606. Forensic observations of recovered perimeter column

sections showed that failed bolts had from zero to four threads exposed. In addition, to obtain

unambiguous load-displacement data, the NIST tests did not employ the wedge required by A 325.

20 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

8.00

Dimensions are in Inches

Uniforr^i Test Section

Flat 2"

Specimen thickness

is original section

thickness

Figure 3-1. Flat tensile specimen typically used for standard room-temperature quasistatic tensile tests.

5. CO

Uniform Test Section

2.500

Specimen thickness is original section

thickness

o

ol

Dimensions are in Inches Flat 1"

Figure 3-2. Flat tensile specimen typically used for elevated-temperature tensile tests.

NISTNCSTAR 1-3D. WTC Investigation 21
Chapter 3

4.10 - 4.25

Specimen

Uniform Test Section thickness is original section

Dimensions are in Inches Flat-C 1

" thickness

Figure 3-3. Flat tensile specimen used for room and elevated-temperature tensile tests.

3.94

Specimen

thickness is 0.118 in. (3 mm)

Dimensions are in Inches Flat-C2 1"

Figure 3-4. Flat tensile test specimen used for some creep tests.

22 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

Dimensions are in inches

Figure 3-5. Flat tensile test specimen used for some creep and elevatedtemperature tests.

Norm. 3.0— Inches Overall Length

Uriform Test Section

Dimensions are in Inches Rd 1"

Figure 3-6. Round tensile specimen used for room-temperature and elevated

temperature tensile tests.

NIST NCSTAR 1-3D. WTC Investigation 23
Chapter 3

Uniform Test Section

Dimensions are in Inches Rd 2"

Figure 3-7. Round tensile specimen used for room-temperature tensile testing.

00d

4.3

Specimen thickness

is 0.172 in.

oo

Uniforrri Test Section

Dimensions are in Inches Fiat-W 1

Figure 3-8. Flat tensile specimen typically used for tensile testing of all-weld metal

specimens.

24 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

Uniform Test Section 1.125

5.0

Dimensions are in inches Heat affected zone weld specimen HAZ-1

Figure 3-9. Heat affected zone tensile test specimen with flange/web weld intact. The

flange is the specimen portion that is in tension.

Uniform Test Section 1.125

Weld Removed

and Surfaced

5.0

Dimensions are in inches Heat affected zoHB wsld Specimen

HAZ-2

Figure 3-10. Heat affected zone tensile specimen with weld and web machined flush to

the flange surface. The flange is the specimen portion that Is in tension.

NISTNCSTAR 1-3D, WTC Investigation 25
Chapter 3

3/4-10 "breads

Typ. both ends

NRB - 1 NRB - 2 NRB - 3

Dimensions are in Inches

Figure 3-11. Notched tensile specimens.

3.2.3 Welds

Tests on specimens of all-weld metal from perimeter columns followed ASTM E 8 and used flat specimen type Flat-Wl", Fig. 3-8.

Transverse tensile tests on perimeter column welds also followed ASTM E 8 and used specimen types

K-1 and K-2, Figs. 3-9 and 3-10. In these flange specimens, the weld and the two heat affected zones (HAZ) are oriented transverse to the pull direction. The reduced section region extends from within the

base metal (plate), through the weld and surrounding HAZ, and well into the base metal on the other side

of the web. The type K-1 specimen retains a stub of the perimeter column outside web about 0.5 in. long,

which extends at a right angle from the center of the specimen. In the type K-2 specimen, the stub of the

web is removed entirely.

The shear strength of the resistance welds in the floor trusses was evaluated on a segment of the lower

chord of a truss. A push rod gripped in the upper hydraulic grip of a 220,000 Ibf closed-loop, servo- hydraulic test machine loaded the truss rod at 0.001 in./s while the angles of the lower chord rested on a

clevis, Fig. 3-12.

26 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

Source: NIST. Figure 3-12. The resistance weld shear strength test (a) before loading, (b) after failure.

NISTNCSTAR 1-3D, WTC Investigation 27
Chapter 3

3.3 RESULTS

3.3.1 Steel

Figure 3-13 displays some example stress-strain curves for perimeter column, core column, and truss

steels. Appendix A of this report summarizes the measured yield and tensile strengths, total elongations,

and reductions of area for all the specimens tested. It also contains stress-strain curves for most

specimens. Sections 3.4.1 and 3.5.1 analyze these results in terms of engineering specifications.

Figure 3-14 shows stress-strain curves for two sets notched bar tension tests on core column steels. Specimen C71-1-F2 is from the flange of a F, = 36 ksi 12WF190 shape from WTC 1 column line 904

between floors 77-80, oriented longitudinally. The un-notched specimen, labeled R = <x), has an unexpectedly high yield strength and lacks the expected yield point behavior, probably from damage

incurred in the collapse and subsequent recovery. The flange was 1.736 in. thick. Specimen B6152-1-F1

is from a flange plate of a F, = 36 ksi type 380 box column from WTC 1 column line 803 between

floors 15-18. The original plate was 2.06 in. thick.

Stresses were calculated on the 0.25 in. diameter reduced cross section. The axial strain was calculated

using the notch height, 2R, as the gauge length, see Fig. 3-1 1. The diametral strain was calculated using

the 0.25 in. reduced cross section diameter, which is the same for the three specimen geometries.

Figure 3-14 plots the diametral strains in the negative direction and includes the results of tests on

unnotched specimens for comparison. Because the operator sometimes removed the extensometers before

failure, the strains may not represent failure strain. All fractures occurred by ductile hole growth; there was no evidence of a transition to cleavage fracture. The increasing tensile strength with decreasing notch

radius shows that neither steel is notch-sensitive under the test conditions.

3.3.2 Bolts

Table 3-1 summarizes the results of tests on bolts. Figure 3-15 plots the load-displacement data for the

four WTC A 325 bolts tested. All seven A 325 bolts failed in the threads, as expected.

3.3.3 Welds

Perimeter Columns

The yield strengths of two specimens of all weld metal produced by submerged arc welding from

specimen N-9 (WTC 1, column 154, floors 101-104 specified F, 55 ksi) are F, = 85.1 ksi and

Fy = 84.8 ksi, Table 3-2. Their tensile strengths are TS^ 103 ksi and TS = 103.3 ksi.

Transverse tests on weld specimens were conducted on fillet welds from specimen N-8 (WTC 1, column

142, floors 97-100, specified F, = 60 ksi). The yield and tensile strengths are independent of the specimen

type, Tables 3-3 and 3-4, so the presence of the stub of the web did not create a stress concentration.

Because none of these transverse specimens broke in the weld, the strengths do not represent the weld

strength, but rather represent the strength of the weakest region across the entire joint, which encompasses

base metal to weld to base metal.

28 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

120

100 -

80 -

20 -

0

T—I—I—I—

r

1—I—I—I—I—I—

r

1: C132-TA-7-W04a F =50 ksi

2: B6152-2-F1-2-L1 (box) F =36 ksi

3: C30-W1-1-05 (web) F =36 ksi

4: HH-FL-2-L1 F,=42 ksi Core Columns

and trusses

_i 1 I I I I 0.00 0.05 0.10 0.15 0.20 0.25 0.30

Engineering strain

120

100

80

60

(0

ina>

i_ ^—

CO

_c

(DO

40

O)c

LU 20

-I—I—I—

r

1: S9-C1T-S1-L1 F =36 ksi

2: M26-C1B1-RF-L1 F^=50 ksi

3: N6-C1B1-LF-L2 F,=60 ksi

4: C46-C2B-FL-1-L1 F,=70 ksi

5: C25-C1B-FR-2-L1 F =80 ksi

6: ClO-ClM-Fi.-1-h F=iOO kSi

_L

Perimeter Columns

-I I L. 0.00 0.05 0.10 0.15 0.20 0.25 0.30

09:55:03 - Wednesday September 29. 2004 Engineering strain

Figure 3-13. Examples of stress-strain curves for perimeter column, core column, and

truss steels. In most cases, the strains do not represent failure.

NISTNCSTAR 1-3D. WTC Investigation 29
Chapter 3

120

20

-| 1 1 1 1 1 r n I I I r

R=0.031 in.

"^=0.062 in.

R=0.125 in.

Strain

Core box column B6152-1-FL-1-2 WTC 1 column 803 15-18

Fy = 36 ksi

_1 I l_l I \ I I I I L -0.2 0.0 0.2 0.4 0.6 Engineering strain

0.8

120

100

CO 80 w

0)

00O) 60

0)

<X) 40 c

LU 20

1

\—^—I\—\—I—I—I—I

I R=0.031 in. /

R=0-063 in.

R=0,125 in.

Axial

Strain

Core wide-flange C71-1-F1 WTC 1 Column 904 77-80

Fy = 36 ksi

I I I I I I I L -0.2 0.0 0.2 0.4 Engineering strain

0.6 0.8

10:25:34 - Wednesday September 29, 2004

Figure 3-14. Stress-strain curves for notched round bar tests using the specimens

in Fig. 3-11.

30 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

Table 3-1. Results of tensile tests on bolts.

Tests on A 325 Bolts

Test Type Bolt Size

1 ensue

Strength

(Ibf)' Notes A<sTM F f,C\fi n"* i i>-i r uvju-u— / o in.x4.zj in. failed in threads

/\^ A<sTM F fiOf^ D"' I Xvi r uuO-u_ / o in.x4.^ J in. AO nnn failed in threads A^sTM F f,Of^-()'> 1 iM r uuu u— 1 JS in.x4._j in. idiieu in inreaus

1 esi Rate

(in./s)

Tensile

Strength

(Ibf)

load-elongation 7 8 in.x4.5 m. 0.00065 67.900 failed in threads

load-elongation 7/8 in.x4.0 in. 0.00065 68,300 failed in threads

load-elongation 7/8 in.x4.0 in. 2.0 68.800 failed in threads

load-elongation 7 8 in.x4.0 in. 2.0 68.200 failed in threads

Test on an A 490 Bolt

Test Bolt Size (psi)

TS

(psi)" Notes A 370 tensile' 1 in.x5 in. 153.500 163,400 £1, = 16 % (2 in. gage length)

ROA = 59.5 %

a. For A 325 bolts 7"Sm,n = 120,000 psi. For 7 8 in. bolts minimum tensile strength = 55,450 Ibf based on^ = 0.462 in." for 7/8 in. diameter bolts.

b. For A 490 bolts 7"Sm,n = 150.000 psi; minimum F, = 130,000 psi; minimum EL, = 14 percent (based on 2 in. gage

length ); minimum reduction in area ROA = 40 percent.

c. A 490 bolt was tested as a round tension specimen machined from the bolt.

8x10'

6

2

0

1

1 1 1 1

1

I 1 11 1

1

1 1 1

1

1 1 1 1

- r \

\

\

rate = 0.00066 in./s

1 1 1 1 1 1 1

- - - rate = 2.0 in./s average

Elastic loading 2.5x10** Ibf/in.

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 II I I I I I

I I I I I I I I I \ I \ i I I I—I—I—I

\—I—I—

I

0.00 0.05 0.10 0.15 0.20 0.25 0.30 displacement, In.

Figure 3-1 5. Load-displacement curves from four tensile tests of A 325 bolts, and the

average curve.

NISTNCSTAR 1-3D. WTC Investigation 31
Chapter 3

Table 3-2. Room-temperature weld properties as measured.

Weld Location and Type (ksi)

TS

(ksi) Comments

Perimeter column fillet weld

from N9 WTC 1 column 154 101-104

base metal Fy =55 ksi test 1

85.1 103.0 All weld metal

NIST test

Perimeter column fillet weld

fromN9 WTC 1 column 154 101-104

base metal Fy =55 ksi test 2

84.8 103.3 All weld metal

NIST test

Perimeter column lower strength steels 81.0 92.3 From PC&F Procedures

Perimeter column higher strength steels 105.0 118.0 From PC&F Procedures

Truss resistance welds (in shear) 46.4 NIST test Total Load 104,000 Ibf

5/8 in. Core column weld in shear Test 1 82.9 From Stanray Pacific Procedures

5/8 in. Core column weld in shear Test 2 84.9 From Stanray Pacific Procedures

0.5 in. submerged arc weld for core column in shear

129.6 From Stanray Pacific Procedures

Table 3-3. Results of transverse tensile tests on welds from specimen N-8 (WTC 1,

column 142, floors 97-100 , specil led Fy = 60 ksi).

Specimen Specimen Type (ksi)

TS

(ksi)

Total

Elongation

(%) Comments

N8-C3B1-RF-HAZ-2 HAZ-1 (with

stub Fig. 3-9)

70.0 95.8 12 Broke at weld root

N8-C3B1-RF-HAZ-4 HAZ-1 (with

stub Fig. 3-9)

n/a 92.6 n/d

N8-C3B1-RF-HAZ-1R HAZ-2 (without

stub Fig. 3-10)

65.3 92.2 15 Broke 0.5 in. from weld

N8-C3B1-RF-HAZ-3R HAZ-2 (without

stub Fig. 3-10)

67.8 92.9 17

Key: n/d, not detennined.

Table 3-4. Fillet weld sizes for various plate thicknesses in the core box columns.

Plate Thickness Range (in.) Fillet Weld Size (in.)

0.75 to 1.5 3/8

1 5/8 to 1 7/8 7/16

2 1/8 to 2 5/8 1/2

32 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

Trusses

Although much of truss Tl was seriously deformed, it was possible to select several 5 in. to 6 in. long,

relatively undamaged lower chord sections with resistance welds. The load for shear failure was

104.000 Ibf using the geometry of Fig. 3-12. On the fracture surface, the two weld areas that make up the

resistance weld measure 0.45 in." and 0.67 in.". Assuming the two angles shared the load equally, and that the tu'o welds on the side that did not fail have the same area as those on the side that did fail, the weld

shear strength is 46.4 ksi.

3.4 COMPARISON WITH ENGINEERING SPECIFICATIONS

3.4.1 Steel

There are a number of issues in testing recovered steel that complicate answering the question, "Are the

NIST-measured properties of the steel consistent with the appropriate specification?" They fall into three

main groups: differences between the NIST test procedures and those in the mill test, natural variability of

steel properties, and confounding effects from the post-fabrication handling of the steel. This section first discusses these complicating factors, and then summarizes the mechanical property data for each

representative steel specimen in relation to the requirements of the original specifications.

Test Method Differences

One issue that complicates comparing the NIST-measured properties of the steel to the requirements of

the original specification arises from differences between the test methodology used in establishing the

mill test report and the test methodology that the NIST Investigation team used.

For structural steel, ASTM A 6 specifies that mill tests are conducted in accord with ASTM A 370. Most

of the NIST tests on recovered steel were conducted in accord with ASTM E 8, which is a more generic

standard for tensile testing. The major differences between the two are in the testing rate. E 8 prescribes

that the testing rate, r, lie in the range 1.15 MPa/s < r < 11.5 MPa/s. A 370 prescribes only that the

maximum strain rate in the uniform cross section to be 0.00104 s"'. In general, the operator calculates this

strain rate by assuming that all the actuator motion produces elastic strain in the specimen, which is frequently a poor assumption, because the test machine is not infinitely stiff It is a reasonable

assumption, however, that the A 370 rate used for the mill test report is between 5 and 10 times the rate used in the E 8 tests at NIST. A rule of thumb is that the measured yield strength increases between 2 ksi

to 5 ksi per decade of strain rate. (Rao 1966. Johnson 1967, Barsom, 1987) On the average, then, the yield

point measured in the NIST Investigation could be up to 3 ksi less than the one measured in the mill test report from rate effects alone. Unfortunately, the "rule of thumb value" is approximate: some steels exhibit almost no rate sensitivity, so it is impossible to predict the magnitude of the effect.

Another problem is that A 370 specifies three alternative methods for determining yield point and does

not require the mill to report which method it used. In any case, very few mill test reports have survived,

and none exist for the particular plates or shapes that are in the NIST inventory. The three methods are

• Drop of the beam method (for quasi-load controlled systems)

• Position of the knee or first maximum load in the curve

NISTNCSTAR 1-3D, WTC Investigation 33
Chapter 3

• Total extension under load

In any given test, these three methods all yield slightly different values for the yield point. If a definite

yield point exists, it will generally be the highest of the possible values. NIST is unaware of any studies

that document whether one of the other methods consistently yields higher values, however.

A 370 requires that plates and shapes with / < 1.5 in. must be tested full thickness with an 18 in. long,

1.5 in. wide specimen. Because of test machine limitations and lack of suitable, undamaged material,

NIST frequently tested smaller specimens than ASTM A 370 calls for. NIST generally used 8 in. long,

0.5 in. wide full thickness flat specimens for plates with /<0.5 in. For most other plates, NIST employed

0.5 in. diameter round specimens. Most ASTM standards prescribe that total elongation is to be measured

on the 18 in. long specimen using an 8 in. gauge length, instead of the 2 in. gauge length used for the 8 in. specimen. For tests on samples taken from the same plate, the total elongations measured using 8 in. long

flat specimens will be larger than those measured using 1 8 in. long specimens, because the necked region

in the shorter specimen will occupy a greater fraction of the total gauge length. There is no absolute

scaling law between the two gauge lengths, however.

During the WTC era ASTM A 6 prescribed that specimens for mill tests of wide-flange shapes originate

from a section of the web. In some cases due to material quality problems, NIST harvested specimens

from the flanges of wide-flange shapes. In general, because they are thicker than the webs, and therefore

cool more slowly, the flanges will exhibit a yield point that can be several ksi lower than tests from the

webs. In the 1974 AISI report on property variability, the average for the difference between product

flange tests and official mill test reports is nearly zero for low strength (F, < 40 ksi) steels, but is about

4 ksi for higher strength steels.

Natural Variability

ASTM steel standards require very little tension testing for mill acceptance. For example A 36-66

required only two tension tests from each heat, unless the heat was less than 50 tons, where only one test was necessary. The standard does not clarify how the yield point shall be reported when the two tests yield different values. In the early 1970s the American Iron and Steel Institute (AISI) conducted an

extensive study on the variability of mechanical properties and chemistry of structural steel. That report

has a great deal of information on product tension tests as they relate to the mill test report. The critical aspect is that the mill test report is a single tension test, conducted on one piece of a large lot of steel. It is a check that the quality of the steel is acceptable. The contemporary version of A 6 specifically calls out

the AISI report (1974) and points out that a mill test report of YP > 36 ksi is not a guarantee that all the

product supplied has YP > 36 ksi:

"These testing procedures are not intended to define the upper or lower

limits of tensile properties at all possible test locations within a heat of

steel. It is well known and documented that tensile properties will vary

with a heat or individual piece of steel. It is, therefore, incumbent on

designers and engineers to use sound engineering judgement when using

tension test results shown on mill reports. The testing procedures of

Specification A6/A6M have been found to provide material adequate for normal structural design criteria."

34 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

The AISI property variability study (1974) has a substantial amount of data on product tests on both wideflange and plate steels, where the product tests are taken from the same plate or shape as the mill test specimen. The variability within the entire heat, which encompasses many plates or shapes, will certainly

be larger than this. The chapter on wide-flange sections indicates that the standard deviation of the

difference between the mill test report and a product test from the same shape is about 2.3 ksi for shapes

with YP < 40 ksi. In other words, it is not uncommon for product tests to be significantly lower than the

mill test report. Barsom and Reisdorf (1988) reached similar conclusions from tension tests on

heavyweight rolled A 36 wide-flange shapes.

Effects Arising From Prior Deformation

All steels relevant to the WTC Investigation except the F,. = 100 ksi steels were qualified at the mill based on yield point, YP, rather than on yield strength F, or YS. Figure 3-16 defines yield point, yield strength,

and yield point elongation graphically. Alpsten (1972, Fig. 5) reported a typical difference between the

yield point (the maximum stress reached before yielding) and the yield strength (for instance defined by

the 0.2 percent offset method) as about 3 ksi. Individual values were as large as 10 ksi. Testing at NIST

has borne out this observation. Plain carbon steels, such as A 36, will almost always exhibit a yield point.

Microalloyed steels (A 572, A 441) may or may not exhibit a yield point, but there is no a priori method

to predict the existence or magnitude.

Q I I I I I I I I I I

I I I I I I I I I I 1 1 1 1 1 1 0

0.00 0.01 0.02 0.03 0.04 0.05

Strain

Figure 3-16. Schematic diagram of the various definitions of yield behavior in

mechanical testing of steel.

NISTNCSTAR 1-3D. WTC Investigation 35
Chapter 3

The yield point elongation, or Liiders' strain, in Fig. 3-16 arises because of inhomogeneous deformation

in the gauge length. Mobile interstitial impurities, mostly carbon and nitrogen, can diffuse to the cores of

dislocations and pin them in place. An overstress is necessary to break the dislocation free of its atmosphere of interstitials. Rather than homogeneous deformation throughout the gauge length, the early

stages of deformation of such steels proceeds as deformation bands sweep through the gauge section. A

pre-strained specimen, where dislocations have already been freed from their interstitial atmospheres, will not exhibit yield point elongation.

Any prior deformation of the material for the test specimens typically removes the yield point. Two

scenarios exist. The prior deformation may be sufficient to remove the yield point, but only partially remove the yield point elongation. In this case the NIST-reported value will almost certainly be less than

what the mill test report would have stated. Larger prior deformation will eliminate the yield point

elongation entirely and result in a work-hardened steel. In this case, the NIST-measured yield strength

may be more or less than what the mill test would have reported, and it may still be less than the specified

value. • \

The A 36 steels should exhibit a yield point and yield point elongation if undeformed. If a low-strength

steel does not exhibit yield point behavior, and the resulting yield strength is expressed as 0.2 percent

offset yield strength, there is a significant probability that the NIST-reported yield strength will be less than the specification called for. This does not imply, however, that the NIST-measured mechanical

properties of the steel are inconsistent with original specification. In general, a fraction of a percent strain will remove the yield point, and a further (1-3) percent will remove the yield point elongation. The strains induced by the collapse and subsequent handling are probably sufficient to remove the yield point

elongation. Any test of a F,. = 36 ksi steel that does not resuh in the appearance of a yield point and yield

point elongation should be regarded with caution.

The pre-strain that removed the yield point elongation can further reduce the measured yield strength

through the Bauschinger effect (Uko 1980). The measured yield strength of a specimen that has been

strained in the positive direction and subsequently tested in the negative direction will be less than

originally measured in the positive direction. The yield strength of a deformed flange may be lower if the

direction of the tensile test is opposite to the direction of its pre-strain. The literature is not clear on the

possible magnitudes of the reduction for pre-strains less than the typical yield point elongation, however.

Identification Methodology

The results of the NIST-conducted chemical and mechanical characterizations can be compared to the

requirements of the various ASTM steel specifications allowed in the steel contracts. In some cases, it is possible to determine with reasonable confidence that the properties of the steel are consistent with the

requirements of individual ASTM or proprietary specifications. In other cases, especially for the

perimeter columns where the historical record indicates that many proprietary steels were used, this chapter attempts to identify the specific mill that rolled the steel. Identification of the specific mill cannot

be made with the same level of certainty as correspondence with intended specifications, because it requires combining information from several disparate sources and making reasonable assumptions based on historical data and steelmaking practice. No documents exist that identify whether a given column was

to be fabricated from steel meeting a given ASTM standard or proprietary specification. Instead, the

structural engineering plans specify only the minimum yield strengths of the steel. The fabricator was free

36 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

to choose from a large list of acceptable steels. One exception to this rule is for columns with specified

Fy = 36 ksi. ASTM A 36 is the only structural steel specification in the list of approved steels that supplies

steel with this yield point.

Figure 3-17 illustrates the method for identifying possible steel grades graphically. The Port of New

York Authority' (PONYA) contract with the steel fabricators allowed them to use steels that met

enumerated ASTM standards, as well as specific proprietary steels, without further approval. In addition,

fabricators could request permission from PONYA to use certain unlisted steels after providing property

data. The Port Authority of New York and New Jersey (PANYNJ) documents reviewed during the

Investigation contain many examples of these requests. These documents do not provide sufficient

information to identify an unknown steel, however, because the standards generally only specify the allowable ranges of a few chemical elements. Steel mills have much more specific recipes for individual

steels. Occasionally, NIST has identified literature examples of named steels that can help identify

unknown steels. The boundaries of the diagram should be considered porous, since NIST has incomplete

information on construction era approvals of steels, as well as chemical and mechanical characterization

of proprietary steels. Furthennore, as discussed above, each steel exhibits a natural variability in properties. From the chemical and mechanical analyses and the historical record, it is never possible to

state with certainty that a specific mill rolled a specific plate. Where the historical record indicates that one mill supplied a proprietary steel, the analysis discusses whether the specimen in the NIST inventory was likely to have met that specification.

Figure 3-17. Methodology for identifying recovered steels.

The rest of this section summarizes the representative specimens and attempts to discern whether it was

likely that the steel met the original specifications, subject to the constraints that the complicating factors detailed above induce. A separate document (NIST NCSTAR 1-3B) summarizes the chemistries of all

steels characterized as part of the Investigation. Another document (NIST NCSTAR 1-3 A) summarizes

the standard specifications for the steels used in the construction.

Perimeter Columns

Contemporaneous construction documents (White 1967; Symes 1968) indicate that Pacific Car and

Foundry (PC&F) fabricated the perimeter columns primarily with steel it purchased from Yawata Iron and Steel, and possibly Kawasaki Steel. One contemporaneous document (White 1967) indicates that

NIST NCSTAR 1-3D, WTC Investigation 37
Chapter 3

Yawata was to supply the steels with F, > 36 ksi and Kawasaki was to supply steels with F,, = 36 ksi. At

least one surviving shipping manifest (Mitsui 1968) shows PC&F received 190 t of A 36 spandrel plates

that originated in Chiba, Japan, the location of a Kawasaki Steel mill. One contemporaneous construction

document clearly indicates that Yawata also supplied A 36 for spandrels (Symes 1969b).

Another contemporaneous document (Symes 1969) also indicates that PC&F used domestic steel for plate 3, the inner web, of the perimeter columns. By weight, the plate 3 steel represents less than

10 percent of the mass of the perimeter columns, because the spandrel plates make up much of the inside

wall of the perimeter column.

Except for the F,. = 36 ksi steel, which it supplied to meet A 36, Yawata supplied all other grades of steel to its own specifications, which the PONYA chief engineer approved. NIST was unable to locate the

complete specification for these steels. The only document that NIST located (White 1967b) details required changes to the complete specifications for Yawata steels during the negotiations over acceptance.

Unfortunately, NIST could not locate the original specification document that describes the tensile

strength and total elongation requirements of any of the Yawata steels with F, > 36 ksi. It is therefore

impossible to compare the measured tensile strengths and total elongations of perimeter column steels to specification documents. Generally, the measured total elongations of the intermediate-strength steels are greater than the 24 percent minimum that A 441-66 specified.

In general, the measured yield strengths perimeter column steels exceed the minimum requirements.

Figure 3-18 plots the ratio of the measured yield strength to the specified minimum yield strength for all

tests using specimens oriented parallel to the original rolling direction of the plate, which was the

specified test orientation for plates in the WTC construction era. The points that lie below the line of

unity ratio at specified F, = 60 ksi originated in two inner web plates from the same perimeter column

panel, and thus probably came from the same plate.

38 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

1.8

>-1.6

LL

D

.0

0 1.4

Q.

C/D

^ 1.2

CO

CO

CD ^ 1.0

0.8

values < 1 arise from

natural variation

Perimeter Columns

L orientation only

A Plates 1,2,4

o Plate 3

F =(55 60 70 100) ksi values offset for clarityI Q

bars represent max and min values

points without bars represent single measurements

40 60 80

Specified F ksi

100

Figure 3-18. Ratio of measured yield strength to specified

minimum yield strength for all longitudinal tests of perimeter

column steels.

Fy = 36 ksi S-9 Spandrel

The representative F, = 36 ksi specimen, S9-C1T-S1, came from the 100th floor spandrel of recovered

perimeter column S-9 (WTC 1 Column line 133 floors 97-100), which is just above the airplane impact

hole. The rolling, or longitudinal, direction of the spandrel plates lies in their long direction, or perpendicular the axis of the building. The measured yield strengths in longitudinal tests exceed the

requirements of A 36; the average F, = 38.2 ksi. The tensile strength and total elongations are also

consistent with the requirements of A 36.

The chemistry of specimen S9-C1T-S1, Table 3-5 column 1, is also consistent with the requirements of

A 36, Table 3-5. The Mn and Si contents are higher, and the C content is lower, than most of the other

Fy = 36 ksi spandrels, but A 36 has no requirements for Mn or Si for plates that are 0.75 in. thick or less. Two of the thirteen characterized spandrels specified as Fy = 36 ksi have this chemistry; the chemistry of

the other eleven is different, but also consistent with the requirements of A 36.

NISTNCSTAR 1-3D. WTC Investigation 39
Chapter 3

Table 3-5. Summary of mechanical properties and chemical compositions for steels

from low-strength perimeter columns.

(note a) ClB-FL-2

aI4-CiDS-1 Mzo-C 1 1 - FR-1 FR-1 N8-C1B-F-1iNo-Ci 1 - OW-1

c 0.10 0.19 0.16 0.16 0.17 0.16 0.18 C

Mn 1.41 1.21 1.18 1.36 1.27 1.50 1.39 Mn

P 0.010.020 <0.005 0.024 0.016 0.023 0.018 P

S 0.016 0.016 0.024 0.022 0.011 0.016 0.012 S

Si 0.55 0.25 0.36 0.29 0.40 0.44 0.40 Si Ni 0.02 0.01 0.06 0.01 0.01 0.01 0.02 Ni

Cr 0.04 0.03 0.05 0.02 0.02 0.02 0.04 Cr

Mo <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 Mo

Cu 0.08 0.05 0.14 0.04 0.05 0.05 0.05 Cu

V 0.018 <0.005 0.024 <0.005 <0.005 0.005 0.011 V

Nb <0.005 0.040 <0.005 1 0.047 0.042 0.064 <0.005 Nb

Ti 0.026 <0.005 0.018 <0.005 <0.005 0.007 <0.005 Ti Zr <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 Zr

Al <0.005 <0.005 <0.005 0.018 0.024 0.042 0.029 Al

B <0.0005 <0.0005 <0.0005 <0.0005 <0.0005 <0.0005 <0.0005 B

N 0.0032 0.0070 0.0060 0.0186 0.0065 0.0033 0.0050 N

F, (ksi) specified

36 45 46 50 55 60 60 F, (ksi) specified

Fy avg

(long)

38.2 56.7 55.8 61.7 61.8 69.2

(note b)

60.8 Fy avg

(long)

Mm.) 3/8 1 7/8 3/8 9/16 1/4 5/16 1/4 /(in.)

a. S9-C3T-S-1 and S9-C1T-S-1 originate from different locations on the same plate

b. The average of 4 longitudinal tests of N8-C1B-F-1 is Fy = 68.7 ksi. Value reported is the average of 4 tests on N8-C1B-F-1 and

3 tests on N8-C3B-RF

Key: lon^, longitudinal^ Note: Siighlighted entries are particularly relevant for steel identificationj

Fy = 45ksiASCE3

The representative F, = 45 ksi specimen ASCE3-C1B-FL came from an unidentified location. A starnping

at one end identified the yield strength of the plates, however. The yield strength measured in the single

longitudinal test, F, = 56.7 ksi, far exceeds the specified F, = 45 ksi. The yield and tensile strengths and

total elongation are consistent with values from a group of recovered Yawata mill test reports for Fy = 45 ksi steels of similar thickness. In that group of mill test reports, the averages of 25 individual

plates are F, = 5 1 .6 ksi, TS = 72.8 ksi; the total elongation in 8 in. is EJ, = 28.0 percent.

The chemistry of ASCE3-C1B-FL is consistent with the Yawata "A441 -modified" chemistry

specification. Table 3-7 under the heading "Yawata 50." The important features of that specification are

the high allowable Si content and the use of Nb as a micro-alloying agent. That specification also

expresses the Cu content as a maximum rather than the usual minimum.

40 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

Fy = 46 ksi S-14 Spandrel

The representative F, = 46 ksi specimen S14-C3B-S1 came from the 91st floor spandrel of a panel from

the north face of WTC 2 on column line 217. The structural engineering plans specify this spandrel as F, = 46 ksi. For unknown reasons, the plans include spandrel plates specified as F,. = 45 ksi and

Fy = 46 ksi, sometimes even in the same perimeter column panel. However, none of the surviving

contemporaneous construction communication between PC&F and SHCR and PONYA ever mention

Fy = 46 ksi steel. All surviving Yawata specifications describe Fy = 45 ksi and F, = 50 ksi steels only.

Furthermore, none of the contemporaneous ASTM structural steel specifications describe a steel supplied

with Fy = 46 ksi for the thickness of this spandrel: / = 3/8 in. For example, plates meeting A 242, A 440,

or A 441 would be supplied with Fy = 50 ksi for this thickness. A 572 does not have a F,. = 46 ksi entry.

In any case, the measured yield strength far exceeds the specified minimum; the average value of three

tests is Fy = 55.8 ksi, which is also consistent with any of the F, = 50 ksi specifications.

The chemistry of the specimen is consistent with the requirements for the specification for the Yawata

Fy - 50 ksi steel. Table 3-5. It does differ from the chemistry of the other two F, = 50 ksi plates

recovered, however.

Fy = 50 ksiM-26

The representative F, = 50 ksi specimen M26-C1T-FR came from the north face of WTC 1 on column

line 131 near the 93rd floor, which is just below the impact zone. The test specimen originated in a

section of flange (plate 1) 9/16 in. thick. The average measured longitudinal yield strength is more than

10 ksi higher than the specified minimum: F, = 61 .7 ksi.

The chemistry of the specimen is consistent with the Yawata specification for F, = 50 ksi. Like many of

the other perimeter column steels, it is strengthened by addition of Nb. The Yawata hot-rolled steels in the

range 50 ksi < F, < 60 ksi share a common chemistry specification, which the contemporaneous

construction correspondence denotes as "A441 -modified." That specification only puts an upper limit of

0.15 percent on Nb+V, so many different chemistries could satisfy this specification.

Fy = 55ksiy-9

The representative F, = 55 ksi specimen N9-C1M-FR came from the north face of WTC 1 column

line 155 near the 102nd floor. The test specimen originated from a section of flange (plate 1) 0.25 in.

thick. The measured average yield strength is consistent with the specification: F,. = 61.8 ksi.

The chemistry is consistent with the Yawata F, = 55 ksi specification, and is very similar to those of

perimeter column steels specified as F, = 50 ksi and F,. = 60 ksi.

Fy = 60 ksiN-8

The representative F, = 60 ksi specimens came from several of the flange and outer web plates in specimen N-8, which is from the north face of WTC 1, centered on column line 142 between floors 97-

100. This panel was just above the impact zone. The average yield strength of seven tests of longitudinal

specimens taken from two different 5/16 in. thick flanges (plate 1) is F,, = 69.2 ksi, which is much greater

than the specified minimum. The average yield strength of three longitudinal specimen taken from the

NISTNCSTAR 1-3D, WTC Irivestigation 41
Chapter 3

outer web (plate 2) of N8-C3-B1 (column line 141, near the bottom) is F, = 60.8 ksi. One of these tests was shghtly lower than the specified minimum (F, = 59.2 ksi vs. F, = 60 ksi).

The chemistries of the flange (plate 1) and outer web (plate 2) are consistent with the Yawata

specification for F, = 60 ksi steel. Table 3-7, but they are not identical. In particular, the outer web

(plate 2) does not contain Nb, and the V content is rather low (0.01 1 percent). But because the Yawata

specification only limits the sum of Nb+V, both chemistries are consistent with this specification.

Fy = 65ksiN-99

The representative F, = 65 ksi specimen came from the flange (plate 1) of specimen N99-C3T1-RF,

which originated in WTC 1, column line 147 near the 102nd floor. This location is several floors above

the impact zone. The average measured yield strength of two longimdinal specimens is F, = 71.8 ksi,

which exceeds the minimum F, = 65 ksi in the Yawata specification.

The chemistry of N99-C3M-FR-1, which is from a different location in the same plate, is also consistent

with the Yawata F,. 65 ksi specification. Table 3-6, which contemporaneous construction documents

denote as WEL-TEN 60R. According to these documents, this was the highest strength hot-rolled grade,

and it was only used at thicknesses up to 0.5 in. Thicker plates required the quenched and tempered

WEL-TEN 60. The hot-rolled WEL-TEN 60R is alloyed with V instead of Nb, and has added Cr.

Fy = 70 ksi C-46, S-1, and S-14

NIST tested three different examples of F, = 70 ksi perimeter-column steels. They are S1-C2M-FL, a 7/16 in. thick flange (plate 1) from the west face of WTC 1 column line 433 near floor 89, C46-C2M-FL,

a 3/4 in. thick flange from west face of WTC 2 column line 157 near floor 69, and S14-C3M-FR, a 1/4 in. thick flange (plate 1 ) from the north face of WTC 2 column line 218 near floor 92. The global average

longitudinal yield strength is F,. 77.7 ksi, which exceeds the minimum requirements of the Yawata

specification for F, = 70 ksi WEL-TEN 62.

The chemistries of all three specimens are similar. Table 3-6. All are consistent with the F, = 70 ksi

WEL-TEN 62 specification. Table 3-7, which has V and Cr additions.

Fy = 75 ksi C-22

The representative F, = 75 ksi specimen C22-C2M-FL came from a 1/4 in. thick flange (plate 1) from the

north face of WTC 1 column line 157 near floor 94. This location is just above the impact opening. The

average longitudinal yield strength, F,. = 83.7 ksi exceeds the requirements of the F, = 75 ksi

WEL-TEN 62 specification.

The chemistry is consistent with the F,. = 75 ksi WEL-TEN 62 specification. Table 3-6, and is similar to the chemistry of the F, = 70 ksi steels.

42 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

Table 3-6. Summary of mechanical properties and chemical compositions for steels

from high-strength perimeter columns.

N99-C3MFR-1

(note a) S1-C2MFL-1

C46-C2MFL-1S14-C3MFR-1

C22-C2MFL-1

C25-C1BFR-1MlObC3IVI-FR-1 c 0.16 0.13 0.12 0.13 0.15 0.13 0.14 C

Mn 1.25 1.08 1.10 1.07 1.13 0.97 0.84 Mn

P 0.018 0.008 0.014 0.010 0.016 0.014 0.009 P

S 0.018 0.013 0.015 0.013 0.013 0.015 0.016 S

Si 0.38 0.41 0.40 0.39 0.39 0.33 0.23 Si Ni 0.01 0.01 0.04 0.01 0.01 0.01 0.01 Ni

Cr 0.19 0.22 0.21 0.21 0.23 0.32 0.88 Cr

Mo <0.01 <0.01 < 0.01 < 0.01 <0.01 0.20 0.40 Mo

Cu 0.05 0.04 0.05 0.05 0.04 0.28 0.23 Cu

Vj 0.053 j 0.020 0.024 0.020 0.041 < 0.005 <0.005 V

Nb <0.005 < 0.005 < 0.005 < 0.005 <0.005 0.024 <0.005 Nb

Ti <0.005 < 0.005 < 0.005 < 0.005 <0.005 < 0.005 <0.005 Ti Zr <0.005 < 0.005 < 0.005 < 0.005 <0.005 < 0.005 <0.005 Zr Al 0.031 0.110 0.097 0.110 0.110 0.100 0.075 Al

B <0.0005 < 0.0005 < 0.0005 < 0.0005 <0.0005 0.0016 <0.0005 B

N 0.0120 0.0060 0.0070 0.0070 0.0020 0.0070 0.0051 N

F, (ksi) 65 70 70 70 75 80 100 F, (ksi) Fy avg(long) 71.8 77.9 75.9 79.4 83.7 95.7 107.7 F, avg(long)

?(in) 1/4 7/16 3/4 1/4 1/4 1/4 1/4 l(in)

a. N99-C3M-FR-1 and N99-C3T1-RF are from different locations in the same plate. In the specimen designation system, occasionally "FR" and "RF" are used interchangeably to indicate specimens from the right flange. Kev: long, loncitudinal. c~

— —

1 Note: !Hij;hli_^hted entries are particularly relevant for_steel identifi_c_ationj

Fy = 80 ksi C-25

The representative F, = 80 ksi specimen C25-C1B-FR-2 came from a 1/4 in. thick flange (plate 1) from

the east face of WTC 1 column line 207 near floor 89. The average longitudinal yield strength for two

specimens, F, = 95.7 ksi, is significantly higher than the minimum requirement of the Yawata

WEL-TEN 70 specification.

The chemistry, which identifies it as a chrome-molybdenum steel, is also consistent with the WELTEN 70 specification, Table 3-7 The chemistries of three other F,. = 80 ksi plates from different columns

are similar. A thicker F,. = 80 ksi section, C46-C1B with / = 5/8 in. contains 0.43 percent Ni and small

amounts of V instead of the small amounts of Nb of the other F,. = 80 ksi steels. The specification for WEL-TEN 70 has no limits on Ni, Nb, or V, however, so the chemistry of C46-C1B is also consistent

with the WEL-TEN 70 specification.

NISTNCSTAR 1-3D, WTC Investigation 43
Chapter 3

Fy = 85 ksi to Fy = 100 ksi M-lOb, C-10, B-1043, and C-14

Contemporaneous construction documents (White 1968) indicate that PC&F requested and received

permission to use F, = 100 ksi steel wherever the plans called for steels with yield strengths greater than

85 ksi. Forensic investigation of the recovered steel confirmed this finding. Ten of the fourteen columns

of recovered steel that were specified to have F,. > 85 ksi or F, > 90 ksi had legible yield strength

stampings. In each case the columns were stamped as F, = 100 ksi.

NIST tested four different specimens of nominally Fy = 100 ksi perimeter column steel.

Specimen M-10b-C3Bl-RF came from 1/4 in. thick flange (plate 1) from the north face of WTC 2 column line 205 near floor 83. Although the stmctural engineering drawings specify it as F,. = 85 ksi,

markings on the plate indicate that it is F, = 100 ksi. Specimen ClO-ClM-FL came from a 1/4 in. thick

flange from the west face of WTC 1 column line 452 near floor 86. Although specified as F, = 90 ksi,

markings on the plate indicate that it is F, = 100 ksi. Specimen C14-C1-RF came from a 1/4 in. thick

flange from the northeast comer of WTC 2 column line 301. Specimen B1043-C2T-FL is an example of a thicker section, / = 1.125 in., specified as F,. = 100 ksi from the south face of WTC 2 column line 406

near floor 43.

As a group, the average yield strength is F, = 109.2 ksi; the minimum of the group is F, = 107.5 ksi. All

the yield strengths exceed the minimum requirement of F, = 100 ksi for Yawata WEL-TEN 80C,

Table 3-6 and Table 3-7. The total elongations to failure for specimen C14-C1-RF are less than those of

the other specimens: 16 percent and 17 percent compared to an average of 22 percent for the others. Some

contemporaneous construction correspondence (White 1967b) indicates that the F, = 100 ksi WELTEN 80C was to meet the strength and total elongation requirements of A 514-66, which mandates a minimum total elongation of 18 percent in a 2 in. gauge length. A 1966 Yawata steel advertisement

(WEL-TEN 1966) states a 16 percent minimum. The total elongation of each specimen from F, = 100 ksi

plate is larger than this value.

Inner Web (Plate 3)

Contemporaneous construction documents (Symes 1969) indicate that PC&F generally fabricated the

flanges, outer webs and spandrels (plates 1, 2 and 4) from Japanese steel (primarily Yawata) and the inner

web (plate 3) from domestic steel. For this reason, this section reviews the properties of steel specimens

from the inner webs separately. Chemical analyses of specimens from the inner web confirm that the steel

is frequently different from the steel used in plates 1 and 2. In some cases, however, the composition of

the inner web plate is identical to the other plates in the column, indicating that it was fabricated frorh a Yawata steel. All inner web plates with F,. > 75 ksi have chemistries consistent with the Yawata grades. In terms of mass, the inner web represents less than 1 0 percent of the total steel used in the perimeter

columns, because the spandrel plates cover part of the inside of the perimeter column, and because the

inner web is often thinner than the flange plates.

Other contemporaneous construction documents provide evidence that Bethlehem steel supplied most of

the domestically produced steel for the PC&F perimeter column contract. The actual fraction is unknown,

and those same construction documents also show that PC&F purchased or requested approval to purchase plate from nearly every domestic steel company. None of the recovered plate 3 steels with

Fy < 100 ksi contain Nb, which means that they do not correspond to USS Ex-Ten, which specifies a

44 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

minimum Nb coment. Because of the lack of information in the construction record, it is impossible to unambiguously identify the source of the steel used for plate 3.

Table 3-7. Summary of mechanical properties, chemical compositions, and relevant

ASTM and Yawata specifications for steels from high-strength perimeter columns.

FL-1 C2T-FL-1 A 441 Yawata50

WFTI >^ tL-TP\I I iLiy 62 70

W tLj- 1 80C c 0.17 0.15 0.22 0.22 0.18 0.18 0.18 C

Mn 0.89 0.80 0.85-1.25 1.1-1.6 1.40 1.20 0.6-1.2 Mn

P 0.015 0.005 0.040 0.040 0.035 0.030 0.030 P

S 0.012 0.012 0.050 0.050 0.035 0.030 0.030 S

c

;

U._4 n Ti U.13 U.jU U.J J A ^ C U.J J A < C U.J J A 1 ^ A 7 ^ U.l J—U.JJ

U.U 1 U.U 1 n on u.vu U.oo A 1A U. oU A 1A U. jU 1 .3 max AU. 7l — 11l .J Mo A /I 7 U.J J 0.6 max 0.6 max MO

U.zo U.J 1 u.zu A OA U.ZU A 1 ^ A ^U. 1 J—U.J A 1 A ^U. 1 J—U.J L u

V U.UU / <U.UUj U.uz min 0.12 max V <U.UUj <-U.UUj U. J J IN D+ V max

In D

Ti <0.005 <0.005 Ti Zr <0.01 <0.005 Zr

Al 0.081 0.053 Al

B <0.0005 <0.0005 0.006 max B

N 0.0040 0.0080 N

F, (ksi) 100 100 50 50-60 70 75-80 100 F, (ksi)

F, avg (long) 108.7 110.3 F, avg (long)

/ (in.) 1/4 1 1/8 /(in.)

Key: long, longimdinai.

N9-C1T1-IW is a specimen of plate 3 with specified F, = 55 ksi from the north face of WTC 1 column

line 155 near the 102nd floor. The original plate is 0.25 in. thick. The average yield strength of three

longitudinal specimens is F, =61.3 ksi, which is larger than the specified F,. = 55 ksi Chemically, the

steel has slightly too much Mn to meet the Bethlehem V-series specification (1.31 percent vs. a maximum

1.25 percent) even after accounting for the relaxed requirements of product testing, assuming the

increased compositional ranges allowed by A 572-70 for Mn. This difference would have an insignificant

effect on mechanical properties.

NIST tested three examples of F, = 60 ksi plate 3 steel. N8-C1B1-A-IW is a specimen of inner web from

the north face of WTC 1, column line 143 near floor 98. The average yield strength of two longitudinal

specimens of N8-C2M-IW is F, 56.7 ksi, which is below the specified minimum: F, = 60 ksi. The

average of three transverse specimens, an orientation which would not have been used for mill testing is similarly low: F, = 56.8 ksi. N8-C2M-E-IW came from the adjacent column 142. The average yield

strengths of longitudinal and transverse specimens are similarly low: Fy = 56.7 ksi and Fy = 57.2 ksi

NISTNCSTAR 1-3D, WTC Investigation 45
Chapter 3

respectively. Given that the columns are adjacent and the inner web plate thickness is the same, it is likely that these two inner web plates came from the same original plate. Specimen C40-C2M-IW originated

from the north face of WTC 1, column line 136 at floor 100. The average yield strength of two

longitudinal specimens of C40-C2M-IW, F, = 64.6 ksi, exceeds the specified minimum.

The chemistries of the two plates are very similar to each other, as well as to the compositions of many of

the other plate 3 specimens. All are high in V, and contain Cr and Ni. The magnitudes of the alloy contents are consistent with the Bethlehem V-series specification.

Specimen N99-C3M1-IW is an example of an inner web plate with specified F, = 65 ksi from WTC 1 column line 147 between floors 99 and 102. The average yield strength of three longitudinal specimens is Fy - 66.6 ksi, which exceeds the specified minimum. The chemistry of the plate is very similar to all of

the other plate 3 specimens. It has vanadium added for strength, as well as trace amounts of Ni and Cr.

Specimen S14-C3T-IW-1 is an example of an inner web plate with specified F, = 70 ksi from WTC 2

column line 217 between floors 91-94. The average yield strength of two longitudinal specimens is Fy = 76.0 ksi, which exceeds the specified minimum: F, = 70 ksi. The chemistry is similar to the rest of

the plate 3 specimens and consistent with the construction correspondence on the chemistry specifications

for the Bethlehem V-series steels.

The measured yield strengths of two of the inner web plates are less than the specified minimum. The

relevant question is, "If the measured yield strength of a plate measured in the course of the investigation

is lower than the specified minimum, is this an anomalous or an expected event?" That question is the

convolution of at least two other questions:

1 . What is the distribution of yield strengths from mill test reports for a given specified

minimum yield strength?

2. What is the distribution of product test values of yield strength for a given strength in a mill

test report?

The literature data on variation in structural steel properties (Alpsten 1972, Galambos 1978, AISI 1974),

while reasonably large, is not structured in a manner that makes it easy to use. Alpsten (1972) and

Galambos (1978) provide partial answers to question 1, and the AISI report (1974) on "Variation of

Product Analysis and Tensile Properties. . helps answer question 2. One might also attempt to estimate

how frequently in construction that steels with inappropriate specified minimum yield strengths are used,

but this question is fundamentally different from the objective answers to questions 1 and 2. Ideally, we

would like to answer the question by calculating the probability that a plate specified as F, = 60 ksi would

actually have a measured (product) F, 56.7 ksi, the average yield strength of test specimens from the

inner web of perimeter column N-8.

Several 1960's era studies (Alpsten 1972, Galambos 1978) summarize the distribution of mill-test-report

yield strengths for given steel grades. One shortfall of these studies in applying them to the questions at hand is that they usually report just a mean value and a standard deviation or coefficient of variation (C,.). Because each grade of steel is supplied to a specified minimum value, the distributions of mill test report

values are non-normal. Instead, they are truncated at the specified minimum and therefore skewed to the

positive values. Estimating the fraction of plates with strength near the specified minimum is not accurate.

46 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

A second problem is that no studies exist that examine the variabihty in these distributions between steel mills. It is likely that individual steel mills have different production practices that result in different

shapes to the distributions. The reported values for the distributions usually represent the output of a

single mill.

The AISI report on product variability (1974) appears at first glance to be the answer to these questions,

but it codes its data in a manner that removes some of the utility. The probabilities that it expresses for product tests are given as the expected difference between the measured value and the mill test report

value. In other words, knowledge of the original mill test report value is necessary, which is impossible

for the WTC steels. In that sense, it answers question 2, but a good answer to question 1 is still absent.

Given the uncertainties in the AISI report (1974), it is not prudent to attempt a detailed statistical calculation. Instead, the calculation should simply establish whether it is reasonable to have found a product tension test result that is less than the specified minimum in the suite of tests that characterize the

WTC steels.

To proceed with the estimation, define the following variables:

Variable Value Definition

Fy 60 ksi the specified minimum yield strength for the plate in question, N-8

Fy 56.7 ksi the NIST-measured (product) yield strength

„mtr the historical average of the ratio of yield strength in mill test reports, r

kp ^

- 1 .092 F^" , to the specified minimum yield strength, F, , for high-strength

^ steel plates

^ r.. . the coefficient of variation of the historical average of mill test reports

c 0.054

of a given specified minimum yield strength

Galambos (1978) estimated and C, from Alpsten's (1973) data on Swedish F,. ^ 58 ksi plates.

The AISI report on variability ( 1 974) expresses its statistics for results of product tests in terms of

deviation from the official value (i.e. the value stated on the mill test report). To estimate this value,

calculate the deviation, Ao, from the mill test report as

Act = k/^. - F/ = 1 .092 x 60 ksi - 56.7 ksi = -8.82 ksi

From Table 1 1 of study SU/18 in the AISI report (1974), the minimum fraction, / of tests higher than this difference from the official test is/= 0.87 for steels with F,>50 ksi. Therefore 13 percent of the time,

product tests of plates would have produced values such as those for plates N8-C2M-IW and

N8-C1B1-IW. Given that they were physically located next to each other, and are the same thickness and

specified minimum yield strength, it is likely that these inner web plates originated from a common plate.

NISTNCSTAR 1-3D, WTC Investigation 47
Chapter 3

In comparison, NIST tested more than 20 distinct perimeter column plates. Of these, only the inner web

plates from N8-C2M-IW and N8-C1B1-IW, or about 5 percent of the total, had a product tension test that was less than the specified minimum. Therefore, it is not unreasonable or anomalous that the measured

yield strengths of inner web plates from perimeter column N-8 are less than the specified minimum.

The previous calculation is approximate and should be regarded only as an order-of-magnitude

calculation. It shows that it is possible that a product tension test of WTC steel could be less than the

specified minimum without being considered anomalous. Comprehensive data on the range of possible

values of kp and C, do not exist. The data from study SU/18 of the AISI report (1974) is based on only

168 mill test reports. That report does not contain sufficient infonnation to decide if those 168 mill test reports came from steels made to high-strength specifications, or if they were simply very high mill test reports for steels made to low strength specifications.

Core Columns

NIST tested representative samples of the four most common core columns: box and wide-flange in Fy = 36 ksi and F, = 42 ksi. Table 3-8 summarizes these results. Appendix A of this report tabulates the

complete test data and plots the stress-strain curves for the individual tension tests. Mechanically, the

yield stress of some of the specimens is less than the specified minimum, but these differences probably

arise from damage to the column that removed the yield point or test differences between the mill test report and the NIST protocol. Figure 3-19 plots the ratio of measured yield strength or yield point to the

specified minimum for longitudinal tests.

Table 3-8 and Table 3-9 also summarize the chemistry data on specimens of identified core column steel. With one exception, the steels are unremarkable. The chemistry of specimens from all the columns

(shapes and plates) specified as F, = 36 ksi are consistent with ASTM A 36. The chemistries of the highstrength shapes are consistent with A 441 or A 572. Although A 572 is not listed in the PONYA steel contract, both HH-Fl-1 and C-26 (a beam from the 107th floor) could have been supplied to the

Bethlehem V-series requirements. That steel is listed in the PONYA steel contract and could be sold to meet A 572. The combination of chemistry and measured yield strength of specimen C-88c is inconsistent

with every ASTM specification listed in the PONYA contract, but that contract lists proprietary steels for which NIST does not have typical compositions for comparison.

Most of the core columns recovered were significantly deformed, which made it difficult to select undeformed regions to harvest test specimens from. Even the relatively straight sections were often

slightly bent. Appendix B summarizes the extent of the deformation in the regions from which specimens were taken for colunms C-30, C-65, C-70, and C-155. From the geometry of the deformed colurrm, it was

possible to estimate the magnitude of the changes to the stress-strain curves for the web and flange

specimen. Generally, the disappearance of the yield point and the elevated strengths of the flange

specimens are consistent with the magnitude of the deformation measured from the shape of the core columns.

This section describes each column and the individual specimens harvested from it.

48 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

Fy = 36 ksi Wide-Flange C-30

Specimen C-30 is a F, = 36 ksi 14WF237 shape from WTC 2 column line 1008, between floors 104-106.

This location is on the opposite side of the building and well above the impact zone. All six tensile specimens, three from the web and three from the flange, exhibit both yield point and yield point

elongation behavior. All six tests produced YP > 36 ksi. The segment of the column from which the test specimens originated had no measurable deformation, see Appendix B, which is consistent with the

appearance of the yield point and the similarity of the stress-strain curves from the webs and flanges.

Figure 3-20 plots the stress-strain curves for the longitudinal tests of C-30, along with the corresponding

curves for the three other wide-flange core columns specified as F,. = 36 ksi for which web tests were

conducted. The chemistry, tensile strength, and total elongation of C-30 meet ASTM A 36.

Table 3-8. Summary of mechanical properties and chemical compositions for steels

from core column wide-flange shapes.

Shapes

C30-F1-1

WTC 2 1008

104-106

C65-F1-1

WTC 1 904

86-89

C71-FL-2

WTC 1 904

77-80

C80-F1-1

WTC 1 603

92-95

C155-F1-1

WTC 1 904

83-86

HH-Fl-1

WTC 1 605

98-101

C26-F1-1

WTC 1 beam

107

C 0.17 0.23 I 0.23

1 0.23 0.23 0.17 0.19 C

Mn 1.06 0.74 0.73 0.90 0.87 1.08 1.19 Mn

P <0.005 0.006 0.027 0.008 <0.005 <0.005 <0.005 P

S 0.014 0.015 0.016 0.008 0.017 0.014 0.015 S

Si 0.10 0.02 0.03 0.03 0.03 0.03 0.03 Si Ni 0.05 0.02 0.02 0.01 0.02 0.02 0.04 Ni

Cr 0.04 0.02 0.02 0.02 0.03 0.02 0.03 Cr

Mo <0.01 0.01 0.04 0.01 <0.01 <0.01 <0.01 Mo

Cu 0.24

L _1 0.05 0.08 U.Uj U.Uo U./4 U.uz u

\'] 0.036

I <0.005 < 0.005 <0.005 <0.005 0.065 0.059 V

Nb <0.005 <0.005 < 0.005 <0.005 <0.005 <0.005 <0.005 Nb

Ti <0.005 <0.005 < 0.005 <0.005 <0.005 <0.005 <0.005 Ti Zr <0.005 <0.005 < 0.005 <0.005 <0.005 <0.005 <0.005 Zr

Al <0.005 <0.005 < 0.005 <0.005 <0.005 <0.005 0.018 Al

B <0.0005 <0.0005 < 0.0005 <0.0005 <0.0005 <0.0005 <0.0005 B

N 0.0067 0.0041 0.003 0.004 0.0049 0.0099 0.0065 N

V/N ratio 5.4 6.6 9.1

specified

F, (ksi)

36 36 36 36 36 42 50

F^ avg

(web) (ksi)

41.9 31.1 33.2 n/d 42.2 n/d 50.3

Yield

behavior'' YP NYP YP NYP YP YP NYP

Fy avg

(flange) (ksi)

39.6 32.4 53.4 34.4 50.9 54.1 n/d

NISTNCSTAR 1-3D. WTCInvestigation 49
Chapter 3

C30-F1-1 C65-F1-1 C71-FL-2 C80-F1-1 C155-ri-l HH-Fl-1 C26-F1-1

Shapes

WTC 2 1008

104-106

WTC 1 904

86-89

WTC 1 904

77-80

WTC 1 603

92-95

WTC 1 904

83-86

WTC 1 605

98-101

WTC 1 beam

107 Type 14WF237 12WF161 ^ 12WF190 14WF184 12WF161 12WF92 27WF94

^. (in.)(b) 1.090 0.905 1.060 0.840 0.905 0.545 0.576

//(in.)(b) 1.748 1.486 1.736 1.378 1.486 0.865 0.872

a. Yield behavior YP = yield point behavior exists in stress-strain curve NYP = no yield point behavior exists in stress-strain curve

b. tf = flange thickness; /„ = web thickness. Key: n/d, not determined. Note: [Highlighted items are particularly relevant for steel identification.!

1.8

1.6

D

'o

Q)

Q.

CO

DCDW

cd0

1.4

1.2

1.0

0.8

1 r T1 1 1

1

1 1 1 1

1

1 1 1 r F = 36 ksi specimens offset for clarity

-| 1 1 r r 1

35

Core Columns

L orientation only

^plates

owide-flange webs

values < 1 arise from

loss of yield point

-obars represent max and min values

points without bars represent single measurements

40 45

Specified F ksi

50 55

Figure 3-19. Ratio of measured yield strength to specified minimum yield strength for all longitudinal tests of core column steels.

50 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

Table 3-9. Summary of mechanical properties and chemical compositions for steels

from core box columns.

Specimen B6152-

2-FL-3

B6152-

1-FL-l

Fuji mill

test report^

C88CF2-1C88AFl-1

C88BFl-1C88BF2-1C88BF3-1

1 nr^tion WTC 1 504

33-36

WTC 2 803

15-18

WTC 2 801

80-83

WTC 2 801

80-83

WTC 2 801

77-80

WTC TT 1 \_ A.2 801

77-80

WTC 2 801

77-80 c 0.17 0.16 0.20 0.18 0.19 0.15 0.18 0.18 Mn 0.81 0.98 0.96 0.98 1.15 1.11 0.86 0.87 P <0.005 0.024 0.013 0.029 0.013 <0.005 <0.005 <0.005

S 0.011 0.013 0.008 0.020 0.021 0.011 0.014 0.016

Si 0.20 0.24 0.20 0.04 0.05 0.09 0.03 0.03 Ni 0.02 0.01 0.02 0.02 0.02 0.02 0.02 Cr 0.03 0.02 0.02 0.03 0.01 0.01 0.02 Mo <0.01 <0.01 0.06 0.02 <0.01 <0.01 <0.01 Cu 0.05 0.05 0.05 0.05 0.02 0.02 0.03 V <0.005 < 0.005 < 0.005 <0.005 <0.005 <0.005 <0.005

Nb <0.005 < 0.005 < 0.005 <0.005 0.030 0.011 0.013

Ti <0.005 < 0.005 < 0.005 <0.005 <0.005 <0.005 <0.005 Zr <0.005 < 0.005 < 0.005 <0.005 <0.005 <0.005 <0.005

Al 0.013 0.031 < 0.005 <0.005 <0.005 <0.005 <0.005

B <0.0005 < 0.0005 < 0.0005 0.0024 <0.0005 <0.0005 <0.0005

N 0.0100 0.0070 0.005 0.004 0.006 0.004 0.006

specified F, (ksi) 36 36 36 42 42 42 42 42

F, avg (ksi) 40.4 38.8 38.4 49.7 n/d n/d n/d n/d

Yield behavior'' YP YP n/d NYP n/d n/d n/d n/d

Column Type Type 354 Type 380 n/a Type 378 n/a n/a n/a n/a

/(in.r 1.88 2.06 3.0 1.5 1.5 1.44 1.44

a. Data for 25.263 lb plate 65.5 in, by 453.75 in. tested Aug 5 1969, rolled at Fuji Hirohata Works,

TS= 64,9 ksi; total elongation measured on gage length of 8 in: = El, = 32 percent,

b. Yield behavior: YP = yield point behavior exists in stress-strain cur\ e. NYP = no yield point behavior exists in stress-strain curve.

c. / = plate thickness. Key: n/a, not applicable; n/d, not determined.

NISTNCSTAR 1-3D, WTC Investigation 51
Chapter 3

Figure 3-20. Yield behavior in tests of webs of four wide-flange core columns specified as Fy = 36 ksi.

Fy = 36 ksi Wide-Flange C-65

Specimen C-65 is a F, = 36 ksi 12WF161 shape from WTC 1 column line 904 between floors 86-89.

This location is on the opposite side of and well-below the impact zone. None of the eight specimens

tested, six from a flange and two from the web, exhibit either yield point or significant yield point

elongation. There is a small yield point elongation in the two specimens from the web (Fig. 3-20), but it

is only one tenth as long as the usual (1-3) percent. Only one of the eight tests produced a yield strength

above 36 ksi. The section that is the source of the web specimens is deformed into an "S" shape, see Appendix B, although the specimens were harvested away from the large tear in the web. The section that

is the source of specimens from the flange, see Appendix B, shows no evidence of significant

deformation, but the lack of yield point and yield point elongation indicate that the material has more than

several percent prior strain.

There are several possible explanations for the low yield strength, given the absence of yield point

behavior. Galambos (1978) reported the mean value of yield strength in mill test reports for English A 36

shapes as 1 .22x36 ksi = 44 ksi, which is consistent with data in Barsom (1988) on U.S. heavy A 36 wideflange shapes from slightly after the WTC construction era. For such a mean large excess strength, the

AISI variability study (1974) predicts virtually no possibility of finding under- strength product tests, unlike the estimate for high-strength perimeter column plates in the previous section. The measured yield

strengths of the other WTC wide-flange specimens tested that exhibit yield point phenomena are all less than this mean value. One limitation of the AISI variability study is that 97 percent of the members of its data set for wide-flange shapes are ASTM Group 1 and 2 shapes, while the WTC core columns tested are

all much heavier Group 3 and 4 shapes. Because these heavier shapes undergo much less reduction during

rolling, and cool more slowly from the rolling temperature, their average strengths may be much closer to the specified minimum than the heavily rolled, fast-cooling lighter shapes in the AISI study. It is also

52 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

possible that this difference represents different steelmaking practice for the wide-flange columns, which

probably came from Yawata Iron and Steel.

There are several other possible origins for the low value of the measured yield strength. The typical

difference between yield point YP and yield strength YS or F,. (3 ksi or more) could account for part of the

shortfall, although the mean of the two tests is only 30.5 ksi. The slower testing rate in the NIST tests could account for an additional 2 ksi. Finally, were the prior strain in the opposite direction from the

tensile tests, the Bauschinger effect (Uko 1980, Tipper 1952, Elliot 2004) could further reduce the

measured yield strength (10-20) percent from the expected value. In particular, the flow stress in the first 3 percent strain after pre-straining in the opposite direction is much less than in the forward direction, but

the difference decreases with increasing strain. The sum of these three effects could raise the yield

strength above F, = 36 ksi.

Although the yield strength of column C-65 is less than the specified minimum, the stress-strain behavior

is very similar to the other F, = 36 ksi columns. Fig. 3-20. By approximately 3 percent strain, the flow

stress is similar to column C-30. Furthermore, the tensile strength and total elongation measured in specimens from C-65 are consistent with the requirements of A 36.

The chemistr>' of C-65 meets ASTM A 36.

Fy = 36 ksi Wide-Flange C-71

Specimen C-71 is a 12WF190 shape from WTC 1 column line 904 between floors 77-80 specified to have F, = 36 ksi. Both web specimens exhibit yield point and yield point elongation behavior, but the

measured yield point is less than 36 ksi; the mean value is YP = 33.2 ksi. In contrast the flange specimens

exhibit significant evidence of substantial prior deformation: F, = 53.3 ksi and absence of yield point

elongation. The measured chemistry, total elongation, and tensile strength are all consistent with the

requirements of A 36.

Arguments made for explaining the low measured yield strengths of specimen C-65 apply to specimen

C-71 as well.

It is likely that the high strength and lack of yield point behavior of the specimens from the flanges arose from bending of the column about the web axis. Appendix B summarizes an analysis of the shape of the

deformed columns and estimates the strain in the flanges from the bending of the column. The measured

deformation of the column is consistent with the elevated strength of the flange specimens, which came

from the portion of the flange with tensile strain.

Fy = 36 ksi Wide-Flange C-80

Specimen C-80 is a 14WF184 shape from WTC 1 column line 603 between floors 92-95 specified to have F, = 36 ksi. The yield strength of the single flange specimen tested is slightly lower than the

specified minimum: F, = 34.4 ksi. The stress-strain curve does not exhibit yield point or yield point

elongation. Reintroducing the expected yield point behavior to the measured yield strength would

probably increase the yield strength enough to be consistent with the requirements of A 36. The

chemistry, tensile strength, and total elongation of C-80 are consistent with the requirements of ASTM

A 36.

NISTNCSTAR 1-3D. WTC Investigation 53
Chapter 3

Fy = 36 ksi Wide-Flange C-155

Specimen C-155 is a 14WF184 shape from WTC 1 column line 603 between floors 92-95 specified to have Fy = 36 ksi. All three web specimens exhibit yield point and yield point elongation behavior, and the

yield points of all three are well above 36 ksi; the average yield point is YP — 42.2 ksi. The chemistry,

tensile strength, and total elongation all meet A 36.

Like specimens from C-71, the flange specimens show evidence of prior deformation: elevated yield

strength, and lack of yield point and yield point elongation. Like column C-71, the magnitude of the

elevated strength and lack of yield point in the flanges is consistent with the measured defomiation in the

column. The tensile specimens came from the portion of the flange that had been strained in tension.

Appendix B details the calculation.

Fy = 42 ksi Wide-Flange HH

Specimen HH is a 12WF92 shape from WTC 1 column line 605 between floors 98-101 specified to have

Fy = 42 ksi. The two longitudinally oriented specimens from the flange exhibit yield point and yield point

elongation behavior with YP = 54.1 ksi. Although the mill test report requires data from a web specimen,

given that the flanges usually have lower strength than the webs, it is likely that a web test from specimen

HH would have produced a yield point YP > 42 ksi. The chemistry of specimen HH is consistent with the

requirements of both ASTM A 441 and A 440 with its Cu content and A 572 types 2 and 4 because of its high vanadium content. The chemistry of HH-FL-1 is inconsistent with USS Ex-Ten, because it lacks Nb.

Both ASTM A 441 or A 440 require a group 2 shape like HH to have YP = 50 ksi, which is less than the

measured yield point of HH-FL-1. The measured yield point is also consistent with the requirements of

A 572 Grades 42, 45, 50, and 55, but A 572 does not appear in the list of approved steels in the PONYA

contract. However, A 572 Type 4 is written to be satisfied by the Bethlehem V-series of steels, which

were permitted under the PONYA contract and presumably could be supplied in 42 ksi and 45 ksi grades.

Historical documents (Betts 1967) indicate that Yawata supplied about half of the mass of the steel for the

rolled shapes in the core. Other documents (Monti 1967) indicate that Yawata requested and was granted an exception to supply its own "A 441 modified" composition, which HH-FL-1 does not meet. The

historical documents do not indicate whether Yawata intended to supply all rolled shapes with F,. > 36 ksi to this specification or not. It is most likely that HH-FL-1 represents a Yawata A 441 shape, which would

have been supplied to F, = 50 ksi.

Fy = 50 ksi Wide-Flange C-26

Specimen C-26 is a 27WF94 shape from a beam in the WTC 1 107th floor specified to have F, = 50 ksi. The measured yield strengths of two of the three web specimens are less than 50 ksi, but both by less than

1 ksi. None of the stress-strain curves exhibit yield point or yield point elongation. The web that is the source of the specimens is quite deformed. Indeed, the flat, full-thickness, 8 in. tensile specimens bowed

about 1/8 in, so the absence of a yield point and significant yield point elongation is not surprising.

The chemistry of C-26 is consistent with the requirements of A 572 Type 2 and Type 4 as well, based on

the vanadium and nitrogen content. Because it lacks of sufficient Cu, the chemistry is inconsistent with

the requirements of A 44 1

54 NISTNCSTAR 1-3D. WTC Investigation
Room-Temperature Tensile Properties

Given the yield strength and chemistry C-26 could be a Bethlehem V-series shape, which the PONYA

steel contract did allow.

Fy = 36 ksi Box B6152-1

Specimen B6 152-1 is a 2.06 in. thick flange plate from a type 380 box column from WTC 1 column line 803 betw een floors 15-18 specified to have Fy = 36 ksi. The two stress-strain curves are very similar, but

only one exhibits a definite yield point. The measured yield strengths are both F, > 36 ksi, however.

The chemistry is consistent with the requirements of ASTM A 36 and is rather similar to the chemistry of

a Fuji steel A 36 plate (Morris 1969) shipped to Stanray Pacific, Table 3-8. That similarity and its thickness (2.06 in) are consistent with the historical documents (Warner 1967) that Fuji Steel supplied

most of the plates with ? < 3 in. in the Stanray Pacific contract for the welded core columns.

Fy = 36 ksi Box B6152-2

Specimen B6 152-2 is a 1.88 in. thick flange plate from a type 354 box column from WTC 1 column line 504 between floors 33-36 specified to have F, = 36 ksi. Both longitudinal specimens exhibit a yield point

and yield point elongation. The yield points exceed the requirements of A 36: YP = 40.4 ksi. The

chemistry of B6152-2-F1-2 is similar to B6152-1-F1 and consistent with A 36.

Fy = 42 ksi Box C-88c

Specimen C-88c is a 1.5 in. thick flange plate from a type 378 box column from WTC 2 column line 801

between floors 80-83 specified to have F, = 42 ksi. Originally, it was part of C-88a, Fig. 3-21. Note that the junction between C-88a and C-88b is the intact welded splice at the 80th floor. The yield strength of

the single longitudinal test, 7S = 49.7 ksi, (0.2 percent offset) is much larger than the F,. = 42 ksi yield

strength requirement.

The chemistry of C-88c is anomalous. Table 3-9. It is inconsistent with all ASTM specifications hsted in the PONYA steel contract. Its lack of vanadium makes it inconsistent with the requirements of A 441.

Because it does not contain Nb either, it inconsistent with the chemistry requirements of all the variants of

the A 572 specification. Although the PONYA steel contracts do not mention A 572, there is archival

evidence (Marubeni-Iida 1967) of shipments of A 572 steels to Stanray Pacific. Its low Cu content is inconsistent with the requirements of A 440. There are several steels listed in the PONYA contract, most

notably proprietary Lukens grades, for which NIST cannot locate typical or specified compositions, so possibly it could have been shipped to one of those specifications. Summaries such as Woldman's (1990)

indicate only specified minimum compositions, and are thus of limited utility in identifying specific

alloys. Contemporaneous documents (Warner 1967) indicate that Fuji steel supplied all plates with

? < 1 .75 in. The thickness of the C-88 plates (1.5 in.) identifies Fuji as a possible source. However, plates

with F, > 42 ksi for the box columns were rather uncommon; only 69 of the 1 202 columns per building

are specified as F, = 42 ksi. Because of their rarity, Stanray Pacific could have purchased these outside

their general contract. The PANYNJ documents reviewed by NIST yielded no correspondence on

substitutions or approvals of such a plate, but it is certainly possible that the PANYNJ documents do not

contain a complete record of the construction correspondence. NIST was not been able to identify which

mills might have supplied this plate.

NISTNCSTAR 1-3D. WTC Investigation 55
Chapter 3

The other F, = 42 ksi steels from the adjacent plate and column respectively, C-88a, are chemically

similar, but not identical. Table 3-9. The chemistry of the flange opposite C-88c (C-88a-FI) is different

and contains B, but much less Mo. The plates of C-88b, which are slightly thinner and originate from the

floors below, contain Nb; their chemistries are consistent with the requirements of A 572, grade 42.

Circled locations denote approximate specimen locations for chemical analysis

Source: NISI.Figure 3-21. Section of C-88c that is the source of the specimens.

Trusses

Contemporaneous construction documents indicate that Laclede (1968b, 1968c) supplied the steel in the

floor trusses to two specifications: A 36 and A 242. Although modem versions of A 242 specify

requirements for corrosion resistance, A 242-66 left the buyer and seller to set these requirements. A 36

specifies a minimum yield point, F, = 36 ksi, while A 242 specifies a higher yield point, F, = 50 ksi. Table 3-10 summarizes the dimensions of the main truss components, the specifications they were

supplied to, and where they were corrmionly used. Figure 3-22 plots the ratio of the NIST-measured yield

strength or yield point to the specified minimum value for all truss steels tested.

56 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

The chemistries of all of the truss components tested are consistent with the requirements of both A 36

and A 242. Their strengths all exceed the specified minima. Table 3-1 1 summarizes the chemistry and

mechanical property data for the truss steels.

Ta ble 3-10. Common truss component dimensions and standards.

Specification Shape 1 I^pH F'or A 36 3 in. X 2 in. bulb angle Lower chord of many common 60 ft trusses A 36 1.09 in. round bars Main web of many 60 ft tmsses A 36 1 13/16 in. round bars Unknown

A 242 2 in. X 1.5 in. bulb angle Upper chord of many common 60 ft trusses Upper and lower chord of many common 36 ft trusses A 242 0.75 in round bars Main web of bridging trusses A 242 0.92 in. 0.98 in. 1.14 in. round bars Main web of many common 36 ft trusses

Truss Angles

Four of the seven 2 in. x 1 .5 in. bulb angles have added vanadium, which makes them similar to a modem

A 572 steel. The measured yield strengths of all of the truss angles tested are all Fy > 50 ksi, even if they were supplied to A 36. which is consistent with the requirements of both A 36 and A 242. The tensile strength of one of the three bulb angles specified as A 36 exceeds the required maximum by 1 ksi.

Truss Round Bars

The yield strength of an A 36 web round bar with D ^ 1 .09 in. is Fy = 47.4 ksi. The chemistries of both

the D= 1.09 in. diameter bars are consistent with the requirements of A 36. The chemistries and

mechanical properties of all round bars originally specified as A 242 are consistent with the requirements

of that standard.

NISTNCSTAR 1-3D, WTC Investigation 57
Chapter 3

Table 3-11. Summary of mechanical properties and chemical compositions, and

specifications for truss steels tested.

Truss Bulb Angles

Tl-BA- C53- C53- C132- C137f- T-l-TA- C137a- C53-

Plate"" TA-1 TA-2 TA-3 TA-5 Plate" TA-3 BA3

c 0.20 0.22 0.18 0.19 0.18 0.18 0.19 0.17 Mn 0.77 0.46 0.84 0.82 0.84 0.86 0.90 0.60 p 0.009 0.024 0.010 0.020 0.016 0.021 0.013 s 0.032 0.028 0.024 0.029 0.020 0.028 0.029 0.040

Si 0.06 0.15 0.04 0.07 0.07 0.04 0.06 0.02 Ni 0.09 0.10 0.08 0.08 0.06 0.08 0.08 0.09 Cr 0. 12 0.09 0. 10 0.10 0.05 0.12 0 08 0V/. 08V/O Mo <0.01 < 0.01 < 0.01 < 0.01 0.01 0.02 0.05 0 01 Cu 0.26 0.48 0.28 0.32 0.22 0.29 0.30 0.32 VV < fi on^ ^ U.UUJ U.UJO ^ \J.U\JJ 0 04.4 0U.UH04^J U.UJ 7 l\0 ^ u.uuj U.U 1 o ^ U.uUj ^ U.UUj u.uuy ^ U.UUj ^ U.UUj

Ti1 1 <rn City's *^ u.uuj ^ U.UUj ^ U.UUj ^ U.UUj ^u.uuj ^ U.UUj ^ U.UUj

U.U 1 1 ^ U.UUJ ^ \j .\}\} J <n^U.UUJ DOS ^ U.UUJ ^ U.UUJ Al 0.045 < 0.005 0.028 0.032 0.038 0.015 0.029 n/d

B <0.0005 <0.0005 <0.0005 <0.0005 <0.0005 <0.0005 < 0.0005 < 0.0005

N 0.0080 0.0080 0.0010 0.0080 0.0100 0.0066 0.0160 0.0090

dimensions 1.5 in. X 2 in. X 2 in. X 2 in. X 2 in. X 2 in.x 2 in. X 2 in. X 3 in. X

1.25 in. 1.5 in. X 1.5 in. X 1.5 in. X 1.5 in. X 1.5 in.x 1 .5 in. X 1.5 in. X 2 in. X

0.25 in. 0.25 m. 0.25 in. 0.25 in. 0.25 in. 0.37 in. 0.25 m 0.37 in. Spec A 242 A 242 A 242 A 242 A 242 A 242 A 242 A 242 A 36

Fv(ksi) n/d n/d n/d 58.8 n/d ^2 2 n/d 55.5 57.2 Truss Round Bars

A242-

€53- C137a- C53- M32- C106- 66

Tl-SR-1 MR-1 SR-3 Tl-LR-l** LR-r LR-1 SR-1 A 36-66 Type V

C 0.20 0.22 0.24 0.21 0.18 0.20 0.21 0.26 0.22 Mn 0.84 0.92 0.99 0.79 0.79 0.86 0.64 1.25 P 0.008 0.018 0.007 0.008 0.018 0.020 0.011 0.04 S 0.027 0.031 0.025 0.019 0.030 0.032 0.023 0.05 0.05

Si 0.07 0.08 0.08 0.05 0.06 0.07 0.05 Ni 0.05 0.07 0.08 0.04 0.03 0.08 0.06 Cr 0.07 0.05 0.11 0.04 0.10 0.06 0.09 Mo <0.01 0.03 < 0.01 <0.01 0.01 0.03 0.01 Cu 0.12 0.22 0.27 0.08 0.27 0.04 0.26 V 0.035 0.041 0.038 0.038 <0.005 <0.005 0.031 Nb <0.005 <0.005 < 0.005 <0.005 <0.005 <0.005 <0.005

Ti <0.005 <0.005 < 0.005 <0.005 <0.005 <0.005 <0.005

Zr <0.005 <0.005 < 0.005 <0.005 <0.005 <0.005 <0.005

Al 0.044 0.032 0.033 0.033 <0.005 0.033 0.017

B <0.0005 <0.0005 < 0.0005 <0.0005 <0.0005 <0.0005 <0.0005

N 0.0083 0.0100 0.0110 0.0080 0.0110 0.0100 0.0110

diameter 0.75 m. 0.92 m. 0.92 in. 0.92 m. 1.09 in. 1.09 m.

Spec. A 242 A 242 A 242 A 242 A 36 A 36 A 36 A 242

Fv(ksi) n/d n/d n/d 60.0 47.4 n/d n/d 36 50

a. From a bridging truss.

b. Thickness approximately 0.23 in.

c. Originally attached to perimeter column N8. Chemistry not determined.

d. Originally assigned as D = 1 .0 in. e. Yield point reported. Other alloying elements must be reported.

Key: n/d not determined

58 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

1.8

1.6

D

g

Q 1.4

Q.

CO

LJ_ D

(D

O)

(00

1.2

1.0

0.8

1 I—I—I—I—I—I

\—I—I—I—I—T—I

\—I—I—

I—I—I

]—I—

I—I—

r

Fy = 50 ksi specimens offset for clarity

r 1

Truss Components

L orientation only

I

bars represent max and min values

points without bars represent single measurements

J I I I I I I I I I I I L J I I L 35 40 45 50

Specified F ksi

55 60

Figure 3-22. Ratio of measured yield strength to specified

minimum yield strength for all longitudinal tests of truss steels.

Truss Seats

The structural engineering plans specify that all truss seats were to have a minimum yield strength

Fy = 36 ksi. Presumably, the fabricators would have used A 36 steel. The plans do not require that the

steel conform to a specific ASTM standard, however. Figure 3-23 plots the ratio of the NIST-measured

yield strength or yield point to the specified minimum for all longitudinal tests of truss seat steels. Table 3-12 summarizes the chemistry and yield data for all truss seats characterized. The chemistry data

are summarized in terms of averages and ranges because the mechanical data come from one set of

specimens and the chemistry data come from another. The chemistries of all the perimeter truss seats characterized are consistent with the requirements of A 36. All the perimeter column truss seats meet

A 36 for yield and tensile strength and total elongation. The yield point of the core truss seats (C128-T1)

is slightly less than A 36 allows: F,. = 34 ksi. Because of the natural variability in product tests, this slightly low value is not remarkable. One of the other core column truss seats tested is substantially

stronger than required: F, - 58.6 ksi and TS - 82.2 ksi. The tensile strength slightly exceeds the

TS = 80 ksi maximum that A 36 allows. This truss seat is strengthened with vanadium, see Table 3-12;

the high strength is consistent with the chemistry.

NISTNCSTAR 1-3D. WTC Investigation 59
Chapter 3

1.8

T3

0)

Q 1.4

Q.

CO

DCD

03

CO0^ 1.0

1.2

0.8

T 1 1 ]

\ 1 1

1

1 1 1

1 r

Fy = 36 ksi specimens offset for clarity

n'

! Truss seats L orientation only

32

bars represent max and min values -

points without bars represent single measurements

J I I I I I I I I I I 34 36 38 40

Specified F^, ksi

Figure 3-23. Ratio of measured yield strength to specified minimum yield strength for al longitudinal tests of truss seat steels.

3.4.2 Bolts

Table 3-1 summarizes the results of tests of the A 325 and A 490 bolts. The measured strengths exceed

the requirements of their respective standards. The chemistry of an A 325 bolt recovered from perimeter

panel M-2 is consistent with the requirements of the specification for A 325 Type 1.

3.4.3 Welds

One problem in comparing weld properties to those reported in fabrication documents is that the

fabrication documents usually reference welding procedure data. These data are taken from test welds

with very large gaps between the base plates. The advantage of this large gap is that it makes it easy to remove mechanical test specimens (tensile, impact and bend) that are purely weld metal, and so accurately reflect the performance of the weld metal itself The disadvantage is that this large gap does

not simulate the conditions of the production weld in the component, where dilution of the weld with some of the base metal will change its properties. The change is usually to higher strength since the

carbon content in the base metal, which is higher than that of the filler metal, increases the hardenability

of the weld composition.

60 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

Table 3-12. Summary of mechanical properties, chemical compositions for truss seat

steels tested.

Perimeter Seat ChemistrA

N8-C1B1 N9-C1T1 N99-C1B

M34-TSA-L5

%

C128-T1

%

Average

%

Max

%

Min

%

c 0.19 0.24 0.16 0.24 0.19 Mn 0.44 0.47 0.41 1.16 0.83 P 0.006 0.008 <0.005 0.013 0.01 S 0.022 0.030 0.015 0.006 0.005

Si 0.06 0.07 0.06 0.05 0.038

Ni 0.09 0.16 0.06 0.022 0.014

Cr 0.07 0.10 0.05 0.053 0.018

Mo <0.01 <0.01 0.035 0.02 Cu 0.24 0.32 0.20 0.27 0.027

V <0.005 <0.005 0.048 0.002

Nb (a) 0.046 <0.005 0.002 n/d

Ti <0.005 <0.005 0.002 n/d Zr <0.005 <0.005 n/d n/d

Al (b) 0.034 <0.005 n/d n/d

B <0.0005 <0.0005 n/d n/d

N 0.0090 0.0280 0.0040 n/d n/d

Description Perimeter

seat

Perimeter

seat

Perimeter

seat

Perimeter

seat'

' Perimeter

seat*"

Perimeter

seat**

Core seat Core seat

Specification A 36 A 36 A 36 A 36 A 36

(ksi) 40.7 40.9 41.1 58.6 34.0

Yield behavior YP YP YP YP YP

a. One specimen with [Nb] =0.046 percent, all others <0.005.

b. One specimen with [Al]=0.034 percent, all others < 0.005.

c. Longitudinal orientation.

d. Transverse orientation.

e. Two of three specimens have YP. Key: n d, not determined; NYP; no yield point behavior; YP. yield point behavior in stress-strain curve. Notes: Chemistry averages based on 7 specimens of perimeter truss seats not characterized for strength. Chemistry expressed in mass fraction.

Welds in the Perimeter Columns

The perimeter column panels were large and complicated, so a variety of welding processes were used

during the various steps in their fabrication. Although welding procedures were sometimes changed

during the construction (SHCR 1968), the floors that the aircraft struck appear to have been constructed

using gas-shielded flux-core electrodes to make the butt welds that joined the spandrels to the inner web

of the panels, shielded-metal-arc electrodes for tacking the diaphragm plates inside the columns, tandem

submerged arc to make the long longitudinal fillet welds that formed the box shape, and a combination of

flux-core and shielded-metal-arc electrodes to finish the ends (PCF 1967).

NISTNCSTAR 1-3D, WTC Investigation 61
Chapter 3

Pacific Car and Foundry requested permission (SHCR 1968) to change to a different flux-cored electrode

that produced Fy = 84.5 ksi, TS = 94.5 ksi, total elongation EI, = 24 percent, and impact toughness of

52 ft lbf at 0 °F, when welded with 14 passes on 0.75 in. thick plate. Apparently, they wished to use this electrode for the butt welds that connected the spandrels to the inner web. It is likely that they used this procedure for the columns at the level of impact. Pacific Car and Foundry's welding procedure (PCF

1967) specifies a series of weld strength ranges that correspond to the base metal strength ranges, and

typical properties of the flux-cored electrodes to be used.

Similarly, the submerged-arc fillet welds used two types of flux, one with higher alloying levels, to produce two strength levels: F, = 81 ksi, TS = 92.3 ksi, and F, = 105 ksi, 75 = 1 18 ksi (PCF 1967). The

lower-strength flux would have been specified for the F,. = 55 ksi (nominal) strength of perimeter column

N-9 (WTC 1, column 154). These procedure data compare well with the measured strengths for a fillet weld in N-9 ofF,, = 85.1 ksi, TS = m ksi, and F,- 84.8 ksi, TS^ 103.3 ksi. The strengths are slightly

higher than those listed in the procedure document (F,. = 81 ksi, TS = 92.3 ksi), probably because of

dilution of the weld by the higher-carbon base metal.

Tests of the transverse strength of the perimeter columns at the region of the fillet welds in column N-8,

Table 3-3, show that the weld region was stronger than the adjacent plate material. With the weld metal

milled from the surface of the flange, the failure occurred outside the weld region. Without the weld

removed, the failure occurred at the weld root, indicating that residual stress or geometrical effects explain the rips that occurred adjacent to the longitudinal welds in many of the recovered columns.

Together, these transverse data provide a measure of the strength and ductility of the longitudinal fillet welds in the columns. Since none of these transverse specimens actually broke in the weld, the results are not a true measure of the weld strength, but rather of the weakest region across the entire joint (base metal

to weld to base metal). The failure location, which is adjacent to the weld, matches the general

observations on the recovered sections, where rips progressed through the regions adjacent to the welds,

but seldom through the welds.

Welds in the Core Columns

The longitudinal fillet welds used to fabricate the core box columns from plates tended to be much

smaller than the plates. An example (Stanray Pacific 1967) is a nominal 5/8 in. fillet used to join a 1 in. thick A 36 plate to a 1.25 in. thick A 36 plate using a tandem submerged arc weld. The average shear load

at failure for two transversely loaded fillet weld tests was 42,400 Ibf per linear inch. The equivalent shear

strengths, calculated using the actual cross-sectional areas, are 82.9 ksi and 84.9 ksi. Stanray Pacific also

measured longitudinal shear loads for these fillet welds of 25,600 lb per linear inch and 26,200 Ibf per

linear inch in two tests. Another document, (Stanray Pacific, undated) provides transverse fillet weld

property data on a 0.5 in. triple-wire submerged-arc weld used to join a 1.5 in. A 36 plate to a 1.25 in. plate to fabricate a column. Just over 5 linear inches of this weld failed at 236,000 Ibf, for a shear load of

45,900 Ibf per linear inch.

Because submerged arc welding was not suitable for the ends of the columns or for the tack welds used to align plates during construction. Stanray Pacific (1967b) used a self-shielded FCAW electrode for these

locations, with E7018 used as a root pass when full penetration welds were required. The weld procedure was developed on a 1.25 in-thick plate with a 20 degree bevel on a butt weld, and a root gap of 0.5 in.

62 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

Two transverse tensile tests gave tensile strengths TS = 84.4 ksi (failure in the HAZ) and TS = 84.25 ksi

(failure in the weld). The wide gap also permitted determination of the all-weld-metal tensile strength

75=81 ksi.

Welds in the Trusses

Truss Tl is built from 2 in. x 1.5 in. x 0.25 in. thick bulb angle with a 0.92 in. diameter rod. The

manufacturer, Laclede (1968) designated this resistance weld assembly as type R-21. Unfortunately there

are no strength data for this assembly type, but it is very similar to several others for which data do exist. Extrapolating from the data for R-12 (56,900 Ibf) and R-17 (56,800 Ibf) suggests an average load at failure of 56,700 Ibf for the R-21 assembly (per the manufacturers shear test procedure). This procedure was based on loading of an entire and intact truss.

The shear load of 104.000 Ibf in the test of the section from truss Tl is significantly higher than the

56,700 Ibf suggested by the manufacturer's data. Some strengthening may have occurred by aging during

the lifetime of the truss. The different test methods between the manufacturer, who tested an entire truss with a slightly different loading geometry, and NIST is another possible source of the difference, but the

measured load at failure is still higher than expected.

There were a few non-resistance welds in the trusses, primarily fillet welds near the ends, but the weld

geometry of these welds was not suitable for test. However, documents (Laclede 1969) provide weld

strength data from procedure and welder qualification tests for both shielded-metal-arc and flux-cored-arc

electrodes. They indicate shear strengths of 6.8 ksi/in to 7.5 ksi/in for about 9 in. of a 1/8 in. fillet weld

(total shear load of 60,500 Ibf to 67,000 Ibf) produced with an E7018 shielded metal arc electrode, and

about 9 ksi/in for about 9 in. of a 1/8 in. fillet weld (total shear load near 80,000 Ibf) produced with a

fluxed cored electrode.

3.5 MECHANICAL PROPERTY VALUES

3.5.1 Steel

Early in the Investigation, other NIST Investigation Team members requested estimates for the stressstrain behavior of the many WTC steels. Responding to these requests in a timely manner necessitated

estimating some properties based solely on historical averages, rather than on experimentally determined

properties. Experimental measurements on recovered steels formed the basis for estimating the stressstrain behavior for perimeter column and truss steels. The stress-strain curves for core column steels,

however, are based mostly on literamre estimates of the properties of WTC-construction era plates and

shapes.

Estimating the constitutive behavior of the WTC steels required two steps. The first step was to make the

best estimate of the yield strength for each grade of steel by combining measured values, literature estimates of expected properties, and historical mill test reports. The second step was to estimate the

work-hardening behavior for each grade using stress-strain relations measured from WTC steels, because

work-hardening behavior is not available in the literature for any WTC steel.

For step 1 , two types of correcfion are necessary to estimate the yield strength for input into a constitutive

model. The first is to correct the experimentally derived or mill test report data for artifacts of testing.

NISTNCSTAR 1-3D, WTC Investigation 63
Chapter 3

These corrections are expressed as factors to be added to the measured yield strengths. If only the

specified minimum yield strength is available, a second type of correction is necessary. This was the case

for the core column steels at the time that the modeling groups needed the constitutive behavior. In this case, correction must be made for the fact that the yield strength of the as-supplied steel exceeds the

specified minimum. This correction is expressed as a multiplicative correction factor to the specified

minimum yield.

An important additive correction to the estimated yield strength is necessary because the experimental and

mill test report values are enhanced because the tests are not conducted at zero strain rate (Beedle 1960).

Properly, the yield strength in a mill test report or experimental determination is a dynamic yield strength,

Oyd. For modeling building performance, the appropriate strength is the static, or zero strain rate, yield

strength a, Starting from the static yield strength, all the strength enhancements due to strain rate effects,

such as produced in the aircraft impact, can be calculated. From tests on several different structural steels,

Rao et al. (1963) estimated that for strain rates, 16x1 0~s~ >£">2x 10~^s~' , the difference between

and static, Oys, and dynamic Oyj, yield strength can be represented by

^v.-^,. =-(3-2 + 1000^)= 3-1

where the strain rate, £ , is measured in inverse seconds, and the strengths are measured in ksi. Rao

(1963) measured the static yield strength by straining the specimen past yield, and then locking the

actuator. With the actuator locked, the load drops slightly at nominally constant strain. This constant value is the static yield strength, a,,. The difference between the static and dynamic yield strengths,

termed ^dynamic, is an additive correction factor, which is less than zero. Galambos (1978) used this relation

to estimate the properties of stmctural steel for use in Load and Resistance Factor Design (LRFD). In computing the static yield strength for this Investigation, the strain rate in mill test reports is assumed to be 6x10"^ s"', and the strain rate in NIST-conducted tests is assumed to be 3.3x10 '* s"'.

For estimating the yield strength of steel in rolled wide-flange shapes, it is necessary to make an another

additive correction for the variation in yield strength with location from which the test specimen is taken.

During the WTC era, but not currently, ASTM standards specified that the test specimen for the mill test report be taken from the web section (in the "cross bar" of an "H" shaped specimen) of the rolled shape,

rather than the flange. In typical rolled shapes, however, the flange is thicker than the web, and accounts

for most of the load-carrying capacity of the column. Because it is thicker, it cools more slowly from the

rolling temperature, and its yield strength is generally lower than the web. Many studies, summarized by

Alpsten (1972), have characterized this strength difference between web and flange as 2 ksi to 4 ksi for nominally 36 ksi shapes. It was not uncommon for the measured yield strength of a flange to be I ks-i to 2 ksi below the specified nominal value reported in the mill test report for samples from the web. The

correction for variation with location, Anange- is also an additive correction factor:

'

flange _ web _ K - ^ ^

'^^flange • -^"^

When insufficient experimental or mill test report data are available for a steel grade, it is necessary to use

a second, multiplicative correction factor to estimate the yield strength from the historical averages of that grade. During the 1960s and 70s, several studies (AISI 1973; Galambos 1976; Alpsten 1972)

characterized the variability in properties of steels supplied to various standards. These studies answered

questions like, "What is the mean value of the yield strength of the plates or shapes that meet A 36?"

64 NISTNCSTAR 1-3D. WTC Investigation
Room-Temperature Tensile Properties

They answered this question by examining thousands of mill reports, but not by doing independent

product testing. Because the tension test to certify the mechanical properties is conducted near the end of

the production process, scrapping a heat of steel because it did not meet the intended specification is undesirable. Thus, steel mills generally strive to make steels in which the strength exceeds the intended

specification. Typically, the yield strengths in the mill reports from the WTC-construction era exceed the

specified minimum values by 5 percent to 10 percent. The exact value depended on the specified

minimum yield strength, and whether the steel was supplied as plates or shapes. Of course, the yield

strength in the mill report will never be less than the standard calls for, because the steel could not have

been sold as meeting the standard. The results of these studies are necessary in estimating the properties

of WTC steels when no other corroborating evidence is available.

The correction, kp, from specified minimum to historical average yield strength is a multiplicative factor. For example, for rolled plates, the mean yield strength from the mill test reports, YS, is

yS = k^F^., 3-3

where F, is the specified minimum yield strength.

Estimating the yield strength of a given WTC steel requires some or all of these tliree correction factors. The static yield strength, o",.^ , for perimeter columns and truss steels was calculated from the average

value of the NIST-measured yield strengths, YS, corrected for dynamic effects.

a =YS + k, , 3-4

vi dynamic '

where ^dvnamic is calculated from Eq. 3-1. Where mill test report data exist for a grade of perimeter column

steel, these data were combined with the NIST-measured data. No correction for location effects or

historical averages was necessary.

The static yield strength for rolled shapes was calculated using the expected historical value from

literature data (Alpsten 1972) rather than the measured values, with correction for dynamic and location

effects:

<^.Vi ~ ^s^y + ^'dynamic + ^'nangc '

where is the specified minimum yield strength. The factor ks^ 1.2 corrects for the fact that,

historically, the mill test report value of the yield strength of rolled shapes exceeds the specified minimum

value by 20 percent. (Alpsten, 1972). The factor Anange = -2.6 ksi corrects for the fact that, until recently,

the specimen for the mill test report came from the higher-strength web, but the flange represents the

majority of the load-carrying area. The factor ^dynamic is calculated from Eq. 3-1.

For core rolled plates no correction for location effects was necessary:

^ys ~^p^y'^ ^dynamic '

NISTNCSTAR 1-3D, WTC Investigation 65
Chapter 3

where kp = 1.092 (Alpsten 1972). Table 3-13 summarizes the estimated static yield strengths for all the

steels in the fire and impact zones.

The second step in estimating constitutive behavior was to estimate the work hardening of the steel after yield. There is no historical or literature infonnation on work hardening behavior of WTC steels, so the work hardening behavior for all WTC steels was modeled using stress-strain data generated in the

Investigation. Early in the Investigation, based on requests from other Investigation Team members,

NIST selected the Voce hardening law to represent the increase in stress, a, with plastic strain, Sp, in the

plastic regime. In this hardening rule the flow stress, c, reaches a limiting stress, , with an

exponential decay:

3-7

This relation is available in most finite element packages. ^

Table 3-13. Estimated static yield strengths and work-hardening parameters, Eq. 3-5, for

Description

f

ksi ksi

TS

ksi ''max b ksi

Rn

ksi

All Fy =36 ksi perimeter column steels in plates 1 2 4 (i.e.. not inner plate - plate 3)

ib jj.D D 1 .1 (\ 1 OA u. 1 yu it\ 111 14Ao 1 D /.y /3

All Fy -45 ksi perimeter column steels in plates 1 2 4 (i.e.. not inner plate - plate 3)

45 53.1 74.9 0.153 24.965 27.198 62.245

All Fy -50 ksi perimeter column steels in plates 1 2 4 (i.e.. not inner plate - plate 3)

50 54.0 75.6 0.220 28.659 27.870 74.790

All Fy =55 ksi perimeter column steels in plates 1 2 4 (i.e.. not inner plate - plate 3) with thickness < 1.5 in.

55 60.8 82.6 0.259 18.479 24.698 76.770

All Fy =60 ksi perimeter column steels in plates 12 4 (i.e.. not inner plate - plate 3) witht< 1.25 in.

60 62.0 87.3 0.176 27.535 24.543 74.925

All Fy =65 ksi perimeter coluinn steels in plates 1 2 4 (i.e.. not inner plate - plate 3) with t < 0.5 in.

65 69.6 90.4 0.138 38.284 23.847 89.520

All Fy =70 ksi perimeter coluinn steels in plates 1 2 4 (i.e.. not inner plate - plate 3)70 76.7 92.0 0.103 19.499 26.777 10.714

All F, =75 ksi perimeter column steels in plates 1 2 4 (i.e.. not inner plate - plate 3)

75 82.5 96.8 0.070 29.008 17.463 17.826

All Fy =80 ksi perimeter column steels all plates

80 91.5 99.4 0.079 32.567 14.203 29.522

All Fy =85 Fy =90 Fy =100 ksi perimeter

column steels, regardless of plate

85 104.8 116.0 0.081 13.857 32.500 1.793

Perimeter Plate 3 (inner web) 42 42.6 67.2 0.153 24.974 26.843 62.110

Perimeter Plate 3 (inner web) 45 45.9 69.8 0.153 24.977 26.785 62.147

Perimeter Plate 3 (inner web) 50 51.4 74.2 0.220 28.777 24.729 73.369

Perimeter Plate 3 (inner web) 55 56.9 78.5 0.259 18.455 25.244 76.857

66 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

F,> "is TS

Description ksi ksi ksi •'max h ksi ksi

Perimeter Plate 3 (inner web) 60 62.4 82.9 0.176 27.547 24.298 74.922

Perimeter Plate 3 (inner web) 65 67.9 87.3 0.138 38.407 21.774 88.546

Perimeter Plate 3 (inner web) 70 78.9 96.0 0.070 31.698 29.734 0.000

Perimeter Plate 3 (inner web) 75 78.9 96.0 0.070 31.698 29.734 0.000

a. is the true strain at the tensile strength, TS.

The procedure for determining the parameters of Eq. 3-7 was multi-step:

1. For each experimental stress-strain curve for a given steel, remove yield point and yield point

elongation or truncate curve at the 0.2 percent offset yield strength for stress-strain curves without this yield point behavior.

2. Truncate the curve at the tensile strength, TS, which is the beginning of necking.

3. Shift the stress-strain cur\'e to zero stress and strain.

4. Fit the parameters of Eq. 3-7 to the resulting curve segment.

5. For the set of replicated tests, determine the smallest of the strains at tensile strength, e^^x,

which is the point at which necking begins.

6. Create a fitted plasticity curve for each experimental curve that terminates at Cmax.

7. Numerically average each family of fitted curves at each strain point to produce an average

behavior.

8. Refit the parameters of Eq. 3-7 to the average curve produce the average work-hardening

properties.

Tables 3-13 and 3-14 and Figs. 3-24 to 3-27 summarize the representative tensile stress-strain curves for the 33 WTC steels characterized in the Investigation. The elastic portion of those curves use the value of

elastic modulus (205 GPa = 29.7 Msi) extended to the value of the static yield strength. The strain at which this occurs will be close to, but not equal to the 0.2 percent offset yield strength, YS.

3.5.2 Bolts

The vertical column connections between exterior wall panels were fastened using either four or six highstrength bolts, depending on the vertical location on the building. The bolts were specified to conform to

either the ASTM A 325 or A 490 standards. The joints lower on the building were generally composed of

six A 490 bolts, and those higher up of four A 325 bolts. These bolt types are designed for use in general

structural assembly in buildings and other structures, and are not designed for either high-temperature or severe environmental exposure.

Bolts are characterized by a smooth section, the shank, and a threaded section. Because the two sections

have different diameters, the load bearing section is not uniform along the length of the bolt. As a

NISTNCSTAR 1-3D. WTC Investigation 67
Chapter 3

consequence, the tensile stresses that each section of the boh feels are not uniform, and are higher in the

threaded area. In addition, the threads complicate the characterization of the mechanical behavior in three

ways. First, the roots of the threads concentrate stresses in those areas and cause preferential locations for

both plastic flow and fracture initiation. Second, the interaction of the bolt threads and the nut threads in making the mechanical joint is complex. The first three or four tlireads toward the bolt shaft take up most

of the load of the joint. Finally, the threads deform under shear, rather than under tension, Fig. 3-28.

These peculiarities both require the mechanical properties of bolts to be specified differently from

ordinary structural steel and introduce additional variations in bolt performance in service.

The use of stress-strain constitutive behavior to describe the mechanical behavior of bolts does not fully

capture their behavior. The aforementioned effects of the threads in localizing plastic response require

that the mechanical behavior be described in terms of load-elongation curves from a tensile test of an

actual bolt and nut sample. Nevertheless, for each bolt diameter ASTM A 325 specifies a minimum

tensile strength in pounds derived from a minimum tensile strength, TS = 120,000 ksi, calculated on the

load-bearing area in the threads for bolts less than 1.125 in. diameter. For ASTM A 490 bolts, the

standard specifies minimum values for yield strength, F, , tensile strength, TS, total elongation to failure and reduction of area, ROA. Tests on A 490 bolts are conducted on specimens machined from the bolts

rather than on the boUs themselves.

Table 3-14. Estimated static yield strengths and work-hardening parameters, Eq. 3-5, for core column and truss steels.

Description ksi ksi TSksi *'niax b ksi Roksi

Truss rounds specified as A 36 36 38.1 59.6 0.126 20.523 31.539 0.000

Truss angles (regardless of ASTM

specification) and all rounds specified as A 242

36 55.3 74.1 0.190 31.113 20.583 64.369

Fy =36 ksi core WF shapes 36 37.0 n/d 0.190 30.337 24.467 67.973

Core Group 1 &2 shapes 42 53.8 n/d 0.202 23.023 32.116 30.495

Core Group 3 shapes 42 49.0 n/d 0.202 23.023 32.116 30.495

Core Group 4&5 shapes 42 44.2 n/d 0.202 23.023 32.116 30.495

Core Group 1&2 shapes 45 53.8 n/d 0.202 23.023 32.116 30.495

Core Group 3 shapes 45 49.0 n/d 0.202 23.023 32.116 30.495

Core Group 4&5 shapes 45 47.8 n/d 0.202 23.023 32.116 30.495

Core Group 1 &2 shapes 50 53.8 n/d 0.202 23.023 32.116 30.495

Core Group 3 shapes 50 53.8 n/d 0.202 23.023 32.116 30.495

All 36 ksi core box column steels 36 36.7 64.5 0.204 21.723 30.729 58.392

Core Plates with t < 0.75 in. 42 51.4 n/d 0.202 23.023 32.116 30.495

Core Plates with 0.75 in. < t < 1.5 in. 42 47.0 n/d 0.202 23.023 32.116 30.495

Core Plates with 1 .5 in. < t < 4.0 in. 42 42.6 n/d 0.202 23.023 32.116 30.495

Key: n/d, not determined.

a. Smax is the true strain at the tensile strength, TS.

68 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

0 I 1 I I I I I I I I I I I I I I I I I I I I 1 I I 1 I I I I 0.00 0.05 0.10 0.15 0.20 0.25 0.30 True strain

14:40:01 - Saturday August 21 , 2004

Figure 3-24. Representative tensile stress-strain behavior for perimeter column steels

from flanges, outer webs, and spandrels (plates 1, 2, and 4).

Figure 3-25. Representative tensile stress-strain behavior for perimeter column steels

from inner webs (plate 3).

NISTNCSTAR 1-3D, WTC Investigation 69
Chapter 3

Figure 3-26. Representative tensile stress-strain behavior for selected core column

steels.

Q I I I I I I 1 I 1 I I I I I I I I 1 I I I I I I I I I \ i I 0.00 0.05 0.10 0.15 0.20 0.25 0.30 True strain

14:40 13 - Saturday August 21, 2004

Figure 3-27. Representative tensile stress-strain behavior for truss steels.

70 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

Figure 3-28. Representation of deformation that occurs in exposed bolt threads (left)

and in bolt threads coupled with nut threads (right).

In simulating the response of the bohs as a feature of the finite element model, one would take stress- strain behavior measured for the base metal of the bolt, mesh the bolt with all of its non-uniformities, and

then generate the load-displacement behavior up to failure. However, it is easier and more accurate to simply test bolts in tension and supply the actual load-displacement curves.

Figure 3-15, a plot of the four load-displacement curves for the A 325 bolts, shows that the bolt strength

is independent of the strain rate. The high-rate Kolsky bar tests and quasi-static compression tests on

specimens taken from the same bolt, described in Chapter 4, produced similar results. In consequence, an

average load-displacement curve was calculated from all four tests and is plotted in bold. This average

curv e was produced by calculating the average failure displacement for the four tests, then modifying

each curve by truncating at or extending to this displacement (in the case of extension, using a spline fit). The load values of the four tests were then averaged to produce the bold curve, which should be used in calculating the mechanical response of the bolted joints at room temperature.

Relatively few experimental studies of A 325-type bolts have been published. Rumpf and Fisher (1963)

presented load-elongation data for A 325 bolts and measured the actual change in length of the bolt

during the test. Other smdies (KJrby 1995, Li 2003, Sakumoto 1993, Salih 1992) reported proof loads or

failure loads while investigating temperature or other effects on strength.

The tensile strengths of A 325 bolts from the WTC towers, Table 3-1, are higher than measured in previous studies and much larger than the A 325 specification. The average tensile strength of the WTC

A 325 bolts is (67,700±300) Ibf, or 22 percent greater than the specified minimum. The failure loads

taken from ten batches of 7/8 in. bolts in the 1960s (Rumpf 1963) range from 53,000 Ibf to 67,500 Ibf

with an average of 58,500 Ibf or 10 percent greater than the specified minimum. When Rumpf et al. conducted these tests, the A 325 standard was based on TS = 1 15,000 Ibf, so the specified minimum

tensile load was 53,200 Ibf The strength of the WTC bolts is comparable to the 1990s era 7/8 in. A 325

NIST NCSTAR 1-3D, WTC Investigation 71
Chapter 3

bolts characterized by Salih et al. (1992), whose mean tensile strength was (65,000± 400) Ibf, or 17 percent greater than the specified minimum.

Bolt strengths that exceeded the requirements of A 325 were apparently quite common, but the high

values for the WTC bolts are unexpected. Use of the literature values for the bolt strengths in the models

would under predict the strength of the vertical perimeter column connections, and might tend to incorrectly predict a global collapse mode.

Studies in the literature have established the effect of the number of exposed threads above the nut on

both ultimate load and elongation of the bolt. Figures 3-29 and 3-30 show the results from the literature studies graphically. Because bolts have a non-uniform cross section, deformation tends to concentrate within the exposed threads above the nut. Increasing the number of exposed threads increases the length over which this deformation occurs and increases the total elongation to failure. Exposing fewer threads

has the opposite effect. In terms of critical load at failure, the threads act as stress concentrators, and

failure will occur at the location within the threads that has the greatest stress concentration. Increasing

the number of exposed threads increases the number of stress concentrators, and increases the chance of

loading a more severe stress concentrator, which produces failure at lower loads. The data of Rumpf

et al. (1963) indicate that the linear strength decrease stops above nine exposed threads, however.

55

g.

-6

COo

50

45

1—I—I—

1—I—I—

1—i—I—

1—I—I—

1—I—I—

O m = (-1.05±0.15) kip/thread: Maximum Tensile load

m = (-1.10±0.16) kip/thread: Load at Yield

O

J I I I I I I I I L Source: Salih 1992J I 1 I I I I I I I I I I L

1 2 3 4 5

Number of Threads Exposed

Figure 3-29. Tensile strength change of A 325 bolts

with exposed thread length.

72 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

0.34

0.32

0.30 CL

LO

Gi

O

"cc 0.28

-4—' c

CDE 0.26

(DO

0.24

0.22 -

0.20

1 I I

I

^ I I I

1—I—I—I—I—I—

"I I

I

I I I I

I

I I I r

o

o

o

0.00 0.05 O.lO 0 15 0.20 0.25 0,30 displacement, in. O

O

o

o

o

J I L Source: Salih 1992J I I I I I I I I I I L 0 1 2 3 4 5 6

Number of Threads Exposed

Figure 3-30. Displacement near failure of A 325 bolts as a function of number of threads exposed.

The Investigation generally followed the procedures in ASTM F 606 for testing threaded fasteners which

mandates leaving four threads exposed during the load-displacement test of heavy head bolts. The study

deviated from F 606 in that only two threads were exposed, which is an average value of the number of

exposed threads on failed bolts seen in the WTC recovered components. During the WTC construction

era, A 370 governed the test requirements for A 325 bolts, and mandated six exposed threads. In the

absence of other data, it is reasonable to assume that the statistical variation of the percent change in strength with thread exposure is the same as literature values from contemporary materials.

The slopes of the linear portions of the experimentally measured load-displacement curves are

significantly lower than those published in the literature. Both Rumpf (1963) and Salih (1992) used an extensometer on the bolt. The data of Fig. 3-15 represent the displacement of the crosshead of the testing

machine, which includes the elastic compliances of the load frame, grips, etc. To reproduce the loaddisplacement curve of the bolt itself, the load-displacement curve from the test. Fig. 3-15 has been

modified in Fig. 3-3 1 to increase the slope of the linear portion to match that reported in the literature,

2,500,000 Ibf/in. (Rumpf 1963).

Using Bolt Data

Use the average curve of the actual tests. Fig. 3-31, for the A 325 bolts from the World Trade Center. Using literature values, which are lower, may result in inaccurate failure mode predictions.

NISTNCSTAR 1-3D, WTCInvestigation 73
Chapter 3

Use literature values for the linear portion of the load-displacement curve for A 325 bohs. Figure 3-3

shows this corrected data.

3x10'' I—I—I—I—I—

I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I

0.00 0.05 0.10 0.15 0.20 0.25 0.30 displacement, in.

Figure 3-31. Data for load-displacement of A 325

bolts from Fig. 3-15 corrected for initial elastic slope

from literature.

Assume no strain rate effects on strength, because the Kolsky data in Chapter 4, and the tensile data,

Fig. 3-15, do not show a strong effect.

Use the data of Figures 3-29 and 3-30 to estimate the effect of different exposed thread lengths on tensile

strength and elongation at failure. Assume no further strength reduction for more than nine exposed

threads.

3.5.3 Welds

To assist in modeling the structural perfonnance. Tables 3-2 and 3-15 summarize the as-measured and

as-designed weld data. Table 3-2 contains data generated as part of this study. Table 3-15 contains data

for the strength ranges listed in the construction documents.

Table 3-15. Room-temperature weld metal properties as designed.

For Applications With Weld Metal Properties'

F, < 60 ksi Fy = 65 ksi, TS = 80 ksi 60 ksLF,75 ksi Fy =77.5 ksi, TS = 85 ksi

F, > 75 ksi Fy= 107.5 ksi, r5= 115 ksi

a. Theoretical weld properties (not including the strengthening due to dilution by base metal).

74 NISTNCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

3.6 SUMMARY

The Investigation made several findings regarding the properties of the recovered steel and the original

specifications and expectations. The yield and tensile strengths of the perimeter columns in 22 of 24 cases

are larger than the minimum requirements. The number of slightly under-strength plates and the amount

by which they fall short are consistent with expected values for the average strength and coefficient of

variation of yield strengths of plate steels from the WTC-construction era. The ratio of measured yield

strength to specified yield strength, approximately 1 . 1 , is also consistent with literature estimates from the

WTC-construction era. Unlike the perimeter columns, the measured yield strengths of 2 of 8 samples of

wide-flange shapes recovered from the core are less than minimum F,. = 36 ksi specified in A 36. It is possible that these low values arose from damage that removed the yield point behavior. The average

measured yield strength and yield point for the wide-flange specimens is about 10 percent lower than

expected value predicted from the literature of the WTC-construction era, however. Many of the steels in the floor trusses were high-strength, low-alloy (HSLA) steels, even when they were specified to conform

to A 36. and so were much stronger than specified. Recovered bolts were stronger than the minimum

specified in A 325, and were much stronger than expected based on reports from the contemporaneous

literamre. The welds tested had strengths consistent with the requirements of the design documents.

3.7 REFERENCES

3.7.1 References Available from Publicly Available Sources

AISI. 1974. The Variation of Product Analysis and Tensile Properties Carbon Steel Plates and Wide

Flange Shapes, in Contributions to the Metallurgy ofSteel. American Iron and Steel Institute. Washington. DC.

Alpsten G. A. 1972. Variations in Mechanical and Cross-Sectional Properties of Steel, in Planning and

Design of Tall Buildings volume lb. American Society of Civil Engineers. New York. 755-805.

Banovic S. W., C. McCowan and W. E. Luecke. 2005. Federal Building and Fire Safety Investigation of

the World Trade Center Disaster: Physical Properties ofSteels. NIST NCSTAR 1-3E. National

Institute of Standards and Technology. Gaithersburg, Md. April.

Banovic S. W., Foecke T. 2005b. Federal Building and Fire Safety Investigation ofthe World Trade

Center Disaster: Damage and Failure Modes ofStructural Steel Components. NIST NCSTAR 1-3C.

National Institute of Standards and Technology. Gaithersburg, Md. April.

Barsom, J. M. 1988. Characteristics of Heavyweight Wide-Flange Structural Shapes. Welding Research

Council Bulletin. 332. 1-19 April.

Barsom, J. M. 1987. Material Considerations in Structural Steel Design. AISC Engineering Journal. 24[3]

127-139.

Beedle, L. S. and L. Tall. 1960. Basic Column Strength. ASCEJ. Struct. Div. ST 7. 139-173.

Elliot, R. A., E. Orowan, T. Udoguchi, and A. S. Argon. 2004. Absence of yield points on strain reversal

after aging, and the Bauschinger overshoot. Mechanics ofMaterials. 36. 1 143-1 153.

NIST NCSTAR 1-3D, WTC Investigation 75
Chapter 3

Johnson, R. F. 1967. The Measurement of Yield Stress. In Strong tough structural steels: proceedings of

the joint conference organized by the British Iron and Steel Research Association and the Iron and

Steel Institute at the Royal Hotel Scarborough, 4-6 April 1967. ISl Publication 104. Iron & Steel

Institute. London. 51-60.

Kirby, B. R. 1995. "The Behaviour of High-strength Grade 8.8 Bolts in Fire." J. Construct. Steel. Research. 33. 3-38. -

Li, G.-Q., S.-C. Jian, Y.-Z. Yin, K. Chen and M.-F. Li. 2003 "Experimental Studies on the Properties of

Constructional Steel at Elevated Temperatures." J. Struct. Eng. 129[12]. 1717-1721.

Luecke, W. E, T. A. Siewert, and F. W Gayle. 2005. Federal Building and Fire Safety Investigation ofthe

World Trade Center Disaster: Contemporaneous Structural Steel Specifications. NIST NCSTAR

1-3 A. National Institute of Standards and Technology. Gaithersburg, Md. April.

Rao, N. R. N, M. Lohrmann, and L. Tall. 1966. Effect of Strain Rate on the Yield Stress of Structural

Steels. Journal ofMaterials. 1 [2] 24 1-26 1

Rumpf, J. L. and J. W. Fisher. 1963. "Calibration of A325 Bolts." J. Struct. Div. ASCE. 89[ST6].

215-234.

Sakumoto, Y., K. Keira, F. Furuyama and T. Ave. 1993. "Tests of Fire-Resistant Bolts and Joints."

J. Struct. Eng. 1 1 9[ 1 1 ]. 3 1 3 1-3 1 50.

Salih, N., J. Smith, H. M. Aktan and M. Usmen. (1992) "An Experimental Appraisal of the LoadDeformation Properties of A325 High-Strength Bohs." J. Test. Eval. 20[6]. 440-448.

Tipper, C. F. 1952. Effect of Direction of Rolling, Direction of Straining, and Ageing on the Mechanical

Properties of Mild-Steel Plate. J. Iron Steel Inst. 172[10]. 143-148.

Uko, D. R. Sowerby, and J. D. Embury. 1980. "Bauschinger Effect in Structural Steels and Role in Fabrication of Line Pipe: Part 1 Analysis of Bauschinger Effect in Structural Steels." Metals

Technology. 1. 359-367.

Voce, E. 1948. "The Relationship between Stress and Strain for Homegeneous Deformation. J. Inst. Metals. 74 537-562.

WEL-TEN. 1966. Iron Age 198 p 18 Advertisement showing representative properties of Yawata

WEL-TEN steels. "Nickel may be added to WEL-TEN 60 and 62 if necessary." October 6.

Woldman's Engineering Alloys. 1990. Frick J.P. editor. ASM International. Materials Park, OH.

3.7.2 References Available from Nonpublic Sources

Betts, Jr, W. E. 1967. Letter to R.M. Monti (PONYA) that details Montague-Betts steel purchase.

August 25. "We have had five representatives of Yawata Iron & Steel, Ltd. of Japan in our office for

several days discussing the Contract which we are placing with them for a minimum of 12,000 tons of

wide flange beams and columns."

76 NIST NCSTAR 1-3D, WTC Investigation
Room-Temperature Tensile Properties

Laclede. 1968. Document QC-101 "Quality Control Data Resistance Weld Designation and Strength."

March 23.

Laclede. 1968b Memorandum No. 4. Interoffice memorandum from R.D. Bay that contains specifications

and drawings for bulb angles used to construct floor trusses. January 16.

Laclede. 1968c Memorandum No. 11. Interoffice memorandum from R.D. Bay that contains

specifications and drawings round bars used to construct floor trusses. February 28.

Laclede. 1969. Memorandum from E.M. Elgan. General Welding Foreman, to Fred Deppe, St Louis

Testing Laboratories, that contains qualification data on fillet welds for welder and procedure

certification. December 18. NIST Document WTCI-509-LERA

Marubeni-Iida. 1967. Memorandum from H. Yamada (Marubeni-Iida (America)) to I. Soffer (PONYA)

that contains copies of shipping orders for first three shipments of steel plate to Stanray Pacific. The

shipping order lists three plates marked as A 572 Grade 42. 5 pages, July 18.

Mitsui. 1968. Shipping invoice from Mitsui & Co. to Pacific Car and Foundry for 382.892 lb of steel plate conforming to A 36-66 for spandrels. Originating port was Chiba, Japan, site of a Kawasaki

steel mill. June 10.

Monti, R.M. 1967. Memorandum from R.M. Monti (PONYA) to William Clarkson (Montague-Betts) that contains approval to use the Yawata "A 441 modified" steel composition. Includes supporting

correspondence from Yawata Steel to Montague Betts (dated November 2, 1967) with chemical and

mechanical requirements. 5 pages. November 21.

Morris, R. E. 1969. Letter to James White (SHCR) that contains a mill test report H9-5386 for an A 36

plate from Fuji Steel Hirohata works. Aug. 5.

PCF. 1967. Pacific Car and Foundry. Documents WS-1 lA through WS-1 IC "Weld Procedure." June 1. NIST document number WTCI-158-LERA

Skilling-Helle-Christiansen-Robertson. 1968. Inter-office memo, October 25.

Stanray Pacific. 1967. Memorandum from H. Kjerulf, VP of Engineering (Stanray Pacific), to L. Feld

(PONYA) that describes the use of tandem submerged arc welding to join core column plates.

May 23.

Stanray Pacific. 1967b. Memorandum from F. Allen (Controller, Stanray Pacific) to L. Feld (PONYA)

that describes the use of a self-shielded FCAW electrode for welding the ends of columns. June 2.

Stanray Pacific. 1968. Memorandum from R.E. Morris (Contract Administrator, Stanray Pacific) to

J. White (SHCR) that describes modifications to the welding procedure SA-4 during the contract. April 8.

Stanray Pacific, undated. "Stanray Pacific Welding Procedure Qualification SA-2 (revised)," Report

includes transverse fillet weld strength data. Filed with documents dated October 1967.

NISTNCSTAR 1-3D, WTCInvestigation 77
Chapter 3

Symes. R.C. 1968. PCF Memo #666-90 Control Procedure for Material Substitution. Memorandum from

R.C. Symes (PCF) to R.M. Monti (PONYA) that details the procedure for substituting steel with yield

strength larger than specified. May 29.

Symes R.C. 1969 Memorandum PCF D-666 # T-40. Letter to M. Gerstman, Tishman Realty and

Construction, FebiTiary 5. ''Our original buying plan for this contract was that plates #1, 2 and 4

would be imported and stripped in our plant, while plate #3 would be domestic. In fact, we have

occasionally bought minor quantities of domestic material instead of planned imported, to maintain

fabrication schedules and so on, but in general we have adhered to our original concept."

Symes. R.C. 1969b. PCF Memorandum D-666 -#T-178. Memorandum from Symes (PCF) to R.M. Monti

(PONYA) that contains the results of a Yawata Iron and Steel investigation (dated Dec. 18, 1968) into

"minor laminar discontinuities" in A 36 spandrel plates, "all FY 36 spandrels incorporated into panels

up to the present time meet the contract specifications for this material and that discontinuities found

do not represent injurious defects."

Warner, H. L. 1967. Letter to Malcolm Levy (PONYA) that details Stanray Pacific steel purchasing plan.

July 7. The memo shows 10,240 tons of "British" steel 1.75 in. and thicker and 21,760 tons of

"Japanese" steel of all gauges. Other documents identify Fuji as the Japanese supplier and Colvilles

as the British supplier.

White, J. 1967. Memorandum from James White (SHCR) to R.M. Monti (PONYA) on sources of steel for Pacific Car and Foundry. September 6. "Infomiation now in our hands indicates that only

Kawasaki and Yawata are producing steel for PCF."

White, J. 1967b. Memorandum from James White (SHCR) to R.M. Monti (PONYA) that details required

changes to original specification for proprietary Yawata steel grades, "A441 -modified,", WEL-TEN

60R, WEL-TEN 60, WEL-TEN 62, WEL-TEN 70, WEL-TEN 80C (described as "A 514-modified.")

White, J. 1968. Letter on SHCR letterhead to R.M. Monti (PONYA) that approves the substitution of

Fy =100 ksi steel for F,. = 90 ksi in all instances. Feb. 15.

78 NISTNCSTAR 1-3D, WTC Investigation
Chapter 4

High-Strain-Rate Properties

4.1 introduction

It is well known that the yield and ultimate strengths of steels increase with increasing strain rate (Harding 1979. Hutchinson 1977, Wiesner 1999). In terms of the mechanical response of the steels in the

towers, the high-speed impact of the aircraft is a high-strain-rate event, which is well above the

conventional strain rates normally associated with mechanical properties measurements. It is estimated

that the strain rates on the World Trade Center (WTC) steels, due to the aircraft impacts, were up to 1000 s"'. Accurate models of the aircraft impact and the resulting damage to the towers require knowledge

of the mechanical properties of the steels at high strain rates.

The goal of the high-strain-rate testing plan was to develop a database of high-strain-rate properties on WTC steels for use in modeling the extent of structural damage to the perimeter and core columns as a

result of the aircraft impacts. For strain rates up to 500 s'', the test plan employed high-rate tension tests on eight perimeter- and five core-column steels. These data were used to develop models for strain rate sensitivity for the Investigation. For higher rates up to 2500 s

' the plan employed high-rate compression

tests, using a split Hopkinson pressure bar or Kolsky test, on one bolt and two perimeter-column steels.

4.2 TEST PROCEDURES

4.2.1 High Strain-Rate Tension Tests

High-strain-rate tension tests were conducted on eight perimeter-column steels and five core-column

steels at rates between 50 s"' and 500 s

'. High-strain-rate tension specimens. Fig. 4-1, had a gauge length

of 1 .0 in., a width of 0.25 in., and a thickness of either 0.125 in. or 0.063 in. Strain was measured using

open-grid, high-elongation strain gauges attached to the specimen test section with high-elongation strain gauge adhesive. An additional strain gauge attached to the fixed-grip end of the specimen was used to measure the load and to validate that the specimen grip-end did not deform plastically.

7.00

Uniform Test Section

0.125

TK TN

Dimensions are in Inches

0.053

Flat-HSR-1 " TK Flat-HSR-1 " TN

Figure 4-1. Specimen used for high-strain-rate tension tests.

NISTNCSTAR 1-3D, WTC Investigation 79
Chapter 4

The high-strain-rate test machine is a closed-loop, servo-hydraulic machine with an 11, 240 lb capacity

actuator mounted in a 1 10,000 lb load frame (Bruce 2003). A 400 gal per min. servo-valve, suppUed by a

5 gal capacity accumulator, controls the high rates for this machine. A digital controller and associated

computer system provide system control. The specified peak velocity for the system is 13.5 m/s at zero

load and 10 m/s at 50 percent of the maximum load. Data are acquired using commercially available

software and a data acquisition board capable of acquiring 500,000 data points per second. The typical

acquisition rate is 10 ms per point, which allows 5,000 data points per test. The system employs two load

measurement devices: a piezoelectric load washer attached to the fixed end of the load train and a strain gauge bonded to the grip end of the specimen.

A slack-adaptor in the load train. Fig. 4-2, allows the actuator to achieve the desired displacement rate before loading the specimen. The tension specimen is gripped using bolt-tightened wedge grips.

Triggering the system starts the data acquisition and accelerates the actuator to the desired testing speed.

At the end of its travel, the slack adapter engages the lower grip rod and loads the specimen at the desired

extension rate. The targeted strain rates were not always achieved, but the resulting rates could be

calculated and are used in this report.

Piezoelectric load '"

i 1""^ ^ '

'

^

I

Slack adaptor

Figure 4-2. Schematic diagram of the slack adaptor apparatus.

Data collected during high-strain-rate tension tests include the load from the piezoelectric load washer,

the actuator displacement, the strains in the test section and the grip-end of the specimen from the strain gauges, and the time. The reported strain rate realized by the specimen is obtained from the slope of the

strain vs. time curve recorded for each test.

80 NISTNCSTAR 1-3D, WTC Investigation
High-Strain-Rate Properties

4.2.2 Analysis of High-Strain-Rate Tension Test Data

The data were analyzed using a multi-step technique (Bruce 2003):

1 . Correct the specimen gauge section and grip section strain data for Wheatstone bridge nonlinearity.

2. Plot the engineering strain, e, versus time, using the gauge-section strain gauge data. Determine the engineering strain rate, . in the plastic region by fitting a line through the

linear part of the data over a time range t\ <i < Calculate the engineering strain at each

time increment, At , for / > ti as

e(t-\-At) = e(t) + At-e^, 4-1

3. Convert the grip-section strain data to load, using the calculated modulus and cross-sectional area of the grip-end of the specimen. Overlay the grip section load curve on the data from the

piezoelectric load washer cur\'e, and, if necessary, adjust the modulus in the grip load signal

to align it with the mean of the load washer signal to produce the corrected load signal.

4. Calculate the engineering stress based on the corrected load signal and the cross-sectional area of the test section of the specimen.

5. Calculate the true strain and true stress values from the engineering strain and engineering

stress values.

6. Choose the uniform strain limit as the strain value at the tensile strength.

7. Compute the total elongation, £/„ or engineering strain at fracture, e/, from the distance, Lo,

between gauge marks made on the specimen prior to testing to that following testing, Lf.

El, ^

' 4-2

Compute the reduction of area, ROA, from the original cross-sectional area, Ao, of the

specimen test section prior to testing, and that measured after testing, Ap

An ~ A f ROA = — 4-3

9. Plot the engineering- and true-stress-strain curves up to the point of instability (uniform strain

limit).

10. Determine the yield strength by visually estimating the mean value of stress just prior to the onset of strain hardening. In cases where the ringing of the load signal precludes reliable

visual estimation, fit a third order polynomial to the yielding and strain hardening regions

( 1 percent <e<5 percent) of the curve. Draw a line parallel to the linear-elastic part of the

NISTNCSTAR 1-3D, WTC Investigation 81
Chapter 4

stress-strain curve at an offset of 1 percent. The intersection of these two constructed lines is the estimated yield strength, see Fig. 4-3. The value of 1 percent was chosen because the

downside of the peak maximum load curve often fell slightly less than this offset point and

significantly skewed the data. The ESIS Procedure for Dynamic Tensile Tests (2000)

describes the procedure in greater detail.

1 1 . Determine the tensile strength, TS, by dividing the maximum load by the original crosssectional area of the test section.

150

w 100

50

3rd order

polynomial fit for 0.01<e<0.05

1 I I I I I \ I I I I I I I I I I I L 0.00 0.02

10.57,25 - Friday February 1 1 , 2005

0.04 0.06 0.08 0.10 True strain

Figure 4-3. Schematic of the procedure for

estimating the tensile yield strength when ringing in

the load signal precludes reliable visual estimation.

4.2.3 Kolsky Bar Tests

Tests at very high strain rates employed the split Hopkinson pressure bar, or Kolsky, test. This test method is well known, and the literature of high-rate testing describes it in detail, for example Follansbee

(1985). The device consists of two hardened steel bars lightly supported in a frame as shown

schematically in Fig. 4-4. The test specimen, which is lubricated to reduce friction, is placed between two

hardened (58 HRC) maraging steel bars, 15 mm in diameter and 1.5 m long. A gas gun fires a 0.5 m long

striker bar at the end of the input bar. The impact generates an elastic compressive pulse in the input bar

as shown in Fig. 4-5. This pulse, which travels down the input bar at nearly 5000 m/s, compresses the test

strain gage strain gage

m m

striker bar Input bar specimen Output bar

Figure 4-4. Schematic diagram of Kolsky bar apparatus.

82 NISTNCSTAR 1-3D, WTC Investigation
High-Strain-Rate Properties

0.010

CO ~ 0.005

CDe 0.000 -

0)

reo>

-0.005

-0.010

I I

I I I

Input bar Output bar

Reflected strain wave in Input bar ^

(proportional to strain in specimen |

11 PV\A

Transmitted st.'ain wave in output bar

(proportional to stress in specimen

oading stress wave in input bar

2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1

t, ms

J:16 46 - Tuesday December 14, 2004

Figure 4-5. Oscillograph record of an incident pulse

that is partially transmitted to the output bar and

partially reflected in the input or incident bar.

specimen between the input and output bars at strain rates between 200 s"'and 10,000 s"'. Thin copper

disks placed between the striker bar and the input bar shape the loading pulse and reduce oscillations

during dynamic loading of the specimen. The exact strain rate depends on parameters such as the length

and velocity of the striker bar, and the test specimen dimensions. Some of the loading pulse on the test specimen enters the output bar as a transmitted strain wave, and some is reflected back into the input bar, see Fig. 4-5. The reflected and transmitted strain waves are recorded using strain gauges mounted on the

bars, dynamic Wheatstone resistance bridges, and dual trace oscilloscopes. The stress, a, in the test specimen at time / is related to the transmitted strain, er, in the output bar by

A

4-4

where E is Young's modulus of the bar, Ao is the cross-sectional area of the bar, and A is the cross- sectional area of the test specimen. The strain rate in the test specimen at any instant is related to the reflected strain, £/?, in the incident bar by —d£ ^ C , , = -2 —£Jt) 4-5

where C is the longitudinal sound speed in the bar and L is the length of the test specimen. The strain rate

is integrated to obtain the strain at any time during the test. Combining the strain rate data with the stress record provides the high rate constitutive behavior, or stress-strain curve. In the Kolsky test, strains in the

range 0.01 < £ < 0.20 are usually considered reliable. At strains below this range, which includes the

elastic and early yielding regions, the test specimen stress has not reached equilibrium. At strains greater

than this range, the test specimen may deform nonuniformly.

NISTNCSTAR 1-3D. WTC Investigation 83
Chapter 4

The Kolsky bar test specimens were cylinders, 4 mm or 6 mm in diameter and 2 mm high, whose

longitudinal axis was parallel to the rolling direction of the plate. Specimens originated from two

perimeter columns and an A 325 bolt. Specimen C22-C2M-FL3 is a F, = 75 ksi plate from WTC 1 column line 157 between floors 93-96. Specimen N8-C3M-FR2 is a F, = 60 ksi plate from WTC 1 column line 141 between floors 97-100. The original location of the A 325 bolt is unknown. Chapter 3 summarizes the results of the bolt tests.

4.2.4 Quasi-Static Compression Tests

Because the lowest strain rate that the Kolsky test can achieve is about 200 s ', quasi-static compression

tests using Kolsky specimens were conducted to extend the strain rate data to low rates. These tests employed an ordinary servo-hydraulic test machine in extension control at constant extension rate. In most cases, the specimen strain was estimated from the crosshead displacement, after correcting for machine compliance. Tests on the A 325 boh steel employed an extensometer clipped to the ends of the

loading rams as well.

4.3 RESULTS

4.3.1 High Strain-Rate Tension Tests

Figure 4-6 is an example of a set of high-strain-rate stress-strain curves for F,. = 50 ksi perimeter column

M26-C1-RF, from WTC 1 column 131 floors 90-93, north face, just above impact zone. Although the

stress measurement was taken from the strain gauge on the grip-end of the test specimen, the ringing in the stress signal is not completely eliminated.

Tables 4-1 and 4-2 summarize the results of the high strain-rate tension tests of the eight perimetercolumn steels and five core-column steels. Appendix A tabulates the individual test data and presents the

stress-strain curves for each test.

84 NISTNCSTAR 1-3D, WTC Investigation
High-Strain-Rate Properties

Table 4-1. Summary of specimens and results for high strain-rate tests of

perimeter columns.

(ksi) Specimen

Location^ in Building and

Shape

Specimen

Orientation

m

Eq. 4-9

50 M26-C1-RF

WTC 1 131 90-93

9/16 in. flange plate

longitudinal

F,: w=0.0121±0.0033

TS: w=0.0118±0.0023

transverse

F,: w=0.0178±0.0044

TS: W7=0.0 15810.0026

60 N8-C1-RF

WTC 1 143 97-100

5/16 in. flange plate

longitudinal

F,: w=0.0160±00033

TS: w=0.0133±0.0032

transverse

F,: w=0.0115±0.0032

TS: w=0.0 12710,0012

65 N99-C3-RF

WTC 1 147 99-102

1/4 in. flange plate

longitudinal

F,: w=0.0125±0.0023

TS: w=0.01 3 1+0.0028

transverse

F,.: w=0.0094±0.0036

TS: w=0.0112±0.0022

75 C22-FL

WTC 1 1 57 93-96

1 /4 in. flange plate

longitudinal

F,: A«=0.0067±0.0021

TS: w=0.0138±0.0003

transverse

F,.: OT=0.0061±0.0016

TS: w=0.0145±0.0038

75 C14-SP

WTC 2 300 86

j/6 in. spandrel plate

longitudinal

Fy. w=n/d

TS: »7=0.0 10810.00 15

transverse

F,: w=0.0083±0.0021

TS: w=0.0098±0.0016

100 MIOB-RF

14 in. flange plate

longitudinal

F,: w=0.0053±0.0014

TS: w=0.0085±0.0013

transverse

F,: w=0.0039±0.0038

TS: w=0.0083±0.0010

100 ClO-IW

WTC 1 451 85-88

1 /4 in. inner web plate

longitudinal

F, : w=0.0077±0.0033

TS: w=0.0113±0.0006

transverse n/d

100 CIO-FL

WTC 1 451 85-88

1/4 in/ flange plate

longitudinal

F,: w=0.0044±0.0014

TS: w=0.0080±0.0026

transverse

F, : w=0.0051±0.0014

TS: w=0.0091 ±0.0021

a. WTC ft XXX YY-ZZ: ff=tower 1 or 2: XXX=column line; YY-ZZ=floor range Key: n/d, not determined, generally because of a lack of low strain rate data Note: Reported uncertainties are defined as the estimated standard uncertainty of w, s,

NISTNCSTAR 1-3D. WTC Investigation 85
Chapter 4

Table 4-2. Summary of specimens and results for high strain-rate tests of core CO umns.

ksi Specimen

Location" in Building and

Shape

Specimen

Orientation

m

(Eq. 4-9)

36 C65-WEB

WTC 1 904 86-89 web of 12WF161

longitudinal

F,: OT=0.0336±0.0030

TS: w=0.0207±0.0010

transverse n/d

36 C65-FL

WTC 1 904 86-89

flange of 12WF161

longitudinal

F,: w=0.0290±0.0042

TS: OT=0.0130±0.0014

transverse n/d

36 B6152-2-FL

WTC 1 504 33-36

flange of box column

longitudinal

F,: »;=0.0239±0.0038

TS: /w=0.0165±0.0012

transverse

F,: w?=0.0396±0.0009

rc- rii=0 01 96+0 001

36 C80

WTC 1 603 92-95

flange of 14WF184

longitudinal

F,: «;=0.0317±0.0027

TS: ff?=0.0175±0.0039

transverse n/d

42 HHl-WEB WTC 1 605 89-101 web of 12WF92

longitudinal

F,.: w=0.0265±0.0014

TS: w=0.0200±0.0058

transverse n/d

a. WTC # XXX YY-ZZ: #=tower 1 or 2; XXX=column line; YY-ZZ=floor range. Key: n/d, not determined, generally because of a lack of low strain rate data Note: Reported uncertainties are defined as the estimated standard uncertainty of m, s.

4.3.2 High Strain-Rate Kolsky Bar Tests

Figure 4-7 is an example of the results of a Kolsky test. The plot shows both the stress-strain behavior

and the corresponding strain-rate-strain behavior for a perimeter column steel. Note that the strain rate is not constant during the test. Table 4-3 summarizes the Kolsky bar tests for the two perimeter column

steels and the A 325 boh steel. Appendix A presents the results of the other tests.

86 NISTNCSTAR 1-3D, WTC Investigation
High-Strain-Rate Properties

(0

0)

120

100

80

60

40 c

'cn c

LU 20

(1) L8-06 de/dt=417 1/s

(2) L7-06 d£/dt= 260 1/s

(3) L6-06 dt7dt= 100 1/s

(4) L2 di:-.'dt= 65 1/s

(5) LI d£ydt= 6.06x1 0"M/s

(4) 11(2)

(1)

M26-C1B1-RF WTCl Column 130 90-93

-I I L. 0.0

10:1450 - Friday February ll. 2005

0.1 0.2 0.3 Engineering strain

(3)

0.4

Figure 4-6. Examples of tensile high-rate stressstrain curves for Fy = 50 ksi perimeter column steel

M26-C1B1-RF.

Figure 4-7. Example stress-strain and strain rate-strain curves for Kolsky tests.

NISTNCSTAR 1-3D, WTC Investigation 87
Chapter 4

Table 4-3. Summary of Kolsky bar tests.

Test Specimen

and Location

Nominal Fy

(ksi) Test #

Average

Flow

Stress" (MPa)

Average

Strain

Rate''

(1/s) 434 943±43 15621408

/IOC 435 895±55 11341123

C22-C2M-FL3

WTC 1

436 804±46 579142

100

437 675±65 389195

93-96

438 1015+43 21021283

439 1029±31 25711423

440 829+42 529176

441 835±53 1 1411125

450 831164 15331159

451 808164 1609180

452 776171 1016181

N8-L3JVl-rR2

WTC 1 Column 141

453 621182 7591119

60 454 453195 443187

97-100 455 860152 21691204

456 906155 23541246

457 921155 27991344

458 898156 26891206

324 691136 9171335

1 T c 325 7951179 71 11434

A 325 bolt unknown

location

92

326 8361227 6881445

327 992170 11451338

328 957186 10601382

329 1077144 23951566

a. Average flow stress represents the average in the range

b. Average strain rate represents the average in the range

0.2en,ax < e < 0.8 Eniax- Note: Uncertainties represent the standard deviation of the value;

\N-] '

4.3.3 Quasi-Static Compression Tests

Table 4-4 and Fig. 4-8 summarize the quasi-static compression test conditions and results that are used

for analyzing the strain rate sensitivity from the Kolsky bar tests. The plots are graphically truncated at

1 0 percent strain.

88 NISTNCSTAR 1-3D, WTC Investigation
High-Strain-Rate Properties

Table 4-4. Summary of quasi-static compression tests

(ksi) Specimen Location in Building

Initial Strain Rate

(1/s) 60 N8-C3M-FR2-1 WTC 1 142 97-100

5/16 in. flange plate 0.001

0.01

75 C22-C2M-FL3 WTC 1 157 93-96

1/4 in. flange plate

0.01

0.001

100 C10-C1M-FL2 WTC 1 451 85-88

1/4 in. flange plate

0.01

n/a A 325 bolt unknown 0.002 Key: WTC n XXX YY-ZZ: #=Tovver 1 or 2; XXX, column line; YY-ZZ, floor range; n a = not applicable.

1500

^ 1000

c/53 500

—I—^ \ 1

1 1 1—I1 1 1

1 r—I 1—I

1 1 1

(1) N8-C3M-FR2-2 diVdt = 0.01 s"' (2) N8-C3M-FR2-1 ds/dt = 0.001 s"' (3) A 325 bolt strain from machine compliance de/dt = 0.0023 s"' (4) A 325 bolt strain from extensometer

0.00 0.02

1500

0.04 0.06 True strain

^ 1000

CO3 500

-| 1 r -I 1 T"

0.08 —I0.10

(1) C10-C1M-FL2-1 de/dt = 0.01 s

(2) C22-C2t</1-FL3-1 de/dt = 0.01 s

'

(3) C22-C2M-FL3-2 dtVdt = 0.001 s'

0.00 0.02 0.04 0.06 True strain

0.08 0.10

Figure 4-8. Quasi-static compression stress-strain curves for the tests summarized in Table 4-4.

NISTNCSTAR 1-3D. WTC Investigation 89
Chapter 4

4.4 DISCUSSION

There are several important limitations in understanding the data from the high-rate tensile tests. At

testing rates greater than about 200 s"' (Follansbee 1985b) the propagation of the stress wave that loads

the specimen through the loading train begins to influence the measurement of the test load. At these

rates, the time necessary for the test specimen to achieve an equilibrium stress state throughout the gauge

section can be a significant portion of the total time of the test. In this case, the experimentally derived

low strain behavior does not represent the defonnation behavior of the steel. An upper limit for the rate at which the stress wave propagates through the specimen (Follansbee 1985b) is the elastic wave velocity v^:

4-6

where E is the elastic modulus and p is the density. A lower limit is the velocity of the plastic stress wave:

y ds y 4-7

where dcr/ds is the instantaneous slope of the true-stress tme-strain curve. Follansbee (1985b) asserts that the stress wave velocity is closer to the elastic wave velocity even after yielding. The minimum valid

strain can be estimated (Follansbee 1985b) from the time necessary for three passages of the stress wave:

''min

through the specimen of gauge length lo deforming at a strain rate £ . For the high rate tension tests, the

1 percent offset strain used for the yield strength is more than sufficient strain to ensure an equilibrium

stress state in the gauge section.

A second issue in high-rate tests is the heating of the test specimen by the work of plastic deformation

(Follansbee 1985b). Quasistatic tests are often assumed to be isothermal, where the heat generated by the

work of deformation can escape from the specimen via the grips. In contrast, high-rate tests are generally

assumed to be adiabatic. In the short time of the test, the heat generated in defonning the specimen carmot

escape the specimen. An upper limit estimate, following Follansbee (1985b), for the temperature rise under adiabatic deformation is about 80 °C. The Investigation made no attempt to correct for changes in yield and tensile strength or elongation resulting from adiabatic heating.

Ringing of the load signal. (Gillis 1985) introduces a third problem in interpreting the stress-strain curves. The ringing generally occurs at about the natural frequency of the load cell. The problem can be

particularly severe for high-rate tests that employ conventional load cells (Bruce 2004). Even with test specimens instrumented with strain gauges in the grip section away from the load application, like those

used in the Investigation, it is not always possible to eliminate the ringing (Bleck 2000; ESIS 2000;

Wiesner 1999; Yan 2002). Many tests reported in the literature have been heavily filtered to remove the

90 NISTNCSTAR 1-3D, WTC Investigation
High-Strain-Rate Properties

load ringing (Gillis 1985). In reporting the data, the Investigation did not attempt to mathematically

remoN e the ringing.

4.4.1 Calculation of Strain-Rate Sensitivity for Tension Tests

Strain-rate sensitivity (SRS) is an indication of the effects of strain rate on material properties such as yield and tensile strength. This report defines the SRS as the slope of the log(stress)-log( strain rate) curve. Although there are other representations of the stress-strain rate behavior, this analysis expresses the

effect of strain rate on yield, F, . and tensile strength, TS, via a simple relation:

(7

^8^

(I V ^0 J

4-9

where a is the yield, F, . or tensile, TS, strength, ao = 1 ksi is a normalizing parameter, s^^ is a fitted

reference strain rate, and m is the strain rate sensitivity. From the parameters of Eq. 4-9, it is straightfon\'ard to calculate the parameters of the other possible representations.

To calculate the strain rate sensitivity, the natural logarithm of Eq. 4-9 is fit to the data:

log.

<7 = m log^, (£) - w log,, (fy ) 4-10

where <t^^ — 1 ksi. The slope of the fit is the strain rate sensitivity, w, and the intercept is the second term

in Eq. 4-10.

Tables 4-1 and 4-2 summarize the fitted value of the strain rate sensitivity, m. In all cases the yield and

tensile strength increase with increasing strain rate. Appendix A contains strain rate sensitivity plots of

both F, and TS for each steel. Some of the calculations utilize data developed using the standard Flat 2"

and round specimens for the quasi-static rates. The tables in Appendix A identify which specimen was

used for each test. Appendix E provides justification for combining the data from the different specimens

to evaluate the strain rate sensitivity. Figure 4-9 plots the measured strain rate sensitivity, m, for the yield

and tensile strengths as a function of specified minimum yield strength. The uncertainties in Fig. 4-9 and

Tables 4-1 and 4—2 are the estimated standard uncertainties of the calculated strain rate sensitivities, w, as described in the footnote of Table 4-1. There is no significant difference in the strain rate sensitivity, w, measured in the longitudinal and transverse directions.

Because the total elongation, £/„ depends on the specimen geometry, it is not possible to use all the quasistatic test data to evaluate the strain rate sensitivity. For many of the core column steels, however, there is quasi-static test data obtained using the high-rate style specimens of Fig. 4-1. Many of the quasi-static

tests on perimeter column steels employed the / = 0.25 in. Flat 2" specimen, which is geometrically

identical to the ? = 0.125 in. Flat-HSR T'specimen. Appendix E provides theorefical and experimental

justification for combining data generated using these two specimens for evaluating the total elongation

behavior.

NISTNCSTAR 1-3D, WTC Investigation 91
Chapter 4

4.4.2 Calculation of Strain-Rate Sensitivity for Kolsky Tests

Figure 4-10 shows the strain rate dependence of the flow stress for the three steels evaluated in the

Kolsky bar tests. For the tv>'0 perimeter columns, the flow stress is evaluated at a 1 percent offset strain. For the A 325 bolt steel the flow stress is evaluated at a 5 percent offset strain. Table 4-5 summarizes the data plotted in Fig. 4-10. Table 4-6 compares the strain rate sensitivities evaluated in tension with those

evaluated in the compression Kolsky bar tests. The absolute values of the strain rate sensitivities evaluated by the two techniques are similar. The larger scatter of the Kolsky data, Fig. 4-10, results in uncertainties in the calculated strain rate sensitivities, m, that are roughly two times larger than those from

the tensile data.

0.07

0.06

^0.05

if) 0.04 c

CDW

B 0.03

03C

« 0.02

00

0.01

0.00 20 40

'

1

' —1

1

1

'

' '

1

' 1

1

I 1 1 Tensile strength:

o°c(de/clt)"'

- o TS longitudinal A TS transverse [

I f

9

1 , , 1 , 1 1 1 , , , 1 , 1 1 1 1 1 60 80 100 120

Fy, ksi

0.07

0,06

w 0.04 c

0wB 0.03 c

'to 0.02

to

0.01

0.00 20

9'

40

-i 1 r

5^,

Yield strength

O F,, longitudinal A F, transverse

60 80

09 13:00 - Thursday February 10, 2005 Fy, ksi

A

100 120

Figure 4-9. Strain rate sensitivity of yield and tensile

strength as a function of specified minimum

yield strength.

92 NISTNCSTAR 1-3D, WTC Investigation
High-Strain-Rate Properties

Figure 4-1 1 plots the strain rate dependence of the flow stress at different strain levels for the A 325 bolt. Note that by 5 percent strain the flow stress is independent of the strain rate. This behavior, summarized

in Table 4-6, agrees with the strain rate independent strength found in the full bolt tests described in Section 3.5.2.

Tabl e 4-5. Summary of stress-strain rate data plotted in Fig, ^ N8-C3M-FR2

Fy = 60 ksi WTC 1 142 97-100

C22-C2M-FL3

Fy - 75 ksi WTC 1 157 93-96A 325 Bolt

Location Unknown

Stress (MPa)

Strain

Rate

(1/s)

Stress (MPa)

Strain

Rate

(1/s)

Stress (MPa)

Strain

Rate

(1/s) 500 0.001 650 0.001 990 0.0023

460 0.01 650 0.01 988 24

470 500 700 500 1020 384

550 750 800 600 968 1150

550 750 700 600 992 915

650 2400 700 1200 991 2970

575 1500 720 1200

650 2200 740 1250

620 2400 840 1750

620 2200 800 2400

550 3000

-10.

Notes: N8-C3M-FR2 and C22-C2M-FL3 evaluated at 1 percent offset strain. A 325 bolt evaluated at 5 percent offset strain. Slow rate tests evaluated using method of Section 4.2.4.

Figure 4-10. Flow stress as a function of strain rate for Kolsky tests.

NISTNCSTAR 1-3D, WTC Investigation 93
Chapter 4

3.1

CO CL 3.0

COO 2.9

CO

CD

13 ^ 2.8

O

2.7

m = -0.000010.0016 evaluated at e=0.05

rr, = +0.0119+0.0041 evaluated at c:=0.03 m = +0.0260+0.0061 evaluated at S:-=0.01

A 325 bolt

19:26.26 - Monday December 20, 2004

0 2

log^o (strain rate), 1/s

Figure 4-11. Flow stress as a function of strain rate evaluated at different strains for

Kolsky tests on the A 325 bolt.

Table 4-6. Comparison of strain rate sensitivity measured in tension and compression.

Specimen ksi

Strain Rate Sensitivity from

Tension Tests"

Strain Rate Sensitivity from Kolsky

Bar and Compression Tests" C22-C2M-FL3 75 F,: «;=0. 006710.0021 w=0.0114±0.0041

N8-C3M-FR2 60 Fy. w=0.0 160100033 m=0.0152±0.0057

A 325 Bolt TS^ ^ mm

120 ksi TS: w=0.001 ±0.001

m=0.0000±0.0016

Stress evaluated at 5 % plastic strain

a. Strengths evaluated at 1 percent offset strain for both tension and compression.

4.5 HIGH-STRAIN-RATE DATA PROVIDED TO THE INVESTIGATION

Early in the Investigation, before the tensile testing was complete or tlilly analyzed, it was necessary to provide estimates of the high-strain rate behavior of the WTC steels to other Investigation team members.

These estimates were based on a provisional set that contained only six perimeter-column and no core- column steels. The provisional model predicted the increase in flow stress as a function of the strain rate and the quasi-static yield strength, using the Cowper-Symonds equation. After the full set of high strain- rate data from the thirteen steels was complete, the strain rate sensitivities for the fiall set were analyzed

using the same methodology. The prediction of the high-strain-rate behavior of the perimeter column

steels was generally similar for the provisional and complete models. The provisional model based on the

six perimeter-column steels over-predicts the increase in flow stress in the low-strength core-column

steels. Appendix C summarizes the analysis of both data sets.

94 NISTNCSTAR 1-3D, WTC Investigation
High-Strain-Rate Properties

4.6 COMPARISON WITH LITERATURE DATA

Table 4-7 summarizes the strain rate sensitivities of several structural steels for comparison with those of

the WTC steels in Table 4-2. It is not a comprehensive list, but it does include several plain-carbon

(Couque 1988. Krafft 1962, Manjoine 1944) and HSLA (Chatfield 1974, Davies 1975, Langseth 1991)

structural steels. The data from Chatfield ( 1 974) and Davies ( 1 975 ) are for automotive rather than

strucmral steels, but the chemistries are similar to the WTC steels. Most of the references, except Krafft

(1962) and Couque (1988) provide little information on the microstructure of the steels characterized.

Table 4-7. Literature data for strain rate sensitivities of structural steels.

Steel Reference (ksi)

/;/ Determined

for Fy

(Eq. 4-9)

m

Determined

for TS

(Eq. 4-9) Notes Chemistry% mass

1020 ir Couque

1988

28 0.043 n/r (a) to Not reported

1020 T7' Couque

1988

30 0.059 n/r (a) to Not reported

Commercial

low-carbon

open hearth''

Manjoine

1944

31 0.054±0.007 0.019±0.005 (a) te Not reported

HSLA-40 Chatfield

1974

40 0.045±0.004 0.031+0.007 te C 0.06 Mn 0.34 Si 0.02 Cu 0.04 Cr 0.03 Ni0.03 Al 0.055

EM-3'' Krafft 1962

44.6 0.064 0.053 (b)c C 0.20 Mn 1.12 Si 0.31

HSLA-45-1 Chatfield

1974

45 0.035±0.003 0.03410.0009 te C 0.18 Mn 0.78 Si 0.06 Cu 0.04 Cr 0.03 Ni0.03 Mo 0.016 Al 0.001 HSLA-45-2 Chatfield

1974

45 0.024±0.002 0.023±0.005 te C 0. 1 8 Mn 0.69 Si 0.63 Cu 0.07 Cr 0.03 Ni 0.12 Mo 0.012 Al 0.007

HbLA-50 LnatiieJd

1974

30 0.026±0.003 0.01910.0001 te L 0.J3 Mn U.DD Si U.94 Lu 0.04 Lr 0.42 Ni 0.12 Mo 0.020 Al 0.012

YST-50 Davies

1975

50 0.014±0.002 0.01010.0018 te CO.lOMn 0.54 Ti0.12

ST52-3N Langseth

1991

51 0.021±0.002 0.009810.0005 te C0.12 Mn 1.30 Si 0.40 Cu 0.20 Cr 0;08 Ni 0.20 Mo 0.03 Al 0.03

Hot-rolled Davies

1975

60 n r 0.02210.003 te C0.19Mn 0.75

HSLA-80-1 Chatfield

1974

80 0.020±0.002 0.01610.003 te C 0. 14 Mn 0.76 Si 0.60 Cu 0.03 Cr 0.49 Ni 0.03 Mo 0.005 Al 0.02 HSLA-80-2 Chatfield

1974

80 0.018±0.007 0.01810.005 te C0.05 Mn 1.55 Si 0.16Cu 0.01 Cr 0.02 NiO.Ol Mo 0.29 Al 0.068

YST-80 Davies

1975

80 0.012±0.0007 0.008710.001 te CO.ll Mn 0.54 Ti 0.30 V 0.018

a. Actual \ aiue forF,.

b. Reported value. Key: te, tension test; to. torsion test; c, compression test; n/r = not reported. Note: See Table 4-1 for definition of uncertainties of parameters.

NISTNCSTAR 1-3D. WTC Investigation 95
Chapter 4

Figure 4-12 plots the strain rate sensitivities, m, from Table 4-7, as a function of yield strength, F,,, along

with the data from the WTC steels. The behavior and magnitude of the strain rate sensitivities of the WTC

steels is entirely consistent with those from the literature of similar steels.

0.07

0.06

0.05

w 0.04

CD

COc2 0.02

CO

0.01

0.00 20

Tensile strength:

ooc(de/dt)'^ WTC

LiteratLire

1 1

I t65

-I I L. J I L. J I 1_ 40 60 80 100 120

Fy, ksi

0.07

0.06 -

0.05 -

w 0.04

CO

CO

'2 0.02 W

0.01

0.00 I—

-| 1 1

1

1 1 r

20

1 1?

1

~9

Yield strength

G vnc

• Literature

40 60 80 100 120

09:05:55 - Thursday February 10, 2005 Fy, ksi

Figure 4-12. Comparison of strain rate sensitivities of

NIST WTC steels to values for structural steels from

the literature.

One limitation of the data from the WTC steels is that the strain rate sensitivity is determined using data

only at the extremes of the strain rates. Gilat et al. (1997) found that the strain rate sensitivity of

AISI 1020 steel at strain rates near 1 s"' was negative. They argued that it arose from dynamic strain aging. Other investigations of plain-carbon and HSLA steels either did not probe this strain rate range

(Couque 1988) or did and found that the strength increases monotonically with strain rate (Chatfield

1974, Davies 1975, Krafft 1962). Even steels likely to be susceptible to dynainic strain aging {Chatfield

96 NISTNCSTAR 1-3D, WTC Investigation
High-Strain-Rate Properties

1974. Krafft 1962. Manjoine 1944) exhibit this monotonic increase in strength with strain rate. In contrast

to Gilat (1997). Harding (1972) found three regimes of constant, but positive, strain rate sensitivity of the

yield strength of BS 968, a ferrite-pearlite structural steel. At strain rates less than 0.1 s"' the strain rate sensitivity is nearly zero. At rates up to 10"^ s"' the strain rate sensitivity is w~0.07. At rates above 10"^ s"' the strain rate sensitivity is even higher.

There is no consistent relation between total elongation and strain rate in structural steels in the strain-rate range used in this in\'estigation. Ahhough Bleck et al. (2000) found that total elongation, £/„ in mild steel (C = 0.05 percent) decreases with increasing strain rate, they also found that values for other steels do not depend equally on strain rate. Manjoine (1944), Kendall (1965), Chatfield (1974), Davies (1975),

Langseth (1991). and Harding (1972) also found no strong correlation between strain rate and total elongation. Bruce (2003) reported that ductility increases with increasing strain rate in HSLA steels.

Figures 4-13 and 4-14 compare the strain rate sensitivities of the total elongation of various structural

steels from the literature to those obtained in the Investigation. The data for the WTC steels were all obtained using specimens of constant, identical geometry, but possibly different size, or using identical

Flat-HSR 1" specimens at all rates. The data tables in Appendix A identify the specific specimen types

employed. The elongation behavior of the steels from the perimeter and core columns is similar to the

behavior of other structural steels. Uncertainties in the strain rate sensitivities of the total elongation are

large for both literature and WTC steels. In many cases, the measured strain rate sensitivities are

statistically indistinguishable from zero.

4.7 SUMMARY

The strain rate sensitivity of the yield and tensile strength of the perimeter and core column steels tested is similar to that of other structural steels reported. The total elongation of perimeter and core column steels generally increases slightly with increasing strain rate. The magnitude of the increase is several absolute

percent and is similar to behavior reported for other structural steels.

NISTNCSTAR 1-3D, WTC Investigation 97
50 -

UJ 40

30

20

10

1 ' ' ' \

' ' '

\ '

F^,= 50 ksi e,=(+1 .97±0.G0) M26-C1B-RF

F,,= 60 ksi e, =(+0.61 ±0.07) N8-C1B-RF

F^.= 100 ksi e -.;-f0,86±0.18) M10B-C3B-RF

F: = 100 ksi e.=(+1. 20+0.01) C10-C1M-LF

Total Elongation

EI,=e(j+e,logj(d£/dt)

All test specimens have A /Lo = 0.177 M26. N8 are Flat HSR 1", M10B, CIO are mixed

perimeter steels longitudinal orientation Fy>=50 ksi

I I I I I I I I I I I -2 0 log^Q strain rate, 1/s

50

40

CO§30 1-

A

_

20

10

1—'—'—'—I—'—'

'—

F^=51 ksi e,={-Q.00±0.71) ST52-3N

F.,=50 ksi e,=!-0.60±0.84) HSLA50

F^=80 ksi e;=(-0,53±0 63) HSLA80A

F.=80 ksi e.,=(+1.08±0 30) HSLA80B

Data sources ST52-3N Langseth (1991) HSLA... Chatfield (1974)

, \ , I , L

Literature Steels with f^>=50 ksi

I I I I I I I

1 4:43:1 5 - Thursday December 30, 2004

-2 0 log^o strain rate, 1/s

gure 4-13. Total elongation, Elt, as a function of strain

rate for high-strength, perimeter column steels and

high-strength steels in the literature.

NISTNCSTAR 1-3D, WTC Investigation
High-Strain-Rate Properties

50

lD 40 c

o

00§ 30

0)o

20

10

Total Elongation

EI.=ep+e,logj(clE/clt)

1

' ' '

\

' ' ' 1 <- A F„=36 ksi e,=(+0.83±0.27) B6152-2 0 F,=36 ksi e,=(+0.50±1 .37) C65-web • r.=36 ksi e.=!+0.62±0.56) C65-fiange F„=42 ksi e.=(-0.19±0.02) HH-web

All test specimens have A /Lo = 0.125

All test specimens are Flat HSR 1" with t=0.0625 in. core steels longitudinal orientation F^,<50 ksi

I I I I I I \ \ I I i_ -4 -2 0 log^o strain rate, 1/s

50 -

UJ 40 - c

o

O) I

30

o

20 h

10

1 0

•

1

1 1 1 1

1 1 1

1 F„=29 ksi e.=(+l .31x0.39) Manjoine

F.=40 ksi e,={+0.90±1.09) HSLA40

Ff.=45 ksi e.=(+0.52i:1 .89) HSLA45A

F;.=45 ksi e,=(+0.15±0.64) HSLA45B

1 1 1

1 I _

^—-——"u

-—"— ^

—

•

- Data sources —Manjoine (1941)

- HSLA... Chattield (1974)

a •

Literature Steels with F^<50 ksi

1 1 1 1 1 1 1 1 1 1 1 1 1 1 -4

14 43.23 • Thursday December 30, 2004

-2 0 loQiQ Strain rate, 1/s

Figure 4-14. Total elongation, E/,, as a function of

strain rate for low-strength, core column steels and

low-strength steels in the literature.

4.8 REFERENCES

Bleck, W and I. Schael. 2000. Determination of crash-relevant material parameters by dynamic tensile

tests. Archivfur das Eisenhutlenwesen. 71 173-178.

Bruce, D. M. 2003. Dynamic Tensile Testing ofSheet Steels and Influence ofStrain Rate on

Strengthening Mechanisms in Sheet Steels. Ph.D. Thesis. Colorado School of Mines.

NISTNCSTAR 1-3D, WTC Investigation 99
Chapter 4

Bruce, D. M. D. K. Matlock, J. G. Speer, and A. K. De. 2004. Assessment of the Strain-Rate Dependent

Tensile Properties of Automotive Sheet Steels. SAE Technical paper 2004-01-0507. Presented at the

SAE 2004 World Congress. March 8-11.

Chatfield, D. and R. Rote. 1974. Strain Rate Effects on Properties of High Strength, Low Alloy Steels. Society of Automotive Engineers, pubhcation no. 740177.

Couque, H., R. J. Asaro, J. Duffy, and S. H. Lee. 1988. Correlations of Microstructure with Dynamic and

Quasi-Static Fracture in a Plain Carbon Steel. Met. Tram. A. 19A .2179-2206.

Davies, R. and C. Magee. 1975. The Effect of Strain-Rate Upon the Tensile Deformation of Materials.

JErig. Malei: Tech. 97[2]. 151-155.

ESIS Designation P7-00. 2000. "Procedure for Dynamic Tensile Tests," European Structural Integrity

Society. KH. Schwalbe, Editor. Approved August 2000.

Follansbee, P. S. 1985. High Strain Rate Compression Testing, The Hopkinson Bar. p.p 198-203. in Metals Handbook 9''' ed. Volume 8 Mechanical Testing. American Society for Metals, Metals Park,

Ohio.

Follansbee, P. S. 1985b. High Strain Rate Compression Testing, Introduction, in Metals Handbook 9''' ed. Volume 8 Mechanical Testing. American Society for Metals, Metals Park, Ohio. 190-192.

Gilat, A. and X. Wu. 1997. Plastic Deformation of 1020 steel over a wide range of strain rates and

temperatures. Int. J. Plasticity. 13[6-7]. 611-632.

Gillis. P. P and T. S. Gross. 1985. Effect of Strain Rate on Flow Properties. Metals Handbook Ninth

Edition, Volume 8 Mechanical Testing. American Society for Metals. Metals Park, OH. p.p. 38-46.

Harding, J. 1972. Effect of High Strain Rate on the Room-Temperature Strength and Ductility of Five

Alloy Steels. J. Iron Steel Inst. 2 1 0. 425-432.

Harding J. 1987. The Effect of High Strain Rate on Material Properties. Chapter 4 of Materials at High

Strain Rates. T.Z. Blazynski, ed. Elsevier. London.

Hutchinson, J. W. and K. W. Neale. 1977. Influence of Strain-Rate Sensitivity on Necking Under

Uniaxial Tension. Acta Metallurgica. IS. 839-846.

Krafft, J. M. and A. M. Sullivan. 1962. On Effects of Carbon and Manganese Content and of Grain Size on Dynamic Strength Properties of Mild Steel. Trans. A.S.M. 55. 101-118.

Langseth, M. U. S. Lindholm, P. K. Larsen, and B. Lian. 1991. Strain Rate Sensitivity of mild steel grade

ST-52-3N. J. Eng. Mech. 117[4]. 719-731.

Manjoine M. J. 1944. Influence of Rate of Strain and Temperature on Yield Stresses of Mild Steel. Trans. A.S.M. 66. A-2\\-A-2\S.

100 NISTNCSTAR 1-3D, WTC Investigation
High-Strain-Rate Properties

Wiesner, C. S. and H. MacGillivray. 1999. Loading Rate Effects on Tensile Properties and Fracture,

Proceedings ofthe Seventh Symposium Toughness ofSteel, Fracture, Plastic Flow and Structural

Inlegiity: April 29, Abingdon, UK. 149-173.

Yan. B. and K. Xu. 2002. High-strain-rate behavior of advanced high strength steels for automotive

applications. 44" Mechanical Working and Steel Processing Conference Proceedings. Volume 40.

Association for Iron and Steel Technology. Warrendale, PA.

NiSTNCSTAR 1-3D, WTCInvestigation 101
Chapter 4

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102 NISTNCSTAR 1-3D, WTC Investigation
Chapter 5

Impact Properties

5.1 INTRODUCTION

Charpy impact tests were performed on steels from the World Trade Center (WTC) to provide data for comparison with the WTC-construction era literature data, and to provide additional information on how

strain-rate affects the fracture mode of the steels. Estimates of the energy absorbed during an impact and

the characteristic fracture mode of the steel are both of interest. As strain rates increase, the strength of

steel tends to increase, and the fracture behavior tends to be less ductile and more notch-sensitive. Charpy

impact testing is one of the methods used to evaluate these effects. This chapter discusses the data from

the impact tests with respect to the transition temperature from ductile to brittle behavior, the absorbed

energy levels associated with a given test temperature, and the influence of specimen orientation on the

absorbed energy. The impact data generated in the Investigation were not used quantitatively used in the

modeling efforts, however.

Figure 5-1 is a schematic example of the definition of the transition temperature expressed in terms of the

energy absorbed. At high temperature there is an upper shelf of relatively constant absorbed energy where

fracture is ductile. At low temperature the absorbed energy is also relatively constant, but low, and

fracture is brittle. An S-shaped curve connects the two shelves. The transition temperature from ductile to

brittle behavior can be defined in many different ways. Common definitions based on absorbed energy

include the midpoint of the S-curve, a specific fraction of the upper shelf energy, or some fixed energy

level, such as 20 ft Ibf (27 J) or 30 ft Ibf (41 J). The transition temperature can also be characterized by

examining the change in appearance of the fracture surface with temperature. The fracture appearance

transition temperature (or FATT) represents the temperature at which fibrous features occupy a specific

fraction of the fracmre surface. In this chapter, transition temperatures based on energy are defined as the

midpoint between the upper and lower shelf energies. Transition temperatures based on fracture

appearance are defined as the temperature at which ductile fracture features occupy 50 percent of the

fracmre surface. For any steel, the transition temperatures measured by these two definitions will rarely

be identical, but frequently they will be similar.

5.2 PROCEDURES

To obtain the most useful data, representafive samples of the steels found near the impact zone were

evaluated. These samples do not represent the thicker secUons and the types of steels found in the lower

stories of the towers. Impact test specimens were oriented according to the requirements of ASTM

International (ASTM) Standard Test Method A 370-97a. Tests were conducted according to the

requirements of ASTM Standard Test Method E 23-02. Some components were thick enough to permit

full-size (10 mm x 10 mm) specimen cross sections to be taken, but for many components the plates were

too thin, and sub-size specimens were used. The ASTM Type A specimen, with 5 = 5 mm thickness

(5 mm X 10 mm cross section), was used for the sub-size specimens. Figure 5-2 shows the geometries of

the two specimens.

NISTNCSTAR 1-3D, WTC Investigation 103
Chapter 5

The specimens were oriented according to Section 4 of A 370. Figure 5-2 illustrates the sample

orientations with respect to the rolling direction of the steel using a composite micrograph of the

microstructure of a plate from perimeter column N9-C1TI-LF (F, =55 ksi). In the figure, the dark

features are the bands of pearlite. The grains are the slightly elongated shapes with varied contrast. The

inclusions are also elongated in the direction of rolling, but are not visible on this scale. The longitudinal

orientation is such that the long dimension of the specimen is parallel to the rolling direction, and the

fracture direction is perpendicular to the rolling direction. The long dimension of the transverse specimen

is across the plate or structural shape, perpendicular to the rolling direction, and the fracture direction is parallel to the rolling direction. These longitudinal (L) and transverse (T) specimens are often referred to as L-T and T-L specimens respectively in other specimen orientation designation systems. For the

samples taken at or near welds that join flanges and webs on perimeter columns, the welding direction

coincides with the rolling direction of the steels. A transverse specimen can be located with the notch tip

adjacent to the fusion line of the weld and promote crack growth in a direction parallel to the weld

(parallel) in the heat affected zone (HAZ).

All specimens were tested on a 300 ft lbf capacity impact machine that was verified to the requirements of

E 23-02. The striker velocity at impact was approximately 17.7 ft/s. Strain rates at the notch are estimated

(Poussard 2003) to approach 5000 s"'. The average strain rate across the ligament of the sample is estimated to be about 100 s'', which is near that of the high-rate tensile tests. When testing sub-size

specimens, the standard anvil supports for 10 mm thick specimens were replaced with higher ones, so the center of the striker met the center of the specimen.

Specimens were tested over a range of temperatures to characterize the transition behavior of the steels. Because of the scatter that is common in impact testing, multiple specimens were tested at most

temperatures. All impact energy data were collected digitally using a precision encoder. The fracture

surface appearance was assessed to estimate the fracture appearance transition temperature (FATT),

which was defined as the temperature at which the fracture surface exhibited 50 percent ductile fracture

features. Evaluations were made using the chart comparison technique of ASTM E 23 Aimex A6. These

estimates of percent shear provide qualitative information, complementary to the absorbed energy data.

This investigation employed only one sub-size specimen geometry, so these results can be compared to each other, but they cannot be directly compared to handbook data, which are usually developed on

standard, full-thickness specimens. There is a large literature on methods for predicting the results of full- size specimen Charpy tests using sub-size specimens. Lucon (2001) presents a concise summary of the

various methods for predicting the upper shelf energy, as well as the ductile-to-brittle transition

temperature. When unlimited material is available and plate thickness is greater than the Charpy specimen

thickness, many methods simply develop an empirical correction factor by analyzing both full- and subsize specimens. Because many of the plates of interest for the Investigation were less than 10 mm thick,

this type of analysis was not possible.

For the scaling of energies, methods generally define a correction factor, K, that scales the upper shelf

energy obtained from sub-size specimen. Ess, to the expected energy from full-size specimens, Efs:

E, ^KE 5-1

104 NISTNCSTAR 1-30, WTC Investigation
Impact Properties

The correction factors. K. take many different forms. Table 5-1 summarizes some common ones. All

include some permutation of the specimen width, 5, and ligament depth, b. Fig. 5-2. More recent correction factors incorporate details of the notch, including the notch radius, R, and notch angle, 6.

The individual studies on specimen size effect (Corwin 1986, Kayano 1991, Lucas 1986, Lucon 2001,

Louden 1988. Manahan 1990, Schubert 1995, Schwartzbart 1954, Uehira 2004) reach different,

incompatible conclusions about the suitability of the different correction factors. These discrepancies

probably arise because each investigator characterized different steels, using test specimens with different

notch and loading geometries. Several of the correction factors (Corwin 1986, Lucas 1986) do not

incorporate information about the notch geometry.

This investigation uses a correction factor, K. first proposed by Lucas (1986)

^ ^ ^kMs ^ 10mmx(8mm)- ^ ^

B^^b^ 5 mm x (8 mm)"

Note that nearly all the proposed correction factors in Table 5-1 predict K=2 because the full-size and

sub-size specimens have identical notch geometries. These scaling factors do not explicitly consider the

effect of increased constraint for larger specimens, as ASTM E 23-04 (2004) Appendix XI discusses.

Different scaling methods must be used for estimating the full-size specimen transition temperature from

sub-size specimens (Lucon, 2001). The transition temperatures in this report, which were estimated from

fracture appearance, have not been scaled.

5.3 RESULTS

Appendix A of this report tabulates the data from individual impact tests. Table 5-2 summarizes some of

the important experimental results. The rest of this section summarizes the results for the steels arranged

by location in the building.

5.3.1 Perimeter Columns

The upper shelf energy of longitudinal specimens from flange material from perimeter column N8-C1M1

(Fy = 60 ksi specified strength, F, =68.8 ksi measured), located between the 97th and 100th floors of

WTC 1, is 38.3 ft-lbf, Fig. 5-3. At about -5 °F the absorbed energy begins to decrease as the fracture mode enters the transition region. For the transverse specimens, the absorbed energy drops from an upper

shelf of 10.4 ft Ibf beginning at about -40 °F. For both orientations, fracture surface evaluations indicate

that the transition temperature (FATT) is about -95 °F. After correcting for sub-size specimen effects using Eq. 1-2, the estimated upper shelf absorbed energy is 76.6 ft lbf and 20.8 ft lbf for full-thickness

longitudinal and transverse orientations respectively. The ratio of the upper shelf energies for the longitudinal to transverse orientations is approximately 3.7.

The impact data for the flange of the ClO-ClMl perimeter column from WTC 1 west face, column 452,

floors 85-88 in Fig. 5-3 show that the upper shelf for both the longitudinal and transverse orientations

extends throughout the temperature range evaluated. Fracmre surface evaluations confirm this result and

indicate that the transition temperamre (FATT) is below -100 °F for this quenched and tempered steel

NISTNCSTAR 1-3D, WTC Investigation 105
Chapter 5

(specified F,. = 90 ksi, supplied F,. = 100 ksi, measured F,. = 109.4 ksi). As is the case for the N8-C1M1

flange, there are two upper shelves: 47.1 ft lbf for longitudinal specimens and 1 1.4 ft lbf for the transverse specimens. After correcting for sub-size specimen effects, the upper shelf absorbed energies for full-size longitudinal and transverse specimens are 94.2 ft-lbf and 22.8 ft-lbf respectively.

The transition curve of steel from the perimeter column spandrel N8-C1B1 (specified F,, = 42 ksi,

measured F, = 54.4 ksi). Fig. 5^, is similar to that for the flange material from the same column. The

absorbed energy drops from the upper shelf at temperatures below about -40 °F. The upper shelves for the longitudinal and transverse specimen orientations are 40.9 ftlbf and 13.3 ft-lbf respectively. Fracture

surface evaluations indicate the transition to 100 percent brittle fracture is nearly complete at -1 12 °F. After correcting for the effects of the sub-size specimen using Eq. 1-2, the estimated upper shelf energy

for full-size longitudinal specimens is 81.8 ft lbf, and the upper shelf energy for the transverse specimens

is 26.6 ft lbf

5.3.2 HAZ Materials from Perimeter Columns

The upper-shelf absorbed energy values of HAZ (heat affected zone) specimens, notched near the toe of a

fillet weld on the flange of perimeter column N8-C1M1, are similar to those of the unaffected flange

materials. Fig. 5-3. The upper shelf is 44.3 ft lbf for the longitudinal specimens, and 9.4 ft lbf for the transverse specimens. The data and fracture surface evaluations indicate that the specimens with

longitudinal orientations have entered the transition region at 0 °F, but the transverse specimens have not. Again, after correcting to full-size using Eq. 1-2, the upper shelf absorbed energies for the longitudinal

and transverse specimens are 88.6 ft-lbf and 18.8 ft lbf respectively. The transition temperature of the

HAZ region appears to be higher than the unaffected flange material, but this has little practical

significance considering the transition temperature is still quite low and likely below 0 °F.

The upper shelf absorbed energies from the HAZ material from the ClO-ClMl flange, tested using

transversely oriented specimens are similar to those of the unaffected ClO-ClMl transverse flange

material, Fig. 5-3. The average absorbed energy is 11. 0 ft-lbf down to temperatures below- 100 °F. After

correcting for sub-size specimen effects, the upper shelf absorbed energy for the transverse specimens is 22.0 ft lbf

5.3.3 Core Columns

Impact properties of specimens from the web of core column C-80, a 14WF184 shape from WTC 1 column 603 floors 92-95, (specified F, = 36 ksi, measured F,. = 34.4 ksi) also vary as a function of

sample orientation. Fig. 5-4. The average absorbed energy of the room temperature tests is (53.8±6.3) ft-lbf for the longitudinal specimens, and (26.1±1.2) ft lbf for the transverse specimens. The

absorbed energy decreases with temperature throughout the range tested, indicating transition behavior over the range of 0 °F < r< 172 °F. Fracture surface evaluations show approximately 95 percent ductile

features at 1 72 °F for both longitudinal and transverse specimen orientations, and less than 50 percent

ductile features at room temperature. The transition temperatures (FATT) for both orientations are greater

than room temperature and less than 140 °F.

106 NISTNCSTAR 1-3D, WTC Investigation
Impact Properties

5.3.4 Trusses

The data for the D = 0.92 in. diameter truss rod longitudinal samples (specified Fy = 50 ksi, measured

Fy = 60.0 ksi ) show a gradual transition in impact behavior throughout most of the temperature range

tested (Fig. 5-5). The absorbed energy drops from 107 ft lbf at 172 °F to 4.9 ft lbf at -4 °F. At room

temperature the absorbed energy is (32.4±4.8) ft lbf Fracture surface evaluations show about 90 percent

ductile fracture at 172 °F and about 30 percent ductile fracture at room temperature. The transition

temperature (FATT) is higher than room temperature, likely about 100 °F.

The steel from the floor-truss angles (2 in. x 1.5 in.xO.25 in.) (specified Fy = 50 ksi, measured

Fy = 52.2 ksi-58.8 ksi) also shows transition behavior throughout the temperature range tested (Fig. 5-5).

The absorbed energy drops from (31.1±1.1) ft lbf at room temperature to 1.6 ft lbf at -76 °F. Fracture

surface evaluations indicate that the transition temperature (FATT) is likely below room temperature

(50 percent to 70 percent ductile at room temperature). After correcting for sub-size specimen effects using Eq. 1-2, the absorbed energy is (62.2±2.2) ft lbf at room temperature, (17.8±1.1) ft lbf at -4 °F, and

3.2 ft lbf on the lower shelf at -76 °F.

5.3.5 Truss Seats

The Fy - 36 ksi truss seats from perimeter columns, like the floor-truss angle and rod, show transition

behavior throughout the temperature range tested, Fig. 5-5. The absorbed energies for transverse specimens range from 27.5 ft lbf to 36.8 ft lbf at 172 F, and drop to about 2.3 ft lbf at -40 °F. At room

temperamre the absorbed energies of truss seats N13-C3B1, M4-C3T, and N8-C3T are (7.7±1.1) ft lbf,

(1 1.9±0.5) ft lbf, and (21.2+0.5) ft lbf respectively. Fracture surface evaluations indicate that the N13 and

M4 samples were about 10 percent ductile at 77 °F, 50 percent ductile at 140 °F, and about 80 percent

ductile at 172 °F. The N8 samples were 20 percent ductile at 77 °F, 50 percent ductile at a temperature

between room temperamre and 104 °F, and nearly 100 percent ductile at 172 °F.

5.3.6 Bolts

At 68 °F. the averaged absorbed energy for the A 325 bolt material in Fig. 5-6 is (41.1±12.4) ft lbf, and

the fractures were 100 percent ductile. The scatter and scarcity of data prevent a more accurate estimate of

the average. Again, correcting for sub-size specimen effects, the room temperamre absorbed energy is (82.2±24.8) ft lbf As the temperamre drops to 32 °F, fracture surface evaluations indicate the transition

temperature region is entered. At -40 °F the fracture mode is approximately 75 percent brittle, so the

lower shelf energy starts somewhere below this temperature.

Figures 5-7 and 5-8 summarize the impact toughness data for the various building components tested. Data for sub-size specimens are normalized for comparison with data for ftiU-size specimens using

Eq. 1-2. The summary plots use the same scales to facilitate comparison.

5.4 DISCUSSION

5.4.1 Perimeter Columns

NISTNCSTAR 1-3D. WTC Investigation 107
Chapter 5

The transition temperatures of the N8 perimeter coluinn steels. Fig. 5-7 and Table 5-2 are in the expected

range based on the relationships shown in Fig. 5-9 for the strength and toughness of HSLA steels in 1967 (Irvine 1967b). These data show that a fine-grain ferrite-pearlite steel (or a controlled rolled steel)

with a strength 50 ksi < F,. < 65 ksi would be expected to have a transition temperature in the range

-50°F<r<-100°F.

The impact data for the higher strength quenched and tempered ClO-ClMl flange steel {Fy = 109.6 ksi),

Fig. 5-7, show no indication of transition behavior down to -120 °F. The transition temperature of this material, as well as the HAZ samples tested from it, is well below -100 °F, which is far below the

temperatures predicted by the historic data in Fig. 5-9.

The transition temperatures of the perimeter column steels are similar and are lower than those predicted

by historical data. These steels are very tough at 70 °F, the temperature at the time of the aircraft impact.

There is no reason to expect that they would fail in a brittle manner at high strain rate.

The upper shelf energies of the perimeter column longitudinal specimens, which have a notch orientation

that is transverse to the rolling direction, are approximately three times larger than those of the transverse specimens. By the mid 1970's this effect was understood (Irvine 1967, Wells 1975, Kozasu 1975,

DeArdo 1975, Farrar 1975, Tamura 1988). The toughness of steel in the upper shelf region is influenced

by the morphology of inclusions, pearlite, grains and other constituents in the steel. Banded pearlite and

increasing inclusion lengths, for example, were known to reduce the through-thickness (short transverse)

impact toughness of structural steels (Fig. 5-2).

One of the key issues in understanding the effect of microstructure on the toughness of these plate steels

is that sulfide inclusions become more plastic relative to the steel with decreasing rolling temperatures

and approach the deformability of the ferrite matrix at 1650 °F (900 °C) (Baker 1972). In the 1960's, the

development of controlled rolled steel technologies resulted in the use of lower rolling temperatures than

had been common in the past. This approach to plate production, for which a lower rolling temperature was key, produced high strength, fine grain, ferrite-pearlite steels, with the adverse effect of elongating

the sulfide inclusions along the rolling direction. Since inclusions and other microstructural features that can reduce toughness are oriented with their longest lengths and largest areas on planes parallel to the

plane of rolling, toughness can depend strongly on sample orientation in heavily rolled or controlled

rolled steels.

Figures 5-10 and 5-1 1 show that the fracture surfaces of longitudinal and transverse specimens are

characteristically different, which reflects the influence of the microstructure on the fracture process. When the plane of crack growth is perpendicular to the rolling direction, the fracture surface is convoluted and has circular ductile dimple features where voids initiated at manganese sulfide inclusions

grew and coalesced just prior to failure. When the direction of crack growth is parallel to the rolling

direction, the fracture surface has ductile linear features, which form a corduroy texture with long parallel

peak-valley features, Fig. 5-12. For this orientation, the void growth is tubular rather than spherical,

because the lengths of the elongated inclusions are parallel to the crack growth direction. Fracture features on the surfaces of these elongated ductile dimples. Figs. 5-1 Ic and 5-1 Id, have morphologies that might

be expected for disrupted pearlite colonies and remnants of manganese sulfide inclusions. Figure 5-13

shows the locations and morphologies of manganese sulfides on the fracture surface of a transverse impact specimen using an elemental x-ray map. These resuhs indicate that many of the film or plate-like

108 NISTNCSTAR 1-3D, WTC Investigation
Impact Properties

features on the fracture surface are manganese sulfides. As the number and length of the manganese

sulfide inclusions in the steels increase, the resistance to crack growth in this orientation decreases.

The impact energies of these perimeter column flange and spandrel steels are in the range expected,

compared with data from the literature, see Table 5-3. The corrected upper shelf impact energies for the

longitudinal specimens, for example, are about 75 ft-lbf, which is similar to the 84 ft-lbf reported for a 0.012 S low carbon structural steel (Hulka 1993).

Although the upper shelf energies for the flange and spandrel steels are within the range expected for

steels of the late 1960's. the ratio of longitudinal to transverse energy is near the maximum reported in literamre. Ratios of longitudinal to transverse properties of about 3 have been reported in WTC-era HSLA

ferrite-pearlite steel (Korchynsky 1970) and also for heavily rolled niobium X60 steels (Kozasu 1973).

These differences in the impact toughness of longitudinal and transverse orientations were of concern at the time, but were not improved significantly for HSLA structural steels until the 1970's, when sulfur control in steelmaking became common practice (Gladman 1997). As late as 1970 the reduction in sulfur content to a level below 0.01 mass percent, where transverse toughness begins to increase significantly, was cited as uneconomical in large scale steel production (Korchynsky 1970).

One of the common concerns for low through-thickness toughness was cracking under welds on structural members, which was being discussed in relation to controlled rolled steels in the 1960's. The problem was primarily viewed as a concern for thick sections, in which the high post-weld stresses could produce

large cracks under the welds due to delamination. In the case of the WTC perimeter columns and

spandrels, delaminations due to welding have not been observed to be a problem, but the lower transverse toughness relates to the ductile tearing that occurred along the direction of rolling and the "pull-out"

failure modes at welded components on the faces of the columns.

5.4.2 HAZ Materials from Perimeter Columns

The impact energies of the perimeter column HAZ samples indicate that the welding operation had little

practical effect on the toughness, Fig. 5-7. The lower impact toughness for the transverse HAZ specimen

orientation is relevant to the fractures seen along the edges and at the joints of some of the columns,

presumably due to overload during the collapse.

5.4.3 Core Columns

The absorbed energy at 77 °F of transverse web specimens from core column C-80, (26.1 ±1.2) ft lbf, are similar to those that Barsom (1988) reported for slightly heavier wide-flange shapes. Table 5-3. In those

tests, the average room-temperature absorbed energy was 37 ft lbf, but the data ranged over

12 < CVN < 80 ft lbf The estimate that the FATT is between room temperature and 140 °F is consistent

with the average value of 1 13 °F that Barsom (1988) reported.

5.4.4 Trusses

The impact energies for the truss angle and rod. Fig. 5-8, increase with increasing temperature, which

indicates transition behavior in the temperature range tested. The upper shelf is not apparent in these data,

but fracture surface evaluations indicate that the FATT is near room temperature. Literature data (Wilson

NISTNCSTAR 1-3D, WTC Investigation 109
Chapter 5

1978) indicate that a range in transition temperature (FATT) from about 70 °F to 145 °F might be

expected for steel plate grades such as A 36 and A 537, having thickness of 0.75 in. to 1.5 in. The

transition temperatures of other late 1960's, 0.17 C - 0.8 Mn steels are also at or above room temperature

(Duckworth 1967). Considering the nominal composition of the WTC truss angle and rod (0.04 Al, 0.20

C, 0.8 Mn, 0.004 N, 0.05 Si, and 0.01 to 0.03 S) and the other factors known to influence the transition

temperature, such as pearlite content, precipitation strengthening, and free nitrogen content, only pearlite content would be expected to raise the transition temperature higher than that predicted for the 0.17 C -

0.8 Mn steels. This is because the higher carbon content would increase the pearlite content, if all other

factors remained unchanged. The nitrogen and silicon contents of the truss steels are low, and the sulfur content affects only the absorbed energy and not the transition temperature. So, the near-roomtemperature transition temperatures of the angle and rod shapes from the trusses are consistent the

properties expected for this steel. Clearly, these materials were ductile enough for fabrication, which was

the primary consideration.

5.4.5 Truss Seats

Figure 5-8 summarizes the impact data for the F, = 36 ksi seat materials from perimeter columns N8,

N13, and M4. The transition temperatures for the truss seats are all above room temperature, and fracture

surface evaluations indicate very low ductility at room temperature. In Fig. 5-14, for example, the

fracture surfaces of the N- 1 3 and M-4 samples show that fracture occurred in a predominantly brittle manner at room temperatures. This finding may be relevant to possible scenarios for floor collapse.

5.4.6 Expected Values of Impact Toughness

The measured impact properties of the recovered steels must be kept in proper context to be useful. One

context is their similarity to the properties of other structural steels of the era. For this context, it is important to remember that there were no specified requirements for impact properties on the steels used

to build the WTC. No ASTM specifications for structural steel intended for use in buildings, then or now,

require Charpy impact tests. The current AISC LRFD manual (2001) only requires impact testing in one

very limited case: steels for heavy Group 4 and 5 rolled shapes used in tension and spliced by welding

must have a minimum CVN impact toughness of 20 ft lbf at 70 °F. Furthermore, the literature discussions of the era indicate that not even the simplest of minimum toughness requirements for

structural steels had been agreed upon in the late 1960's (Irvine 1967). One criterion, that the steel possess a CVN impact toughness of 15 ft lbf at the operating temperature, originated with the WWII

experience of the catastrophic failure of the Liberty ships. (Barsom 1987b)

Table 5-3 summarizes the historical data on Charpy impact toughness of structural steel supplied to ASTM specifications. The impact tougliness of WTC steel evaluated at room temperature is quite similar

to the historical values. The data for core wide-flange column C80 can be compared to the heavyweight

A 36 wide-flange data in Table 5-3. The perimeter column plate steels N8-C1M1 and N8-C1-B1 can be

compared to the A 572 plate steels. Finally the F, = 100 ksi perimeter column plate ClO-ClM has

properties similar to the A 514 Type E steel. The impact toughness of all the of the historical and WTC

column steels exceeds 15 ft lbf at 70 °F. Only the impact toughnesses of some of the truss seats steels are

less than this value.

110 NISTNCSTAR 1-3D, WTC Investigation
Impact Properties

5.5 SUMMARY

Table 5-2 summarizes the results of the Charpy tests. The upper shelf energies of the perimeter column

steels, including material from the heat-affected zones of welds, average 76 ft lbf longitudinal and

19 ft-lbf transverse. Their transition temperatures are all well below room temperature, and room

temperamre is on the upper shelf The room temperature absorbed energies of the truss and core-colunm

wide-flange steels are about 40 ft lbf longitudinal, and their transition temperatures are at room

temperamre or above. The room-temperamre transverse absorbed energies of the truss seat steels are as low as 7 ft lbf, and their transition temperatures are all above room temperature. Fracture-surface

e\'aluations demonstrated that the failure of the truss seat steels at room temperature was predominantly

brittle.

The impact properties of the steels, except some of the truss seat steels, are consistent with

contemporaneous structural steels intended for use in buildings.

5.6 REFERENCES

AISC (American Institute of Steel Construction). 2001. Load and Resistance Factor Design Specification

for Structural Steel Buildings. Chicago, II. Section A.3.1c, page 3 and commentary on p.p. 168-169.

Sept. 4. ASTM (American Society for Testing and Materials). 2004. E 23-04 Standard Test Methods for Notched

Bar Impact Testing of Metallic Materials. Book of Standards volume 03.01. American Society for Testing and Materials. West Conshohocken, Pa.

Baker, T. J. 1975. Use of Scanning Electron Microscopy in Studying Sulfide Morphology on Fracture

Surfaces. In Sulfide inclusions in steel: proceedings ofan international symposium, 7-8 November

1974, Port Chester, New York. J. J. DeBarbadillo and E. Snape, eds. American Society for Metals.

Metals Park, Ohio. 135-158.

Baker T. J. and J. A. Charles. 1972. Deformation of MnS Inclusions in Steel. Journal ofthe Iron and

Steel Institute. 210. 680-690.

Barsom J. M. and S. R. Novak. 1977. Subcritical Crack Growth and Fracture ofBridge Steels. National

Cooperative Highway Research Program Report 181. Transportation Research Board, National

Research Council.

Barsom J. M. 1987. Material Considerations in Structural Steel Design. AISC Engineering Journal. 24[3]

127-139.

Barsom, J. M. and S. T. Rolfe. 1987b. Fracture and Fatigue Control in Structures. Applications of

Fracture Mechanics. 2"^* edition. Prentice-Hall, Inc. New Jersey. Section 15.4.

Barsom J. M. and B. G. Reisdorf. 1988. Characteristics of Heavyweight Wide-Flange Structural Shapes.

Welding Research Council Bulletin. Number 332. 1-19. April.

NISTNCSTAR 1-3D. WTC Investigation 111
Chapter 5

Corwin, W.R. and A.M. Hoagland 1986. Effect of Specimen Size and Material Condition on the Charpy

Impact Properties of 9Cr-lMo-V-Nb Steel, p.p. 32-388in The Use ofSmall-Scale Specimens for

Testing Irradiated Material, ASTM STP 888. W.R. Corwin and G.E. Lucas, eds. American Society

for Testing and Materials. Philadelphia, Pa.

DeArdo Jr. A. J. and E. G. Hamburg. 1975. Influence of Elongated Inclusions on the Mechanical

Properties of High Strength Steel Plate. In Sulfide inclusions in steel: proceedings ofan international

symposium, 7-8 November 1974, Port Chester, New York. J. J. DeBarbadillo and E. Snape, eds. American Society for Metals. Metals Park, Ohio. 309-337.

Duckworth, W. E. 1967. Metallurgy of Structural Steels: Present and Future Possibilities. 1967. In Strong

tough structural steels: proceedings ofthejoint conference organized by the British Iron and Steel

Research Association and the Iron and Steel Institute at the Royal Hotel Scarborough, 4-6 April

1967. ISI Publication 104. Iron & Steel Institute. London. 61-73.

Farrar J. C. M. and R. E. Dolby. 1975. Lamellar Tearing in Welded Steel Fabrication. The Role of Sulfide

Inclusions. In Sulfide inclusions in steel: proceedings ofan international symposium, 7-8 November

1974, Port Chester, New York. J. J. DeBarbadillo and E. Snape, eds. American Society for Metals.

Metals Park, Ohio. 253-285.

Frank, K. H., J. M. Barsom, and R. O. Hamburger. 2000. State ofthe Art Report, Base Metals and

Fracture. SAC-2000-5a. Federal Emergency Management Agency.

Gladman, T. 1997. The Physical Metallurgy ofMicroalloyed Steels. The Institute of Materials. London

Hulka, K. and F. Heisterkamp. Low Carbon Structural Steels, The Key to Economic Constructions. 1993.

Low Carbon Steels for the 1990's. R. Asfahani and G. Tither, eds. TMS. 211-218.

Irvine, K. J. 1967. The Development of High-Strength Structural Steels. In Strong tough structural steels: proceedings of the joint conference organized by the British Iron and Steel Research Association and

the Iron and Steel Institute at the Royal Hotel Scarborough, 4-6 April 1967. ISI Publication 104. Iron & Steel Institute. London. 1-10.

Kayano, H., H. Kurishita, A. Kimura, M.Narui, M. Yamazaki, and Y. Suzuki. 1991. Charpy Impact

Testing using Miniature Specimens and its Application to the Study of Irradiation Behavior of LowActivation Ferritic Steels. J. Nucl. Mater. 179-181 425-428.

Korchynsky, M. and H. Stuart. 1970. The role of strong carbide and sulfide forming elements in the

manufacture of formable high strength low alloy steels. Symposium low alloy high alloy steels. Symposium jointly organized by the Metallurg Companies held at Nuremberg, BRD, on May 21-23,

1970. 17-27

Kozasu, I. and T. Osuka. 1973. Processing Conditions and Properties of Control-Rolled Steel Plates. In Processing and Properties of Low Carbon Steel, J. M. Gray, ed. The Metallurgical Society of AIME.

New York. 47-68.

112 NISI NCSTAR 1-3D, WTC Investigation
Impact Properties

Kozasu. I. and J. Tanaka. 1975. Effects of Sulfide Inclusions on Notch Toughness and Ductility of

Structural Steels. In Sulfide inclusions in steel: proceedings of an international symposium, 7-8

November 1974, Port Chester, New York. J. J. DeBarbadillo and E. Snape, eds. American Society for Metals. Metals Park, Ohio. 287-308.

Kumar. A.S.. B.S. Louden, F.A. Gamer, and M.L. Hamilton. 1993. Recent Improvements in Size Effects

Correlations for DBTT and Upper Shelf Energy of Ferritic Steels. /?./>. 47-61 in Small- Specimen Test Techniques Applied to Nuclear Reactor Vessel Thermal Annealing and Plant Life Extension. ASTM

STP 1204. W.R. Corwin et al., eds. American Society for Testing and Materials. Philadelphia, Pa.

Louden, B.S.. A.S. Kumar, F.A. Gamer, M.L. Hamilton, and W.L. Hu. 1988. The Influence of Specimen

Size on Charpy Impact Testing of Unirradiated HT-9. J. Nucl. Mater. 155-157. 662-667.

Lucas, G.E., G.R. Odette, J.W. Sheckherd, P. McConnell, and J. Perrm. 1986. Subsized Bend and Charpy

V-Notch Specimens for Irradiated Testing. 305-324in The Use ofSmall-Scale Specimens for

Testing Irradiated Material, ASTM STP 888. W.R. Corwin and G.E. Lucas, eds. American Society

for Testing and Materials. Philadelphia, Pa.

Lucon, E. 2001. Material Damage Evaluation of Primary Power Plant Components Using Sub-size

Specimens. Advanced Engineering Materials. 3[5]. 291-302.

Manahan. M.P. and C. Charles. 1990. A Generalized Methodology for Obtaining Quantitative Charpy

Data from Test Specimens on Nonstandard Dimensions. Nucl. Tech. 90. 245-259.

Poussard, C, C. Sainte Catherine, P. Forget, and B. Marini, 2003. On the Identification of Critical Damage Mechanisms Parameters to Predict the Behavior of Charpy Specimens on the Upper Shelf in Predictive Material Modeling: Combining Fundamental Physics Understanding, Computational

Methods and Empirically Observed Behavior. M. T. Kirk and M. Erickson Natishan, eds. ASTM

STP 1429. American Society for Testing and Materials, West Conshohocken, Pa.

Schubert. L.E. A.S. Kumar, S.T. Rosinski, and M.L Hamilton. 1995. Effect of Specimen Size on the Impact Properties of Neutron Irradiated A533B Steel. J. Nucl. Mater. 225. 231-237.

Schwartzbart, H and J. P. Sheehan. 1954. Effect of Specimen Size on Notched Bar Impact Properties of

Quenched-and-Tempered Steels. ASTMProc. 54 939-963.

Tamura, I., C. Ouchi, T. Tanaka, and H. Sekine. 1988. Thermomechanical Processing of High Strength

Low Alloy Steels. Butterworth & Co. Ltd. London.

Uehira A. and S. Ukai. 2004. Empirical Correlation of Specimen Size Effects in Charpy Impact Properties

of 1 lCr-0.5Mo-2W, V. Nb Ferritic-Martensitic Stainless Steel. J. Nucl. Sci. Tech. 41 [10] 973-980.

Wells, R. G. 1975. Metallographic Techniques in the Identification of Sulfide Inclusions in Steel. In Sulfide inclusions in steel: proceedings of an international symposium, 7-8 November 1974, Port

Chester, New York. J. J. DeBarbadillo and E. Snape, eds. American Society for Metals. Metals Park,

Ohio. 123-134.

NISTNCSTAR 1-3D, WTC Investigation 113
Chapter 5

Wilson A. D. 1978. Comparisons of Dynamic Tear and Charpy V-notch Impact Properties of Plate Steels. Trans. ASME. 100. 204-211.

114 NISTNCSTAR 1-3D, WTC Investigation
Impact Properties

Table 5-1. Common sub-size to full-size upper shelf energy correction factors.

Reference Correction Factor, K

K

Note a Definitions

Convin 1986

B bss ss

2 B = specimen widthb = ligament depth

(see Figure 5-2)

Corwin 1986

2.8

Lucas 1986

bX 2

Louden 1988

Bjsbl IbJI

2 L = span = 40 mm per Fig. A 1.2 of ASTM E 23

K, = notch stress concentration factor''

Schubert 1995 bX IbjI

lX I lX 2 K, ' = alternate notch stress concentration factor

a. K is evaluated for the NIST Charpy specimen geometries.

b. K, and K, ' depend only on the notch angle, 9. root radius, R, and ligament depth, b. These three parameters are identical in the

full-size and sub-size specimens of the Investigation. Formulas for AT, can be found in Kumar (1993). Note: Subscripts 55 and fs refer to sub-size and full-size specimens, respectively.

NISTNCSTAR 1-3D, WTC Investigation 115
Chapter 5

Table 5-2. Summary of Charpy data.

Specimen Description ksi Location"

Eus

longitudinal

ftlbf

(Note c)

trans- verse

ftlbf

Transition

T

(FATT)

°F

(Note b)

E77

longitudinal*^ ftlbf (Note d)

E77

transverse'' ftlbf

N8-C1M1 flange

/=0.3125 in. plate

sub-size specimen

60 WTC 1 142 97-100 76.6 20.8 -95 76.7±2.9 21.6+0.3

ClO-ClMl flange

/=0.25 in. plate

sub-size specimen

100 WTC 1 452 85-88 94.2 22.8 <-100 92.0±2.4 23.3±0.7

N8-C1B1 spandrel

7=0.375 m. plate sub-size specimen

42 WTC 1 142 99 81.8 26.6 n/d 83.6±2.2 26.4±0.8

N8-C1M1 Heat-affected zone

sub-size specimen

n/a WTC 1 142 97-100 88.6 18.8 <0 87.6±6.2 19.2±0.4

ClO-ClMl Heat-affected zone

sub-size specimen

n/a WTC 1 452 85-88 n/d 22.0 <-100 n/d 23.1±1.2

C80 core WF web

14WF184

full-size specimen

36 WTC 1 603 92-95 n/d n/d 11<T< 140 53.8+6.3 26.1+1.2

truss rod Z)=0.92 m.

full-size specimen

50 unknown n/d n/d ^100 32.4±4.8 n/d

truss angle 2 in. X 1.5 in. xO.25 m.

sub-size specimen

50 unknown n/d n/d <11 62.2+2.2 n/d

N13 truss seat

full-size specimen

36 WTC 1 130 99-102 n/d n/d 140 n/d 7.7+1.1

N8 truss seat

full-size specimen

36 WTC 1 142 97-100 n/d n/d 77 < T < 104 n/d 21.2±0.5

M4 truss seat

full-size specimen

36 unknown n/d n/d 140 n/d 11.910.5

A 325 bolt D=7/S m. bolt sub-size specimenn/a unknown 82.2 n/d -40 < T<32 n/d n/d

a. Location code example; WTC 1 142 97-100: Tower 1. column line 142, between floors 97 and 100.

b. Transition temperature evaluated from fracture surface appearance (FATT).

c. = upper shelf absorbed energy, corrected using Eq. 1-2 where appropriate.

d. £77 =room temperature absorbed energy, average and standard uncertainty, corrected using Eq. 1-2 where appropriate.

Key: HAZ, heat affected zone; long, longitudinal orientation; trans, transverse orientation. n/a = not applicable; n/d, not determined. Note: The reported uncertainty, is the estimated uncertainty of the mean of n values:

1 n{n - 1)

116 NISTNCSTAR 1-3D, WTC Investigation
Impact Properties

Table 5-3. Historical data on Charpy impact toughness of structural steel

Reference Year Steel Type

CVN

ftlbf

T

°F Orient Notes

Barsom and

Novak 1977

1977 A3 6 plate 28 72 L&T single plate t=] in. Figure C-1 and

Table 3 Barsom and

Reisdorf 1988

1988 A? 6 wide-flange

W14 shapes

>342 Ib/n

37 70 28 individual shapes.

Range: 12 ft lbf<CVN<80 ft lbf

Frank 2000 1994 A3 6 wide-ilange

shapes

95 70 91% of the specimens had

CVN>30ftlb @70 °F, Table 6-4 Barsom 1987 1972 A572 Gr 50 plate 55 70 L also appears in (Frank 2000)

52 plates from 6 mills Barsom 1987 1972 A572 Gr 50 plate 37 40 L

Barsom 1987 1972 A572 Gr50 plate 21 0 L

Barsom 1987 1972 A572 Gr50 plate 25 70 T

Barsom 1987 1972 A572 Gr 50 plate 18 40 T

Barsom 1987 1972 A572 Gr 50 plate 12 0 T

Barsom and

Reisdorf 1988

1988 A572 wide-flange

W14 shapes

>342 lb/ft

38 70 T 22 individual shapes

Range: 11 ft-lbf<CVN<81 ftlbf

Frank 2000 1994 A572 wide-flange 61 70 77% of specimens had CVN>30ftlb

r, 1 dole D-H Barsom and

Novak 1977

1977 A514 Type E plate 65 70 L single plate, ? = 1 in. Figure C-10 and

Table 3 Barsom and

Novak 1977

1977 A514 Type E plate 40 70 T

a. Year refers to first year of publication for references that cite other sources.

b. CVN: absorbed energ>' in Charpy test.

c. Orient: specimen orientation with respect to rolling direction Key: L, longitudinal: T, transverse.

NISTNCSTAR 1-3D. WTC Investigation 117
Chapter 5

80 1—I—I—I—

I—I—I—I—I—I—I—I—I—I—

I

\—\—I—I—

I—I

\—\—I—

|-

transition region

Lower shelf

I I I I I I

' I' ' '

'

Upper shelf

J I I 1 I I I I 1 I L

-150 -100 -50 0 50 100 150 200

T, °F

11 02:01 - Tuesday February 15. 2005

Figure 5-1. An example transition curve.

Specimen Orientation

rolling direction

ASTM A 370 longitudinal impact specimers

L-T orientation ASTfvl A 370 transverse impact specimen

T-L orientation Specimen Geometry

55 mm B=5 mm

, ,

, . „ ASTM E 23 Type A

b=8 mm I ! i II 0 mm sub-ssze speamen

55 mm B=10 mm

b=8 mm

T ASTM E 23 Type A numm fyji-size ^ecimen

Figure 5-2. Charpy impact specimen geometries and orientations with respect to the

plate rolling direction.

118 NISTNCSTAR 1-3D, WTC Investigation
Impact Properties

60

50

40 h

§ 30 oCDO 20 <

10

T—^—I—I—

I—I—I

\—I—

I—I

\—I—I—I—I—

r

I

' ' ' '

I

o o

C10-C1M1

F =100 ksi WTC1 452 85-1 Q © § 0 C10-C1M1 L O O

. • a 1=— 1 t C10-C1M1 T

"= i »= — ClO-C1f/n HAZ T

subsize specimen

J > I I I I I I I I I i_ -150 -100 -50 0 50 100 150 200

T, °F

60

50

40

>.O)

oio 30 oCDo 20 w<

10

0

T—I—I—TT—I—I—I—I—I I I—n—I—I—I—Ir

N8-C1M1

F =60 ksi WTC1 142 97-100

NS-CIWil-HAZ L Q N8-C1M1-FL L

•1^1= H

NS-CIMI-FL T o N8-C1M1-HAZT

L=longitudinal; T=transverse

subsize specimen

I I I I I I i_J I L. -150 -100 -50 0 50 100 150 200

T,°F

10:05:35 • Tuesday March 15, 2005

Figure 5-3. Longitudinal and transverse Charpy impact data of samples from the flange

and adjacent HAZ of perimeter column N8-C1M1, the flange and adjacent HAZ of

perimeter column C10-C1M1.

NISTNCSTAR 1-3D, WTC Investigation 119
Chapter 5

60

50

40 h

>.

CDS 30

D

CDO 20

CO <

10 1-

T—I—I—I—

I—I—I—I—I—

I—I

\—I—I—

I—I—I—I—

N8-C1B1 -spandrel F =42 ksi WTC1 142 '

/ /o

N8-C1B1-spandrel L

/. J I

I i

_l I I I I I I I L I .... I

1 N8-C1 El -spandrel

subsize specimen J I \ I I L_J I I L -150 -100 -50 0 50 100 150 200

T, °F

150

100

cn

CDc

CD D

CDO

CO <

50

T—1—I—I—

I—I—I—I—r-|—I—I—I—I—[-

:80-web L

I , , , Q8p-y.-Qb,T,itc80-web

F =36 ksi WTC1 603 92-95

T=transverse

L=longitudinal

full-size specimen

-150 -100 -50 0 50 100 150 200

T, °F

15:13:17 - Friday March 11. 2005

Figure 5-4. Longitudinal and transverse Charpy impact data for the spandrel associated

with perimeter column N8 and the web of wide-flange core column C-80.

120 NIST NCSTAR 1-3D, WTC Investigation
Impact Properties

150

100

0)c

<D DCDO

CO <

50

T—I—I—1—

I—I—I—I—I—I—I—I—I—

r

Truss seats F =36 ksi M4

N13 N8

I =? r-,-1- r.-^r I

full-size specimen

I I , I I I I I I -150 -100 -50 0 50

T, °F

100 150 200

Figure 5-5. Transverse Charpy impact data from samples from perimeter column truss

seats M4, N13, and N8, and from floor truss components.

NISTNCSTAR 1-3D, WTC Investigation 121
Chapter 5

60

50

40

>.

D)

0)o 30

-o

0) X3O 20

CO <

10

-1—I—I—I—

I—I—I—I—I—

l-

_L

O O

' I' ' '

' A 325 bolt

I I subsize specimen

I , I I I I I I I -150 -100 -50 0 50 100 150 200

18 09,15 - Tuesday Oclober 05, 2004 T, °F

Figure 5-6. Longitudinal Charpy impact data for A 325 bolts.

122 NISTNCSTAR 1-3D. WTC Investigation
Impact Properties

Figure 5-7. Summary plot of the dependence of absorbed energy on test temperature for

all perimeter and core column steels. The absorbed energy values of the sub-size

specimens have been corrected using Eq. 5-2 to compare them to data from full-size

(10 mm by 10 mm) specimens.

NISTNCSTAR 1-3D. WTC Investigation 123
Chapter 5

120

100

>. 80

D)

i_

<DC

0^ 60

X3

§ 40

<

20

- « M4

A N13

m N8

T—I—I—

I—TT—

Truss seats F =36 ksi

1 I—I I I——I I I I I—I—I—I——I—I—I

I——I—I—I—

I-" §/ .i'" ^ ^ ^ -^A full-size specimen

_i I I I I I I I I I i I ~T" I I E_i I I I I I I I I I I I I I I L -150 -100 -50 0 50 100 150 200

120 -

100

>; 80

c

0^ 60

o

i 40

<

20

0

—I I

I—I I I I—|—

Truss components

F =50 ksi

T—I—I—

I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—

D=0.92 in, rod

O

I I PrTT

o / '^,2 iry, X 1 .5 in. angle

0 corrected to full-size specimen

I , I I I I I I I I I I -150 -100 -50

16:28:08 - Tuesday October 05. 2004

0 50 100 150 200

T, °F

Figure 5-8. Summary plot of the dependence of absorbed energy on test temperature for

all truss component and truss seat steels. The absorbed energy values of the sub-size

specimens have been corrected using Eq. 5-2 to compare them to data from full-size

(10 mm by 10 mm) specimens.

124 NISTNCSTAR 1-3D. WTC Investigation
Impact Properties

c

50 1 1 1 1

1

1 1 1 1

1

1 / 11 1

\ A 1 1 1

1 I 1 ^"zy^ / Bainite

0 _ Fine Grained /

Ferrite-Peaiiite / / Controlled — / Rolled /.'^ Bainite,^^

-50 / /

Quenched and /Controlled^^ Tempered

/Rolled

-100 _ /Fentte- /

- / Pearlite/

-150

-200 J 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 50 60 70 80 90 100

Yield Strength, ksi

Figure 5-9. Strength-toughness relationships for several types of structural steels from

the WTC construction era, after Irvine (1969).

c Source: NIST

Figure 5-1 0. Scanning electron micrographs of the fracture surface of a Charpy V-notch

longitudinal specimen orientation from an N8-C1M1 perimeter column (WTC 1-142-97-

100). (a) ductile dimples (oval features) and general surface morphology, (b) low

magnification view of large and small ductile dimples on the fracture surface, (c) higher

magnification view.

NISTNCSTAR 1-3D, WTC Investigation 125
Chapter 5

Source: NIST. Figure 5-11. Scanning electron micrographs of the fracture surface of an N8-C1M1

perimeter column sample showing ductile tearing features that form due to fracture

initiation and growth at elongated inclusions and pearlite on planes parallel to the rolling

plane. The "ductile dimples" in this case are linear features with a peak-valley

morphology

Figure 5-12. Perspective view of the fracture surface of sample N8-C1M1 showing the

long peak-valley features characteristic of the fracture surface for transversely oriented

impact specimens. The green line indicates the topography of the fracture surface.

126 NIST NCSTAR 1-3D, WTC Investigation
Impact Properties

C D Source: NIST. Figure 5-13. A gray-scale image (a) and compositional maps from fracture surface of an N8-C1M1 perimeter column Charpy V-notch specimen. The relative concentrations of (b)

iron, (c) manganese, and (d) sulfur. The surface of the "ductile dimple" is littered with the

remnants of manganese sulfide inclusions.

Source: NIST. Figure 5-14. The fracture surface of a perimeter truss seat, N13-C3B1 that was tested at room temperature shows cleavage facets, which indicate a brittle fracture mode.

NISTNCSTAR 1-3D. WTC Investigation 127
Chapter 5

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128 NIST NCSTAR 1-3D, WTC Investigation
Chapter 6

Elevated Temperature Properties

6.1 INTRODUCTION

The high temperature tensile deformation and creep behavior of structural steels are important inputs to

finite element models of the response of the World Trade Center (WTC) structures to the fires. This

segment of the Investigation used two approaches to provide that information. The first was to experimentally characterize the high-temperature mechanical properties of the steels that were most likely

to have been exposed to the fires. The second was to develop methodologies to estimate the tensile and

creep properties of those steels that were not experimentally characterized.

The experimental characterization of the high-temperature tensile properties focused on the steels that were most likely to have been exposed to high temperature from the fires: the floor trusses and truss seats, the core columns, and the inner web plates of the perimeter columns. The experimental characterization

of creep properties focused on the steel from the floor trusses

The methodology to estimate high-temperature tensile properties of untested steels recognizes that the

yield and tensile strength of structural steels, normalized to their room temperature values, degrade with

temperature along a master curve. To estimate the dependence of the shape of the stress-strain curves with

temperature, the methodology uses both literature and experimental data.

For estimating creep properties of all WTC structural steels, except those in the floor trusses, the

methodology uses 1970s literature data for creep of structural steel scaled by the ratios of their tensile strengths. For the properties of the truss steels, the methodology uses data generated from tests on

recovered truss steels themselves.

High-temperature tensile tests employed two different machines. The tests on the floor truss steels employed an electromechanical testing machine (Instron 8562) with a contact extensometer that had a 12.5 mm gauge length. The furnace is a split, MoSii-element design. The specimens, designated type

Flat C2, conform to Fig. 1 of ASTM International (ASTM) E 21. The uniform cross section is 32 mm

long and 3 mm x 6 mm wide. They were loaded using pin-aligned, superalloy, wedge grips. The

specimen temperature was monitored using a K-type thermocouple mounted within 1 mm of the specimen

surface. The specimens were loaded as soon as possible after the furnace temperature stabilized at the test temperature, which was generally within 20 min. During the temperature stabilization step, the specimen

temperature was always within 5 °C of the desired temperature. The crosshead speed in these tests was

0.0325 mm/s, which produced a strain rate after yielding of approximately 0.001 s\ measured using the extensometer. Prior to yield, the crosshead displacement rate produced a specimen stressing rate of

4±2 MPa/s. These rates meet the requirements of ASTM E 8 and E 21.

6.2 TEST procedures

6.2.1 Tensile Tests

NISTNCSTAR 1-3D, WTC Investigation 129
Chapter 6

Most other tests used a second electromechanical machine with a high-temperature extensometer with a

1 .0 in. gauge length. The furnace is a split design in which quartz lamps heat the specimen. The

specimens. Type Flat CI, have a 1.1 in. uniform long cross section 0.25 in. wide. The specimen thickness was usually the original thickness of the plate. These tests followed the loading protocol of ASTM E 21.

The initial crosshead displacement rate was 0.00167 mm/s. After several percent strain, the crosshead

rate was increased to 0.0167 irmi/s, as required by E 21,which produces a discernible step in the flow

stress for elevated temperature tests.

6.2.2 Creep Tests

Creep testing employed two types of machines. Some employed the same electromechanical test machine

with MoSi2 furnace used in the high-temperature tensile tests. Most tests employed two identical,

traditional dead-load, lever-ann creep frames (Applied Test Systems) with split, wire-wound, three-zone

NiCr furnaces. Extensometers in these furnaces are averaging, clamp-on types with a nominal gauge

length of 25.4 mm. The specimens were loaded by hand by adding masses to the weight pan. Generally, it took less than three minutes to reach full load. During the test, the operator removed mass to maintain

constant stress from the weight pan at approximately 3 percent strain increments. These corrections

manifest themselves as kinks in the strain-time curves. Specimen C132-2w-2 shows such a correction at approximately 6,000 s (as shown later in Fig. 6-3). The test specimens were the type Flat C2. Creep tests were conducted at temperatures 400 °C, 500 °C, 600 °C, and 650 °C. One stress-temperature condition was used per specimen. Individual specimen stresses lay in the range 100 MPa < a < 445.8 MPa. Tests were generally discontinued after 2 h if the specimen had not failed, though some tests were allowed to run longer.

6.3 RESULTS

6.3.1 Tensile Tests

Table 6-1 describes the specimens from the three perimeter column, two core column and two floor truss

steels tested at high temperature and their original locations in the buildings. The curves of Fig. 6-1,

which illustrate data from specimen N8-C1B1 A-FL, are typical of the high-temperature stress-strain curves for all the specimens tested. Appendix A contains the stress-strain curves for the other specimens.

In most cases, the experimenter removed the extensometer prior to failure, so the end points of the curves do not necessarily represent failure. Because some of these tests followed the loading protocol of ASTM

E 21, there is a jump in the flow stress at about £ = 0.02 that corresponds to the mandatory extension rate change. At low temperatures, where the strain rate sensitivity is low, the jump is small, but at elevated

temperatures, it can be as large as 5 ksi.

6.3.2 Creep Tests

Truss steel from a 2 in. x 1.5 in. x 0.25 in. truss top chord angle from specimen C-132 was characterized

for creep. This steel was specified to conform to A 242 with F, 50 ksi. Chemical and mechanical

analyses of the larger lower chord angles, which were specified to conform to A 36, indicate that they were made from the same steel, however.

130 NISTNCSTAR 1-3D, WTC Investigation
Elevated Temperature Properties

Table 6-1. Specimens and locations for high-temperature tensile tests with full stress-strain data.

Specimen Description Source (ksi)

C40-C2M-IW-2 Perimeter coluntn inner web WTC 1 Column 136 floors 98-101 60

N8-C1B1A-FL Perimeter column flange WTC 1 Column 142 floors 97-100 60

ClO-ClM-FL-1 Perimeter column flange WTC 1 Column 451 floors 85-88 100

C65 12WF161 core column WTC 1 Column 904 floors 86-89 36

HH-FL-1 12WF92 core column WTC 1 Column 605 floors 98-101 42

C-132 2 in. X 1.5 inx0.25in. truss bulb

angle

Unknown 50

C-53 3 in. x2 inxO.37 in. truss bulb

angle

Unknown 36

Note: F, is the specified \'alue. rather than the NIST-measured value.

Figure 6-1. Elevated-temperature stress-strain curves. Specimen N8-C1B1A-FL is from a Fy = 60 ksi perimeter

column flange plate from WTC 1 column 142 between

floors 97-100. Annotations refer to individual test

specimen numbers.

NISTNCSTAR 1-3D. WTC Investigation 131
Chapter 6

The creep test matrix consisted of 20 creep tests taken to failure or discontinued after several hours.

Figures 6-2, 6-3, 6-4, and 6-5 display the creep curves for specimens from the truss upper chord.

t,h

0.0

0.05

c

ffl 0.04

D)

CD

CDC

0.02

LU

0.01

0.00

0.5 ~1~ — 7w-1 147 MPa — 6n-2 145.4 MPa — 2n-3 126 MPa — 2w-8 125 MPa

1.0

~i

' n—' ' 650 °C-

/

[/

' J I I I I I I I I I I \ I I I I I I I I I I L

discontinuedtailed creep test

fit —

0 1000 2000 3000 4000 5000

t, s

19;23 07 - Thursday October 14, 2004

Figure 6-2. Creep curves of A 242 truss steel from specimen C-132 at 650 °C. Dashed

lines represent the fit from Eq. 6-14 using the parameters in Eqs. 6-16, 6-17, and 6-18.

Experimental curves are graphically truncated at £ = 0.05.

t, h

0.0 0.5 1.0 1.5 2.0 2.5 3.0

"T—m—I—

I—I—I—I—I—

I—I—I—I—I—

I—I—I—I—I—I—I—1—I—I—I—I—I—I—n—I—I—I—

600 "C

discontinued"

tailed creep test -

fit

5w-2 164 MPa

' 4w-6 150 MPa [ 3w-3 151.8 MPa

2VV-2 131.4 MP£

1 w-2 123 MPa .

1.0

xlO'

19:231 6 - Thursday October 14, 2004

Figure 6-3. Creep curves of A 242 truss steel from specimen C-132 at 600 °C. Dashed

lines represent the fit from Eq. 6-14 using the parameters in Eqs. 6-16, 6-17, and 6-18.

Experimental curves are graphically truncated at s = 0.05.

132 NISTNCSTAR 1-3D. WTC Investigation
Elevated Temperature Properties

t, h

2.0

1 1 1 r 1

500 'C 1^ discontinue3"

failed creep test -

fit

1n-2 300 MPa

2n-2 296.8 MPa

4W-.S. 277.9 MPa

lw-4 272.2 MPa

3n-l 252 MPa

4v»,'-3 250 MPa

3w-2 200 MPa -

0 2000

1923 19 - Thursday Oclober 14 2004

4000

t, S

6000

Figure 6-4. Creep curves of A 242 truss steel from specimen C-132 at 500 °C. Dashed

lines represent the fit from Eq. 6-14 using the parameters in Eqs. 6-16, 6-17, and 6-18.

Experimental curves are graphically truncated at e = 0.05.

Figure 6-5. Creep curves of A 242 truss steel from specimen C-132 at 400 °C. Solid lines

represent measured creep strain. Dashed lines represent the fit from Eq. 6-14 using the

parameters in Eqs. 6-16, 6-17, and 6-18. Experimental curves are graphically truncated

at e = 0.05.

NISTNCSTAR 1-3D. WTC Investigation 133
Chapter 6

6.4 MECHANICAL PROPERTY VALUES FOR STEELS

6.4.1 A Universal Curve for Elevated-Temperature Tensile Properties

Early in the Investigation, before any significant tensile testing was completed, it was necessary to provide other NIST Investigation Team members with a relation to describe the yield and tensile strength

of structural steel as a fiinction of temperature. This relation forms the basis for estimating the elevatedtemperature stress-strain behavior of the recovered steels. Because it was not experimentally feasible to characterize all of the different grades, the relation is necessary to estimate the properties of the steels that were not characterized. The WTC steels tested for elevated temperature behavior were those that were

particularly relevant for modeling the fire-affected floors. These were primarily the steels in the core columns and the floor tmsses, though several perimeter column steels were also characterized.

The literature contains very few summaries of the properties of structural steels at elevated temperature.

The small data set that is available is particularly appropriate to the Investigation, however, because it comprises structural steels from the WTC construction era. A strong disadvantage of the sources is that they do not typically report the strain rate used in the tests.

A useful and often recognized relation is that the strength of structural steel as a function of temperature,

normalized to the room-temperature value, follows a universal curve, independent of strength level. Both

the normalized yield, F,., and tensile, TIS, strength curves are roughly sigmoidal. The normalized curve for yield strength drops away from unity more quickly than does the curve for tensile strength, which has a plateau out to about 300 °C. A function that represents this behavior with a smooth curve and a minimum

of parameters is

/

= FAT)

F,(23°C) = (1 - A2)e\p 6-1

JJ

and

TSjT)

TS{23°C)

(1 - ^Oexp 6-2

where 7 is measured in "C. The parameters in Eqs. 6-1 and 6-2 are completely empirical and have no

physical significance.

Figures 6-6 and 6-7 plot the master curves for yield and tensile strength overlaid on the literature data,

which are the small solid symbols (Chijiiwa 1993, Goda 1964, Harmathy 1973, Holt 1964, Melloy 1963,

USS 1972). The model curves, Eq. 6-1 and 6-2, are not fits to the literature data. Instead, the parameters

of Eqs. 6-1 and 6-2 were adjusted to provide a visually accurate representation of the bulk of the data,

particularly in the region of steep slope in the range 500 °C < r< 700 °C. Furthermore, the values of the

parameters, summarized in Table 6-2, have no physical significance. The scatter in the literature data

arises from several sources. At intermediate temperatures, the sensitivities of the different steels to dynamic strain aging differ. Dynamic strain aging can actually raise the flow stress above the room

temperature value. Furthermore, the literature sources do not report the testing rates employed, which

134 NISTNCSTAR 1-3D, WTC Investigation
Elevated Temperature Properties

may differ between the sources. Rate effects can make a significant contribution to the measured flow

stress at temperatures above about 500 °C.

Figure 6-6. Ratio, f, of room- to high-temperature yield strength (Fy) for all steels

characterized. The data spread at room temperature exists because the individual test

results are normalized to their mean. The solid line is the expression, Eq. 6-1, developed

using literature data on structural steels, which are denoted by the smaller symbols.

oo

OCM

1.0

0.8 -

C/)

CO

0.6

0.4

0.2

0.0

Model

. » literature da'.a

• core truss seats

0 core columns A truss components

perimeter truss seats o perimeter columns

200 400

T, °C

600 800

Figure 6-7. Ratio of room- to high-temperature tensile strength (TS) for the steels in

Table 6-1. The data spread at room temperature exists because the individual tests are normalized to their mean. The solid line is the expression developed for literature data on

structural steels, Eq. 6-2, denoted by the smaller symbols.

NISTNCSTAR 1-3D, WTC Investigation 135
Chapter 6

Table 6-2. Values for the parameters in the strength reduction equations

(Eqs. 6-1 and 6-2).

Yield Strength

Reduction Parameters

(Eq. 6-1)

Tensile Strength

Reduction Parameters

(Eq. 6-2)

A. = 0.074692 m, = 8.325929 m. = 1 .000000

5,= 638.384277 T

S2 = 523.523132 "C

A2 = 0.086327

«, = 5.0004

H2= 4.992176

/, = 531.403687 T

/.= 1032.864014 T

The recent Eurocodes (EN 1993-1-2 Fire) recommend expressions for the proportional limit,^ and flow

stress, /max,of stRictural steel as functions of temperature. These two parameters define specific points on

the recommended stress-strain behavior for steel at elevated temperature in the Eurocode. That curve has

four regions:

1 . a small region of linear-elastic behavior up to the proportional limit, /,,

2. a work-hardening region that extends to e = 0.02,

3. a plastic regime that extends at constant, maximum flow stress, /^nax, and

4. a failure range where the strength decreases to zero.

An important feature of this stress-strain curve is that it reaches the maximum stress, /nax. at e = 0.02. Figure 6-6 compares both of these to the Investigation data. Neither the Eurocode proportional limit,^ nor the e = 0.02 yield strength, /i^nax, corresponds to the e = 0.002 offset yield strength reported in Figure 6-6, but they do represent lower and upper bounds. There is no analogous definition for the tensile

strength, TS.

6.4.2 Analysis of Tensile Data

Figures 6-6 and 6-7 also summarize the normalized yield (F,) and tensile (TS) strength for all the steels tested, segregated by component. At temperatures up to 400 °C the data from the WTC steels generally lie in the same range as the literature data. At 500 °C and above, the WTC data average only 75 percent of

the curve that represents the literature data. This deviation probably arises from the differences in the

testing rates. The strain rates used for the WTC data were as low as £" = 8.3 x 1 0' s" while the literature data were probably generated using rates appropriate to ASTM A 370, which are ten times higher:

£ = 8.3 X lO'^'s"' . The expected difference between the two testing rates can be evaluated by recognizing

that the high-temperature flow stress, o, depends on the strain rate £" in a power-law relation:

cr = £,£'" 6-3

where £„ is a constant and m is designated as the strain rate sensitivity.

The necessary strain rate sensitivity, w, to raise the average of the WTC data to the prediction of the

model curve is w = 0.13. Reported elevated-temperature strain rate sensitivities for low-carbon structural

136 NISTNCSTAR 1-3D, WTC Investigation
Elevated Temperature Properties

steel are near this value: m = 0.105 at 600 °C (Manjoine 1944), m = 0.08 at 649 °C (Gowda 1978). An

analysis of the strain rate sensitivities of six WTC steels, from the stress jump that occurs when the

extension rate is changed at about 2 percent strain yielded similar values. The strain rate sensitivity, /», evaluated from the individual strain rate jumps, increases with temperature: m = 0.055±0.029 at 500 °C,

777 = 0.119±0.020 at 600 °C, and 77? = 0.146±0.029 at 650 °C.

6.4.3 Estimating Elevated-Temperature Stress-Strain Curves

A complete stress-strain curve as a function of temperature is necessary as input for finite element

modeling. Because it was not feasible to characterize all twenty different WTC steels for their hightemperature stress-strain beha\ior. a methodology was necessary to estimate the stress-strain curves for each steel. The stress-strain (a-s) curves of the more than twenty different WTC structural steels can be

grouped into two classes based on their shape and specified minimum yield strength. The shapes of the a8 curves of low yield strength (A 36, F, = 36 ksi) steels differ from those of higher yield strength

{Fy > 36 ksi) steels. The low strength steels work harden over a larger strain range, even when they

develop the same tensile strength {TS) as the higher strength steels.

At elevated temperatures, of course, the yield and tensile strengths decrease, see Figs. 6-6 and 6-7. Some

steels initially increase in strength with increasing temperature, through the process of dynamic strain- aging, (Baird 1970) but this behavior is not a priori predictable. The analysis does not attempt to predict

the effects of dynamic strain aging. The limited quantity of literature stress-strain curves, as well as those

generated in the Investigation, indicates that the shape of the work-hardening portion of the stress-strain curve changes with increasing temperature. The model to describe the stress-strain curves must capture

this shape change.

The methodology for creating stress-strain curves for WTC steels has three steps.

1 . Choose a representative low-strength (A 36 F,. = 36 ksi) and a high-strength (F,. > 36 ksi)

steel for which literature or experimental stress-strain curves at different elevated

temperatures are available.

2. De\ elop a model that can predict the stress-strain behavior of these steels as a function of

temperature.

3. Apply this model stress-strain behavior to all other WTC steels by appropriately scaling the

resulting stress-strain behavior of the steel in question to that of the steel used to develop the

model.

The stress-strain curves for 0 °C < r< 650 °C for all steels are represented using a power-law workhardcnmg model:

a = Rj^R^K{T)£"^'^^ 6-4

where a has units of ksi, Thas units of °C and

NISTNCSTAR 1-3D, WTC Investigation 137
Chapter 6

KiT) = {k,-k,)exp

Vk2 J + k, 6-5

and

ii{T) = («4 -/?o)exP

"I / ^ A"

V '»2 y 6-6

The parameter i?rs is the ratio of the room temperature tensile strength of the steel of interest, TSi , to the room temperature tensile strength, TSreu of the steel used to develop the model expressed by Eqs. 6-5

and 6-6.

TS,

6-7

Because the work hardening decreases with increasing temperature, scaling by the room-temperature

tensile strength, TS, is more appropriate than scaling by the room-temperature yield strength, F,,- The

parameter Rc corrects for two additional phenomena. The temperature-dependent functions (Eqs. 6-5

and 6-6) were originally developed using data that were not corrected to zero strain rate, whereas the

stress-strain curves supplied to the Investigation teams for room temperature behavior are corrected to the zero strain rate. Secondly, because the temperature dependence of all the possible steels is represented

using the behavior of only two different steels, it is possible that the stress-strain behavior predicted by

Eq. 6-4 at room temperature could differ from that predicted by the already generated and supplied roomtemperature stress-strain curve. The parameter Rc corrects the elevated-temperature stress-strain curve so

that the value of stress predicted by Eq. 6-4 for r= 25 °C at 8 = 0.05 equals that of the room temperature

model curve. In general, the correction is small: 0.9 < Rc< 1.04.

The functions K(T) and n(T) have the same form as the functions that describe the reduction in yield and

tensile strength with temperature in Figs. 6-6 and 6-7. Although they are not based on a physical model,

they represent the behavior of n(T) and K(T), and are relatively easy to evaluate. Their functional form

produces a monotonic decrease in the strength of the work hardening with temperature, which agrees with

the trend of the literature data. The maxima of the functional forms of n(T) and K(T) occur at 7 = 0 °C.

To calculate the six parameters in each expression fox K(T) and n(T), the plastic portion of each tensile

test stress-strain curve, i.e. the strain at stress greater than the yield strength, was modeled with a powerlaw work hardening expression.

cj = Ke" 6-8

using a non-linear least-squares fitting method, to estimate an individual Ki and n, . The individual A', and

n, were then fitted with Eqs. 6-5 and 6-6 respectively. The fits for n(T) were subject to constraints on the

maximum values for n / and ri: , which were necessary to avoid introducing anomalous behavior in Eq. 6-4 at intermediate temperature. Note also that the functional form of Eqs. 6-5 and 6-6 does not

capture the small strength increase caused by dynamic strain aging at temperatures less than 300 °C. In

138 NISTNCSTAR 1-3D, WTC Investigation
Elevated Temperature Properties

addition to depending sensitively on testing rate, the magnitude of this increase varies from steel to steel.

It may be absent in many steels, particularly the higher strength, microalloyed steels. Computationally, one of the modeling groups required stress-strain behavior in which the strength decreased monotonically

with temperature, which this functional form supplies.

The model for the low-strength steel stress-strain curves is based on the A 36 data of Harmathy ( 1 970).

Table 6-3 summarizes some of the properties of this steel. Table 6—4 and Figs. 6-8 and 6-9 show the

individual Ki and u, to which Eqs. 6-5 and 6-6 were fit. Table 6-5 summarizes the values of those

parameters in Eqs. 6-5 and 6—6 for steels with F,. = 36 ksi. Figure 6-10 shows the prediction of the model

plotted o\ er the original data used to generate the parameters for Eqs. 6-5 and 6-6. Note that although some of the K, and n, in Figs. 6-8 and 6-9 deviate strongly from the fitted curves, the prediction of the

model for the stress-strain behavior. Fig. 6-10, agrees with the data quite well.

Table 6-3. Property data for the A 36 steel reported in Harmathy (1970).

Grade A 36

Composition C 0.19%

P 0.007 %

S 0.03 %

Mn 0.71 %

Si 0.09 %

semi-killed, from 1/2 in. plate

compositions e.xpressed in mass percent

(20 °C) 44 ksi (reported, not corrected to zero strain rate)

TS (20 °C) 64 ksi (reported, not corrected to zero strain rate) Range of data 20 °C - 649 °C

Number of tests 10

Table 6-4. Individual K, and n, used for steels with Fy = 36 ksi.

T

°C ksi «, 24 92.95 0.1404

99 82.2 0.1335

149 108.8 0.1947

204 140.0 0.2458

260 155.9 0.2766

315 152.6 0.2928

427 105.3 0.2352

535 54.98 0.1402

593 38.41 0.1316

649 24.81 0.1167

NISTNCSTAR 1-3D. WTC Investigation 139
Chapter 6

150

100 -

50

T—I—)—I—

I—I—I—)—I—I—I—I—I—I—r~i—I

'~i—

I

f

A K measured in individual test K(T)

J I I I I I I I I I I I I I I I L. J I I I I I i I I I I I 1 l_ 100 200 300 400 500 600 700

T, °C

15.58.08 - Monday October 11, 2004

Figure 6-8. K(T), Eq. 6-5, for the A 36 steel of Harmathy (1970), used to model the

behavior of steel with F„ = 36 ksi.

0.30

0.25

0.20

c 0.15

0.10 -

0.05

0.00

1—I—I—I—

I—I—I—I—I—I—I—I—I—I—nn—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—

A

A

A

A n measured in individual test n(T)

, I , I I , I I I I I , I I I I I

I . I , I I , I I 0 100 200 300 400 500 600 700

T, °C

1 5:58:1 1 - Monday October 1 1 . 2004

Figure 6-9. n(T), 6-6, for the A 36 steel of Harmathy (1970) used to model the behavior of

steel with Fy = 36 ksi.

140 NISTNCSTAR 1-3D. WTC Investigation
Elevated Temperature Properties

Table 6-5. Values of the parameters of

Eqs. 6-5 and 6-6 for steels with = 36 ksi . 29.0492 ksi

hi 524.1812 °C

tk2 523.6799 °C

k, 9.4346

k: 9.3532

k. 121.6056 ksi

nil 0.1235

tnl 524.4304 °C

521.2410 °C

a 19.0000

19.0000

n4 0.2168

a. Fits on /?/ and /7j were constrained to have n< 19.

0 I I I , I I I I 0.00 0.05 0.10 0.15 0.20 True strain

16 59 05 - Monday February 07 2005 Figure 6-10. Predictions for the model (dashed lines) for A 36 steel (Fy = 36 ksi nominal)

overlaid on the original data used to generate the model (solid lines). The model

(Eqs. 6-4, 6-5, and 6-6 and Table 6-4) makes essentially identical predictions for

0°C < T < 300 °C, so only one line is plotted. Note that the model should not be used for

strains in the elastic region (e~ < 0.003), but the curves are shown in this region. Instead,

elastic lines of the appropriate modulus should be used.

The model for the high-strength steel stress-strain curves is based on measurements made in the NIST

laboratory of steel taken from the truss angles C-132 and C-53, used in WTC 1 and WTC 2 (location in building unknown). The Laclede truss steel, summarized in Table 6-6 is a good model for most of the high strength steels. Its composition and mechanical behavior are similar to an ASTM A 572 Grade 50

steel. Chemical and mechanical characterization indicates that although the truss angles of C-132 were

NISTNCSTAR 1-3D, WTC Investigation 141
Chapter 6

supplied to A 242 and the tmss angles of C-53 were supplied to A 36, the steels are identical, highstrength, low-alloy (HSLA) steels similar to current ASTM A 572. Most of the perimeter steels in the

towers are also micro-alloyed steels, especially those up to about F, = 60 ksi.

Table 6-6. Property data for the A 242 Laclede truss steel tested as part

of the Investigation.

Grade

Composition

F, (20 "O

r5(20"C)

Ranee of data Number of tests

A 242 (F, - 50 ksi) c 0.19%

Mn 0.82 %

P 0.010%

S 0.029 %

Si 0.07 %

Ni 0.08 %

Cr 0.10%

Cu 0.32 %

V 0.038 %

from 2.5 in. x 1.5 in. x 0.25 in. open web floor truss angle

compositions expressed in mass percent

59 ksi (static F, -corrected to zero strain rate)

77.8 ksi (static ZlS-corrected to zero strain rate) 20 "C - 650 "C

14

Table 6-7 and Figs. 6-1 1 and 6-12 show the individual Ki and to which Eqs. 6-5 and 6-6 were fit. Table 6-8 suirmiarizes the values of those parameters in Eqs. 6-5 and 6-6 for steels with F, > 36 ksi. Figure 6-13 shows the prediction of the model plotted over the original data used to generate the

parameters for Eqs. 6-5 and 6-6. The model captures the shapes of the stress-strain curves. The actual

data show the reduced ductility characteristic of the Laclede truss steel.

Table 6-7. Individual K, and n, used for steels with Fy > 36 ksi.

T K

rc) (ksi) n

25 117.52 0.1276

25 122.9 0.1699

25 113.6 0.1141

300 137.4 0.1763

400 95.48 0.1460

400 111.3 0.1614

500 64.72 0.0815

500 68.12 0.0833

600 35.32 0.0415

600 30.16 0.0267

600 3 1 .0 0.0250

600 37.03 0.045

650 23.0 0.0327

650 27.46 0.0353

142 NISTNCSTAR 1-3D, WTC Investigation
Elevated Temperature Properties

150

100

50

0

0 100 200 300 400 500 600 700

T, °C

16:39 11 - Monday Oclober 11, 2004

Figure 6-1 1 . K(T), Eq. 6-5,for the A 242 Laclede steel used to model the behavior of steel

with Fy > 36 ksi.

0.30

0.25

0.20

c 0.15

0.10

0.05

0.00

0 100 200 300 400 500 600 700

T, °C

16 39 18 - Monday Ociobei 11 2004

Figure 6-12. n(T), Eq. 6-6,for the A 242 Laclede steel, used to model the behavior of

steel with Fy > 36 ksi.

NISTNCSTAR 1-3D. WTC Investigation 143
Chapter 6

Table 6-8. Values of the

parameters of Eqs. 6-5 and 6-6 for

steels with Fy > 36 ksi.

ko IbAlli ksi

hi

c 1 1 O'") ii/i or^ Ml.OiOD L

hj

1 1 OHIO o/^

1ki

O.DV /

1 Zz. J i O / KSI

'hi 0.0342

519.6340

499.603 1 "C

a "/ 10.0000

10.0000

0.1511

a. fits on ;?/ and z?! were constrained to have n< 10. 100

80

• 60 w

(/) CD to^ 40

20

0 1—1 1 I I I I I I I I I I——I I I

^ I I 0.00 0.05 0.10 0.15 0.20 True strain

16 55 48 - Monday Oclober 1 1 , 2004

Figure 6-13. Predictions for the model (dashed lines) for steel with Fy > 36 ksi overlaid

on the original data used to generate the model (solid lines). Note that the model should

not be used for strains in the elastic region (e- < 0.003), but the curves are shown in this

region. Instead, elastic lines of the appropriate modulus should be used.

T

1 1 1—I

1 1 1 1 1 1—^1 1 1 1 25°C -

144 NISTNCSTAR 1-3D, WTC Investigation
Elevated Temperature Properties

A small disadvantage of this method is that the very low strain (8 < 0.001) behavior is neither Unear nor

does it ha\'e a slope equal to the Young's modulus appropriate to that temperature, hi terms of the stress- strain beha\ ior. this is a ver>' small error, however. One way of introducing linear stress-strain behavior

for small strains is to solve Eq. 6-4 for the intersection point, e,,, with the correct Young's modulus, E(T),

which can be evaluated using Eq. 1-2 of the Chapter 1

R^sRc^(T^)^r"'^' = EiT)£, 6-9

For £ < £y use the linear Young's modulus relation; for s > Syuse Eq. 6-4. Figure 6-14 shows stress-strain cur\ es calculated using the method of Eq. 6-9. Figure 6-15 compares the yield points calculated by that method with the expression for the yield point as a function of temperature for many structural steels. Fig. 6-6. The agreement is within the band of the scatter of the data, confirming the suitability of this method. Further accuracy is not warranted, because as temperature increases the yield strength depends

increasingly strongly on the testing rate. In these cases, the proportional limit may be much less than the

0.02 percent offset yield strength.

0.000 0.005 0.010 0,015 0.020 True Strain

1 7 34;S1 ' Monday Octobei 1 1 , 2004

Figure 6-14. Simulated elevated temperature stress-strain curves for the Laclede A 242

truss steel. The small-strain behavior is modeled using the appropriate Young's

modulus, while the large-strain behavior comes from Eq, 6-4.

Generally, the correspondence between the original room temperature stress-strain curves and those

predicted by Eq. 6-4 is quite good. For the steels with F,, > 75 ksi, the agreement is less satisfactory

because these steels do not strongly work harden. During the development of this methodology NIST

could not locate any elevated temperature stress-strain curves for such high-strength steels, and had not

yet conducted the tests reported in Appendix A.

NISTNCSTAR 1-3D, WTC Investigation 145
Chapter 6

120 -

100

20

T—I—I—I—

I—I—I—I—I—

I—I—I—I—I—

I—I—I—I—I

\—I—

- F =60 ksi perimeter T=20°C _ _ - -20 C _ - 350°C _ _ - -400°C _ - - -450°C

500°C -

-550X

- -600 C

-650°C

Q 1 I I I I I I I I \ I I I I \ I I I ! I I I I \ 1 I I I I I I I \ I L 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 True Strain

09 31 ;59 - Tuesday October 12, 2004

Figure 6-15. Example of predicted stress-strain curves for Fy = 60 ksi perimeter columns

calculated using Eq. 6-4.

Limitations of Elevated-Temperature Stress-Strain Curves

There are important limitations to the high-temperature stress-strain curves. At room temperature the

stress-strain curves are time-invariant, and, to a good approximation rate-invariant as well. At elevated

temperature, beginning at about 400 °C, where thermally activated dislocation motion is easier, these

assumptions are no longer valid. In particular the yield strength, defined as the stress below which no

permanent deformation occurs, is very poorly defined.

Summary of Data for All WTC Steels

Table 6-9 shows the values of Rts and Rc for all the different steels in WTC 1 and WTC 2.

Example Calculations

To estimate the stress-strain behavior of a F,. = 60 ksi perimeter column steel use the supporting data from

Table 6-8 and Table 6-9 with Rjs = 1.1800 and Rc = 0.8900. Figure 6-16 shows the stress-strain curves

generated using Eqs. 6-4, 6-5, and 6-6 and these parameters. Note, of course, that there is no distinct

yield point in the curves generated by Eq. 6-4.

146 NISTNCSTAR 1-3D, WTC Investigation
Elevated Temperature Properties

Table 6-9. Scaling parameters (Eg. 6-4) for all WTC steels.

Static Fy

(ksi)

Static TS

(ksi) He Summary Description

36.7 64.5 1.09 0.86 All 36 ksi core box column steels

37.0 63.5 1.07 0.95 All 36 ksi core WF

51.4 79.2 1.07 0.88 All 42 ksi box column steels t < 0.75m

47.0 74.8 1.01 0.88 All 42 ksi box column steels 0.75in < t < 1 .5 in

42.6 70.4 0.95 0.88 All 42 ksi box column steels t > 1.5 in

53.8 74.4 1.00 0.98 42 ksi Group 1&2 shapes WF Core Columns

49.0 71.1 0.96 0.95 42 ksi Group 3 shapes WF Core Columns

44.2 66.6 0.90 0.95 42 ksi Group 4&5 shapes WF Core Columns

53.8 74.4 1.00 0.98 45 ksi Group 1&2 shapes WF Core Columns

49.0 71.1 0.96 0.95 45 ksi Group 3 shapes WF Core Columns

47.8 71.1 0.96 0.94 45 ksi Group 4&5 shapes WF Core Columns

53.8 74.4 1.00 0.98 50 ksi Group 1&2 shapes WF Core Columns

53.8 74.4 1.00 0.98 50 ksi Group 4&5 shapes WF Core Columns

35.6 61.2 1.03 0.88 All 36 ksi perimeter column steels plates 1 2 4 ( not inner web = plate 3)

53.1 74.9 1.01 0.95 All 45 ksi perimeter column steels plates 1 2 4 ( not inner web = plate 3)

54.0 75.6 1.02 0.98 All 50 ksi perimeter column steels plates 1 2 4 ( not inner web = plate 3)

60.8 82.6 1.11 0.90 All 55 ksi perimeter column steels plates 1 2 4 (i.e. not -plate 3) with

thickness < 1 .5 in

62.0 87.3 1.18 0.89 All 60 ksi penmeter column steels plates 1 2 4 (i.e. not inner plate -plate

3) with t < 1.25 m

69.6 90.4 1 22 0.98 All 65 ksi perimeter column steels plates 1 2 4 (i.e. not inner plate —plate

3) with t < 0.5 in

76.7 92.0 1.24 0.96 All 70 ksi perimeter column steels plates 1 2 4 ( not inner web = plate 3)

82.5 96.8 1.31 0.94 All 75 ksi perimeter column steels plates 1 2 4 ( not inner web = plate 3)

91.5 99.4 1.34 0.99 All 80 ksi perimeter column steels all plates

104.8 116.0 1.57 0.98 All 85, 90, 100 ksi perimeter column steels, regardless of plate

38.1 59.6 1.00 0.94 Truss rounds specified as A 36

55.3 74,1 1.00 0.96 Truss angles (A 36 or A 242) and all rounds specified as A242

42.6 n d 0.90 0.91 All = 42 ksi perimeter column plate 3 steels

45.9 n/d 0.94 0.92 All Fy = 45 ksi perimeter column plate 3 steels

51.4 n/d 1.00 0.93 All F, = 50 ksi perimeter column plate 3 steels

56.9 n/d 1.07 0.91 All = 55 ksi perimeter column plate 3 steels

62.4 n/d 1.13 0.95 All Fy = 60 ksi perimeter column plate 3 steels

67.9 n/d 1.19 0.98 All Fy = 65 ksi perimeter column plate 3 steels

78.9 n/d 1.31 1.00 All F, = 70 ksi and F, = 75 ksi perimeter column plate 3 steels Key: n/d. not determined.

NISTNCSTAR 1-3D. WTC Investigation 147
Chapter 6

oO

O

(M

1.2

1.0

0.8

.0.6

0.4

0.2

0.0

Intersection of E(T) with elevated temperature a-c curve Equation for f(T)

Rts = 1 -00 Rc = 0.96 FJ23°C) = 59.0 ksi I J 1 I I I I I L. 200

17 35 05 - Monday October 11, 2004

400

tCc)

600

Figure 6-16. Yield point calculated from intersection of appropriate Young's modulus

and Eq. 6-4 compared with the expression for the decrease in yield strength for

structural steels in general Eq. 6-1. The correspondence is within the uncertainty of

either expression.

To estimate the stress-strain behavior of a Fy = 36 ksi core column WF steel use the supporting data from

Table 6-5 and Table 6-9 with Rjs = 1.0700 and Rc = 0.9500. Figure 6-17 shows the stress-strain curves generated using Eqs. 6-4, 6-5, and 6-6 and the parameters above. Note that Eq. 6-4 slightly overpredicts

the already supplied room temperature curve (the solid line), but this difference is within the range of

behavior NIST has observed for other A 36 grades of steel.

100 -|—I—I—

I—I—I—I—I—I—I—I—I—I—

I—I—I—I—I—

I—I—I—I—I—

I—I—I—

FY=36 ksi core WF T=20°C —

'

I

' ' ' - -20°C ^""^-^ 350X - -400°C _ _ - -450°C

-500°C

_ _ -550°C

600°C

Q 1

I I I

I 1 I I I I I I I 1 1 L_l 1 I I I I 1 I I I I I I I I I I I L 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

(0 :^

inCO 60 - /^''

- // ^

CD CO .///

CD 40 /// /

i_ P/

f//

20 1^ ^ _ - - -

True Strain

16 10 30 - Tuesday October 12, 2004 Figure 6-17. Example stress-strain curves for Fy = 36 ksi WF core columns calculated

using Eq. 6-4.

148 NISTNCSTAR 1-3D, WTC Investigation
Elevated Temperature Properties

6.4.4 Analysis of Creep Data

Background

The literature contains \"er>' few strain-time curves for short-temi creep of structural steel. One source is Knight (1971) who characterized four Austrahan steels: AS A135, AS AI49, X-60, and Aus-Ten 50. The

first two are similar to US ASTM A 36 steels. X-60 is a pipeline steel conforming to API 5LX. AusTen 50 is a weathering steel with Cr and Ni additions. A second source is Fujimoto (1979), who

characterized the creep of three Japanese structural steels: SS41, SM50, and SM58. SS41 is similar to A 36. SM50 is a higher strength grade (F, = 350 MPa (51 ksi) , TS = 530 MPa). SM58 is a Cr-Mo-V steel (F, = 540 MPa (78 ksi), TS = 660 MPa). Williams-Leir (1983) reanalyzed the data m both these studies

and developed a very useful analytical model for predicting creep strain as a function of time, stress, and

temperature for all seven steels.

Fields and Fields (1988) reanalyzed Knight's creep data of AS A 149 steel. The data set for the AS A 149

steel consists of 44 tests in the range 350 °C < r< 650 °C. They also used their model to predict the

strain-time creep behavior of SS41 in Fujimoto (1979). The analysis of this section preserves the notation

of Fields and Fields, and refers to the predicted strain using these parameters as the "AS A 149 model."

Fields and Fields (1988) represented the creep strain, Sc, using a function that separates the time and stress dependence into independent functions:

= At^a^ 6-10

The functions A, B, and C are all functions of temperature.

B(T)^B,+B,T

where

Bo= -1.1

5, = 0.0035

6-11

where Tis in °C and / is in minutes.

c(r) = Q + c,r

where 6-12

Co- 2.1 C,= 0.0064

where Tis in °C and c is in ksi. The parameter C is the temperature dependent creep stress exponent,

often denoted by the symbol n.

NISTNCSTAR 1-3D, WTC Investigation 149
Chapter 6

A(T):=\0~'

where

for r< 500 °C

Aa^ 6.10 6-13

A^^ 0.00573

for 7 > 500 °C

Ao= 13.25

Ai = -0.00851

where T is in °C and strain is in percent.

The analysis of the remainder of this section has two goals. One is to characterize the actual creep

behavior of the important truss angle steel that made up the chords of the floor trusses. The second is to develop a methodology for estimating from literature data the creep behavior of the other steels in the

trusses, truss seats, perimeter columns, and core columns.

Results for A 242 Fy = 50 ksi Truss-Angle Steel

The A 242 steel tested came from the upper and lower chords of truss specimen C-132, of unknown

location in the buildings. The steel was specified to conform to ASTM A 242. Chemical analysis showed

that, although its chemistry is consistent with the A 242 specification, it contains reduced levels of the

usual Cr and Ni alloying elements added to A 242 steels for corrosion resistance. Chemically it is similar

to a contemporary A 572 steel.

The solid lines in Figs. 6-2 through 6-5 plot the creep curves for A 242. The curves have been

graphically truncated at 5 percent strain. Horizontal arrows denote tests that were suspended. Vertical arrows denote tests that ended with specimen failure.

Analysis of A 242 Fy = 50 ksi Truss-Angle Creep Data

The analysis retains the formalism of the 1988 analysis of Fields and Fields, which describes the creep

(not total) strain as

where A, and C are all functions of temperature.

In practice it was not possible to evaluate both A and C independently in a single creep test. While Fields

and Fields evaluated C from plots of creep strain as a function of stress at different times, we evaluated C

as the stress exponent for creep rate, ds/dt:

= At''(T c 6-14

t ^C{T) 6-15

150 NIST NCSTAR 1-3D. WTC Investigation
Elevated Temperature Properties

Calculating C this way enabled the use of extra strain rate data from the high-temperature tensile tests. A

non-linear least squares fit to the (ds/dt, a, T) data shown in Table 6-10, using the natural logarithm of

Eq. 6-15 resulted in

C{T)^C, + C,T

where

Co= 3.233

C, = 0.0117

where T is in °C and a is in MPa. Using the entire data set simultaneously resuhs in a fit that weights the

temperatures with more data points more strongly. Figure 6-18 shows the strain rate data and the fits to Eq. 6-16. The range of OT^ is 7.9 < C(T) < 10.8 for 400 °C < r< 650 °C.

Table 6-10. Stress-temperature-strain rate data

used to evaluate the parameters of Eq. 6-16.

Strain

Flow Stress T Rate

Specimen MPa °C 1/s C132TA6W-1 165 650 9.02E-04

C132TA7W-1 147.2 650 2.69E-04

C132TA6N-2 145.4 650 2.13E-04

C132TA2N-3 126 650 1.39E-04

C132TA2W-8 125 650 3.20E-04

C132TA2W-4 100 650 2.95E-05

C132TA4N-3 212 600 7.62E-04

C132TA1W-8 202 600 2.50E-04

C132TA3W-4 190 600 2.62E-04

C132TA5W-2 164 600 2.79E-05

C132TA3W-3 152 600 2.85E-05

C132TA4W-6 150 600 1.90E-05

C132TA2W-2 131 600 9.91E-06

C132TA1W-2 123 600 2.44E-06

C132TA1N-3 349 500 8.60E-04

C132TA1N-2 300 500 2.50E-04

C132TA2N-2 297 500 4.11E-05

C132TA4W-4 278 500 1.53E-05

C132TA1W-4 272 500 7.75E-06

C132TA3N-1 252 500 6.77E-06

C132TA4W-3 250 500 3.48E-06

C132TA3W-2 200 500 5.17E-07

C132TA4N-2 445.8 400 2.67E-06

C132TA7W-2 422.4 400 8.81E-07

C132TA7N-1 346.4 400 8.40E-08

NISTNCSTAR 1-3D. WTC Investigation 151
Chapter 6

w .4

o

o -6

1

" 0

1 1

650 °C

1

' 1

' 1

1

'

' 1 1 1 1

- 600 °C

- A 500 °C /6 A

0 400 °C 0 /Q A

_ 0/V /

0 / £J /A - //a - / A

0

A

0

dcydt = A' a^'^'

1 , 1 , , 1 , , 1 , , C(T) = Co+C.T

, 1 \ , 2.0 2.2 2.4 2.6

19-21 36 - Thursday Oclober 14. 2004 logio(cj), MPa

Figure 6-18. Prediction of the function C(T), Eq. 6-16, from strain rate data for A 242

truss steels.

Unlike the data for the AS A 149 steel, the time-hardening exponent, B, does not increase linearly with

temperature, so the representation of Eq. 6-1 1 is inappropriate. To ensure accurate representation of the

creep behavior near and outside the boundaries of the experimental conditions requires choosing a function for B that does not diverge or produce negative values of B at low temperature. Of course, creep

at temperatures less than about 400 °C is insignificant, so the specifics of the behavior at low

temperatures will not affect the measurable strain. The chosen function for B(T) has a minimum value for low temperature:

B{T) = B,,+BJ^'

where

5o= 0.3982 6-17

5,= 3.5531x10""

82= 3.6975

where 7" is in °C and / is in seconds. The parameters have no physical significance. Figure 6-19 shows the

behavior of B(T) over the temperature range where creep is important.

152 NISTNCSTAR 1-3D. WTC Investigation
Elevated Temperature Properties

2.0

1.5

m 1.0

0.5

T\—I—I—I—I—

r

I ' ' I

' ' ' '

I

' ' ' '

I

B(T) = Bo+B,t"^ O

o

o

0.0 ' ' ' ' ' I ' ' ' I I I I I I I I

I I I I I I I I I I I I I I I I I I 350 400 450 500 550 600 650 700

19:22:55 - Thursday Ociober 14 2004 T, °C

Figure 6-19. Variation of the parameter S with temperature. Solid line is Eq. 6-17.

The prefactor, A, in Eq. 6-14 could be represented by a quadratic polynomial.

A{T) = exp(A^^ + A^T + y^,^' ) where

^,,= -55.4504 6-18

Ai= 9.47600x10"^

A2= -3.52064x10-'

where T is in °C and strain is in natural units.

Evaluating the parameters of Eqs. 6-16, 6-17, and 6-18 required several steps. The first step was to evaluate d, and C/ of Eq. 6-16 using the data of Table 6-10. After that, Eq. 6-14 was fit to each

individual creep curve, using the appropriate values of C(T) and a. This resulted in a set of 20 individual

(Ai.Ti) and (B,,!,) pairs, from which the values of the parameters in Eqs. 6-17 and 6-18 were evaluated by

linear regression in the case of Eq. 6-18 and non-linear, least-squares fitting in Eq. 6-17.

Comparison of the A 242 Model with Actual Creep Curves.

Figures 6-2 through 6-5 also plot the model creep curves developed using the methodology of the

previous sections. In general the agreement is acceptable and is of the same magnitude as the model fits of WilHams-Leir (1983) to Knight's (1971) and Fujimoto's (1979) data, as well as Fujimoto's ( 1979) fits to his own data. It is important to realize that the model curves in the figures are not fits to the individual

creep cur\ es. but are rather the prediction of the model from the global fit, using the methodology of the

previous section.

NISTNCSTAR 1-3D. WTC Investigation 153
Chapter 6

Estimating Creep Properties of WTC Steels.

The general goal of the second part of this analysis is to develop a methodology to estimate the creep

properties of the other WTC steels using either literature-generated models or models generated from

creep data from the NIST laboratory.

Previous analysis has indicated that both the yield (F,) and tensile (TS) strengths follow a universal curve with temperature, when normalized to their room temperature values. This behavior suggests that it is possible to use the creep data developed for one steel to predict the creep behavior of another steel by

appropriate scaling by one of these parameters. A second advantage of this approach is that F,., TS, and

total elongation to failure are frequently the only steel parameters reported in the literature. To use the

AS A 149 model to predict creep behavior of an untested steel, Eq. 6-14 uses a corrected stress, ac, equal

to the applied stress, cr,„ times the ratio, R^, of the yield or tensile strength of the AS A 149 to that of the

untested steel. For example,

(^c ^^a^a 6-19

^ ^ um

So, for an untested steel that is stronger than the reference AS A 149 steel, the corrected stress used to predict the creep behavior will be less than the actual applied stress (Ra< !)• An alternative approach,

which is even simpler, is to set = 1 , which is equivalent to assuming that all steels have identical creep

properties.

The best of the three possible scaling ratios, R^. (Fy , TS, and no scaling^ was evaluated subjectively.

Williams-Leir's (1983) analytical expressions for creep strain as a function of time for the four Australian

steels of Knight (1971) and three Japanese steels of Fujimoto (1979) facihtated this intercomparison.

Although Williams-Leir used an expression for creep strain as a function of time, stress, and temperature

that differs from Eq. 6-14, his fits produced creep curves of similar fidelity. Using his analytical

expression removed the need to re-fit all seven data sets. At four temperatures (500 °C, 550 °C, 600 °C,

and 650 °C) the ratio, R, of the creep strain at 3600 s predicted by the scaling ratio approach of Eq. 6-19

to that predicted from the Williams-Leir analytical expression was plotted as a function of stress for each

steel. The range of stresses plotted was constrained to produce strains in the range 5xlO"^<£<0.25.

Strains outside this range are irrelevant for creep modeling. This procedure produced 28 plots (seven

steels at four temperatures) where each plot has seven plotted ratio lines. Perfect prediction by one steel of

another's creep strain would produce a horizontal ratio line with R= I. For each steel at each scaling

ratio, Ra-, the quality of the prediction was ranked by visually judging the distance from the distance of the

predicted line from unity.

The results of the ranking established the tensile strength ratio scaling as the best approach. In about 1/2

of the cases, tensile-strength ratio scaling and no scaling produced similar results. In about 1/3 of the

cases, tensile-strength ratio scaling produced the best results, and in the remaining 1/6 of the cases, no

scaling produced the best results. Thus, in about 5/6 of the cases, scaling the creep behavior by the room

temperature tensile strength ratio produced creep curves that most closely approximated the measured

curves. Scaling by the yield strength ratio almost always produced the worst predicted strains. Frequently

it produced estimates that differed from the actual data by more than a factor of ten. The only times that

154 NISTNCSTAR 1-3D. WTC Investigation
Elevated Temperature Properties

scaling by the yield strength ratio produced acceptable results was when the scaling ratio was fortuitously

identical to the scaling ratio produced by the tensile strengths.

Values for Creep Parameters

To estimate the creep strain as a function of time for all truss bulb angles use Eq. 6-14 with the

parameters in Eqs. 6-16, 6-17 and 6-18.

To estimate the creep strain as a function of time for all other steels use Eq. 6-14 with the parameters of

Eqs. 6-11. 6-12 and 6-13. Scale the applied stress by the ratio of the tensile strength of the AS A149

steel {TSai49 = 70.6 ksi) to that of the steel in question using Eq. 6-19.

6.4.5 Values for Bolts

Background

NIST did not test any bolts at elevated temperatures. Given the lack of experimental data, it is necessary

to predict the bolt properties using the methodology of Section 6.4.1.

Because A 325 and A 490 bolts are not intended to be used above room temperature, very little elevated

temperamre property data exist in the literature. The limited strength data for bolts plotted in Fig. 6-20 come from tensile specimens of bolt steels rather than the bolts themselves, made to British, Japanese and

Chinese boh standards. In general, these bolt standards are similar to the corresponding ASTM standards,

however. Table 6-1 1, and the rest of this section, summarize the literature data used to construct

Fig. 6-20.

Ta ble 6-1 1 . Sources of bolt data.

Reference Steel TS (spec)MPa

Fy (Spec)

MPa Steel Chemistry

Kirby Grade 8.8 785-981 628 C-Mn-Cr-Mo-B-Ti

Li 20MnTiB 1040-1240 940 C-Mn-Ti-B

Sakumoto Bol-Ten 981-1177 883 unknown

A 325 827 634 C-Mn-B

Kirby (1995) studied British Grade 8.8 bolts supplied to BS 3692 from two manufacturers. These bolts are very similar to A 325 bolts in both strength and chemistry. He conducted tensile strength tests at temperatures up to 800 °C on actual bolts, as well as on the steels used to fabricate the bolts. In the bolt

tests, the mode of tensile failure depended on choice of nut. Even though the nuts were made to identical

specifications, in one set the bolt failed by ductile necking in the thread, while in the other set, the bolt threads stripped out.

Li et al. (2003) tested bolt steel identified as "20MnTiB," which apparently corresponds to a Chinese bolt

specification. In terms of strength, these bolts are similar to A 490 bolts, rather than to A 325 bolts. They

tested tensile specimens fabricated from bolt steel at temperatures up to 700 °C.

NISTNCSTAR 1-3D. WTC Investigation 155
Chapter 6

Sakumoto (1993) reported the properties of ordinary and "fire-resistant" (FR) bolts made to JSS 11-09- 1981. Both were identified as "BOL-TENl ION" steel. This bolt steel is stronger than the A 325 bolt

steels.

1.0

0.8

E 0.6 cn

t:cn

0.4

0.2

0.0

T

O TS Sakumoto B0LTEN1 10N-FR

5 TS Sakurnoto BOLTENl 10N

« TS: Li 20yr!TIB

6 5% Stress: Kirby Set A Gr 8,8 V-B Recommend value for structural steel Recommend value for bolt steel

_i I i_ _I I L J I l_ 200 400

T, °C

600

09:41 45 - Friday August 27, 2004

Figure 6-20. Comparison of high-temperature yield, Fy, and tensile, TS, strength for bolt

steels and the expression for structural steels in general, Eqs. 6-1 and 6-2. Solid

symbols are bolt steels. Open symbols are "fire-resistant" bolt steels. Expression for bolt

steels is the dashed line.

Figure 6-20 shows the ratio of yield (F,.) and tensile (TS) strengths to their room-temperature values for the various bolt steels. The solid line is the expression for structural steels in general. The bolt steels,

156 NISTNCSTAR 1-3D, WTC Investigation
Elevated Temperature Properties

however, with the exception of the FR BOL-TEN steel, lose strength more rapidly than do the structural

steels.

Results

Because the bolt steels lose strength more rapidly with increasing temperature than do structural steels, it

is necessary to use a different set of parameters for Eqs. 6-1 and 6-2 to describe the strength loss. Table 6-12 shows the values for the parameters for yield (Eq. 6-1) and tensile (Eq. 6-2) strength

reduction. The dashed curve in Fig. 6-20 corresponds to the expression for the strength reduction of bolt

steels.

Table 6-12. Values for the parameters In the strength reduction equations (Eqs. 6-1

and 6-2) for use with bolts.

\'ie\d Strength Reduction Parameters

(Eq.6^1)

Tensile Strength Reduction Parameters

(Eq. 6-2)

.42 = 0.07 A2= 0.09 w, = 6.8221 /7, = 5.0237

W2= 1.0000 «2 = 5.0245

s, = 491.0 = 478.0

S2= 810.0 t2 = 585.0

Values for Bolt Strength Parameters

The yield and tensile strength of bolt steels degrade more rapidly with temperature than do ordinary

structural steels. The reductions can be represented by Eqs. 6-1 and 6-2, using the parameters in Table 6-12.

Limitations

Most of the data in Fig. 6-20 come from tests on bolt steels, rather than on bolts themselves. Only Kirby

(1995) provides data from actual bolt tests. No information exists on changes in failure mechanisms with

increasing temperature for structural steel bolts. The ductility measured in tensile tests of bolt steels cannot be directly applied to predict bolt failure, because bolts generally fail in the threads. There is relatively little data on the expected (versus specified) strength of bolts. Fisher (1963) and Salih (1993)

found very different values for the average strength of A 325 bolts, though as expected, the average

strength was much higher than the specified strength.

In the absence of data to the contrary, the reduction in bolt strength with temperature can be modeled with

Eq. 6-2 using the parameters of Table 6-12.

6.5 SUMMARY

The yield and tensile strength of structural steel follow a master curve when normalized to their room

temperature values, Eqs. 6-1 and 6-2.

NISTNCSTAR 1-3D. WTC Investigation 157
Chapter 6

The shapes of stress-strain curves at elevated temperature can be modeled based on the room-temperature

shape of a model curve scaled by the ratio of their tensile strengths. The model accounts for the decrease

in work-hardening with increasing temperature, Eqs. 6-4, 6-5 and 6-6. The behavior of steels with

F,. = 36 ksi is based on the behavior of A 36 steel from the literature. The behavior of steels with

Fy > 36 ksi is based on the behavior of F, = 50 ksi steel recovered from the WTC floor trusses.

Creep behavior can be modeled using a function that breaks the time and stress dependence of the creep

strain into separate functions, Eq. 6-14. The behavior of steels with Fy = 36 ksi is based on a literature analysis of Australian AS A 149 steel. To apply this model to other F, = 36 ksi steels, it is necessary to scale the applied stress by the ratios of their room-temperature tensile strengths, Eq. 6-19. The creep

behavior of steels with Fy > 36 ksi is based on results of tests on Fy = 50 ksi steel recovered from the

WTC floor trusses.

Bolt properties are estimated from literature data.

6.6 REFERENCES

Baird, J.D. 1971. The effects of strain-ageing due to interstitial solutes on the mechanical properties of

metals. Review 149. Melall. Rev. 16 1-18.

Chijiiwa, R. et al. 1993. Development and Practical Application of Fire-Resistant Steel for Buildings.

Nippon Steel Technical Report 58. 47-55.

Fields. B.A. and R.J. Fields. 1989. Elevated Temperature Deformation of Structural Steel. NISTIR 88-

3899.

Fujimoto, M. e/ al. 1979. Primary Creep of Structural Steel at High Temperatures. Report of the Research

Laboratory of Engineering Materials, Tokyo Institute of Technology. Number 4. p.p. 1-27.

Goda, S. et al. 1964. Present Status of Weldable High Strength Steel. Yawata Technical Report (Yawata

Seitetsu Kenkyu) 248 5086-5163 (in Japanese).

Gowda, B.C. 1978. Tensile Properties of SA 516, Grade 55 Steel in the Temperature Range of 25°-927°C

and Strain Rate Range of lO"'' to 10"' sec"'. In, Characterization ofMaterials for Sen'ice at Elevated

Temperatures. Smith, G.V. ed. American Society of Mechanical Engineers. New York, "presented at the 1978 ASME/CSME Montreal Pressure Vessel & Piping Conference, Montreal, Quebec, Canada,

June 25-29 1978." 145-158

Harmathy T.Z. and W.W. Stanzak. 1970. Elevated-Temperature Tensile and Creep Properties of Some

Structural and Prestressing Steels, in Fire Test Performance, ASTM STP 464, American Society for Testing and Materials. 186-208.

Holt, J.M. 1963. Short-time Elevated-Temperature Tensile Properties of USS "T-l" and USS "T-l" Type

A Constructional Alloy Steels. United States Steel Report (57.019-901)(3)(a-AS-EA-2). Also

reported in (USS 1972).

158 NISTNCSTAR 1-3D. WTC Investigation
Elevated Temperature Properties

Holt. J.M. 1964. Short-Time Elevated-Temperature Tensile Properties of USS Cor-Ten and USS Tri-Ten

High-strength Low-alloy steels, USS Man-Ten (A 440) High-strength Steel and ASTM A 36 Steel. United States Steel Corp. Report 57.19-901(1). Monroeville, Pa, Applied Research Laboratory.

Oct. 16 (much of this data appear in (USS 1972)

Jerath. V.. K.J. Cole and C.L Smith. 1980. Elevated Temperature Tensile Properties of Structural Steels

Manufactured by the British Steel Corporation. Unpublished Report # T/RS/1I89/1 1/80/C. British

Steel Corporation, Teeside Laboratories. July 24.

Kirby, B. R. 1995. The Behaviour of High-strength Grade 8.8 Bolts in Fire. J. Construct. Steel. Research.

33 3-38.

Knight. D.. D.H. Skinner and M.G. Lay. 1971. Short Term Creep Data on Four Staictural Steels. Report

MRL 6/3 Melbourne Research Laboratories. The Broken Hill Proprietary Company. Clayton,

Victoria, Australia. Januar}'.

Li. Guo-Qiang. Shou-Chao Jian, Ying-Zhi Yin. Kai Chen and Ming-Fei Li. 2003. Experimental Studies on the Properties of Constructional Steel at Elevated Temperatures. J. Struct. Eng. 129[12] 1717-

1721.

Manjoine, M.J. 1944. Influence of Rate of Strain and Temperature on Yield Stresses of Mild Steel. Trans. ASME. 66. A-211-A-218.

Melloy, G.F. and J.D.Dennison. 1963. Short-time Elevated-Temperature Properties of A7, A440, A441,

V50, and V65 Grades. Internal memorandum to J.W. Frame. Report 1900-4g. Bethlehem Steel. April 17.

Rumpf John L. and John W. Fisher. 1963. Calibration of A325 Bolts. J. Struct. Div. ASCE. 89[ST6].

215-234.

Sakumoto, Y., K. Keira, F. Furuyama and T. Ave. 1993. Tests of Fire-Resistant Bolts and Joints. J. Struct. Eng. 1 19[1 1] 3131-3150.

Salih, Nazir, John Smith, el al. 1992 An Experimental Appraisal of the Load-Deformation Properties of

A325 High-Strength Bolts. J. Test. Eval. 20[6] 440-448.

(USS) 1972. Steels for Elevated Temperature Sen'ice. United States Steel Corporation. Pittsburgh, Pa.

Williams-Leir, G. 1983. Creep of Structural Steel in Fire: Analytical Expressions. Fire and Materials.

7[2] 73-78.

NISTNCSTAR 1-3D. WTC Investigation 159
Chapter 6

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160 NIST NCSTAR 1-3D, WTC Investigation
Chapter 7

Summary AND Findings

7.1 SUMMARY

The experimentally determined mechanical property data of World Trade Center (WTC) steels can be

divided into five groups:

• Elastic properties as a function of temperature were determined from recovered perimeter

column steels.

• Room temperature yield and tensile strength, and total elongation were determined from

specimens of recovered steel of all grades from the fire and impact zones. Complete stress- strain curves are reported for 29 different steels with 12 yield strength levels. Loaddisplacement curves were measured from recovered A 325 bolts. Stress-strain curves and

strengths of several different perimeter column and truss weld geometries are reported.

• Yield and tensile strengths and total elongations of selected perimeter and core column steels were determined as a function strain rate at rates up to 400 s"'. Strain-rate sensitivities and

complete stress-strain curves for these steels are reported. Several steels were characterized at higher rates using Kolsky bar testing.

• Impact properties as a function of temperature were determined using Charpy tests for selected

perimeter column, core column, truss, and truss-seat steels.

• Elevated-temperature yield and tensile strength, and total elongation were determined on

selected specimens of recovered perimeter and core column, truss, and truss seat steels. Complete stress-stram cur\'es for temperatures up to 650 °C are reported for all steels characterized. Creep deformation as a function of temperature and stress was determined for

the truss chord steel.

By combmmg these measured properties with historical averages from the literature, and in some cases recovered mill test reports. National Institute of Standards and Technology (NIST) developed expressions

and values for properties of the important grades of steel:

• Elastic modulus and Poisson's ratio as a function of temperature,

• Model room-temperature stress-strain curves for all WTC steel grades from the fire and

impact zones corrected for dynamic effects,

• Room-temperature load-displacement curves for A 325 bolts,

• Yield and tensile strength for weld metals,

• Stram rate sensitivity of steel for high strain rate properties.

NISTNCSTAR 1-3D. WTC Investigation 161
Chapter 7

• Elevated-temperature yield and tensile strength and complete stress-strain behavior for all WTC steel grades from the fire and impact zones,

• Elevated-temperature load-displacement curves for A 325 bolts,

• Creep response for all WTC steel grades from the fire and impact zones.

7.2 FINDINGS

This report makes a number of findings concerning the steel used in the WTC:

• Steels, bolts, and welds generally have properties that indicate that they met the specifications

to which they were supplied.

• Infrequently, the measured yield strengths are lower than called for by the appropriate

specifications. The measured yield strengths of 2 of 24 perimeter column samples were lower

than the specified minimum. This probably occurred from a combination of the natural

variability of steel properties, and small differences between the mill and NIST testing

protocols. The measured yield strengths of 2 of 8 core column samples were lower than the

specified minimum. In this case, mechanical damage that occurred in the collapse and

subsequent recovery removed the expected yield point behavior leading to the low values.

• The average measured yield strength of the steels from the perimeter columns exceeds the

specified minimum values by about 1 0 percent, which is consistent with historically expected

values for steel plates.

• The measured yield strengths of the F, - 36 ksi wide-flange core columns are lower than

expected from historical measurements of other structural steels.

• The strain rate sensitivities of the yield and tensile strengths of perimeter and core columns are similar to other structural steels from the WTC construction era.

• The impact properties of the steels, evaluated by Charpy testing, are similar to structural steels from the WTC era. The ductile-to-brittle transition temperatures of the perimeter and core column steels are at or below room temperature.

• The behavior of the yield and tensile strengths of WTC steels with temperature is similar to

that of other structural steels from the WTC construction era.

162 NIST NCSTAR 1-3D, WTC Investigation
Appendix A

Data Tables and Supplemental Figures

A.1 DATA tables FOR ROOM-TEMPERATURE MECHANICAL PROPERTIES

The following tables summarize the room-temperature mechanical properties.

NISTNCSTAR 1-3D. WTC Investigation 163
Appendix A

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NIST NCSTAR 1-3D, WTC Investigation 165
Appendix A

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Appendix A

A.2 DATA TABLES FOR HIGH STRAIN RATE PROPERTIES

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176 NISTNCSTAR 1-3D, WTC Investigation
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180 NIST NCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

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NISTNCSTAR 1-3D. WTC Investigation 181
Appendix A

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182 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

A.3 DATA TABLES FOR IMPACT PROPERTIES

The following table summarizes the Charpy test results.

Table A-15. Charpy V-notch impacit data for samples tesited."

Building Location

Specimen

Thickness

in. SpecimenOrientation

Test Temp

F

Ent'rg>'

ft-Ibf

Energy- Corrected

ft-Ibf

Sp^ci men

ID Specimen Type

1-42-97 100 0.2 T 77 14.2 25.6 SL-1 N8-C1 Bl Spandrel T

1-142-97 100 0.2 T 77 12.8 23,0 SL-2 N8-C1 Bl Spandrel T

1-142-9" 100 0.2 T 77 13.3 24.0 SL-3 N8-ClBlSpaiidrcl_T

1-142-97 100 0.2 T 14.0 25.2 SL-7 N8-C1 Bl Spandrel T

1-142-97 100 0.2 T 32 14.9 26.9 SL-8 N8-CIBlSpandrel_T

1-142-97 100 0.2 T 32 13.4 24.1 SL-9 N8-ClBlSpandrcl_T

1-142-97 100 0.2 T -4 1 1.8 21.2 SL-10 N8-ClBlSpandrcl_T

1-142-97 100 0.2 T -4 13.3 23.9 SL-1

1

N8-ClBlSpandrcl_T

1-142-97 100 0.2 T -4 15.2 27.4 SL-12 N8-C1 Bl Spandrel T

1-142-97 100 0.2 T -40 12.6 22.8 SL-1

3

N8-C1 Bl Spandrel T

1-142-97 100 0.2 T -40 12.6 22.8 SL-14 N8-ClBlSpandrel_T

1-142-97/100 0.2 T -40 1 1.5 20.7 SL-1

5

N8-ClBlSpandrcl_T

1-142-97 100 0.2 T -76 12.2 21.9 SL-4 N8-ClBlSpandrcl_T

1-142-97 100 0.2 T -1 12 2.4 4.4 SL-5 N8-ClBlSpandrcl_T

1-142-97100 0.2 L 77 43.3 78.0 ST-1 N8-C1B1-Spandrcl L

1-142-97 100 0.2 L 77 42.7 76.9 ST-2 N8-ClBl-Spandrcl_L

1-142-97 100 0.2 L 77 39.3 70.7 ST-3 N8-C1B1-Spandrel L

1-142-97 100 0.2 L 32 39.4 70.8 ST-7 N8-C1B1-Spandrel L

1-142-97'100 0.2 L 32 38.6 69.4 ST-8 N8-CIBl-Spandrcl_L

1-142-97 100 0.2 L 32 39.6 71.3 ST-9 N8-C1B1-Spandrel L

1-142-97 100 0.2 L -4 43.0 77.5 ST- 10 N8-C1B1-Spandrcl L

1-142-97 100 0.2 L -4 42.5 76.6 ST-1

1

N8-C1B1-Spandrcl L

1-142-97 100 0.2 L -4 40.0 72.0 ST-12 N8-ClBl-Spandrcl_L

1-142-97 100 0.2 L -40 32.5 58.5 ST- 13 N8-ClBl-Spandrel_L

1-142-97 100 0.2 L -40 39.5 71.2 ST- 14 N8-C1B1-Spandrel L

1-142-97 100 0.2 L -40 40.1 72.2 ST- 15 N8-ClBl-Spandrcl_L

1-1 42-97 100 0.2 L -76 15.3 27.5 ST-4 N8-ClBl-Spandrel_L

1-142-97 100 0.2 L -1 12 9.8 17.6 ST-5 N8-ClBl-Spandrel_L

1-142-97 100 0.2 L 77 35.3 63.6 NL-1 N8-ClMl-Flangc_L

1-142-97' 100 0.2 L 77 41.0 73.9 NL-2 N8-C1 Ml -Flange L

1-142-97'100 0.2 L 77 38.7 69.6 NL-3 N8-ClMl-Flange_L

1-142-97 100 0.2 L 32 37.4 67.4 NL-7 N8-ClMl-Flangc_L

1-142-97/100 0.2 L 32 38.1 68.6 NL-8 N8-ClMl-Flangc_L

1-142-97 100 0.2 L 32 39.2 70.5 NL-9 N8-ClMl-Flangc_L

1-142-97 100 0.2 L -4 38.2 68.8 NL-10 N8-ClMl-Flange_L

1-142-97 100 0.2 1 -4 38.0 68.5 NL-1

1

N8-ClMl-Flangc_L

1-142-97 100 0.2 L -4 37.3 67.2 NL-12 N8-ClMl-Flangc_L

1-142-97 '100 0.2 1 -40 35.2 63.4 NL-13 N8-C1M 1 -Flange 1,

1-142-97 100 0.2 L -40 32.5 58.5 NL-14 N8-ClMl-Flange_L

NISTNCSTAR 1-3D, WTC Investigation 183
Appendix A

Building LocationThickness

in. SpecimenOrientation

Test Temp

F

Energy

ft-lbf

Energ\' Corrected

it-lbf Specimen

ID Specimen Type

1-142-97/100 0.2 L -40 34.1 61.3 NL-15 N8-ClMl-Flangc_L

1-142-97/100 0.2 1 -76 25.5 46.0 NL-4 N8-C 1 Ml -Flange 1,

1-142-97/100 0.2 L -1 12 15.2 27.3 NL-5 N8-C1 Ml -Flange L

1-142-97/100 0.2 T 77 10.4 18.7 NT-1 N8-C1M1-Flangc T

1-142-97/100 0.2 T 77 10.9 19.7 NT-2 N8-C1M1-Flangc T

1-142-97/100 0.2 T 77 10.9 19.7 NT-3 N8-C1M 1 -Flange T

1-142-97/100 0.2 T 32 10.6 19.0 NT-7 N8-C1 Ml -Flange T

1-142-97/100 0.2 T 10.9 19.6 NT-8 N8-C1 Ml -Flange T

1-142-97/100 0.2 T 32 10.1 18.2 NT-9 N8-C1M1-Flange T

1-142-97/100 0.2 1 -4 10.2 18.3 NT-10 N8-C1 Ml -Flange T

1-142-97/100 0.2 T -4 9.5 17,1 NT-1

N8-ClMl-Flange_T

1-142-97/100 0.2 T -4 10.8 19.4 NT- 12 N8-C1M1-Flange T

1-142-97/100 0,2 T -40 9.9 17,8 NT-1

N8-C1M1-Flange T

1-142-97/100 0.2 T -40 10.5 18.9 NT- 14 N8-C 1 Ml -Flange T

1-142-97/100 0.2 T -40 10.5 18.9 NT- 15 N8-C1M1-Flangc T

1-142-97/100 0.2 i -76 10.5 18.9 NT-4 N8-C1M1-Flangc T

1-142-97/100 0.2 T -112 5.6 10.1 NT-5 N8-C1 Ml -Flange T

1-142-97/100 0.2 L 77 44.1 79.4 HL N8-C1M1-HAZ L

1-142-97/100 0.2 L 77 37.4 67.3 HL N8-C1M1-HAZ 1,

1-142-97/100 0.2 L 77 49.8 89.7 HL N8-C1M1-HAZ_L

1-142-97/100 0.2 L 32 32.2 57.9 HL N8-C1M1-HAZ_L

1-142-97/100 0.2 L 32 40.2 72.4 HL N8-C1M1-HAZ_L

1-142-97/100 0.2 L 32 44.2 79.5 HL N8-C1M1-HAZ L

1-142-97/100 0.2 L -4 22.6 40.7 HL N8-C1M1-HAZ_L

1-142-97/100 0.2 L -4 42.8 77.0 HL N8-C1M1-HAZ_L

1-142-97/100 0.2 L -4 23.6 42.6 HL N8-C1M1-HAZ_L

1-142-97/100 0.2 T 77 9.8 17.7 HT N8-ClMl-Weld_T

1-142-97/100 0.2 T 77 9.9 17.8 HT N8-ClMl-Weld_T

1-142-97/100 0.2 T 77 9.1 16.3 HT N8-ClMl-Weid_T

1-142-97/100 0.2 T 32 9.2 16.6 HT N8-ClMl-Wcld_T

1-142-97/100 0.2 T 32 9.4 16.9 HT N8-ClMl-Weld_T

1-142-97/100 0.2 T 32 9.2 16.6 HT N8-ClMl-Weld_T

1-142-97/100 0.2 T -4 9.8 17.6 HT N8-ClMl-Weld_T

1-142-97/100 0.2 T -4 9.6 17.3 HT N8-ClMl-Wcld_T

1-142-97/100 0.2 T -4 8.9 15.9 HT N8-ClMl-Wcld_T

1-451-85/88 0.2 L 77 43.2 77.8 CIO L ClO-ClMl-FlangcL

1-451-85/88 0.2 L 77 47.5 85.5 CIO L C10-C]Ml-Flange_L

1-451-85/88 0.2 L 77 47.2 85.0 CIO L ClO-ClMl- Flange L

1-451-85/88 0.2 L 32 47.2 85.0 ClOL ClO-ClMl - Flange L

1-451-85/88 0.2 L 32 46.5 83.7 ClOL ClO-ClMl-FlangcL

1-451-85/88 0.2 L 32 51.5 92.7 CIO L ClO-ClMl - Flangc L

1-451-85/88 0.2 L -4 47.1 84.8 CIO L ClO-ClMl - Flange L

1-451-85/88 0.2 L -4 48.3 86.9 ClOL ClO-ClMl-FlangcL

l-451_X5/88 0,2 L -4 47 5 85,5 ClOL C10-ClMl-Flange_L

184 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

Building Location

Specimen

Thickness

in. Specimen

Orientation

Test Temp

op Energy

ft-lbf

En cr*'^ Corrected

ft-lbf Specimen

ID Specimen Type

1-451-85 88 0.2 L -40 46.3 83,3 CIO L C 1 0-C 1 M 1 - Flange L

1-451-85 88 0.2 L -40 51.4 92,5 ClOL ClO-ClMl-FlangcL

1-451-85 88 0.2 L -40 46.5 83,7 CIO L ClO-ClMl-FlangcL

1 -45 1 -85 88 0.2 L -76 43.7 78,7 ClOL ClO-Cl Ml - Flange L

1-451-85 88 0.2 L -1 12 44.8 80.6 CIO L ClO-ClMl- Flangc L

1-451-85 88 0.2 T 77 1 1.2 20.2 ClOT ClO-ClMl - Flange T

1^51-85 88 0.2 T 77 12.4 22.3 CIO T ClO-ClMl- Flange T M51-85 88 0.2 T 77 1 1.4 20.5 ClOT ClO-ClMl - Flange T

1-451-85 88 0.2 T 32 12.4 ClOT ClO-ClMl - Flange T M51-85 88 0.2 T 32 1 1.2 20.2 ClOT ClO-ClMl- Flangc T

1-451-85 88 0.2 T 32 1 1.6 20.9 CIO T ClO-Cl Ml - Flange T

1-451-85 88 0.2 T -4 11.8 21.2 ClOT ClO-ClMl - Flange T

1-451-85 88 0.2 T -4 1 1.4 20.5 CIO T ClO-ClMl - Flange T

1-451-85 88 0.2 T -4 1 1.0 19.8 CIO T ClO-Cl Ml - Flange T M5 1-85 88 0.2 T -40 10.7 19.3 ClOT ClO-ClMl - Flange T

1_;51.85 88 0.2 T ^0 10.9 19.6 ClOT ClO-ClMl - Flangc T

M51-85 0.2 T -40 1 1.3 20.3 CIO T C 1 0-C 1 M 1 - Flange T M51-85 88 0.2 T -76 1 1.3 20.3 ClOT ClO-Cl Ml - Flange T

1-45 1-85 '88 0.2 T -1 12 10.7 19.3 ClOT ClO-ClMl - Flangc T

1_451.S5 88 0.2 T 77 11.1 20.0 CIO HAZ ClO-ClMl-HAZ T

1-451-85 88 0.2 T 77 12.0 21.6 ClOHAZ ClO-ClMl-HAZT

1-451-85 88 0.2 T 32 1 1.4 20.5 ClOHAZ ClO-ClMl-HAZT

M51-85'88 0.2 T 1 1.2 20.2 ClOHAZ ClO-ClMl-HAZ T

0.2 T 32 1 1.0 19.8 CIO HAZ ClO-ClMl-HAZ T

1-451-85 88 0.2 T -4 12.3 22.1 ClOHAZ ClO-ClMl-HAZT

1-451-85 88 0.2 T -4 1 1.7 21.1 CIO HAZ ClO-ClMl-HAZT

1-451-85 88 0.2 T -4 1 1.3 20.3 CIO HAZ ClO-ClMl-HAZ T

1-451-85 88 0.2 T -40 10.3 18.5 ClOHAZ ClO-ClMl-HAZT

1-451-85 88 0.2 T -40 1 1,0 19.8 ClOHAZ ClO-ClMl-HAZT

1-451-85 88 0.2 T -40 9.9 17.8 CIO HAZ ClO-CIMl-HAZ T

1-451-85 88 0.2 T -76 10,5 18.9 ClOHAZ ClO-ClMl-HAZT

1-451-85 88 0.2 T -1 12 9.2 16.6 CIO HAZ ClO-ClMl-HAZT

1-603-92/95 0.4 T 172 48.9 48.9 C80_T C80-wcb-column 603_T

1-603-92 '95 0.4 T 172 45.0 45,0 C80 T C80-web-colunin 603_T

1-603-92 '95 0.4 T 140 45,4 45,4 C80_T C80-wcb-column 603_T

1-603-92 '95 0.4 T 140 46,3 46,3 C80_T C80-web-column 603_T

1-603-92/95 0.4 T 77 28,9 28,9 C80_T C80-wcb-column 603_T

1-603-92/95 0,4 T 77 24,4 24,4 C80 T C80-wcb-column 603_T

l-603-92'95 0.4 T 77 25,1 25,1 C80_T C80-wcb-column 603_T

1-603-92/95 0.4 T 32 7,1 7,1 C80_T C80-wcb-column 603_T

1-603-92 '95 0.4 T 32 7,3 7,3 C80_T C80-wcb-column 603_T

1-603-92 95 0.4 T 32 13,2 13,2 C80_T C80-web-column 603_T

1-603-92 '95 0.4 T -4 3,8 3,8 C80_T C80-web-column 603_T

1-603-92 '95 0.4 T -4 4,0 4.0 C80 T C80-wcb-column 603_T

NISTNCSTAR 1-3D. WTC Investigation 185
Appendix A

Building Location

^npri men

Thickness

in. Specimen

Orientation

Test Temp

"F Energy

ft-lbf

Ener^'v Corrected

ft-lbf SpecimenID Specimen Type

1-603-92/95 0.4 T -4 3.6 3.6 C80 T C80-web-column 603_T

1-603-92/95 0,4 L 172 1 10.5 1 1 0.5 C80_L C80-web-column 603 L

1-603-92/95 0.4 L 172 121.1 121.1 C80_L C80-wcb-column 603 L

1-603-92/95 0.4 L 140 115.1 1 15.1 C80_L C80-wcb-column 603_L

1-603-92/95 0.4 L 140 103.9 103.9 C80 L C80-wcb-colunin 603_L

1-603-92/95 0.4 L 77 45.6 45.6 C80 L C80-wcb-column 603 L

1-603-92/95 0.4 L 77 47.4 47.4 C80_L C80-wcb-column 603_L

1-603-92/95 0.4 L 77 68.3 68.3 C80 L C80-wcb-column 603_L

1 -603-92/95 0.4 L 32 36.3 36.3 C80_L C80-wcb-column 603 L

1 -603-92/95 0.4 L 32 44.1 44.1 C80_L C80-wcb-column 603_L

1-603-92/95 0.4 L 32 35.6 35,6 C80_L C80-wcb-column 603_L

1-603-92/95 0.4 L -4 8.9 8.9 C80_L C80-web-column 603_L

1-603-92/95 0.4 L -4 7,0 7.0 C80_L C80-wcb-column 603_L

1-603-92/95 0.4 L -4 3.8 3.8 C80 L C80-web-column 603_L

unknown 0.2 L 77 29.1 52.3 Tl-1 Angle (2X1.5X0.25 inch) unknown 0,2 L 77 33,5 60.4 Tl-2 Angle (2X1.5X0.25 inch) unknown 0.2 L 77 30.7 55.3 Tl-3 Angle (2X1.5X0.25 nich) unknown 0.2 L 32 18.2 32.7 Tl-7 Angle (2X1.5X0.25 inch) unknown 0.2 L 32 27.5 49.4 Tl-8 Angle (2X1.5X0.25 inch) unknown 0.2 L 32 19.0 34,2 Tl-9 Angle (2X1.5X0.25 inch) unknown 0.2 L -4 8.3 14.9 Tl-10 Angle (2X1.5X0.25 inch) unknown 0.2 L -4 9.8 17.7 Tl-1

Angle (2X1.5X0.25 inch) unknown 0.2 L -4 8.6 15.5 Tl-12 Angle (2X1.5X0.25 inch) unknown 0.2 L -40 5.5 9.9 Tl-13 Angle (2X1.5X0.25 inch) unknown 0.2 L -40 1.9 3.4 Tl-14 Angle (2X1.5X0.25 inch) unknown 0.2 L -40 1.6 2.9 Tl-15 Angle (2X1.5X0.25 inch) unknown 0.2 L -76 1,6 3.0 Tl-4 Angle (2X1.5X0.25 inch) unknown 0.4 L 172 100,3 100.3 TR Rod (0.92 inch) unknown 0.4 L 172 1 13,8 1 13.8 TR Rod (0.92 inch) unknown 0.4 L 140 101.2 101.2 TR Rod (0.92 inch) unknown 0.4 L 140 96.9 96.9 TR Rod (0.92 inch) unknown 0.4 L 77 22,4 22.4 TR Rod (0.92 inch) unknown 0.4 L 77 41.7 41.7 TR Rod (0.92 inch) unknown 0.4 L 77 33.1 33.1 TR Rod (0.92 inch). unknown 0.4 L 32 21 .3 21.3 TR Rod (0.92 inch) unknown 0.4 L 32 19.8 19.8 TR Rod (0.92 inch) unknown 0.4 L 32 7.5 7.5 TR Rod (0.92 inch) unknown 0.4 L -4 5.4 5.4 TR Rod (0.92 inch) unknown 0.4 L -4 3.2 3.2 TR Rod (0.92 inch) unknown 0.4 L -4 6.1 6.1 TR Rod (0.92 inch)

0.4 T 172 27.4 27.4 N13 Scat

0.4 T 172 26.9 26.9 N13 Scat

0.4 T 140 21.6 21.6 N13 Scat

0.4 T 140 23,0 23.0 N13 Scat

186 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

Building Location

Specimen

Thickness

in. Specimen

Orientation

Test Temp

F

Energy

ft-lbf Corrected

ft-lbl Specimen

ID Specimen Type

0.4 T 77 6.1 6.1 N13 Scat

0.4 T 77 10.3 10.3 N13 Scat

0.4 T 77 6.7 6.7 N13 Scat

0.4 T 3.6 3.6 N13 Scat

0.4 T

'^") 3.0 3.0 N13 Scat

0.4 T 3.3 3.3 N13 Scat

1-142-97 100 0.4 T 172 36.8 36.8 N8 Scat

1-142-97 100 0.4 T 140 33.2 33.2 N8 Scat

1-142-97 100 0.4 T 140 33.2 33.2 N8 Scat

1-142-97, 100 0.4 T 104 26.3 26.3 N8 Scat

1-142-97 100 0.4 T 104 28.7 28.7 N8 Scat

1-142-97 100 0.4 T 77 20.7 20,7 N8 Scat

1-142-97 100 0.4 T 77 22.4 22.4 N8 Scat

1-142-97 100 0.4 T 77 20.6 20.6 N8 Scat

1-142-97 100 0.4 T -40 1.9 1.9 N8 Scat

1-142-97 100 0.4 T -40 2.9 2.9 N8 Scat

unknov\n 0.4 T 172 34.2 34.2 M4 Scat

unknov\n 0.4 T 140 28.7 28.7 M4 Scat unknown 0.4 T 140 28.3 28.3 1VI4 Scat unknown 0.4 T 104 19.9 19.9 M4 Scat unknown 0.4 T 104 21.1 21.1 M4 Scat unknown 0.4 T 77 10.8 10.8 M4 Scat unknown 0.4 T 77 12.9 12.9 M4 Scat

unknovv n 0.4 T 77 11.9 1 1.9 M4 Scat unknown 0.4 T -40 2.1 2.1 M4 Scat unknown 0.4 T -40 2.5 2.5 M4 Scat

a. The oinentation of spandrel specimens appears to be reversed because the specimen identifications (L and T) were in reference to the building, rather than the rolhng direction of the steel.

NISTNCSTAR 1-3D. WTC Investigation 187
Appendix A

A.4 DATA TABLES FOR ELEVATED-TEMPERATURE PROPERTIES

The following tables summarize the data from the elevated temperature tensile tests.

188 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

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NISTNCSTAR 1-.3D, WTC Investigation 189
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o

Q. D

0 o

c

C

w

o 'c

CJ

CO D)

nued

as

eri

ine

C7) c 0 c

o 0

o i

C/3 o _c

T3 •o0

c

CO 'cD wa Str c0 (/)" 0

im 'o O

o 0 CO o Cl "E 0 (O

ich as c0

LU! 0i_ o 0

0 c x: sp tra

(/) o *

wh erin n

=

At

CC ne 0 'cD E c 1 0 to

CO E 0 J" 0

O

NISTNCSTAR 1-3D, WTC Investigation 193
Appendix A

A.5 SUPPLEMENTAL FIGURES FOR ROOM-TEMPERATURE PROPERTIES

Figures A-1 through A-17 summarize the room-temperature stress-strain behavior of specimens taken

from the perimeter columns. Figures A-1 8 through A-26 sununarize the room-temperature stress-strain behavior of specimens taken from the core wide-flange and box columns. Figure A-27 summarizes the

behavior of the truss steels.

X

s9-c1t-s1-l1 s9-c1t-s1-12

s9-c1t-s1-l3

s9-c1t-s1-t1 s9-c1t-s1-t2

S9-C1 T-S1

WTC 1 Col 133 97-100

—1—I- 1 1 1

1 1--1—1—r-1

1— r TT'-i—r—

Specified = 36 ksi

Ifj 1/

'1/

' 1 . . . . 1 . . . 0 005 0 010 0 015 Engineering Strain

J I I I I I I I 0.00 0.05 0.10 0.15 0.20

Engineering Strain

0.25

15:01:41 Sunday September 12.2004

Figure A-1. Stress-strain curves for Fy = 36 ksi S-9 spandrel.

194 NISTNCSTAR 1-3D. WTC Investigation
Data Tables and Supplemental Figures

Figure A-2. Stress-strain curves for Fy = 45 ksi ASCE3.

Figure A-3. Stress-strain curves for Fy = 46 ksi S-14 spandrel.

NISTNCSTAR 1-3D. WTC Investigation 195
80

w 60

-4—

C/DO)

i 40

<D

<DC

c

LU 20

-|—I—I—I—I—I—I—

"T—I—I—I—I—I—

M26-C1T-RF

WTC 1 Col 131 93

m26-c1b1-rf-11 m26-c1b1-rf-l2

iTi26-c1bi-rf-!1 m26-c1b1-rf-t2

0 000 0 005 0 010 0 015 Engineering Strain J l_ ± _1_ J I I l_ 0.00 0.05 0.10 0.15 0.20 Engineering Strain

0.25

09:34:55 - Friday October 29, 2004

Figure A-4. Stress-strain curves for Fy = 50 ksi M-26.

T—I—I—I—

-1—I—I—I—i—1—I—I—I—I—I—

0

0 000 0 005 0 015 0 020

-I I I I I 1_

Engineering Strain

I I I I I I i I I 1 0.00 0.05 0.10 0.15 0.20 0.25 0.30

Engineering Strain

11:51:19 - Sunday September 12, 2004

Figure A-5. Stress-strain curves Fy = 50 ksi CC-C3T-IW.

NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

80

w 60

03

(Di_ COi 40

CDc

c

LLI 20

0

T 1—1—

r

T—I—I—

r

n I I I

n9-c1t1-rf-l2

n9-cit1-rf-t1:

N9-C1T1-RF

WTC 1 Col 155 102

.

' 1' 1

1 1 1

1

- /

: i1

F, Specified = 55 ksi

T 1

. 1 T i

; / i

•f

1

, , , . 1 , . , . 1 , . , . 0 005 0 010 0 015 Engineering Strain

J I I I I I I I J I I [_ 0.00

15:05:47 - Sunday September 12. 2004

0.05 0.10 0.15 0.20

Engineering Strain

0.25

Figure A-6. Stress-strain curves for Fy = 55 ksi N-9.

Figure A-7. Stress-strain curves for Fy = 55 ksi N-9 inner web.

NISTNCSTAR 1-3D. WTC Investigation 197
Figure A-8. Stress-strain curves for Fy = 60 ksi N-8.

80

COO)

i 40

CD

CDg^ 20

-i—I—I—n I i P I I I F

n8-c1b1-a-iw-l1

N8-C1B1-IW

WTC 1 Col 143 98

60 Specified = 60 l<si

_1_

J I 1 L

0 000 0 005 0 010 0 015 0 020 Engineering Strain

I I I I I I I I I I I I L. 0.00 0.05 0.10 0.15 0.20

Engineering Strain

0.25

15:33:43 - Sunday September 12. 2004

Figure A-9. Stress-strain curves for Fy - 60 ksi N-8 inner web.

NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

100

n99-c3t1-rf-l1 n99-c3t1-rf-l2

n99-c3t1-rf-t1 n99-c3t1 -rf-t2

T—I—I—

r

N99-C3T1-RF

WTC 1 Col 147 102

0 000 0 005 0010 0 015 0 020 0 025 Engineering Strain

0.00 0.05 0.10 0.15 0.20

Engineering Strain

0.25

15:09:41 - Sunday September 12. 2004

Figure A-10. Stress-strain curves for Fy = 65 ksi N-99.

Figure A-11. Stress-strain curves for Fy = 70 ksi C-46.

NISTNCSTAR 1-3D, WTC Investigation 199
Appendix A

100

CO

C/D 0) COc

CD

0)c

"O) c

UJ

80

60

40

20

0

-i—I—I—I—I—I—I—I—I—I—i—i—

s1-c2m-fl-2-l1 s1-c2m-fl-2-l2

s1-c2m-fl-2-t1 s1-c2m-fl-2-t2

I I I I

1 1 1 1 1 r 1 1 1 1 r 1 1 I 1 1 r 1 1

Specified = 70 ksi

- Il /'/

:/// //

y

'

, . . . 1

S1-C2M-LF

WTC 1 Col 433 80

_J I I L

0 005 0 010 0015

Engineering Strain _1 I I

I I I I L. _l I

I I I I I L. 0.00

09:47:15 - Friday October 29, 2004

0.05 0.10 0.15 0.20

Engineering Strain

0.25

Figure A-12. Stress-strain curves for Fy = 70 ksi S-1.

100

inw

CO

0)

<Dc

D)C

LU

80

60

40

20

n I I I I 1 ! r

s14-c3m-fr-1

s14-c3iTi-fr-1-l2 s14-c3m-tr-i-t1 s14-c3m-fr-1-t2

. . . . , .

- J-'

- 1

Specified = 70 ksi

. . 1 . , . , 1 . . , .

S14-C3M-RF

WTC 2 Col 218 92

0.000 0 005 0 010 0 015 0 020 Engineering Strain

I I \ I I \

\ I I 1 I \ I L 0.00

09:58:34 Friday October 29, 2004

0.05 0.10 0.15

Engineering Strain

0.20 0.25

Figure A-13. Stress-strain curves for Fy - 70 ksi S-1 4.

200 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

0.00 0.05 0.10 0.15 0.20 0.25

Engineering Strain

1536:42 -Sunday September 12, 2004

Figure A-14. Stress-strain curves for Fy = 70 ksi S-14 inner web.

Figure A-15. Stress-strain curves for Fy = 75 ksi C-22.

NISTNCSTAR 1-3D. WTC Investigation 201
Appendix A

Figure A-16. Stress-strain curves for Fy = 80 ksi C-25.

T—I—I—I—

I—I—I—I—I—

I—I—I—I—I—I—I—I—I—I—

I—I—I—I—

0,000 0 005 0.010 0 015 0.020 Engineering Strain 0 I \ I I I I I I I I I I I I I I I I I I I I I I I 0.00 0.05 0.10 0.15 0.20 0.25

Engineering Strain

10:14:18 - Friday October 29, 2004

Figure A-17. Stress-strain curves for Fy = 100 ksi C-10.

202 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

Figure A-18. Stress-strain curves for core wide-flange column C-30.

NISTNCSTAR 1-3D, WTC Investigation 203
Appendix A

Figure A-20. Stress-strain curves for core wide-flange column C-71.

Figure A-21. Stress-strain curves for core wide-flange column C-155.

204 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

80

w

o

CO

a3o

c

D)C

LU

60

40

20

HH 12WF92

WTC 1 Col 605 98-101

hh-fl-1-2-l1 flange

hh-fl-1-2-12 flange

hh-fl-1-16 flange

hh-fl-i-2-tl flanae

hh-fl-1-2-t2 flange

J I L

0 005 0 010 0015

Engineering Strain

\ < < ^ \ L. 0.00 0.02 0.04 0.06 0.08 Engineering Strain

0.10

10:57:48 - Fnday October 29 2004

Figure A-22. Stress-strain curves for core wide-flange column HH.

80 -| 1 r

c26-f2-1-1 web

_i_ _i_

0 000 0 005 0,010 0 015 0 020 Engineering Strain

_l 1 I I I I I I 1 1 I I I I L 0.00 0.02 0.04 0.06 0.08 Engineering Strain

0.10

Figure A-23. Stress-strain curves for core wide-flange beam C-26.

NISTNCSTAR 1-3D. WTC Investigation 205
Appendix A

70

60

50

COo^ 40

c

0)

CDc

"CD c

LU

30

20

10

-Ir

iange) B6152-1-F1-1-15 (flange)

y

J 1 L_

B6152-1 box column

WTC 1 Col 803 15-18

/ — — — Specified = 36 ksi

J

1 V

1 , . , , 1 , . . ,

J I L0.000 0.005 0010 0 015 Engineering Strain\ 1 1 \ I I I I 0.00 0.02 0.04 0.06 0.08

Engineering Strain

0.10

11:34:31 - Friday October 29, 2004

Figure A-24. Stress-strain curves for core box column B6152-1.

70

60

(0 ^ 50

CO

C/3 CD

CDC

'g^ 20

LU

10

B6152-2-F1-2-I1 B6152-2-F1-2-!2

B6152-2-F1-2-t1 B6152-2-F1-2-t2 B6152-2 Box column

WTC 1 Col 504 33-36

J I L. J I L

0 005 0 010 0 015 Engineering Strain

I I 1 I I I 0.00 0.02

1 1 33:05 - Friday October 29. 2004

0.04 0.06

Engineering Strain

0.08 0.10

Figure A-25. Stress-strain curves for core box column B6152-2.

206 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

Figure A-26. Stress-strain curves for core box column C-88c.

0.00 0.05 0.10 0.15 0.20 0.25

Engineering Strain

11:39:13 • Friday October 29 2004

Figure A-27. Stress-strain curves for all truss angles.

NISTNCSTAR 1-3D, WTC Investigation 207
Appendix A

A.5 SUPPLEMENTAL FIGURES FOR HIGH STRAIN-RATE PROPERTIES

Figures A-28 through A-37 summarize the stress-strain curves as a function of strain rate for perimeter

column steels. Figures A-44 through A-52 summarize the curves for core-coluinn steels. Figures A-53

through A-68 summarize the strain rate sensitivities for perimeter column steels. Figures A-69

through A-77 suminarize the strain rate sensitivities for core column steels. Figure A-78 through A-80

summarize the dependence of total elongation, El„ on strain rate. Figures A-81 through A- 103 show the

results of high strain-rate Kolsky tests.

208 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

120

M26-RF 50 ksi (Longitudinal)

(A«

10

0)

>*-• COc

0)

0)c

80-

5, 40-

c

HI

Fy strain Rate, s"^ 6.06 E -5 65

100

260

417

\ \ I 0.1 0.2 0.3

Engineering Strain, in/in

-800

-600

re Q.

-400

-200

0.4

Figure A-28. Stress-strain curves for Fy = 50 ksi perimeter column M26-C1B1-RF

longitudinal.

M26-RF 50 ksi (Transverse)

120

V)

V)

(0

O)

c

'!!0)

<Uc

80-

40-

c

liJ

Fy Strain Rate, s 6.06 E -5 99

255

316

360

0

-800

-600

re Q.

-400

-200

1 \ \ \ 0

0 0.1 0.2 0.3 0.4

Engineering Strain, in/in

Figure A-29. Stress-strain curves for Fy = 50 ksi perimeter column M26-C1B1-RF

transverse.

NIST NCSTAR 1-3D, WTC Investigation 209
Appendix A

N8-RF 60 ksi (Longitudinal)

0)

0) •I 40 H

c

Fy Strain Rate, s

1.07 E -4 90

257

277

386

n 1 \ T"

0.1 0.2 0.3 0.4

Engineering Strain, in/in

800

-600

03 Q.

-400

-200

0.5

Figure A-30. Stress-strain curves for Fy = 60 ksi perimeter column N8-C1B1-RF

longitudinal.

(A

0) cnC

100-

75-

= 50-

O

c

"5)

111 25

N8-RF 60 ksi (Transverse)

F„ Strain Rate, s 0.005

54

297

800

-600

-400

-200

0.1 0.2 0.3

Engineering Strain, in/in

0.4

Figure A-31. Stress-strain curves for Fy = 60 ksi perimeter column N8-C1B1-CRFtransverse.

210 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

150

N99-RF 65 ksi (Longitudinal)

V)w

V) 100 H

<3)

(0

0)

c

ui

T

-1000

-800

600

TO Q.

-400

-200

0 0.1 0.2 0.3 0.4

Engineering Strain, in/in

Figure A-32. Stress-strain curves for Fy = 65 ksi perimeter column N99-C3M1-RF

longitudinal.

120

N99-RF 65 ksi (Transverse)

BOCA

0)

D)

_C

a>E

o

40 H

c

LU

Fy Strain Rate, s-^ 6.06 E -5 79

270

515

-800

[-600

Q.

-400

-200

0 0.1 0.2 0.3 0.4

Engineering Strain, in/in

Figure A-33. Stress-strain curves for Fy = 65 ksi perimeter column N99-C3M1-RF

transverse.

NISTNCSTAR 1-3D, WTC Investigation 211
Appendix A

C22-FL 75 ksi (Longitudinal)

150

(A

(ft U)O

OT

S) TMc

0)c

100-

^ 50- cnc

LU

-1000

-800

-600

-400

-200

0.1 0.2

Engineering Strain, in/in

Figure A-34. Stress-strain curves for Fy = 75 ksi perimeter column C22-C2B-FL

longitudinal.

C22-FL 75 ksi (Transverse)

150

(A

to 100

(A

0)

O)

c

HI

Fy strain Rate, s"" 8.33 E -5 121

276

358

- 1000

-800

[-600

ni Q.

-400

-200

\ \ 0 0.1 0.2 0.3

Engineering Strain, in/in

Figure A-35. Stress-strain curves for Fy = 75 ksi perimeter column C22-C2B-FL

transverse.

212 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

C

LU

150

C14-SP 75 ksi (Longitudinal)

w 100-1

v)

k. (O

D)C

'Z!0)o

i 50H

0-]-

0

T

- 1000

-800

-600

(0 Q.

400

-200

0.3

\

0.1 0.2

Engineering Strain, in/in

Figure A-36. Stress-strain curves for Fy - 75 ksi perimeter column spandrel C14-SP

longitudinal.

C14-SP 75 ksi (Transverse)

1000

120-

(/)

(/T

(/) 0) *—

D)

C

'l.0) _c

LU

800

-600

400

-200

0 0.1 0.2

Engineering Strain, in/in

Figure A-37. Stress-strain curves for Fy = 75 ksi perimeter column spandrel C14-SP

transverse.

NISTNCSTAR 1-3D, WTC Investigation 213
Appendix A

M10B-RF 100 ksi (Longitudinal)

(A

0) to

.E

0)0)c

"5) c

Hi

. 120-

1200

-1000

-800

-600

-400

-200

0.05 0.1 0.15 0.2

Engineering Strain, in/in

Figure A-38. Stress-strain curves for Fy = 100 ksi perimeter column M10B-C3B1-RF

longitudinal.

M10B-RF 100 ksi (Transverse)

1000

-800

-600

-400

-200

0 0.1 0.2 Engineering Strain, in/in Figure A-39. Stress-strain curves for Fy = 100 ksi perimeter column M10B-C3B1-RF

transverse.

214 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

C10-FL 100 ksi (Longitudinal)

(A

160-

120

CO

.E 80 H

0)

0)c

"o

c

UJ 40-

Fy Strain Rate, s

^

1.1 E-4

70

241

325

433

I \ I I 0.05 0.1 0.15 0.2

Engineering Strain, in/in

1200

-900

a. -600

-300

0.25

Figure A-40. Stress-strain curves for Fy = 100 ksi perimeter column C10-C1M1-FL

longitudinal.

C10-FL 100 ksi (Transverse)

V)

V)

c

160-

120-

E 80-

0)

c

UJ 40-

1200

-900

-600

300

0 0.1 0.2

Engineering Strain, in/In

Figure A-41. Stress-strain curves for Fy = 100 ksi perimeter column C10-C1M1-FL

transverse.

NISTNCSTAR 1-3D. WTC Investigation 215
Appendix A

150

C10-IW 100 ksi (Longitudinal)

-1000

-800

-600

-400

-200

T

0.05 0.1 0.15 0.2

Engineering Strain, in/in

0.25

Figure A-42. Stress-strain curves for Fy = 100 ksi perimeter column C10-C1M1-IW

longitudinal. :

C10-IW 100 ksi (Transverse)

160-

V)

0)

k.

U)c

o

o

c

'Ui c

HI

: 120-

r 1200

-900

-600

-300

1 \ 1 r 0 0.05 0.1 0.15 0.2 0.25

Engineering Strain, in/in

Figure A-43. Stress-strain curves for Fy = 100 ksi perimeter column C10-C1M1-IW

transverse.

216 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

C65-FL 36 ksi (Longitudinal)

in

o>

(Oc

80-

60-

^40-

0)

0)c

"o) cm

20-

-400

Fy Strain Rate, s

^

8.75 E -5 81

238

^307

~1 1 \ T"

0.1 0.2 0.3 0.4 Engineering Strain, in/in

Q.

200

0.5

Figure A-44. Stress-strain curves for Fy = 36 ksi core wide-flange C65-FL longitudinal.

C65-FL 36 ksi (Transverse)

V)

(/)

COc

Q)C

Ui

c

LU

100

75-

800

-600

-400

50-

25-

-200

1 I r 0 0.1 0.2 0.3

Engineering Strain, in/in

Figure A-45. Stress-strain curves for Fy = 36 ksi core wide-flange C65-FL transverse.

NISTNCSTAR 1-3D, WTC Investigation 217
Appendix A

90

C65-WEB 36 ksi (Longitudinal)

-600

-400

~i 1 \ r

0.1 0.2 0.3 0.4 Engineering Strain, in/in

ns CL

-200

0.5

Figure A-46. Stress-strain curves for Fy = 36 ksi core wide-flange C65-WEB longitudinal.

V C65 WEB 36 ksi (Transverse)

120

(A

<fl (A

Q)

1- V)

ra

0)

0)c

'u> c

HI

80-

-800

-600

-400

40-

-200

0.1 0.2 Engineering Strain, in/in

Figure A-47. Stress-strain curves for Fy - 36 ksi core wide-flange C65-WEB transverse.

218 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

C80 36 ksi (Longitudinal)

w

oJ

(/)c

0)c

'O) c

LU

100-

80

60-

40-

20-

-600

T

800

-400

-200

0 0.2 0.4 0.6

Engineering Strain, in/in

Figure A-48. Stress-strain curves for Fy = 36 ksi core wide-flange C80-A longitudinal.

HH1-WEB 42 ksi (Longitudinal)

800

6.3 E -4 68

233

0 0.05 0.1 0.15 0.2 0.25

Engineering Strain, in/in

Figure A-49. Stress-strain curves for Fy = 42 ksi core wide-flange HH1-WEB longitudinal.

NIST NCSTAR 1-3D. WTC Investigation 219
Appendix A

HH1-WEB 42 ksi (Transverse)

80-

(AS

(0

O)

c

'!Zo

0)c 40-

'5) c

UJ

800

-600

-400

-200

1 r

0.1 0.2 0.3

Engineering Strain, in/in

Figure A-50. Stress-strain curves for Fy = 42 ksi core wide-flange HH1-WEB transverse.

B6152-FL 36 ksi (Longitudinal)

120

F„ Strain Rate, s-^

"~l T T T

0.1 0.2 0.3 0.4 Engineering Strain, in/in

-800

-600

re Q.

400

-200

0.5

Figure A-51. Stress-strain curves for Fy = 36 ksi core box column B6152-FL longitudinal.

220 NISTNCSTAR 1-3D. WTC Investigation
Data Tables and Supplemental Figures

120

B6152-FL 36 ksi (Transverse)

-800

-600

-400

-200

1 r

0.1 0.2 0.3 Engineering Strain, in/in

0.4

Figure A-52. Stress-strain curves for Fy = 36 ksi core box column B6152-FL transverse.

2.2

2.1 -

V)

c 2.0

CD W

o^ 1.9

1.8

- 1

" o

1 1 I —1

1 1 1

1

1 1 1

1

1

m=0.01 21 +0.0033 F Longitudinal

nn=0.01 1 8±0.0023 TS Longitudinal

S O -

] o ^—

.

-—-

' o

1 1 1 1 1 1 1

Perimeter column

M26-C1B1 F =62, ksi

-4 -2 0 2

160

140

120

100 i=c

0}

80

60

19:43:12 - Wednesday Febmary 09, 2005 log.io strain rate, 1/s

Figure A-53. Strain rate sensitivity for Fy = 50 ksi perimeter column M26-C1-RF

longitudinal.

NISTNCSTAR 1-3D, WTC Investigation 221
Appendix A

160

140

120

CO

100 B

c

19:43:13 - Wednesday February 09. 2005

-2 0

log-,0 strain rate, 1/s

Figure A-54. Strain rate sensitivity for Fy = 50 ksi perimeter column M26-C1-RF

transverse.

2.2 160

140

- 120

- 100 i:C

CD CO

19:43:13 - Wednesday February 09. 2005

-2 0

log-io strain rate, 1/s

Figure A-55. Strain rate sensitivity for Fy = 60 ksi perimeter column N8-C1-RF

longitudinal.

222 NISTNCSTAR 1-3D. WTC Investigation
Data Tables and Supplemental Figures

Figure A-56. Strain rate sensitivity for Fy = 60 ksi perimeter column N8-C1-RF

transverse.

Figure A-57. Strain rate sensitivity for Fy = 65 ksi perimeter column N99-C3-RF

longitudinal.

NISTNCSTAR 1-3D. WTC Investigation 223
Appendix A

Figure A-58. Strain rate sensitivity for Fy = 65 ksi perimeter column N99-C3-RF

transverse.

2.2

2.1

V)

§ 2.0

COo^ 1.9

1.8

^

1 1 1

1 1 1 r 1

1 1 1 1 1 _ O m=0.0067±0.0021 F„ Longitudinal o m=0.01 38x0.0003 TS Longitudina!

O —'

' o

. 1 1 1 1 1 1 1 L.

Perimeter column

C^2-C2M Fy=83 Ifsi -4 -2 0 2

160

140

120

100 -hc

(D

C/D 80

60

19:43:14 Wednesday February 09, 2005 log^o strain rate, 1/s

Figure A-59. Strain rate sensitivity for Fy = 75 ksi perimeter column C22-C2M-FL

longitudinal

224 NIST NCSTAR 1-3D. WTC Investigation
Data Tables and Supplemental Figures

2.2 r

2.1

o

TO ^ 1.9

1.8

^

1

1 1

1

1 1 1

1

1 1 1

1

1 O m=0.0061±0.0016 F^, Transve.rse m=0.0145±0.0038 TS Transverse

-——

- ^

' "

- o

-

"

- G-

- 1 , , , 1 , , ,

Perimeter column

C22-C2M F =83 ksi

-4 -2 0 2

160

140

120

100

cnc

0)

80

60

19:43:14 • Wednesday February 09, 2005 log^Q strain rate, 1/s

Figure A-60. Strain rate sensitivity for Fy = 75 ksi perimeter column C22-C2M-FL

transverse.

2.2

2.1 (5

c 2.0

CD W

o

d) ^ 1.9

1.8

- 1

1 1 1

1

1 1 1

1 1 1 1 1 1 0 F^, Longitudinal m=0.0109±0.001 5 TS Longitudinal

^—-o

0" —

% \ 0 0 -

> 1 1 1 1 1 1 1 L_

Perimeter column

CH-SP F,=92 ks)

,

160

140

120

CO

100 i=c

CD

(J) 80

19:43:16 • Wednesday February 09, 2005

-2 0 loQio Strain rate, 1/s

60

Figure A-61. Strain rate sensitivity for Fy = 75 ksi perimeter spandrel C14-SPlongitudinal.

NISTNCSTAR 1-3D, WTC Investigation 225
Appendix A

2.2

2.1 -

CO

cno^ 1.9

1.8

T 1 r O m=0.0083±0,0021 F,, Transverse m=0.0098±0.00l 6 TS Transverse

160

140

120

CO

100 i=O)

c

CD

80

J I L.

Perimeter column

C14-SP F =92 kS|i

19:43:16 Wednesday February 09. 2005

-2 0

log 10 strain rate, 1/s

60

Figure A-62. Strain rate sensitivity for Fy = 75 ksi perimeter spandrel C14-SP transverse.

2.2

2.1 -

CD

o

1.9

1.8

-4

1 1 r 1 1 r T 1 r O n-i=0.0053±0.0014 F„ Longitudinal

m=0.0085±0,001 3 Tfe Lonaitudmal

160

140

120

- 100

80

0) CO

J I L. _l I L

Perimeter column

M10B-C3B1 F =1,08 ksi

19 43:14 • Wednesday Febmary 09, 2005

-2 0

log^Q strain rate, 1/s

60

Figure A-63. Strain rate sensitivity for Fy = 100 ksi perimeter column M10B-C3B-RF

longitudinal.

226 NISTNCSTAR 1-3D. WTC Investigation
Data Tables and Supplemental Figures

2.2

2.1

4—* W

o

TO ^ 1.9

m=0.0039±0.0038 F^. Transverse m=0.0083±0.0010 TS Transverse

160

140

120

100 i=C

CD

80

Perimeter column

M,10B-C3B1 F =1,08 ksi

I I I LI I I -4

19:43:15 - Wednesday February 09, 2005

-2 0

log.io strain rate, 1/s

60

Figure A-64. Strain rate sensitivity for Fy = 100 ksi perimeter column M10B-C3B-RFtransverse.

2.2

2.1

CO

cnc 2.0

CD W

o

TO ^ 1.9

-|

1 I O m=0,0044±0.0014 F Longitudinal m=0.0080±0.0026 TS Longitudinal

-J I L.

160

140

- 120

0)

100 -B

CDC

CD

80

Perimeter column

C10-C1M F =110, ksi

19:43:15 • Wednesday February 09. 2005

-2 0

loQio Strain rate, 1/s

60

Figure A-65. Strain rate sensitivity for Fy = 100 ksi perimeter column C10-C1M1-FL

longitudinal.

NISTNCSTAR 1-3D. WTC Investigation 227
Appendix A

2.2 F

2.1 -

S

2.0

^ 1.9

1.8

1—'—'—'—

O F„ Transverse

ife Transverse

n I F

o

o

160

140

120

100

- 80

_J I L

Perimeter column

C10-C1M-IW F =110 ksi

C

0

C/D

19:43:16 - Wednesday February 09. 2005

-2 0 log^o strain rate, 1/s

60

Figure A-66. Strain rate sensitivity for Fy = 100 ksi perimeter column C10-C1M1-FL

transverse.

2.2 F^—

2.1

CO

g

2.0

•I—

cn^ 1.9

1.8

1 1 r 1 1 r O m=G.0077±0.0033 F., Longitudinal

=0.0113±0,0006 TS Longitudii

_i I [_ _l I L

160

140

- 120

Perimeter column

C10-C1M-IW F =110 ksi

100

C

CD CO - 80

19:43:16 - Wednesday Febmary 09. 2005

-2 0 log^o strain rate, 1/s

60

Figure A-67. Strain rate sensitivity for Fy = 100 ksi perimeter column C10-C1M1-IWlongitudinal.

228 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

2.2

2.1

i

2.0

oO

1.9

T 1 r -1 1 r O F, Transverse o TS Transverse

o

o

-I I L.

Perimeter column

C10-C1M-IW F =110 ksi

I I

' ' I I i__

160

140

120

(0

100 i=c

80

19 43 16 - Wednesday February 09. 2005

-2 0

log^o strain rate, 1/s

60

Figure A-68. Strain rate sensitivity for Fy = 100 ksi perimeter column C10-C1M1-IW

transverse.

2.0

C/3

CDC

§^1.6

1.4

1 - o

D

1 1 I [

1 1 1

1

1 1 1 1- m=0.0290±0.0042 F., Longitudinal m=0.01 3010.001 4 Tfe Longitudinal

1

—1 o

o

o

— '"Q^

§

O

1 1 1 1 1 1 1 1

Core column

C^SJIange f^=3^ ksi

-4 -2 0 2

100

80

CO 60 ^

c0

40

19:48:00 - Wednesday February 09. 2005 log.io strain rate, 1/s

Figure A-69. Strain rate sensitivity for Fy = 36 ksi core wide-flange C65-FL longitudinal.

NISTNCSTAR 1-3D. WJC Investigation 229
Appendix A

2.0

CO 1.8 c0

COoO 1.6

1.4

-| 1 r n I r O F Transverse TS Transverse

_l I L.

Core column

C^SJIange Fy=3:^ ksi

100

80

V) 60 ^

c

CD

40

-4

19:48:01 - Wednesday February 09, 2005

-2 0

log.iQ strain rate, 1/s

Figure A-70. Strain rate sensitivity for Fy - 36 ksi core wide-flange C65-FL transverse.

2.0

CO 1.8 cnc

CD cn

1.4

1 1 r O m=0.0336±0.0030 F Longitudinal m=0,0207±0.0010 TS Longiludinai

J I L

Core column

C^5_web Fy=31 Ifsi

19 47;59 - Wednesday February 09, 2005

-2 0

log.io strain rate, 1/s

100

80

60

c

CD

40

Figure A-71. Strain rate sensitivity for Fy = 36 ksi core wide-flange C65-WEB longitudinal

230 NISTNCSTAR 1-3D. WTC Investigation
Data Tables and Supplemental Figures

2.0

' 1.8 c

§^1.6

1.4

[

1 1 1

[

1 1 T" ~ O F Transverse o TS Transverse ° °o :

oo - o

Core column

1 1 1 1 1 1 1 1 C^5_web Fy=31 Ifsi

19:47:59 • Wednesday Febnjary 09. 2005

-2 0

log.,Q strain rate, 1/s

100

80

CO 60 ^

C

(D

40

Figure A-72. Strain rate sensitivity for Fy = 36 ksi core wide-flange C65-WEB transverse.

2.0

1.8 C

(U

o

§^1.6

1.4

-1 1 r O m=0.0317±0.0027 F Longituciinal o m=0.0175±0.0039 TS Longitudinal

_l I L. _l I L

Core column

C8C1 F,=34 ksi

19:47:55 • Wednesday Febmary 09, 2005

-2 0

log.io strain rate, 1/s

100

80

CO 60 ^

c

CD

- 40

Figure A-73. Strain rate sensitivity for Fy = 36 ksi core wide-flange C80 longitudinal.

NISTNCSTAR 1-3D. WTC Investigation 231
Appendix A

2.0 -

CO 1.8 c

CO

8^1.6

1.4

O m=0.0265±0.0014 F^, Longitudinal m=0. 0200+0. 0058 TS Longitudinai

J I L.

Core column

HH_web F =52 ksi

I I Ul I I

100

80

60 ^

C

CD

(f) 40

19:48:02 - Wednesday February 09, 2006

-2 0 log 10 strain rate, 1/s

Figure A-74. Strain rate sensitivity for Fy = 42 ksi core wide-flange HH1-WEB

longitudinal.

2.0

1.8

CO

I—

O)

c

CD

i_ CO

8^1.6

"1

'—'—'—

O F,, Transverse

Tfe Transverse

1 1 r n 1 r

1.4 I L

-4

19:48 02 - Wednesday February 09, 2005

Core column

HH_web F =52 ksi __] '

I V I I

100

80

CO 60 ^

05c

CD CO 40

-2 0

log.,0 strain rate, 1/s

Figure A-75. Strain rate sensitivity for Fy = 42 ksi core wide-flange HH1-WEB transverse.

232 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

2.0

CO

c

(U

o

1.6

1.4

1 1 1 1 1

1 1 1 1 1 —I 1

1 1 - O m=0.0239±0.0038 F^, Longitudinal -

m=0.01 65+0.001 2 TS Longitudinal

-

- o

/S ' o

1 1 1 1 1 1 1 L

Core column

B^152-2 Fy=38 k^i

-4 -2 0 2

100

80

60 ^

C5) C

CD

40

19:47:57 - Wednesday February 09. 2005 log^o strain rate, 1/s

Figure A-76. Strain rate sensitivity for Fy = 36 ksi box column B6152-FL longitudinal.

2.0

c

w

o

8^1.6

1.4

\ I i O m=0.0396±0.0009 F Transverse m=0.0196±0.001 7 TS Transverse

J I L. J I L

Core column

B^152-2 Fy=38 k^i

100

80

CO 60 ^

C

CD

40

-4

19:47:58 • Wednesday February 09, 2005

-2 0

log.,0 strain rate, 1/s

Figure A-77. Strain rate sensitivity for Fy = 36 ksi box column B6152-FL transverse.

NISTNCSTAR 1-3D. WTC Investigation 233
Appendix A

50

40

c

o

* F=KX) k-«i e.=( 1.98+0.23) MIOB-C^B-RlA F,= IOO ksi e|=(1.24±0.07) ClO-ClM-LF

I + F;= 75 k,sie,=(l. 27+0.1 1)C14-SP

20

10

50

40

-r T-

.2 -"^0

c

O

20

10

T

1 ' \ rTolal Elongation

El =e„+e|log,(dE/dt)

-2 0

log,() Strain rate, 1/s

T 1 r

F -iOO ksi c.=(0.86±0.18) M10B-C3B-RF

F,= 1 00 k.si e,=( 1 .20±0.0 i ) CI 0-C 1 M-LF

perimeter steeks longitudinal orientation

n.HM)!' - Wediicsduy December 15, 2(1114

-2 0

logjo strain rate, 1/s

Figure A-78. Dependence of total elongation, £/,, on strain rate for high-strength

perimeter columns.

234 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

-1

I

I t I

\ I t 1 p

O F, =50 ksi r,=(0.75±0.33) M26-C1 B-RF A F,=60 ksi ri=(1.92±0.21) N8-C1B-RF

0 F,=65 ksi r,=(0.85±0.11 f N99-C3T-RF • f;=75 ksi r,=(0.22±0.18) C22-C2M-LF

Reduction of area ROA = ro+r,log^(de/dt)

-2 0 loQio Strain rate, 1/s

T 1 1 1 \ 1 1 r T 1 r

60

<

o

40

F„=50 ksi r,=(-1.10±0.30) M26-C1B-RF

F'=60 ksi N8-C1B-RF

F.=65 ksi r,=(-2.64±0.85) N99-C3T-RF

F'=75 ksi r,=(-1 .9310.57) C22-C2M-LF

perimeter steels longitudinal orientation

_l I 1 I L. I t I I

18:40.29 - Thursday October 28 2004

-2 0 loQio Strain rate, 1/s

Figure A-79. Dependence of total elongation, £/,, on strain rate for low-strength

perimeter columns.

NISTNCSTAR 1-3D. WTC Investigation 235
Appendix A

50

40

20

10

T 1

1 1 1

1 1 1 1 p- A P =36 ksi e,=(+0.83±0.27) B61 52-2 0 r,,=:36 ksi e.=--{+0.5Q±" .37) C65-weD • F =36 ksi ei=(+0.62±0.56) C65-flange

F,=42 ksi e, ={-0.1 9+0.02) HH-web

—' '

\

' Elongation

El, = eo+e,logp(dE/dt)

A

A

.2 30

D5C

o

CD

J I L.

all Flat-HSR 1" specimens with t=0.0625 in. A"^/Lo = 0.125

core steels longitudinal orientation

J I I I I I I I I I

14 19 20 - Friday December 17. 2004

-2 0

log^o strain rate, 1/s

Figure A-80. Dependence of total elongation, Elt, on strain rate for core columns.

03 D.

c/)

00

CD

13

1200

1000

800

600

400

200

-1 1 1

1

1 r — True stress

- - True strain rate

1 1

1 1 1 r —1

1

1 \ 1 1

test 434 C22-C2M-FL3

WTC 1 Col 157 93-96

J I I I I I L. 0.0 0.1

1 7:21 25 . Monday December 20, 2004

0.2 True strain0.3

4000

3000

cd' CO

2000 §COCD

I- - 1000

0

0.4

Figure A-81. Kolsky-test stress-strain behavior for Fy = 75 ksi perimeter column

C22-C2M.

236 NISTNCSTAR 1-3D. WTC Investigation
Data Tables and Supplemental Figures

CD D.

(0

-—

w

CD

13

1200 -

1000

800 -

test 4k C22-C2M-FL3

WTC 1 Col 157 93-96

600

400 f

200

0.0 0.1

17:21:26 - Monday December 20, 2004

0.2 True strain

4000

3000

c5

2000 2

CO0)

1000

0.4

Figure A-82. Kolsky-test stress-strain behavior for Fy = 75 ksi perimeter column

C22-C2M.

1200 -

T 1 1 11 1 r True stress True strain rate

0.0 0.1

17:21:28 - Monday December 20, 2004

1

1 I I I I test 436 C22-C2M-FL3

WTC 1 Col 157 93-96

_L 0.2 True strain

0.3

4000

3000

•4—

CD

2000 2

COo

3

1000

0.4

Figure A-83. Kolsky-test stress-strain behavior for Fy = 75 ksi perimeter column

C22-C2M.

NISTNCSTAR 1-3D. WTC Investigation 237
Appendix A

C5 CL

w

0)

•4—

(/)

13

1200

1000

800

600

400

200

=—1

1 1 1

1

1 1 1 1 1

1 1 r True stress

1 11 1 1 1 1 test 4J/ \jdd-'^dm-r\-6 True strain rate WTC 1 Col 157 93-96

'

-

-

- /^^

- /

\ -/—

-1

\ J

1

1

\

\

t 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0.0 0.1

17:21:28 • Monday December 20, 2004

0.2 True strain0.3

4000

3000

CO

2000 %

CD3

I- 1000

0.4

Figure A-84. Kolsky-test stress-strain behavior for Fy = 75 ksi perimeter column

C22-C2M.

CT3 CL

CO

0)

00

CD3

1200

1000

800

600

400

200

0

T 1 1 1

1

1 1 r

True stress True strain rate —1—T

s

^ \ ^ f test 438 C22-C2M-FL3 ' WTC 1 Col 157 93-96

J I 1 I I I i_ 0.0 0.1

17,21 29 - Monday December 20 2004

0.2 True strain

0.3

4000

3000

CO

"cD

2000 2

CO

CD3

1000

LIO

0.4

Figure A-85. Kolsky-test stress-strain behavior for Fy = 75 ksi perimeter column

C22-C2M.

238 NISTNCSTAR 1-3D. WTC Investigation
Data Tables and Supplemental Figures

0.0 0.1 0.2 0.3 0.4 True strain

17:21:30 Monday December 20. 2004

Figure A-86. Kolsky-test stress-strain behavior for Fy = 75 ksi perimeter column

C22-C2M.

Q_

CO(0

<D

.*—

0)

0)

13

1200 -

1000

800 -

600

400 -

T 1 1 1

1

1 1 1 1 p

True stress True strain rate

200

test 440 C22-C2M-FL3 " WTC 1 Col 157 93-96

J I I I

' I

I I I I I I I I I L 0.1 0.2 True strain

0.3

4000

3000

CO

0)

2000 2

O

Z3

1000

0.4

17:21:31 - Monday Decen*er 20. 2004

Figure A-87. Kolsky-test stress-strain behavior for Fy = 75 ksi perimeter column

C22-C2M.

NISTNCSTAR 1-3D. WTC Investigation 239
Appendix A

CO Cl

COO

3

1200

1000

800

600

400

200

1 1 1 1

1 1 1 r

True stress True strain rate •

-n 1>—I—I

1 f test 441 C22-C2M-FL3 " WTC 1 Col 157 93-96

r//

I I I I I I 1 I 0.0 0.1

17:21:31 - Monday December 20. 2004

0.2 True strain0.3

4000

3000

C3

2000 S(J) 3

- 1000

0.4

Figure A-88. Kolsky-test stress-strain behavior for Fy = 75 ksi perimeter column

C22-C2M.

03 Q_

CO

0)

CO

CD

Z3

1200 -

1000

800 -

600 -

400 -J

200

0.2 True strain

17:21:32 - Monday December 20. 2004

Figure A-89. Kolsky-test stress-strain behavior for Fy = 60 ksi perimeter column N8-C3M.

240 NISTNCSTAR 1-3D. WTC Investigation
Data Tables and Supplemental Figures

(0 CL

00

1200 -

1000

800 -

test 4^1 N8-C3M-FR2

WTC 1 Col 141 97-100

600

400

200

0.0 0.1

17:21:32 - Monday December 20. 2004

0.2 True strain

4000

3000

c5

2000 2

CO

I- 1000

Figure A-90. Kolsky-test stress-strain behavior for Fy = 60 ksi perimeter column N8-C3M.

1200 -

1000

CO

(0

o

800

600

400

200

I I

I

I \—I I p — True stress True strain rate

I I I 1 I 1

—T > > ' > r test 452 N8-C3M-FR2 " WTC 1 Col 141 97-100

'

_l 1 L. 0.0 0.1

17:21 :33 • Monday December 20, 2004

0.2 True strain

0.3

4000

3000

CO B

c5

2000 ^m

CD

15

- 1000

Lio

0.4

Figure A-91 . Kolsky-test stress-strain behavior for Fy = 60 ksi perimeter column N8-C3M.

NISTNCSTAR 1-3D, WTC Investigation 241
Appendix A

CL

CO

CO

0)

Zi

1200

1000

800

600

400

200

0

—1

1

1

1 1 r

True stress True strain rate

n I r —I 1 1—I

1 1

test 453 N8-C3M-FR2

WTC 1 Col 141 97-100

/

J I I I !_) I I I I I I I I L. 0.0 0.1

1 7:21 :34 - Monday December 20. 2004

0.2 True strain

0.3

4000

- 3000

J—I—U 0

CO

(D 4—

2000 2

(0

0)

1000

0.4

Figure A-92. Kolsky-test stress-strain behavior for Fy - 60 ksi perimeter column N8-C3M.

1200 -

1000

C/3

Q)

CO

0)

13

800

600

400

200

^ I

I

I I r

True stress True strain rate

T I r p-T \ 1

\ r test 454 N8-C3M-FR2

WTC 1 Col 141 97-100

I I

J I I I I I I I I I I I I I I I L

4000

3000

V)

0)o5

2000 5CO

CD

1000

0.0 0.1 0.2 0.3 True strain

Ll 0

0.4

1 7:21 35 - Monday December 20. 2004

Figure A-93. Kolsky-test stress-strain behavior for Fy = 60 ksi perimeter column N8-C3M.

242 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

Figure A-94. Kolsky-test stress-strain behavior for Fy = 60 ksi perimeter column N8-C3M.

Figure A-95. Kolsky-test stress-strain behavior for Fy = 60 ksi perimeter column N8-C3M.

NISTNCSTAR 1-3D. WTC Investigation 243
Appendix A

1200 -

1000

n3 Cl

c/) CO

•4—

W

0)

800

600

400

200

T 1 1 11

1 1 1 1 11 1 r True stress True strain rate

/ \

-1 1

1 I I test 457 N8-C3M-FR2

WTC 1 Col 141 97-100

J I I I 1 I I I I I I I I I I I I 1 L.

4000

3000

B

2000 ^

w

CD3

1000

0.0 0.1

17:21 43 • Monday December 20, 2004

0.2 True strain0.3 0.4

Figure A-96. Kolsky-test stress-strain behavior for Fy - 60 ksi perimeter coiumn N8-C3M.

1200

1000 -

Q_

to

0)

13

800

600 -

400

200 -

4000

- 3000

0)(0

- 2000 5

0)

Z5

- 1000

1 7.21 44 - Monday December 20, 2004

0.2 True strain

Figure A-97. Kolsky-test stress-strain behavior for Fy = 60 ksi perimeter column N8-C3M.

244 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

(0 Q.

(/} CO

CD -—

•

w

0)

1200

1000

800

600

400

200

0

~i 1

1

1 1 1 r

True stress True strain rate

7

1111

0.0 0.1

17:21:46 - Monday December 20. 2004

-T T 1 1 r—

test 324 A 325 bolt unknown location 4000

3000

2000

1000

0)COc

"cO

•4—

'

(00

3

± J I I L 0.2 True strain

0.3 0.4

Figure A-98. Kolsky-test stress-strain behavior for A 325 bolt.

0.0 0.1 0.2 0.3 0.4 True strain

1 7:21 49 • Monday December 20. 2004

Figure A-99. Kolsky-test stress-strain behavior for A 325 bolt.

NISTNCSTAR 1-3D. WTC Investigation 245
Appendix A

1200 -

1000

CL

C/3 if} CD

1

CD3

800

600

400

200

0

T 1 1 11 1 r True stress True strain rate

1 1 11 1 r test 3^6 A

unknown location —>—I—

325 bolt

- ( . /

r / \ I

J I I L I I I I i I I I I I I I I I l_

4000

3000

CD

CO

2000 5CO

CD

=3

1000

0.0 0.1

17:21 50 - Monday December 20. 2004

0.2 True strain

0.3 0.4

Figure A-100. Kolsky-test stress-strain behavior for A 325 bolt.

1200

1000 -

T 1 1 I 1 r

True stress True strain rate

CO Q.

CO

CD

CD

800

V) 600 -

test 327 A 325 bolt unknown location

400

200 L

I I I I 0.0 0.1

17:21:52 - Monday December 20, 2004

0.2 True strain

0.3

4000

3000

"co

H 2000 t

m

CD

I- - 1000

0.4

Figure A-1 01 . Kolsky-test stress-strain behavior for A 325 bolt.

246 NISTNCSTAR 1-3D. WTC Investigation
Data Tables and Supplemental Figures

Q.

CO(0o

^—

•

w

CD

=3

1200

1000

800

600

400

200

0

1 1 1 1 1

1 1 1 1 1

1 1 r True stress

1 1 1 1 1 1

1 test 328 A 325 bolt

" True strain rate unknown location

-

/ I / !

-

-

- tv^ I - \\ \ i \ '

r

I

1 1 1 1 1 1 t 1 1 1 1 1 1 1 1 1 1 1 1 1

4000

3000

2000

1000

COc

'cO

COCD

i_ I0.0 0.1

1 7:21 :54 - Monday December 20. 2004

0.2 True strain

0.3 0.4

Figure A-102. Kolsky-test stress-strain behavior for A 325 bolt.

Figure A-103. Kolsky-test stress-strain behavior for A 325 bolt.

NISTNCSTAR 1-3D. WTC Investigation 247
Appendix A

A.7 SUPPLEMENTAL FIGURES FOR ELEVATED-TEMPERATURE

PROPERTIES

Figures A- 104 through A-1 12 summarize the high-temperature stress-strain behavior of all steels tested.

1 00

I

1 1 1 1

1

1 1 1 1

1

1 11 r

annotations refer to individual specimen numbers

0 ' I I I I 1 I I I I I \ 1 I I I I I I I 0.0 0.1 0.2 0.3 0.4 Engineering Strain

10:57:55 - Friday Octobei 08, 2004

Figure A-1 04. Perimeter column Fy = 60 ksl N8-C1B-LF.

1 00 I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I

t—I—I

0.0 0.1 0.2 0.3 0.4 Engineering Strain

12:00 18 - Frrday Oclober29. 2004

Figure A-1 05. Perimeter column Fy = 60 ksi C40-C2M-IW.

248 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

1 I I I

I

I I I I

I

!

I 1 I

I

I I I I

I

I I I I—

I—I—I—I—I—

I—I—I—I—

r

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Engineering Strain

1733:31 - Fnday October 08. 2004

Figure A-106. Perimeter column Fy = 100 ksi C10-C1M-FL.

T

' 1 1 1

1

1 1 1 1

1

1 1 1 1

1

1 r

Q I I I I I I I I 1 1 I I I I I I I I I I I 1__L 0.0 0.1 0.2 0.3 0.4 Engineering Strain

12:03:37 - Fnday October 29. 2004

Figure A-107. Core wide-flange column Fy - 36 ksi C65-FL longitudinal.

NISTNCSTAR 1-3D. WTC Investigation 249
Appendix A

I I I I I I ,, I I I , I I I I I I I I I 0.0 0.1 0.2 0.3 0.4 Engineering Strain

12:04:46 Friday October 29. 2004

Figure A-108. Core wide-flange column Fy = 36 ksi C80.

HH-FL-1 WTC1 605 98-101 0 I I I I I I I I I I I

I I I I I

I I I I I I I I ' 0.0 0.1 0.2 0.3 0.4 0.5

Engineering Strain

1 1 :51 :49 - Sunday September 12, 2004

Figure A-109. Core wide-flange column Fy = 42 ksi HH.

250 NISTNCSTAR 1-3D, WTC Investigation
Data Tables and Supplemental Figures

1 1

1

1 1 1 r

#7w4; 20 °C

#2w7; 400 °C

#1n3: 500 C

#4n3: 600 °C

600 °C

#6w1; 650 °C

CI 32

2 in. X 1 .5 in. truss angle

_/ I I I I I I I I I L _I I I L 0.0

12:05:57 - Fnday October 29. 2004

0.1 0.2 0.3 Engineering Strain0.4

Figure A-1 1 0. Truss 2 in.x1 .5 in.x0.25 in. angle Fy - 50 ksi C1 32.

n I ! I I I n 1 1 1 r

#2-5w; 300 C

#1-6n; 20 °C

#3-6; 400 °C

C53

3 in. X 2 in, truss angle

#1-3w; 600 C

0 I I I I I I I L. #2-4w; 650 C

J I I I I I I I I 0.00 0.05 0.10 0.15

Engineering Strain

0.20 0.25

Figure A-1 1 1 . Truss 3 in.x2 in.x0.37 in. angle Fy - 36 ksi.

NISTNCSTAR 1-3D. WTC Investigation 251
Appendix A

1—I—1—I—

"1 1 I I

I

I I I I

I r~T I r

#1254-2: 400 C

#1254-1; 20 °C

#1255Y-1; 20 °C -

#1255Y-2: 400 °C

1254 prepared by AWS A5.20-69 255Y prepared by AWS A5.1-69

#1255Y-3; 650 °C

#1254-3; 650 °C

J I I I I I I I I I I I i_ 0.0 0.1

12:08:40 - Friday October 29, 2004

0.2 0.3 Engineering Strain0.4 0.5

Figure A-112. Contemporary all weld metal specimens.

252 NISTNCSTAR 1-3D, WTC Investigation
Appendix B

Effects of Deformation of Wide-Flange Core Columns on

Measured Yield Strength

B.1 INTRODUCTION

Several of the stress-strain curves of specimens taken from wide-flange core columns differ significantly

between the webs and the flanges. In particular, the stress-strain cur\'es of specimens from the flanges of

C-65, C-71. and C-155 and the webs of C-65 do not exhibit yield point or yield point elongation behavior.

The yield strengths, YS, of specimens from the flanges of C-71 and C-155 are also much larger than those

from the corresponding webs. Both effects are evidence of significant prior deformation.

Although National Institute of Standards and Technology (NIST) made every effort to sample the least- deformed material in core column, the condition of the core columns made it necessary to take material

that was slightly deformed. Measurements of the shapes of the columns in their as-recovered state can be

used to estimate the level of strain in tensile specimens harvested from those columns and characterized in the Investigation. This appendix summarizes this analysis.

B.2 analysis

Figure B-1 defines the variables that this analysis uses. In a typical wide-flange column, the moment of

inertia about the .v-x axis is about three times as large as the moment of inertia about the v -v axis. To a

first approximation, a column will buckle about the axis with the lower moment of inertia; the major

deformation will be about the v-v axis. The deformation of a wide-flange shape bent about the x-x axis is called camber, while the deformation about the>'-v is called sweep, see Fig. B-2.

If the column bends around the y-y axis, the strain, £,., at any point in the flange is

f,, =- B-8

R

where is the distance from the neutral axisj-v to the specimen position, and R is the radius of curvature

the beam is bent to. Bending about the y-y axis produces much less strain in the web of the column

because the neutral axis runs down the centerline of the web. Specimens from the web originate only a fraction of an inch from the neutral axis, in contrast to specimens from the flanges, which originate up to

/, = 6 in. from the neutral axis.

In practice, it is difficult to measure the radius of curvature, R. Instead, it is easier to measure the camber,

/?,, and sweep, /?, , over a length /, of the column in the region where the tensile specimens originated. The

radius of curvature, R, and the sweep, hy, are related geometrically:

NISTNCSTAR 1-3D. WTC Investigation 253
Appendix B

R =

h + — Z 4.

Vi.

B-9

Equation B-2 assumes that the radius of curvature, R, is much larger than the thickness of the piece. In

practice, the sweep of a buckled column will usually be much larger than the camber, because the

moment of inertia about the x-x axis is smaller.

Figures B-3 to B-6 are photo montages of the core coluinns C-30, C-65, C-71, and C-155 that describe

the overall condition and specific origins of tensile test specimens. Table B-1 summarizes the important

dimensions of these four columns. Table B-2 summarizes the measured sweeps, /?, , of the flanges and

webs and their resulting strains. The sweep of column C-30 is indistinguishable from zero. The curvatures of columns C-71 and C-155 were verified by measuring the sweep of the flange on both sides of the web.

As expected, it was positive on one side and negative and of approximately equal magnitude on the other. The large uncertainty in the sweep of the column, +/-0.02 in., arises from the rather irregular shape of the

edge of the flanges, which were presumably damaged in the collapse and subsequent recovery.

Table B-3 summarizes the measured cambers of the four core columns. The cambers of the flanges of

columns C-30, C-71, and C-155 are indistinguishable from zero. Although the sweep of column C-65 was

indistinguishable from zero, its camber is measurable. Fig. B-^. Table B-3 also contains an entry for the

deformation of the web of C-65. Its shape does not correspond to either camber or sweep. Instead, the

shape of its web, Fig. B-4, is consistent with the entire web having buckled.

From the measured sweep, /?, , it is possible to estimate the strain, 8, ,, in the flange specimens using

Equations B-1 and B-2. Table B-2 summarizes this infonnation in the column labeled "£,., shape."' The

strain in the flange test specimens is about 1 percent in columns C-71 and C-155, where the sweeps could

be measured. Based on the sign of the sweep on the side of the flange from which the test specimens

originated, the strain is tensile in specimens from both columns. Table B-2 also summarizes the yield

point behavior of the webs and flanges of all four columns. The strain estimated from the sweep of the

flanges of C-71 and C-155 is certainly sufficient to remove the flange yield point behavior, and is the same magnitude as the typical yield point elongation. The flange tests of C-71 and C-155 do not exhibit

any obvious yield point elongation behavior, consistent with the strain calculated from the sweep of the

flange. Column C-30, which has no measurable sweep or camber, exhibits yield point behavior in both

flange and web tests.

254 NISTNCSTAR 1-3D, WTC Investigation

B.3 RESULTS

B.4 DISCUSSION
Effects of Deformation

The sweeps reported in Table B-2 are roughly five times larger than the maximum sweep permissible in ASTM A 370. which is

h""'' =0.125 in. V

/ 120 in. B-10

The maximum allowable sweep and camber in A 370 is smaller than the smallest sweep measurable on

the core columns, due to their short length and poor edge condition.

Both flange and web test specimens for column C-65 originated in deformed material. Because the

deformation for these specimens is predominantly camber, rather than sweep, the magnitude of the prestrain depends sensitively on the exact location of the specimen in the original billet. There were two

types of flange specimens for column C-65. Some were 0.25 in. diameter rounds whose centerlines lay

roughly at the 1 4-depth point of the flange. The others were 0.5 in. diameter rounds whose centerlines lay

roughly on the mid-depth of the flange. Because of the camber of the flange, both types of specimens

have non-uniform pre-strain across their widths. The deformation is probably sufficient to remove any

yield point behavior, however.

It is also possible, neglecting any contribution of the Bauschinger effect, to roughly estimate the strain in the flange specimens of C-71 and C- 155 by assuming that the web represents the undeformed behavior.

This is a coarse assumption because the flanges typically lower yield strength. The flange strains predicted from the web stress-strain behavior are approximately 4 percent and 2 percent for C-71 and

C-155 respectively.

B.5 SUMMARY

The magnitudes of the prior strains estimated from the tensile data are similar to those estimated from the

measured sweep of core columns C-71 and C-155. It is likely, therefore, that the elevated yield strength

and lack of yield point behavior in the flanges of these two columns arises from the prior deformation of

the column. The deformation of column C-65 is sufficient to remove any yield point behavior from the

tensile specimens cut from it.

NISTNCSTAR 1-3D. WTC Investigation 255
Appendix B

3-1. Locations and dimension informaltion for wide-f ange core co

Specimen Shape Location

d

(in.) (in.) (in.) (in.) A

(in.) C-30 14WF237 WTC 2 Col. 1008

Fl. 104-106

16.12 15.91 1.09 1.75 5.0

C-65 12WF161 WTC 1 Col. 904

Fl. 86-89

13.88 12.52 0.91 1.49 2.3

C-71 12WF190 WTC 1 Col. 904

Fl. 79-80

14.38 12.67 1.06 1.74 4.0

C-155 12WF161 WTC 1 Col. 904

Fl. 83-86

13.88 12.52 0.91 1.49 4.0

Note: See Fig. B-1 for definitions of symbols.

Table B-2. Stress-strain behavior and sweep of core co umns.

Web Flange Flange Web

Fy £,,,, £,, K R

Specimen (ksi) (ksi) Shape" Test" (in.) (in.) (in.) (in.) C-30 41.9 (YP) 39.6 (YP) <0.004' n/a <0.02 <0.02 15 > 1,400'

C-65 31.1 (NYP) 32.4 (NYP) <0.0004' n/a <0.03 n/d 36 >5,400'

C-71 33.2 (YP) 53.4 (NYP) +0.011 0.04 0.09 0.09 16 380

C-155 42.2 (YP) 50.9 (NYP) +0.012 0.02 0.13 0.11 18 320

a. Estimated from sweep of flange, /?, , and location of specimens, /, using Eqs. B-1 and B-2.

b. Estimated from stress-strain behavior of flange, relative to the web.

c. Uncertainties in /?, are approximately +/-0.02 in. d. / is the length of the flange over which the sweep is measured, Fig. B-1 13.

e. Estimated from minimum measurable sweep, /?,

Key: n/a, not applicable; n/d, not determined; NYP, no yield point behavior in stress-strain curve; YP,

yield point behavior in stress-strain cur\'e.

256 NISTNCSTAR 1-3D, WTC Investigation
Effects of Deformation

Table B-3. Stress-strain behavior and camber and deformation of core columns.

Web Flange Flange

Fy Measurement '"A Specimen (ksi) (ksi) Type (in.) (in.) C-30 41.9 (YP) 39.6 (YP) flange <0.02 15 C-65 31.1 (NYP) 32.4 (NYP) flange 0.17 36

C-65 31.1 (NYP) 32.4 (NYP) web 0.38 9 C-71 33.2 (YP) 53.4 (NYP) flange <0.015 16 C-155 42.2 (YP) 50.9 (NYP) flange <0.015 18

a. Uncertainties in /?, are approximately +/-0.02 in. b. / is the length of the flange over which the camber is measured. Key: NYP, no yield point behavior in stress-strain ciir\'e; YP. yield point behavior in stress-strain curve.

NISTNCSTAR 1-3D, WTC Investigation 257
Appendix B

Source: NIST. Figure B-3. Montage of specimen C-30 that shows the overall state of the column and the

region that provided the test specimens.

258 NISTNCSTAR 1-3D. WTC Investigation
Effects of Deformation

Source: NIST. Figure B-4. Montage of specimen C-65 that shows the overall state of the column and

the region that provided the test specimens.

NISTNCSTAR 1-3D, WTC Investigation 259
Appendix B

1 6 in

C71-1-VV1

. 4 in.

3 in. C71-1-F1

All tension specimens originated

specimen C71-1-F2, which came from

flange (hidden by web above) with

tensile strain. Note that the curvature

of the specimen area is opposite to the

general curvature of column.

C71-1-F1

C71-1-F2

C71-1-.W1

Source: NIST. Figure B-5. Montage of specimen C-71 that shows the overall state of the column and the

region that provided the test specimens.

260 NISTNCSTAR 1-3D, WTC Investigation
Effects of Deformation

Source: NIST. Figure B-6. Montage of specimen C-1 55 that shows the overall state of the column and

the region that provided the test specimens.

NISTNCSTAR 1-3D, WTC Investigation 261
Appendix B

This page intentionally left blank.

262 NISTNCSTAR 1-3D, WTC Investigation
Appendix C

Provisional Analysis of High-Rate Data

Early in the Investigation, before testing was complete, it was necessary to provide high-strain-rate data to other Investigation Team members modeling the aircraft impact on the building. The accelerated

schedule required that the model for the data be developed on a provisional set of the steels. This

provisional data set includes the six steels listed in Table C-1. The reported values in Appendix A differ

shghtly from the provisional values used in the provisional analysis because the provisional analysis did

not use the ESIS procedure for data reduction.

For use in the finite element model, the data had to be expressed in terms of the Cowper-Symonds

equation: —=i-t6

C-1

The Cowper-Symonds equation describes the increase in flow stress with strain rate using two

parameters. C and p. In the Cowper-Symonds equation s is the strain rate, a is the stress (yield, tensile, or other stress) and ao is the reference stress. Usually, Gq is the stress measured in a low-rate tensile test. The present analysis uses the yield strength.

Equation C-1 can be rewritten as ^^= s =

0 /

' s

^ C

C-2

Taking the natural logarithm of each side produces an equation linear in the Cowper-Symonds

parameters:

log^ S = -log, ^ - -log, C C-3

P P

The value of the stress at the lowest strain rate, Oo, fixes the overall strain rate behavior, but this value

cannot be included in a fit of Equation C-3, because at a = Oo, S = 0

.

To avoid this problem. NIST fit a line to the cr - log, £ data, as is commonly done in analysis of highrate test data. Plotting this semi-log fit on log-log axes results in a cur\'e. A linear fit of Equation C-3 to

this curve yields the Cowper-Symonds parameters p and C.

In these fits for the provisional data, the Cowper-Symonds exponent,/?, was constant among the steels,

but the pre-factor C increased with the yield strength. The increase in C with increasing yield strength, F,., can be represented with a second order polynomial of the natural logarithm of C:

NISTNCSTAR 1-3D. WTC Investigation 263
Appendix C

log, C = C„+C,F,+CF; C-4

where F, is the yield strength measured in the low-rate tensile test in units of ksi. The provisional

Cowper-Symonds model, after incorporating the yield strength dependence of the parameter C is

Table C-2 summarizes the values of the parameters in the provisional model, Eq. C-5.

The predictions of Eq. C-5 to the individual provisional data sets for the six steels were reasonable in that the yield stress increases with increasing strain rate. Furthermore, predicting the behavior of low-strength

steels, outside the strength range of the original data set, using Eq. C-5 does not produce anomalous

behavior.

By analyzing the full data set collected as part of the Investigation in the same manner as the provisional

data set, it is possible to assess the quality of the predictions of the provisional model. As in the analysis

of the provisional data, the exponent p for the full data set does not vary between the steels: p = 4.4053.

The nomializing constant C also increases with increasing yield strength; a second-order polynomial is sufficient to represent the behavior. Table C-2 summarizes the parameters in the Cowper-Symonds model

for the full data set of 13 steels.

The model based on the full data set predicts the behavior of the six steels in the provisional data set about

as well as the provisional model does. Of these six steels, the full model fits more accurately in half the

cases, where the goodness-of-fit is taken as the sum of squares of the residuals. As expected, the full model fits the data of the steels not in the provisional set better than does the provisional model. In the cases of the high-strength steels, the differences are not substantial, however. Figures C-1 through C-13

illustrate the data and the predictions of both models for high and intermediate strength steels.

The provisional model tends to overpredict the strain-rate sensitivity of the low-strength, core column

steels. Figures C-10 through C-13 show the predictions of the model for the low-strength, core-column

steels that were not included in the provisional data set. Because the aircraft struck the perimeter columns

first, the reduced fidelity for the core columns should not be a significant problem.

(7 = 1 + C-5

264 NISTNCSTAR 1-3D, WTC Investigation
Provisional Analysis of High-Rate Data

Table C-1. Specimens and orientations used in the provisional analysis to determine

Fy

(ksi) Specimen Orientation

50 M26-C1B L&T

60 N8-C1B L

65 N99-C3T L&T

75 C22-C2M L

100 M10B-C3B L&T

100 ClO-ClM L&T

Key: L, longitudinal; T, transverse.

Table C-2. Comparison of provisional and final Cowper-Symonds parameters, Eq. C-5,

for the high-strain rate data.

Data Set # of Steels P

Parameters in Equation C—4

G, c, Complete 13 4.4053 -4.7374 0.3614 -0.001707

Pro\isional

(Table C-1)

6 6.7824 -17.391 0.74512 -0.0035253

NISTNCSTAR 1-3D. WTC Investigation 265
Appendix C

Figure C-1. Predictions of the provisional and full Cowper-Symonds model for perimeter

column steel C10-C1M1-FL , which is in the provisional data set.

1.3

1.2

1.0

1—I—I—I—I—I—I—I—I—I—I—I—I—1—I—I—I—I—I—I—I—

provisional set

full set O

o

o o o

N99-C3M F =73.6 ksi 1-148-99/102

I I I I

I I 1 I I I I I I I I I I I I I I L 0 100

16:56:29 - Tuesday December 14. 2004

200 300 400

Strain rate, 1/s

500

Figure C-2. Predictions of the provisional and full Cowper-Symonds model for perimeter

column steel C10-C1M1-IW , which is not in the provisional data set.

266 NISTNCSTAR 1-3D, WTC Investigation
Provisional Analysis of High-Rate Data

1.0 ^

~i—

I—I—I—I—I—

I—I—I—

r

provisional set

full set

1—I—I—

r

"I

I I I I I c

_L

B6152-2-FL F =40.2 ksi 1-803-33/36

J I I I L. J I I I i 1 L. 0 100

15:56:12 - Tuesday December 14. 2004

200 300 400

Strain rate, 1/s

500

Figure C-3. Predictions of the provisional and full Cowper-Symonds model for perimeter

column steel M10b-C3B1-RF, which is in the provisional data set.

1.20

1.15

oD

D

1.10

1.05

1.00

1—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—

r

provisional set

full set

O

O

J 1 I I I I 1 I I I I I I I I I I I I I I I 1 L

M10B-C3B F =112.6 ksi 2-206-83/86

I , 0 100

15:55:36 - Tuesday December 14. 2004

200 300 400

Strain rate, 1/s

500

Figure C-4. Predictions of the provisional and full Cowper-Symonds model for perimeter

column steel C22-C2M, which is in the provisional data set.

NISTNCSTAR 1-3D, WTC Investigation 267
Appendix C

1.15

1.10

1.05

1.00

1—I—I—I—I—I—I—I—I—I

\—I—I

\—I—I—I—I—I—1—I—I—I—I—I—

provisional set

full set

O o

C22-C2B F =83.8 ksi 1-157-93/96

_i I I I I 1 I I'll _L J I I I I I I I I I L 0 100

15;56;34 • Tuesday December 14. 2004

200 300 400

Strain rate, 1/s

500

Figure C-5. Predictions of the provisional and full Cowper-Symonds model for perimeter

column spandrel steel C14-S, which is not in the provisional data set.

1.2

1 1 1 1

1 1 1 1 1 1 r 1 1 1

provisional set

1 1 1 1 1 1 1 1 1 1 1

1 1 T

" o full set

O

o

/

'/

( HH1 -WEB F =52.0 ksi 1-605-98/101

N 1 1 1 t 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 100 200 300 400 500

Strain rate, 1/s

1 5 :56 1 9 - Tuesday December 1 4. 2004

Figure C-6. Predictions of the provisional and full Cowper-Symonds model for perimeter

column steel N99-C3T, which is in the provisional data set.

268 NISTNCSTAR 1-3D, WTC Investigation
Provisional Analysis of High-Rate Data

Figure C-7. Predictions of the provisional and full Cowper-Symonds model for perimeter

column steel N8-C1B, which is in the provisional data set.

1.8

1 I 1

[

I I I I [ 1—1—1—

r

—1—1—1—1—r- .J-...,-.-P-.-I—1—

1—1

1—

r

- provisional set

- full set ^

o ^^^^

O o

o o

O _

^

- /

-/

- C65-WEB F =38.7 ksi

1 -904-86/89

5 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1. 0 100 200 300 400 500

Strain rate, 1/s

15:56:14 - Tuesday December 14, 2004

Figure C-8. Predictions of the provisional and full Cowper-Symonds model for perimeter

column wide-flange steel M26-C1B, which is in the provisional data set.

NISTNCSTAR 1-3D. WTC Investigation 269
Appendix C

Figure C-9. Predictions of the provisional and full Cowper-Symonds model for core column wide-flange steel HH-WEB, which is not in the provisional data set.

Figure C-10. Predictions of the provisional and full Cowper-Symonds model for core column wide-flange steel C80, which is not in the provisional data set.

270 NISTNCSTAR 1-3D, WTC Investigation
Provisional Analysis of High-Rate Data

1.15

o

(0

- I 'I )

1

1 1 1 1 ]

\—1—1—

r

1 1 1 1 1 1

1 1 1 1 [ T'"l

— nrnviQinnal cot

- full set -

- o

-

- o

-

- O - o

-

-

- O ^ " ^ "

-

-

-

/

{ C10-C1M1-IW F = 112.5 ksr

1-451-85/88

\ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I 1 0 100 200 300 400 500

Strain rate, 1/s

15:55:51 - Tuesday December 14. 2004

Figure C-11. Predictions of the provisional and full Cowper-Symonds model for core column steel C65-WEB, which is not in the provisional data set.

Figure C-12. Predictions of the provisional and full Cowper-Symonds model for core column wide-flange steel C65-FL2, which is not in the provisional data set.

NISTNCSTAR 1-3D, WTC Investigation 111
Appendix C

1.5

1.4

- 1 1 1

1

1 1 1 1

1

1 1 1

1

1 1 1 1 |--|-T--| 1

1

1 1 c

provisional set

-

full set o :

- —

-

— o - o

--

-

-

_-

-

' Li \J — — ^^O^ " " ^

"

f /

i

[ M26-C1B F =62.5 ksi

"

1-130-90/93

\ ,

1 1 1 1 ) 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 00 200 300 400 500

Strain rate, 1/s

15:54:54 - Tuesday December 14, 2004

Figure C-13. Predictions of the provisional and full Cowper-Symonds model for core box

column steel B6152-2, which is not in the provisional data set.

272 NISTNCSTAR 1-3D, WTC Investigation
Appendix D

Deformation of Steels Used in WTC 7

D.1 INTRODUCTION

Unlike World Trade Center (WTC) 1 and WTC 2, no recovered steel in the National Institute of

Standards and Technology (NIST) inventory can be unambiguously assigned to WTC 7. Therefore,

properties must be estimated completely from the literature. Building plans called for rolled column

shapes conforming to two ASTM International (ASTM) grades: A 36 and A 572 Grade 50. The heaviest

rolled columns were built up with cover plates that were specified to several different ASTM

specifications, by thickness: (in note 1 to drawing S-17)

/ < 2 in. A 36

2in. </<4m. A 588 Grade 50

4in. <r<6in. A 572 Grade 42

/>6in. A 588 Grade 42

About 26,000 tons of steel went into WTC 7 (Salvarinas 1986). Construction documents collected during

the Investigation did not provide any information on steel suppliers, but a telephone interview with the

former project manager for the construction of WTC 7 yielded some information (Salvarinas 2003).

TradeARBED supplied the jumbo columns for the base of the building to ASTM specifications. British

Steel (now Corns) and Bethlehem Steel supplied A 36 and Grade 50 steel. Algoma Steel supplied most of

the beams used for the floors. These were rolled to a proprietary Algoma shape that was approximately

41 lb ft (Salvarinas 2003). They were supplied to meet Canadian Standard CSA G40.21 ("General

Requirements for Rolled or Welded Structural Quality Steel/Structural Quality Steel") Grade 44W. This was a Fy = 44 ksi steel, where the "W" designates "weldable."

The goal of this report is to estimate the relevant mechanical properties of the steels. The sections of this appendix follows outline of main report on mechanical properties.

D.2 ELASTIC PROPERTIES

Chapter 2 of this report summarizes the values for the elastic properties. NIST found no evidence that the

Young's modulus or Poisson's ratio differs between structural steels, so the values in Chapter 2, Eqs. 2-2

and 2-3 can be used directly.

D.3 ROOM TEMPERATURE PROPERTIES

Because NIST recovered no steel from WTC 7, it is not possible to make any statements about its quality.

The values of the parameters that make up the expressions for the stress-strain behavior can be estimated

using the same methodology that was used for the WTC 1 and WTC 2 steels. This section summarizes

that methodology.

NISTNCSTAR 1-3D. WTC Investigation 273
Appendix D

The static yield strength, a^.^ for rolled shapes was calculated using the expected historical value from

literature data (Alpsten 1972):

where F, is the specified minimum yield strength, The factor = 1 .2 corrects for the fact that,

historically, the mill test report value of the yield strength of rolled shapes exceeds the specified value by

about 20 percent. (Alpsten 1972). The factor Adynamic ^ -3.6 ksi corrects the strength in the mill test report

to the value the static value. The factor Afiange = -2.6 ksi corrects for the fact that, until recently, the

specimen for the mill test report came from the higher-strength web, but the flange represents the majority

of the load-carrying area. For core rolled plates the correction is slightly different:

(T = k F + k ys P y dynamic alSO 3-4

where Ap = 1.092 (Alpsten 1972).

The Voce hardening law can represent the increase in stress, a, with plastic strain, s,,, in the plastic

regime. In this hardening rule the flow stress, o, reaches a limiting stress, , with an exponential decay:

This relation is available in most finite element packages.

Table D-1 summarizes the estimated static yield strengths and the parameters of the Voce hardening law

for the steels from WTC 7.

Table D-1. Parameters for calculating room-temperature stress-strain curves.

a,, TS

Steel Description ksi ksi ksi b ksi ksi Source of Plastic Portion

Fy = 36 ksi WF shapes 36 36.8 61.3 0.190 30.337 24.467 67.973 S9-C1T-S1

36 ksi perimeter steel Fy = 50 ksi WF shapes 50 53.8 74.8 0.202 23.023 32.116 30.495 HH-FL-1

50 ksi core wide-flange

Fy = 36 ksi cover plates 36 35.6 61.2 0.204 21.723 30.729 58.392 B6152-2-FL2

t < 2 in. 36 ksi box column

steels A 588 cover plates 50 50.8 72.5 0.202 23.023 32.116 30.495 HH-FL-1

2 in. < t < 4 in. A 572 cover plates 42 42.0 65.4 0.202 23.023 32.116 30.495 HH-FL-1

4 in. < t < 6 in. A 588 cover plates 42 42.0 65.4 0.202 23.023 32.116 30.495 HH-FL-1

t>6.0 in. Note: £n,ax is the true strain at the tensile strength, TS. The static yield strength, a,,„ is calculated from Eq. D-2. The column

labeled "source of plastic portion" identifies the specific WTC specimen used to develop the voce hardening parameters

D-1

also 3-3

D-3

also 3-5

274 NISTNCSTAR 1-3D, WTC Investigation
Deformation of Steels Used in WTC 7

DA HIGH STRAIN RATE PROPERTIES

Because, prior to collapse, WTC 7 did not suffer any high strain rate events, NIST made no effort to estimate high-strain-rate properties of the steel.

D.5 IMPACT PROPERTIES

Because, prior to collapse, WTC 7 did not suffer any high strain rate events, and because there were no

reco\ ered steels to test, NIST made no effort to estimate impact properties of the steel.

D.6 ELEVATED-TEMPERATURE PROPERTIES

D.6.1 Stress-Strain Curves

The elevated-temperature stress-strain cur\'es can be estimated using the methodology of Section 6.4.3.

The stress-strain cur\'es for 0°C < T< 700^ for all WTC 7 steels is represented using a power-law workhardening model:

also 6-4

where a has units of ksi, Thas units of °C and K(T) and n(T) are defined in Eqs. 6-5 and 6-6. The

parameter Rts is the ratio of the room temperature tensile strength of the steel of interest, TS, , to the room

temperature tensile strength, TS,^f, of the steel used to develop the model expressed by Eq. D-4.

R = D-5

TS^^f also 6-7

The parameter Rc corrects for two additional phenomena. The temperature-dependent functions K(T) and

n(Tj (Eqs. 6-5 and 6-6) were originally developed using data that were not corrected to zero strain rate, whereas the stress-strain cur\ es supplied to the Investigation teams for room temperature behavior are corrected to the zero strain rate. Secondly, because the temperature dependence of all the possible steels is represented using the behavior of only two different steels, it is possible that the stress-strain behavior

predicted by Eq. at room temperature could differ from that predicted by the akeady generated room

temperature stress-strain curve. The parameter Rc corrects the elevated-temperature stress-strain curve so

that the value of stress predicted by Eq. D-4 for T = 25 °C at e = 0.05 equals that of the room temperature

model curve. In general, the correction is small: 0.9 < Rc < 104.

Table D-2 summarizes the values of the parameters necessary to calculate high-temperature stress-strain curves for the WTC 7 steels.

NISTNCSTAR 1-3D. WTC Investigation 275
Appendix D

Table D-2. Parameters for estimating elevatedtemperature stress-strain and creep curves.

Steel Description ksi

TS

ksi Rts Rc

Parameter defined in Equation

6-7 6-19

Fy = 36 ksi WF shapes 36 61.3 1.03 0.90 1.15 Fy =50 ksi WF shapes 50 74.8 1.01 0.97 0.94 Fy -36 ksi cover plates

/ < 2 in.

36 63.4 1.07 0.86 1.11

A 588 cover plates 2 in < / < 4 in.50 72.5 0.98 0.97 0.97

A 572 cover plates

4 in < t < 6 in.

42 65.4 0.88 0.94 1.08

A 588 cover plates

t > 6.0 in

42 65.4 0.88 0.94 1.08

Note: R(j based on a reference tensile strength TSm49 = 70.65 ksi.

D.6.2 Creep Behavior

The creep behavior can be estimated using the methodology of Section 6.4.4. The creep strain as a

function of time is represented by Eq. 6-10 using the parameters of Eqs. 6-11, 6-12, and 6-13.

s^.=At'a' D-6

also 6-10

The applied stress should be scaled by R^, ratio of the tensile strength of the steel in question to that of the

reference AS A149 steel (r5'Ai49 = 70.6 ksi) using Eq. 6-19.

D-7

also 6-19

=R(J^, =

c (7 a TSA 1 49

TS..

(J

Table D-2 summarizes the values of the parameters necessary to calculate high-temperature creep curves

for the WTC 7 steels.

D.7 REFERENCES

D.7.1 References Available from Publicly Available Sources

Salvarinas, John J. 1986. Seven World Trade Center, New York, Fabrication and Construction Aspects.

Canadian Structural Engineering Conference-] 986 Proceedings. Canadian Structural Steel Council,

Willowdale, Ontario, Canada. ISBN 0-88811-062-6 pages 1 1-1 to 1 1-44.

276 NISTNCSTAR 1-3D, WTC Investigation
Deformation of Steels Used in WTC 7

D.7.2 References Available from Nonpublic Sources

Salvarinas. John J. 2003. Telephone interview with WilHam Luecke. Salvarinas was the project manager

for Frankel Steel during the erection of WTC 7. May 16.

NIST NCSTAR 1-3D, WTC Investigation 277
Appendix D

This page intentionally left blank.

278 NISTNCSTAR 1-3D, WTC Investigation
Appendix E

Specimen Geometry Effects on High-Rate Tensile Properties

E.1 introduction

The test plan utilized several different specimen sizes and geometries to evaluate the strain rate sensitivity

of the yield and tensile strength and the total elongation. Two types of high-rate specimens were used in an attempt to minimize the ringing in the load signal that comphcated the data analysis. In addition, data

from ordinary test specimens for assessing the quality of the steel and for characterizing roomtemperature stress-strain behavior already existed. To evaluate the strain rate sensitivity, it was

convenient to combine these data sets. This appendix describes the limitations of combining data

developed from different specimen geometries and sizes and summarizes the calculations of the strain rate dependence of the total elongation to failure.

The analysis reaches two conclusions. Results from specimens with different geometry can be combined

to evaluate the strain rate sensitivity of yield and tensile strength. To evaluate the strain rate sensitivity of

the total elongation, the specimens may be different size, but they must be geometrically self-similar.

E.1.1 Estimating Specimen-Size Effects on Yield and Tensile Strength

To a first approximation, the details of the specimen shape should not affect the measured yield and

tensile strength. If they did, the applicability of tension tests to determine material properties for use in constitutive models would be questionable.

E.1.2 Estimating Specimen-Geometry Effects on Ductility

The total elongation to failure of a specimen depends on the gauge length. In specimens with small gauge

lengths, the neck occupies a greater fraction of the total elongation, which leads to greater measured total elongations. In general, geometrically similar specimens develop similar total elongations. This relation is called Barba's law (Metals Handbook, 1985), which relates the total elongation, £/,, to the specimen area

A(, and gauge length Lo:

El,-/3-^ + e^ E-1

The parameter p is a proportionality coefficient, and e„ is the engineering strain at the point of necking.

Generally, specimens with different geometries will produce similar total elongations if the first term in Eq. E-1 is held constant. This relation underlies the choice of the geometries of the various round

specimens in ASTM E 8, where the standard- and sub-size specimens have ^"'^/Z-o = 0.177.

NISTNCSTAR 1-3D, WTC Investigation 279
Appendix E

E.2 PROCEDURE

In the course of the World Trade Center (WTC) Investigation, investigators employed a variety of

different specimen geometries for tests at quasi-static rates to satisfy the constraints of the starting

material and the test equipment available. The quasi-static strain rates, which were not identical between

specimen geometries, were in the range 6x10"" s"' < ds/dt < 1.2x10"^ s"'. Some of the quasi-static tests that used the ordinary tensile specimens followed E 8, which allows for an increased extension rate after the

yield strength has been determined. For these specimens, the strain rate plotted is the rate appropriate for the tensile strength determination.

Table E-1 summarizes the different specimen geometries used for determining strain rate sensitivities. Note that the / = 0.25 in. Flat 2" specimen and the Flat-HSR-l'TK specimen not only have the same

A^'~/Lo ratio, but their gauge lengths have the same width to thickness ratio. They are geometrically self- similar.

Table E-1. Specimen geometries used for establishing strain rate sensitivities.

Specimen

Description

Specimen

Designation in.

H'

in. in. in. Ain.' High rate Flat-HSR-1" TK 0.125 0.25 1.25 1 0.031 0.177

High rate Flat-HSR-1" IN 0.0625 0.25 1.25 1 0.016 0.125

3 in. round for

1 in. elongation

measurement

Rd 1" D = 0.25 in n/a 1.5 1 0.049 0.220

8 in. long flat' Flat 2" 0.250 0.50 2.75 2 0.125 0.177

8 in. long flat'' Flat 2" 0.5625 0.50 2.75 2 0.281 0.260

4 in. long flat Flat-C 1" 0.250 0.25 1.1 1 0.0625 0.250

a. Material thickness for Flat 2 in. specimens was generally 0.25 in. b. Specimen M26-C1B1-RF came from a plate with / = 9/16 in. Key: n/a, not applicable; A, area; L„, length of uniform cross section; Lo, elongation gauge length; /, specimen thickness; w, specimen gauge section width.

E.3 RESULTS

Figures E-1 through E-7 show the quasi-static stress-strain curves for the different specimen geometries.

In general, the agreement between the different specimen geometries is good for 1 percent offset yield

strength and tensile strengths. Table E-2 summarizes the data from those figures. Total elongations

should be taken from the table, because in soine cases the operator removed the extensometer prior to

failure. Although the yield and tensile strengths are not identical between the specimens, the differences

are small enough so that the differences can be ignored and the data combined to evaluate the strain rate

sensitivity.

The total elongation data generally follow Barba's law. Fig. E-9. The total elongation, El„ in specimens

with larger ^ ' ^/Lfi is correspondingly longer in five of the six cases. Specimen MlOB provides a direct

comparison between the large, Flat-2" specimen and the 1/4-size Flat-HSR TTK specimen. The A^ '/Lq ratio is identical for both test specimens. The total elongations for each specimen are statistically

indistinguishable, which is consistent with the expected Barba's law behavior.

280 NISTNCSTAR 1-3D, WTC Investigation
Specimen Geometry Effects on High-Rate Tensile Properties

hsr M10B-C3B1-L4

hsr M iOB-C3B1-L5

Isr M10B-C3B1-L1

Isr M10B-C3B1-L2

20

0 _l 1 1 L. J I I L.

Isr Flat 2 in. hsr t = 0.125 in. J I I I 1 0.0

12:57:04 • Tuesday December 21 . 2004

0.1 0.2 0.3 Engineering Strain

0.4

Figure E-1. Comparison of quasi-static stress strain curves obtained with high-rate style

(HSR) and ordinary specimens for specimen M10B-C3B1-RF, Fy = 100 ksi.

100

C/DO)

c

I

40

c

"d> c

LU 20

1 1 1 1 r

hsr N99-C3

Isr N99-C3

Isr N99-C3

J I L. _l I I L. 0.0

12:57:01 - Tuesday December 21. 2004

Isr Flat 2 in. hsr t = 0.0625 in.

0.1 0.2 0.3 Engineering Strain

0.4

Figure E-2. Comparison of quasi-static stress strain curves obtained with high-rate style

(HSR) and ordinary specimens for specimen N99-C3M1-RF, Fy = 65 ksi.

NISTNCSTAR 1-3D, WTC Investigation 281
Appendix E

100

CO

CO

C/) Q) COCD

_c

0)

CDc

'O) c

LU

40

20

\ I I n

hsr N8-C1B1-L2 N8-C1B1-LF

ordinary specimen is LF!

Isr Flat 2 in. hsr t = 0.0625 in. _^ , \ , , I I 0.0

12;57:02 • Tuesday December 21 , 2004

0.1 0.2 0.3 Engineering Strain0.4

Figure E-3. Comparison of quasi-static stress strain curves obtained with high-rate style

(HSR) and ordinary specimens for specimen N8-C1B1-RF, Fy = 60 ksi.

Figure E-4. Comparison of quasi-static stress strain curves obtained with high-rate style

(HSR) and ordinary specimens for specimen M26-C1B1, Fy = 50 ksi.

282 NISTNCSTAR 1-3D, WTC Investigation
Specimen Geometry Effects on High-Rate Tensile Properties

Figure E-5. Comparison of quasi-static stress strain curves obtained with high-rate style

(HSR) and ordinary specimens for specimens from the flange of WF core shape C65-FL2,

Fy = 36 ksi.

100

CO

COo

COD)

CD

0}

c

'U) c

LU

80

20 -

0

.J I L. J I L.

Isr Round 1 in. hsr t = 0.125 in. J < < < 1 0.0

12:57:04 - Tuesday December 21 , 2004

0.1 0.2 0.3 Engineering Strain

0.4

Figure E-6. Comparison of quasi-static stress strain curves obtained with high-rate style

(HSR) and ordinary specimens for specimens from the web of WF core shape C65-WEB,

Fy = 36 ksi.

NISTNCSTAR 1-3D, WTC Investigation 283
Appendix E

Figure E-7. Comparison of quasi-static stress strain curves obtained with high-rate style

(HSR) and ordinary specimens for specimens from core box column B6152-2 Fy = 36 ksi.

284 NISTNCSTAR 1-3D, WTC Investigation
Specimen Geometry Effects on High-Rate Tensile Properties

Table E-2. Summary data for tests using different specimen geometries at quasi-static rates.

Strain Rate for Fy

1/s

1%

offset

ksi

Strain Rate for TS

1/s

TS

L-ci

El,

/o in. ROA

/o

/

in.

fy

ksi Specimen type sample ff Location

1 7 I U 1 1 El-J O J. J 7A A 30-U

-) 0 1 n CATC 0.265 50 Flat 2" lvI26-C IBI-RF-Ll 1 1 ") c\ A A /A1

1 -1 30-90/93

O.VOII-J A0 1 7 1./ 1 7 1 p 1 oD. J 1 s A 3 J.U

-) 61 0.5625 0,265 50 Flat 2" M26-C 1 Bl -RF-L2 t 1 1 A AA /A 1 1-130-90/93

I 07E-4 70 f\ 1 07F-A 7 1 .o 77 A I U. 1 IZI A 1 11 U.I// 50 Hat HbK 1 1 K. Ivl26-C 1 B 1 -L

1

1 1 T A AA / A 1

] -130-90/93

J -U n —

r

If. -) QC 0 "''11

1 J

1

0. 1 25 65 Hat HSR 1 1 N N99-C3]vl 1 -Rh-L6 1-148-99/102 70 R 1

1 —7 11 pL-J Q 5 1 70 A -) An A 0 ^ A 1 in U.I// 65 Flat 2" N9y-L3 i 1 -Rr-Ll 1 \ AO AA / 1 AT l-l4;s-99/IUz

f, OAF <, 7"; Q 1 7 I p Qzl 0 TA A -> Andu A A T Tl U. 1 / / 65 Flat 2" N99-L J I

1 -RF-L2 1 1/10 AA/IAT

1 -148-99/1 02

1 1 ^.J" 1 n7P /I 1 77 A 1 c c

1 O.J I A 1 T ^

0. 1 77 85 Flat HbK 1 1 K

\/Il/lD M IJH \ DC I A

1 OB-L 3B 1 -RF-L4 "> ")AA 01/0A 2-/06-83/86

1 .u / c. —

1 1 -I-. J

1 n^p 1

I _3 .4

"> A 1

I c /I A 1 1 C

U. 1 25 0. 1 77 85 Flat HSR 1 1

K

M 1 OB-C 3B 1 -RF -L5 2-206-83/86

D.UDt-J 1 1 1 A 1 1 1 .0 I "> t p I

I I n-j i _U.4 7A A -) 55 0.25 0. 1 77 85 Flat 2" MI0B-C3B1-Rr-Ll 2-206-83/86

D.UOt-j I 1 J.J 1 7 I P 7

1 1 C-J 1 7A 1 0 A J o.U

-> A T 6z 0.25 0. 1 77 85 Flat 2" M 1 0B-C3B 1-RF-L2 2-206-83/86

I . I ut

\ 1 1 1 A I 1 J.D 7 CCP A 1 7 'I 7 77 A - /.U I A "7 6 / 0.25 0.250 90 Flat-C 1

C 1 0-C 1 M-FL-l -1 1 -45 1 -85/88

1 1 QF 1

1 . I 711 —1^ 1 1 "> Q t! 1 HP 1 I "> 1 1

1 _4.4 _ J.U 1 A A 66 0.25 0.250 90 Flat-C 1

L lO-C 1 Ivl-rL-1-2 1 /let OC/OO1 -45 1 -85/88

fi 'i'^F ^ 1 OQ Q O. J JCJ

I I V.J 1 C A A 1 0 1 0, 1 77 n Ayu Flat 2" L 1 U-L 1 D-rL-l -LI 1 /1C1 OC/OO

1 -43 1 -83/50

fs.3 jt-j I 1 U.J Js.j Jh-j 1 1 fl A 1 iv.v 1 0 A 1 O.U 60 0.25 0. 1 77 90 Flat 2" ClO-C lB-hL-l-L2 1 A c y OC/OO 1-451-85/88

I A7F-^ jy.o I n7F 1 04. J J 3.

3

i ^1j3 A AAT C A 1 "K U. 1 Zj 1 A JO r lat HSK 1 1 IN DO i j2-i-rL-2-L4 1 C A/I "3 /O A

I -jU4-33/3d

1 ri'^t 1

J -U / 1-4 4U.fS 1 .U / fc-4 AA 1 00. 1 ^A A JO.O 1 5

1

0.0625 0. 125 36 Flat HSR 1 1 N B61 j2-2-rL-2-Lj 1 -504-33/30

1 1 7F 1 1.1/ t-J J V.o I 1 7P 1 A< 7 DJ. / 7< A 3 J.U

-) AO A C U.J A on 1 A 36 Rd 2" bol jz-i-rL-2-L 1 1 C A/1 11 /I ^

I -jU4-33/3o

1 "^AF 1 17 A J /.D I 7 AP 7 OJ.4 lA A 3d.U

-) A"? 0 / A C 36 Rd 2" DAIO TCI Tl") Dol j2-z-rL-2-L2 1 C A/1 1 T /I A

1 -jU4-33/3d

5. / J t-j 1 7 Q /I 17P A 03. J 4 1 .U 1 AA66 0.25 0.222 36 Rd 1

CDJ-2-WhD-Ll 1 AA A OA /OA1 -yu4-co/oy

jy. J 4.

J

64.0 4_.U 1 63 0.25 0.222 36 Rd 1

Loj-2- whb-L2 1 AA A O £ /O A1 -yu4-oo/oy

1 .U /h-4 1 o o JO.O 1 A "7 C A 60.9 TIC 3 1 .J ] 55 0.0625 0. 1 25 36 Flat HSR 1 IN L65-WtB-L4 1 AA A O /O A

1 -904-86/89

1 n "7 jy. / A 11X1 A 4.3 / 1-4 66.6 43.0 1 63 0.25 0.222 36 Rd I

C65-H.25-LI 1 AAyl OA /OA

1 -yu4-86/8y

O. / Jt-J 1 7 C /I 7 "TC A 4.3 / c-^ AC A OJ.U 4_.U 1 A 1 0 I A C

v. -21 36 Rd 1(- 65-H.25-L2 1 AAyl OA /OA

1 -yU4-a6/8y 38 3 4 37E-4 70 0 43 0 ] 62 0

""5 0

''"'2 Jv Rd 1

PAS H I

1 QAA SA/8Q

8.75E-5 47.0 4.37E-4 68.0 38.0 1 63 0.25 0.222 36 Rd 1" C65-H.5-L1 1-904-86/89

8.75E-5 42.5 4.37E^ 66.0 42.0 1 62 0.25 0.222 36 Rd 1" C65-H.5-L2 1-904-86/89

8.75E-5 37.1 4.37E-4 64.0 43.0 1 62 0.25 0.222 36 Rd 1" C65-H.5-L3 1-904-86/89

2.19E-3 38.7 2.19E-3 65.6 42.0 1 62 0.25 0 222 36 Rd 1" C65-H.5-L4 1-904-86/89

1.07E-4 40.1 I.07E-4 65.3 39.2 1 54 0.0625 0.125 36 Flat HSR 1" TN C65-FL2-L4 1-904-86/89

E.4 DISCUSSION OF TOTAL ELONGATION BEHAVIOR

The excellent correspondence between the different-sized specimens of M1 OB indicates that, where

possible, the different sized specimens with identical /i' '^/Lo ratios can be combined to gain insight into the behavior of the total elongation with strain rate.

E.4.1 Limitations

Although the specimens are geometrically similar, the analysis does not account for the temperature rise

in the specimen as the work of deformation is transformed into heat. High rate tests are generally assumed

NISTNCSTAR 1-3D, WTC Investigation 285
Appendix E

to be adiabatic. Quasi-static tests have additional time for the specimen to cool during deformation, so the

temperature rise will be less. For adiabatic conditions the temperature rise, AT, is

1 Ar = \s(e)de E-2

' p e

for a material with density, p, heat capacity C,,, which deforms at engineering stress s(e) to a final strain of (?/. For steel, assume that the failure strain is = 0.3, the flow stress constant s = 830 MPa (120 ksi),

C;, = 415 J/kg/K, and p = 7860 kg/ml Then

S6

AT = —^=76.5K E-3

which represents an upper limit. The temperature rise of lower-strength specimens will be

correspondingly smaller. Because the temperature rise of the quasi-static specimen is not zero, however,

the effect of the temperature rise on the ductihty of the high-rate specimens is probably not significant.

E.5 CONCLUSIONS

It is possible to combine ordinary and high-rate style specimens for determining the yield and tensile

behavior as a function of strain rate. For determining the total elongation behavior as a function of strain

rate, because of the Barba's law behavior, specimens can be mixed only if their width-to-thickness ratios are identical and their ^' "/Lo ratios are identical.

E.6 REFERENCES

Metals Handbook. 1985. Metals Handbook Ninth Edition. Volume 8 Mechanical Testing. American

Society for Metals. Metals Park, OH. USA. p. 26.

286 NISTNCSTAR 1-3D, WTC Investigation
Specimen Geometry Effects on High-Rate Tensile Properties

: 150

CO

C

0) ^ 100

c

50

O

DA

0

I- ,.=65 ksi e. =1-85. 91±1 5.05} N99-C3M1-Rr

F^,=85 ksi M10B-C3B1-RF

F,.=90 ksi e. =(66.2613.42) ClO-CilVi-FL Fy=36 ksi e,=(2.59±8.43) B6152-2-FL

r,=36 ksi e, =(29. 50x4.48) C6S-web

F„=36 ksi e,={11.98±22.16) c65-flange

-0

0.10

15:04:56 - Thursday December 16 2004

0.15 0.20 0.25

17. ^50

c

0}

i— •—

»

p 100

0)

CD

(0

50

T

F =50 ksi ei=(-92.19±14.70) M26-C1B1-RF

F =65 ksi e-=(-74.32±51.83) N99-C3tVi1-RF

F,.=85 ksi M10B-C3B1-RF

F'=90 ksi e-,=f41.67±6.30i C10-C1M-FL

Fy=36 ksi e,=(-15.53±12.97) B6152-2-FL

F.,=36 ksi e,=(-i .04114.34) C65-web

f'=36 ksi ei=(0. 59138. 60) c65-fiange

J I I L _l I I L 0.10 0.15

15:04:56 - Thursday December 16. 2004

0.20 0.25

Figure E-8. Yield (1 percent offset) and tensile strength as a function of specimen

geometry for specimens tested at quasistatic rates.

NISTNCSTAR 1-3D, WTC Investigation 287
Appendix E

60

50

40

•4—* OH

o

A

F =50 ksi e,=(145.93±9.80) M26-C1B1-RF

l\=(^5 ksi c,=(104.:3±!6.72) N99-C3Mi-RF

F;=85 ksi M10B-C3Bi-RF

F;=90 ksi e,=( 95.63127.32) Cl(t-C1M-FL

F,=36 ksi e,=(5.69±17.85) B6152-2-FL

F;=36 k.si e.=(l03.52±8.97) C65-wcb F =36 ksi e,=(27.5 1+19.62) c65-flange *

20

10 I—'—I

>—

0.10

_1 I I L. 0.15 0.20 0.25

l5:l).S:(lf) - Tllursday Dcccmhcr 16, 20(14

Figure E-9. Total elongation, Elt, as a function of specimen geometry for specimens

tested at quasistatic rates.

288 NISTNCSTAR 1-3D. WTC Investigation