NIST NCSTAR 1-3

Federal Building and Fire Safety Investigation of the

World Trade Center Disaster

Mechanical and Metallurgical

Analysis of Structural Steel

Frank W. Gayle

Richard J. Fields

William E. Luecke

Stephen W. Banovic

Timothy Foecke

Christopher N. McCowan

Thomas A. Siewert

J. David McColskey

National Institute of Standards and Technology • Technology Administration • U.S. Department of Commerce

NIST NCSTAR 1-3

Federal Building and Fire Safety Investigation of the

World Trade Center Disaster

Mechanical and Metallurgical

Analysis of Structural Steel

Frank W. Gayle

Richard J. Fields

William E. Luecke

Stephen W. Banovic

Timothy Foecke

Christopher N. McCowan

Thomas A. Siewert

J. David McColskey

Materials Science and Engineering Laboratory

National Institute of Standards and Technology

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 (JSC 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 1-3

Natl. Inst. Stand. Technol. Natl. Constr. Sfty. Tm. Act Rpt. 1-3, 184 pages (September 2005)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2005

For sale by the Superintendent of Documents, U.S. Government Printing Office

Internet: bookstore.gpo.gov — Phone: (202) 512-1800 — Fax: (202) 512-2250

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

This report is an overview of the results of the mechanical and metallurgical analysis of structural steel from the World Trade Center (WTC), part of the National Institute of Standards and Technology

Investigation of the WTC disaster of September 11, 2001.

The goal of the study was threefold:

Determine mechanical properties of WTC structural steel,

Determine the quality of the steel and if design requirements were met, and

Analyze the recovered steel to provide insight into failure mechanisms to guide and/or validate models of

building performance.

Structural steel recovered from the WTC site was analyzed for composition, microstructure, and

mechanical properties, including room temperature properties (for modeling baseline building

performance), high temperature properties (for modeling structural response of the building to fire), and

behavior at high strain rates (for modeling airplane impact). Failure analysis of the recovered steel, complemented by pre-collapse photographs of the damaged building, was used to establish failure modes

and temperature excursions experienced by the steel. In addition, documents from the construction era covering issues ranging from steel specifications to engineering design drawings were used to help

interpret the results and supplement models of mechanical properties used in the models of building

performance.

The analysis focused on the WTC 1 and WTC 2. Although no steel was recovered from WTC 7, a 47-story building that also collapsed on September 11, properties for steel used in its construction were

estimated based on literature and contemporaneous documents.

Keywords: Failure analysis, mechanical properties, specifications, structural steel, World Trade Center.

NISTNCSTAR 1-3, WTC Investigation in
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iv NISTNCSTAR 1-3, WTC Investigation
Table of Contents

Abstract iii

List of Figures xi

List of Tables xvii

List of Acronyms and Abbreviations xix

Metric Conversion Table xxi

Preface xxv Acknowledgments xxxv

Executive Summary xxxvii

Chapter 1 Introduction 1

1 . 1 Overview and Scope 1

1.2 Approach 2

Chapter 2 Tower Design - Structural Steel Documents 5

2.1 Introduction 5

2.2 Specification of Steel Grades (Minimum Yield Strength) 5

2.3 Structural Overview 5

2.3.1 Perimeter Columns 7

2.3.2 Core Columns 10

2.3.3 Flooring System 10

2.3.4 Floors 107 to 110 13

2.3.5 Impact Zone 14

2.3.6 Floors Involved in Post-Impact Fires 14

2.4 References 14

Chapter 3 Contemporaneous Steel Specifications 15

3.1 Introduction 15

3.2 Floor Trusses 15

3.3 Perimeter Columns and Spandrels 16

3.4 Core—Welded Box Columns 17

NISTNCSTAR 1-3, WTC Investigation v
Table of Contents

3.5 Core—Rolled Wide flange Shapes 17

3.6 Findings 18

3.7 References 18

Chapter 4

Contemporaneous Construction Specifications 21

4.1 Introduction 21

4.2 Fabrication of the Various Components 21

4.2.1 Floor Trusses 21

4.2.2 Exterior Wall Columns and Spandrels 21

4.2.3 Core—Welded Box Columns 22

4.2.4 Connections (Bolts and Welds) 23

4.2.5 Construction (On-Site Assembly) 23

4.3 References 24

Chapter 5 Steel Inventory and Identification 27

5.1 The Recovery of World Trade Center Structural Steel 27

5.2 Structural Elements Recovered from WTC Buildings 28

5.2. 1 Location and Labeling of Structural Steel Elements 28

5.2.2 Identification Methods of WTC Structural Steel Elements 29

5.2.3 Identified Perimeter Panels 33

5.2.4 Identified Core Columns 35

5.2.5 Other Structural Pieces 36

5.3 Structural Steel Elements of Special Importance 36

5.3.1 Samples Located in or around the Floors of the Airplane Impact 36

5.3.2 Samples Representing the Various Types of Steel Specified in the Design Drawings 38

5.4 Summary 39

5.5 References 39

Chapter 6

Damage and Failure Analysis of Structural Steel 41

6.1 Background 41

6.2 Observable Pre-Collapse Damage of Exterior Column Panels 42

6.2. 1 Photographic Analysis of Pre-Collapse Damage to Exterior Panels 42

6.2.2 Photographic Evidence of Damage to the Flooring System of WTC 2 45

6.2.3 Damage to Sprayed Fire Resistive Material in WTC 1 45

vi NISTNCSTAR 1-3, WTC Investigation
Table of Contents

6.2.4 Damage to Sprayed Fire-Resistive Material in WTC 2 49

6.2.5 Photographic Analysis of Pre-Collapse Fire Exposure to Exterior Panels 55

6.2.6 Photographic Evidence of Pre-Collapse Distortion of Exterior Wall: South Face of

WTC 1 59

6.2.7 Photographic Evidence of Pre-Collapse Distortion of Exterior Walls: East Face of

WTC 2 63

6.2.8 Photographic Evidence of Details of the Collapse of WTC 2 63

6.3 Physical Damage of Recovered Exterior Wall Panels 69

6.3. 1 Overall Damage Patterns of Exterior Wall Panel Sections 70

6.3.2 Damage and Failure Modes of Exterior Wall Columns 70

6.3.3 Exterior Wall Spandrel Connections 74

6.3.4 Exterior Wall Column Splices (End Plate/Butt Plate Connections) 74

6.3.5 Exterior Wall Seats or Floor Truss Connectors 80

6.4 Physical Damage of Core Elements (Columns and Channels) 80

6.4.1 Core Columns 80

6.4.2 Core Channels and Truss Seats 92

6.5 Physical Damage of Floor Truss Material 93

6.6 Fire Exposure of the Structural Elements 94

6.6.1 Visual Inspection of Recovered Structural Components 94

6.6.2 Core Columns Exposed to Fire 95

6.6.3 Metallographic Analysis of Elements Exposed to Fire 95

6.7 Comments Concerning Contract to Wiss, Janney, Elstner 99

6.8 Findings 99

6.8.1 Structural Damage of Exterior Panel Sections 99

6.8.2 Structural Damage of Core Columns 100

6.8.3 Structural Damage of Floor Trusses and Seats 100

6.8.4 Damage to Sprayed Fire-Resistive Material 100

6.8.5 Fire Exposure of Exterior Panel Sections 101

6.8.6 Fire Exposure of Core Columns 101

6.8.7 Time-Dependent Deformation of Perimeter Walls due to Fire and Load

Redistribution 101

6.8.8 Other 102

Chapter 7 Mechanical Properties of Structural Steels 103

7.1 Introduction 103

NISTNCSTAR 1-3, WTC Investigation vii
Table of Contents

7.2 Elastic Modulus , 103

7.3 Room Temperature Tensile Properties 104

7.3. 1 Measured Tensile Properties 104

7.3.2 Measured Properties and Specifications 104

7.3.3 Representative Steel Stress-Strain Curves 107

7.4 High-Strain Rate Tests 1 08

7.5 Charpy Tests 109

7.6 High-Temperature Tests Ill

7.7 Steels Used in WTC 7 113

7.7.1 Steels Used in Construction of WTC 7 113

7.7.2 Mechanical Properties of WTC 7 Steel 1 14

7.7.3 Physical Properties of WTC 7 Steel 115

7.8 Findings 115

7.8. 1 Room Temperature Tensile Properties 1 1

7.8.2 High-Strain Rate Properties 116

7.8.3 Impact Properties 1 16

7.8.4 High-Temperature Properties 1 16

7.9 References 116

Chapter 8 Physical Properties of Structural Steels 119

8.1 Introduction 119

8.2 Chemical Analysis of Recovered Steel 119

8.3 Metallography 120

8.4 Thermal Properties 122

8.5 Summary 124

8.5.1 Chemistry of WTC Steels 124

8.5.2 Microstructure of WTC Steels 125

8.5.3 Thermal Property Values of WTC Steels 126

8.5.4 Other 126

8.6 References 126

Chapter 9

Findings and Issues 129

9.1 Introduction 129

9.2 Findings - Steel Specifications, Standards, and Sources 129

viii NISTNCSTAR 1-3, WTC Investigation
Table of Contents

9.3 Findings - Inventory of Recovered Structural Steel 1 29

9.4 Findings - Damage and Failure Analysis 1 30

9.4. 1 Structural Impact Damage - Perimeter Panels 130

9.4.2 Structural Impact Damage - Core Columns 1 3

9.4.3 Impact Damage to Sprayed Fire-Resistive Material 1 3

9.4.4 Structural Damage - Floor Trusses and Seats 131

9.4.5 Fire Exposure and Temperatures Reached by the Steel 132

9.4.6 Time-Dependent Deformation of Perimeter Walls Due to Fire and Load

Redistribution 132

9.5 Findings - Mechanical Properties 133

9.5. 1 Room Temperature Tensile Properties 133

9.5.2 High-Strain-Rate Properties 1 34

9.5.3 Impact Properties 134

9.5.4 High-Temperature Properties 134

9.5.5 Conclusions - Mechanical Properties 134

9.6 Issues 134

NISTNCSTAR 1-3, WTC Investigation ix
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X NIST NCSTAR 1-3, WTC Investigation
List of Figures

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

disaster xxvii

Figure 2-1 . Schematic diagram of the tower structure 6

Figure 2-2. WTC 1 tower floor plan and column numbers 7

Figure 2-3. Cross-section of perimeter columns; sections with and without spandrels 8

Figure 2-4. Characteristic perimeter column panel consisting of three full columns connected by

three spandrels 8

Figure 2-5. Partial elevation of exterior bearing-wall frame showing exterior wall module

construction. Highlighted panel is three stories tall (36 ft) and spans four floors. Distance between panels has been exaggerated 9

Figure 2-6. Core column layout in WTC towers 10

Figure 2-7. Typical welded box members and rolled wide flange shapes used for core columns

between floors 83 and 86 1

Figure 2-8. Schematic diagram of a floor truss 12

Figure 2-9. Hat truss in upper floors 13

Figure 5—1. Example of stampings on the interior base of the middle column for each panel.

Shown here on panel M-lOb are the stampings "B206 86 83," indicating that it is from WTC 2, column number 206, spanning floors 83 to 86 30

Figure 5-2. Example of stampings placed on one end of a core column. Shown here on sample

C-90 are the stampings "18 TON, 701 B 12 - 16, 549 WEST," indicating that it is from WTC 2, core column number 701, and spans floors 12 to 16. Also indicated are

the approximate weight of the piece (18 tons) and the derrick division <549> that was to lift it into place 3

Figure 5-3. a) and b) Characteristic stenciling found on the lower portions of the exterior column

panels for sample M-27. Markings indicate this piece was in WTC 1, column

number 130, spanning floors 93 to 96. Also seen are two other indicators, "5T" and

"<63>," indicating the estimated piece weight in tons and erector's derrick division

number, respectively, c) Characteristic stenciling found on core column for sample

B-6 152-1. Markings indicate this piece was in WTC 1, column number 803,

spanning floors 15 to 18 with a derrick division number of <52> 32

Figure 5-4. Location of the exterior panels recovered from the top third of the WTC towers. Recovered WTC 1 panels are shown in light shading/green and WTC 2 panels are in dark shading/blue 34

NISTNCSTAR 1-3, WTC Investigation xi
List of Figures

Figure 5-5a. Interpreted column damage, from photographic evidence, to north face of WTC 1, with overlay of recovered steel identified by NIST. Colored regions represent

recovered portions. Both core columns (#603 and #605) are in the second row from

the north face of WTC 1 37

Figure 5-5b. Interpreted column damage, from photographic evidence, to south face of WTC 2,

with overlay of recovered steel identified by NIST. Samples shown represent

recovered portions. Core column line 801 is in the row closest to the south face of

WTC 2 38

Figure 6-1 . (top) Enhanced image showing the impact hole in the north face of WTC 1, and

(bottom) overlay of an outline of a Boeing 767 that has been distorted to get a best

approximate fit with damage to WTC 1 . The location and type of localized damage

has been indicated 43

Figure 6-2. (top) Enhanced image showing the impact hole in the south face of WTC 2 and

(bottom) overlay of the location and type of localized damage 44

Figure 6-3. Damage to northeast corner of WTC 2 at 81st floor. The red arrow indicates an

intact column 300 at the 82nd floor. Other photographs confirm that the spandrel

connecting column 300 to columns 259 and 301 is also intact (location indicated by

green arrows) 46

Figure 6—4. Damage to columns on north face of WTC 2 near northeast corner due to internal

impact by debris. Broken vertical column bolted connections (green arrows) are

visible, along with an intact outer web on column 254 (red arrows) and missing

column sections (blue arrows) 47

Figure 6-5. Dropped floors on north face of WTC 2 48

Figure 6-6. Diagrams showing how the fireproofing was to be applied to the exterior columns,

and the exterior aluminum panels were attached to the window frames 50

Figure 6-7. Fireproofing that had been applied to the exterior columns. The depression formed

by the outer web and the flange tips are completely filled in with fireproofing at the arrow. North face of WTC 2 51

Figure 6-8. Port side of impact hole in north face of WTC 1 showing missing fireproofing. The

red arrows indicate a few of the many instances of missing fireproofing around the

impact hole. The region roughly approximated as the area where the aluminum

panels were dislodge has been digitally enhanced for clarity 52

Figure 6-9. Missing fireproofing on floor trusses that fell into impact hole on north face of WTC

1 . The arrows labeled "1" are three trusses where the lower or upper chords can be

seen, and can distinguish the line that separates the two angle iron components. The

arrows labeled "2" lie at either end of a lower chord of a truss that fell farther inside

the building, and is made up of four angle irons (a double truss). The red paint color

being visible and the fact that the line between the angle iron can be seen indicated

that the fireproofing was displaced 53

xii NISTNCSTAR 1-3, WTC Investigation
List of Figures

Figure 6-10. Missing facade and fireproofing on north face of WTC 2. Two regions have had

contrast and gamma adjusted to enhance visibility of the features. Red arrows

indicate columns with missing or damaged fireproofing. Missing or damaged

fireproofing determinations were made from this and several other photos taken at different times and angles. Blue arrows indicate white features where some sort of

coating has preserved the fireproofing in place 54

Figure 6—11. Missing fireproofing on flanges of exterior columns of north face of WTC 2. Fireproofing has come off, revealing red Tnemec paint of the column (red arrows).

This image also shows how the outer web regions have been partially or completely

denuded of fireproofing. Damage is evidenced by shading by sunlight - compare

green (undamaged fireproofing) and white (damaged fireproofing) arrows. Blue arrow indicates same type of white coated area of fireproofing as previous figure that adhered when material above and below was dislodged 56

Figure 6-12. Damage to fireproofing on east face of WTC 2 due to internal impact. Red arrows

highlight areas where fireproofing has been damaged. The blowup to the right shows

a column where the red Tnemec paint of the column is visible 57

Figure 6-13. Time-fire exposure map for panel K-2 (A236: 92-95). Fire can be observed on the 92nd and 94th floors of the panel 58

Figure 6-14. Inward bowing of south face (at right, with dark lines overdrawn) of WTC 1. Taken

at 10:23 a.m. from an NYPD helicopter 61

Figure 6-15. Pull in of south face of WTC 1 at 10:23 a.m. showing considerable displacements of

outer columns into the building. Faint white vertical features are the aluminum

facade of the exterior. Maximum visible inward displacement of the columns is 55

in. +/- 6 in. Poor resolution leads to large uncertainties in measurements: a large

fraction of the south face of the building is obscured by smoke 62

Figure 6-16. East face of WTC 2 at 9:53:04 a.m. showing a larger amount of pull in. The vertical

black lines were drawn to establish the original line of the exterior wall columns, and

the shorter line segments are drawn at the same angle as the nearest observable joint

in the exterior aluminum panels to establish the inward bowing distance. For actual

column numbers, add a "3" to the front of the two-digit designations in the image 64

Figure 6-17. Map of inward displacements of the east outer face of WTC 2 inward at 9:21:29 a.m. Each measurement refers to the upper left corner of the window within which the

number resides. Measurements were in inches. The colors refer to groups of 5 in. displacements (0-5: black. 6-10: blue) for easier visualization 65

Figure 6-18. Map of pull in displacements of the east outer face of WTC 2 inward shortly before

collapse. Each measurement refers to the upper left corner of the window within

which the number resides. Measurements were in inches. The colors refer to groups

of 5 in. displacements (0-5: black, 6-10: blue. 1 1-15: orange. 16-20: red) for easier

visualization. This data was combined from two images taken by the same

photographer at nearly the same location 10 s apart, and it is assumed that the inward

deflection did not change appreciably during this time 66

Figure 6-19. Initial seconds of collapse of WTC 2 viewed from the east-northeast, and kink in the

southeast edge of the building near the 106th floor (arrow) 67

NISTNCSTAR 1-3. WTC Investigation xiii
List of Figures

Figure 6-20. A close-up of the kink area in the previous figure (southeast corner, WTC 2), with

overlaid lines to highlight the details of the kink geometry. The edge of the building

(1) bends an angle to another direction (2), then bends back somewhat toward the

original direction (3) 68

Figure 6-21. Northeast corner of WTC 2, several seconds before collapse. The corner shows no

distortion of the type in the previous figure, implying that the distortion accompanied

the collapse and did not precede it. The region in the upper right is enhanced for

visibility 69

Figure 6-22. a) Overall view of panel M-2 (A130: 94-97). Bottom of panel is on the left-hand side

of the picture and the outside of the panel is facing down, b) major bend in panel

located at the 97th floor level, and c) splayed bottoms of columns 129 and 130 71

Figure 6-23. Thinning of outer web from column 130 of panel M-2 (A130: 96-99). a) Photograph

showing thinning, b) graph indicating plate thinning near fracture surface 72

Figure 6-24. Damage images from sample M-2 (A130: 96-99). a) Large buckles observed on the

flange plates of column 130, b) cracking of the flange plates on column 130,

c) cracking of spandrel at 97th floor level, and d) no cracking of the welds in this area on column 130 73

Figure 6-25. Photographs of recovered spandrel connections from sample M-2 (A130: 96-99) 75

Figure 6-26. Photographs of recovered end plate connections from sample M-2 (A127: 97-100).

Both ends of column 131 had bolt failure that resulted in deformation of the bolt

holes in the end plates 77

Figure 6-27. Floor truss connectors for panel M-2 (A 130: 96-99) 81

Figure 6-28a. Damage diagram overlaid with recovered samples and damage of connectors 84

Figure 6-28b. Damage of connectors. At or below the 95th floor, all connectors are either bent

down or missing 85

Figure 6-29a. Positioning of the recovered core columns with the core area for WTC 1 86

Figure 6-29b. Positioning of the recovered core columns with the core area for WTC 2 87

Figure 6-30. Core column C-80 (603 A: 92-95). a) Overall view of recovered column, b) view of

connection in the 92nd floor region, failure as a result of bolt fracture, and c) torn end

of column near the 94th floor region 88

Figure 6-31. Overall view of recovered core columns C-88a (801B: 80-83) and C-88b (801B:

77-80) 89

Figure 6-32. Damage associated with core column C-88b (801B: 77-80). a) Overall view of failed

end. b) bottoms of "south" flange and both webs bent towards the east just below the

80th floor level, ends were flame cut, and c) "north" flange bent towards the north,

majority of plate was fractured with some flame cutting 90

Figure 6-33. Core column HH (605A: 98-101). a) Fracture at both ends and b) separation between

flange and web in the 99th floor region 91

Figure 6-34. Failure modes of core channels, a) Fracture through channel as indicated by arrows,

b) failure associated with end connector, and c) seat ripped off at welded connection

to channel 92

Figure 6-35. Examples of recovered floor truss material, a) Small sections of rod and chord

(sample C-53) and b) "balled-up" sections of rod and chord (C-137f) 93

xiv NISTNCSTAR 1-3, WTC Investigation
List of Figures

Figure 6-36. Crushed portion of column 210 in the area of the 98th floor (from sample K-l , A209:

97-100). Paint had mud cracking characteristic in this region 96

Figure 6-37. Spandrel at the 93rd floor of column 236 from sample K-2 (A236: 92-95). Paint mud

cracking was found below the 93rd floor line, no evidence of paint mud cracking was

found above floor line 97

Figure 6-38. Truss seat of the 99th floor on column 143 of sample N-8 (A 142: 97-100). a) Top

view showing the re-solidified black plastic with embedded papers, most likely a binder of some type and b) bottom view showing the drip pattern. It was unknown

when the high-temperature excursion may have occurred 98

Figure 7-1 . Elastic modulus as a function of temperature determined by NIST compared with

literature data for structural steel 1 04

Figure 7-2. Examples of longitudinal (11, 12, and 13) and transverse (tl) room-temperature, stress- strain curves for a Fy =46 ksi perimeter column spandrel plate 105

Figure 7-3. Ratio of measured yield strength or yield point to specified minimum yield point for WTC perimeter column steels 105

Figure 7-4. Ratio of measured yield strength or yield point to specified minimum yield point for WTC core column steels 106

Figure 7-5. Examples of representative true-stress true-strain curves for perimeter column steels 108

Figure 7-6. Strain rate sensitivity as a function of specified minimum yield strength for WTC

steels 109

Figure 7-7. Summary data for Charpy tests. Lower figure: perimeter column steels; upper figure: core column steels 110

Figure 7-8. Normalized yield strength as a function of temperature for WTC steels, the

recommended function to describe behavior (solid line), and the literature data used

to develop that relation 1 1

Figure 7-9. Normalized tensile strength as a function of temperature for WTC steels, the

recommended function to describe behavior (solid line), and the literature data used

to develop that relation 1 1

Figure 7-10. Typical high-temperature stress-strain curves for specimen HH, a Fy = 42 ksi wideflange shape from near the fire and impact zone of WTC 1 113

Figure 7-1 1. Typical creep curves for different stresses at 600 °C from truss steels specified as A 242, from specimen C132-TA 1 14

Figure 8-1 . Microstructure of ferritic-pearlitic WTC perimeter column flange plate steels, a) Fy =

45 ksi (unidentified perimeter column with "45"' flange stamping) and b) Fy = 60 ksi

(WTC 1, column 126, 97th floor) 121

Figure 8-2. Examples of "quenched and tempered" WTC perimeter column flange steel, a) Fy =

70 ksi (WTC 1 , column 224, 94th floor) and b)Fy = 80 ksi (WTC 1 , column 207,

89th floor) 122

Figure 8-3. Thermal conductivity as a function of temperature for twelve low-alloy steels 125

NISTNCSTAR 1-3, WTC Investigation xv
List of Figures

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xvi NISTNCSTAR 1-3, WTC Investigation
List of Tables

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

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

Table 3-1 . Steel companies involved in WTC construction and their contracts 16

Table 5-1. Identified perimeter column panel pieces from WTC 1 and WTC 2 33

Table 5-2. Identified pieces of core column material from WTC 1 and WTC 2 35

Table 6-1. Recovered exterior panel sections with known as-built locations, separated by pre- collapse location and environmental conditions 60

Table 6-2. Recovered core columns with known as-built locations, separated by pre-collapse

conditions 86

NISTNCSTAR 1-3, WTC Investigation xvii
List of Tables

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xvm NIST NCSTAR 1-3, WTC Investigation
List of Acronyms and Abbreviations

Acronyms

AISC American Institute of Steel Construction

ASCE American Society of Civil Engineers

ASTM ASTM International

AWS American Welding Society

CTE coefficient of thermal expansion

DTAP dissemination and technical assistance program

FEMA Federal Emergency Management Agency

GMS Gilsanz Murray Steficek, LLP

HAZ heat affected zone HSLA high-strength, low-alloy

JFK John F. Kennedy International Airport

LERA Leslie E. Robertson Associates

NCST National Construction Safety Team

NIST National Institute of Standards and Technology

NYPD New York City Police Department

PANYNJ Port Authority of New York and New Jersey

PC&F Pacific Car and Foundry

PONYA Port of New York Authority

R&D research and development

SEAoNY Structural Engineers Association of New York

SFRM sprayed fire-resistive materials

SHCR Skilling, Helle, Christiansen, & Robertson

SMA shielded metal arc USC United States Code

WJE Wiss, Janney, Elstner Associates, Inc. WTC World Trade Center

WTC 1 World Trade Center 1 (North Tower)

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

WTC 2 World Trade Center 2 ( South Tower)

WTC 7 World Trade Center 7

Abbreviations

± plus or minus

°C degrees Celsius

°F degrees Fahrenheit

um micrometer

cm centimeter

Cr chromium

ft foot

Fy yield strength (AISC usage)

h hour

in. inch

kg kilogram

kip a force equal to 1,000 pounds

ksi 1 ,000 pounds per square inch

L liter

lb pound

m meter

m square meter

mm millimeter

min minute

s second

NIST NCSTAR 1-3, WTC Investigation
Metric Conversion Table

To convert from

AREA AND SECOND MOMENT

square foot (ft")

square inch (in.

2

)

square inch (in.")

FORCE

kilogram-force (kgf)

kilopond (kilogram-force) (kp)

kip(l kip= 1.000 Ibf)

kip (1 kip= 1.000 lbf)

pound-force (lbf)

FORCE DIVIDED BY LENGTH

pound-force per foot (lbf/ft)

pound-force per inch (lbf/in.)

LENGTH

foot (ft)

inch (in)

inch (in.)

micron (m)

MASS and MOMENT OF INERT

kilogram-force second squared

per meter (kgf • s"/m)

pound foot squared (lb • ft")

pound inch squared (lb • in.")

ton, metric (t)

ton. short (2,000 lb)

MASS DIVIDED BY AREA

pound per square foot ( lb/ft")

pound per square inch

(not pound force) (lb/in.

2

)

to

AREA

square meter (m")

square meter (m")

square centimeter (cm2

)

newton (N)

newton (N)

newton (N)

kilonewton (kN)

newton (N)

newton per meter (N/m)

newton per meter (N/m)

meter (m)

meter (m)

centimeter (cm)

meter (m)

kilogram (kg)

kilogram meter squared (kg • m")

kilogram meter squared (kg m")

kilogram (kg)

kilogram (kg)

kilogram per square meter (kg/m2

)

kilogram per square meter (kg/m")

Multiply by

9.290 304 E-02

6.451 6 E-04

6.451 6 E+00

9.806 65 E+00

9.806 65 E+00

4.448 222 E+03

4.448 222 E+00

4.448 222 E+00

1.459 390 E+01

1.751 268 E+02

3.048 E-01

2.54 E-02

2.54 E+00

1 .0 E-06

9.806 65 E+00

4.214 011 E-02

2.926 397 E-04

1 .0 E+03

9.071 847 E+02

4.882 428 E+00

7.030 696 E+02

NIST NCSTAR 1-3, WTC Investigation xxi
Metric Conversion Table

To convert from to Multiply by

MASS DIVIDED BY LENGTH

pound per foot (lb/ft)

pound per inch (lb/in.)

pound per yard (lb/yd)

kilogram per meter (kg/m)

kilogram per meter (kg/m)

kilogram per meter (kg/m)

1.488 164E+00

1.785 797E+01

4.960 546 E-01

PRESSURE or STRESS (FORCE DIVIDED BY AREA)

kilogram-force per square centimeter (kgf/cnr ) pascal (Pa)

kilogram-force per square meter (kgf/irr) pascal (Pa)

kilogram-force per square millimeter (kgf/mnr) pascal (Pa)

kip per square inch (ksi) (kip/in.~) pascal (Pa)

kip per square inch (ksi) (kip/in.") kilopascal (kPa)

pound-force per square foot (lbf/fr) pascal (Pa)

pound-force per square inch (psi) (lbf/in.") pascal (Pa)

pound-force per square inch (psi) (lbf/in.

-

) kilopascal (kPa)

psi (pound-force per square inch) (lbf/in.") pascal (Pa)

psi (pound-force per square inch) (lbf/in.") kilopascal (kPa)

9.806 65 E+04

9.806 65 E+00

9.806 65 E+06

6.894 757 E+06

6.894 757 E+03

4.788 026 E+01

6.894 757 E+03

6.894 757 E+00

6.894 757 E+03

6.894 757 E+00

TEMPERATURE

degree Celsius (°C)

degree centigrade

degree Fahrenheit (°F)

degree Fahrenheit (°F)

kelvin (K)

kelvin (K)

degree Celsius (°C)

degree Celsius (°C)

kelvin (K)

degree Celsius (°C)

T/K = t/°C + 273.15

t/°C * t /deg. cent.

t/°C = (t/°F- 32)/1.8

T/K = (t/°F + 459.67)/ 1.8

t/°C = T/K 2 273.15

TEMPERATURE INTERVAL

degree Celsius (°C)

degree centigrade

degree Fahrenheit (°F)

degree Fahrenheit (°F)

deuree Rankine (°R)

kelvin (K)

degree Celsius (°C)

degree Celsius (°C)

kelvin (K)

kelvin (K)

1.0

1.0

5.555 556 E-01

5.555 556 E-01

5.555 556 E-01

xxn NISTNCSTAR 1-3, WTC Investigation
Metric Conversion Table

To convert from to Multiply by

VELOCITY (includes SPEED)

foot per second (ft/s)

inch per second (in./s)

kilometer per hour (km/h)

mile per hour (mi/h)

mile per minute (mi/min)

meter per second (m/s)

meter per second (m/s)

meter per second (m/s)

kilometer per hour (km/h)

meter per second (m/s)

3.048 E-01

2.54 E-02

2.777 778 E-01

1.609 344 E+00

2.682 24 E+01

VOLUME (includes CAPACITY)

cubic foot (ft

3

)

cubic inch (in.

3

)

cubic yard (yd3

)

gallon (U.S.) (gal)

gallon (U.S.) (gal)

liter (L)

ounce (U.S. fluid) (fl oz)

ounce (U.S. fluid) (fl oz)

cubic meter (m )

cubic meter (m3

)

cubic meter (m3 )

cubic meter (m3

)

liter (L)

cubic meter (m3

)

cubic meter (m3

)

milliliter (mL)

2.831 685 E-02

1.638 706 E-05

7.645 549 E-01

3.785 412 E-03

3.785 412 E+00

1.0 E-03

2.957 353 E-05

2.957 353 E+01

NISTNCSTAR 1-3, WTC Investigation xxiii
Metric Conversion Table

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xxiv NISTNCSTAR 1-3, WTC Investigation
Preface

Genesis of This Investigation

Immediately following the terrorist attack on the World Trade Center (WTC) on September 1 1, 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 investigate the building construction, the materials used, and the technical conditions that contributed to the outcome of the WTC disaster.

• To serve 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 1-3, WTC Investigation \\\
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-l, and the key interdependencies among the projects are illustrated in Fig. P-l.

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 performance 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

Environment; 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. ;

XXVI NISTNCSTAR 1-3, WTC Investigation
Preface

NIST WTC Investigation Projects

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 1-3, WTC Investigation xxvii
Preface

• Robert Hanson, Professor Emeritus, University of Michigan

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

Inc.

• Kathleen Tierney, 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 public 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.

xxvin NIST NCSTAR 1-3, WTC Investigation
Preface

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

Date Location Principal Agenda

June 24, 2002 New York City, NY Public meeting: Public comments 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 bnefinf? on release of the Public I Indaip anH NIST rpnuesl

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 Advisory Committee meeting on plan for and progress on WTC Investigation with a public comment session. May 7. 2003 New York City, NY Media briefing on release of May 2003 Progress Report. Anrmst ">6-">7 ">003 (^i^itHpr^hi ire*" TVID 1NV_ l_) 1 AUV JaUlj v, U111I11I I ICC HILL 1 IIIL Ull MalUo Ul LUC VV 1 investigation with a public comment session. September 17, 2003 New York City, NY Media and public briefing on initiation of first-person data

iT\l CU11CCIIU11 1 pf*ti An nrniprtc IJl U | tv to

Dpremher 1—3 ">ft03 fiJiitriPtxhiircT v_J Ci J L11C1 DUtll lVIL/ K/1D MilNV^OsT An\'icnn/ inmniitlpp rripptiriti r*n ctnfiic anH initial rpciiltt 1 rVUv 15U1 V v. Ulllllll UCC lllCCllilL; Uil r>talUj alJU llllLlaJ ICMlJlo and release of the Public Update with a public comment session. February 12, 2004 New York City, NY Public meeting on progress and preliminary findings with public

L K

1

1 1 1 1 1 IL 1 1 1 > Ull loiUCS IU UC LUIIMUCICU 111 1 Ul 1 1 1 11 1 a 11 11 u llllal recommendations.

JU11C Tunp JO, 1 8 *-UUH ">0ft4 Npw INCVv Yr>rV I OIK Titv v„ny, MYIN I IVlCUla/ UUU11L UllCllil^, Ull 1 ClCaoC Ul Ji/Z/C Z \J\JH it U^r too J\t.fJUr I. June 22-23, 2004 Gaithersburg, MD NCST Advisory Committee meeting on the status of and

pi Cllllllllal yilllUlll^o 11U111 111C VV 1 V_ ill VCMl£,allUll Willi a pUUHC

comment session. August 24, 2004 Northbrook, IL Public viewing of standard fire resistance test of WTC floor oVMClll al U I1UC1 W1 1 IC1 o L^aVXJL alUl JCd, IIIL . 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 Advisory Committee discussion on draft annual report to Congress, a public comment session, and a closed session to

piicp*i ice t~\ rp> pi »q T-f »*p» c f\ miTipnrl citi Anc t pr \a/ 1 I ln\/(^ctio, 5itiP\n LUoCLloo UI C ill all i CLUlllIllCllUallUllo 1U1 VV IL. Ill VCMlu,allUll. April 5, 2005 New York City, NY Media and public briefing on release of the probable collapse

aCUUCIlCC lOI U1C VV 1 V. lUWCIo allU UJall ICpUlla 1U1 LI1C piUJCClD Ull 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,

2005

Gaithersburg, 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 1-3, WTC Investigation XXIX
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 1A. 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 Safely

Investigation ofthe World Trade Center Disaster: Final Report on the Collapse of World Trade Center 7. NIST NCSTAR 1A. 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 1-1B. 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 1-1C. 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

Construction of World Trade Center I, 2, and 7 and Post-Construction Provisions Applied after Occupancy. NIST NCSTAR 1-1 D. 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 ofCodes, Standards, and Practices in Use at the Time ofthe

Design and Construction of World Trade Center I, 2, and 7. NIST NCSTAR 1 -IE. 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: Comparison ofthe 1968 and Current (2003) New

xxx NIST NCSTAR 1-3, WTC Investigation
Preface

York City Building Code Provisions. NIST NCSTAR 1-1F. 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 1-1G. 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 I and 2. NIST NCSTAR 1 - 1 H. 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 1-1 1. 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 1-2A. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Kirkpatrick, S. W., R. T. Bocchien, 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 1-3, WTC Investigation XXXI
Preface

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

Disaster: Steel Inventory and Identification. NIST NCSTAR 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 11,

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.

xxxn NIST NCSTAR 1-3, WTC Investigation
Preface

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

J. Yang, G. Mulholland, 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 ofStructural 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 1-5D. 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. Starnes, J. L. Gross, J. C. Yang, S. Kukuck, K. R. Prasad, and R. W. Bukowski.

2005. Federal Building and Fire Safet}> 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 1-3. WTC Investigation XXXlll
Preface

T. P. McAllister. 2005. Federal Building and Fire Safety Investigation of the 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.

McAllister, 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 1-6H.

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 ofthe 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 Survey Administration. 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.

xxxiv NIST NCSTAR 1-3, WTC Investigation
Acknowledgments

The National Institute of Standards and Technology (NIST) thanks the many people who contributed

valuable skills, facilities, and information that made this report possible, including the following.

Maureen Williams, Kil-Won Moon, Rodney Jiggetts, Michael Savage, Donald Kramer, David Kelley,

Deb Basak, Tessa Dvorak, Brian Goudy, Raymond Santoyo, Mark Iadicola, Lonn Rodine, Don Harne,

Jim Fink, Sandy Claggett, Carelyn Campbell, Richard Ricker, David Dayan, Paul Brand, Mike Kennedy,

Greg Turk, John Sieber, and Michael Winchester, all of NIST, are thanked for numerous technical

contributions.

Volunteers of the Structural Engineers Association of New York are thanked for their efforts in the steel recovery: Amit Bandyopadhyay, Anamaria Bonilla, Peter Chipchase, Anthony Chuliver, Edward

DePaola, Louis Errichiello, James Fahey, Jeffrey Hartman, David Hoy, Dean Koutsoubis, Andew

McConnell, Rajani Nair, Alan Rosa, Gary Steficek, Kevin Terry. Countless hours were unselfishly spent

searching for these invaluable pieces that are an integral component of this investigation. Further, the

following people are recognized for outstanding leadership roles in the recovery effort:

• Ramon Gilsanz, primary leader of the recovery effort

• David Sharp, coordinated volunteer activities; authored guide selection of suitable pieces

• Audrey Massa of the Federal Emergency Management Agency (FEMA), documentation and

cataloguing of efforts and pieces to be saved.

The FEMA/American Society of Civil Engineers Building Performance Study team, Professor A.

Astaneh-Asl (University of California, Berkeley), and the National Science Foundation are recognized for

their help in the recovery effort; Blanford Land Development Corporation, Hugo Neu Schnitzer, Inc., and

Metal Management, Inc., for their assistance and patience during the review, abatement, and removal of

pieces of interest from the salvage yards; and the Port Authority of New York and New Jersey for its assistance in surveying, sectioning, and transporting the technically significant steel collected and stored

at John F. Kennedy International Airport.

Dr. George Krauss (University Emeritus Professor, Colorado School of Mines), Dr. Elliot Brown (EB

Consulting, Golden, CO), and Mr. Arlan O. Benscoter and Dr. Arnold R. Marder (Lehigh University), are recognized for technical discussions regarding the metallurgy of the steels in the World Trade Center

(WTC) towers.

Professor Hugh MacGillivray (Imperial College, London, UK) and Professor David Matlock (Colorado

School of Mines) are recognized for technical discussions regarding high strain rate testing, and Professor

Matlock for use of the high-rate tensile testing machine at the Colorado School of Mines.

Dr. Amelia Logan and Dr. Edward P. Vicenzi of the National Museum of Natural History, Smithsonian

Institution, are thanked for analytical examination of the corrosion products observed on the recovered

WTC steel.

NIST NCSTAR 1-3, WTC Investigation \\\\
Acknowledgments

Dr. Kazushige Tokuno (Nippon Steel USA) is recognized for supplying historical infonnation on Yawata

steel.

Former employees of Laclede Steel (St. Louis, Missouri) are recognized, including David McGee and

Larry Hutchison, for locating Laclede records for NIST investigators during a visit to the Laclede steel

mill.

Professors John Fisher and Al Pense (Lehigh University), and Drs. Keith Taylor and Bruce Bramfitt

(International Steel Group), are thanked for discussions of WTC era steels and properties.

XXXVI NISTNCSTAR 1-3, WTC Investigation
Executive Summary

E.1 OVERVIEW

The World Trade Center (WTC) towers collapsed on September 1 1, 2001, as a result of damage inflicted by aircraft and the ensuing fires. The properties of the steel played an important role in how the building

performed, from the initial impact to the final collapse. Structural steel recovered from the site has been a valuable resource in the investigation of the disaster, providing information ranging from details of

structural response to the aircraft impact to data on steel properties for insertion into models of building

performance.

This report is an overview of the mechanical and metallurgical analysis of structural steel from the WTC,

part of the National Institute of Standards and Technology (NIST) Investigation of the WTC disaster. The purpose was to analyze structural steel available from WTC 1, 2, and 7 to determine the metallurgical

and mechanical properties and quality of the metal, weldments, and connections, and provide these data to other parts of the NIST WTC Investigation for insertion into models of building performance. The

analysis focused on steel from the towers to provide data for modeling building performance and

characterizing the steel quality. In addition, the steel used in the construction of WTC 7 is described

based solely on data from the literature, because no steel from the building was recovered.

The three goals were to:

1. Determine mechanical properties of WTC structural steel,

2. Determine the quality of the steel and if it met its design requirements, and

3. Analyze and provide insight into failure mechanisms for guiding the development of models

of building performance and validating their output.

E.1.1 Building Design and Steel Specifications

Building plans and material specifications from the construction era provided a starting point for the

study. Thousands of pages of design documents were reviewed. Most valuable were the structural steel design drawings for the WTC towers provided by the Port Authority of New York and New Jersey. In addition, Laclede Steel Company, the fabricator of the floor trusses, provided construction-era documents

that showed, amongst other information, that steels with higher strength than specified were used in the

floor truss systems. Numerous other sources, including Yawata (now Nippon) Steel documents on

perimeter column steel, provided essential insights into the steel fabrication and properties.

From the standpoint of building design, the towers were unique based on the number of different steels specified for construction. Fourteen different strengths of steel were specified in the design drawings,

although only 12 were actually used. Most modern buildings use no more than two or three different

strengths of steel.

NISTNCSTAR 1-3, WTC Investigation XXXVll
Executive Summary

Furthermore, more than a dozen suppliers and fabricators supplied steel for the buildings. As a result, even when the different steels met a single specification, their properties could be significantly different. These complications resulted in more than forty different steels being used in the tower structures, all of

which are characterized to some extent in this study.

A total of 236 recovered pieces of WTC steel were cataloged; the great majority belonging to the towers,

WTC 1 and WTC 2. These samples represented a quarter to half a percent of the 200,000 tons of

structural steel used in the construction of the two towers. The NIST inventory included pieces from the

impact and fire regions, perimeter columns, core columns, floor trusses, and other pieces such as truss seats and wind dampers.

The original, as-built locations of 42 recovered perimeter panels and 12 recovered core columns were

determined, based on markings and geometry of the columns. Samples were available of all 12 strength

levels of perimeter panel steel, the two strength levels of the core column steel that represented 99 percent

of the total number of columns, and both strength levels used in the floor trusses. A number of structural pieces were recovered from locations in or near the impact- and fire-damaged

regions of the towers, including four perimeter panels directly hit by the airplane and three core columns

located within these areas. These pieces provided opportunity for failure and other forensic analyses.

The collection of steel from the WTC towers was sufficient for determining the quality of the steel and, in combination with published literature, for determining mechanical properties as input to models of

building performance.

Extensive failure analysis of the recovered steel was conducted to determine damage characteristics,

failure modes, and fire-related degradation of the recovered structural components. In addition, pre- collapse photographic evidence of the impact damage and location and intensity of the fires was used to distinguish between pre- and post-collapse damage.

Two sets of observations were made:

1 . Pre-collapse analysis concentrated on impact damage sustained by the perimeter panel

sections. The analysis employed enhanced photographic and video images of the towers and was largely limited to the perimeter panels, because the core columns and most floor trusses are not visible in the images. The images were also used to determine the location of pre- collapse fires, damage to fire proofing, and possible fire-related damage to the panels. In

addition, some details of pre-collapse, time-dependent deformation of the structure, including

bowing of perimeter columns across wide areas of the perimeter walls, were characterized.

2. Post-collapse analysis concentrated on the damage characteristics of the recovered structural

steel elements. The perimeter panels were again evaluated, as well as the core columns and

the connectors or seat assemblies used to attach the floor trusses to the core columns and

E.2 INVENTORY OF RECOVERED STEEL

E.3 DAMAGE AND FAILURE MODES OF THE STRUCTURAL STEEL

XXXVlll NISTNCSTAR 1-3, WTC Investigation
Executive Summary

perimeter column panels. Of particular importance were the samples located near the airplane impact regions and those where fires burned before the collapse of the buildings.

In addition to the NIST analysis, an outside contractor made an independent study of the recovered steel elements. In general, the observations concerning local failure mechanisms by the contractor agreed with

those that NIST found.

The observations of fracture and failure behavior, described below, were used by the groups that modeled

the building performance during impact and subsequent fire to guide the development of their models and

validate their results.

E.3.1 Structural Impact Damage - Perimeter Panels

Correlation between pre-collapse photographs and the recovered exterior panels from the impact zone

indicated that two of the four recovered impact-damaged panels were in a condition similar to that just

prior to building collapse. Some damage can be attributed to the events during and after collapse, but the general shape and appearance of the recovered pieces match the damage photographs.

The fracture behavior of the plates from the recovered columns that were hit directly by the aircraft was

of particular interest. Transverse fractures of these plates (e.g., fracture through the outer webs of two

impacted panels that occurred perpendicular to the rolling direction) exhibit ductile characteristics,

including necking and thinning away from the fracture. These features indicate that the steel behaved in a ductile manner under very high strain rates.

Conversely, fractures that occurred parallel and directly adjacent to welded joints exhibited little or no

ductile characteristics. Diminished properties of the heat-affected zone in the base plate, the geometry of

the joint with respect to the direction of impact, stress concentrations due to the geometry, and the

orientation of the crack propagation with respect to the rolling direction of the plate contributed to the

lack of ductility. There was no evidence to indicate that the joining method, materials, or welding

procedures were inadequate. The welds appeared to perform as intended.

In general, perimeter columns severed by the aircraft wings fractured at the internal stiffener or diaphragm plate associated with the spandrel connection to column. Other perimeter columns hit by the

plane tended to fracture along heat-affected zones adjacent to welds. Perimeter columns outside the

impact zone did not exhibit this behavior.

The failure mode of spandrel connections on perimeter panels differed above and below the impact zone. At or above the impact zone, bolt hole tear-out was more common. Below the impact zone, it was more common for the spandrels to be ripped off from the panels. The change in mode may be due to shear

failures as the weight of the building during collapse came down on these lower panels. There was no

evidence that fire exposure changed the failure mode for the spandrel connections

With the exception of the mechanical floors, the perimeter panel column splices failed by fracture of the

bolts. At mechanical floors, where splices were welded in addition to being bolted, the majority of the

splices in the several recovered columns did not fail.

NISTNCSTAR 1-3, WTC Investigation XXXIX
Executive Summary

E.3.2 Structural Impact Damage - Core Columns

Failure was noted in a limited number of recovered core columns as a result of both splice connection

failures and fracture of the columns themselves. One recovered core column (WTC 2, column line 801,

floors 77-80) may have sustained damage as a direct result of the airplane impact; however, the welded

splice to the column above survived intact.

In both towers, most of the perimeter panel floor truss connectors (perimeter truss seats) below the impact

floors were either missing or bent downward. Above this level, the failure modes were more randomly

distributed. This behavior apparently resulted from the building collapse sequence.

Failure of the welds associated with the perimeter panel floor truss connectors (perimeter seats) typically

occurred as a result of the weld geometry, where the fracture occurred at the smallest cross-sectional area. Typically, these were the standoff plates. However, there was no evidence to indicate that the type of

joining method, materials, or welding procedures were inadequate.

Of the 31 core floor truss connectors (core seats) recovered, about 90 percent were still intact, although

many were extensively damaged. Only two were completely torn from the channel. This distribution

may have resulted from the process used to select recovered steel from at the salvage yards, however.

In the recovered floor trusses, a large majority of the electric resistance welds at the web-to-chord

connections failed. Failure of the connection between the floor truss and the perimeter panel floor truss connectors was typically a result of tab plate weld and bolt failure.

E.3.4 Damage to Fireproofing Due to Aircraft Impact

Pre-collapse photographs indicated that, as expected, fire-proofing was removed from pieces struck by the

incoming aircraft or debris exiting the far side of the buildings. In addition, the impact caused fire- proofing and aluminum facade panels to spall off many perimeter columns which were not directly struck nor severed, but apparently suffered strong accelerations and forces transmitted through the structure. This indirect damage to the sprayed fire-resistive materials (SFRM) was observed on the north and east faces of WTC 2. A coating on the SFRM prevented the loss of the SFRM in some locations on the perimeter columns.

This coating appeared as a band of white features on the SFRM wherever two aluminum panels met on

the exterior columns of the buildings, becoming visible when the panels were dislodged. This may be a coating applied to protect the SFRM from moisture infiltration at the aluminum panel joints, acting to preserve the SFRM even when the SFRM was knocked off both above and below those locations.

E.3.5 Time-Dependent Deformation of Perimeter Walls Due to Fire and Load

Images of WTC 1 showed gross deformations of the south wall prior to final collapse. Images taken

approximately 5 min prior to collapse showed inward bowing of the exterior columns, reaching an

observable maximum of about 55 in. near column 3 16 on the 96th floor. The inward deflection extended

E.3.3 Structural Damage - Floor Trusses and Seats

Redistribution

xl NISTNCSTAR 1-3, WTC Investigation
Executive Summary

over the entire south face of the building at this time, and was visible between the 94th and 1 00th floors. Photographs taken approximately 35 min prior to collapse did not show any inward bowing of the south

faceofWTC 1.

Approximately 18 min after the impact of the aircraft, the east face of WTC 2 exhibited inward bowing of

up to 10 in. in the region of the 79th to 83rd floors. This inward bowing increased to 20 in. at 5 min

before collapse of the tower.

Sagging floor slabs at the 82nd and 83rd floors were visible in window openings on the east and north

faces, respectively, of WTC 2 and the positions of these slabs changed over time. This suggests a progression of failure of certain parts of the flooring in this area of the tower.

At the moment of collapse of WTC 2, the top portion of the building was found to have moved to the

west as it tilted to the southeast. During this tilting, a complex kink developed at the southeast corner of

the top of the building, in the region of the 106th floor. In addition, the portion of the building above the

aircraft impact site twisted slightly clockwise (as viewed from above) as the collapse progressed.

E.3.6 Fire Exposure and Temperatures Reached by the Steel

The pre-collapse photographic analysis showed that 16 recovered exterior panels were exposed to fire prior to collapse of WTC 1. None of the nine recovered panels from within the fire floors of WTC 2 were

observed to have been directly exposed to pre-collapse fires.

NIST developed a method to characterize maximum temperatures experienced by steel members using

observations of paint cracking due to thermal expansion. The method can only probe the temperature

reached and it cannot distinguish between pre- and post-collapse exposure. More than 170 areas were examined on the recovered perimeter column panels; however, these columns represented only 3 percent

of the perimeter columns on the floors involved in fire and cannot be considered representative of other

columns on these floors. Only three locations had evidence that the steel reached temperatures above

250 °C. These areas were:

• WTC 1, east face, floor 98, column 210, inner web,

• WTC 1, east face, floor 92, column 236, inner web,

• WTC 1, north face, floor 98, column 143, floor truss connector

Other forensic evidence indicates that the last example probably occurred in the debris pile after collapse.

Annealing studies on recovered steels established the set of time and temperature conditions necessary to

alter the steel microstructure. Based on the pre-collapse photographic evidence, the microstructures of

steels known to have been exposed to fire were characterized. These microstructures show no evidence

of exposure to temperatures above 600 °C for any significant time.

Similar results, i.e., limited exposure if any above 250 °C, were found for the two core columns recovered

from the fire-affected floors of the towers, which had adequate paint for analysis. Note that the perimeter

NISTNCSTAR 1-3, WTC Investigation xli
Executive Summary

and core columns examined were very limited in number and cannot be considered representative of the

majority of the columns exposed to fire in the towers.

Perimeter columns exposed to fire had a greater tendency for local buckling of the inner web than those

known to have no exposure. A similar correlation did not exist for weld failure.

E.4 MECHANICAL PROPERTIES

Mechanical properties of the steel were determined using tests at room temperature (for modeling

baseline performance), high temperature (for modeling structural response to fire), and at high strain rates (for modeling the airplane impact). In addition, the structural steel literature and producer documents were used to establish a statistical basis for the variability expected in steel properties and for modeling

steel properties as a function of temperature and strain rate for insertion into building performance

models. A total of 32 distinct steels (various strengths and suppliers) were modeled in this manner. A comparison of mechanical properties with relevant specifications and properties required by the design

documents is included. The steel used in the construction of the WTC generally met the expectations of

the designers and the specifications called for in the steel contracts. Approximately 87 percent of all tested steel exceeded the required minimum yield strengths specified in design documents, and

approximately 1 3 percent of the damaged steel tested did not meet the required minimum yield strengths.

The occurrence of test results below the specified minimum values is not unexpected because differences

in test procedures from those in the qualifying mill tests could account for 2 ksi to 3 ksi lower values in the NIST tests, the loss of a yield point due to damage to the steel accounts for 2 ksi to 4 ksi lower values

in the NIST tests in several cases, and variability exists within a heat of steel relative to the ASTM

specified test location.

The yield strengths of the perimeter column steels generally exceeded their specified minimums by

10 percent to 15 percent. The tensile properties of the perimeter columns are consistent with literature estimates for average properties of construction steel plate during the WTC construction era. The number

of occurrences of plates with tensile properties at or slightly below the specified minimum is consistent

with the historical variability of steel strength.

The yield strengths of steels in the core columns, with a few exceptions, exceeded the specified minimum.

The yield strengths of some wide-flange shapes were lower than called for in the specifications but, as

stated above, this probably arose from a combination of mechanical damage that removed the yield point,

differences between the NIST and original mill test report testing protocols, and variability within a heat

of steel relative to the ASTM International (ASTM) specified test location. Regardless of the source, the

observed distributions are accounted for in the typical design factor of safety for allowable stress design.

The yield strengths of undamaged steels in the rolled core columns, however, were lower than the

historical literature indicates as typical.

The strength of the steel in the floor trusses was higher than called for in the original specifications. Many

of the truss steels that were specified as low strength A 36 were supplied as high-strength, low-alloy steels with much higher strengths. Laclede Steel Company's substitution of 50 ksi yield strength steel for A 36

in the lower chord of the trusses is expected to have provided significantly improved performance at high

temperature.

xlii NISTNCSTAR 1-3, WTC Investigation
Executive Summary

The limited tests on bolts indicated that their strengths were greater than the specified minimums, and

they were stronger than contemporaneous literature suggests as typical.

Limited tests on recovered welds and weld material indicated that their mechanical properties and

chemistry were consistent with their intended specifications.

The strain rate sensitivity and high-strain rate ductility of the perimeter and core column steels were

similar to other construction steels of the WTC era. The impact toughness of steels from the perimeter

and core columns, and floor trusses was consistent with literature values for the WTC construction era.

The high-temperature yield and tensile strength behavior of WTC steels was similar to behavior of

construction steels from the WTC construction-era literature. The creep behavior of WTC steels was

found to be best modeled by scaling WTC-era literature data using room temperature tensile strength

ratios.

In summary, the steel used in the construction of the WTC towers met the expectations of the designers

and the specifications called for in the steel contracts. Material substitutions of higher strength steels were common in the perimeter columns and floor trusses. The safety of the WTC towers on

September 11, 2001, was most likely not affected by the fraction of steel that, according to N1ST testing,

did not meet the required minimum yield strength. The typical factors of safety in allowable stress design can accommodate the measured property variations below the minimum.

E.5 PHYSICAL PROPERTIES

A number of physical properties of the structural steel from the towers were either measured or estimated.

Of these, composition and microstructure were characterized for many of the recovered pieces in order to identify the specifications to which the steel was fabricated, enabling a better characterization of the

mechanical properties of the steel. In addition, thermal properties were estimated as a function of

temperature in order to provide input to models of the steel thermal response to the fires.

Chemical analyses of the flange, outer web, and spandrel plates of the exterior panel sections were found

to be nearly identical for a given plate gauge and yield strength, as were inner web plates with yield

strengths equal to 80 ksi or 100 ksi. In contrast, inner web plates with yield strengths less than 80 ksi were found to be chemically distinguishable from the other plates. These results support the

contemporaneous documents stating that Yawata Steel (now Nippon Steel) produced all flange, outer web, and spandrel plates, and that inner web plates were primarily supplied domestically. The alloying

practices were typical for steels of this era.

Chemistry results for the core columns varied for any given strength and shape (built-up box, rolled wideflange). This result supports the contemporaneous documents that state numerous suppliers produced the

steel to be used for these structural components. All core columns tested met chemistry specifications for one of the numerous ASTM structural steel grades available during the construction era.

Floor truss rods and chords, manufactured by Laclede Steel Co., met chemistry specifications for ASTM A 242. Contemporaneous construction documents indicated that Laclede Steel Co. routinely

upgraded A 36 components to A 242 steel.

NISTNCSTAR 1-3, WTC Investigation
Executive Summary

Two types of bolts were specified for construction of the towers, A 325 and A 490 bolts. Stampings on

the bolt heads clearly indicated the bolt type. Chemical analysis indicates the A 325 bolts met the A 325

Type 1 chemistry specifications.

A majority of the other structural components (floor truss seats, diagonal bracing straps, gusset plates, core channels, etc.) met chemistry specifications for ASTM A 36 grade steel, which was the default steel when strengths were not specified on the design drawings.

No steel was recovered from WTC 7; however, construction-related documents describe the structural

steel as conventional 36 ksi, 42 ksi, and 50 ksi steels. The building plans called for rolled column shapes

conforming to two ASTM grades: A 36 and A 572 Grade 50. Cover plates for the heaviest plates were

specified as A 588 Grades 42 and 50, and A 572 Grade 42. There is a substantial literature base on the

properties of these steels.

The literature of the construction period was researched in order to estimate properties and provide

models of room temperature mechanical properties of the various steels. Methods developed for the

WTC tower steels were used to model the high temperature mechanical performance for the building

performance models of WTC 7 response to the fires.

Based on the Investigation findings, NIST identified a detailed set of issues related to practice, standards,

and codes that provided the basis for formulating the Investigation's draft recommendations. The

Investigation team has studied practices ranging from those used during construction of the towers to newly available practices that could improve the safety and performance of high-rise buildings. The

recent development and use of "fire-resistant" steel in Europe and Japan falls in the latter category.

Fire-resistant steels are reported to retain a higher fraction of their room temperature strength at temperatures expected in building fires, and are used either with or without fire protection depending on

the application. If fire-resistant steels do indeed retain improved high temperature properties, then

improved fire resistance would be expected even in cases where conventional fire protection has been

damaged, whether during normal construction and modifications, or due to other damage as in the attack on the WTC towers.

Each issue identified by NIST was divided into three levels with between two and five categories each:

• Categories in Level 1 : practices; standards, codes, and regulations; adoption and enforcement;

research and development or requiring further study; and education and training.

• Categories in Level 2: all tall buildings (buildings over 10 stories in height); selected tall buildings (buildings over 10 stories in height that are at risk due to design, location, use, iconic status, contents, etc.); selected other buildings (buildings that are at risk due to design,

location, use, historic/iconic status, contents, etc.).

xliv NISTNCSTAR 1-3, WTC Investigation

E,6 STRUCTURAL STEEL IN WTC 7

E.7 ISSUES
Executive Summary

• Categories in Level 3: related to the outcome on September 1 1

, 2001 (i.e., could have

changed the outcome); or unrelated to the outcome on September 1 1, 2001 (i.e., would not have changed the outcome yet is an important building and fire safety issue that was

identified during the course of the Investigation).

Under Level 1, the fire-resistant steel issue is considered to include practices and research and

development or requiring further study. The fire-resistant steel issue under Level 2 applies to all tall buildings and selected other buildings. Under Level 3, the use of fire-resistant steel in the WTC towers

may have increased the time to collapse or even prevented the collapse of the towers.

Discussion of this issue is not intended to suggest that fire-resistant steels should have been used in the construction of the WTC towers, or even that fire-resistant steels were available commercially at the time

of their construction.

NISTNCSTAR 1-3, WTC Investigation xlv
Executive Summary

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xlvi NISTNCSTAR 1-3, WTC Investigation
Chapter 1 Introduction

1 .1 OVERVIEW AND SCOPE

The collapses of the World Trade Center (WTC) towers on September 11, 2001, were a result of damage

inflicted by the aircraft impact and the ensuing fires within the two buildings. The performance of the

steel within the structures played a key role in how the buildings performed, from impact to final collapse.

Structural steel recovered from the site has been a valuable resource in the investigation of the disaster,

providing information ranging from details of structural response to impact, to data on steel properties for insertion into models of building performance.

This report provides an overview of the mechanical and metallurgical analysis of structural steel from the

WTC, part of the National Institute of Standards and Technology (NIST) Investigation of the WTC

disaster. The purpose was to analyze structural steel available from WTC 1, 2, and 7 to determine the metallurgical and mechanical properties and quality of the metal, weldments, and connections.

The analysis focused on steel from the towers to provide:

1. Mechanical properties of WTC structural steel for insertion into building performance

models,

2. An assessment of the quality of the steel and design requirement compliance, and

3. Analysis and insight into failure mechanisms for guiding and validating models of building

performance.

A total of 236 pieces was recovered and cataloged, and the original, as-built location for many of the pieces has been identified. These pieces include perimeter columns, core columns, trusses, and other

pieces such as truss seats and wind dampers, including many pieces from the fire and impact regions.

NIST has examples of the many grades of perimeter columns and spandrels, the two grades of wideflange and built-up box core columns (representing 99 percent of the core columns), the two grades of

steel in the floor trusses, and samples of core and perimeter truss seats. The collection of steel from the

WTC towers was sufficient for determining the quality of the steel and, in combination with published

literature, for determining mechanical properties as input to models of building performance.

WTC 7 was constructed in 1986, using conventional 36 ksi, 42 ksi, and 50 ksi steel produced to ASTM

International (ASTM) specifications. Since no steel from WTC 7 was recovered from the site, the steel used in the construction of this building is described based on data from the literature of the period.

From the standpoint of building design, the towers were unique based on the number of different steels specified for construction. Fourteen different strengths of steel were specified in the design drawings,

compared with two or three different steels in most modern buildings. Furthermore, there were numerous

suppliers and fabricators of the steel used in the buildings, sometimes with significantly different steel

NISTNCSTAR 1-3, WTC Investigation 1
Chapter 1

properties even when supplying to the same specification. These complications resulted in at least 32 different steels being used in the tower structures, all of which are characterized based on the approach

described in the following section.

1.2 APPROACH

This Investigation analysis was divided into six tasks as follows:

• Task 1 —Colled and catalog the physical evidence (structural steel components and connections)

and other available data, such as specifications for the steel, the location ofthe steel pieces

within the buildings, and the specified steel properties.

This task consisted of two major areas. The first was the collection and analysis of the enormous amount of contemporaneous ( 1 960s era) documentation, ranging from design, specification and

construction documents to data from the numerous structural steel fabricators. Results of the

analysis are summarized in

Chapter 2 - Tower Design - Structural Steel Documents

Chapter 3 - Contemporaneous Steel Specifications

Chapter 4 - Contemporaneous Construction Specifications

The second major area under this task involved cataloging the structural steel recovered from the WTC site. The 236 recovered pieces included many examples of the structural elements of major

importance, such as core columns, perimeter panels, floor trusses, and truss seats. These pieces,

and in many cases the precise as-built location within the buildings, are described in

Chapter 5 - Steel Inventory and Identification

• Task 2—Document failure mechanisms and damage based on visual obsei-vations ofrecovered

steel, especiallyfor available columns, connectors, andfloor trusses.

Extensive failure analysis of the recovered steel was conducted. In addition, pre-collapse

photographic evidence of the impact damage and location and intensity of the fires was used to characterize damage to the buildings due to aircraft impact and details of damage to structural elements and fire-proofing. These images were also used to distinguish between pre- and postcollapse damage. The response of the building to the fire and redistribution of loads resulted in bowing of perimeter columns, which was characterized as a function of time. Details of fracture and failure behavior were supplied to the NIST staff who were modeling building performance

during impact and subsequent fire to provide guidance and validation of model results. These

failure studies are summarized in

Chapter 6 - Damage and Failure Analysis of Structural Steel

• Task 3—Determine the metallurgical and mechanical properties ofthe steel, weldments, and

connections, including temperature dependence ofproperties. Identify the grades ofsteel in the columns, welds, spandrels, trusses, truss seats, and fasteners, based on composition,

microstructure, and mechanical and impact properties. Provide steel property data, including

models ofelevated temperature behavior for relevant steels, to those Investigation efforts

2 NISTNCSTAR 1-3, WTC Investigation
Introduction

modeling damage to the structural steel members from aircraft impact, structural fire response,

and initiation andpropagation ofstructural collapse.

The mechanical test program included testing of representative samples of the numerous types of

steel in the WTC towers. A variety of test methods were used to characterize room temperature

properties, tensile properties at high-strain rates, and performance at high temperatures typical of

building fires. In addition, the structural steel literature and producer documents were used to establish a basis for the statistical variability expected in steel properties and for modeling steel properties as a function of temperature and strain rate for insertion into building performance

models. A total of 32 steels were modeled in this manner. A comparison of mechanical

properties with relevant specifications and properties required by the design documents is included. These are reported in

Chapter 7 - Mechanical Properties of Structural Steels

Composition and microstructure were characterized in order to fully identify the various steels. Other physical properties, needed for the finite element models of the buildings, were determined

from the literature, including specific heat, coefficient of thermal expansion, and thermal

conductivity, all as a function of temperature. These analyses, including relationships suitable for insertion into the building models, are reported in

Chapter 8 - Physical Properties of Structural Steels

Analogous information on mechanical properties of the structural steels of WTC 7. as derived

from the literature, is also summarized in Chapter 8.

• Task 4—Coirelate determined steel properties with the specified properties for constmction of

the buildings. The quality ofthe steel used in the buildings will be compared with that specified.

This determination depended upon mechanical properties and, thus, is included in Chapter 7.

• Task 5—Analyze the steel metallographically to estimate maximum temperatures reached,

recognizing that high-temperature exposure before the collapse may be difficult to distinguish

from exposure during post-collapse fires.

This analysis was closely tied to failure analysis and. thus, is reported in Chapter 6.

• Task 6—Report preparation.

The present report summarizes the results of the investigation of the recovered steel. In addition, the

following technical reports provide supporting technical details: Contemporaneous Structural Steel Specifications (NISTNCSTAR 1-3A')

Steel Inventory and Identification (NISTNCSTAR 1-3B)

Damage and Failure Modes of Structural Steel Components (NISTNCSTAR 1-3C)

Mechanical Properties of Structural Steels (NTST NCSTAR 1-3D)

Physical Properties of Structural Steels (NISTNCSTAR 1-3E)

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

to this report.

NISTNCSTAR 1-3. WTC Investigation 3
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NISTNCSTAR 1-3, WTC Investigation
Chapter 2

Tower Design - Structural Steel Documents

2.1 INTRODUCTION

This chapter describes the World Trade Center (WTC) tower structure and critical structural elements

relevant to the analysis of the structural steel from the towers. The description includes the structural design and properties specified by the structural engineers for columns, floor systems, and connections.

A more detailed description can be found in National Institute of Standards and Technology (NIST)

NCSTAR 1-3 A.

Information needed for the metallurgical characterization was obtained from the structural steel drawings

provided by the Port Authority of New York and New Jersey and the structural database assembled by

Leslie E. Robertson Associates for the NIST Investigation (NIST NCSTAR 1-2A). The complexities

introduced by the size of the structure and the specification of fourteen different strengths of steel led the designers to a tracking system whereby the steel fabricators stamped and/or stenciled each structural element with a unique identifying number (see Chapter 5). The structural engineering drawings included

these identifying numbers as well as the yield strength of the individual steel components. Thus, when

NIST was able to find the identifying number on an element such as a perimeter column panel, then the

particular steel specified for each component of the element was known.

2.2 SPECIFICATION OF STEEL GRADES (MINIMUM YIELD STRENGTH)

Specifications for structural steel (ASTM International [ASTM], American Iron and Steel Institute, etc.) typically place limits on chemical composition or mechanical properties or, most commonly, both.

Various mechanical properties may be specified, such as tensile strength, minimum yield strength,

ductility, and toughness. Other material properties may not appear in a specification, yet are critical in building design; the most important such property is perhaps the elastic modulus, or stiffness, which does

not appear in specifications because there is little variability amongst various steels.

In practice, the material property of greatest importance for characterizing a particular steel is the yield

strength (Fy

). In the United States, steel is often referred to according to its yield strength; for example, a "50 ksi steel" is steel with a minimum yield strength of 50,000 pounds per square inch. Skilling, Helle,

Christiansen, & Robertson (SHCR), structural engineers for the WTC towers, followed this convention,

and the design drawings were marked with the minimum yield strength for each piece of structural steel.

The WTC tower buildings had a frame-tube construction consisting of closely spaced perimeter columns

coupled to a rectangular service core (Fig. 2-1). The buildings had a square footprint, 207 ft 2 in. (63.14 m) on a side with chamfered corners. From the 9th to 107th floors, the perimeter columns

consisted of closely spaced built-up box columns. The service core at each building's center was

approximately 87 ft by 137 ft (26.5 m by 41.8 m) and connected to the perimeter columns by a floor panel

2.3 STRUCTURAL OVERVIEW

NIST NCSTAR 1-3, WTC Investigation 5
Chapter 2

system that provided an essentially column-free office space, see Fig. 2-1. In addition to showing the

location of perimeter and core columns, Fig. 2-2 describes the column numbering scheme used to identify

each column on a given floor. WTC Tower

Technical Services

if

j j

Skylobby

Technical Services

Skylobby

Technical Services

Express Elevators

Technical Services

Local

Elevators

Express

Elevators

Local

Bar

<9g

Exterior Framed-Tube

!0

Central Core Columns Floor Slab

59 columns Der side

Concrete

Floor Trough

Covering Decking

mm

AirElectric conditioning Duct Plaza Level "TH«itLLr' Elevators

Figure 2-1. Schematic diagram of the tower structure

The WTC tower structural steel plans (SHCR 1967) point out the major structural elements of interest. These features include the perimeter columns, the core columns and associated framing, the trusses that supported the floors, and the connections between and within these elements. In addition, a "hat truss" located within the 107th to 1 10th floors tied the core to the perimeter columns and provided a base for the

television mast atop WTC 1 and support for a proposed mast atop WTC 2.

The structural steel drawings provide the location, cross-sections, and grade of steel (i.e., required

minimum yield strength) for each of the thousands of structural elements in the buildings. In all, 14 different grades of steel were specified, ranging in yield strength from 36 ksi to 100 ksi. In addition to yield strength requirements, Port of New York Authority (PONYA) documents specified allowable steels, bolts and welds using ASTM or other standards.

6 NISTNCSTAR 1-3, WTC Investigation
Tower Design - Structural Steel Documents

N

101 103 106 109 112 115 118 121 124 127 130 133 136 139 142 145 148 151 154 157 159

459 457-

454-

451-

448-

445-

442-

439-

436-

433-

430-

427-

424-

421-

418-

415-

412-

409-

406-

403-

401

100v l n n i i n n l n n i n a 1 n in 1 1 n n 1 | n I I n n 1 o ,n 1 1 n 1 n i - | - J- - | - i ti - 1

Ml 502

f601 602

503

603

504 505

604 605

506

606

507

508i

608

607

N701

801

>901

702

802

1001

902

1002

703 yog 706

803 804 805

303 904 305 906

1003 1004 1005 1006

707

806

907

1007

708i

807*

908"

1008i

201

-203

-206

-209

-212

-215

-218

-221

-224

-227

-230

-233

-236

-239

-241

-245

-248

-251

-254

-257

259

; T TTTTTTTTTTTTTTTTTTTI 300

! 357 354 351 348 345 342 339 336 333 330 327 324 321 318 315 312 309 306 303 301 Figure 2-2. WTC 1 tower floor plan and column numbers.

2.3.1 Perimeter Columns

Between the 9th and 107th floors, the perimeter structure consisted of closely spaced, built-up box

columns. Each building face consisted of 59 columns spaced at 40 in. (1 .02 m). The columns were

fabricated by welding plates of steel to form an approximately 14 in. (0.36 m) square section (Fig. 2-3).

Adjacent columns were interconnected at each floor level by deep spandrel plates, typically 52 in. (1.32 m) deep (Fig. 2-4). The spandrels formed an integral part of the columns: there was no inner web

plate at spandrel locations. Spandrels were generally specified with a yield strength lower than that of the

column webs and flanges, as well as a heavier gauge than the adjacent inner webs. Spandrel thicknesses

ranged from 0.375 in. to 1.375 in.

NISTNCSTAR 1-3, WTC Investigation
Chapter 2

14.0 in. Outer Web (Plate 2)

Flange , (Plate 1) 11.0 in. 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 2-3. Cross-section of perimeter columns; sections

with and without spandrels.

; •- <i--

• C3 C2 ':m^^mm 2, <m Source: Unknown. Enhanced by NIST. Figure 2-4. Characteristic perimeter column panel consisting of three full columns

connected by three spandrels.

8 NISTNCSTAR 1-3, WTC Investigation
Tower Design - Structural Steel Documents

The perimeter columns were prefabricated into panels, typically three stories tall and three columns wide

(Fig. 2-4). Other than at the mechanical floors, panels were staggered (Fig. 2-5) so that only one-third of

the units were spliced (i.e., connected) in any one story. Heavy end, or "butt" plates with Fy = 50 ksi and

1.375 in. to 3 in. (3.5 mm to 7.6 mm) thick were welded to the top and bottom of each column. Fillet welds were used inside the columns along three edges, with a groove weld on the fourth, outside edge.

During erection, abutting spandrels were bolted together, and columns were bolted to the adjacent

columns, all using ASTM A 325 bolts except for the heaviest butt plates, which used ASTM A 490 bolts.

Floor

Floor

Floor

Floor

Floor

Floor

III III Ml Ml Ml Ml

iilliiii iiiiiiiiiiiiiiiiiiiii

Ffrf

iiliiiiiiiiiiiiiiiiil

Figure 2-5. Partial elevation of exterior bearing-wall frame showing exterior wall module

construction. Highlighted panel is three stories tall (36 ft) and spans four floors.

Distance between panels has been exaggerated.

Fourteen grades of steel were specified in the design documents for the perimeter columns, with

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

grades of steel were specified for the spandrels, with the same strength levels as the columns but without

the two highest strength steels. The structural steel drawings indicate that the flanges and webs of a given

column section consisted of a single grade (i.e., minimum yield strength) of steel, but each column and

spandrel within a single pre-fabricated panel could be fabricated from different grades of steel.

Perimeter columns in the upper stories were typically fabricated of lighter gauge steel, most commonly

0.25 in. (6.35 mm), with the grade of steel presumably dictated by the stresses induced by the calculated

gravity and wind loads. In this manner, the weight on the lower stories would be minimized (reducing

elastic distortion during construction), a design goal important in very tall buildings. A further

consideration may have been to reduce the weight and thus the difficulty in raising heavy structural pieces

as high as 1,300 ft above ground level. In contrast to the upper stories, in the lower stories, the perimeter

column flanges were as thick as 3 in. (76 mm) and typically made of the lower strength steels. The larger cross sections and lower applied stresses associated with the lower strength steels also served to decrease

the elastic deflections in the building.

NISTNCSTAR 1-3, WTC Investigation 9
Chapter 2

2.3.2 Core Columns

Core columns were of two types: welded box columns and rolled wide flange shapes. The columns in the

lower floors were primarily huge box columns as large as 12 in. by 52 in. (0.30 m by 1.32 m) composed

of welded plates up to 7 in. (178 mm) thick. In the upper floors, the box columns transitioned to the

rolled wide flange shapes. Fig. 2-6 indicates the transition floors for each of the core columns, and

Fig. 2-7 shows typical geometries of box columns and wide flange columns at the 84th floor. Core

columns were typically spliced at three-story intervals. The splices in the impact and fire zones of both

buildings were at floors 75, 77, 80, 83, 86, 89, 92, 95, 98, and 101. Diagonal bracing was used at the

mechanical floors and in the area of the hat truss. Core box columns were 36 ksi or 42 ksi. Core wide

flange columns were specified to be one of four grades, but were primarily 36 ksi and 42 ksi steel; only

about 1 percent of all the core columns were made of 45 ksi or 50 ksi steel.

302}

83 I

Column #

Floor of transition from box to WF columns

}

Shape of column at 84th floor WF Column

Box Column t Typical enclosed area for elevators and other services

H I I I I

83 83 <503} ( 83 504}86 C505"> ^86

<506"' 83 C507^

83 I I I I I I I

<602> <603} < (605} C606~>

i 80 80 80 80 80 80 80

j83

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7 I

<703)

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C704)

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C706>

24" I

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^1

P 95 95 cm} I

83 I

80

92l dot

891

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(3001) dpo^'

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(903> <904> <^905"> C966T' C9Q7> C^&S

I 80 177 I 86 I 48 ITT 48l|

(3003) CujgJ) </joo£r< (Joog) (3po?> (1qo| I 77 _86 _ 86 77 77 80 I I 1 I I H

Figure 2-6. Core column layout in WTC towers.

The core area was framed conventionally with beams. There were numerous openings in the core area

floor for elevators and stairwells. Since fewer elevators served the upper floors, part of the core area was

not needed for services. In Fig. 2-6 the dashed line shows the perimeter of the core, and shaded areas

indicate typical enclosed areas for elevators and other services.

2.3.3 Flooring System

In the great majority of floors, the floor area outside the central core was supported by a series of 29 in. (0.74 m) deep floor trusses that spanned between the core and perimeter wall, Fig. 2-8. At the core, the

floor trusses were bolted to seats welded to channels that ran continuously along the edge of the core. At

the perimeter columns, the floor trusses were bolted and then welded to seats, mounted on spandrels at every other column. The floor trusses were approximately 60 ft ( 1 8.3 m) or 35 ft (10.7 m) long

K) NISTNCSTAR 1-3, WTC Investigation
Tower Design - Structural Steel Documents

(depending upon the relative orientation of the building core), spaced at 6 ft 8 in. (2.0 m). There were

dozens of variants, with differences in length, top and bottom chord size, bearing angles at the truss ends,

gauge of web diagonals, and presence or absence of chord plates.

j, 22.0 in. J ~ CZZL. : 1.31 in.

22.0 in Column 504

14.0 in. I

I I

15 0 in Column

701

L J

1.50 in

Columns 1001 1008 9J160$

14WF730

17.89 in. Note: To scale.

j 4.91 in. J-L

Columns 607 906 Column 705 14WF219 14WF61

L62jn. j 0 64 in

15.825 in. 10 00 in

Figure 2-7. Typical welded box members and rolled wide flange shapes used for core columns between floors 83 and 86.

The prefabricated floor modules were typically 20 ft (6.1 m) wide, containing two sets of doubled trusses

in the interior and a single truss along each edge. Thus, each seat supported either a double truss within a floor panel, or two single trusses from adjacent floor panels. In addition, the bottom chord of each pair of

trusses was attached to perimeter spandrels with visco-elastic dampers. Bridging trusses ran perpendicular to the main bar trusses and were spaced at 13 ft 4 in. (4.06 m). The floor panels were

covered with a corrugated steel floor deck that rested on the bridging trusses and deck support angles

running intermediately between bridging trusses. Flutes in the deck ran parallel to the main trusses. Once

in place, 4 in. (100 mm) of lightweight concrete was poured for the floor. Figure 2-8 shows an assembled

floor panel before the concrete floor was poured.

The minimum yield strength of the steel for the floor trusses was specified to be 50 ksi "unless otherwise

noted." In practice, several of the designs specified 36 ksi steel as well as 50 ksi steel (see Chapter 3 for

details).

All seats were specified to be of 36 ksi minimum yield strength. There were more than 30 varieties of

perimeter seats, with various thicknesses from 3/8 in. to 7/8 in. in 1/8 in. increments (9.5 mm to 22.2 mm

in 3.2 mm increments). Core seats were 7/16 in., 1/2 in., 5/8 in., or 3/4 in. (11.1 mm, 12.7 mm, 15.9 mm.

or 19 mm) thick.

NISTNCSTAR 1-3, WTC Investigation 11
Chapter 2

Exterior Column Interior Column

Detail A Detail B

Gusset plate Truss upper chord

I

Viscoelastic 'Spandrel damper

Splice plate Saddle

Detail A - Exterior Wall End Detail

Truss lower

chord

Floor Line interior Column •

Detail B - Interior Wall End Detail

Figure 2-8. Schematic diagram of a floor truss.

Certain floors outside the core were supported by rolled structural steel shapes rather than trusses. These

included the mechanical floors (e.g., floors 75 and 76) and the floors just above the mechanical floors

(e.g., floor 77). Beam framing was typically W272 beams in the long span region and W16 beams in the

short direction with beams spaced at 40 in. The floor was 5.75 in. thick, normal-weight concrete poured

on a 1.5 in. fluted steel deck, acting compositely with the steel beams. The concrete on the beam-framed

floors above the mechanical floors was 8 in. thick, normal-weight concrete in the core area and 7.75 in. thick normal-weight concrete outside the core.

The areas inside the core for other floors were framed with rolled structural steel shapes with welded

shear studs acting compositely with 5 in. normal-weight concrete slabs.

" The "W" in W27 beam denotes the shape of the beam, which is like the letter "I". The number following the "W" is the nominal depth of the beam in inches. The second number denotes the weight of the beam in pounds per linear foot. A W27 by

1 14 beam is 27.28 in. high and weighs 1 14 lb/ft.

12 NISTNCSTAR 1-3, WTC Investigation
Tower Design - Structural Steel Documents

2.3.4 Floors 107 to 110

At the top of each tower (floors 107 to the roof), a "hat truss" interconnected the core columns (Fig. 2-9).

Diagonals of the hat truss were typically W12 or W14 wide flange members. In addition, four diagonal

braces (18 in. by 26 in. box beams spanning the 35 ft. gap, and 18 in. by 30 in. box beams spanning the

60 ft. gap) and four horizontal floor beams connected the hat truss to each perimeter wall at the

108th floor spandrel. The hat truss was designed to provide a base for antennae atop each tower, although

only the North Tower (WTC 1) antenna was actually built.

110th Boor

109S) Floor 108th Floor

107fri Floor 106th Floor 105th Floor

1041*1 Floor 103rd Floor l

^-

ZL

i—

| LL i - I

Diagonal Braces

(box sections)

Section Plan

Figure 2-9. Hat truss in upper floors.

Perimeter columns for floors 107 to 110 also differed from the lower floors and were alternating small

tube columns or wide flange columns, with the wide flange columns supporting the floor system.

NISTNCSTAR 1-3, WTC Investigation 13
Chapter 2

2.3.5 Impact Zone

The impact zones of the two towers were of particular interest, and special testing of the steels in this region was conducted. High-strain rate mechanical tests and high-temperature mechanical property tests focused on those steels most prominent in the impact zones, as indicated below.

In WTC 1, the perimeter columns torn out or otherwise damaged by the airplane impact (see Chapter 6) were predominantly specified as 55 ksi and 60 ksi steel. In WTC 2, the damaged columns ranged from

50 ksi to 100 ksi, although most were specified in the 55 ksi to 65 ksi range. Core columns in the impact zone of WTC 1 were almost entirely wide flange shapes. The core in the WTC 2 impact zone contained

about equal numbers of box and wide flange columns, and 1 7 of the 49 core columns transitioned from

box column to wide flange on these floors (Fig. 2-6). As is typical of all core columns, the steel was

predominantly specified as 36 ksi and 42 ksi minimum yield strength.

2.3.6 Floors Involved in Post-Impact Fires

Special attention was given to characterizing the performance of the structural steel found in floors engaged in the post-impact fires. There were large fires on floors 92 through 100 and 104 in WTC 1 and

floors 77 through 82 in WTC 2. In these floors, about 73 percent of the perimeter columns had strengths

in the range 55 ksi to 70 ksi, and most of the remainder from 75 ksi to 100 ksi.

SHCR (Skilling, Helle, Christiansen and Robertson). 1967. World Trade Center Structural Drawing

2.4 REFERENCES

Books.

14 NISTNCSTAR 1-3, WTC Investigation
Chapter 3

Contemporaneous Steel Specifications

3.1 INTRODUCTION

As is common practice, the structural engineering plans (obtained from the Port Authority of New York

and New Jersey) only specify the minimum yield strengths and dimensions of the beams and columns.

The steel contracts that the Port of New York Authority (PONYA or Port Authority)

(PONYA 1967:Ch. 2) awarded for the fabrication provided specifications for the steels to meet those

minimum yield strengths. Those contracts allowed the fabricators to use steels that conformed to certain

ASTM International (ASTM) Standard Specifications including A 36, A 440, A 441 and A 514.

Fabricators could also use certain proprietary steels from the major U.S. steel mills including the

Bethlehem V-series and the United States Steel Ex-Ten series. These proprietary grades were required to conform to specific dated and published data sheets that the steel mills provided. Finally, the contracts

also allowed other proprietary steels not listed in the contract, provided that the Port Authority chief

engineer of the project reviewed and formally approved their specifications (PONYA 1967:

Section 203. 100). It was by this third method that the Japanese steels were accepted for use. In all cases, extensive documentation was required for the steels to be acceptable.

This chapter focuses on the steels used in the area of the impact and fire: the floor trusses, the perimeter

columns, the welded core box columns, and the rolled core columns, fabricated by Laclede, Pacific Car &

Foundry (PC&F), Stanray Pacific, and Montague-Betts, respectively. It does not consider any of the sections of the buildings remote from the impact and fire sites, so fabricators of sections below the

9th floor (Mosher. Drier, Levinson, Pittsburgh-Des Moines, and Atlas) are not addressed. Table 3-1

summarizes some background information on the steel fabricators that worked on the upper stories of the towers. None of the four fabricating companies exist in the steel industry today.

In this document, "contemporaneous" refers to the standards in effect at the time of construction, in contrast to contemporary (or present-day) standards. ASTM standards are modified and renewed at regular intervals, so the current requirements of a standard may not have been in force during the World

Trade Center (WTC) era. This distinction is also important because historical versions of standards can be

difficult to locate. Although ASTM structural steel standards have evolved since the construction of the

towers, the changes have been minor and do not represent changes to the basic mechanical properties of

the steels.

Laclede Steel manufactured the trusses for the composite floor panels for both WTC 1 and WTC 2 from

steel they made and rolled at their mill in Alton, Illinois. The chords were fabricated from hot-rolled

angles, while the web was fabricated from hot-rolled round bar, Fig. 2-7.

3.2 FLOOR TRUSSES

NISTNCSTAR 1-3, WTC Investigation 15
Chapter 3

Table 3-1. Steel companies involved in WTC construction and their contracts.

Fabricator Current Status Component Tons

Pacific Car & Foundry Co. Sold in 1974 Exterior columns and spandrels,

floors 9 to 107

55,800

Montague Betts Co., Inc No longer a steel fabricator

Rolled core columns and beams above

9th floor

25,900

Pittsburgh-Des Moines Steel Co.Bifurcation columns ("trees") 4th to 9th floor

6,800

Atlas Machine & Iron Works No longer in business Box columns from 4th floor to the bifurcation columns

13,600

Mosher Steel Co. Currently active Core box columns below the 9th floor 13,000

Stanray Pacific Corp. Closed in 1971 Core box columns above the 9th floor 31,100

Levinson Steel Co. Sold in 1997, parent

company in bankruptcy

Supports for slabs below grade 12,000

Laclede Steel Co. Bankrupt in 2001, new

owners of rolling mill

Floor trusses Unknown

Drier Structural Steel Co., Inc. Unknown Grillages Unknown

Source: Feld 1971.

According to internal Laclede documents (Bay 1968), the top chord angles, as well as most round bars, were fabricated to meet ASTM A 242 (Fy = 50 ksi). Only 1.09 in. (27.7 mm) and 1 13/16 in. (46.0 mm)

round bars and the bottom chord angles were specified as ASTM A 36. Conversations with Laclede

metallurgists (Brown 2002) active during the WTC construction revealed that for many components

specified as ASTM A 36, Laclede supplied a higher-strength micro-alloyed steel with a typical

Fy = 50 ksi, similar to a contemporary A 572 steel. National Institute of Standards and Technology

(NIST) mechanical and chemical analyses (NIST NCSTAR 1-3D and 1-3E) have confirmed that this substitution was made in many cases.

3.3 PERIMETER COLUMNS AND SPANDRELS

PC&F of Seattle, Washington, fabricated the perimeter wall panels. The perimeter panels were

comprised of three important subassemblies: the columns, the spandrels, and the truss seats. The

structural plans called for the columns to be fabricated from 14 grades of steel with Fy = 36, 42, 45, 46,

50, 55, 60, 65, 70, 75, 80, 85, 90, and 100 ksi, although PC&F received approval to upgrade all 85 and

90 ksi steels to 100 ksi. Above the 75th floor, more than half of the columns had yield strengths between

55 ksi and 70 ksi, inclusive. The spandrels were fabricated from twelve grades of steel with Fy = 36, 42,

45, 46, 50, 55, 60, 65, 70, 75, 80, and 100 ksi (again with all 85 ksi steel upgraded to 100 ksi). The truss seats were specified to be fabricated from steel with Fy = 36 ksi minimum.

Yawata Iron and Steel Co. (now part of Nippon Steel) supplied most of the steel to PC&F for the

perimeter columns and spandrels. In general, the exterior (or web) and side (or flange) plates of each

column and the spandrels were fabricated from Japanese steel, and the interior web plate (plate 3) was

fabricated from domestic steel (Symes 1969a; White 1969a). Searches of archival material yielded no

information on the steels for the truss seats beyond the fact that they were specified as F} = 36 ksi.

16 NIST NCSTAR 1-3, WTC Investigation
Contemporaneous Steel Specifications

During the 1960s, Yawata produced a number of named, proprietary grades (such as WEL-TEN and

YAW-TEN series) of weldable steels with specified minimum properties. Several of these named grades

supplied to PC&F (WEL-TEN 60, WEL-TEN 62, WEL-TEN 80C) are common in the contemporaneous

literature; open literature publications (Ito 1965a, 1965b; Goda et al. 1964) describe many of their physical and mechanical properties quite extensively. For two of the proprietary grades that Yawata

supplied to PC&F (WEL-TEN 60R and WEL-TEN 70), NIST has been unable to find corroborating

specifications or mechanical property data, even in consultation with Nippon Steel. It is possible that these names were assigned simply for convenience for the WTC construction. Chemically, WEL-TEN 60,

60R and 62 are similar to contemporary ASTM A 588, with their Cr additions and high silicon contents,

though none would meet that specification exactly. WEL-TEN 60, 62, and 70 are heat-treated steels,

while WEL-TEN 60R is a hot-rolled steel. WEL-TEN 80C is a steel containing chromium and

molybdenum that is very similar to A 514 steels, and possibly could have been manufactured to meet that specification.

Contemporaneous documents indicate that PC&F also purchased V-series (White 1968a, 2003) and

modified V-series plate from Bethlehem Steel (Symes 1967a), EX-TEN and modified EX-TEN from

U.S. Steel (Symes 1967a; White 2003; Barkshire 1968a), and various Kaisaloy grades (Barkshire 1968b)

from Kaiser steel, for use in the interior plates. The interior plate (plate 3 - see Fig. 2-3) was usually half

the thickness of the flanges (when the flanges exceeded 0.5 in.), and never exceeded 15/16 in. thick, and

so represents a small fraction (at most 5 percent) of the mass of steel in the entire contract. Status reports

from mid- 1968 indicate that PC&F phased out U.S. Steel and Kaiser and replaced them with Bethlehem

as the only domestic supplier (Barkshire 1968c). Based on this information, it appears that most of the

inner web plates in the columns near the impact floors were made from hot-rolled Bethlehem V-series

steels.

Stanray Pacific Corp. fabricated the welded core columns above the 9th floor in both buildings. The

plans called for two grades of steel with 36 ksi and 42 ksi minimum yield strengths. Contemporaneous

documents (Morris 1967; Warner 1967) indicate that Stanray Pacific purchased at least 10,240 tons (of an

estimated total contract for 32,000 tons) of plate from Colvilles Ltd. (rolled in the Dalzell Works,

Motherwell, Scotland). The rest of the plate (21,760 tons) came from Fuji Iron and Steel, Hirohata Works

(Morris 1967; Warner 1967).

A mid 1967 document (Warner 1967) indicates that Fuji Steel supplied all plates thinner than 1.75 in. Both Fuji and Colvilles supplied plates 1.75 in and thicker, with Fuji supplying about 60 percent of the

total mass of steel used. In the fire and impact floors of WTC 1 (94 to 98), only three of the columns are welded box columns, and all three were made from plate thinner than 1.75 in. In the fire and impact

floors of WTC 2 (floors 78 to 84) only 9 of 52 welded box columns are made from plate 1.75 in or

thicker. In terms of steel properties for modeling, the columns can, therefore, be modeled with the

properties of the Fuji-supplied plates alone.

Montague-Betts Steel fabricated all the rolled wide flange shapes for the core columns, as well as all the

beams above the 9th floor in both towers. These rolled shapes represent a significant fraction of the total

3.4 CORE—WELDED BOX COLUMNS

3.5 CORE—ROLLED WIDE FLANGE SHAPES

NISTNCSTAR 1-3, WTC Investigation r
Chapter 3

core columns in the fire and impact zone. Above the 80th floor in WTC 2, more than half of the core columns were wide flange shapes, and above the 94th floor in WTC 1, 43 of the 46 columns were wide

flange shapes. The plans called for steels with 36 ksi, 42 ksi, 45 ksi, and 50 ksi minimum yield strengths,

but very few of the rolled shapes used the 45 ksi or the 50 ksi material. Various sources (Davis 2002;

Yawata 1969) confirm that Montague-Betts purchased about 12,000 tons (of a total contract of

25,900 tons) of A 36 and A 441 wide flange shapes from Yawata Iron and Steel, Sakai Works. An

additional 1200 tons came from Dorman-Long, Lackenby Works, Middlesborough, England

(Gallagher 1968; Goode 1967). Given the size of the Yawata contract, it is likely that it represents the

majority, if not all, of the wide flange core columns. Montague-Betts CEO William Davis (2002), who

worked on the project, confirmed that Montague-Betts also purchased steel from Bethlehem and

U.S. Steel, the only two domestic mills that produced 14W rolled sections heavier than 87 lb/ft (AISC 1973).

3.6 FINDINGS

Fourteen nominal strengths of steel were specified in the structural engineering plans, but only

12 strengths of steel (minimum yield strengths of 36, 42, 45, 46, 50, 55, 60, 65, 70, 75, 80, and 100 ksi) were actually used in construction due to an upgrade of two steels (85 and 90 ksi steel upgraded to 100 ksi).

Ten different steel companies fabricated structural elements for the towers, using steel supplied from at least eight different suppliers. Four fabricators supplied the major structural elements of the 9th to 107th

floors: Laclede Steel (floor trusses), PC&F (perimeter column panels), Stanray Pacific (welded core box

columns), and Montague-Betts (rolled core columns).

Although ASTM structural steel standards have evolved since the construction of the towers, the changes

have been minor and do not represent changes to the basic mechanical properties of the steels.

3.7 REFERENCES

American Institute of Steel Construction (AISC). 1973. Manual ofSteel Construction. New York:

American Institute of Steel Construction.

Barkshire, A. 1968a. Internal SHCR report to J. White on fabrication at PC&F. Contains five page

attachment showing instances of U.S. S. steel used in columns mostly in floors 20 to 30. Tower not

specified. Seven pages. May 15.

Barkshire, A. 1968b. Internal SHCR to J. White showing spandrel plate of Kaisaloy 50-SG in panel 203-

16-19A. Six pages. December 4.

Barkshire, A. 1968c. Internal SHCR memo to J. White stating that USS and Kaiser are being phased out

as suppliers with Bethlehem (Seattle) and Japanese mills furnishing all steel. Two pages with five

pages attached. June 5.

Bay, R. D. 1968. Memorandum No. 1 1, Laclede internal memorandum showing grades of steel for bar

joists, February 28.

is NISTNCSTAR 1-3, WTC Investigation
Contemporaneous Steel Specifications

Brown, D. 2002. Telephone interview with Richard Fields. Brown, the Laclede metallurgist during the WTC construction, confirmed that for ASTM A 36, Laclede would have specified a microalloyed

steel similar to current ASTM A 572. November 20.

Davis, W. 2002. Telephone interview with William Luecke. Montague Betts furnished rolled beams for the core of both towers as well as the antenna base for one tower. He remembered about 60 percent

of the steel was American, and the rest was Japanese or British. Confirmed that they did buy steel from Yawata, and that Yawata had better tolerances than the U.S. steel mills did. All records of the job were destroyed after 6 years. Montague Betts closed its steel operations in 1992. November 5.

Feld. L. S. 1971. Superstructure for the 1,350-ft World Trade Center. Civil Engineering-ASCE. 41:66-

70.

Gallagher. H. B. 1968. Internal PONYA memo W. Borland detailing inspection trip to Great Britain to

visit Colvilles mills at Motherwell and Mossend. and Dorman-Long. May 15.

Goda. S.. T. Ito. H. Gondo, I. Kimura. and J. Okamoto. 1964. Present status of weldable high strength

steel. Yawata Technical Report (Yawata Seitetsu Kenkyu) 248:5086-5163.

Goode, B. 1967. Internal SHCR memo to Leslie Robertson on Worthington, Skilling, Helle. Jackson

letterhead stating that Dorman-Long will produce 1.200 t of wide flange (W) sections for MontagueBetts. September 8.

Ito, T., K. Moriyama, and Y. Sogo. 1965a. Usability of WEL-TEN 80C welded joints of high-tension

steel (in English). Yawata Technical Report (Yawata Seitetsu Kenkyu). 253:6307-6316.

Ito. T.. K. Moriyama, and Y. Sogo. 1965b. Usability of WEL-TEN 80C Welded Joints of high-tension

steel (In Japanese). Yawata Technica] Report (Yawata Seitetsu Kenkyu). 253 6232-6244 (1965).

Morris. R. E. 1967. Letter from Stanray Pacific to R. M. Monti (PONYA) showing Colvilles (British

Steel Export Assn.) and Fuji Steel as source of plate for contract. September 8.

Symes. R. C. 1967a. Memo from PC&F to R. Monti (PONYA) requesting approval of Bethlehem

V-series steels outside of the published plate sizes. Two pages. Denied without full information on September 8. 1967. requested again with further documentation on November 2, 1967. Provisionally

approved November 18, 1967 (no PCF letter #). August 14.

Symes. R. C. 1 969a. Memo from PC&F to M Gerstman (Tishman) requesting adjustment to payment

because of steel changes. States that plates 1. 2, and 4 (flange, outside web, and spandrel) were made

from imported steel (presumably Yawata) and plate 3 (inside web) was fabricated from domestic

steel. Also contains a table showing tons of steel used by grade and thickness. 6 pages PCF#T-40.

February 5.

Warner, H. L. 1967. Memo from Stanray Pacific to Malcolm Levy (PONYA) detailing distribution of

plate thicknesses between British and Japanese steels. Total is 32.000 tons. July 7.

White, James. 1968a. Memo from SHCR to RM Monti (Port Authority) approving PC&F substitution of

Fy = 100 ksi steel for Fy = 90 ksi steel in exterior columns. February 15.

NISTNCSTAR 1-3. WTC Investigation 19
Chapter 3

White, James. 1969a. Memo from SHCR to R. Monti (PONYA) documenting use of heat-treated steel above (PC&F) and below (PDM) the 9th floor splice. Contains statement that plate 3 (inside web) was fabricated from domestic steel, while plates 1, 2, and 4 (flange, outside web, and spandrel) are imported steel. Also contains table that shows where ASTM 441 -modified and WEL-TEN grades were used, by thickness and yield strength. 28 pages. July 28.

White, J. 2003. Telephone interview with William Luecke. PC&F used (in addition to Yawata) U.S.S.

and Bethlehem Steel for plates (probably for Fy less than 60 ksi). February 1 1

Yawata Iron and Steel Co. Ltd. 1969. New York World Trade Center Building. Internal Yawata

document.

20 NISTNCSTAR 1-3, WTC Investigation
Chapter 4

Contemporaneous Construction Specifications

4.1 introduction

Chapter 3 traces the sources and grades of steel used to fabricate structural steel components for the World Trade Center (WTC) towers. This chapter extends further into the construction process,

specifically adding information on the fabrication (welding) of components and the erection of the

buildings.

The web and the chord angles of the floor trusses (Fig. 2-8) were joined by resistance welding

(Laclede 1969). Little information is available on the standards used for fabrication of the floor trusses; however, floor joist standards have existed since 1929. The American Institute of Steel Construction

(AISC) Steel Construction Manual (1972) "Standard Specifications for Open Web Steel Joists" specifies

that 36 ksi and 50 ksi minimum yield strength steel are permitted in such bar joists and that "Joint

connections and splices shall be made by attaching the members to one another by arc or resistance

welding or other approved methods." A Technical Digest from the Steel Joist Institute (Somers 1980) also

confirms the use of resistance welding.

4.2.2 Exterior Wall Columns and Spandrels

A Welding Design and Fabrication article (1970a) describes the fabrication sequence of the perimeter

columns. The fabrication began with forming the inside wall of the modules using a butt joint to link the

spandrel plates to the inner column webs, followed by the addition of the sides and outer face of the

columns by six simultaneous submerged arc welds. Pacific Car and Foundry (PC&F) constructed a

16-station automated production line to keep up with the schedule of 55,800 tons of perimeter column

panels between November 1967 and August 1970, an average of 1,400 tons per month.

The construction contract states that the submerged arc electrodes used in the WTC were purchased to the

requirements of ASTM Standard A 558 "Specification for Bare Mild Steel Electrodes and Fluxes for Submerged Arc Welding." This Standard was withdrawn in 1969, and was replaced by an equivalent

American Welding Society (AWS) Standard A 5.17 "Bare Mild Steel Electrodes and Fluxes for Submerged Arc Welding." The period 1965 to 1969 was one of transition, during which AWS assumed

the responsibility of maintaining the standards for welding filler materials. Because the contract was

awarded in 1967, the fabrication was likely started with the requirements of the 1965 version of the

ASTM International (ASTM) Standard (ASTM A 558-65T, jointly published by AWS as AWS A

5.17-65T), but later perimeter column panels may have included some minor changes associated with the

conversion to the 1969 version of the AWS Standard (AWS A 5.17-69).

4.2 FABRICATION OF THE VARIOUS COMPONENTS

4.2.1 Floor Trusses

NISTNCSTAR 1-3, WTC Investigation 21
Chapter 4

Distorted columns were straightened in the conventional manner by heating just after column assembly.

Such treatment can result in minor changes in properties.

The inner wall assembly (the spandrels and inner plates of the perimeter column panels) was joined with

full penetration welds according to the requirements of AWS D 2.0 "Specifications for Welded Highway

and Railway Bridges." This probably refers to the 1966 version of AWS D 2.0. This standard may have

been chosen over D 1 .0 "Code for Welding in Building Construction" because, at the time, D 1.0 was

limited to steel strengths under 60 ksi (Fenton 1966). AWS D 2.0 specifies various dimension and

strength requirements for the assemblies and their welds (e.g. paragraph 302 and 403). This standard, like most standards, lagged behind the steel technology of the time. Thus, it seems to be mostly designed

around the application of fairly old steels like A 7, A 36, and A 373.

Once the inner wall was ready, the columns were assembled from side plates, butt plates, diaphragm

plates, and flange plates (Welding Design 1970a). Once assembled and preheated, the plates were joined

in the main fillet weld gantry, a station that made six 0.75 in. (19 mm) fillet welds simultaneously along

the length of the perimeter column panel. Then the module was jacked 90 degrees, and the other six fillet welds were made along the length of the panel. At full production, this gantry laid down 2,900 lb

( 1 ,300 kg) of weld metal a day. These large fillet welds started 6 in. ( 1 50 mm) from the ends of the

columns, so manual welding was used to finish the welding of the ends as well as to make any repairs.

4.2.3 Core—Welded Box Columns

To fabricate the core box columns, Stanray Pacific, like PC&F, used large assembly fixtures and triple

submerged arc welding stations to achieve high production rates. Correspondence generated during the

initial stages of the fabrication showed the level of attention to welding and inspection details needed to meet the requirements of Port of New York Authority (PONYA or Port Authority) and Skilling, Helle,

Christiansen, & Robertson (SHCR) as described below.

A September 1967 draft of the contract between PONYA and The United States Testing Laboratory (a

third-party inspector) lists the documentation that would be required of the work at Stanray Pacific

Corp (White 1967c). This contract prescribes daily and weekly written reports of components that were

accepted, those that were rejected, and a summary of any problems, with copies going both to the

construction manager and to SHCR. In addition, a weekly report was sent with all the chemical and

physical (mechanical) tests performed. The inspectors checked the various steps from plate delivery

(checking heat number, specification conformance, and condition), through fabrication (alignment,

100 percent visual inspection of the welds, and selection of regions for non-destructive testing), to final inspection (perpendicularity of milled ends, overall length, cleaning, and marking). PONYA also had a procedure to inspect the steel from all sources. The procedure included double-checking the mill

certificates by performing a tensile test and a check analysis on one out often heats selected at random (Monti 1967b). The requirements were still higher for steel with strengths above 50 ksi or from

foreign sources. The welding procedures, welders, and welding operators were required to be qualified in accordance with requirements of Appendix D of AWS Codes Dl.1-66 and D 2.0-66. The welding

electrodes for manual metal arc welding conformed to ASTM A 233-64T, E60 and E70 series (also

AWS A 5.1-64T). Mild steel electrodes and fluxes for submerged arc welding conformed to ASTM

A 588-65T (also AWS A 5.17-65T) and to Sec. 1.17.3 of the AISC Specification for Structural Steel Buildings.

22 NISTNCSTAR 1-3, WTC Investigation
Contemporaneous Construction Specifications

By October 1967, welders were being qualified, magnetic particle inspector qualification was being

discussed (based on a minimum of 40 h of training), and chemical analysis of the steel was underway

(Chauner 1967a). The level of inspector oversight continued to increase until by November 10, 1967

"U.S. Testing inspectors are all over the place and recording a lot of information" (Chauner 1 967b). The

level of attention to detail increased even more after a surprise visit to Stanray by Hugh Gallagher, a PONYA inspector, on November 20, 1967 (Gallagher 1967).

While reading the correspondence, one senses that toward the beginning of the contracts, the various

fabricators faced major (and perhaps unexpected) challenges introduced by both the tight production

schedule and PONYA and SHCR's strict quality requirements.

4.2.4 Connections (Bolts and Welds)

The Port Authority contract allowed the use of ASTM A 307, A 325, and A 490 fasteners. The WTC

Design Standards book (p. DS1-6) calls for the use of ASTM A 325 bolts with no indication of type.

According to the standard, they would have been supplied as Type 1 . As in the contemporary version of

ASTM A 325, Type 1 bolts in 1970 had Fy = 120 ksi for diameters up to and including 1 in, and

Fy = 105 ksi for larger diameters. ASTM A 325-70 differs significantly from the current ASTM A 325-02

in several ways. In particular, the specification for Type 2 bolts was withdrawn in 1991 . ASTM A 325-02

also admits three new chemistries for Type 1 bolts. In ASTM A 325-02, the specification for Type 1 Carbon Steel bolts most closely approximates the Type 1 bolts of A 325-70.

Spandrels of adjacent perimeter column panels were connected with high-strength bolted shear

connections. Adjacent spandrels were butted to each other with splice plates on the inside and outside

(Fig. 2-3). For floors 9 to 107, each spandrel was connected to the splice plates with anywhere from 6

to 32 bolts, depending on design load. Splice plates were all 36 ksi steel regardless of spandrel grade.

Bolts for all connections between spandrels conformed to ASTM A 325. Minutes of a May 1967

(Feld 1967a) meeting between PC&F, PONYA, and Koch, state that no A 490 bolts were to be used for the spandrel splice plates, and that only A 325 bolts were to be used there.

Perimeter columns were bolted via the butt plates to those immediately above and below, with four bolts

in the upper stories and six bolts in the lower stories. Other than at the mechanical floors, panels were

staggered (Fig. 2-5) so that only one-third of the units were spliced in any one story. At the top and

bottom of the mechanical floors, every column contained a splice, and columns were welded together as well as bolted.

Seats for the trusses that supported the floor were welded to spandrels in the perimeter column panels and

to channels attached to core columns. At the core, trusses were bolted to the seats. At the perimeter, the

trusses were positioned on the seats and held in place at the perimeter with construction bolts, and then

welded to the seats. The construction bolts generally remained in place after welding.

4.2.5 Construction (On-Site Assembly)

During fabrication, Karl Koch Erecting Co. used a combination of bolting, shielded metal arc (SMA)

welding (E7018), and gas metal arc welding (semiautomatic Fab Co 71 with CO: shielding) to join the components (Welding Design 1970b). The E7018 low-hydrogen SMA electrode would likely have been

NISTNCSTAR 1-3, WTC Investigation 23
Chapter 4

produced to ASTM Standard A 233-64T (also published by AWS as A 5.1-64T), then AWS

Standard A 5.1-69 for the later parts of the fabrication. The 3/32 in. (2.4 mm) diameter Fab Co 71 (sic,

probably should be FabCO 71, a trademark of Hobart Brothers Company) was an E70T-1 flux cored arc electrode and would likely have been produced according to ASTM A 559 (withdrawn in 1969), then

AWS A5.20-69. Higher-strength SMA electrodes (ASTM A 316 until 1969, then AWS A 5.5-69) were

also permitted by the contract. More than 48,000 lb (22,000 kg) of electrodes were used in each of the

towers (Welding Design 1970b). Koch used a combination of visual and ultrasonic inspection on the

joints. They estimated that rework would cost three times as much as the original weld, so they inspected

early and often to minimize any rework. One reason that rework was so expensive is that some welds

took as many as 200 passes, so they wanted to catch any problems before the later passes made access more difficult.

Perhaps the most common construction standard for buildings of the period was AWS D 1 .0 "Welding in Building Construction" (Fenton 1966). This document was subject to frequent revisions by the

responsible committee. Some versions that may have been specified for parts of the WTC towers were the

versions published in 1966, 1967, and 1968. The 1967 and 1968 revisions addressed issues such as the

details on the use of multiple-electrode submerged arc welding, more requirements on qualification of the

welders (especially tack welders), and the addition of radiographic inspection. Many of these revisions

may have been driven by the needs of the WTC design. Since the D2.0 code referenced in the discussion on fabrication of perimeter column panels above only covered the use of submerged arc and shielded

metal arc welds (unless through special application of Sec. 5), use of Dl .0 (specifically through the use of

Sec. 502) might have been the easiest way to cover the use of FabCO 71 electrode.

Examination of the perimeter columns shipped to National Institute of Standards and Technology

revealed arc welds at the ends of the trusses, where they were attached to the columns during erection.

These welds were probably produced by gas metal arc or shielded metal arc electrodes.

4.3 REFERENCES

American Institute of Steel Construction ( AISC). 1 972. Standard specifications and load tables for open

web steeljoists, longspan steeljoists, deep longspan steeljoists. New York: American Institute of

Steel Construction.

Chauner, R. 1967a. Internal SHCR memo to James White. October 27.

Chauner, R. 1967b. Internal SHCR memo to James White. November 10.

Feld, L. S. 1967. Internal PONYA memo to H. Tessler summarizing meeting between PC&F, Koch,

Tishman, SHCR, and PONYA to discuss engineering changes. Discusses spandrel plate splices,

A 325 bolts, not using A 490 bolts for the spandrel splices, and a statement by N. Soldano (PC&F)

that Yawata would furnish imported steel with Fr>36 ksi, and Kawasaki would furnish 36 ksi steel. Five pages. May 9.

Fenton, E. A. 1966. Codefor welding in building construction. New York: American Welding Society.

Gallagher, H. 1967. Internal PONYA memo to D. Brown. December 1 1.

24 NISTNCSTAR 1-3, WTC Investigation
Contemporaneous Construction Specifications

Laclede Steel. 1968. Standard Resistance and Arc Weld Connections used in Truss Manufacture.

February 28.

Monti, R. M. 1967b. Memo to R. Morris (Stanray Pacific). November 13. PONYA (Port of New York Authority) 1967. 'The World Trade Center Contract WTC-2 17.00 Fabricated

Steel Box Core Columns and Built-up Beams from the 9th Story Splice to the Penthouse Roof, North

and South Towers." Chapter 2 "Materials." Sections 201-203. Contract between Stanray Pacific Corp

and PONYA. May 18.

Somers, R. E. and T. V. Galambos. 1980. Welding ofOpen Web Steel Joists. Technical Digest No. 8. Steel Joist Institute Richmond, Va. Based on inclusion of statistics on U.S. joist production in 1979.

Welding Design and Fabrication. 1970a. Welding and fabricating 55,000 tons of steel for the World Trade

Center, July:50-52.

Welding Design and Fabrication. 1970b. Welding and inspecting the World Trade Center-giant among

giants, February:53-56.

White, J. 1967c. Memo to PONYA. September 1.

NISTNCSTAR 1-3, WTC Investigation 25
Chapter 4

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26 NISTNCSTAR 1-3, WTC Investigation
Chapter 5

Steel Inventory and Identification

5.1 the recovery of world trade center structural steel

Beginning in October of 2001 , nearly a year prior to the start of the National Institute of Standards and

Technology (NIST) Investigation, members of the Federal Emergency Management Agency (FEMA) - American Society of Civil Engineers (ASCE) Building Performance Assessment Team (BPAT), members

of the Structural Engineers Association of New York (SEAoNY), and Professor A. Astaneh-Asl of the

University of California, Berkeley (with support from the National Science Foundation), began work to identify and collect World Trade Center (WTC) structural steel from the various recovery yards where

debris, including the steel, was taken during the WTC site clean-up effort. Dr. J. Gross, a research

structural engineer at NIST and a member of the FEMA/ASCE Building Performance Study, was

involved in these early efforts.

There were four major sites where debris from the WTC buildings was shipped during the clean-up effort:

• Hugo Neu Schnitzer, Inc., Fresh Kills Landfill in Staten Island, New Jersey,

• Hugo Neu Schnitzer East, Inc., Claremont Terminal in Jersey City, New Jersey,

• Metal Management, Inc., in Newark, New Jersey, and

• Blanford and Co. in Keasbey, New Jersey.

The volunteers searched through enormous unsorted piles of steel and other debris for pieces from the

WTC buildings, specifically searching for (McAllister 2002):

• Exterior column panels and interior core columns from WTC 1 and WTC 2 that were exposed

to fire and/or impacted by the aircraft,

• Exterior column panels and interior core columns from WTC 1 and WTC 2 directly above

and below the impact zones,

• Badly burned pieces from WTC 7,

• Connections from WTC 1, 2, and 7 (e.g., seat connections, single shear plates, and column

splices),

• Bolts in all conditions,

• Floor trusses including stiffeners, seats, and other components,

• Any pieces that in the engineers' professional opinion might be useful.

NISTNCSTAR 1-3, WTC Investigation 27
Chapter 5

Once selected for recovery, the samples were marked as "SAVE" and given an alphanumeric code that identified the recovery yard where they were located and an accession number. Some pieces were not saved in their entirety, but instead, small portions were removed, hereafter called coupons and held at Gilsanz Murray Steficek, LLP (GMS). Coupons were also removed in the field for WTC 5 and held at GMS (all coupons were later sent to NIST).

Facing concern that the selected steel may not be properly preserved in the recovery yards, NIST arranged

for the steel to be shipped to its campus located in Gaithersburg, MD, starting in February of 2002, about

six months before launching its investigation.

Before the samples were shipped to the NIST campus, environmental testing for asbestos and analysis of

the paint for lead was conducted. SEAoNY volunteers and NIST personnel continued to select and ship

steel specimens to NIST through October 2002. Mr. S. Cauffman of NIST coordinated all environmental

testing, transportation, and delivery of the recovered steel to NIST. The recovered structural components

constituted the material base for analysis in the NIST Investigation.

Structural steel elements were also collected and held by the Port Authority of New York and New Jersey

(PANYNJ or Port Authority ) in Hangar 17 of John F. Kennedy International Airport. The main goal of

the Port Authority project was to decontaminate and preserve the steel, as well as other WTC artifacts, for

future exhibits and memorials. A complete listing of the pieces held by PANYNJ can be found in the

Presen'ation and Inventory Report prepared by Voorsanger and Associates Architects, PC

(Voorsanger 2002). NIST personnel visited the hangar and identified 12 pieces considered important to the Investigation. Six whole pieces were moved to the NIST campus in Gaithersburg, Maryland, as well

as portions of six additional pieces.

NIST obtained 236 samples from the WTC buildings. The report Steel Inventory and Identification

(NIST NCSTAR 1-3B) gives a complete list of each sample with a brief description and characteristic

photographs. The samples ranged from full exterior column panels to pieces of bolts and bags of glass

and other debris fragments. The pieces were classified into one of eight categories:

5.2 STRUCTURAL ELEMENTS RECOVERED FROM WTC BUILDINGS

5.2.1 Location and Labeling of Structural Steel Elements

Classification # of Pieces

Exterior column panel sections (flat wall or corner) 94

"Bow-tie" pieces

Rectangular built-up box column (not perimeter column)

Wide flange sections

2

1

44

Floor trusses 23

Channels 25

Coupons from WTC 5 7

Miscellaneous (isolated bolts, floor hanger components, other) 30

2S NIST NCSTAR 1-3, WTC Investigation
Steel Inventory and Identification

5.2.2 Identification Methods of WTC Structural Steel Elements

Information from Leslie E. Roberts Associates (LERA) indicated that all structural steel pieces in WTC 1 and WTC 2 were uniquely identified by stampings (recessed letters and numbers) and/or painted stencils (Faschan 2002). N1ST was successful in finding these markings on many of the perimeter panel sections, core columns, and other wide flange members, thereby identifying the original, as-built location of the

pieces. (Based on identification of damaged panels in precollapse photographs, NIST believes that the as- built locations were correctly identified; for the majority of identified pieces, however, it is the intended

location within the building that was identified.)

The positive identification of the structural elements was made possible by deciphering the stampings

and/or stencils found on them. During fabrication, the exterior panel sections were stamped at the bottom

of the center column on the inside face. These stampings indicated the building, center column line number, and floors spanned by the columns. The core columns were stamped at the lower end of the

component. The building was typically represented as "A" for WTC 1 and "B" for WTC 2. An example

of a stamping found on an exterior column is shown in Fig. 5—1, where the stamping indicates that the

piece was from WTC 2 with center column line number of 206, spanning floors 83-86. Core columns

were found to have similar markings as seen in Fig. 5-2. Other stampings found on the flanges of the

perimeter columns indicated the column type (individual plate dimensions) as well as the specified

minimum yield strength of the column. Additional stampings were sometimes located on the flanges,

although NIST, PANYNJ, and LERA were unable to determine the significance of these codes. All

stampings were typically located within one meter of the bottom of the column.

The perimeter panels and core columns were often additionally stenciled in white or yellow lettering with

similar building information. For the exterior panel sections, the stenciling was located on or near the

lowest spandrel on the interior face. Figure 5-3a shows typical stenciling found on a perimeter panel,

indicating this piece was in WTC 1 with center column line number of 130, spanning floors 93 to 96. For

the core columns, both stenciling and hand written codes were observed on the recovered pieces.

Figure 5-3b shows one of these stencilings from a core column located in WTC 1.

Also seen in Fig. 5-3a are two other markings, "5T" and "<63>", found on the exterior panel sections.

These markings are the estimated piece tonnage ( 1 ton equals 2,000 lbs, or approximately 907 kg) and

erector's derrick division number, respectively. This information was also stamped on some of the core column pieces, see Fig. 5-2. The erector, Karl Koch Erecting Co., Inc., assigned derrick divisions

47 through 70 for WTC 1 and derrick divisions 547 through 570 for WTC 2 (PONYA 1967). Each

division was assigned to a specific area of the building and shared one of four cranes with other nearby

derrick divisions. For example, a single crane may have lifted pieces from derrick divisions 65, 67, and

69. Several derrick divisions that hoisted the specific columns for both buildings have been identified,

based on derrick numbers found on structural elements with positive identification, providing additional

information to help identify partially identified panels (NIST NCSTAR 1-3B).

NISTNCSTAR 1-3, WTC Investigation 29
Chapter 5

Source: NIST. Figure 5-1 . Example of stampings on the interior base of the middle column for each

panel. Shown here on panel M-10b are the stampings "B206 86 83," indicating that it is from WTC 2, column number 206, spanning floors 83 to 86.

30 NIST NCSTAR 1-3, WTC Investigation
Steel Inventory and Identification

Source: NIST. Figure 5-2. Example of stampings placed on one end of a core column. Shown here on

sample C-90 are the stampings "18 TON, 701 B 12 - 16, 549 WEST," indicating that it is from WTC 2, core column number 701, and spans floors 12 to 16. Also indicated are the

approximate weight of the piece (18 tons) and the derrick division <549> that was to lift it into place.

NISTNCSTAR 1-3, WTC Investigation 31
Chapter 5

Source: NIST. Figure 5-3. a) and b) Characteristic stenciling found on the lower portions of the

exterior column panels for sample M-27. Markings indicate this piece was in WTC 1,

column number 130, spanning floors 93 to 96. Also seen are two other indicators, "5T"

and "<63>," indicating the estimated piece weight in tons and erector's derrick division

number, respectively, c) Characteristic stenciling found on core column for sample

B-61 52-1. Markings indicate this piece was in WTC 1, column number 803, spanning

floors 15 to 18 with a derrick division number of <52>.

32 NISTNCSTAR 1-3, WTC Investigation
Steel Inventory and Identification

5.2.3 Identified Perimeter Panels

Of the 94 pieces of recovered perimeter panel, 90 panels are distinct. The other four pieces of perimeter

column had become separated from their main panels during salvage and were subsequently labeled

C-13a, C-16a, C-28b, and K-16a. Of the 90 panels, 42 distinct exterior column panels have been

unambiguously identified, and one has been partially identified. Table 5-1 lists the fully identified panels

with Fig. 5^1 showing the relative location of those identified exterior panels within the top third of the

buildings. Significantly more pieces were recovered from WTC 1 than WTC 2.

Table 5-1. Identified perimeter column panel pieces from WTC 1 and WTC 2. NIST Derrick

Name Description Bldg Column Floors Division

ASCE-2 One full column WTC 2 330 40-43 NA

B-1024 Full panel WTC 2 154 21-24 NA

B-1043 Full panel WTC 2 406 40^13 NA

B-1044 Full panel WTC 2 409 40^13 NA

C-10 Full panel WTC 1 451 85-88 5x

C-13 Rectangular column with spandrel WTC 2 200 90-92 569

C-13a Partial of single column WTC 2 159 90-92 569

C-14 1 column, lower 1/3 WTC 2 300 85-87 570

C-18 3 columns, bottom 2/3 WTC 2 230 93-96 NA

C-22 3 columns, lower lA WTC 1 157 93-96 69

C-24 3 columns, upper 1/3 WTC 2 203 74-77 NA

C-25 1 column, lower V2 WTC 1 206 89-92 69

C-40 2 columns, lower 2/3 WTC 1 136 98-101 6x

C-46 Nearly full panel WTC 2 157 68-71 569

C-48 Nearly 2 full columns WTC 2 442 91-94 NA

C-55 1 column, lower 1/3 WTC 1 209 94-97 NA

C-89 2 full columns WTC 2 215 12-15 NA

C-92 1 column, lower 1/3 WTC 2 130 93-96 NA

C-93 1 column, lower 1/3 WTC 1 339 99-102 NA

CC 2 full columns WTC 1 124 70-73 NA

K-l 3 columns, lower 1/3 WTC 1 209 97-100 NA

K-2 1 column, lower 2/3 WTC 1 236 92-95 NA

M-2 Full panel WTC 1 130 96-99 63

M-lOa 3 columns, middle section 1/3 WTC 2 209 82-85 NA

M-lOb 3 columns, lower V2 WTC 2 206 83-86 569

M-20 2 columns, lower 1/3 WTC 1 121 99-102 63

M-26 Full panel WTC 1 130 90-93 6x

M-27 2 columns, lower 3/4 WTC 1 130 93-96 63

M-28 3 columns, lower 1/4 WTC 2 345 98-101 NA

M-30 2 columns, lower 1/3 WTC 1 133 94-97 65

NISTNCSTAR 1-3, WTC Investigation 33
Chapter 5

NIST

Name Description Bldg Column Floors

Derrick

Division

N-l 2 full columns WTC 1 218 82-85 NA

N-7 Full panel WTC 1 127 97-100 NA

N-8 Full panel WTC 1 142 97-100 67

N-9 Nearly full panel WTC 1 154 101-104 69

N-10 2 columns, lower 2/3 WTC 1 115 89-92 6x

N-12 2 full columns WTC 1 206 92-95 69

N-l 3 3 columns, lower 1/3 WTC 1 130 99-102 63

N-99 Nearly full panel WTC 1 148 99-102 67

N-101 Full panel WTC 1 133 100-103 65

S-l 2 columns, lower 1/3 WTC 1 433 79-82 47

S-9 Full panel WTC 1 133 97-100 NA

S-10 2 columns, lower lA WTC 1 224 92-95 NA

S-14 Full panel WTC 2 218 91-94 557

Key: NA. information not available. Note: "x" in the derrick division column indicates an unreadable number.

400 series 300 series 200 series 1 00 series

WTC 1 West face

WTC 2 South face

WTC 1 South face

WTC 2 East face

WTC 1 East face

WTC 2 North face

WTC 1 North face

WTC 2 West face

Figure 5-4. Location of the exterior panels recovered from the top third of the WTC

towers. Recovered WTC 1 panels are shown in light shading/green and WTC 2 panels

are in dark shading/blue.

34 NISTNCSTAR 1-3, WTC Investigation
Steel Inventory and Identification

Of the 42 positively identified exterior panels, 25 had specific markings giving all the information needed

(building, column, floors) to locate the structural element within the buildings from one or both codes

(i.e., stampings or stencils). The flange stampings, which indicated the specified yield strength and

column type, were used to confirm the findings (NIST NCSTAR 1-3B). The only deviation noted was

that 100 ksi steel was substituted for grades that were specified as 85 ksi and 90 ksi. This observed

substitution is consistent with Port Authority documents from the construction period indicating that 100 ksi steel was used for all steel specified as 85 or 90 ksi (NIST NCSTAR 1-3 A).

Sixteen other panels were identified through analysis of flange and web thickness and identifying marks

in the partial stampings and stenciling legible on the panel. These markings included various

combinations of specified minimum yield strength, column type, and derrick division numbers.

The 42 identified panels include three panels from the mechanical floors and two panels from the corners of the buildings. The identification process and complete documentation of all panels are detailed in NIST NCSTAR 1-3B.

5.2.4 Identified Core Columns

Table 5-2 lists the 12 core columns that have been positively identified. An additional sample, C-83, is also listed in this group. Though no markings were found on the piece, the shape and dimensions of this sample conform to the design drawings for core columns and have a similar appearance to core column

C-90. In addition, there were 13 pieces of wide flange sections that have stampings or markings, although

NIST, PANYNJ, and LERA were unable to determine the significance of these codes. These pieces are believed to be floor beams or other non-column pieces, which were not numbered in a systematic way;

indeed, these pieces may not even be from the towers.

Table 5-2. Identified pieces of core column material from WTC 1 and WTC 2.

NIST Name Type3 Description Bldg Column Floors

Derrick

Division (ksi)

B-1011 RB Heavy rectangular column WTC 1 508 51-54 55 36

B-6152-1 RB Heavy rectangular column WTC 1 803 15-18 52 36

B-6 152-2 RB Heavy rectangular column WTC 1 504 33-36 51 36

C-83b RB Heavy rectangular column NA NA NA NA NA

C-88a RB Heavy rectangular column WTC 2 801 80-83 550 42

C-88b RB Heavy rectangular column WTC 2 801 77-80 550 42

C-90 RB Heavy rectangular column WTC 2 701 12-15 549 36

C-30or S-12 W Wide flange section WTC 2 1008 104-106 NA 36

C-65 or S-8 W Wide flange section WTC 1 904 86-89 52 36

C-71 W Wide flange section WTC 1 904 77-80 NA 36

C-80 W Wide flange section WTC 1 603 92-95 51 36

C-155 W Wide flange section WTC 1 904 83-86 52 36

HH or S-2 W Wide flange section WTC 1 605 98-101 53 42

a. RB is a rectangular box

b. C-83 was not positively

Key: NA, information not

column consisting of four welded plates; W is a rolled wide flange shape,

identified, but due to similar size and shape was deemed a core column,

available.

NIST NCSTAR 1-3, WTC Investigation 35
Chapter 5

Of the 12 identified core columns, all but two were clearly marked. Sample C-30 had markings that clearly indicated the building and column, however, the floors were partially obscured: "x04 - 10x." As

the 24 ft section had both connector ends, it spanned only two floors and fit with the floor levels of

104—106. The second incompletely marked sample was C-88b which did not have any stampings or markings, but was welded to C-88a (identified by stampings).

5.2.5 Other Structural Pieces

Floor trusses were also recovered; however, attempts to identify their specific as-built locations within the

buildings were not successful because no stampings were found. Of the 23 pieces held by NIST, eight were of significant size but were badly tangled and twisted as a result of the collapse and subsequent

handling. The remaining pieces consisted of shorter sections of chord and rod material in addition to welded sections that connected the trusses to the floor seats.

Seven samples were recovered from WTC 5 (NIST NCSTAR 1-3B). These are coupons that were removed at the WTC site and held by GMS, LLP. They were subsequently sent to NIST once the

Investigation officially began.

No structural elements have been positively identified from WTC 7. However, the columns were

fabricated from conventional 36 ksi, 42 ksi, and 50 ksi steel produced to ASTM specifications.

5.3 STRUCTURAL STEEL ELEMENTS OF SPECIAL IMPORTANCE

Of the 42 exterior column panels and 1 2 core columns that were positively identified, many were

considered especially important to the NIST Investigation. Two major categories of steel are considered

to be of special value:

• Samples located in or around the floors impacted by the airplanes,

• Samples that represent one of 14 grades of steel specified for the exterior columns, one of

four grades of steel specified for the core columns, or one of the two grades of steel for the

floor trusses.

5.3.1 Samples Located in or around the Floors of the Airplane Impact

Photographic evidence indicated that damage to WTC 1 due to aircraft impact occurred from the 94th to 99th floors and was bounded by columns 1 1 1 through 152. For WTC 2, the impact area was lower, with

damage found from the 77th to 85th floors. While the damage appeared to be bounded by column

lines 41 1 and 440, the starboard wing of the aircraft may have struck columns even closer to the southeast comer of the building. This area was persistently obscured by smoke, however, and no clear images were

available. From this information, NIST was able to determine which perimeter panels and core columns

could be used to comment on damage (physical and fire) and possible failure mechanisms in this area. Fig. 5-5a shows an overlay of the recovered exterior panels in and around the impact zone of WTC 1. Samples C-80 and HH, both core columns, were also identified as residing near the impact zone. The

recovered portion of each column is approximately represented in this image. No similar impacted

exterior panels were recovered for WTC 2; however, two core columns were recovered, Fig. 5-5b.

36 NIST NCSTAR 1-3, WTC Investigation
Steel Inventory and Identification

Figure 5-5a. Interpreted column damage, from photographic evidence, to north face of

WTC 1, with overlay of recovered steel identified by NIST. Colored regions represent

recovered portions. Both core columns (#603 and #605) are in the second row from the

north face of WTC 1.

NISTNCSTAR 1-3, WTC Investigation 37
Chapter 5

Damage in shaded area cannot be accurately determined. Figure 5-5b. Interpreted column damage, from photographic evidence, to south face of

WTC 2, with overlay of recovered steel identified by NIST. Samples shown represent

recovered portions. Core column line 801 is in the row closest to the south face of

WTC 2.

5.3.2 Samples Representing the Various Types of Steel Specified in the Design

Drawings

The other class of samples important to the investigation included representative samples of the different

strengths of steel specified for the buildings. From the recovered steel, sufficient representative samples

from each important class of steel were available for a full examination (i.e., chemical, metallurgical, and

mechanical property analyses) to provide data for modeling the behavior of the towers during impact

through collapse. Detailed information can be found in NIST NCSTAR 1-3B. Considering both column

and spandrel material, samples of all strengths specified for the perimeter panels were available. While

only two (36 ksi and 42 ksi) of the four grades of steels were recovered for the core columns (Table 5-2),

38 NIST NCSTAR 1-3, WTC Investigation
Steel Inventory and Identification

99 percent of core columns were fabricated from these two grades. Samples of floor truss could not be

identified as to their precise, as-built location within the buildings; however, chemical and mechanical

property analyses showed that both specified grades of steel were recovered.

5.4 SUMMARY

N1ST has 236 samples from the WTC buildings, with the vast majority belonging to WTC 1 and WTC 2. These samples represent roughly 0.25 percent to 0.5 percent of the 200,000 tons of structural steel used in the two towers. The collection of steel from the WTC towers was sufficient for determining the quality of

the steel and, in combination with published literature, for providing mechanical properties as input to models of building performance. The original as-built location of 42 recovered perimeter panels and

12 recovered core columns were determined. Representative samples exist for all 12 grades of perimeter

panel material, two grades of the core column material (representing 99 percent of the total number of

columns), and both grades of the floor truss material. Furthermore, a number of structural pieces were

recovered from locations in or near the impact and fire damaged regions of the towers, including

four exterior panels directly hit by the airplane and three core columns located near these areas.

5.5 REFERENCES

Faschan, W. 2002. Leslie E. Robertson Associates, New York City, NY, personal communication to

F. Gayle, National Institute of Standards and Technology, Gaithersburg, MD, May 21.

McAllister, T., ed. 2002. World Trade Center Building Performance Study: Data Collection,

Preliminaiy Obsen'ations, and Recommendations. FEMA Report 403. Federal Emergency

Management Agency. Washington, DC, May.

Voorsanger and Associates Architects, PC. 2002. WTC Archives Interim Storage Facility, JFK

International Airport: Presewation and Inventoiy Report, Draft 2. New York City, NY, November.

NISTNCSTAR 1-3, WTC Investigation 39
Chapter 5

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40 NIST NCSTAR 1-3, WTC Investigation
Chapter 6

Damage and Failure Analysis of Structural Steel

6.1 background

Damage characteristics, failure modes, and fire-related degradation of the recovered structural steel components from the World Trade Center (WTC) towers are important in:

1. Ascertaining the response of the structural steel of the towers to the impact of the planes;

2. Providing information on perimeter column damage for correlation with impact damage

modeled in the aircraft impact damage analysis;

3. Providing information on thermal excursions experienced by the structural steel for use in the

reconstruction of thermal and tenability environment. (The model of the fire was not intended to match the actual fire on a column by column basis, and thus the observations on

thermal excursions of the steel are not expected to match the model in detail); and

4. Providing input to the National Institute of Standards and Technology (NIST) structural fire response and collapse analysis covering observations of missing fire proofing on the

perimeter columns due to airplane impact. This information is vital as fire proofing was the

primary factor determining whether fire exposed steel reached temperatures where there was

significant strength loss. In addition, NIST used the detailed characterization of damage

observations as a base for modeling the damaged towers and the subsequent collapse of the buildings.

To document the damage to the structural steel of the towers, two sets of observations were made:

1 . Pre-collapse analysis concentrating on impact damage sustained by the exterior panel

sections. This analysis was conducted using photographic and video images of the towers

and was limited to the perimeter panels as the interior structural components (e.g., core columns, floor trusses) could not be clearly observed. The photographic and video images were also used to determine the location of pre-collapse fires and possible fire-related damage

to the panels.

2. Damage characteristics ofthe recovered structural steel elements. The exterior panels were

again evaluated, as well as the connectors or seat assemblies used to attach the floor trusses to the exterior column panels, the inner core columns, and the seat assemblies attached to the channels of the core columns. Of particular importance were the samples from locations near

the airplane impact region (north face of WTC 1 and south face of WTC 2) and those where

fire was known to exist before the collapse of the buildings.

This information is summarized below, and the complete analysis is found in the report Damage and

Failure Modes ofStructural Steel Components (NIST NCSTAR 1-3C).

NISTNCSTAR 1-3, WTC Investigation 41
Chapter 6

While the physical damage and failure mechanisms were reported for the present condition of the steel, in most cases it was difficult to conclude which extreme loading event may have led to the condition.

Observed damage may have been a result of the aircraft impact, the pre-collapse fires, the ensuing

collapse of the buildings, post-collapse fires, or the subsequent handling related to the recovery efforts. In

particular, it was difficult to separate degradation from the relatively brief fire exposure experienced prior

to the collapse of the buildings from the fire exposure in the debris pile at the WTC site, which for some

samples was as long as 4 months.

6.2 OBSERVABLE PRE-COLLAPSE DAMAGE OF EXTERIOR COLUMN

PANELS

Photographic images, video images, engineering drawings, and other relevant resources were used to determine pre-collapse damage to the exterior column panels. Through this analysis, impact damage,

failure modes, and possible exposure to fire of the panels prior to the collapse of the towers was

established. The analysis focused primarily on the perimeter panels since damage to the core columns

and floor trusses prior to collapse could not be discerned from pre-collapse images.

6.2.1 Photographic Analysis of Pre-Collapse Damage to Exterior Panels

Multiple photographs were meshed together, with extensive image processing, to produce detailed images

of impact damage for both WTC 1 and WTC 2. Figure 6-1 (top) shows a resulting image of WTC 1. An

outline of a Boeing 767 aircraft was superimposed and distorted to approximately match the outline of the

hole in the building, Fig. 6-1 (bottom). Further, markers denote observable failure modes in the exterior

columns. The specific failure modes identified include bolt failure, weld failure, and tearing of the steel plates. For visualization purposes, the black lines on the image indicate the location of the bolted column

splices. Of the 42 recovered panels with known as-built locations, five from WTC 1 had been directly

impacted by the airplane or had damage as a result of the impact. These are the NIST labeled panels

M-30, S-9, M-27, M-2, and N-7 shown in Fig. 6-1 (bottom). Damage features and failure modes of these

five panels and individual components of the panels have been documented in NIST NCSTAR 1 -3C.

The availability of images with details of the impact hole in WTC 2 was much more limited than for WTC 1 (refer to NIST NCSTAR 1-5 A). This was primarily due to geography, where the land area south

of the WTC complex was much less extensive than to the north, and tall buildings to the south made it

difficult to get clear views. In addition, most of the professional and amateur photographers who moved

toward the area came from the north and did not try to go south of the WTC complex. Fig. 6-2 (top)

shows one of the few images where smoke shrouded only a portion of the damage to WTC 2 caused by

the airplane impact. In Fig. 6-2 (bottom), markers were again added to identify observable failure modes

in the exterior columns. The damaged region encompassing the starboard wing tip area (outlined roughly

in yellow) was unobservable in all available photographs and video footage due to the large and persistent

volume of smoke emitted from those floors. For visualization purposes, the black lines indicate the

location of the bolted column joints in the exterior wall. Detailed post-collapse analysis of the failures was not possible around the impact hole in WTC 2 because no identified panels in NIST's possession came from this area.

42 NIST NCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

I Broken Bolt Connection Split Longitudinal Welds — Wall Splice

I Cut Metal Unknown / Obscured

Figure 6-1. (top) Enhanced image showing the impact hole in the north face of WTC 1,

and (bottom) overlay of an outline of a Boeing 767 that has been distorted to get a best

approximate fit with damage to WTC 1. The location and type of localized damage has

been indicated.

NISTNCSTAR 1-3, WTC Investigation 43
Chapter 6

Figure 6-2. (top) Enhanced image showing the impact hole in the south face of WTC 2

and (bottom) overlay of the location and type of localized damage.

44 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

Parts of the aircraft that impacted the south face of WTC 2 exited the building on the north face and

landed some distance from the building. In particular, it was believed that an engine exited the northeast corner of the building on floor 81, and a landing gear strut component cut through the wall on the north

face near the northeast corner on the same floor. Images were examined to determine damage caused by

these pieces in order to feed this information back to the impact damage and building stability models.

Several images taken from different angles and with different smoke and lighting conditions were examined to characterize the damage. Fig. 6-3 is a view of the northeast corner of WTC 2 centered on

approximately the 82nd floor. The 81st floor, where the engine exited, was piled full of burning debris. The corners of the buildings were designed such that every other floor had a column in this space, an example of which can be seen above the hole, indicated by the arrow. The plans of the building show that the 81st floor was one without a column, consistent with the image, so that the exiting engine did not remove one as it exited the hole. Also, examination of this and two other images showed that the columns that border the hole to the left and right appear undamaged, so that the engine appears to have

not damaged the building structurally as it exited.

On the right side of the image in Fig. 6-3 is the north face of WTC 2. It was believed that a large

component of a landing gear exited the building on this face near the northeast corner in the region of the 80th floor. On Fig. 6-4, the arrows indicate where the columns have been cut (regions of missing column

denoted by blue arrows) or where a bolted connection has broken and bent the column ends outward

slightly (green arrows). The bolted connections were broken at the 81st floor in columns 257 and 258.

The entire column was cut through at columns number 254 and 255, also at the 81st floor. (Column 254

retained an intact outer web, however [red arrows]. The web had fractured free of the rest of the column

over a distance of slightly more than two floors. Since the flange tips can be seen on either side of the outer web, the fractures occurred within the flange plates.) These exterior columns were no longer load

bearing.

6.2.2 Photographic Evidence of Damage to the Flooring System of WTC 2

Several images, such as Fig. 6-5, showed what appear to be sagging or hanging floor slabs in the

windows on the east and north faces of WTC 2. The thickness and color of the feature indicated by

arrows in Fig. 6-5 are consistent with that of the concrete slab of the floor assembly (see NIST

NCSTAR 1-6C). The floor slab in Fig. 6-5 appeared in the windows bounded by columns 241 and 248 on the 80th floor, and in earlier images was bounded by columns 245 and 248, indicating that the floor

slabs moved over time.

The details and implications of the presence and locations of these floor slabs are discussed in greater

detail in NIST NCSTAR 1-6E.

6.2.3 Damage to Sprayed Fire Resistive Material in WTC 1

According to construction documents (Feld 1971), the sprayed fire-resistive materials (SFRM)

(i.e., fireproofing) applied to the exterior of the outer columns within the impact regions of both towers was the same material and application process used for core columns and floor trusses. One of the

parameters in the impact and fire analysis was the condition of the SFRM, so available photographs were examined for evidence of its condition after impact.

NIST NCSTAR 1-3, WTC Investigation 45
Chapter 6
Damage and Failure Analysis of Structural Steel

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Chapter 6

48 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

Areas of particular interest were around the impact holes in each tower, as well as regions where the exterior walls were impacted from the inside by debris, causing the exterior facade to be damaged. The

photos were enhanced digitally to reveal the condition of the SFRM. The image processing techniques

involve changing the range of displayed levels and effective gamma, the application of filters such as unsharp mask and edge sharpening, and subtracting filtered images from the original to highlight specific

features. For an explanation of these image processing techniques, the reader is referred to Russ (1995).

Figure 6-6 schematically shows how the SFRM was to be applied to the exterior columns, with a relatively even thickness coating to all exterior portions of the column. In practice (Fig. 6-7), it appears

the SFRM was sprayed so as to fill the depression formed by the outer web of the column and the

protrusions of the two column flanges, so that the outer face of the column appeared flat (Fig. 6-7).

Figure 6-6 shows details of the architectural aluminum panel attachment to the window frames,

suggesting how the blowout of windows and frames could possibly pull off the aluminum panels as well.

There was extensive photographic evidence of SFRM damage around the impact hole on the north face of

WTC 1. This was not surprising considering the energy of the impact. However, some of the damage

was a considerable distance from the impact site.

Figure 6-8 shows the east side of the impact hole in the north face of WTC 1. Arrows indicate where the

SFRM was damaged, and in several locations the underlying paint showed through. Examining these

images further, it was possible to view the trusses on the underside of the floors that collapsed into the impact hole (Fig. 6-9). In the two instances indicated on the figure, the details of the structure and the

paint color of the lower chord of the trusses were quite visible, indicating that most if not all of the SFRM

had been dislodged.

6.2.4 Damage to Sprayed Fire-Resistive Material in WTC 2

In WTC 2, there was substantial damage to perimeter wall SFRM due to impact of debris from inside the

building, as discussed below.

North Face Near Northeast Corner

The north face of WTC 2 contained the exit holes for some of the debris from the collision of the aircraft. This debris consisted of aircraft components, building contents, and possibly structural parts of the

building. This debris impacted the interior of the north wall, blowing out windows, damaging exterior

columns and removing aluminum exterior panels and SFRM from the columns.

A broad region of the north face of WTC 2, spanning columns 230 through 259 and the 79th through

82nd floors, had the exterior aluminum panels and SFRM damaged or removed due to debris impact from

within the building. In particular, a rectangular region consisting of columns 239 through 249 on

floors 79 through 81 was found to be completely free of aluminum panels, and the best observations of

SFRM damage were made here (Fig. 6-10). Red arrows indicate where the SFRM was dislodged from

the outer web between the protruding tips of the flanges. This determination was made using this image

and other higher resolution images. The appearance of white linear features on the edge of the columns,

at this angle of solar illumination, shows the absence of at least some, if not most SFRM from the center region of the outer web of the column. In contrast, column 246 (green arrow) shows bright contrast

NISTNCSTAR 1-3, WTC Investigation 49
Chapter 6

1 -6s

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Typica! Column

Aluminum Cover Figure 6-6. Diagrams showing how the fireproofing was to be applied to the

exterior columns, and the exterior aluminum panels were attached to the window frames.

50 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

© 2001 Lyle Owerko/Wonderlust

Figure 6-7. Fireproofing that had been applied to the

exterior columns. The depression formed by the outer

web and the flange tips are completely filled in with

fireproofing at the arrow. North face of WTC 2.

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Damage and Failure Analysis of Structural Steel

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54 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

across the entire face, and this and other photos confirm that the SFRM was nearly intact along this column for the entire exposed three floors, despite the fact that the aluminum facade has been dislodged.

Additional images taken of the region at higher resolution (Fig. 6-1 1) showed more subtle details about

the SFRM damage. The red arrows indicate where the SFRM has been dislodged completely from the flanges, as evidenced by the exposed red Tnemec paint on the columns. It was not possible to make an unambiguous determination as to whether the insulation on the exteriors of the spandrels was damaged in any of the photographs. This photo also reveals details about the loss of the SFRM. The blue arrow in Fig. 6-1 1 points to an example of a band of white features that appear wherever two aluminum panels

met on the exterior of the building, the white feature becoming visible when the panels were dislodged.

This appears to be a coating applied to protect the SFRM from moisture infiltration at the aluminum panel

joints. This coating appears to have prevented the loss of SFRM in a great many locations where the SFRM was knocked off both above and below this location.

East Face of WTC 2

The interior of the east face of the tower was not impacted as directly as the north face, but nevertheless

experienced damage to both aluminum exterior panels and SFRM. The damage was scattered across a large fraction of the east face on the 79th through 82nd floors (Fig. 6-12), including areas where the

SFRM has been damaged or removed to the level of the paint (red arrows). Loss of SFRM was not found

south of column 316 on this face of WTC 2, but since dislodged SFRM can only be seen if the aluminum

panels were also dislodged, and the panels south of column 316 were intact, this does not mean that the

insulation was necessarily undamaged.

6.2.5 Photographic Analysis of Pre-Collapse Fire Exposure to Exterior Panels

The NIST Investigation Team used photographic images, video images, and other relevant information to develop detailed time lines for the spread and growth of fires in the towers (NIST NCSTAR 1-5 A). This

data was utilized to ascertain the location and extent of fire exposure for the recovered exterior panels.

The methodology used to develop the information below is described in (NIST NCSTAR 1-3C).

In WTC 1, the 89th through 1 10th floors were analyzed for fire exposure. From the 26 recovered panels

identified from WTC 1, 22 panels fall within this range. In general, fire was observed on floors 92

through 99 as well as the 104th floor. Sixteen of the 22 panels were exposed to fire during the 1 h and

42 min that WTC 1 stood after the impact. For WTC 2, the 77th through the 1 10th floors were analyzed

for visual observations of fire. Nine of the 15 panels recovered from WTC 2 were from this region. In general, fire was observed from floors 78 through 83. No visual evidence of fire was observed near any

of the recovered panels prior to the collapse of WTC 2.

Figure 6-13 displays a time-fire exposure map for panel K-2 (WTC 1, center column line 236, floors 92

to 95). On the right-hand side is a schematic showing the recovered portion of the panel, drawn as if viewed from outside of the building, with the floors and column lines labeled. The "grayed-out" sections

of the panel were not recovered, resulting in portions of only six different windows available for inspection. On the left-hand side of the figure, the time line analysis for exposure to fire can be observed

for that given panel, along with the color-coded, numerical key. The location of each window for a given

NIST NCSTAR 1-3, WTC Investigation 55
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56 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel
Chapter 6

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Figure 6-13. Time-fire exposure map for panel K-2 (A236: 92-95). Fire can be observed

on the 92nd and 94th floors of the panel.

58 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

time is consistent with the recovered panel diagram. Similar to the "graying-out" of the panel schematic,

the corresponding windows were "grayed-out" in the time sequence, however, the numeric key is still visible. In the analysis, the windows were numbered for a given floor (similar to the original column

lines of the perimeter panels) and the determination was made from the resources (photographic and

video images) as to which category each individual window fell into for the given time frame. The

categories were labeled as:

0: No fire was visible

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2: Fire visible inside

3: External flaming

9: Unable to be determined (typically as a result of smoke obscuring the window).

For the recovered portion of panel K-2, fires were observed on the 92nd and 94th floors around

column 236. This information was correlated with the visual evidence for fire exposure on the recovered

panels as well as the paint mapping study discussed in Sec. 1.6.

Table 6-1 lists the panels recovered from the floors involved in fire and a description of the extent of

direct fire exposure based on the pre-collapse images.

6.2.6 Photographic Evidence of Pre-Collapse Distortion of Exterior Wall: South Face

of WTC 1

As a result of the fires and load redistribution, exterior walls in each tower bowed inward well before the

initiation of global collapse. Approximately 6 min before collapse, a New York City Police Department

(NYPD) helicopter image captured such bowing in the south face of WTC 1. The image is shown in nearly its entirety in Fig. 6-14, and the analysis is shown in Fig. 6-15.

The magnitude of the bowing on the south face was calibrated using column spacing on the west wall as follows. A true vertical column line in the bowed region was established by extending the column lines along 20 floors below the damaged region. This was extended along the same line through the smoke

filled regions. The distance the columns were deformed into the building perpendicular to the south face was established by measuring angles and distances on the west face at the southwest comer of the

building (see Fig. 6-15, lower left). This established the angle (167 degrees from right horizontal) and

base magnitude (120 ft per 88 pixels) of the inward bowing vector. The foreshortening of the south face

due to perspective was calculated and applied to the distances measured on the south face, and increased

the measurements by a maximum of approximately 1 5 percent over the physical distance measured on the image.

The region over which the exterior columns appear to bow inward spans nearly the entire face, and

extends vertically over at least four floors, if we assume continuity between the inwardly bowed sections on the east and west sides of the south face. The maximum inward bowing at this time is 55 in. +/- 6 in. near column 316 on the 97th floor. Note that this is the maximum that is visible, but a large portion of the

south face is obscured by smoke. An earlier image taken at 9:44 a.m., or 58 min after aircraft impact,

shows that the columns were straight, indicating that the inward bowing developed quite some time after impact.

NISTNCSTAR 1-3, WTC Investigation 59
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60 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

Figure 6-14. Inward bowing of south face (at right, with dark lines overdrawn) of WTC 1. Taken at 10:23 a.m. from an NYPD helicopter.

NISTNCSTAR 1-3, WTC Investigation 61
Chapter 6

Figure 6-15. Pull in of south face of WTC 1 at 10:23 a.m. showing considerable

displacements of outer columns into the building. Faint white vertical features are the

aluminum facade of the exterior. Maximum visible inward displacement of the columns

is 55 in. +/- 6 in. Poor resolution leads to large uncertainties in measurements; a large

fraction of the south face of the building is obscured by smoke.

62 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

6.2.7 Photographic Evidence of Pre-Collapse Distortion of Exterior Walls: East Face

of WTC 2

Several high-resolution photographs taken of the east face of WTC 2 in the region of the 80th floor show

inward bowing of the exterior columns. Three photographs were of sufficiently high resolution to enable

measurement of the inward bowing over the surface of the face of the building.

As for the north tower images, vertical lines to establish original alignment of the columns were drawn by

tracing from unbowed sections well above and below the bowed section. However, for the south tower

images, because of the high resolution of the images and the more normal viewing angle, the calibration

for extent of bowing was based on the 1 1 in. depth of the aluminum facade (N1ST NCSTAR 1-3C for

details). Corrections were made for barrel distortion of the camera lens. The estimated error in each

measurement was approximately 1 in. to 2 in.

One image was taken at 9:21:29 a.m., or 18.5 mm after impact, and the other two images were taken by

the same photographer at nearly the same location 10 s apart beginning at 9:52:54 a.m., or roughly 6 min

before collapse of WTC 2 (Fig. 6-16 shows one of these images). The images were analyzed extensively

to spatially map the inward bowing of the exterior columns across the face of the building to provide

insight into when this bowing occurred and how it changed with time. (Additional figures with much

lower resolution were examined to determine the amount of deflection as close to the moment of collapse

as possible. These did not produce any usable results, however, due to the low resolution and

obscurations.)

The resulting data are plotted in Figs. 6-17 and 6-18. (The results from the two images that went into

Fig. 6-18 were combined because they were taken from the same location with the same camera only 10 s apart, and it was assumed that the inward bowing of the columns did not change appreciably in that time).

It was clear that only 18.5 min after the impact of the plane, there was significant inward bowing of the

east face of the building. After an additional 31 .5 min, the deflections nearly doubled in most locations.

6.2.8 Photographic Evidence of Details of the Collapse of WTC 2

Some details of the early stages of collapse of WTC 2 were found in the photographic record and

analyzed for validation of the models of collapse. The image in Fig. 6-19 was taken about one second

after the tower began to collapse, and shows the east face of the building. A noticeable kink was visible

along the southeast comer of the building in the region of the 106th floor (arrow).

A more detailed analysis of the geometry has been made of this distortion, shown in Fig. 6-20. The kink was in fact a double-kink, and the directions of the axes of bending are shown in the figure. It was

believed that this distortion was formed by the sideways shift of loads transmitted via the hat truss and

rigid, beam-framed floors of floors 107 to 110, and a more detailed structural analysis can be found in NIST NCSTAR 1-6E. Examination of the right side of the image, after considerable image processing,

shows that the NE comer of the building bent, but had no sharp discontinuity. Thus, the sharp kink had

not propagated across the face of the building. An image taken approximately 2 s before collapse

(Fig. 6-21) shows no bending, so it is believed that the kink developed at either the moment of collapse

initiation or during the earliest moments of the collapse process.

NIST NCSTAR 1-3, WTC Investigation 63
Chapter 6

Figure 6-16. East face of WTC 2 at 9:53:04 a.m. showing a larger amount of pull in. The

vertical black lines were drawn to establish the original line of the exterior wall columns,

and the shorter line segments are drawn at the same angle as the nearest observable

joint in the exterior aluminum panels to establish the inward bowing distance. For actual

column numbers, add a "3" to the front of the two-digit designations in the image.

64 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

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66 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

©2001 Gulnara Samoilova/AP

Figure 6-19. Initial seconds of collapse of WTC 2 viewed from the east-northeast, and

kink in the southeast edge of the building near the 106th floor (arrow).

NISTNCSTAR 1-3, WTC Investigation 67
Chapter 6

Figure 6-20. A close-up of the kink area in the previous figure (southeast corner,

WTC 2), with overlaid lines to highlight the details of the kink geometry. The edge of the

building (1) bends an angle to another direction (2), then bends back somewhat toward

the original direction (3).

68 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

Figure 6-21. Northeast corner of WTC 2, several seconds before collapse. The corner shows no distortion of the type in the previous figure, implying that the distortion

accompanied the collapse and did not precede it. The region in the upper right is enhanced for visibility.

6.3 PHYSICAL DAMAGE OF RECOVERED EXTERIOR WALL PANELS

Damage surveys were conducted of the recovered exterior wall panels to identify and inventory the

various failure modes associated with impact and collapse of the building. The individual columns,

spandrel plates, weld connections, panel splice connections (column end plate and spandrel splices), and

floor truss connections were examined. In this overview, the damage to one WTC 1 panel which was hit by the airplane fuselage is shown as an example, followed by a summary of findings for all panels. As

only five of the recovered panels were directly hit or otherwise sustained damage as a result of the impact,

damage to the other recovered exterior panel sections was a result of events that occurred during or after the collapse of the buildings.

NISTNCSTAR 1-3, WTC Investigation 69
Chapter 6

6.3.1 Overall Damage Patterns of Exterior Wall Panel Sections

Panel M-2 (WTC 1, column line 130, floors 96 to 99) was struck by the upper part of the airplane's

fuselage and the vertical stabilizer of its tail. In the photographic images (Fig. 6-1 [top]), the condition of

the panel appeared remarkably similar to its as-recovered condition, Fig. 6-22a. The damage observed on

the recovered panel was largely a result of the impact. The lower part of all three columns was bent into

the building with the 97th floor concrete slab acting as a fulcrum point, Fig. 6-22b. Also, the lower end

plates of columns 129 and 130 were ripped off with column plate fracture occurring near welds with to the butt plates, Fig 6-22c. This gave the columns a "splayed" appearance. The end plate for column 131 was still intact, and the lower portion of the column was relatively undeformed. In the pre-collapse

images, the lower portion of this column is also bent inward just below the 97th floor spandrel; however,

it was bent back out sometime during or after the collapse.

Comparison of the four recovered panels that were directly impacted with pre-collapse photographs

showed that two panels (M-2 and M-30) remained in a condition similar to just after impact. Some of the

extraneous damage can be attributed to the events during and after the collapse, but the general shape and

appearance of the recovered pieces agree with the damage observed in the photographs. Due to the lack

of detailed pre-collapse visual evidence of impact damage sustained by panels M-27 and S-9, the same

assertion could not be made. Finally, the lower portion of N-7 incurred some damage; however, it could

not be positively correlated with pre-collapse images.

6.3.2 Damage and Failure Modes of Exterior Wall Columns

Damage to the lower portion of panel M-2 (WTC 1, column line 130, floors 96 to 99) was clearly

associated with impact, which led to the missing endplates for columns 129 and 130. The inner and outer web plates of both columns have thinned in this area due to plastic deformation, Fig. 6-23a. The

fracturing of the plates propagated as a 45 degree slant crack, oscillating between positive and negative

45 degrees with respect to the normal of the plate. The necking (thinning) of the outer web plate, plotted

in Fig. 6-23b, indicates significant ductility, even at the high-strain rates associated with impact. Large

buckles were observed on both flanges for columns 129 and 130 with some splitting occurring in the base

material of the flanges (Figs. 6-24a and 6-24b) and spandrels (Fig. 6-24c). However, the welds found

near this bending point remained intact (Fig. 6-24d).

The five panels located in the airplane impact zone had greater damage than those outside of this region.

There did not appear to be any difference in failure modes or damage characteristics whether the panels were exposed to fire or not, with the exception of the buckling phenomenon observed for the inner web

plates. For the fractures that occurred near fillet welds, metallographic evaluation showed that the

primary mode of fracture was initiation of a crack in the heat affected zone (HAZ) followed by

propagation that was restricted to the HAZ of the base plates. Based on visual observations of all the

recovered exterior panel columns, this was the dominant mode of fracture that occurred near the welds.

In one case, the fracture initiated in the HAZ and subsequently propagated into the unaffected base

material, although this was a rare occurrence.

70 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

Source: NIST. Figure 6-22. a) Overall view of panel M-2 (A130: 94-97). Bottom of panel is on the left- hand side of the picture and the outside of the panel is facing down, b) major bend in panel located at the 97th floor level, and c) splayed bottoms of columns 129 and 130.

NISTNCSTAR 1-3, WTC Investigation 71
Chapter 6

Source: NIST.

b ) 0.30

0.00 J '

1 1 1

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60

Distance from fracture surface (in)

Figure 6-23. Thinning of outer web from column 130 of panel M-2 (A130: 96-99).

a) Photograph showing thinning, b) graph indicating plate thinning near fracture surface.

72 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

Source: NIST. Figure 6-24. Damage images from sample M-2 (A130: 96-99). a) Large buckles observed

on the flange plates of column 130, b) cracking of the flange plates on column 130,

c) cracking of spandrel at 97th floor level, and d) no cracking of the welds in this area on

column 130.

NISTNCSTAR 1-3, WTC Investigation '3
Chapter 6

Of particular interest was the fracture behavior of the plates composing the columns that were directly

impacted by the airplane. Fractures of the plates in areas away from a welded joint (e.g., fracture occurring perpendicular to the rolling direction of the outer webs of panels M-30 and S-9) exhibited

ductile behavior, including necking and thinning away from the fracture, despite the high-strain rates.

Conversely, fractures occurring parallel and directly next to a welded joint exhibit little or no ductile

characteristics. Diminished properties of the heat affected zone in the base plate, the geometry of the

joint with respect to the direction of impact, stress concentrations due to the geometry, and the orientation

of the crack propagation with respect to the rolling direction of the plate are expected to contribute to the

lack of ductility.

6.3.3 Exterior Wall Spandrel Connections

As the entire panel M-2 (A130: 96-99) was recovered, all six spandrel connections were available for inspection. From pre-collapse photographs, it appears that both connections located on floor 97 failed

upon impact of the airplane. Both failures were associated with tearing of the bolt holes (Fig. 6-25); the connection associated with column 131 had bolt hole tear out, and the connection nearest column 129 still retained the splice plate without the neighboring spandrel plate. The connections associated with floor 99

appeared intact, and failure may have occurred upon collapse. Those connections located on the

98th floor were obstructed by smoke, and the condition could not be determined prior to collapse. Their

failures were associated with bolt hole failure, as well as fracture of the spandrel plate.

From the observations made of the spandrel connections on recovered panels, there was no difference in failure mode whether or not the panels were exposed to pre-collapse fires. There was a statistical difference in the failure modes when the location of the panels was considered. For panels at or above the impact zone, bolt hole tear out was a more common failure mode. For those samples below the impact

zone, there was a higher propensity for the spandrels to be ripped off from the panels.

6.3.4 Exterior Wali Column Splices (End Plate/Butt Plate Connections)

The state of the recovered bottom portion of columns 129 and 130 was nearly identical to the condition

after the impact for panel M-2 (Al 30: 96-99). Both end plates were ripped completely out of the column.

Fig. 6-26. Failure was a mixture of fracture in the base plates (portion of the flange/web still attached to the end plate) and clean separation of the end plate from the flange/web (complete flange/web recovered

with internal weld bead visible). Bolt hole deformation for column 131 indicated that the connection was

broken by bending inward of the lower column of the connection. Finally, while bolt failure was

observed at the 99th floor splice of column 129, it can be noted that some separation existed between the

flange plates of the column and the endplate.

The major failure mechanism for the column splices was fracture of the bolts. Considering the

120 connections that were exclusively bolted, there were only six samples where the end plate was ripped

either partially or wholly out from the column. Two of these were a direct result of the impact. For

samples where the connections were welded as well as bolted (mechanical floors of the towers), fracture of the flange and web plates of the columns was the predominant mode of failure.

74 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

M2-C1T

Column 131, 99lh floor

Spandrel Detail : 101

Failure mode: Type 4a, fracture of spandrel

Orientation of image: From inside building looking out.

M2-C1M

Column 131, 98th floor

Spandrel Detail : 101

Failure mode: Type 3, mixed mode of fracture and

bolt hole tear out, portion of spandrel from S9-C3B

still attached

Orientation of image : From inside building looking

out.

M2-C1B

Column 131, 97th floor

, | Spandrel Detail : 101

Failure mode: Type la, bolt hole tear out

Orientation of image: From inside building looking out.

Source: NIST. Figure 6-25. Photographs of recovered spandrel connections from sample

M-2 (A1 30: 96-99).

NISTNCSTAR 1-3, WTC Investigation ~5
Chapter 6

M2-C3B

Column 129, 97th floor

Spandrel Detail : 101

Failure mode: Type lb, bolt hole tear out

Orientation of image : From inside building looking out.

Source: NIST. Figure 6-25. Photographs of recovered spandrel connections from sample M-2

(A130: 96-99) (continued).

76 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

M2-C1T

WTC1

Col 131

Fl 99

M2-C1B

WTC1

Col 131

Fl 96

Source: NIST. Figure 6-26. Photographs of recovered end plate connections from sample M-2

(A127: 97-100). Both ends of column 131 had bolt failure that resulted in deformation of

the bolt holes in the end plates.

NISTNCSTAR 1-3, WTC Investigation
Chapter 6

No change in bolt

hole shape

M2-C2T

WTC1

Col 130

Fl 99

M2-C2B

WTC1

Col 130

Fl 96

Source: NIST. Figure 6-26. Photographs of recovered end plate connections from sample M-2

(A127: 97-100). The upper end plate experienced bolt hole failure. The bottom end plate

of column 130 was ripped out upon impact of the airplane. Failure was found to be a mixture of fracture in the base plates (portion of the flange/web still attached to the end

plate) and clean separation of the end plate from the flange/web (complete flange/web

recovered with internal weld bead visible) (continued).

78 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

Separation between flange

plates and butt plate

M2-C3T

WTC1

Col 129

Fl 99

M2-C3B

WTC1

Col 129

Fl 96

Source: NIST. Figure 6-26. Photographs of recovered end plate connections from sample M-2

(A127: 97-100). The upper end plate experienced bolt hole failure. The bottom end plate

of column 129 was also ripped out upon impact of the airplane. Failure was found to be a mixture of fracture in the base plates (portion of the flange/web still attached to the end

plate) and clean separation of the end plate from the flange/web (complete flange/web

recovered with internal weld bead visible) (continued).

NISTNCSTAR 1-3, WTC Investigation
Chapter 6

6.3.5 Exterior Wall Seats or Floor Truss Connectors

Panel M-2 (A 130: 96-99), Fig. 6-27, is shown as an example of perimeter wall truss connections. Of the

six truss seats on the panel, four were damaged but intact (e.g., bending up/down, bolt hole failure), and

the other two were completely ripped off of the column. The center column of this panel also had three

gusset plates, used to anchor diagonal bracing straps to provide horizontal shear transfer between the floor

slab and exterior wall, as well as out-of-plane bracing for the perimeter columns not directly supporting

the floor trusses. Only portions of the gusset plates remained on the middle column of the panel with the

diagonal bracing straps ripped from these plates.

The most significant observations were made when the panels were categorized by their as-built elevation

within the building, Fig. 6-28. Of the 28 floor truss connectors at or below the impact floors for WTC 1, 93 percent were either missing or bent downwards. Only 37 percent of the 38 floor truss connectors above the impact floors had similar characteristics. Comparable results were found for WTC 2 where

88 percent of the floor truss connectors below the impact floors were bent down or missing, while only

55 percent of the floor truss connectors above this region had such damage characteristics. This occurrence was most likely a result of the overloading of the lower floors ("pancaking" mechanism)

during collapse of the building.

Missing seats were typically a result of fracture near a welded joint associated with the standoff plates.

Inspection of the failures associated with the floor truss connectors showed that failure typically occurred

in the component with the lowest cross-sectional area with respect to the high loading forces of the

collapse. This was usually located near a welded joint. However, there was no evidence to indicate that the type ofjoining method chosen was improper, that the materials chosen were inadequate, or that inappropriate welding procedures were used.

6.4 PHYSICAL DAMAGE OF CORE ELEMENTS (COLUMNS AND

CHANNELS)

Damage surveys were conducted on the core elements to identify and inventory the various failure modes.

The identified core columns, channel material that connected the trusses to the core, and floor truss connectors (truss seats welded to the channels) were examined. A full discussion of these results can be

found in NIST NCSTAR 1-3C.

6.4.1 Core Columns

Table 6-2 lists the 12 identified core columns, including as-built locations and possible conditions to which they may have been exposed prior to the collapse of the buildings. Due to the small number of

samples, statistical data analysis of the damage features and failure modes would be of little use. Therefore, in-depth descriptions of the four significant pieces (C-80, C-88a, C-88b, and HH) that were

located within the fire zone floors are provided below. For reference, Fig. 6-29 displays the position of

the columns within the core and with respect to the perimeter panel damage.

80 NIST NCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

Exterior Column Panel: M-2. Column 1

S M2-ClT(Co 131, FI99)

Type A

Seat Detail: 1411

Tab plate remains;

Seat intact, 1 bolt hole remains intact, other ripped

out;

Gusset plate for damper unit ripped off at weld.

M2-ClM(Col31,Fl 98)

Type A

Seat Detail: 1411

Tab plate intact;

1 standoff plate remains, seat and other standoff plate

ripped off at welds;

Gusset plate with damper unit intact, bent up.

M2-ClB(Co 131, Fl 97)

Type A

Seat Detail: 1411

Tab plate ripped off at weld

Seat intact, 1 corner bent downwards, 1 bolt hole

intact with bolt, other ripped out;

Gusset plate with damper unit intact, bent up.

Source: NIST. Figure 6-27. Floor truss connectors for panel M-2 (A130: 96-99).

NISTNCSTAR 1-3, WTC Investigation 81
Chapter 6

Exterior Column Panel: M-2, Column 2

Source: NIST. Figure 6-27. Floor truss connectors for panel M-2 (A130: 96-99) (continued).

82 NIST NCSTAR 1-3, WTC In vestigation
Damage and Failure Analysis of Structural Steel

Exterior Column Panel: M-2. Column 3

I^H M2-C3T (Co 129, Fl 99)

Type A

Seat Detail: 1411 Tab plate ripped off at weld:

Standoff plates ripped off at welds:

M2-C3M (Co 129, Fl 98)

Type A

Seat Detail: 1411

Tab plate remains:

Seat intact, bent down. 1 bolt hole remains intact with bolt, other ripped out;

M2-C3B(Co 129, Fl 97)

Type A

Seat Detail: 1411

Tab plate ripped off at weld:

Seat intact, bent down. 1 bolt hole remains intact,

other ripped out;

Source: NIST. Figure 6-27. Floor truss connectors for panel M-2 (A130: 96-99) (continued).

NISTNCSTAR 1-3. WTC Investigation 83
Chapter 6

«

a) fll (R\ fTj M {H f^l ^ (SfS /ST) (£\ 0 lC I Seat intact, relating undefoiaed

' vy c/ l "/ O w \- / v V' \J V C/ v v M I *. .

T T . T T . 1 I . f. r T .IB Seat mtart, bent up

Figure 6-28a. Damage diagram overlaid with recovered samples and damage of

connectors.

S4 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

b)

80 SOI

1D1

ID

ill

if y n

010

SIC

SID

I88DBQ88

1 001 10 DO:

OiGQlifl;

EG

Seat intact, relatively undeformed

Seat intact, bent up

Seat intact, bent down

Seat intact, bent towards column

Seat gone, atleast one standoff plat* remains

Seat and standoff plates gone

Tab plate intact, relatively undeformed

Tab plate intact, bent up

Tab plate intact, bent down

Tab plate gone

95

th floor level

IflSB

01

ye

BQ

Figure 6-28b. Damage of connectors. At or below the 95th floor, all connectors are

either bent down or missing.

NISTNCSTAR 1-3, WTC Investigation 85
Chapter 6

Table 6-2. Recovered core columns with known as-built locations, separated by

pre-collapse conditions.

Column Description iilrl" JN liS 1 Name Brief Description Column #

Story at Splice Lower Upper

Core columns within impact region

WTC 1 C-80 Wide flange 603 92 95 36 WTC 2 C-8Sb Built-up box beam 801 77 80 42 WTC 2 C-88a Built-up box beam 801 80 83 42 Core columns within fire WTC 1 HH orS-2 Wide flange 605 98 101 42

exposure zone WTC 2 none WTC 1 B-6152-2 Built-up box beam 504 33 36 36 WTC 1 B-1011 Built-up box beam 508

. ^ . 54 36 WTC 1 B-6152-1 Built-up box beam 803 15 _ }JL 36 Core columns outside of WTC 1 C-71 Wide flange 904 77 80 36

fire exposure zone WTC 1 C-155 Wide flange 904 83 86 36 WTC 1 C-65 or S-8 Wide flange 904 86 89 36 WTC 2 C-90 Built-up box beam 701 12 15 36 WTC 2 C-30 or S-12 Wide flange 1008 104 106 36

300 m m m turn < i a a > » a i• HUB a m m m 400

\

a

1

I 1008 1007 1006 1005 1004 1003 1002 1001

11 908 907 906 905 904 903 902 901

1

1 a a a

I 807 806 805 804 803 802 801

1 a a a

1

1 708 707 706 705 704 703 702 701

«

1

I

1•

a a a

1 608 607 606 605 604 603

602 601

1 a a a t I a

I1

11 508 507 506 505 504 503 502 501

I a a a

HH, 605A: 98-101 {wide flange) C-80, 603A: 92-95 (wide flange)

Estimated impact damage to exterior wall

100

159 157 1 54 S151 148 145 142 139 136 133 1 30 127 124 121 118 115 112 109 106 103 101

Figure 6-29a. Positioning of the recovered core columns with the core area for WTC 1.

86 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

200

501 601 i em

508 608 708 807 908 1008

507 607

707 806

w907

B 1007

506 606

706 805

906 1006

•505

•605 705 804 905 1005

504 604 704 904 1004

703 803

503 603 903 1003

502 602

702 802

902 1002

\

701 801 901 1001

C-88a, 801 B: 80-83

(built-up box column)

C-88b, 801 B: 77-80

(built-up box column)

Estimated impact damage to exterior wall

100

459 457 454 451 448 445 442 [ 439 436 433 430 427 424 421 418 415 412

400

409 406 403 401

Figure 6-29b. Positioning of the recovered core columns with the core area for WTC 2.

Sample C-80 (Fig. 6-30a), from WTC 1, was a 36 ksi wide-flange located in the middle of the second row of core columns along the north face. Only the lower third of the column was recovered

(approximately 13 ft). The location of the column indicated that it did not sustain impact damage.

Consistent with the column's position, modeling results indicated that this column was not in the path of

the aircraft debris (NIST NCSTAR 1-2). Different failure modes were observed for each end

(Figs. 6-30b and 6-30c). Bolt fracture occurred at the lower portion of the column (92nd floor region),

while the upper portion of the wide-flange was torn through its section (near the 93rd floor region). No

visible necking (localized reduction in area) was observed in the immediate vicinity of the fracture.

NIST NCSTAR 1-3, WTC Investigation 8"
Chapter 6

Source: NIST. Figure 6-30. Core column C-80 (603A: 92-95). a) Overall view of recovered column,

b) view of connection in the 92nd floor region, failure as a result of bolt fracture, and

c) torn end of column near the 94th floor region.

Core columns C-88a and C-88b, from WTC 2, were unique among the recovered core elements in that the columns were still connected at the welded column splice, Fig. 6-31. Both columns were 42 ksi built-up

box columns with their shared splice in the 80th floor level. The lower 16 ft of C-88a was recovered

(floors 80 through 82), whereas the upper 8 ft of C-88b was still attached (primarily within the

80th floor). The location of the column C-88b suggested that impact damage may have been sustained.

Both webs and the "south" flange were severely bent to the east and had flame cut ends, Fig. 6-32. The

bends occurred just below the connector for the channel associated with the 80th floor. The "north

1 '

88 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

Source: NIST. Figure 6-31. Overall view of recovered core columns C-88a (801 B: 80-83) and C-88b

(801 B: 77-80).

NIST NCSTAR 1-3, WTC Investigation 89
Chapter 6

Source: NIST. Figure 6-32. Damage associated with core column C-88b (801 B: 77-80). a) Overall view

of failed end, b) bottoms of "south" flange and both webs bent towards the east just

below the 80th floor level, ends were flame cut, and c) "north" flange bent towards the

north, majority of plate was fractured with some flame cutting.

90 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

flange had a less significant bend toward the north, with approximately 80 percent of the failed cud oi the plate as a fracture. There was no visible necking of the plate in the area of failure, with the remaining

portion of the plate having been flame cut during the recovery effort. This flange was fractured roughly

2 ft below the splice in the 80th floor level. The fracture surfaces were too corroded for analysis. It was

believed that the damage of the south flange and the two webs occurred during collapse of the building as the direction of bending was not consistent with the travel of the airplane. However, the damage to the north flange may have been a result of impact damage, as the severity and location of the plate failure was

different from the other three plates of the column, and the direction of the bend was consistent with the travel of the airplane. The baseline analysis of the damage model indicated that this core column was not damaged as a result of the impact (NIST NCSTAR 1-2). However, another analysis with parameters set to increase the amount of damage indicated that impact damage was sustained. Core column C-88a did not sustain impact damage as (1) the damage (81st floor level) was above the floors of impact in this region (79th floor level), (2) the columns were bent downward indicating overloading of the column from

above, and (3) the bending direction of the plates was opposite to the direction of the travel of the plane.

This is also consistent with the modeling results (NIST NCSTAR 1-2).

Aside from the three columns discussed above, only one other core column was recovered from within

the fire region. Sample HH from WTC 1, Fig. 6-33, was a 42 ksi wide-flange positioned near the middle

of the second row of core columns along the north face. The bottom half of the column was recovered

(approximately 16 ft). The location of the column was such that damage could not have occurred during

the impact. This was corroborated by the modeling results that indicated the core column was not hit during the impact (NIST NCSTAR 1-2). The failure mode of each end was different (Fig. 6-33a).

Fracture of the plates used in the bolted connection occurred at the lower portion of the column

(98th floor region), while the upper portion was torn through the material ( 100th floor region).

Additionally, the sample was nearly bent in half (99th floor region), with complete fracture of one flange.

Figure 6-3 3b shows that the flanges have become separated from the web in the region of the bend.

Source: NIST. Figure 6-33. Core column HH (605A: 98-101). a) Fracture at both ends and b) separation

between flange and web in the 99th floor region.

NIST NCSTAR 1-3, WTC Investigation 91
Chapter 6

6.4.2 Core Channels and Truss Seats

An analysis was conducted to evaluate the failure modes of the recovered channel material and the

condition of the core truss seats. Of the 24 channels used to support the floor trusses, half failed at both

ends within the channel itself, Fig. 6-34a, while the others failed at one or both of the end connectors,

Fig. 6-34b. Twenty-one pieces of channel had seats or remnants of seats attached. Over 90 percent of

the seats (31 total) were still intact with the majority of these somewhat deformed. Only two seats had

been ripped completely from the channel at the welded joint between the seat and the channel, as shown

in Fig. 6-34c. Half of the retained seats were observed to have both bolt holes still intact, while the other

half had at least one bolt hole ripped out.

Source: NIST. Figure 6-34. Failure modes of core channels, a) Fracture through channel as indicated

by arrows, b) failure associated with end connector, and c) seat ripped off at welded

connection to channel.

42 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

6.5 PHYSICAL DAMAGE OF FLOOR TRUSS MATERIAL

A damage survey was conducted of the floor truss material to identify and inventory the various failure- modes. A full discussion of these results can be found in NIST NCSTAR 1-3C.

Samples received at NIST were either small sections (less than 3 ft to 4 ft in length) of chord and rod

material (Fig. 6-35a) or large, entangled masses in the form seen in Fig. 6-35b. According to Structural

Engineers Association of New York (SEAoNY) volunteer members involved in the recovery effort, it was

necessary to "ball up" any truss pieces of significant size to facilitate removal from the WTC site and

subsequent handling in the recovery yards. This was necessary since the primaiy and bridging trusses for

a given floor were welded together into huge grids. During the collapse, these lightweight floor sections were severely damaged and could not be easily removed. Thus, it is unknown when specific damage

features occurred. Further, it is likely that sections of multiple trusses were "balled" together and labeled

as a single unit rather than that NIST labeled samples consisting of just one pre-fabricated truss unit.

Source: NIST. Figure 6-35. Examples of recovered floor truss material, a) Small sections of rod and

chord (sample C-53) and b) "bailed-up" sections of rod and chord (C-137f).

NIST NCSTAR 1-3, WTC Investigation 93
Chapter 6

Attempts were made to identify the as-built location of the truss material, but unlike the other major

structural elements, identifying marks could not be found on the trusses. Difficulty was also encountered

in trying to measure the length of the truss members to determine if they were of the 35 ft or 60 ft variety.

It appears that no whole length sections were recovered. A large majority of the compressive resistance

welds between the chords and rods were observed to have failed. This was evident from the large amount

of chord material recovered without rods attached and vice versa. It is unknown when these failures

occurred or what caused them.

6,6 FIRE EXPOSURE OF THE STRUCTURAL ELEMENTS

Examination of the structural steel components for indications of fire damage is important to provide an

indication of the location and intensity of the pre-collapse fires in the towers, as well as the effect of the

fires on mechanical properties of the structural steel. This portion of the analysis attempts to

( 1) determine the temperature excursions experienced by the steel components, (2) determine when the

excursion occurred (pre- or post-collapse), (3) determine if pre-collapse fires significantly affected the

mechanical properties of the structural elements such that the structural integrity (load bearing

capabilities) of the component may have been compromised, and (4) provide this information for input/validation of the fire and thermal models of the reconstruction of thermal and tenability

environment (NIST NCSTAR 1-5). Analytical techniques used to carry out these tasks involved

assessment of the condition of the primer paint, microstructure, chemistry, and hardness of the steel. A

full discussion of these results can be found in Chapter 6 of NIST NCSTAR 1-3C.

6.6.1 Visual Inspection of Recovered Structural Components

NIST has developed a novel approach to evaluating the primer paint on the structural components for evidence of exposure to high-temperature excursions (see Appendix D of NIST NCSTAR 1-3C). This

method was found to be relatively easy to implement and robust enough to examine an entire component

in the field. Calibration tests in the laboratory showed that, although there was little or no change in color, the primer paint used on the WTC steels that reached temperatures over 250 °C cracked (similar to a "mud cracking" pattern) from the difference in thermal expansion between the paint and the steel. Since

deformation and environmental effects can also cause mud-cracking, the absence of mud-cracking

indicates the steel has not exceeded 250 °C, but the presence of mud cracks cannot be assumed to be

caused by high temperature.

Visual inspection for the fire effects on recovered steel was conducted solely on the perimeter panels and

core columns, as these were the only structural elements that had known as-built locations. Twenty-one

panels were selected with numerous locations on the inner webs, flanges, spandrels, and floor truss connectors for each floor level analyzed, providing that sufficient paint was available for the analysis.

Core columns C-80 and HH from WTC 1 were examined while C-88a and C-88b were tested from

WTC 2; these columns resided within the fire floors for their respective buildings. The entire length of

each core column was examined, and evaluations of the primer paint were made when sufficient paint was available for inspection.

Over 1 70 areas associated with the 2 1 exterior panels were analyzed, and the results may be found in Appendix E of NIST NCSTAR 1-3C. These 21 panels represent only 3 percent of the panels on floors

94 NIST NCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

involved with fire and cannot be considered representative of other columns on these floors. Only three locations showed evidence of paint mud cracking:

1 . Panel K- 1 , WTC 1 , column 2 1 0, flange and inner web of 98th floor region. Prior to the collapse of WTC 1, the panel was observed to have experienced varying degrees of fire exposure for a minimum of 3 1 cumulative minutes in this area. A unique feature of this panel

is that the upper portion of the column was crushed while generally maintaining concentric

axial alignment with the lower portion of the column, Fig. 6-36. This suggests that deformation occurred at the onset of collapse, while the lower portion of the column was still constrained due to the bolted splice (endplates).

2. Panel K-2, WTC 1 , column 236, 93rd floor spandrel. This area was observed to have fire exposure for nine cumulative minutes prior to collapse, Fig. 6-13. A positive reading was made directly below the truss seat while above the seat in the same location a negative result was obtained, Fig. 6-37. This suggests that the concrete floor may have shielded the upper

portion of the column from the high-temperature exposure experienced by the lower portion.

3. Panel N-8, WTC 1, column 143, seat and standoff plates of the 99th floor. This seat

(Fig. 6-38) was exposed to fire for a minimum of 18 cumulative minutes before collapse.

Mud cracking was not observed on the spandrel plate to which the seat was welded.

6.6.2 Core Columns Exposed to Fire

Four of the core columns with known as-built locations were examined for mud cracking of the paint.

For columns C-88a and C-88b, sufficient paint for analysis was not available. For columns HH and C-80,

few areas of paint were observed (three to five spots per column) with no indication of temperatures over 250 °C. Note that these core columns represent less than 1 percent of the core columns on floors involved

with fire and cannot be considered representative of any other core columns.

6.6.3 Metallographic Analysis of Elements Exposed to Fire

Some aspects of the thermo-mechanical history of a structural steel element may be revealed through a

careful assessment of the component's microstructure. Identification of phases, whether stable or metastable, and their characteristics (e.g., size, morphology, distribution) can help lead to an understanding of the possible rolling conditions and/or cooling rates used during fabrication. These

factors are directly related to the room temperature mechanical properties of the component. Likewise,

high-temperature excursions due to fire can also alter the microstructure and the mechanical properties.

Therefore, if knowledge of the as-fabricated microstructure is available, then a review of the "affected"

microstructure may give an indication of the level of elevated temperature exposure while in service. Thus, an assessment was made to determine if information could be compiled on the extent and effect of

fire exposure through changes in the microstructure. When appropriate, chemical analysis and hardness

evaluations were used to supplement the results. A full discussion of these results can be found in NIST NCSTAR 1-3C.

Samples for analysis were chosen based upon the panel exposure to pre-collapse fires. These included

floor truss connectors (seats) and the lower portion of spandrels where external flaming was observed in

NIST NCSTAR 1-3, WTC Investigation 95
Chapter 6

the window directly beneath the location of sample removal. The microstrucrural results were compared

with the paint mapping analysis, to confirm the apparent dissonance between the results of the paint

mapping study and those from the pre-collapse fire exposure results reported in Sec. 1.2.5.

Source: NIST. Figure 6-36. Crushed portion of column 210 in the area of the 98th floor (from

sample K-1, A209: 97-100). Paint had mud cracking characteristic in this region.

96 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

Source: NIST. Figure 6-37. Spandrel at the 93rd floor of column 236 from sample K-2 (A236: 92-95).

Paint mud cracking was found below the 93rd floor line, no evidence of paint mud

cracking was found above floor line.

NISTNCSTAR 1-3, WTC Investigation
Chapter 6

Source: NIST. Figure 6-38. Truss seat of the 99th floor on column 143 of sample N-8 (A142: 97-100).

a) Top view showing the re-solidified black plastic with embedded papers, most likely a binder of some type and b) bottom view showing the drip pattern. It was unknown when

the high-temperature excursion may have occurred.

98 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

The spandrel steels identified as having been exposed to fire prior to the collapse of the building showed

no microstructural evidence of change. Similar results indicated that three of the four scats observed to be

exposed to severe pre-collapse fire conditions did not experience significant microstructural changes as a

result of the exposure. However, the seat with the melted binder (Fig. 6-38) did show signs of

microstructural alteration as a result of elevated temperature exposure, though it was unknown when this exposure occurred. Finally, in the several columns with known pre-collapse fire exposure, mctallographic

analysis provided no conclusive evidence that the steel exceeded 625 °C, based on calibrations in furnace

exposure studies of WTC steel reported in NIST NCSTAR 1-3E.

6.7 COMMENTS CONCERNING CONTRACT TO WISS, JANNEY, ELSTNER

Consulting engineers from Wiss, Janney, Elstner Associates, Inc. (WJE) provided NIST investigators

with an independent evaluation of the steel recovered from the WTC site. This report, concentrating on

failure mode observations and indications of fire exposure to the structural steel, is appended to NIST

NCSTAR 1-3C. The conclusions drawn by the WJE team were in general agreement with the findings of

the NIST Investigation team, with a few specific examples of disagreement that are discussed in NIST

NCSTAR 1-3C in light of additional forensic evidence developed by NIST and not available to the

WJE team at the time of their examinations.

6.8 FINDINGS

The following findings have been made based on pre-collapse photographic analysis and observations of

the recovered steel. It should be noted that observations were made of a small fraction of the structural

steel in the towers and that pieces were selected focusing on apparent fire and impact damage.

6.8.1 Structural Damage of Exterior Panel Sections

Observations of pre-collapse photographs and the recovered panels from the impact zone indicated that two of the four recovered impact-damaged panels (M-2 and M-30) appear to be in a condition similar to their condition just prior to building collapse. Some damage can be attributed to the events during and

after collapse, but the general shape/appearance of the recovered pieces agree with the damage

photographs.

In general, severing of the perimeter columns occurred at an internal stiffener plate or diaphragm plate

(associated with the spandrel connection to column).

There was a greater tendency for perimeter columns impacted by the airplane to have experienced base

plate fracture near welds than for those outside of the impact zone.

There was no evidence to indicate that the type ofjoining method, materials, or welding procedures used

to fabricate the perimeter columns was improper. The welds appeared to perform as intended during

service.

With the exception of the mechanical floors, the major failure mechanism for perimeter panel column

splices was fracture of the bolts. At mechanical floors, where splices were welded in addition to being

bolted, the majority of the splices did not fail.

NIST NCSTAR 1-3, WTC Investigation 99
Chapter 6

Spandrel connections on exterior panels at or above the impact zone were more likely than those below

this region to experience bolt hole tear out as a failure mode. For those exterior panels below the impact

zone, there was a higher propensity for the spandrels to be ripped off from the panels.

There did not appear to be any difference in failure mode for the spandrel connections whether the

exterior panels were exposed to fire or not.

6.8.2 Structural Damage of Core Columns

Failure of core columns was a result of both splice connection failures and fracture of the columns

themselves.

Core column C-88b (801B: 77-80) may have sustained damage as a result of the airplane impact;

however, the welded splice to the column above survived intact.

6.8.3 Structural Damage of Floor Trusses and Seats

The majority of perimeter panel floor truss connectors (perimeter seats) below the impact floors were

either missing or bent downward. Above this level, the failure modes were randomly distributed. This

trend was observed for both towers.

Failure of perimeter panel floor truss connectors (truss seats) typically occurred in the component that had

the lowest cross-sectional area with respect to the high loading forces experienced during the collapse of

the buildings. Fracture of the standoff plates was typical. There was no evidence to indicate that type of

joining method, materials, or welding procedures used to fabricate the truss seats was improper. The truss

seats and associated welds appeared to have performed as intended during service.

Failure of the connection between the floor truss and the perimeter panel floor truss connectors was

typically a result of tab plate and bolt failure.

Of the 31 core floor truss connectors (core seats) recovered, 90 percent were still intact, though extensive

damage may have occurred. Only two were observed to have been completely torn from the channel.

(This distribution may have resulted from the selection process at the salvage yards).

In the floor trusses, failure of a large majority of the electric resistance welds at the web-to-chord

connections was observed.

6.8.4 Damage to Sprayed Fire-Resistive Material

Pre-collapse photographs indicated that, as expected, fire-proofing was removed from pieces struck by the

incoming aircraft or debris exiting the far side of the buildings. In addition, the impact caused fire- proofing and aluminum facade panels to spall off many perimeter columns which were not directly struck nor severed, but apparently suffered strong accelerations and forces otherwise transmitted through the

structure. This indirect damage to the SFRM was observed on the north and east faces of WTC 2. A coating on the SFRM prevented the loss of the SFRM in some locations on the perimeter columns.

This coating appears as a band of white features on the SFRM wherever two aluminum panels met on the

100 NISTNCSTAR 1-3, WTC Investigation
Damage and Failure Analysis of Structural Steel

exterior columns of the buildings, becoming visible when the panels were dislodged. This may be a coating applied to protect the SFRM from moisture infiltration at the aluminum panel joints, acting to preserve the SFRM even when the SFRM was knocked off both above and below those locations.

6.8.5 Fire Exposure of Exterior Panel Sections

Portions of 16 recovered exterior panels were observed to have been exposed to fire prior to the collapse

of WTC 1. This was determined using video and photographic images. None of the nine panels from

within the fire floors of WTC 2 were observed to have been directly exposed to fire.

It is difficult or impossible to determine if high-temperature exposure occurred prior to or after the

collapse. Of the more than 170 areas examined on 21 exterior panels, only three locations had mudcracking of the paint, indicating that the steel may have reached temperatures in excess of 250 °C. The

21 panels represent only 3 percent of all panels on the fire floors, however, and cannot be considered

representative of other columns on these floors.

Based on microstructural analysis of the recovered structural steel, there was no evidence indicating that the pre-collapse fires were severe enough to affect the steel microstructure of these pieces. Based upon

this evidence, it is believed that no steel was recovered which experienced temperature excursions above

600 °C for any significant length of time as a result of the pre-collapse fires.

Perimeter columns exposed to fire had a great tendency for local buckling of the inner web; a similar

correlation did not exist for weld failure.

6.8.6 Fire Exposure of Core Columns

Two of the core columns with as-built locations in the fire-affected floors were examined for paint

cracking. The few areas with sufficient paint for analysis did not show mud cracking patterns, indicating

the columns did not exceed 250 °C. (It must be recognized that the examined locations represent less than one percent of the core columns located in the fire-exposed region, and thus these temperatures cannot be

considered representative of general conditions in the core).

6.8.7 Time-Dependent Deformation of Perimeter Walls due to Fire and Load

Redistribution

Images of WTC 1 showed gross deformations of the south exterior wall prior to final collapse. Images of

the south face of the tower taken approximately 5 min prior to collapse showed inward bowing of the

exterior columns, reaching an observable maximum of about 55 in. near column 316 on the 96th floor. The inward deflection appears to extend over the entire south face of the building at this time, and is visible vertically between the 94th and 100th floors. Photographs taken approximately 30 min prior to

this time do not show any inward bowing of the south face of WTC 1.

Approximately 18 min after the impact of the aircraft, the east face of WTC 2 exhibited inward bowing of

up to 10 in. in the region of the 79th to 83rd floors. This inward bowing increased to 20 in. at a time

5 min. before collapse of the tower.

NISTNCSTAR 1-3, WTC Investigation I0l
Chapter 6

Hanging floor slabs at the 82nd and 83rd floors were visible in window openings on the east and north

faces, respectively, of WTC 2 and the positions of these slabs changed over time. This suggests a progression of failure of certain parts of the flooring in this area of the tower.

At the moment of collapse of WTC 2, the top portion of the building was found to have moved to the

west as it tilted to the southeast. During this tilting, a complex kink developed at the southeast corner of

the top of the building, in the region of the 106th floor. In addition, the portion of the building above the

aircraft impact site twisted slightly clockwise (as viewed from above) as the collapse progressed.

6.8.8 Other

In general, the observations made by an outside contractor concerning possible local failure mechanisms

of the WTC steel were in agreement with those found by NIST.

102 NIST NCSTAR 1-3, WTC Investigation
Chapter 7

Mechanical Properties of Structural Steels

7.1 introduction

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

properties of the steels, bolts, and welds. 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 structural steel from the World

Trade Center (WTC) construction era. The third was to determine the constitutive behavior for input into

finite element models of the response of the building components to the aircraft impact and to the high

temperatures that the subsequent fires produced.

In support of these three goals, the steel was characterized in five main areas:

1 . Elastic properties as a function of temperature,

2. Room temperature tensile stress-strain behavior measured using conventional tensile tests,

3. Impact properties measured using Charpy tests,

4. High-strain rate stress-strain behavior measured using conventional, high-rate tensile tests as well as compressive Kolsky bar tests, and

5. Elevated-temperature stress-strain behavior of perimeter, core, and truss steels, as well as creep behavior of truss steels.

7.2 ELASTIC MODULUS

Figure 7-1 shows the dependence of elastic modulus, E, as a function of temperature along with other

determinations reported in the literature. The NIST data were determined from three different perimeter

column steels using a dynamic mechanical analyzer in flexure at a frequency of 1 Hz. The maximum

deviation of the experimental data from the fit to the modulus in Fig. 7-1 is 1.5 percent. The enormous

scatter in the literature data arises primarily from the test methods employed. The literature values greater

than the NIST determination generally come from ultrasonic measurements, which are inappropriate for modeling the building, while those less than the NIST value generally come from the loading portions of

tensile tests. At high temperatures, such values include significant plastic deformation and are thus not appropriate for elastic calculations. The modulus curve in Fig. 7-1 is a third-order polynomial fit to the

data for 0 °C < T < 600 °C and extrapolated to 723 °C.

Other elastic properties needed for modeling (Poisson's ratio and shear modulus) have also been

calculated as a function of temperature and are reported in NIST NCSTAR 1-3D.

NISTNCSTAR 1-3, WTC Investigation 103
Chapter 7

T, °C

Figure 7-1. Elastic modulus as a function of temperature determined by NIST compared

with literature data for structural steel.

7.3 ROOM TEMPERATURE TENSILE PROPERTIES

7.3.1 Measured Tensile Properties

To characterize the room temperature stress-strain behavior of the steels in the fire and impact zones of

the buildings, NIST tested specimens from 52 different steel plates and shapes: 31 from perimeter

columns and spandrels, 1 1 from core columns, 5 from truss components, 3 from perimeter truss seats, and

2 from core truss seats. Tests generally followed the ASTM International (ASTM) E 8 protocol.

Figure 7-2 is an example set of room-temperature, stress-strain curves for a perimeter column spandrel

plate from WTC 2. All test results are given in NIST NCSTAR 1-3D. Figures 7-3 and 7-4 graphically

present measured yield strength as a function of specified minimum yield strength for perimeter and core columns.

7.3.2 Measured Properties and Specifications

Assessing the measured tensile properties in relation to the requirements of the various specifications was

not straightforward in many cases. NIST was able to quantify the mechanical properties and chemical

composition of the steel pieces in their recovered state. These "product" tension tests on representative

specimens of the 52 different recovered steels established their yield and tensile strengths and elongations

to failure. Chemical characterization established whether the composition of the recovered steel lay

within the limits set by the various standards.

104 NIST NCSTAR 1-3, WTC Investigation
Mechanical Properties of Structural Steels

j5 60

to

</> o

CO

ra 40

.«* c

<DO

c

iS 20

S14-C3B-S1

WTC2COI217S1

Sl4-c3b-s1-!1 1 «, Sl4-C3b-81-I2*

s14-c3b-s1-I3 ,0 si4-e3b-si-ii

Ertgiouanng Strain

0.00 0.05 0.10 0.15 Engineering Strain

0.20 0.25

Figure 7-2. Examples of longitudinal (11, 12, and 13)

and transverse (t1) room-temperature, stress-strain curves for a Fy =46 ksi perimeter column

spandrel plate.

1.8

»1.6

u_

.9?

(D 1 .4 a

"S 1-2 W

TO

0) ^ 1.0

0.8

values < 1 arise from j natural variation Perimeter Columns

L orientation only ;

* Plates 1 ,2,4 o Plate 3

F =(55 60 70 100) ksi values offset for clarity--

i M *

_ i o„:

i

bars represent max and min values

40 60 80 Specified F ksi

100

Figure 7-3. Ratio of measured yield strength or yield

point to specified minimum yield point for WTC

perimeter column steels.

NISTNCSTAR 1-3, WTC Investigation 105
Chapter 7

1.8

.1.6 UL

X3

.92 1 1. 00

ULT3

=3W)

TO

1.2

1.0

0.8

Fy = 36 ksi specimens offset for clarity _ r i

_J I i I L.

I Core Columns

[ L orientation only

j opiates

j owide-fiange webs

[ values < 1 arise from

i loss ot yield point

bars represent max and min values

35 40 45 Specified F ksi50 55

Figure 7-4. Ratio of measured yield strength or yield

point to specified minimum yield point for WTC core column steels.

Four issues complicated the task of positively assessing the likelihood that the properties of the recovered

steel were consistent with the requirements of the relevant specifications. The first was that the WTC

steels could be supplied to many different, overlapping specifications. No documents exist that identify

the ASTM specification for a given column. Only the specified minimum yield strength was known.

Secondly, differences between the NIST test procedures and the mill test procedures that originally

qualified the steel, including testing rates, specimen sizes, and test locations, could result in values for the

NIST-measured yield strengths that were less than the requirements of the relevant specification. Thirdly,

the natural variability of steel properties within a heat can cause some locations to have yield strengths

that are lower than the minimum required by the relevant standard. Finally, even slight damage to the

steel, caused by the collapse of the WTC towers or subsequent recovery efforts, could remove the yield

point behavior in the tensile test. Most low- and medium-strength steels are qualified using the stress measured at the yield point, rather than the 0.2 percent offset yield strength. For standards that define the

yield strength as the yield point, the absence of yield point behavior in a NIST-conducted test can lead to values of yield strength that are less than the minimum required by the specification.

The investigation made several findings regarding the quality of the recovered steel. The yield and tensile

strength, Fig. 7-3, of the perimeter columns, with only a few exceptions, easily exceeded the strength

requirements intended, typically by 10 percent to 15 percent. The number of slightly under-strength

plates and the amount by which they fall short were consistent with expected values for the average

strength and coefficient of variation of plate steels from the WTC era. The ratio of measured yield

strength to specified yield strength, Fig. 7-3, was also consistent with literature estimates from the WTC

era. The chemistries of the individual perimeter column steels, lay within the ranges specified in the

relevant Yawata Steel proprietary compositions.

The NIST-measured yield strengths of several of the wide-flange shapes recovered from the core were

less than specified in A 36, Fig. 7-4. It is likely that these low values arose from a combination of

106 NISTNCSTAR 1-3, WTC Investigation
Mechanical Properties of Structural Steels

damage to the column and differences between the protocol NIST used for determining yield strength and

the protocol under which the steels were originally qualified. Because these effects are testing artifacts,

they do not indicate that the steel was of insufficient quality. The recovered core wide-flange columns

were more heavily distorted than the core and perimeter box columns; in most cases, it was not possible

to find undeformed sections to test. Low strength steels, like the core wide-flange columns are qualified

based on the yield point, or the maximum stress reached before permanent deformation. After yielding,

the stress drops immediately by as much as 5 ksi. Small strains can remove this yield point behavior

entirely, and result in yield strengths that are less than the specified minimums. In each case where

tensile test specimens from the wide-flange columns did not exhibit a definite yield point, the measured

distortion of the column was sufficient to induce a strain that could remove that yield point. A second

confounding factor that could have further reduced the reported yield strength arose because NIST tests employed a slightly lower testing rate than would have been used in the mill test. Because the strength of

steel increases slightly with the testing rate, this difference could further reduce the NIST-measured yield

strength by several ksi.

The ratio of measured yield strength to specified yield strength in Fig. 7-4 was also lower than some

literature estimates from the WTC era. Those historical estimates, however, were based on measurements

of shapes with much smaller dimensions, in contrast to the massive wide-flange shapes in the core of the towers. Because of their size, the strengths of those massive shapes might be expected to exceed the specified minimums by less than those for the more heavily rolled, lighter shapes from the literature.

Many of the steels in the floor trusses were high-strength, low-alloy (HSLA) steels, with yield strengths

above 50 ksi, even when they were specified to conform to the lower-strength A 36.

Recovered and tested bolts not only met the A 325 specification, but also were much stronger than

expected based on reports from the contemporaneous (1960s) literature.

The welds in the perimeter and core columns and floor trusses had properties consistent with the intended

specifications.

7.3.3 Representative Steel Stress-Strain Curves

To satisfy the third objective of the room temperature tensile testing program, NIST produced models of

stress-strain curves for each of the grades of steel in the fire and impact zones of the towers, for a total of

29 different steels. The modeling groups associated with the investigation required complete stress-strain behavior, rather than just yield and tensile strengths and total elongations. Figure 7-5 shows examples of

representative stress-strain behavior provided for the perimeter column steels. For the perimeter column

and truss steels, the yield strengths were corrected for dynamic effects during testing and, where possible,

experimental data were combined with surviving WTC mill test reports. Experimentally derived stressstrain curves, modeled using the Voce work-hardening law, were used to describe the plastic deformation

of each steel. For the core columns, the yield strength was assumed to be the historical average from the

literature of the WTC era, corrected for dynamic effects. Actual tests on WTC core column steels,

modeled using Voce work-hardening, supplied the plastic behavior. For the A 325 bolts, NIST supplied

load-displacement curves measured on recovered bolts.

NISTNCSTAR 1-3, WTC Investigation 107
Chapter 7

20

0

0.00 0.05

_3 1 I L. 0.10 0.15 0.20 True strain

0.25 0.30

Figure 7-5. Examples of representative true-stress

true-strain curves for perimeter column steels.

7.4 HIGH-STRAIN RATE TESTS

Because the strength of steel depends on the rate at which it is deformed, it is necessary to quantify the

relation between stress and strain rate for the steels in the impact zone. Failing to properly account for the

increased strength with deformation rate could lead to incorrect estimates of the internal damage to the

buildings caused by the airplane impact.

In support of this goal, the investigation employed two different types of high-strain rate tests. Tests at rates in the range 50 s~ < £ < 500 s~ , used a servohydraulic tensile test machine and special flat tensile specimens. Tests at higher rates employed a Kolsky bar test machine in which the test specimen is a cylinder loaded in compression.

The sensitivity of the flow stress to strain rate is expressed in a simple manner from which all other strain rate effect relations can be calculated. The strain rate sensitivity, m, is calculated from the slope of a loge(a)-loge( £ ) plot:

log.

f \

cr

K u oJ = ffjlogi .(£-)-wlogi

, (i0 ) 7-1

where CTo= 1 ksi, and the a in the numerator is either the yield or tensile strength.

Figure 7-6 compares the strain rate sensitivities for the WTC steels characterized to structural steels from

other studies. The behavior of the WTC steels is entirely consistent with the literature data. The

magnitudes of the strain rate sensitivities are similar and lie in the range 0.01 < m < 0.06. The strain rate sensitivity also decreases with increasing yield strength. The total elongations, El,, of the WTC steels, not shown, are not a strong function of rate. The total elongation increased with rate in seven of the eight

WTC steels characterized, but the increase was only 2 to 3 absolute percent. This behavior is similar to

108 NISTNCSTAR 1-3, WTC Investigation
Mechanical Properties of Structural Steels

the behavior of structural steels reported in other studies. Kolsky bar testing demonstrated that the strength of the recovered A 325 bolts is insensitive to strain rate.

0.07

0.06 E

^0.05 >

'I

0.04 o

i/> £ 0.03

tocS 0.02 55

0.01 -

0.00 20 40

i

I ?

Tensile strength' ooc(dt/dt) m

D WTC

Literature

60 80 Fr

ksi

100 120

0.07

0.06

0.05 -

£ 0.04 h

CD B 0.03

COc2 0.02 -

CO

0.01 -

0.00 20

1

Yield strength

o«(de/dtf o WTC

• Literature

. T

0

40 60 80 F

y

, ksi

6

100 120

Figure 7-6. Strain rate sensitivity as a function of

specified minimum yield strength for WTC steels.

7.5 CHARPY TESTS

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

in the presence of notches before fracturing. As such, Charpy tests are particularly relevant to the airplane impact and complement the high-strain rate tests. In the Charpy test, the energy used to break a notched specimen is measured as a function of temperature. At the higher temperatures the absorbed

energy is large and relatively constant, and forms what is called the "upper shelf." Failure on the upper

NISTNCSTAR 1-3, WTC Investigation 109
Chapter 7

shelf is ductile. At the lower temperatures the absorbed energy is small and again constant with

temperature, and forms the lower shelf. Failure on the lower shelf is brittle.

Figure 7-7 summarizes the results of Charpy tests on perimeter and core column steels. All the perimeter

column steels have large upper shelf energies and transition temperatures well below 0 °F. The transition

temperatures of the web of the wide-flange specimen, Fig. 7-7, and the truss rods and angles, not shown,

are near room temperature, but they still possess adequate toughness for use as structural steels. The

transition temperatures of the truss seat steels are above room temperature, and the absorbed energy of the

steels even at room temperature is low, indicating a propensity for low-energy failure.

120

100

>-

i~cc

03

<a -9own

<

80

60

40

20

Core columns

C80-web terns

/

-150 -100 -50

corrected to Mi-size specimen

1 50 200

120

100

>-OC

o©

o

X)<

80 -

60

40

20

Persmeter columns

2

o

N8L

N8T

NS-HAZ L N8-HA2 T C10 L C10T

C10-HAZ1 m so I N8s.pT

"

,/ 4

r/ / i

5

L Songitudinat T « transverse

corrected to fuli-size specimen

150 -100 -50 50 100 1 50 200 T.°F

Figure 7-7. Summary data for Charpy tests. Lower

figure: perimeter column steels; upper figure: core column steels.

Current building construction specifications, as well as those of the WTC construction era, do not

generally specify minimum requirements for Charpy absorbed energies. The room temperature absorbed

energy values of the WTC steels tested are similar to those of construction steels of the WTC era.

110 NISTNCSTAR 1-3, WTC Investigation
Mechanical Properties of Structural Steels

7.6 HIGH-TEMPERATURE TESTS

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, which are the steels most likely to have been subject to the conditions where creep deformation can be significant. In each of these two areas, in addition to the experimental characterization, NIST

developed methodologies to predict the behavior of the untested steels.

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

temperature. Figures 7-8 and 7-9 illustrate this behavior for the yield and tensile strengths of a large

group of structural steels from the literature. The solid curves are the mathematical representations of the behavior. Overlaid on these are the data collected as part of the WTC investigation and the literature data

used to develop the master-curve. At the highest temperatures the WTC data lie slightly below the master

curve, which was developed by NIST early in the investigation, before significant testing was complete.

Like some of the room-temperature data, this difference probably arises because tests in the investigation

used a strain rate that was lower than the rate used to generate the literature data. Strain rate effects on the flow stress, which are small at room temperature, become increasingly significant with increasing

temperature.

1.0

^ 0.8 cO

OCM

II

II

0.6

0.4

A .

:<W ENV 1992-1-2

0.2

0.0

• core truss seats 0 core columns

& truss components

perimeter truss seats c perimeter columns

200 400

T, °C

600 800

Figure 7-8. Normalized yield strength as a function of

temperature for WTC steels, the recommended

function to describe behavior (solid line), and the

literature data used to develop that relation.

NISTNCSTAR 1-3, WTC Investigation 111
Chapter 7

1.0

oa

oCM

li

0.8

0.6

COt

CO

0.4

0.2

0.0

A,

Q

K i

- ~ Euroasete ENV 1992-1-2

• core truss seals

0 core columns

^ truss components

perimeter tnjss seats 0 perimeter columns

-

8

ft #

200 400

T, °C

600 800

Figure 7-9. Normalized tensile strength as a function of

temperature for WTC steels, the recommended function

to describe behavior (solid line), and the literature data

used to develop that relation.

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

strength degradation of the bolts with temperature.

NIST characterized the high-temperature tensile behavior deformation of 15 different steel specimens:

three perimeter columns, four core columns, three truss components, and five truss seats. Figure 7-10 is an example of typical high-temperature tensile stress-strain curves from a wide-flange core column in the

fire and impact zone of WTC 1

To produce the elevated-temperature stress-strain curves of WTC steel, NIST developed a methodology

for accounting for the change in the work-hardening behavior, using literature data for structural steels scaled by the ratio of room temperature tensile strengths.

Data exist for creep and failure of hundreds of different high-temperature alloy steels intended for use at high temperature. These studies typically focus, however, on the long-time, low-stress conditions these

steels see in power-generation and chemical plant operation, rather than the short-time, high-stress

conditions structural steels experience in building fires. Consequently, NIST also characterized the creep

deformation behavior of the floor truss steels. Figure 7-1 1 illustrates a set of creep curves for the

Fv = 50 ksi floor truss steel from the truss chords. The dashed lines, which are not fits to the individual

creep curves, represent the predicted creep strains from the global model for creep of the truss steel, which was developed for all stresses and 400 °C < T< 650 °C. The global model represents the creep

strain, sc, as independent functions of time, /, and stress, a:

£c =At B a

c 7-2

112 NISTNCSTAR 1-3, WTC Investigation
Mechanical Properties of Structural Steels

where A, B, and C are all functions of temperature. For estimating the creep properties of untested steels,

NIST developed a methodology to use either existing literature data or the investigation-generated floor

truss steel data after scaling by the ratios of room temperature tensile strength.

80

jg 60

05W

CD CO

O) 40 c

<D

COc

iS 20

0

0.0 0.1 0.2 0.3 0.4 0.5 Engineering Strain Figure 7-10. Typical high-temperature stress-strain curves for specimen HH, a Fy = 42 ksi wide-flange

shape from near the fire and impact zone of WTC 1

7.7 STEELS USED IN WTC 7

Unlike WTC 1 and WTC 2, no recovered steel in the NIST inventory can be unambiguously assigned to WTC 7. Therefore, properties of the steel were estimated completely from the literature.

7.7.1 Steels Used in Construction of WTC 7

Building plans called for rolled column shapes conforming to two ASTM grades: A 36 and A 572

Grade 50. The heaviest rolled columns were additionally built up with cover plates that were specified to several different ASTM specifications, including A 36, A 572 (Grade 42), and A 588 (Grades 42 and 50).

About 26,000 tons of steel went into WTC 7 (Salvarinas 1986). Construction documents in the NIST

archives 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 Corus) 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, an Fy - 44 ksi steel, where the "W" designates "weldable."

NISTNCSTAR 1-3, WTC Investigation 1 13
Chapter 7

7.7.2 Mechanical Properties of WTC 7 Steel

NIST has found no evidence that the elastic modulus or Poisson's ratio differs between structural steels. Figure 7-1 describes elastic modulus for structural steel as a function of temperature.

Because NIST recovered no steel from WTC 7, it is not possible to make any statements about its quality.

The recommended values for the stress-strain behavior were estimated using the same methodology that was used for the WTC 1 and WTC 2 steels (NIST NCSTAR 1-3D). The static yield strengths were

estimated from historical averages and corrected for testing rate effects.

Because, prior to collapse, WTC 7 did not suffer any high-strain rate events, NIST made no effort to estimate high-strain-rate or impact properties of the steel.

The elevated-temperature stress-strain curves were estimated using the same methodology as used for the

steels in WTC 1 and WTC 2 (NIST NCSTAR 1-3D). The elevated temperature stress-strain curve is modeled using power-law work-hardening, where the degree of work-hardening depends on temperature

and the calculated stress is scaled by the ratio of the estimated tensile strength of the WTC 7 steel to that of the reference steel used to develop the model.

Creep behavior was estimated using the same methodology that was used for the steels in WTC 1 and

WTC 2, in which the creep strain as a function of time depends on stress and temperature

(NIST NCSTAR 1-3D). The applied stress was scaled by the ratio of the tensile strength of the steel in question to that of the reference AS A 149 steel used to develop the creep model.

t, h

0 0 0.5 1.0 1.5 2.0 2.5 3.0

0.0 0.2 0.4 0.6 0.8 1.0

Figure 7-11. Typical creep curves for different stresses

at 600 °C from truss steels specified as A 242, from

specimen C132-TA.

114 NIST NCSTAR 1-3, WTC Investigation
Mechanical Properties of Structural Steels

7.7.3 Physical Properties of WTC 7 Steel

No metallography could be carried out because no steel was recovered from WTC 7. Other physical

properties are the same as those estimated in Chapter 8 for the WTC steels.

Steels of the same nominal strength supplied by different steel producers often have different mechanical

behavior. There were over 40 different steels in the towers, based on the various combinations of

suppliers and nominal strengths. Thirty-two steels were sufficiently different to require distinct models of

constitutive behavior.

The steel used in the construction of the WTC generally met the expectations of the designers and the specifications called for in the steel contracts. Approximately 87 percent of all tested steel exceeded the

required minimum yield strengths specified in design documents; approximately 13 percent of test results on the damaged steel did not meet the required minimum yield strengths. The occurrence of test results below the specified minimum values is not unexpected since differences in test procedures from those in the qualifying mill tests could account for 2-3 ksi lower values in the NIST tests, and the loss of a yield

point due to damage to the steel accounts for 2-5 ksi lower values in the NIST tests in several cases.

The mechanical properties and chemistry of the perimeter column steels were consistent with the requirements of the intended specifications. Their yield strengths generally exceeded their specified

minimums by more than 10 percent. Their tensile properties were also consistent with literature estimates

for average properties of structural steel plate during the WTC construction era. The number of

occurrences of plates with tensile properties at or slightly below the specified minimum is consistent with

the historical variability of steel strength.

The mechanical properties and chemistry of the core column steels were generally consistent with the

requirements of the intended specifications. Some, primarily wide-flange shapes, had NIST-measured

strengths that were lower than called for in the specifications, but this may have arisen from mechanical

damage that removed the yield point, the natural variability of structural steel, and slight differences

between the NIST and original mill test report testing protocols. However, the yield strengths of

undamaged steels in the rolled core columns were lower than the historical literature indicates as typical.

The observed distributions are accounted for in the typical design factor of safety for allowable stress design.

The mechanical properties and chemistry of the steels in the floor trusses were consistent with the

requirements of the intended specifications. The strength of the steel in the floor trusses was higher than

called for in the original specifications. Many of the truss steels specified as low strength A 36 were

supplied as HSLA steels with much higher strengths. Laclede Steel Company's substitution of 50 ksi

yield strength steel for A 36 in the lower chord of the trusses is expected to have provided significantly

improved performance at high temperature.

7.8 FINDINGS

7.8.1 Room Temperature Tensile Properties

NISTNCSTAR 1-3, WTC Investigation 115
Chapter 7

Limited tests on recovered bolts indicated that their mechanical properties and chemistry were consistent

with their intended specifications. Furthermore, the recovered bolts were stronger than contemporaneous

literature suggests as typical.

Limited tests on recovered welds and weld material indicated that their mechanical properties and

chemistry were consistent with their intended specifications.

In summary, the steel used in the construction of the WTC towers met the expectations of the designers

and had properties consistent with the requirements of the specifications called for in the steel contracts. The observed distributions of yield strength are accounted for in the typical design factor of safety for

allowable stress design. Material substitutions of higher strength steels were common in the perimeter

columns and floor trusses. The safety of the WTC towers on September 11, 2001, was most likely not

affected by the fraction of steel that, according to NIST testing, did not meet the required minimum yield

strength. The typical factors of safety in allowable stress design can accommodate the measured property

variations below the minimum.

7.8.2 High-Strain Rate Properties

The strain rate sensitivity and high-strain rate ductility of the perimeter and core column steels were

similar to other structural steels of the WTC era.

7.8.3 Impact Properties

The impact toughness of steels from the perimeter and core columns, and floor trusses was consistent

with literature values for the WTC construction era.

7 8.4 High-Temperature Properties

The high-temperature yield and tensile strength behavior of WTC steels was similar to behavior of

structural steels from the WTC construction era literature.

The creep behavior of WTC steels was modeled by scaling WTC-era literature data using room

temperature tensile strength ratios.

Laclede Steel Company's substitution of 50 ksi yield strength steel for A 36 in the lower chord of the

trusses is expected to have provided significantly improved performance at high temperature.

7.9 REFERENCES

ASM (American Society of Metals) International. 1978. Properties and Selection: Irons and Steels. Metals Park, OH.

Clark, C.L. 1953. High-Temperature Alloys. Pitman, NY.

Cooke, G.M.E. 1988. An Introduction to the Mechanical Properties of Structural Steel at Elevated

Temperatures. Fire Safety Journal, 13, 45-54.

116 NISTNCSTAR 1-3, WTC Investigation
Mechanical Properties of Structural Steels

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. 1980. Fracture-Mechanisms in Pure Iron, Two Austenitic

Stainless Steels, and One Ferritic Steel. Met. Trans. A. 1 1 A, 333-347.

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

1-9.

Salvarinas, John J. 2003. Telephone interview with William Luecke, National Institute of Standards and

Technology, Gaithersburg, MD. Salvarinas was the project manager for Frankel Steel during the erection of WTC 7. May 16.

Salvarinas, John J. 1986. Seven World Trade Center, New York, Fabrication and Construction Aspects.

Canadian Structural Engineering Conference-1 986 Proceedings. Canadian Structural Steel Council.

Willowdale, Ontario, Canada. ISBN 0-88811-062-6, pages 1 1-1 to 1 1-44.

Uddin T., and C. G. Culver. 1975. Effects of Elevated Temperature on Structural Members. J. Struct. Div. ASCE. 101(7). 1531-1549.

NISTNCSTAR 1-3, WTC Investigation 117
Chapter 7

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118 NISTNCSTAR 1-3, WTC Investigation
Chapter 8

Physical Properties of Structural Steels

8.1 introduction

A number of physical properties of the structural steel from the towers were either measured or estimated.

Of these, composition and microstructure were characterized for many of the recovered pieces in order to identify the specifications to which the steel was fabricated, enabling a better characterization of the mechanical properties of the steel. In addition, various thermal properties were estimated as a function of

temperature based on literature data in order to provide input to models of the steel thermal response to the fires.

8.2 CHEMICAL ANALYSIS OF RECOVERED STEEL

In addition to mechanical properties, steel specifications give chemical composition limits for particular

steels. Thus knowledge of the composition can help identify the specification to which the steel was

produced. In addition, individual steel producers often have favored composition ranges which are a subset of those allowed by the steel specification, and a narrow range of analyzed compositions for a given group of steel products suggests that they were produced by the same steel mill.

The most common elements in modern steels include the base iron (Fe), carbon (C), manganese (Mn).

silicon (Si), niobium (Nb), vanadium (V), nickel (Ni), chromium (Cr), molybdenum (Mo), copper (Cu),

aluminum (Al), boron (B), and nitrogen (N). In addition, steels contain the undesirable tramp elements

sulfur (S) and phosphorus (P).

Chemical analysis of the recovered World Trade Center (WTC) structural steel was conducted to help

identify the various types and fabricators of the steel. The complete analysis, found in the report Physical

Properties ofStructural Steels (NIST NCSTAR 1-3E), is summarized below. Nearly every important

component of the recovered structural elements was analyzed (385 chemical analyses). Since many of the

structural elements had stampings indicating the yield strength (Fy

) of the steel, grouping of the different

strength levels for comparison was possible. Tests were conducted in accordance with ASTM

International (ASTM) E415 for all elements except carbon; ASTM E 1019 was used for this element.

The company through which the analysis was done has an annually renewed accreditation with the

American Association of Analytical Laboratories.

Additionally, two independent laboratories were sent samples from nearby locations on perimeter column

plates. This was done as an independent check on accuracy. Both laboratories obtained similar results. Further, two samples from this group were also analyzed at National Institute of Standards and

Technology (NIST) in the Analytical Chemistry Division using X-ray fluorescence (XRF) and Glow

Discharge Optical Emission Spectroscopy (GDOES). Both instruments were calibrated with NIST

standard reference materials for steel. Again similar results were observed.

NIST NCSTAR 1-3, WTC Investigation 119
Chapter 8

The analyses of the flange, outer web, and spandrel plates of the exterior panel sections (all Yawata steel) were found to be nearly identical for a given plate grade (Fy

) and gauge. In contrast, inner web plates

with Fyless than 80 ksi were found to be chemically distinguishable from the other plates. For Fy equal to 80 ksi and 100 ksi, the inner web plates were consistent with the flange compositions, and these plates are therefore believed to be Yawata steel. These results support the contemporaneous documents stating the

flange, outer web, and spandrel plates were produced by the Yawata Steel Co., and the inner web plates were primarily supplied domestically. Regardless of supplier, Mn and Si were the primary strengthening

additive of the lower strength components. Higher strength components contained Cr and Mo additions.

Nb and V additions were used in most of the steels with 45 ksi to 80 ksi specified yield strengths. These

alloying practices were typical for steels of this era.

For the limited number of recovered core columns available for evaluation, the chemistry results varied

for any given Fy and shape (built-up box, rolled wide-flange), consistent with the contemporaneous

documents stating that numerous suppliers produced the steel used for these structural components. All

tested core columns met chemistry specifications for one of the numerous ASTM structural steel grades

available during the construction era.

Floor truss rods and chords, manufactured by Laclede Steel Co., met chemistry specifications for ASTM

A 242; the Mn contents were typically too high to meet ASTM A 36 specifications. Contemporaneous

construction documents indicated that Laclede Steel Co. routinely upgraded A 36 components to A 242

steel. A majority of the other structural components (floor truss seats, diagonal bracing straps, gusset plates, core channels, etc.) met chemistry specifications for ASTM A 36 grade steel, which was the default steel when strengths were not specified on the design drawings.

Two types of bolts were specified for construction of the towers, A 325 and A 490 bolts. Stampings on

the bolt heads clearly indicated the bolt type. Only one A 325 bolt was tested for chemistry with the

results indicating that it met the A 325 Type 1 chemistry specifications.

A piece of the aluminum facade was analyzed and found to be consistent with the non-heat treatable aluminum-manganese alloy AA 3003.

8.3 METALLOGRAPHY

Metallography, or study of the microstructure (details of the distribution of elements and phases), can be

used to identify how a piece of steel was processed. This information can be useful in identifying the type

of steel, certain expected properties, and in a few special cases, help identify the producer. In addition,

metallography can be used to identify exposure to extreme temperatures (see Chapter 6).

Thus, in addition to the chemical analysis, metallography of the recovered WTC structural steels was used

to confirm the identity of the types of steels. This information is summarized below, with the complete

analysis found in the report Physical Properties ofStructural Steels (NIST NCSTAR 1-3E). The

microstructure of every sample removed from the structural elements was evaluated ( 862 metallographic

samples). This number included base material, viewed in two different orientations, and welded

connections. Standard metallographic procedures for grinding, polishing, and etching were used to prepare the samples for examination with light optical microscopy.

120 NIST NCSTAR 1-3, WTC Investigation
Physical Properties of Structural Steels

For the exterior panel sections, two different microstructural types were observed. Flange, web, and

spandrel plates with specified minimum yield strengths less than 70 ksi were ferrite-pearlite (Fig. 8-1).

Additionally, inner webs with Fy of 70 ksi and 75 ksi were ferrite-pearlite. For the Yawata produced

plates (flange, outer web, and spandrel), this type of microstructure was expected given the measured

plate thickness and specified minimum yield strength (refer to NIST NCSTAR 1-3 A). Plates with F. less than 70 ksi were typically hot rolled unless the following sizes were exceeded for a given Fy

: 0.5" for 65 ksi, 1.25" for 60 ksi, and 1.5" for 55 ksi. For these specified plates, as well as those with minimum

yield strengths equal to or greater than 70 ksi, "quenched-and-tempered" microstructurcs were observed

(Fig. 8-2).

Figure 8-1. Microstructure of ferritic-pearlitic WTC perimeter column flange plate steels,

a) Fy = 45 ksi (unidentified perimeter column with "45" flange stamping) and b) Fy = 60 ksi

(WTC 1, column 126, 97th floor).

Microstructure varied within each of the two structure types, depending on product type, gauge, and

strength level. Ferrite-pearlite steels had variations in ferrite grain size, ferrite grain shape, pearlite

lamellar spacing, and banding of microstructural constituents with a change in specified Fy and gauge of

the plate. Examples of these microstructures are shown in Fig. 8-1. Similarly, the quenched and

tempered steels had variations with Fy

. Plates with Fy = 70 ksi had a coarse Widmanstatten morphology

of ferrite with coarse cementite precipitates while the higher strength plates had what appeared to be a tempered martensitic structure, as shown in Fig. 8-2.

With the exception of the column end plates and construction bolts, all other structural elements evaluated

had ferrite-pearlite microstructures. These include the core columns, floor truss connectors, floor truss rods and chords, and the core channels. The end plates and bolts had quenched-and-tempered

microstructures.

NIST NCSTAR 1-3, WTC Investigation 121
Chapter 8

Figure 8-2. Examples of "quenched and tempered" WTC perimeter column flange steel,

a) Fy - 70 ksi (WTC 1, column 224, 94th floor) and b) Fy = 80 ksi (WTC 1, column 207,

89th floor).

For each of the designated minimum yield strengths, the type of structures observed was expected for

steels of that era. While the microstructural features may have varied from plate to plate, there was no

indication that the practices followed to produce the plates were improper.

8 4 THERMAL PROPERTIES

The following thermal properties as a function of temperature were determined, based on literature data, as needed for modeling of the building response to the fires.

• Specific heat (Cp)

• Coefficient of thermal expansion (a)

• Thermal conductivity, k,

Of these, only the thermal conductivity varies by more than a few percent among the different steels used

in the building.

In general, properties are reported from room temperature up to 600 °C or 700 °C. Near 720 °C ironcarbon alloys such as structural steel transform from a mixture of a-phase (ferrite) and cementite (Fe3 C)

to a mixture of a and y (austenite). Most thermostructural properties are discontinuous at this transition,

and the heat capacity becomes infinite. Furthermore the literature contains much less information on

properties at temperatures higher than the phase transformation. Generally, though, the strength of steel above the phase transformation is reduced to 1 0 percent or less of the room-temperature value.

122 NISTNCSTAR 1-3, WTC Investigation
Physical Properties of Structural Steels

The specific heat at constant pressure is defined as

dq

" \dTj (8-1

)

where q is the heat added per unit mass. Because structural steels generally contain very low fractions of

alloying elements it is possible to use the heat capacity as a function of temperature for pure iron without

introducing an error of more than 3 percent.

Of the many experimental determinations, that of Wallace (1960) spans the temperature range of interest,

is relatively recent, and has an uncertainty statement. To represent the data we fit a third-order polynomial

to the data in the range 0 °C < T < 650 °C:

. Cp (T) = c{] +cx T + c2 T

2 +c,T>

where

c0 = 51.1U33.39 (8-2)

ci= 2.019±0.185

c2 = (-3.0135±0.320)xl0 3

c3 = (1.829±0.175)xl0 6

where Cpis expressed in J/(kg K) and Tin kelvins.

The coefficient of thermal expansion (CTE), a, is used to predict the length of a member, L, as a function

of temperature, T. There are two common definitions. The mean CTE, aM, is simply the two point slope

of the length of the specimen at two temperatures. As such, it ignores any changes in slope between the

two fixed points.

au= I^) ,8-3)

The instantaneous CTE, a, which permits integration along the temperature path to find the length of a member at any temperature is defined as

1 a = —

L

dL_

dT

Because the literature provides no evidence of a relation between steel chemistry and thermal expansion

coefficient, the recommended value is that for pure iron (Touloukian 1977)

a{T)-a{) + ax T + a2 T

2 + a} T'

where

a0 = 7.3633 x 10"6 (8-5)

a, = 1.8723 x 10"8 a2 = -9.8382 x 10" 12 a3 = 1.6718 x 10" 16

and T is measured in kelvins, for 300 K < T < 900 K.

NISTNCSTAR 1-3, WTC Investigation 123
Chapter 8

For one-dimensional heat flow, the thermal conductivity, k, relates the quantity of heat per unit time, O,

transported per unit area, A, to the temperature gradient dT I dx: — (8-6) \ox )

and has units of W/(m K). For one-dimensional situations where the temperature depends on time, the

thermal diffusivity, a, is appropriate

ST d

2 — T

= a-—r (8-7)

dt dx

The thermal diffusivity, a,

(X (8-8)

is the ratio of the thermal conductivity, k, to the density, p, and specific heat Cp

. Thermal diffusivity has

units of a diffusion coefficient: m

2/s.

Unlike the other thermal properties (modulus, CTE, specific heat), thermal conductivity is affected by the

microstructure of the steel: identical compositions processed differently will yield steels with different

thermal conductivities. Because the various alloying elements and processing steps modify the

microstructure, there is no generic relation between grade of steel and thermal conductivity. For instance,

the plot in Fig. 8-3 shows the thermal conductivity as a function of temperature for 12 low-alloy steels. Even near room temperature, the data span 40 percent of the mean value.

Using literature data from a statistically planned experiment, it was possible to develop an expression for the thermal conductivity that attempts to account for chemical variations. This expression is only

partially useful because it requires that the chemistry of the steel be known, which is not possible for a generic construction element in the WTC towers. Using the specified minimum values for chemistries in the steels does not help, because the unspecified elements can make a contribution equal to or greater than

the ones in the specification.

8.5 SUMMARY

8.5.1 Chemistry of WTC Steels

Chemical analyses of the flange, outer web, and spandrel plates of the exterior panel sections were found

to be nearly identical for a given plate gauge and yield strength. In contrast, inner web plates with yield

strengths less than 80 ksi were found to be chemically distinct from the other plates; inner webs with

strengths equal to 80 ksi and 100 ksi were the same steel as used for the flanges. These results support the

contemporaneous documents stating that the flange, outer web, and spandrel plates were produced by a

single company (Yawata Iron and Steel Co.), and the inner web plates were primarily supplied

domestically. End plates of the perimeter panel met chemistry specifications for Yawata steels, as well. The alloying practices observed were typical for steels of this era.

124 NISTNCSTAR 1-3, WTC Investigation
Physical Properties of Structural Steels

0 200 400 600

400 600 800

T(K)

Source: Numbered curves correspond to steels in a report from the National Physical Laboratory (NPL 1946). Curves A-C correspond to curves 22, 24, and 79 of Specification Table 334 in Touloukian (1970b).

Curve D appears in Specification Table 335 in Touloukian (1970b); the

original data are from Powell (1956). Curve W comes from a data sheet

for WEL-TEN 80. (Yawata 1969).

Figure 8-3. Thermal conductivity as a function of temperature for twelve low-alloy steels.

The recovered core columns were found to vary in composition for a given strength and shape (built-up

box, rolled wide flange). This result supports the contemporaneous documents stating that numerous

suppliers produced the steel to be used for these structural components.

Construction bolts were either A 325 or A 490 bolts. Bolt types were identified by stampings on the

head. Chemical analysis indicated the A 325 bolts met the Type 1 chemistry specifications.

Chords and rods that composed the floor trusses were either ASTM A 36 or ASTM A 242. Laclede

Steel Co., the truss fabricator, supplied these materials.

Other structural elements (floor truss connectors, core channels, spandrel splice plates, etc.) met

specifications for ASTM A 36. Compositions varied significantly, and it appears that numerous suppliers

completed the steel orders for these components.

8.5.2 Microstructure of WTC Steels

Two different microstructural types were observed for the recovered WTC steels: ferrite-pearlite for low

strength components and quenched-and-tempered for high-strength components.

Discernible changes to the microstructure of furnace exposed WTC steel were observed when exposed to a temperature of 625 °C for as little as 0.25 h. At or below 500 °C, no microstructural change was

NIST NCSTAR 1-3, WTC Investigation 125
Chapter 8

apparent using light optical microscopy. The hardness of these samples varied for a given temperature

and material type, but correlated well with microstructural observations.

Thermal property values as a function of temperature were provided for modeling the structural steel response to fire. These properties include specific heat (Cp

), coefficient of thermal expansion (a), and

thermal conductivity (k).

8.5.4 Other

A piece of the aluminum facade was analyzed and found to be consistent with the non-heat treatable aluminum-manganese alloy AA 3003.

A sample of fireproofing material was chemically analyzed. While the chemical composition of the

material was proprietary, the results were found to be consistent, on a qualitative basis, with the

information presently available on the manufacturer's website.

American Institute of Steel Construction. 1973. Manual ofSteel Construction, American Institute of

Steel Construction, New York, p. 6-1 1

ASM (American Society of Metals) International. 1978. Properties and Selection: Irons and Steels,

ASM International, Metals Park, OH.

Clark, C.L. 1953. High-Temperature Alloys, Pitman, NY.

Cooke, G. M. E. 1988. An Introduction to the Mechanical Properties of Structural Steel at Elevated

Temperatures, Fire Safety Journal, 13, 45-54.

Dever, C.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. 1980. Fracture-Mechanisms in Pure Iron, Two

Austenitic Stainless Steels, and One Ferritic Steel, Met. Trans., A 1 1 A, 333-347.

Harmathy, T.Z., and W. W. Stanzak. 1970. Elevated-Temperature Tensile and Creep Properties of Some

Structural and Prestressing Steels, Fire Test Performance, vol. ASTM STP 464, American Society for Testing and Materials, 186-208.

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

Ludwigson, D. C, and F. C. Schwerer. 1971. The Effect of Composition on the Resistivity of Carbon

Steel Sheet, Metall. Trans., 2, 3500-3501.

8.5.3 Thermal Property Values of WTC Steels

8.6 REFERENCES

1-9.

126 NISTNCSTAR 1-3. WTC Investigation
Physical Properties of Structural Steels

NPL. 1946. The Physical Properties of a Series of Steels.-Part II, ./. Iron Steel Inst., 1 54, 83P-1 2 1 P.

Powell, R. W., and R. P. Tye. 1956. Thermal Conductivity and Electrical Resistivity of a 0.4-0.5%

Manganese Steel, J. Iron Sieel Inst., 184, 286-288.

Schwerer, F. C, and R. M. Fisher. 1986. Steels: Electrical Resistivity and Thermal Conductivity, The

MIT Press, Cambridge, MA, p. 4601

Souder, W., and P. Hidnert. 1922. Thermal Expansion of a Few Steels, Sci. Pap. Bur. Stand., 1 7, 611-626.

Touloukian, Y. S., and Buyco, E. H. 1970a. Specific Heat Metallic Elements and Alloys, IFI/Plenum.

NY.

Touloukian, Y. S., Powell, R. W., Ho, C. Y, and Klemens, P. G. 1970b. Thermal Conductivity Metallic

Elements and Alloys, IFI/Plenum, NY.

Touloukian, Y. S., Kirby, R. K., Taylor, R. E., and Desai, P. D. 1977. Thermal Expansion Metallic

Elements and Alloys, IFI/Plenum, NY.

Uddin, T., and C. G. Culver. 1975. Effects of Elevated Temperature on Structural Members, J. Struct. Div. ASCE 101 (7), 1531-1549.

Wallace, D.C., P. H. Sidles, and G. C. Danielson. 1960. Specific Heat of High Purity Iron by a Pulse

Heating Method, J. Appl. Phys., 31(1), 168-176.

Yawata Iron and Steel Co. 1969. WEL-TEN 80, Japan, 142-164.

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128 NISTNCSTAR 1-3, WTC Investigation
Chapter 9

Findings and Issues

9.1 INTRODUCTION

This chapter summarizes the findings and conclusions from the NIST mechanical and metallurgical

analysis of the WTC structural steel.

9.2 FINDINGS - STEEL SPECIFICATIONS, STANDARDS, AND SOURCES

Fourteen nominal strengths of steel were specified in the structural engineering plans, but only

12 strengths of steel (minimum yield strengths of [36, 42, 45, 46, 50, 55, 60, 65, 70, 75, 80, and 100] ksi) were actually used in construction due to an upgrade of two steels (85 ksi and 90 ksi steel upgraded to 100 ksi).

Ten different steel companies fabricated structural elements for the towers, using steel supplied from at least eight different suppliers. Four fabricators supplied the major structural elements of the 9th to 107th

floors: Laclede Steel (floor trusses), Pacific Car & Foundry (perimeter column panels), Stanray Pacific

(welded core box columns), and Montague-Betts (rolled core columns).

Although ASTM International structural steel standards have evolved since the construction of the towers,

the changes have been minor and do not represent changes to the basic mechanical properties of the

steels.

9.3 FINDINGS - INVENTORY OF RECOVERED STRUCTURAL STEEL

National Institute of Standards and Technology (NIST) has 236 samples from the World Trade Center

(WTC) buildings, the majority belonging to WTC 1 and WTC 2. These samples represent roughly

0.25 percent to 0.5 percent of the 200,000 tons of structural steel used in the construction of the two towers.

The collection of steel from the WTC towers was sufficient for determining the quality of the steel and, in combination with published data, for providing mechanical properties as input to models of building

performance.

The original as-built locations of 42 recovered perimeter panels and 12 recovered core columns were

determined. Representative samples exist for all 12 grades of perimeter panel material, two grades of the core column material (representing 99 percent of the total number of columns), and both grades for the

floor truss material.

A number of structural pieces were recovered from locations in or near the impact and fire damaged

regions of the towers, including four exterior panels directly hit by the airplane and three core columns

located within these areas.

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Material substitutions of higher strength steels were not uncommon in the perimeter columns and floor

trusses.

9.4 FINDINGS - DAMAGE AND FAILURE ANALYSIS

Damage and failure analysis findings were made based on pre-collapse photographs of the recovered

steel. It should be noted that the recovered steel represents only a small fraction of the structural steel in the towers and that the recovery effort focused on apparent fire and impact damage.

9.4.1 Structural Impact Damage - Perimeter Panels

Damage modes due to direct aircraft impact were documented based on pre-collapse photographs.

Correlation between pre-collapse photographs and the recovered panels from the impact zone indicates

that two of the four recovered impact-damaged panels (M-2 and M-30) are in a condition similar to that just prior to building collapse. Some damage can be attributed to the events during and after collapse, but

the general shape and appearance of the recovered pieces match with the damage photographs.

Of particular interest was the fracture behavior of the plates composing the recovered columns that were

directly impacted by the airplane. Fractures of the plates in areas away from a welded joint (e.g., fracture occurring perpendicular to the rolling direction of the outer webs of panels M-30 and S-9) exhibited

ductile behavior, including necking and thinning away from the fracture, indicating that the steel behaved

ductilely under very high-strain rates.

Conversely, fractures occurring parallel and directly next to a welded joint exhibit little or no ductile

characteristics. Diminished properties of the heat affected zone in the base plate, the geometry of the

joint with respect to the direction of impact, stress concentrations due to the geometry, and the orientation

of the crack propagation with respect to the rolling direction of the plate are expected to contribute to the

lack of ductility. There was no evidence to indicate that the type ofjoining method, materials, or welding

procedures were improper. The welds appeared to perform as intended.

In general, severing of the perimeter columns hit by an aircraft wing occurred at an internal stiffener plate or diaphragm plate (associated with the spandrel connection to column). There was a tendency for the

columns hit by the plane to have experienced metal fracture paths along heat-affected zones adjacent to welds. This behavior was not observed for columns outside of the impact zone.

The failure mode of spandrel connections on perimeter panels differed above and below the impact zone. Spandrel connections on exterior panels at or above the impact zone were more likely to experience bolt hole tear out as a failure mode. For those exterior panels below the impact zone, there was a higher

propensity for the spandrels to be ripped off from the panels. This may be due to shear failures as the

weight of the building came down on these lower panels. There did not appear to be any difference in

failure mode for the spandrel connections whether the exterior panels were exposed to fire or not.

With the exception of the mechanical floors, the major failure mechanism concerning perimeter panel

column splices was fracture of the bolts. At mechanical floors, where splices were welded in addition to being bolted, the majority of the splices did not fail.

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9.4.2 Structural Impact Damage - Core Columns

Failure of core columns was a result of both splice connection failures and fracture of the columns

themselves. One recovered core column (WTC 2, column line 801, floors 77—80) may have sustained

damage as a direct result of the airplane impact, however the welded splice to the column above survived

intact.

9.4.3 Impact Damage to Sprayed Fire-Resistive Material

Pre-collapse photographs indicated that, as expected, fire-proofing was removed from pieces struck by the incoming aircraft or debris exiting the far side of the buildings. In addition, the impact caused fire- proofing and aluminum facade panels to spall off many perimeter columns which were not directly struck,

but apparently suffered only strong accelerations and forces otherwise transmitted through the structure. This indirect damage to the sprayed fire-resistive materials (SFRM) was observed on the north and east faces of WTC 2. A coating on the SFRM prevented the loss of the SFRM in some locations on the perimeter columns.

This coating appears as a band of white features on the SFRM wherever two aluminum panels met on the

exterior columns of the buildings, becoming visible when the panels were dislodged. This may have been

a coating applied to protect the SFRM from moisture infiltration at the aluminum panel joints, acting to preserve the SFRM even when the SFRM was knocked off both above and below those locations.

9.4.4 Structural Damage - Floor Trusses and Seats

The damage to truss seats on perimeter panels differed above and below the impact zone. The majority of

perimeter panel floor truss connectors (perimeter seats) below the impact floors were either missing or bent downwards. Above this level, the failure modes were more randomly distributed. This trend was

observed for both towers, presumably resulting from the building collapse sequence.

Failure of the welds associated with the perimeter panel floor truss connectors (perimeter seats) typically

occurred as a result of the weld geometry (i.e., in the component with the lowest cross-sectional area)

with respect to the high loading forces of the collapse. These areas with the lowest cross-sectional area were typically the standoff plates. However, there was no evidence to indicate that the type ofjoining

method, materials, or welding procedures were improper.

Of the 31 core floor truss connectors (core seats) recovered, about 90 percent were still intact though

extensive damage may have occurred. Only two were observed to have been completely torn from the

channel. (This distribution may have resulted from the selection process at the salvage yards).

In the floor trusses, failure of a large majority of the electric resistance welds at the web-to-chord

connections was observed. Failure of the connection between the floor truss and the perimeter panel floor

truss connectors was typically a result of tab plate weld and bolt failure.

In general, the observations concerning possible local failure mechanisms viewed on the WTC steel from

an outside contractor were in agreement with those found by NIST.

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9.4.5 Fire Exposure and Temperatures Reached by the Steel

The pre-collapse photographic analysis showed that 16 of the 33 exterior panels recovered from WTC 1 were exposed to fire prior to building collapse. None of the nine recovered panels from within the fire floors of WTC 2 were observed to have been directly exposed.

It is difficult or impossible to determine if high-temperature exposure occurred prior to or after the

collapse. Of the more than 170 areas examined on 21 exterior panels, only three locations had mudcracking of the paint, indicating that the steel may have reached temperatures in excess of 250 °C. The

21 panels represent only 3 percent of all panels on the fire floors, however, and cannot be considered

representative of other columns on these floors.

Annealing studies on recovered steels established the set of time and temperature conditions necessary to

alter the steel microstructure. Based on the pre-collapse photographic evidence, the microstructures of

steels known to have been exposed to fire were characterized. These microstructures show no evidence

of exposure to temperatures above 600 °C for any significant time for the recovered pieces.

Perimeter columns exposed to fire had a great tendency for local buckling of the inner web; a similar

correlation did not exist for weld failure.

Two of the core columns with as-built locations in the fire-affected floors were examined for paint

cracking. The few areas with sufficient paint for analysis did not show mud cracking patterns, indicating

the columns did not exceed 250 °C. (It must be recognized that the examined locations represent less than about one percent of the core columns located in the fire-exposed region, and thus these temperatures

cannot be considered representative of general conditions in the core).

9.4.6 Time-Dependent Deformation of Perimeter Walls Due to Fire and Load

Redistribution

Images of WTC 1 showed gross deformations of an exterior wall prior to final collapse. Images of the

south face of the tower taken approximately 5 min prior to collapse showed inward bowing of the exterior

columns, reaching an observable maximum of about 55 in. near column 3 16 on the 96th floor. The

inward deflection appears to extend over the entire south face of the building at this time, and is visible

between the 94th and 100th floors. Photographs taken approximately 35 min prior to collapse do not show any inward bowing of the south face of WTC 1.

Approximately 1 8 min after the impact of the aircraft, the east face of WTC 2 exhibited inward bowing of

up to 10 in. in the region of the 79th to 83rd floors. This inward bowing increased to 20 in. at a time

5 min before collapse of the tower.

Hanging floor slabs at the 82nd and 83rd floors were visible in window openings on the east and north

faces, respectively, of WTC 2 and the positions of these slabs changed over time. This suggests a progression of failure of certain parts of the flooring in this area of the tower.

At the moment of collapse of WTC 2, the top portion of the building was found to have moved to the

west as it tilted to the southeast. During this tilting, a complex kink developed at the southeast corner of

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the top of the building, in the region of the 106th floor. In addition, the portion of the building above the

aircraft impact site twisted slightly clockwise (as viewed from above) as the collapse progressed.

9.5 FINDINGS - MECHANICAL PROPERTIES

9.5.1 Room Temperature Tensile Properties

There were over 40 different steels in the towers, based on the various combinations of suppliers and

nominal strengths. Thirty-two steels were sufficiently distinct to require distinct models of constitutive

behavior.

The steel used in the construction of the WTC generally met the expectations of the designers and the specifications called for in the steel contracts. Approximately 87 percent of all tested steel exceeded the

required minimum yield strengths specified in design documents; approximately 13 percent of test results on the damaged steel did not meet the required minimum yield strengths. The occurrence of test results below the specified minimum values is not unexpected since differences in test procedures from those in the qualifying mill tests could account for 2-3 ksi lower values in the NIST tests, and the loss of a yield

point due to damage to the steel accounts for 2^4 ksi lower values in the NIST tests in several cases.

The steels in the perimeter columns met their intended specifications for both chemistry and mechanical

properties. The yield strengths of the perimeter column steels generally exceed their specified minimums

by 10 percent to 15 percent. The tensile properties of the perimeter columns are consistent with literature estimates for average properties of construction steel plate during the WTC construction era. The number

of occurrences of plates with tensile properties at or slightly below the specified minimum is consistent

with the historical variability of steel strength.

The steels in the core columns generally met their intended specifications for both chemical and

mechanical properties. Some, primarily wide-flange shapes, had NIST-measured strengths that were

lower than called for in the specifications, but this may have arisen from mechanical damage that removed the yield point, the natural variability of structural steel, and slight differences between the NIST

and original mill test report testing protocols. Regardless of the source, the observed distributions are accounted for in the typical design factor of safety for allowable stress design. However, the yield

strengths of undamaged steels in the rolled core columns are lower than the historical literature indicates

as typical.

The steels in the floor trusses met their intended specifications. The strength of the steel in the floor trusses is higher than called for in the original specifications. Many of the truss steels that were specified

as low strength A 36 were supplied as high-strength, low-alloy steels with much higher strengths.

Laclede Steel Company's substitution of 50 ksi yield strength steel for A 36 in the lower chord of the

trusses is expected to have provided significantly improved performance at high temperature.

Limited tests on bolts indicated that they met specifications. The recovered bolts were stronger than

contemporaneous literature would suggest as typical.

Limited tests on recovered welds and weld material indicated that their mechanical properties and

chemistry were consistent with their intended specifications.

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In summary, the steel used in the construction of the WTC towers met the expectations of the designers

and the specifications called for in the steel contracts. Material substitutions of higher strength steels were common in the perimeter columns and floor trusses. The safety of the WTC towers on

September 11, 2001, was most likely not affected by the fraction of steel that, according to NIST testing,

did not meet the required minimum yield strength. The typical factors of safety in allowable stress design can accommodate the measured property variations below the minimum.

9.5.2 High-Strain-Rate Properties

The strain rate sensitivity of the perimeter and core column steels was similar to other construction steels of the WTC era.

9.5.3 Impact Properties

The impact toughness of steels from the perimeter and core columns, and floor trusses was consistent

with literature values for the WTC construction era.

9.5.4 High-Temperature Properties

The measured high-temperature yield and tensile strength behavior of WTC steels is similar to behavior

of construction steels from the WTC construction era literature.

The creep behavior of WTC steels can be modeled by scaling WTC-era literature data using room

temperature tensile strength ratios.

Laclede Steel Company's substitution of 50 ksi yield strength steel for A 36 in the lower chord of the

trusses is expected to have provided significantly improved performance at high temperature.

9.5.5 Conclusions - Mechanical Properties

In general, the steel used in the construction of the WTC met the expectations of the designers and the

specifications called for in the steel contracts.

9.6 ISSUES

Based on the Investigation findings, NIST identified a detailed set of issues related to practice, standards,

and codes that provided the basis for formulating the Investigation's draft recommendations. The

Investigation team has studied practices ranging from those used during construction of the towers to newly available practices which could improve the safety and performance of high-rise buildings. The

recent development and use of "fire-resistant" steel in Europe and Japan falls in the latter category.

Fire-resistant steels are reported to retain a higher fraction of their room temperature strength at temperatures expected in building fires, and are used either with or without fire protection depending on

the application. If fire-resistant steels do indeed retain improved high-temperature properties, then

improved fire resistance would be expected even in cases where conventional fire protection has been

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damaged, whether during normal construction and modifications, or due to intentional damage as in the attack on the WTC towers.

Each issue identified by NIST was divided into three levels with between two and five categories each:

• Categories in Level 1: practices; standards, codes, and regulations; adoption and enforcement;

research and development or requiring further study; and education and training.

• Categories in Level 2: all tall buildings (buildings over 20 stories in height); selected tall buildings (buildings over 20 stories in height that are at risk due to design, location, use, iconic status, contents, etc.); selected other buildings (buildings that are at risk due to design,

location, use, historic/iconic status, contents, etc.).

• Categories in Level 3: related to the outcome on September 11, 2001 (i.e., could have

changed the outcome); or unrelated to the outcome on September 1 1, 2001 (i.e., would not have changed the outcome yet is an important building and fire safety issue that was

identified during the course of the Investigation).

Under Level 1, the fire-resistant steel issue is considered to include practices and research and

development or requiring further study. The issue, under Level 2, applies to all tall buildings and selected

other buildings. Under Level 3, the use of fire-resistant steel in the WTC towers may have increased the time to collapse of the towers.

• Discussion of this issue is not intended to suggest that fire-resistant steels should have been

used in the construction of the WTC towers, or even that fire-resistant steels were available

commercially at the time of the construction.

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