NISTNCSTAR1-1A

Federal Building and Fire Safety Investigation of the

World Trade Center Disaster

Design and Construction of

Structural Systems

David A. Fanelia

Arnaldo T. Derecho

S. K. Ghosh

National Institute of Standards and Technology Technology Administrofion • U.S. Deportment of Comi

NISTNCSTAR1-1A

Federal Building and Fire Safety Investigation of the

World Trade Center Disaster

Design and Construction of

Structural Systems

David A. Fanella

Arnaldo T. Derecho

S. K. Ghosh

S. K. Ghosh Associates, Inc.

September 2005

U.S. Department of Commerce

Carlos M. Gutierrez, Secretary

Technology Administration

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

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

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

Disclaimer No. 3

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

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

provided to NIST under nondisclosure agreements.

Disclaimer No. 4 NIST takes no position as to whether the design or construction of a WTC building was compliant with any code

since, due to the destruction of the WTC buildings, NIST could not verify the actual (or as-built) construction, the

properties and condition of the materials used, or changes to the original construction made over the life of the

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

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

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

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

Use in Legal Proceedinqs

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 1-1A

Natl. Inst. Stand. Technol. Natl. Constr. Sfty. Tm. Act Rpt 1-1 A, 166 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 describes the provisions that were used to design and construct World Trade Center 1, 2, and 7. Included is a summary of the major provisions in the codes and standards together with the loads

and load combinations that were used to design the buildings. Methods used to proportion structural members and other components of the buildings are also discussed, as well as tests that were perfomied to support the design. It is shown that the loads that were used to design the members were at least equal to those prescribed in the applicable codes and standards, and that the methods used to proportion the

structural members followed the requirements in the applicable material design standards available at that time.

Also included in this report are the innovative systems, technologies, and materials that were used in the buildings, and the Port Authority's acceptance procedures for such items. Fabrication and inspection

requirements at the fabrication yard and inspection protocol during construction are discussed. Also

covered are the details of the deviations to contract documents that were granted by the Port Authority,

including the justifications for those deviations.

The inforaiation contained in this report is based on documents and structural drawings that were

acquired from the following locations: ( 1) the offices of the Port Authority of New York and New Jersey

in Newark, New Jersey, and New York City and (2) the National Institute of Standards and Technology in Gaithersburg, Maryland. Paper, iTiicrofilm, and electronic versions of these documents were obtained

from these locations. Appendixes to this report include copies of referenced documents.

Keywords: Analysis, codes, construction, design, fabrication, innovative systems, inspection, loads, load

combinations, materials, standards, tests, deviations. World Trade Center.

NISTNCSTAR 1-1A, WTC Investigation iii
Abstract

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iv NISTNCSTAR 1-1A, WTC Investigation
Table of Contents

Abstract iii

List of Figures ix

List of Tables xi

List of Acronyms and Abbreviations xiii

Preface xv

Executive Summary xxv

Chapter 1 Introduction 1

Chapter 2 Provisions Used to Design and Construct the Buildings 3

2. 1 Building Codes Used in Design 3

2.1.1 WTClandWTC2 3

2.1.2 WTC7 3

2.2 Summary of Code Provisions 4

2.2.1 Loads 4

2.2.2 Structural Design Requirements of the Code 30

2.3 Summary of Methods Used to Proportion Structural Members and Components 33

2.3.1 Overview 33

2.3.2 Exterior Columns 34

2.3.3 Floor Trusses 37

2.3.4 Composite Steel Beams 46

2.3.5 Connections 49

2.3.6 Concrete Floor Slabs 5

1

2.3.7 Steel Deck 54

2.3.8 Hat Trusses 54

2.4 References 58

Chapter 3 Tests Performed to Support Design Innovations 61

3.1 Exterior Wall Panel Tests 61

3 .2 Wind Tunnel Tests 65

3.2. 1 Meteorological Program 65

NISTNCSTAR 1-1A, WTC Investigation v
Table of Contents

3.2.2 Wind Tunnel Program 68

3.3 Damping Unit Tests 76

3.4 Floor Truss Tests 78

3.4.1 Full-Scale Flexural Tests 78

3.4.2 Shear Knuckle Tests 80

3.4.3 Interior Panel Connection Tests 82

3.4.4 Bearing Capacity Tests 85

3.5 Stud Shear Connector Tests 88

3.6 References 88

Chapter 4

Port Authority Policies and Agreements with New York City Department of

Buildings... 91

Chapter 5 Innovative Systems, Technologies and Materials, and Acceptance Procedures Used

by the Port Authority 93

5.1 Innovative Features of the Structural System 93

5.2 Lateral-Force-Resisting System of WTC 1 and WTC 2 94

5.3 Damping Units 97

5.3.1 Overview 97

5.3.2 Specifications 100

5.4 Floor Trusses 105

5.4.1 Overview 105

5.4.2 Specifications 108

5.5 References 1 1

1

Chapter 6 Fabrication and Inspection Requirements at the Fabrication Yard 113

6.1 Overview 1 13

6.2 Suminary of Code Requirements for Fabrication and Inspection 113

6.2. 1 Fabrication Requirements 1 14

6.2.2 Inspection Requirements 1 14

6.3 Summary of Fabrication and Inspection Requirements at the Fabrication Yard for WTC 1 and WTC 2 114

6.3.1 Floor Trusses 115

6.3.2 Box Core Columns and Built-up Beams 115

6.3.3 Exterior Wall from Elevation 363 ft to the 9th Floor Splice 117

vi NISTNCSTAR 1-1A. WTC Investigation
Table of Contents

6.3.4 Exterior Wall Above 9th Floor Splice 1 1

8

6.3.5 Rolled Columns and Beams 1 1

8

6.3.6 Other Requirements 119

6.4 Summary of Fabrication and Inspection Requirements at the Fabrication Yard for WTC 7 1 19

6.4.1 Fabrication 120

6.4.2 Inspection 121

6.5 References 121

Chapter 7 Inspection Protocol During Construction 123

7.1 Overview 123

7.2 Erection Marks and Marking System 123

7.3 Quality Control and Inspection Program 123

7.4 Reference 124

Chapter 8 Deviations Granted by the Port Authority 125

8.1 Overview 125

8.2 Deviations Relating to Fabrication/Erection Tolerances 125

8.3 Deviations Relating to Defective Components 1 27

8.4 Deviations Relating to Alternate Fabrication/Erection Procedures 129

8.5 Deviations Relating to Product Substitutions 129

8.6 Deviations Relating to Inspection Practice 130

8.7 Reference 130

Appendix A

Supporting Documents for Chapter 2 131

Appendix B

Supporting Documents for Chapter 3 1 37

Appendix C

Supporting Documents for Chapter 4 215

Appendix D

Supporting Documents for Chapter 5 225

Appendix E Supporting Document for Chapter 6 275

NISTNCSTAR 1-1A, WTC Investigation vii
Table of Contents

Appendix F Supporting Documents for Chapter 7 389

Appendix G

Supporting Documents for Chapter 8 .405

Vlll NISTNCSTAR 1-1A, WTC Investigation
List OF Figures

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

disaster xvii

Figure 2-1 . Definitions used in design criteria for WTC 1 and WTC 2 5

Figure 2-2. Core area in a representative floor plan of WTC 1 and WTC 2 6

Figure 2-3. Design criteria for WTC 1 and WTC 2: floor inside of core - unit dead load 7

Figure 2—4. Design criteria for WTC 1 and WTC 2: floor outside of core - unit dead load 1

1

Figure 2-5. Design load criteria for WTC 7 13

Figure 2-6. Design criteria for WTC 1 and WTC 2: floor inside of core - live load 14

Figure 2-7. Design criteria for WTC 1 and WTC 2: typical floor slabs outside of core - live load 1

9

Figure 2-8. Design criteria for WTC 1 and WTC 2: columns outside of core - live load 20

Figure 2-9. Design criteria for WTC 1 and WTC 2 - live load reduction 22

Figure 2-10. Design criteria for WTC 1 and WTC 2: floors inside of core, except for tenant areas -

live load reduction 23

Figure 2-1 1 . Design criteria for WTC 1 and WTC 2: floors inside of core, tenant areas - live load

reduction 24

Figure 2-12. Design criteria for WTC 1 and WTC 2: floors outside of core 25

Figure 2-13. Design criteria for WTC 1 and WTC 2: floors outside of core - live load reduction 26

Figure 2-14. Design criteria for WTC 1 and WTC 2: columns outside of core - live load

reduction 27

Figure 2-15. Total deflections (ft) at top of WTC 1 and WTC 2 due to wind 36

Figure 2-16. Design method for exterior columns in WTC 1 and WTC 2 38

Figure 2-17. Design method for floor trusses in WTC 1 and WTC 2 40

Figure 2-18. Tabulation of component capacities of floor truss connections in WTC 1 and WTC 2 44

Figure 2-1 9. Design standard for composite sections in WTC 1 and WTC 2 46

Figure 2-20. Design standard for bolted connections in WTC 1 and WTC 2 49

Figure 2-2 1 . Schedule of welding electrodes for connections in exterior columns in WTC 1 and

WTC 2 50

Figure 2-22. General notes for structural concrete in WTC 1 and WTC 2 51

Figure 2-23. Design assumptions for concrete floor slabs in WTC 1 and WTC 2 52

Figure 2-24. Reinforced concrete one-way slab design in WTC 1 and WTC 2 53

Figure 2-25. Specification for reinforcing steel used in WTC 1 and WTC 2 54

Figure 2-26. Design criteria for steel deck in WTC 1 and WTC 2 55

Figure 2-27. Design method for hat trusses in WTC 1 and WTC 2 56

NIST NCSTAR 1-1A. WTC Investigation ix
List of Figures

Figure 3-1. Subassembly used for testing external wall panel in WTC 1 and WTC 2 61

Figure 3-2. Loads applied to model of exterior wall panel 62

Figure 3-3. Test rig used for testing model of external wall panels in WTC 1 and WTC 2 63

Figure 3-4. Spandrel flanges used in some test models of exterior wall panels 63

Figure 3-5. Displacements and rotations measured during model test program of exterior wall

panels 64

Figure 3-6. Location of anemometers in wind study for WTC 1 and WTC 2 69

Figure 3-7. Wind directions that produced the greatest displacements at the top of the tower

during the wind tunnel tests 72

Figure 3-8. Definition of grid system and tower configurations for wind tunnel tests at CSU 73

Figure 3-9. Comparison of the variation of the N-S deflection (amplitude) of WTC 1 subjected to E-W wind for different degrees of damping (y) and flow conditions 75

Figure 3-10. Location of concentrated loads in the full-scale testing of the floor trusses in WTC 1 and WTC 2 79

Figure 3-1 1 . Results from full-scale flexural tests of 32 in. deep floor trusses 79

Figure 3-12. Maximum midspan deflections from full-scale flexural tests of 32 in. deep floor trusses 80

Figure 3-13. Test setup for longitudinal shear knuckle tests 81

Figure 3-14. Results from longitudinal shear knuckle tests 82

Figure 3-15. Test setup for interior panel connection test - horizontal load on welds 83

Figure 3-16. Results from interior panel connection tests - horizontal load on welds 84

Figure 3-1 7. Test setup for interior panel connection test - vertical load on welds 84

Figure 3-1 8. Test setup for first set of bearing capacity tests on floor trusses 85

Figure 3-19. Results from the first set of bearing capacity tests on floor trusses 86

Figure 3-20. Test setup for second set of bearing capacity tests on floor trusses 87

Figure 3-2 1 . Results from the second set of bearing capacity tests on floor trusses 87

Figure 5-1 . Exterior wall panels in WTC 1 and WTC 2 94

Figure 5-2. Exterior wall panel transition in WTC 1 and WTC 2 95

Figure 5-3. Representative structural framing plan on a typical floor of WTC 1 or WTC 2 96

Figure 5^. Floor truss member with Type A damping units 98

Figure 5-5. Wide-flange beam floor member with Type B damping units 99

Figure 5-6. Damping unit details - Types A and B 100

Figure 5-7. Parameters related to mechanical properties of damping units 101

Figure 5-8. Prefabricated floor unit used in WTC 1 and WTC 2 106

Figure 5-9. Section through the main double trusses in the floor system of WTC 1 and WTC 2 107

X NISTNCSTAR 1-1A, WTC Investigation
List of Tables

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

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

Table 2-1. Percentage of live load per the 1968 Code 21

Table 2-2. Design wind pressures on vertical surfaces per the 1968 Code (Table RS 9-5-1 ) 28

Table 2-3. Design wind pressures on horizontal and inclined surfaces per the 1968

Code (Table RS 9-5-2) 28

Table 2-4. Equivalent design wind velocity for WTC 1 and WTC 2.^ 34

Table 3-1 . Constants used in wind study of WTC 1 and WTC 2 67

Table 3-2. Wind directions that produced the largest displacements at the tops of the towers from the twin-tower wind tunnel tests 74

Table 5-1 . Material specifications for damping units per WTC Contract WTC-224.0.'' 103

Table 5-2. Acceptance requirements for damping units per WTC Contract WTC-224.0 104

Table 5-3. Five-year acceptance requirements for damping units per WTC Contract WTC-224.0.... 105

NISTNCSTAR 1-1A, WTC Investigation xi
List of Tables

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xii NISTNCSTAR 1-1A, WTC Investigation
List of Acronyms and Abbreviations

Acronyms

Minnesota Mining and Manufacturing Company

ALl American Concrete Institute AISC American Institute of Steel Construction

ASTM ASTM International

A Wo American Welding Society

/^Cl T Colorado State University

JrK John F. Kennedy (Airport)

rUvb Karl Koch Erecting Company

LERA Leslie h. Robertson Associates

X TI C HP NIST XTa* it a.' i. a. C a. J J J T" 1 1 National Institute oi Standards and Technology

NrL National Physical Laboratory

TI A X TA^X T T PANYNJ T*_i.Ail 'a. JTXT X/ 1 JXT T Port Authority oi New York and New Jersey

PDM Pittsburgh-Des Moines Steel Company

rON YA Port OI New York Authority

rr(j Pittsburgh Plate Glass

SHCR Skilling, Helle, Christiansen, & Robertson

TRCC Tishman Realty & Construction Company

WSHJ Worthington, Skilling, Helle, & Jackson

WTC World Trade Center

WTC 1 World Trade Center 1 (North Tower)

WTC 2 World Trade Center 2 (South Tower)

WTC 7 World Trade Center 7

Abbreviations

°F degrees Fahrenheit

cps cycles per second

ft foot

square foot

NISTNCSTAR 1-1A, WTC Investigation xiii
List of Acronyms and Abbreviations

ft^ cubic foot

in, inch

kip a force equal to 1 ,000 pounds

ksi 1 ,000 pounds per square inch

lb pound

m meter

m/s meters per second

min minute

mph miles per hour

pcf pounds per cubic foot

plf pounds per linear foot

psf pounds per square foot

psi pounds per square inch

s second

XIV NISTNCSTAR 1-1A, 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 perfonnance study of the disaster. The week of October 7, as soon as the rescue and

search efforts ceased, the Building Perfonnance 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 detennine 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 I, 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. Detennine 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 I, 2, and 7; and

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

practices that warrant revision.

NISTNCSTAR 1-1A, WTC Investigation XV
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 resuUed 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 Constioiction Safety Team Act may be used in any suit or action

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

Law 107-231).

Organization of the Investigation

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

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

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

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

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

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

Technical Area and Project Leader Project Purpose

Analysis of Building and Fire Codes and

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

and Mr. Richard W. Bukowski

Document and analyze the code provisions, procedures, and

practices used in the design, construction, operation, and

maintenance of the structural, passive fire protection, and

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

Aircraft Impact Damage Analysis; Project Leader: Dr. Fahim H. Sadek

Analyze the baseline 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.

XVI NISTNCSTAR 1-1 A, WTC Investigation
Preface

NIST WTC investigation Projects

Nisr

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

investigation of the WTC disaster.

National Construction Safety Team Advisory Committee

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

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

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

Team Advisory Committee Chair

• John Barsom, President, Barsom Consulting, Ltd.

• John Bryan, Professor Emeritus, University of Maryland

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

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

• Philip DiNermo, President, Hughes Associates, Inc.

NISTNCSTAR 1-1A, WTC Investigation XVII
Preface

• Robert Hanson, Professor Emeritus, University of Michigan

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

Inc. ,

• Kathleen Tiemey, Director, Natural Hazards Research and Applications Infonnation 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.

xviii NISTNCSTAR 1-1A, WTC Investigation
Preface

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

Date Location Principal Afjenda

June 24, 2002 New York City, NY Public meeting! Public comments on the Drctfi Plan for the pending WTC Investigation.

August 21, 2002 Gaithersburg, MD Media briefing announcing the f ormal start of the Investigation.

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

for photographs and videos. April 8, 2003 New York City, NY Joint public forum with Columbia University on first-person interviews.

Anril 70-^0 '>0(n vj ui II ici ouui iviLy Mr^^T Ar1\/icnr\/ r^Arnmittpp mpptino on nlfin lAr i^nH nroorpcc on WTC Investigation with a public comment session. Mav 7 ''003 New York Citv NY

August 26-27, 2003 Gaithersburg, MD NCST Advisory Committee meeting on status of the WTC

investigation with a public comment session. beptember 1 /, 2003 New York Lity, NY JVIeaia and public bneiing on initiation oi iirst-person data

PAllPPtlAn T^fAIPPtC December 2-3, 2003 Gaithersburg, MD NCST Advisory Committee meeting on status and initial results and release of the Public Update with a public comment session. rebruary 12, 2004 New York City, NY Public meeting on progress and preliminary findings with public

comments on issues to be considered in formulating final

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

preliminary findings from the WTC Investigation with a public

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

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

April 5, 2005 New York City, NY Media and public briefing on release of the probable collapse sequence for the WTC towers and draft reports for the projects on

codes and practices, evacuation, and emergency response. June Zj, zuuj iNew Y orK Lity, in y 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 1-1A, WTC Investigation xix
Preface

National Construction Safety Team Reports on the WTC Investigation

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

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

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

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

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

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

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

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

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

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

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

September.

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

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

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

September.

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

Trade Center Disaster: Comparison ofBuilding Code Structural Requirements. NIST

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

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

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

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

MD, September.

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

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

Construction of World Trade Center I, 2, and 7 and Post-Construction Provisions Applied after Occupancy. NIST NCSTAR 1-lD. 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-lE. 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

XX NIST NCSTAR 1-1A, WTC Investigation
Preface

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

Technology. Gaithersburg, MD, September.

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

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

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

Technology. Gaithersburg, MD, September.

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

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

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

Standards and Technology. Gaithersburg, MD, September.

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

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

Gaithersburg, MD, September.

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

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

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

September.

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

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

Technology. Gaithersburg, MD, September.

Kirkpatrick, S. W., R. T. Bocchieri, F. Sadek, R. A. MacNeill, S. Holmes, B. D. Peterson, R. W. Cilke, C. Navarro. 2005. Federal Building and Fire Safety Investigation ofthe World Trade

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

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

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

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

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

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

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

Investigation ofthe World Trade Center Disaster: Contemporaneous Structural Steel

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

Gaithersburg, MD, September.

NIST NCSTAR 1-1A, WTC Investigation XXI
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 of the World

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

NCSTAR 1-3C. Nationallnstitute 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 of the 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 of the World Trade Center Disaster: Post-Construction Fires Prior to September II, 2001. NIST NCSTAR 1-4A. National Institute of Standards and Technology. Gaithersburg, MD,

September.

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

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

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

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

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

and Technology. Gaithersburg, MD, September.

Ferreira, M. J., and S. M. Strege. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center Disaster: Smoke Management Systems. NIST NCSTAR 1-4D. National

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

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

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

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

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

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

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

September.

xxn NIST NCSTAR 1-1A, WTC Investigation
Preface

Hamins, A., A. Maranghidcs, K. B. McGrattan, E. Johnsson, T. J. Ohlcmillcr, 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. FilHben, and R. G. Gann. 2005. Federal

Building and Fire Safety Investigation of the 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 of the World Trade Center

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

September.

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

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

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

Gaithersburg, MD, September.

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

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

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

MD, September.

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

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

NIST NCSTAR 1-1A, WTC Investigation xxin
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 Safet}> 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 of the World

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

Center?. (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 I-6G.

National Institute of Standards and Technology. Gaithersburg, MD.

Nelson, K. 2006. Federal Building and Fire Safet\' 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 of the World Trade Center

Disaster: Technical Documentation for Swvey 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.

XXIV NIST NCSTAR 1-1A, WTC Investigation
Executive Summary

E.1 OVERVIEW

This report contains a summary of the requirements that governed the design and construction of World

Trade Center (WTC) buildings 1, 2, and 7. It includes specific information related to the following items:

(1) Provisions used to design and construct the buildings; (2) Tests performed to support the design;

(3) Criteria that governed the design of the vertical and lateral load resisting systems and the hat-truss systems; (4) Methods used to proportion structural members and other components of the buildings;

(5) Innovative systems, technologies and materials, and acceptance procedures used by Port Authority of

New York and New Jersey (PANYNJ); (6) Details of variances to contract documents granted by

PANYNJ; (7) Fabrication and inspection requirements at the fabrication yard; and (8) Inspection protocol

during construction. Documents and structural drawings that were used to accomplish these tasks were

acquired from the following locations: (1) the offices of the PANYNJ in Newark, New Jersey, and New

York City and (2) the National Institute of Standards and Technology in Gaithersburg, Maryland. Paper,

microfilm, and electronic versions of the documents were obtained from these locations. Due to the physical condition of some of the documents, certain portions of some of the documents were illegible.

Such items are noted throughout this report. Appendixes to this report include copies of referenced

documents.

E.2 PROVISIONS USED TO DESIGN AND CONSTRUCT THE BUILDINGS

E.2.1 WTC 1 and WTC 2

Minora Yamasaki & Associates and Worthington, Skilling, Helle & Jackson (WSHJ), the architectural and structural engineering firnis, respectively, for the project, were instructed by the Port of New York

Authority (Port Authority or PONYA) in May of 1963 to prepare their designs for WTC 1 and WTC 2 in accordance with the New York City Building Code. At that time, the 1938 edition of that Code was in effect. In September of 1965, the Port Authority instructed the consultants to revise their designs for WTC 1 and WTC 2 to comply with the second and third drafts of the new New York City Building Code

that was under development. The new Code was adopted on December 6, 1968.

Design criteria for WTC I and WTC 2 were established for structural members located inside the core

area and outside the core area. The design dead loads and live loads specified in the design criteria were

greater than or equal to corresponding design loads in the 1968 edition of the New York City Building

Code. Live load reduction requirements given in the design criteria were equal to or more stringent than

Code requirements.

Wind forces on the towers were determined based on a series of wind tunnel tests that were conducted at the Colorado State University (CSU) and the National Physical Laboratory (NPL), Teddington,

Middlesex. United Kingdom. Such tests were permitted by the Code to determine wind pressures in lieu of those tabulated in the Code. Design shear forces and overturning moments on the exterior columns and

spandrel beams due to the wind forces were computed at each floor level from data obtained from the wind tunnel tests.

NISTNCSTAR 1-1A, WTC Investigation XXV
Executive Summary

According to the 1968 edition of the New York City Building Code, structural steel members were to be

designed and detailed in accordance with the requirements in the 1963 edition of the American Institute of

Steel Construction (AISC) Specification for the Design, Fabrication, and Erection ofStructural Steel for

Buildings, with some modifications.

The allowable stress method in the 1963 AISC Specification for the Design, Fabrication, and Erection of

Structural Steeffor Buildings was used to proportion the exterior columns and spandrels for the combined

effects of axial compression, bending moment, and shear due to gravity and wind forces. Composite floor

trusses that were used outside of the core area and the truss seat connections at the core and the exterior

columns were also sized based on the AISC Specification. The allowable stress method was also used to proportion the members in the hat trusses that were located between the 107th floor and the roof in WTC 1 and WTC 2. In the core area, composite steel beams, columns, and their connections were

designed by the appropriate requirements in the 1963 AISC Specification as well. The ultimate strength

method in the 1963 edition of the American Concrete Institute (ACI) Building Code Requirements for

Reinforced Concrete was used to design the concrete floor slabs in WTC 1 and WTC 2. This edition of

the ACI Standard was referenced for concrete design in the New York City Building Code.

E.2.2 WTC 7

WTC 7 was designed and constructed as a "Tenant Alteration"' project of a consortium comprised of

Seven World Trade Company and Silverstein Development Corporation. The specifications for the WTC 7 project required that the structural steel be designed in accordance with the 1968 edition of the New York City Building Code, edited and amended through January 1, 1985, and the 1978 edition of the AISC Specification for the Design, Fabrication, and Erection ofStructural Steel for Buildings.

Design load criteria for WTC 7 were found on one of the structural drawings for this building. In the case of dead loads, the reasonableness of the design values for the superimposed dead loads could not be

ascertained, since the actual materials used for partitions, flooring, and ductwork were not specified. The

live loads in the design criteria were equal to those in the 1 968 New York City Building Code at the

floors where the type of occupancy was noted. No documents were found that indicated what live load

reduction was used.

No design criteria or calculations were found for WTC 7 with respect to wind loads. However, a wind

tunnel study of WTC 7 was carried out in 1983 by the University of Western Ontario at the request of the

structural engineer of record, Irwin G. Cantor, Consulting Engineers. No document is available to show

how the wind tunnel test results were used in the design of WTC 7.

E.3 TESTS PERFORMED TO SUPPORT DESIGN INNOVATIONS FOR WTC 1 AND WTC 2

A series of five different test programs were performed on components used in WTC 1 and WTC 2. A

brief description of these tests follows.

xxvi NISTNCSTAR 1-1A, WTC Investigation
Executive Summary

E.3.1 Exterior Wall Panel Tests

Scale model tests were perfomied at the University of Western Ontario to detennine elastic loaddeflection characteristics of typical exterior wall panel units along the height of the building. One of the main goals of this test program was to determine how the overall stiffness of the wall panels changed as changes were made in the columns, spandrels, and stiffeners that made up the wall panels.

A subassembly of a wall panel was tested, which, according to the researchers, was chosen for its simplicity, flexibility, and low cost. Models were built to a scale of one-quarter of full size and were

fabricated from sheets of thennoplastic polymer. The forces that were applied to the test models simulated

the forces acting on a unit of the actual wall panel.

Deflections and rotations were measured during the tests, and the shear stiffness of a unit was determined

by dividing the load by the deflection. A number of conclusions from these tests, such as the thickness

and depth of the spandrel increases the shear stiffness of the wall panel, were reported to WSHJ.

E.3.2 Wind Tunnel Tests

Wind tunnel tests were part of a four-pronged wind program that was developed by WSHJ for WTC 1 and

WTC 2. The elements of this program were:

• Meteorological Program

• Wind-Tunnel Program

• Structure Damping Program

• Physiological Program

One of the goals of the meteorological program was to detennine the variation of extreme wind speed

with respect to direction at the WTC site. Data from five different sources were examined to help

accomplish this. A statistical model for estimating extreme wind velocity was developed, and it was

reported that the agreement between the observed distributions based on the data from the five sources and the theoretical distribution was satisfactory.

Another goal of the meteorological program was to detennine a suitable mean wind velocity profile as a function of surface roughness. A relationship was found that was reported to represent adequately the

distribution of wind speed with respect to height and exposure, based on data from two of the sources mentioned above. A suitable averaging period for the design wind speed was also studied. A 20 min

averaging period was chosen based on the following considerations: ( 1 ) based on wind tunnel

observations, a 20 min averaging time allowed steady-state response of the towers to develop, and (2) the sampling period used for the CSU wind tunnel tests generally corresponded to approximately 20 min.

In order to obtain representative measurements of wind in the neighborhood of the WTC, anemometers

were mounted on two buildings, close to the WTC site in lower Manhattan.

NISTNCSTAR 1-1A, WTC Investigation xxvn
Executive Summary

Wind tunnel tests were conducted at the CSU and the NPL located in Teddington, Middlesex, United

Kingdom. Tests were conducted on single-tower and twin-tower configurations subject to unifomi and

turbulent flow.

Over 2,000 tests were conducted at CSU to study the behavior of rigid and aeroelastic models. The

directions chosen for the wind tunnel testing of the models of lower Manhattan corresponded to the most

turbulent (southeast direction over Brooklyn) and the least turbulent (southwest over open water)

directions. These two directions were simulated in the wind tunnel. It was found that the models of both

towers oscillated in the wind due to vortex shedding, gust buffeting, and wake buffeting under certain

combinations of key variables in the tests.

Two hundred tests were perfonned at CSU to study the effect of tower spacing on the response of the

buildings. It was concluded that the "as planned" spacing was satisfactory.

Part of the purpose of the aeroelastic tests performed at CSU was to provide a comparison between the

results obtained from the CSU and NPL aeroelastic tests. According to the report by WSHJ, the results from these two locations were in good qualitative and quantitative agreement. In general, these tests indicated that large lateral deflections at the top of the buildings occurred transverse to the direction of the wind for wind velocities in the range of 125 mph to 130 mph for angles of incidence within

approximately 10 degrees of nonnal to a building face.

Tests were also conducted at CSU on the southeast and southwest models of lower Manhattan subjected

to turbulent flow conditions. Similar to the other tests, the most severe oscillations were transverse to the wind and occurred with the wind blowing within a small range of angles on either side of the nonnal to a building face.

Pressures were measured at various points on the model based on an equivalent design wind velocity of

approximately 98 mph. The equivalent design wind velocity was defined as the mean wind velocity

averaged over a 20 min period at a height of 1,500 ft above the ground and based a 50-year return period.

An averaging process was used to detennine average pressure coefficients on the towers in the two

principal directions. Shear force and overturning moment coefficients were detennined from these

average pressure coefficients. As discussed above in Sec. E.2.1, these coefficients were used to design the

exterior columns and spandrels.

No documentation was found on the structure damping program or the physiological program.

E.3.3 Damping Unit Tests

Two programs were carried out to test certain important properties of the damping units that were used in WTC 1 and WTC 2. The purpose of the damping units was to supplement the tubular steel frame in limiting wind-induced oscillations to levels below human perception. The Minnesota Mining and

Manufacturing Company (3M), the manufacturer of the damping units, conducted one series of tests, and

Dr. S. Crandall conducted the other set of tests at the Massachusetts Institute of Technology. The main

goal of these tests was to verify the mechanical and physical properties of the damping units that were

given in the specifications.

XXVIIl NISTNCSTAR 1-1A, WTC Investigation
Executive Summary

WSHJ produced a report that compared the resuUs from the two test programs. Major differences

occurred with respect to the ultimate shear strength of the damping units. According to the tests conducted by 3M, the shear strength of the units was satisfactory with respect to the design parameters,

whereas, the tests conducted by Crandall showed that about twenty percent of the damping units would be

near or over the ultimate shear strength, which implies that they would fail in shear. According to the WSHJ report, the reason for this discrepancy may have been due to the differences in the test set up used

in the two programs.

E.3.4 Floor Truss Tests

Full-scale flexural tests were perforaied on the floor trusses used in WTC 1 and WTC 2, in accordance

with the design specifications. A minimum of one test was required for each of the 23 different types of

floor trusses designated in the design drawings. The Laclede Steel Company, the manufacturer of the

floor trusses, performed all of the tests. Results were found for one of the floor truss shipments in May 1969, which included a comparison of the design deflection (camber) versus the measured

deflections from the tests for various target loads.

Tests were also performed on the shear knuckles (i.e., the floor truss diagonals that extended above the top chord and embedded in the concrete slab). These knuckles acted like shear studs, which made the

floor trusses and concrete slab act in a composite manner. The Laclede Steel Company performed all of

the transverse and longitudinal shear knuckle tests. Results from these tests showed that the shear

strengths of the knuckles embedded in concrete were well above the allowable values assumed in design.

The Laclede Steel Company also conducted tests to verify the horizontal and vertical design loads for two

welded connections between the 32 in. deep floor trusses and the 24 in. deep bridging trusses. Average

measured failure loads for both types of connections were equal to at least twice the design values.

Two types of tests were performed by the Laclede Steel Company to determine the bearing capacity at the ends of the floor trusses. The bearing strength of the as-designed floor trusses and the bearing strength of

repaired bearing ends were both detennined. For example, bearing ends were repaired because they were damaged during transportation from the manufacturer. In both cases, it was shown that the bearing

capacities of the floor truss ends were greater than the design loads.

E.3.5 Stud Shear Connector Tests

A testing program was established to determine the horizontal shear capacity of 3/4 in. diameter by

4 1/2 in. long stud shear connectors welded through the troughs of Roll Form Type "B" steel deck and

embedded in a lightweight aggregate concrete slab. Such tests were required by the 1963 AISC

Specifications, since the lightweight aggregate used in the concrete slabs for the WTC buildings did not conform to the ASTM International specification for nonnal weight aggregates. A work order from the Port Authority was sent to the Fritz Engineering Laboratory at Lehigh University to perform the tests. It has not been possible to locate any results from this testing program. No evidence was found that this system was used in WTC 1 and WTC 2.

NISTNCSTAR 1-1A, WTC Investigation xxix
Executive Summary

E.4 PANYNJ POLICIES AND AGREEMENTS WITH NEW YORK CITY

DEPARTMENT OF BUILDINGS

In 1993, a memorandum of understanding was established between the Port Authority and the New York

City Department of Buildings. The purpose of this document was to restate the "long-standing'" Port Authority policy that its facilities meet or exceed New York City Building Code requirements. Specific

commitments were made by the Port Authority to ensure that any building construction project

undertaken by the Port Authority or by any of its tenants at buildings owned and operated by the Port Authority would confonn to the New York City Building Code. For example, the Port Authority was to thoroughly review and examine all plans for conformance with the requirements of the then-current New

York City Building Code. Plans for projects undertaken by Port Authority tenants were to be prepared

and sealed by a New York State licensed professional engineer or architect retained by the Port Authority.

Also, the Port Authority was to maintain a file containing the most recent drawings, plans, and other

documents required in connection with the review of the project for code conformance. Any variances

from code requirements on a project were to be reported by the Port Authority to the New York City

Department of Buildings, and the Port Authority was required to perform building inspections and

structural integrity inspections on a cyclical basis for all of its buildings located in New York City.

A supplement to this agreement was executed in 1995. The supplement added that the design professional

responsible for perfonning the review and certification of plans for WTC tenants must not be the same

design professional providing certification that the project had been constructed in accordance with the plans and specifications.

E.5 INNOVATIVE SYSTEMS, TECHNOLOGIES AND MATERIALS, AND

ACCEPTANCE PROCEDURES USED BY THE PANYNJ

E.5.1 Innovative Features of the Structural System

The structural system, comprising the lateral-force-resisting as well as the gravity-load-carrying systems,

of WTC 1 and WTC 2 towers incorporated several innovative features including the following:

1. The towers represented one of the earliest applications of the framed-tube lateral-force- resisting system to super high-rise buildings.

2. Unifonn perimeter column geometry (14 in. by 14 in. cross-section) was maintained over most of the height of the 1 1 0-story buildings.

3. Fourteen different specified grades of steel were used to allow the perimeter column

geometry to remain unifonn throughout the heights of the buildings.

4. Deep spandrel plates were used as beam elements cormecting perimeter columns, enabling

framed tube action by strapping around the structure.

5. Prefabrication of steel construction was extensively used, through using 3 -column-wide by

3-stories-high panels, bolted butt-plate column splices, and high-strength bolted shear

connections of the spandrel beams (plates).

XXX NISTNCSTAR 1-1A, WTC Investigation
Executive Summary

6. Specially designed corner panels with chamfered edges were used to facilitate force transfer around the corners of the framed-tubes.

7. Long-span floor trusses were used for the floor systems. Composite action was achieved

between the floor trusses and the concrete floor slab by extending the truss diagonals above

the top chord into the slab. The concrete floor slab acted as a rigid diaphragm, which

distributed the lateral forces to the elements of the tube according to their stiffnesses.

8. Viscoelastic dampers connecting the floor trusses to the perimeter framed tube system were

used in each tower to control dynamic response.

9. Extensive wind tunnel testing was perfonned to establish the lateral wind loads used in the design of the towers.

Except for Items 7 and 8 above, the innovative features were not appraised by acceptance procedures. Tests to support the design innovations were done for Items 5, 7, 8, and 9.

E.5.2 Lateral-Force-Resisting System of WTC 1 and WTC 2

The structural system that resisted lateral loads in WTC 1 and WTC 2 was considered to be a framed-tube

system (closely spaced columns and deep spandrel beams). The exterior walls were composed of steel columns and spandrel plates, and were designed to resist the lateral wind forces and a portion of the gravity forces. The welded steel plate box columns were spaced 3 ft-4 in. on center above the 7th floor. The columns and spandrels were shop assembled and welded into 36 ft high by 10 ft wide panels that consisted of three columns and spandrel beams. These panels were erected on site. Below the 7th floor, the columns were spaced 10 ft-O in. apart, and bracing was used in the core area to increase lateral stiffness. WTC 1 and WTC 2 were early examples of super high-rise buildings that were designed based on the framed-tube concept. The first application of this type of system was in a concrete apartment building in Chicago that was completed in 1965. Many variations of this system were used subsequently in a number

of buildings between the mid-1960s through the early 1970s.

E.5.3 Damping Units

Viscoelastic damping units were part of the structural system in WTC 1 and WTC 2 to supplement the tubular steel frame in hmiting wind-induced building oscillations to levels below human perception. This

may have been the first application of damping units for this purpose in tall building structures.

The damping units were located between the bottom chords of the floor trusses (and bottom flanges of the beams on certain floors) and the columns of the exterior wall. Approximately 100 dampers were used on

each floor from the 7th to the 107th floor in both buildings. As the buildings oscillated from the wind,

part of the energy of oscillation was dissipated by shear defoniiations in the damping units.

As note above, 3M manufactured and tested the damping units for WTC 1 and WTC 2. Working with

WSHJ, 3M wrote specifications for the damping units, which included a prototype test program that would measure key parameters related to the performance of the units. The specifications also included a

NISTNCSTAR 1-1A, WTC Investigation xxxi
Executive Summary

quality assurance program that contained requirements for both initial and long-tenn (5-year) acceptance

and the test methods that were to be used to determine whether damping units met these requirements.

Since this was the first time that this particular type of damping unit was utilized, there was a need to test the units on a long-tenn basis. No information on the design service Hfe of the damping units could be

found.

E.5.4 Floor Trusses

An innovative feature of the floor system used in WTC 1 and WTC 2 outside of the core area was the way

that composite action was achieved between the floor trusses and the concrete slab. Truss diagonals were

extended above the top chord. This "knuckle" acted like a shear stud, which made the floor truss and

concrete slab act in a composite manner.

Working with WSHJ, the Laclede Steel Company, the manufacturer of the floor trusses, wrote

specifications for the floor trusses. Requirements were given for materials, fabrication, welding, bolting,

and painting. Full-scale tests of the floor trusses, which are described above, were also included in the

specifications, as were requirements for quality control and inspection.

E.6 FABRICATION AND INSPECTION REQUIREMENTS AT THE

FABRICATION YARD

E.6.1 WTC 1 and WTC 2

Fabrication and inspection requirements were contained in the contracts for the floor trusses, box core columns and built-up beams, members of the exterior wall, and rolled columns and beams. In general, the inspection requirements from the specifications for the various contracts were at a minimum equivalent to those in the New York City Building Code, and in many cases they were more comprehensive and

stringent than the corresponding provisions in the Code.

E.6.2 WTC 7

The specification for WTC 7 contained the fabrication and inspection requirements for this project. Structural steel for WTC 7 was to be fabricated in accordance with the applicable requirements in the New York City Building Code, the 1963 AISC Specification for the Design, Fabrication, and Erection of

Structural Steelfor Buildings, and other specifications related to bolts, welds, and painting.

The specification also notes that there was a separate contract for testing and inspection. This contract was not found. However, specific requirements for inspection of shop and field welds by a testing agency were located in the specification.

xxxii NISTNCSTAR 1-1A. WTC Investigation
Executive Summary

E.7 INSPECTION PROTOCOL DURING CONSTRUCTION

E.7.1 WTO 1 and WTC 2

Karl Koch Erecting Co., the company that performed the structural steel erection work for WTC 1 and

WTC 2, developed a quality control and safety program. This program included infonnation on 10

different key areas that were to be addressed during construction.

The WTC 7 specifications contain general erection requirements for fasteners, anchor bolts, column

bases, installation, and bracing. No inspection requirements during construction are given in the

specifications.

The Port Authority approved numerous variances in the fabrication and erection of structural members in WTC 1 and WTC 2. The Office of the Construction Manager at the Port Authority approved deviations to the contract documents after Skilling, Helle, Christiansen, & Robertson (SHCR) (a successor firm

established in New York of WSHJ of Seattle, Washington) reviewed the details of the deviations and

granted their approval. In many cases, SHCR submitted alternative methods, which were incorporated

into the deviations.

The variances that were granted for the structural members and their materials may be categorized into the following groups:

• Deviations relating to fabrication/erection tolerances (box columns, box beams, and floor

Deviations relating to defective components (column trees and floor trusses)

Deviations relating to alternative fabrication/erection procedures (core columns, floor trusses, exterior wall columns, and beam seats)

Deviations relating to product substitutions (exterior wall)

Deviations relating to inspection practice (exterior wall and welds).

E.7.2 WTC 7

E.8 DEVIATIONS GRANTED BY PANYNJ

trusses)

NISTNCSTAR 1-1A, WTC Investigation XXXllI
Executive Summary

This page intentionally left blank.

xxxiv NISTNCSTAR 1-1A, WTC Investigation
Chapter 1 Introduction

This report contains a summaiy of the requirements that governed the design and construction of World

Trade Center (WTC) buildings 1, 2, and 7. It includes specific information related to the following items:

• Provisions used to design and construct the buildings

• Tests performed to support the design

• Criteria that governed the design of the vertical and lateral load resisting systems and the hattruss systems of WTC 1 and WTC 2

• Methods used to proportion structural members and other components of the buildings

• Innovative systems, technologies and materials, and acceptance procedures used by the Port Authority of New York and New Jersey (PANYNJ)

• Details of variances granted by PANYNJ

• Fabrication and inspection requirements at the fabrication yard

• Inspection protocols during construction

Documents and structural drawings that were used to accomplish these tasks were acquired from the following locations: (1) the offices of the PANYNJ in Newark, New Jersey, and New York City and

(2) the National Institute of Standards and Technology in Gaithersburg, Maryland. Paper, microfilm, and

electronic versions of the documents were obtained from these sources. Due to the physical condition of some of the documents, certain portions of some of the documents were illegible. Such items are noted

throughout this report.

NISTNCSTAR 1-1A, WTC Investigation 1
Chapter 1

This page intentionally left blank.

2 NISTNCSTAR 1-1A, WTC Investigation
Chapter 2

Provisions Used to Design and Construct the Buildings

2.1 BUILDING CODES USED IN DESIGN

2.1.1 WTC1andWTC2

In 1963, the Port of New York Authority (Port Authority or PONYA) (whose name changed to the Port Authority of New York and New Jersey in 1972) instructed the architect and consulting engineers to prepare their designs for World Trade Center (WTC) 1 and WTC 2 to comply with the New York City

Building Code (hereafter, referred to as the "Code"), although it was not required to comply with this code or any other building code.' The Port Authority, as an interstate agency created under a clause of the U.S. Constitution pemiitting compacts between states with the consent of Congress, was not bound by

local codes. While not explicitly stated in the 1963 letter, the 1938 edition of the Code was in effect at that time. In areas where the Code was not explicit or where technological advances made portions of it obsolete, the Port Authority directed the consultants to propose designs "based on acceptable engineering

practice," and required them to inform the WTC Planning Division when such situations occurred. When

preliminary designs were complete, the Chief Engineer of the Port Authority was to review all design

concepts with the appropriate municipal agencies before the consultants were to proceed with the final design. According to correspondence in 1 975 from Emery Roth & Sons, the architect-of-record for the WTC project, the New York City Building Department reviewed the design drawings of WTC 1 and

WTC 2 in 1968 and "made six comments concerning the plans in relation to the old code."' The

correspondence goes on to state that specific answers on how the drawings conforaied to the new code

with regard to these comments were submitted to the Port Authority in March of 1968. These comments

and the responses to these comments have not been found.

In 1965, the Port Authority instructed the consultants to revise their designs for WTC 1 and WTC 2 to comply with the second and third drafts of the new Code that was under development, and to undertake

any revisions necessary to comply with such provisions."^ The new edition of the Code became effective on December 6, 1968.

2.1.2 WTC 7

Unlike in the cases of WTC 1 and WTC 2, WTC 7 was designed and constructed as a "Tenant Alteration"

project of a consortium comprised of Seven World Trade Company and Silverstein Development

Corporation. Secdon 5A.3 of the project specifications (WTC 7 Project Specifications 1984) required that the structural steel be designed in accordance with the then-current New York City Building Code and the

' Letter dated May 15, 1963 from Malcolm P. Levy (Chief, Planning Division, World Trade Department) to Minoru Yamasaki

(Minoru Yamasaki & Associates) (see Appendix A).

" Letter dated February 18, 1975 from Joseph H. Solomon (Emery Roth & Sons) to Malcolm P. Levy (Chief, Planning Division, World Trade Department) (see Appendix A).

^ Letter dated September 29, 1965 from Malcolm P. Levy (Chief, Planning Division, World Trade Department) to Minoru

Yamasaki (Minoru Yamasaki & Associates) (see Appendix A).

NISTNCSTAR 1-1A, WTC Investigation 3
Chapter 2

latest edition of the Specification for the Design, Fabrication, and Erection ofStructural Steelfor

Buildings published by the American Institute of Steel Construction (AISC 1978). WTC 7 was designed

in the mid-1980s, and the 1968 edition of the Code, edited and amended through January 1, 1985, was in

effect. It is also noted that references were made on the structural drawings (The Office of Irwin G. Cantor 1983) to specific provisions in the Code. In particular. Note 12 on sheet FS-3 states that inspection

requirements for the foundations shall comply with Code Sec. C26-1000 Tables 10-1 and 10-2.

2.2 SUMMARY OF CODE PROVISIONS

This section contains a summary of the structural provisions in the 1968 edition of the Code. As noted

above, the design of WTC 1 and WTC 2 was based on these provisions. The 1968 Code also governed the design of WTC 7. Also provided in this section are the criteria used in the design of WTC 1 and WTC 2 and WTC 7. Wherever applicable, differences between the Code provisions and the corresponding design

criteria are documented. Only those provisions that relate to the design of these buildings are discussed

here. Unless otherwise noted, referenced article and section numbers are from the 1968 edition of

the Code.'

2.2.1 Loads

Article 9 of the Code contains the minimum loads to be used in the design of buildings and parts thereof. According to C26-900.2, Standards, the minimum dead, live, and wind loads prescribed in Reference

Standard RS-9, Loads, are a part of Article 9. In no case was it allowed for the loads used in design to be

less than the minimum values contained in that article.

Dead Loads

Code requirements. Dead loads are defined in sub-article 901.0, Dead Loads, as the actual weight of the building materials or construction assemblies to be supported, based on the unit weights provided in Reference Standard RS 9-1, Minimum Unit Design Dead Loads for Structural Design Purposes

(C26-901 .1 ). Weights in pounds per square foot (psf) of floor area are hsted for various types of (a) walls

and partitions, (b) floor finishes and fills, (c) ceilings, (d) roof and wall coverings, (e) floors (wood joist

construction), and (f) miscellaneous materials. Actual weights may be determined from analysis or from

data in manufacturers' drawings and catalogs, but in no case were the unit weights allowed to be less than

those contained in Reference Standard RS 9-1 unless the Building Commissioner approved them.

Weights from service equipment (plumbing stacks, piping, heating, ventilating, and air conditioning, etc.) and partitions were also to be included in the dead load (C26-901.2 and C26-901.3, respectively).

Design criteria for WTC 1 and WTC 2. The unit dead loads specified for the various structural members are contained in the Design Criteria for WTC 1 and WTC 2 (WSH.T 1965a). Different criteria were established for members located inside the core and outside the core. Definitions for member

locations in the floor plan, as well as other definitions that are used throughout this report, are shown in Fig. 2-1. Note that the definition for "Code wind load" in Item 1 1 of this figure is illegible.

In the 2001 edition of the New York City Building Code, "subchapter" is used in place of "article" and "article" is used in place of "sub-article."

4 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

WORTHINQTON, SKILLINO. HEtLE & JACKSON

Omad^tClvUmdStrnttuaiBni^mm ' 239Pmk AvmM,N»wYo*k,N.Y, KX>t7 • Mii.»SS74

TBE NOSLD TXADS CER1C»

DC

1. "Floor ic«id« of cor*", thic part of cb« floor beuadtd by the oottid*

f«c«« of colvsiss* 501, 508, 1001 m4 1038.

2. "floor outside of core". Tbst part of ehc floor bctveex th*. outsidr

v«lU «»i cb« "Floor i&ai4ie of cor*".

3. "fi»4* ltv« lo*^. tkiit load «fftclfi«d ia Um» Kotr Torfe S«illdlag Co&

for a j^wa occn^aa^.

4. "Llv* lowi for floor desiga". Tb« accuaI Itv« loa4 used for the d«sl^

of tfa* parts of the floor «hicb loml aay aoc li« les» tban thes "Codo live

loAd**, «nd SMqr be reduced for crll>utarjr Area £S di«fined lai "Live lo«d

rtductlon".

$, *'Uv« load for eoium design*'. The cod« l|v« lo«4) r«due«d aa defised

la "Live load reduction" for eotaxm*.

i, "Ca«i8tructici> 4«sBd load". Ibc veij^t of tht b«re strvcture (i.e. xht

bft«M «»d e2i^ tt»«d in thi d«sigft of tmshored ccis^osite bea^.

7. ^te$tx«cti%« live loud"*. Tb« ^ilisumm for th» «eisbt of az^ equtpoeat

*ad/or form ^idh is ooit {t«z^«s«ac «ad dote aot font pi^rt of ch« total

9* "^BiMJl^pofed d««^ loui**. 'Zbe wei^C of ceilis^;;*, floor fisisb, valla

or iMcrtltio^ of l»owa loeatioe, anchaaical «s4 olaccrlcal equipaene asd

• l«'.lAr lt««« aoc included in "Svptxiitp^fti live Icjid" or "Constrccttoa

d«a<5 ioA<r.

5. llb^^ad". avs of iteos 6 m4 8 sbova.

*'Stiii«ri«^0i«d livo lead**. The weight of the dtaige live load, based ce

oecupartey, plua tix ve&ght of particiooa tf thalr loeacios ia aubjacc to domgo.

21. "ChMk tfiod load". 3& eccord^ce vith tb« Kev Yod( Suildine Code cis.

.

Tti« .V • »*' f

*

*Si»"'' low - ' '

12. ^fartttioaa". Ibe wai^t of «oval>la pa«ttttoft« «» a^ociflad by the

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-1. Definitions used in design criteria for WTC 1 and WTC 2.

Detailed documentation is given in the Design Criteria (WSHJ 1965a) for the loads used in the design of

WTC 1 and WTC 2. In this report, samples from the Design Criteria are shown to illustrate the types of

loads that were specified in the various portions of the buildings.

NISTNCSTAR 1-1A, WTC Investigation 5
Chapter 2

• Floor inside of core. The core area in a representative upper floor of WTC 1 and WTC 2 is

illustrated in Fig. 2-2. Unit design dead loads for the beams, columns, and slabs within the core area of the towers are summarized in Fig. 2-3.^

Source: Reproduced with permission of The Port Authority of New Yorl< and New Jersey.

Figure 2-2. Core area in a representative floor plan of WTC 1 and WTC 2.

In all cases, the dead loads in the design criteria were greater than or equal to the corresponding dead loads prescribed in the Code. A list of the dead loads prescribed in the Code is given in Annex Al of NIST NCSTAR 1-lB. References to the "NY Code"

equivalent uniforai loads for partitions (according to C26-901.3(b), the equivalent uniform

^ In Fig. 2-3, "contact" fireproofing is listed. This is a type of fireproofing that is sprayed on to steel members.

6 NIST NCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

partition loads in Reference Standard RS 9-1 may be used in lieu of actual partition weights

when partitions are not shown on the plans) are given in the Design Criteria as well (see

Fig. 2-3).

TORTHMCTOH, SKILUHC, NELLE & JACKSON THE WORLD 'iKSUE CENTER eSSJ S^Homrv CATE S/S/S'S No. MtWOfiU VAMAS**SJ * A550C. e»»«€RY ROTH ft SONfi pwepARtc ^ A/ /- APPROVED ( 7

Confaci (19 per) I" CCr.->cn7iTiQus {24 per) /

lOlO OYC.l'- Zte^J •..v'^cVv;

C.o;-;ci"c7e SIoh , Stone Concrdf^

^U'.->pf^ndeci a'umj'num pan

.

"flic , cone, ioppyir.o . f'2rrocz.7:.o

Ceramic 6'/i'g.h on/y)

7 . \00 r

31.5

42

I50

2

to

' r

pC".

13

/5

5 os^-^-

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-3. Design criteria for WTC 1 and WTC 2: floor inside of core - unit dead load.

NISTNCSTAR 1-1A, WTC Investigation 7
Chapter 2

Mi t (tNMHtttI SngkMH*

THE WCMD TRADE CENTER SiS JJri,o«Tr

UmtHtU VAMAMKt ft ASSOC. EMKRy ROTH * SOMS

/ o

TLOOR - lU^lPZ OF CORE

2 GypQOm plank

^2" G\^psum hoard'

^2' <3^psum bdard

3\Gypsum pfanfC

-'2^ Sypisuhn board

^S" Gypsu/T) hoards

Z^^MpSutr^ p^ank

^2 x$^'yp:>or7i board

/.

2

psf

z

'2 n

-S^G^pSUrrr pfonK

2"

Gypsum board

'

Syp3um board

2 . ^, . 2

2 ^ l&^z psf

Ccram/c 77 'e C6' h/gh on/y) 4^

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-3. Design criteria for WTC 1 and WTC 2: floor inside of core - unit dead load

(continued).

8 NISTNCSTAR 1-1A. WTC Investigation
Provisions Used to Design and Construct the Buildings

irOinHINCTOM, SKtLLWa, Kf LLE & JACKSOK THE WORID "nRADE CENTER ?SS;

:

•tCMBC MOTH, «iic«i-i.cr PBEPAHCD « APPROVED 6

CRITERIA f='OR OZS/Cr.'^J

FLOaR //l,'9/.O.e OF CO.^EL

LOAL

CI !" • P, Ojcji-y-i n rtrc. proo f i /7i

Ce.nn en fifto U c

' ( ?4 por) _ Gyr^sum piazi^r i.'j/rnefc^/ foirh

3": Sos:

Pa"- 10 *

THE Tr»jy jr^ TlUOr. Cl!MlfJr

ft*

FLOOr^ SLAB ' PAR 7mo'V ^ O

5

.'>VO

Source: Reproduced with permission of Tine Port Autinority of New York and New Jersey.

Figure 2-3. Design criteria for WTC 1 and WTC 2: floor inside of core - unit dead load

(continued).

NISTNCSTAR 1-1A. WTC Investigation 9
Chapter 2

IWOSTHINCTOX, SI(ILLIN6, HELL£ t JACKSON THE WORLD TRADE OENTER J:,..«-„.v Me.

tt.nonu fAUAi*'i% « ASSOC. ewEnv hoth a fO\$

.0£AC

So'ip&nJ.^c*' marc-: la^h $

2 p.sf

/oper

Floor

li€ . cone iopptn :? > ierra-zo I't' }3

24

tTh

Ml ofncrz (zKce-ot cxpozzd eohc-') 2 " 'ncchanicctl (/•'cchonir.or, rloor-^ ) weight

(All ather) 'S-psf

Sheds tC n ii 3

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-3. Design criteria for WTC 1 and WTC 2: floor inside of core - unit dead load

(continued).

• Floor outside of core. Unit dead loads for areas outside of the core area are specified in the Design Criteria with respect to the following structural members: one-way long-span floor

trusses, one-way short-span floor trusses, two-way floor trusses, beams on framed floors,

bridging, columns, steel deck, and reinforced concrete slabs. The design criteria also changed

depending upon the floor level. Figure 2-4 contains sample design criteria for the long-span

floor trusses at typical floor levels and for beams on some of the framed floors (i.e., mechanical floors). See WSHJ (1965a) for all of the design criteria. The dead loads in the design criteria for all of the structural members were greater than or equal to the corresponding dead loads prescribed in the Code.

Design criteria for WTC 7. Design load criteria for WTC 7 are summarized in Fig. 2-5. These criteria appear on Sheet S-24, Typical Superstructure Sections and Details, in the structural drawings (The Office

of Irwin G. Cantor 1983). Because the actual materials used for the partitions, flooring, and ductwork were not specified, the reasonableness of these design values cannot be ascertained.

10 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

»ORTHINCTON, SKILLINC. HEULE 6. MCKSOK

jcivil & Sirucrura' Englnocrt

THE WORLD TRADE CENTER ^SSI SCthor.tv DATE « / Ok.J No.

Mlr;;BUI YA»/,ASAKI ft ASSOC. EMERY ROTH A SONS

MINOIOJ V^UAUAKI, *»CHJTCCT IHCHAHO IH^n:, *«CK1TIC7 OSEPAREC BY \/t TF

APPROVED I o

(ZRiT.Z.Rm. f-IOR O..E.3l(3M. 0.£S/OU._ COAO

Conerere SIaJo

3!ah /<'ein-f'.

Sieai Oeok

Sirue -iurai Sieel

3G.S p$f.

1.5 ; loo „ CO/^STRUCriO/^ DEAD LOAD^ 50-0 psf\

M^echdflic Oil t^IecTrlcal

rjrepraz) tlnc^

Flaar Finish

SOPEf^lMPOSEO /DEAD LOAD

\ FLOOR'S

\ lO ihru 14

I 2G ihru 4c

\ Gorhro

1 G&rhro 74

iqihro 9/

<?S Thru lOi. J

2,o

B'd •pt>f

rOTA^L 0£A/D LOAO SS.o psr.

(to he u$eal roi" coh-JoeA)

SUPERlMFOSBFj LIVE LOAO

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-4. Design criteria for WTC 1 and WTC 2: floor outside of core - unit dead load.

NISTNCSTAR 1-1A, WTC Investigation 11
Chapter 2

«8irrNiiiOT«N, wtuwe,

Mlt.l.E4J4eKSOI»

CMI 1, itracml fatvttm.

THE WORLD TRADE C3E>nTR Itl IZr^m-n. O.T, N/t l&>B I- CRITER}Pi FOR D£.SIGi\f - DE5ION LOAL

FLOOR OUrS//D£ OF CORB.

3E/^M FRAME/D hLOOJ^S

Conere ie Slab C&'avd) <? IBOncf' !00 ns^

Slab Rei^A ^ ^ 3 \

Steel Oeck •? Sfe&l B>ean-i 2o

COMSrRUC7/OfJ D£AD LOAD ^I2S psf

FLOOR

1

77

F/oor Finish 2 ps/

Flteproo^inq 3 ' h Mechanlca/ SO

SOPER/MPOSED DEAD LOAD 55 ps/ TOTAL OEAO LOAD IQO ps/

{/o 6e used -for OSmh&r)

SuperiMposBO live load

(Set sht&f fSF7i^

""ISItTll'lS'^"**'; THE WORLD TRAOT OTHSl 3S TSSSSr-e^r, /////4>5 «»

lb. *>Mr*.|i(UI / CRITERIA FOR PES/GAJ DESIGN LOAD

FLOORS OUTSIDE OF COPE.

Concreis Slob (^'2 Ovg) <S>/50pcf pS^

Slob R&inf. 3

Sieel Deck 2

Steel R>eatn 20 -

FLOORS

IB

COUSTRUCTlOkl DEAD LOAD - psf

Tofpirn^ 4 Mcmbt-aine. ("Z'^') 31 psi

Firepnoo fln^ 5 1

Ceiling 3

• '5UPES.IMPOSS.O DEAD lOAP'*^'^^ P^f

70TAL PEAD LOAD */55

SUPEIZIA^PO^EO U\JE LOAD 15 psf ^pl(jz mechanical £<su!pm&n'/ /coof.

(assume cqutv. uni^rm load of 15psf}

Source: Reproduced with permission of The Port Authority of New York and

New Jersey.

Figure 2-4. Design criteria for WTC 1 and WTC 2: floor outside of core - unit dead load

(continued).

12 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

LOADING SCHEOULt

BCAI^^S, TOTAL

iOAt>

Life lCap

rcrAi LOAD

£0 S 7 75 30 los

24"iJlt'XfL. 59 5 12 S 76 60 12$

50 } 12 i$ 8 90 1$

6<f, 5 i2 93

s>vrM fwe SO 1 55 $ s2 27 i4S 50 715

y^r^Kf SfJKtI.— S 12 7 $i> SO

' S2 3& 55 t^i too ess

\ orr^t

• si>4ce !50 33 $ n 210 5Q 260

iiS 3$ 17 2io ISO Utf

75 5 10 it 6 il^O 246

75 !0 iOO 2't6

240 iOQ

Si 60 5 \0 150

1

' 2&1 seo \

1*'noon ilS SO SCO

SUBSTATtOH 3C0 225

Source: Reproduced with permission of Tine Port Authority of New York and New Jersey.

Figure 2-5. Design load criteria for WTC 7. Live Loads

Code requirements. Requirements for live loads are given in sub-article 902.0, Live Loads, of the Code,

with specific requirements for floor live loads given in C26-902.2. Minimum design values for uniforaily

distributed and concentrated floor live loads for various occupancies are contained in Reference Standard

RS 9-2, Minimum Requirements for Uniformly Distributed and Concentrated Live Loads (C26-902.2).

For occupancies that are not listed, design live loads are to be determined by the architect or engineer

subject to approval by the Building Commissioner. Provisions are also given on how to apply

concentrated live loads so as to produce maximum stress.

Design criteria for WTC 1 and WTC 2. Specified live loads are given in the Design Criteria for WTC 1 and WTC 2 (WSHJ 1965a). As in the case of dead loads, different live load criteria were established for members located inside the core and outside the core. Samples from the Design Criteria are shown in this report.

NISTNCSTAR 1-1A, WTC Investigation 13
Chapter 2

• Floor inside of core. Live loads to be used in the design of the beams and the columns in the core area are summarized in Fig. 2-6. As can be seen from the figure, except for Floor 109

and areas occupied by equipment, the design live load varied from 40 psf to 100 psf. A

modification to the design criteria for Floor 109 was made in December of 1976, as indicated on the second page of Fig. 2-6. This modification required that the beams on the 109th floors

in WTC 1 and WTC 2 be designed for a live load of 150 psf Also, notes regarding the design

criteria for WTC 1 were added in June of 1989. These notes were applicable to the beams in the tenant space inside the core on floors 27 through 40, 60, 61, 68 through 74, and 90

through 105, as indicated on the third page of Fig. 2-6. For all occupancies or use of spaces

common to the design criteria and the Code, the live loads in the design criteria were equal to the corresponding live loads prescribed in the Code (which are given in Annex Al of

NISTNCSTAR 1-lB).

KEUf 1 JACKSON

1 MIM^qu VAMASAKl a ASSOC EMERY ROTH 6 SQNS

»|CH*P0 ROTH AltCHITSrT

pn CPARID Ely APPROVED t I/-

l£)0

!0C

Cor", c". too

ICO

/CO

75

Evp:>n:,'on -hfinfr t£:c?rr> loO

IOO

J: 1 <^ 'V •

Uor.-il p:^:rr£^,i.^^ &Jev^ r.ur- (s^<'J

l^ti^'j t^'^

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-6. Design criteria for WTC 1 and WTC 2: floor inside of core - live load.

14 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

WORTHINOTON, SKIIUNC. HEIU « JACKSON THE WORLD TRADE CENTER l,r^''?Si^tvr^on,rY MiKOnU YAMAGAKI & ASSOC. EMEftV ROTH 6 SONS

niCHARO ROTH. ARCHlTtCT

rREI>«RED BY

Shcal I No. I

„.v^£ ^a^D". i^/,oo;-i Or.'S/OM ' c / i 'j^

'

too

7p

/oo

loo

7^ ho-f'^ /

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-6. Design criteria for WTC 1 and WTC 2: floor inside of core - live load

(continued).

-40

/so

NISTNCSTAR 1-1A, WTC Investigation 15
Chapter 2

LESLIE E. ROBERTSON PF.CJECT ^07°i» SHEET NO ASSOCIATES PREPARER ,4 STRUCTURAL ENGfNEERS CHECKER

1"^' ^

74

I ^OO f'Sf^ t*V£

V

1/

(OF ioo psf, Co^mq^^i^/it^ c^piji^ ^^^cpa

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-6. Design criteria for WTC 1 and WTC 2: floor inside of core - live load

(continued).

16 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

mUC I MCKSON CM & Sfrudtirol tn^ntnn

THE WORLD TRADE CENTER ;".^7<^\'Xutho«.t^ ImT' MINORJ YA».-A&AK( A AS^C LUfiny NOTH 6 SONS PBEfABEDBY ^/i/ 2

FLOPA //v5.A0£ Q- CORB

U'jB load FO^ gjiaVfA/

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-6. Design criteria for WTC 1 and WTC 2: floor inside of core - live load

(continued).

NISTNCSTAR 1-1A, WTC Investigation 17
Chapter 2

WOKTMNCTON, SMtUNOi mxt ( MmoN

Qril & S'rucHiial &n«inc«n

THE WORLD"TRADE CENTER l1:V?S^^°^vTHo^,rf CAT* 3/3/GG »<«»

MINOR'J TAMASAKI & ASSOC eM£ftY RCTH ft SONS

ktisomj VAMASAKt. 4SCHITCCT l«CH*l»0 «OTM. *«CMtT«CT PRVARBC or v/A/ CCi 2

/ \3

CRITERIA FOR DESIGN

FLCQQ '\SIDS COR£

fm. 1 »^ti» ' if -t 1

Men's /jp/.'igr'j'

paSS£^^$^ iZ.M-y'i^rc.'' /^OOJ'tf ^^r7i^*7r -r'/ocirs) 76

.40

75

r£j7a^-^ spaces u/if^yhi Cors^ -hCs ^opf- m^^i'd

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-6. Design criteria for WTC 1 and WTC 2: floor inside of core - live load

(continued).

18 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

• Floor outside of core. Like the unit dead loads, design live loads outside of the core area varied with respect to the floor level. At most floor levels, a design live load of 100 psf was

specified for the slabs (see Fig. 2-7 from the Design Criteria). Note that this live load is greater than the 50 psf live load specified in the Code for office occupancies without storage.

At mechanical floors 7, 41, 75, and 108, a 75-psf live load was used (also sec Fig. 2^).

Figure 2-8 contains sample design criteria for the columns at the floor levels noted in the

figure. In this case, live loads specified in the design criteria were equal to the corresponding

live loads prescribed in the Code. Design live loads for the floor trusses, which are specified

in the Design Criteria, are discussed in the following section on live load reductions.

WOSTHiNCTON, SKILtlNCj

H£l.Ue 3, JACKSON

vl! & S(rvc(Kro! Er^immi EMERY ROTH « SONS

DATE '/'3/GG'

PHEPASEC BY A)»P«OVCC CRrrB.R!A FOR OEJolQlJ

i<e.!n-!OrcinQ oms/

Floor h If^/i/?- A-Cf>li.^/:/' 7l/^

OBAD LaAO

Ll\JE LOAD

TcdTAL^ load

FLOORS

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-7. Design criteria for WTC 1 and WTC 2: typical floor slabs outside of core

live load.

Design criteria for WTC 7. As noted previously, design criteria for WTC 7 are summarized in Fig. 2-5.

These criteria appear on Sheet S-24, Typical Superstructure Sections and Details, in the structural drawings (The Office of Irwin G. Cantor 1983). For the floor levels where the type of occupancy was

noted on Sheet S-24, the live loads in the design criteria were equal to those given in the Code.

Live Load Reduction

In general, building codes allow live loads to be reduced below code-prescribed values, since it is unlikely

that an entire floor area will be fully loaded with the design live loads. For example, the probability is small that a column in the lowest floor of a multistory building would have to carry the full codeprescribed live load on all of the supported floors above. The same is true for floor members, such as beams or trusses, that support live loads on only one supported floor: smaller live loads are expected on

NISTNCSTAR 1-1A, WTC Investigation 19
Chapter 2

members that support larger floor areas. It is important to note that codes generally limit the maximum

amount of live load reduction that may be taken on a member (depending on the type of member, the area

it supports, and the type of live load) and that live load reduction is not pennitted in all situations.

mnjHmormi. skillihg, HELLE it JACKSOM

Civil S, STft<e!or«J EnglnMf*

THE WORLD TRADE CENTER I'il S^tho^.t, o«Te 3 /5'<S Ho. MltiORU V«W*SftKI « ASSOC. EMERY ROTH A SONS

M1»0RU \/.%*AtAtH, AtttilTtCr tCHATC. BOTH, fcflCHITECT PHePAREP BY APPROVED r CRiTLP/A "FOR OH^IOKI

FLOOR OU'T'^IDL OP CORE

-LIVE. LOAD . FOR .COLOMU PL^fON

Live Partition

Ffoor ... Lacd AIIoV'/anc<s

9 thru 40

^5 thru > SapSr^ <B p^f

55 thru 14

79 mru IOQ>

G7 4 77 50 psF^ ' 12 psf

.Qecucs per /V<fv'/ Yor/< dfy Bldg

,

Code See Shed A'o. S

Source: Reproduced with permission of The Port Authority of New Yorl< and New Jersey.

Figure 2-8. Design criteria for WTC 1 and WTC 2: columns outside of core - live load.

Code requirements. Provisions for live load reduction are contained in sub-article 903.0, Live Load

Reduction. According to C26-903.1, live load reduction is not pennitted on roofs. The allowable reduced

live load for floor members is detennined by multiplying the basic live load value from Reference

Standard RS 9-2 (see above) by the percentages given in Table 9-1 of the Code, which is reproduced here

as Table 2-1. These percentages are a function of the contributory floor area, which is defined in C26-903.3, and the ratio of live load to dead load. Contributory floor areas are computed as follows

(C26-903.3):

• For one-way and two-way slabs: product of the shorter span length and a width equal to onehalf the shorter span length. Ribbed slabs shall be considered as though the slabs were solid.

• For flat plate or flat slab construction: one-half the area of the panel.

20 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

• For coluiTins, girders, or trusses framing into columns: the loaded area directly supported by

the column, girder, or truss. For columns supporting more than one floor, the loaded area

shall be the cumulative total area of all the floors that are supported.

• For joists and similar multiple members framing into girders or trusses, or minor framing

around openings: twice the loaded area directly supported but not more than the area of the panel in which the framing occurs.

Table 2-1. Percentage of live load per the 1968 Code.

Contributory

Area (ft^)

Ratio of Live Load to Dead Load"

0.625 or less 1 2 or more

149 or less 100 100 100 150-299 80 85 85 300-^49 60 70 75 450-599 50 60 70

600 or more 40 55 65

a. For intermediale \ aliies of live load/dead load, the applicable percentages of live load may be interpolated.

No live load reduction is permitted (C26-903.2(b)) for members and connections (other than columns,

piers, and walls) supporting:

• Floor areas used for storage (including warehouses, library stacks, and record storage);

• Areas used for parking of vehicles; and

• Areas used as places of assembly, for manufacturing, and for retail or wholesale sales.

The maximum live load reduction is 20 percent for columns, piers, and walls supporting such areas.

Live load reduction is also not permitted for calculating shear stresses at the heads of columns in flat slab or flat plate construction (C26-903.2(c)).

As an alternative procedure, live load reduction for columns, piers, and walls may be taken as 15 percent on the top floor, increased successively at the rate of 5 percent on each successive lower floor, with a maximum reduction of 50 percent. For girders supporting 200 ft' or more of floor area, the live load

reduction is 15 percent.

Design criteria for WTC 1 and WTC 2. Sample live load reduction criteria from the Design Criteria of

WTC 1 and WTC 2 are given in Fig. 2-9 (WSHJ 1965a). According to these criteria, live load reductions were to be determined in accordance with C26-348.0 (note: this is the section number of the live load

reduction provisions in the 1938 edition of the Code) subject to the modifications contained in Fig. 2-9. It

is important to note that the live load reduction provisions in C26-348.0 of the 1938 Code are the same as the ahemative provisions contained in C26-903.2(d) of the 1968 Code, except for the provisions related to permissible reductions for certain types of occupancies, which are more comprehensive and more

stringent in the 1968 Code.

NISTNCSTAR 1-1A, WTC Investigation 21
Chapter 2

XrKCRU TA»*««K1 t ASSOC. EMERY RCTM t »0«S

Wve load reductions shall be taken in accordance vith the reiiulreraent

of the Ken Tork Building Code, Section C26-3AS.0 codified as foUows:

a) Coluirais; In structures intended for storage purposes all columna, piers

or valla and ^foundations «ball be designed £or 8SS ol the full aasuned

live load. In structures intended for other uses tlie essuraed live load

used iu deslgnlae coluxms, piers o; vails and foundations shall be

as follows:

1002 of live load on roof •

85Z of live load on top floor

801 of live load on next to top floor'

75J of ItwB load on next floor below -

Oti each succesaive lower floor, there ehall be a SS decrease in the

;>«:centBg« vith a minlmusi of 50% of the live load provided for any

floor. The live loads used as the basis for colunm live load reduction*

are the "Code live Loads" for the epproprlate occupancy, and are not the

reduced live loads used for ocait ox truss design.

b) Girder members, except at roofs, »Alch have e tributary area of 200 s.f.

or greater shall be designed using a reduced live load Ir. accordance with

the design criteria sheets 4 and 5.

c) In designing trusses and girders which support colutmis and in detenslnlng

the area of footings, use the total dead ;oad pius the total live load

(reduced as shOKa above in item a)

.

Source: Reproduced with permission of The Port Authority of New York and

New Jersey.

Figure 2-9. Design criteria for WTC 1 and WTC 2 - live load reduction.

Figure 2-10 shows the percentage of design hve load from the Design Criteria that was to be used in the design of beams in the core area, except for tenant areas, on the floors noted in the figure. These

percentages were the same as those from the 1968 Code (see Table 2-1 of this report), except in the case where the live load to dead load ratio was 2 or more and the loaded area tributary to the floor member was

between 150 ft" and 299 ft"; in this case, the code-prescribed percentage is 85 percent, while the value in the Design Criteria was 90 percent, which is more stringent than the Code requirement.

22 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

.N. SKIUINC, JACNiON . THE WORLD TRADE CIuNTER Tr/v,%'=o"«A'oTHOR tv

Shod

So.

APP«OV£0 /

70

15

Si '^V

£

i ^/p^ ^ 2-0 ar mare

' or ICS'S

3 /i^o 2ao 300 4oa 500

laaahd arsQ frihufory to floor membi^r (sq. ff.)

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-10. Design criteria for WTC 1 and WTC 2: floors inside of core, except for

tenant areas - live load reduction.

Figure 2-1 1 shows the design live loads from the Design Criteria for the tenant areas inside of the core. The solid line represents the reduced live load that was to be used in the design of the beams; these values were computed in accordance with the live load reduction provisions in the Design Criteria (see Item b in Fig. 2-9). Note that the unreduced live load specified in the Design Criteria for tenant spaces inside the core was 100 psf (see Fig. 2-6), which matches the design live load shown in Fig. 2-1 1 for tributary areas up to 200 ff. No live load reduction was to be taken for beams with tributary areas less than 200 ft' in tenant areas Also included in this figure are two other sets of data points: one set represents the reduced

live load computed in accordance with the 1968 Code provisions with a live-to-dead load ratio equal to

NISTNCSTAR 1-1A, WTC Investigation 23
Chapter 2

one (see Table 2-1), and the other set is the Code equivalent uniform load for partitions, which is a constant 6 psf for partition weights up to 100 plf (see Exhibit RS 9-1 in Annex Al of NIST

NCSTAR 1-lB). The Code requires a 50-psf live load in tenant areas (office areas without storage) per Reference Standard RS 9-2 (see Exhibit RS 9-2 in Annex Al of NIST NCSTAR 1-lB). The 50 psf live load plus the 6 psf partition load is shown in the figure for tributary areas up to 150 ft". Figure 2-1

1

clearly shows that the design live loads specified in the Design Criteria, including live load reduction, were greater than those required by the Code for office areas without storage.

' HELLEijiwsoK 1 THE WC«?LD- TRADE CENTER,

I HINOnu YAUASAKI 6 ACSOC.

Mo,

CR/TERfA FOR D£SfGA/^P^3/^M loAP

T£'/S/A^/r SPACE. Load /^^oucrioN

j

FLOORS

7 W^^o 40

11

iqrhrulOG

^N&uj YorJ^: B/J^, ^oJe

£3

10^

700

I —I

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-11. Design criteria for WTC 1 and WTC 2: floors inside of core, tenant areas

live load reduction.

24 NIST NCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

The sheets from the Design Criteria that are shown in Fig. 2-12 give the design dead and hvc loads for the floor trusses (short-span, long-span, and two-way) outside of the core area for the floors that are noted

in the figure. The Design Criteria also specified a live load equal to 1 00 psf that could act over an area of

6 ft-6 in. by 31 ft-0 in. on any of the long-span or short-span trusses in the system.

VORTHMSTON, MILLWC, MfUE 1. JACXSON

Civil ^ SmjcH-ml E»gln*«ri

THE WORLD TRADE CENTER ISS :§SI ;5tm6«.t»

MINOnU VAU*»AKI « ASSOC. eUERY ROTH ft SONS PRtPARto Br ///"^ \ /'!~ APPROVED '

I

' 2

ORITERIA FOR OL9!GKJ. ~..lD.ES/(SA/ load

..FLOOR OUTSIDE OF CORE.

Short Span TrusS<ss -One Way

LL - {S^ii S^i^^.-A6 5^/5 -.'7, /6. 26)

FLOORS

iO thru -r'O

44 ihru 14-

76 -rhrj /o^

D

ilpi

U. ii

n

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-12. Design criteria for WTC 1 and WTC 2: floors outside of core.

NISTNCSTAR 1-1A, WTC Investigation 25
Chapter 2

Figure 2-13 is a reproduction of sheet TF 1/16 from the Design Criteria, which shows the design live

loads, including live load reduction, on the short-span, long-span, and two-way floor trusses in the area

outside of the core for the floor levels that are noted in the figure. Similar criteria were also provided for other floor levels.

The live load reduction criteria for columns outside of the core area are summarized in Fig. 2-14.

j V.lHOHii YAM*SA<i i *i5CC.

Zivil 4. Structure] Eag>o«/i 1 wiaobu rA.MAft*.Ai, A«<*)Tec?

£wc:=?y RaTo a tons f>ft£»»a£c by

loo

\ FLOORS

/O rhru 24-

2G ihro 40

-45 i-hru 5Q

(Bo i'hfO

GB thru 74

lOfhru

Cisfhru/OG

^ 1

/Or

Ne.u> Yafk Cii^u B/J^, C^e I It i y » ' *^ ^

3i.

I '

i

I

' L

ISO ICQ ZOO 40O Goo

Source: Reproduced with permission of The Port Authority of New Yorl< and New Jersey.

Figure 2-13. Design criteria for WTC 1 and WTC 2: floors outside of core - live load

reduction.

26 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

i MINOnU YAMASAKI A ASSDC- EMtftY WCTK A SONS PF«EJ>*I)£D BY

L/K/B LOAO RBOUCT/ON

/oo

90"

60-

70-

FLOOR.S

.£C ^h^DL "0,5 or lese>

JOO 200 300 4oo £00

loaded area inhufary to f/oor member (%q- f/:)

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-14. Design criteria for WTC 1 and WTC 2: columns outside of core -

live load reduction.

Design criteria for WTC 7. Live load reduction criteria used in the design of the structural members in WTC 7 are not listed on any of the structural drawings. However, the project specifications (WTC 7 Project Specifications 1984) require that WTC 7 be designed in accordance with the NYC Building Code.

No documents were found that indicated what hve load reduction was used.

NISTNCSTAR 1-1A, WTC Investigation 27
Chapter 2

Wind Loads

Code requirements. According to sub-article 904.0, Wind Loads, wind forces are computed in accordance with Reference Standard RS 9-5, Minimum Design Wind Pressures. The Code provisions

require that wind shall be assumed to act from any direction, and for continuous framing, the effects of

partial loading conditions shall be considered. Minimum design wind pressures acting on vertical surfaces

are contained in Table RS 9-5-1, which is reproduced here as Table 2-2.

Table 2-2. Design wind pressures on vertical surfaces per

the 1968 Code (Table RS 9-5-1).

Height Zone

(ft above

curb level)

Design Wind Pressure on Vertical Surfaces

(psf of projected solid surface)

Structural Frame Glass Panels

0-50" 15 0-100 20 30

101-300 25 30

301 -600 30 35 601 - 1000 35 40

Over 1000 40 40

a. Signs and similar construction of sliallow depth only.

Table RS 9-5-2 (see Table 2-3) contains the design wind pressures normal to horizontal and inclined

surfaces.

Table 2-3. Design wind pressures on horizontal and

inclined surfaces per the 1968 Code (Table RS 9-5-2).

Roof Slope Design Wind Pressure Normal to Surface

30 degrees or

less

Either pressure or suction equal to 40 % of the values in Table RS 9-5-1 over the entire roof area. More than

3 degrees

Windward slope: pressure equal to 60 % of the values in Table RS 9-5-1. Leeward slope: suction equal to 40 % of the values in Table RS 9-5-1.

For purposes of design, pressures on vertical, horizontal, and inclined surfaces of the building are to be

applied simultaneously.

For the design of wall elements other than glass panels (i.e., mullions, muntins, girts, panels, and other

wall elements including their fastenings), the Code design wind pressure, which includes allowances for

gust, acting normal to wall surfaces is specified as 30 psf pressure or as 20 psf suction for all heights up to 500 ft. Applicable design pressures for heights over 500 ft are to be determined from a special

investigation, but are not allowed to be less than those pressures indicated in Table RS 9-5-1.

Minimum design wind pressures are also given for other building elements; they are to be obtained by

multiplying the pressures in Table RS 9-5-1 by the appropriate shape factors in Table RS 9-5-3. The

28 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct tiie Buildings

shape factors vary from 0.7 for upright, circular cylindrical surfaces to 2.0 for signs with less than

70 percent solid surface.

In lieu of using the wind pressures mentioned above, design wind pressures may be established by

"suitably conducted model tests," subject to review and approval of the Building Commissioner (Item 6

in Reference Standard RS 9-5). The tests are to be based on a basic (fastest-mile) wind velocity of 80 mph

at 30 ft above ground, and are to simulate and include all factors involved in consideration of wind

pressure, including pressure and suction effects, shape factors, functional effects, gusts, and internal pressures and suctions.

Design criteria for WTC 1 and WTC 2. Design wind forces on the towers were determined based on a

series of wind tunnel tests that were conducted at the Colorado State University (CSU) and the National

Physical Laboratory in the United Kingdom. Specific details on these tests can be found in Sees. 2.3.2 and

3.2 of this report.

Design wind pressures were specified in the WTC Design Criteria for external cladding and glazing

(WSHJ 1965a). Outward (negative) pressure acting normal to the surface varied from 65 psf below the 7th floor to 125 psf at the 109th floor. Inward (positive) pressures varied from 45 psf below the 7th floor

to 55 psf at the 108th floor. These pressures are based on the results of a series of wind tunnel tests that were performed specifically for this purpose (WSHJ 1967a).

Design criteria were also established for the antenna mast located on top of WTC 1 (WSHJ 1973). The

antenna and its components were to be designed for the following conditions:

• A mean wind speed of 140 mph in any direction and no ice coating;

• A mean wind speed of 1 10 mph in any direction with an ice coating of 1/2 in. over all exposed unheated metallic surfaces with a minimum air temperature of 20 °F;

• A mean wind speed of 1 1 0 mph in any direction and no ice coating under a range of air temperatures from 1 0 °F to 90 °F;

• A mean wind speed of 40 mph in any direction and no ice coating under a range of air temperatures from -15 °F to 105 °F; and

• The dynamic effects of wind associated with the mean wind speeds specified above (dynamic

effects of wind gusts were obtained by multiplying the mean wind forces by a factor of 5).

The requirement of a 1/2 in. thick coating of ice matches the requirement in C26-905.6 of the Code for the design of open-framed or guyed towers. Also, the Code requires that exterior exposed frames, arches, or shells be designed for the forces and/or movements resulting from an increase or decrease in temperatures of 60 °F for metal construction (C26-905.7). These requirements are less stringent than

those contained in the design criteria. It is not evident from the documents how the wind velocities in the specification were established. The design criteria contain a section on how the wind forces were computed based on these velocities.

The effects of wind on the towers were investigated throughout the years as part of the Structural lntegrit>'

Inspection program. The results from these investigations are discussed in detail in NIST NCSTAR 1-lC

NISTNCSTAR 1-1A, WTC Investigation 29
Chapter 2

(This reference is to one of the companion documents from this Investigation, A hst of these documents

appears in the Preface to this report).

Design criteria for WTC 7. No design criteria or calculations were available for WTC 7 with respect to wind loads. However, a wind tunnel study of WTC 7 was carried out in 1983 by the University of

Western Ontario at the request of the structural engineer of record, Irwin G. Cantor, Consulting Engineers

(Isyumov 1983). No document is available to show how the wind tunnel test results were used in the

design of WTC 7.

Changes in Design Loads

Over the years, the loads imposed on the buildings changed, primarily due to changes in occupancy.

Design guidelines were issued by the Port Authority that pertained to tenant modifications, and included

allowable design loads that could be applied to the buildings. These guidelines are described in detail in Maintenance and Modifications to Structural Systems of WTC 1, 2, and 7 (NIST NCSTAR 1-1 C).

Information on the major structural changes in WTC 1, 2, and 7 can also be found in NIST

NCSTAR 1-lC.

2.2.2 Structural Design Requirements of the Code

The following discussion focuses on the design requirements in the Code as they relate to the design and

construction of the WTC buildings. Only those requirements that are applicable to the structural design of

the members in the WTC buildings are covered. Methods used to proportion structural members and other

components of the buildings are contained in Sec. 2.3 of this report.

General Requirements

Code sub-article 1000.0, Scope and General Requirements, contains the minimum requirements for

materials, design, and construction of structural elements in buildings. NIST NCSTAR 1-lB describes

these minimum requirements. The inspection requirements given in Table 10-1 (Inspection of Materials

and Assemblies) and Table 10-2 (Inspection of Methods of Construction) and the material requirements in sub-articles 1003.0 through 101 1.0 must be satisfied. Reference Standard RS-10, Structural Work, which

contains a list of referenced national standards, is part of the general requirements (C26- 1000.2,

Standards). The list of national standards that were applicable to the design of the WTC buildings can be

found in Annex Al of NIST NCSTAR 1-lB. For example, reference was made to the 1963 edition of

Specification for the Design. Fabrication, and Erection ofStructural Steel for Buildings (AISC 1963b),

which was applicable to the design of WTC 1 and WTC 2. The 1978 edition of the specification was

applicable to steel design in WTC 7 (AISC 1978).

Design methods and materials other than those prescribed in the Code were allowed to be used, as long as

it could be demonstrated to the Building Commissioner that the design would provide a factor of safety

against structural failure consistent with the requirements established for the different building materials

of construction in sub-articles 1003.0 through 101 1.0.

The Code required a signed statement of satisfaction from the architect or engineer when structural elements were detailed on shop or working drawings prepared by someone other than the architect or

30 NIST NCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

engineer. Manufacturers were also required to provide statements or other supporting documentary

evidence of accreditation attesting to the accuracy of fire-resistance ratings data, load tables, or similar

data supplied in catalogues.

General Structural Design Requirements

The general structural design requirements in sub-article 1001 .0 cover, among other things, secondary

stresses, combination of loads, and deflection limitations.

• C26-1001.3, Secondary stresses. Secondary stresses in trusses must be considered in design.

• C26-1001.4, Combination of loads. Dead loads, live loads (including impact), and reduced

live loads are defined in this section as basic loads. Loads of infrequent occurrence are wind

forces, thennal forces, shrinkage, and unreduced live loads (where live load reduction is pennitted by Article 9). Load combinations depend on whether the working stress method or

the ultimate strength method is used to proportion the members.

Where design is based on allowable or working stresses, the loads in Article 9 (discussed

above) are to be multiplied by the following factors: (1) for combinations of basic loads only,

the factor shall be 1 .0; (2) for any combination of one or more basic loads with any one load

of infrequent occurrence, the factor shall be 0.75; and (3) for any combination of one or more

basic loads with two or more loads of infrequent occurrence, the factor shall be 0.67. The

requirements related to the allowable unit stresses for short-time loading design of wood

members are given as an exception to these requirements. NIST NCSTAR 1-lB contains the

specific load combinations for the building materials used in the WTC towers.

Where design is based on ultimate strength criteria (including plastic design of steel structures and proportioning of suspended structures), the loads prescribed in Article 9 are to be multiplied by the factors given in C26- 101 0.5(e) (allowable working loads for suspended

structures, if applicable) and the applicable material reference standards. Two exceptions are given: (1) where load factors are given for wind (or earthquake) forces in reference standards,

the design must additionally consider combinations of loads that include the other loads of

infrequent occurrence substituted for the wind loads and (2) the design shall also consider

combinations of loads where the two most critical loads of infrequent occurrence are combined with the basic loads. The load factors in the reference standards and in C26-1010.5(e) for suspended structures may be reduced 15 percent for the combination of

basic loads plus one load of infrequent occurrence.

• C26-1001.5, Deflection limitations. Vertical deflection limitations for floor and roof

assemblies are provided in the referenced material standards for structural steel and concrete

(see Sec. 5.10 of NIST NCSTAR 1-lB). In addition to those requirements, the total deflection

due to dead load plus live load (including the effects of creep and shrinkage) of members

supporting walls, veneered walls, or partitions constructed of or containing panels of

masonry, glass, or other frangible materials is limited to the span length divided by 360. No

horizontal deflection or drift limitations due to lateral wind forces are prescribed in the Code.

NIST NCSTAR 1-1A, WTC Investigation 31
Chapter 2

Adequacy of the Structural Design

According to sub-article 1002.0, Adequacy of the Structural Design, the design of structural members is to conform to the applicable material standards mentioned in sub-articles 1003.0 through 1011.0

(C26-1002.1). If such computations as prescribed in these standards cannot be executed due to "practical

difficulties," the structural design can be deemed adequate if the member or assembly performs

satisfactorily when subjected to load tests in accordance with 1002.4(a). Provisions to detennine the adequacy of completed or partially completed structures are also provided. Prequalifying load tests (C26-1002.4(a)) can be used to establish the strength of a member or assembly prior to having such

members or assemblies incorporated into a structure. The test specimens are to be a true representation of

the actual members or assemblies in all aspects, including the type and grade of material used. Support

conditions for the members or assemblies being tested are to simulate the conditions of support in the building, except that conditions of partial fixity might be approximated by conditions of full or zero

restraint, whichever produces a more severe stress condition in the member being tested. In regard to strength requirements, the member or assembly must be capable of supporting the following (note: no

specific reference to a particular type of building material is given in this section of the Code):

1. Without visible damage (other than hairline cracks) its own weight plus a test load equal to 150 percent of the design live load plus 150 percent of any dead load that will be added at the

site, and

2. Without collapse its own weight plus a test load equal to 50 percent of its own weight plus

250 percent of the design live load plus 250 percent of any dead load that will be added at the

site.

The latter loading is to remain in place for a minimum period of one week, and all loading conditions in Article 9 of the Code are to be considered. Exceptions to the above load conditions are also given in this section.

The member or assembly is also subject to the following deflection requirements: the recovery of the deflection caused by the superimposed loads listed in item 1 above must be at least 75 percent. Also, the deflection under the design live load is limited to the values prescribed in C26-1001.5.

Requirements are also given for tests on models less than full size. The similitude, scaling, and vahdity of

the analysis are to be attested to by an officer or principal of the firm or corporation making the analysis.

The finn or corporation is to be approved by the Building Commissioner.

Concrete Requirements

According to sub-article 1 004.0, design of reinforced concrete structural members is to confonn to the requirements in that section and Reference Standard RS 10-3, which is the 1963 edition of Building Code

Requirements for Reinforced Concrete (ACI 1963) with modifications, which is apphcable to the design

of WTC 1 and WTC 2. One notable modification made to this standard is that all of the requirements

under ACI 3 1 8 Sees. 902 (Design loads) and 903 (Resistance to wind, earthquake, and other forces) are deleted and replaced with the following: "Building code requirements for loads and infrequent stress conditions shall apply." "Infrequent stress conditions" refer to such conditions as wind and earthquake. In other words, all loads are to be detennined in accordance with the 1 968 Code. In case of concrete

32 NISTNCSTAR 1-1A. WTC Investigation
Provisions Used to Design and Construct tlie Buildings

structures designed by the ultimate strength design method, design (factored) loads are to be determined

in accordance with Sec. 1506 of ACI 318-63.

According to the specifications for WTC 7 (WTC 7 Project Specifications 1984), the 1983 edition of

ACI 318 was applicable (ACI 1983).

Steel Requirements

Design of steel structural members is to confonn to the requirements in sub-article 1005.0 and Reference

Standard RS 10-5, which is the 1963 edition of Specification for the Design, Fabrication, and Erection of

Structural Steelfor Buildings (AISC 1963b) with modifications, which is applicable to the design of

WTC 1 and WTC 2. Similar to the design of reinforced concrete members, the provisions of Sec. 1.3 (Loads and Forces) are deleted and replaced with the following: "The provisions of the building code for loads shall apply." Other notable modifications to the AISC Specification are:

• The following paragraph is added to the definition of composite construction in Sec. 1.11.1: "Concrete materials shall meet the applicable requirements of the building code. Where

concrete having a unit weight less than 130 pcf is used, the capacity of the shear connectors

to resist applied load under the proposed conditions of use shall be investigated..."

• Sec. 1 .25.5 on field connections during erection is deleted and replaced with the following: "...No holes, copes or cuts of any type shall be made to facilitate erection unless specifically

shown on the shop drawings or authorized in writing by the party or parties designated for inspection of such work."

Reference Standards RS 10-6 and 10-7 are to be used for light gage cold formed steel and open web steel

joists, respectively (see NIST NCSTAR 1-lB).

According to the specifications for WTC 7 (WTC 7 Project Specifications 1984), the 1978 edition of the AISC Specification was applicable (AISC 1978).

2.3 SUMMARY OF METHODS USED TO PROPORTION STRUCTURAL

MEMBERS AND COMPONENTS

2.3.1 Overview

This section contains the general methods that were used to proportion the structural members and

components in the buildings. Since design calculations were not available for WTC 7, the discussion that follows covers the design methods employed for WTC 1 and WTC 2. A summary of the design methods is provided for the following structural members in WTC 1 and

WTC 2: exterior columns, floor trusses outside of the core area, composite steel beams in the core area, connections, concrete floor slabs, steel deck, and hat trusses.

NIST NCSTAR 1-1A, WTC Investigation 33
Chapter 2

2.3.2 Exterior Columns

An approximate method was used to estimate the shear forces and bending moments acting on exterior

columns (and spandrels) due to the effects of wind (WSHJ 1966a). In general, design shear forces and

overturning moments were computed at each floor level from an equivalent design wind velocity at the top of the tower and average pressure coefficients that were derived over the height of the tower from the wind tunnel tests (see Sec. 3.2 in this report for details on the wind tunnel tests). The equivalent design

wind velocity was defined as the mean wind velocity averaged over a 20 min period at a height of 1,500 ft above the ground and was based on a 50 year return period (WSHJ 1966c).

A WeibuU probability distribution function was used to predict the maximum deflection (static plus

dynamic deflections) at the top of a tower as a function of return period (see Sec. 3.2 of this report for more details). From the wind tunnel tests, maximum deflections were recorded at the top of a tower for a number of different wind velocities acting in 24 different directions (i.e., 15 degree intervals) around the towers. The equivalent design wind velocity V^^^j^y^ was calculated from the following equation, using a

test wind velocity of 100 mph (WSHJ 1966a):

Vdcs,gn=^^^l-f^ . • (2-1)

where:

Aso = deflection at the top of the tower in the North South or East West direction based on the Weibull probability distribution function using a return period of 50 years

^max = maximum deflection at the top of the tower in the North South or East West direction

obtained from the wind tunnel tests due to a wind velocity equal to 100 mph

Equivalent design wind velocities for both towers in both directions are contained in Table 18 of the wind

report, which is reproduced here as Table 2-4. It can be seen from the table that the equivalent 20 min

design wind velocity was approximately 98 mph in the N-S and E-W directions for both buildings.

Table 2-4. Equivalent design wind velocity for WTC 1 and WTC 2.^

Tower

Direction

of

Movement

50-year

Displacement

(ft)

Critical Direction

for 100 mph

Wind"

Maximum

Deflection

in Critical Direction

(ft)

Equivalent

Design Wind

Velocitj'

(mph)

1 N-S 4.30 70° 4.5 98.0 E-W 3.54 0° 3.7 98.0

2 N-S 4.64 80° 5.0 96.3 E-W 3.66 170° 4.1 95.0

a. Based on critical damping ratio = 2.5 %.

b. Measured clockwise from north; zero angle corresponds to wind blowing from north to south.

34 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

The shear forces S and overturning moments A/ at each floor level due to the equivalent design wind

velocity in each of the principal directions were comprised of static and dynamic components:

S = S±S' _ (2-2)

M=M±M'

where the first tenns in the summations are the mean or steady-state components and the second terms are dynamic components. Mean shear forces and overturning moments at height r above the base, which were

derived from the average pressure coefficients measured in the wind tunnel tests at the CSU on the static twin-tower model, were calculated from the following equations in each principal direction

(WSHJ 1966a):

S(=) = ^pV,"DHCs{z)

(2-3)

where:

p = design air density = 0.0023 slugs/ft^

Vg = mean design wind velocity = 98 mph

= shear force coefficients from wind tunnel tests (WSHJ 1 965b)

= overturning moment coefficients from wind tunnel tests (WSHJ 1965b)

D = plan dimension of building

H = height of building

Dynamic components of the shear forces and overturning moments at any height r, which were based on

the peak dynamic amplitudes of vibration measured in the wind tunnel tests at CSU on the aeroelastic

twin-tower model, were calculated from the following equations in each principal direction

(WSHJ 1966a):

S'iz) = 4K'n;A\"mi=)^{=)±

(2-4)

M'{=)=j^S'iz)dz

In the first of these equations, is the natural frequency of oscillation of the building, which is given in the wind report (WSHJ 1966a), and^ is the amplitude of oscillation at the top of the tower corresponding

to a mean design wind velocity of 98 mph. The quantity m{z) is the mass per unit height of the building

(see Fig. 121 in WSHJ 1966a) and }x(z) is the mode amplitude at height r for unit amplitude at the top of

the building (see Fig. 120 in WSHJ 1966a).

NISTNCSTAR 1-1A, WTC Investigation 35
Chapter 2

As noted above, 24 wind directions at 1 5 degree intervals around the towers were considered in the

analysis. Since there were four possible combinations of static and dynamic components due to wind

(see Eq. 2-2), 96 sets of wind load cases were considered for each tower (WSHJ 1966c). A summary of

the total deflections and dynamic amplitudes at the top of the towers and the adjusted pressure

coefficients over the height of the towers is contained in WSHJ (1966c). Figure 2-15 shows the total

(static plus dynamic) deflections at the top of WTC 1 (A) and WTC 2 (B) in both the north-south and

east-west directions due to wind velocities of 100 mph and 98 mph (design wind speed; see Table 2-4).

Through interpretation of infonnation contained in documents provided by Leslie E. Robertson

Associates (LERA) in July 2004, it is possible to detennine the design wind pressures from the wind

tunnel tests.

TBE WORLD TRADE CENTER lILTo'.'A'wt-c.tv

Br-. Ar-v

o' 3.7 3. I 4-/2. 2. <9i 2.1 i

j 7, txcl \ - - \ . ! /' ~ i * . ^ —- /

- 1 - . _

i 2. 1

«'.-> --. ff 2.nS.3 3. f 3./

7

'^.70 i 9o S.4 S.zC s. ;

ICS' 2X 3. / ¥.3 S. 2 2.£'D 2.9 0 2.0 4.3 5- 7 /.fZ S.2.C 1.75- j.^r S. 7 5.2 : 7.63 /.3f a.ss 3.C-/

/s-o U 2.^5- 2.7S- 1.73 2 *-'i> 2.C-^

/.r 2.3 -5./ 3.3 2.Zt 3.17

/So 2. z 2. 7 ^./^ s.ss- s.cy 2.t-^f

/f 5" i.2f 3.C 2. f

\ 3.0O /.20 2.7s

f.2. 3.3 s. t //£ 2.?2r £2^ Z.^C /. 7 3.0 2: 3. 1 Z. Zvo 7.C 3. 3 2. ! I 2.SC S.ZI 3.4/

j.<?r /. F s. 3r ; a. 35 2. '73 /'73

Z70 2.Z 3.3 2. 2 : 2. it 3.17 /•r? S.OJ

2B5' 3^0 3.0 2./

1

Z.F^ \ /. to 2. Si

SCO 2. 7 Z. 7.C ^.51 2. S-O Si^ 2.1^ 2. 2.?" 2.0 2./t 2./C 1 /«^^

S3o 2.C-f AS / S'X' 2.S¥ t-73 1 2.? 2.04 /3Z 2.i>1 .'9& i 72J 7 97 I i

•

i i ' 1 • ' Source: WSHJ 1966c. Reproduced with permission of The Port Authority of New York

and New Jersey.

Figure 2-15. Total deflections (ft) at top of WTC 1 and WTC 2 due to wind.

36 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct tiie Buildings

Once the total shear forces were computed at each floor level, concentrated forces due to wind were

detennined and applied at each floor level. Member forces were computed based on these applied forces.

Typical hand calculations for an exterior column are given in WSHJ (1967b). These calculations are representative of the allowable stress methods used to proportion exterior columns throughout the towers. The first two pages of the calculations are contained in Fig. 2-16. As can be seen from the figure,

Fonnula (7a) from Sec. 1.6.1 (Combined Stresses, Axial Compression and Bending) of the AISC

Specification (AISC 1963b) was used to proportion the members for the design loads contained in the

tables on the second page of the calculations. For given section properties of the columns, the required

yield strength of the steel was determined from Fonnula (7a).

2.3.3 Floor Trusses

Design data for the composite floor trusses that were used outside of the core area are given in Laclede

Steel Company (1967). Four pages from this document, which are contained in Fig. 2-17, summarize the

loads, materials, design equations, shear connectors, and deflection criteria used in design. As shown on

the third page in the figure, truss members with lengths less than or equal to 24 in. were designed for allowable tension and compression stresses per AISC Specification Sees. 1.5.1.1 (Tension) and 1.5.1.3 (Compression), respectively (AISC 1963b). Top chord members with lengths greater than 24 in. were

designed for combined axial and bending stresses per Sec. 1 .6.1 (Combined Axial Compression and

Bending).

Floor truss panel points were connected by electronically controlled resistance welds providing at least two times the strength of the connected members at full design load (Laclede Steel Company 1967).

As shown in Fig. 2-18, truss seat connection capacities were tabulated for connections at the core and at the exterior columns (SHCR 1971). The governing capacity, which was to be determined in accordance

with the AISC Specification (AISC 1963b) per the Design Criteria (WSHJ 1965a), was taken as the smallest of the capacities of the members and connectors that made up a particular connection.

NISTNCSTAR 1-1A, WTC Investigation 37
Chapter 2

WODIHINOION. SKIltINO, Hint t MCKSON

Civil Strullufot Cn9ln«ttk

THE WOHLD TRADE CENTER r.^v%*c%\*'ru.Ho«.TY No, UINORU TAMASAKl ft ASSOC CMCHT HOt** « SONS ri>(PAn<o BY APPROVED

/

1

5sip- e^Q

.r' -Pgr"

3 t r^irr

S7

EL iia rc

1!'

-I i

^ <

«01

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-16. Design method for exterior columns in WTC 1 and WTC 2.

38 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

WOllHINOtON. SKIUINO, HIIU a JACKSON THE WORLD TRADE CENTEU

MINORU VaMA&AKI * AftSOC.

No.

p Y/cs.s

^^'./ // 7 7 /Co

7.9

CcA r 'r^r^z/^.s^ ^ 73

.-0 J!r

. /e)6

3/7

I- y/ rji JO

/ ^ - /C.iC

/>r.o

7.^

^7.1?2 (/- -m-jjit.

O. i V

/V.'/Z

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-16. Design method for exterior columns in WTC 1 and WTC 2 (continued).

NISTNCSTAR 1-1A, WTC Investigation 39
Chapter 2

ACLEDE Steel Company DlOO-l

Date February 6, 196?

WORLD TRADE CENTER

FLOOR GRID TRUSSES

BASIC DESIGN DATA

Based on double truss units. F^ark 2CT or 2ST_

Single truss components. Mark CT or ST_

DIMENSIONS

:

Unless specifically noted otherwise, see "ST" Details.

Truss clearspan in feet = L. Overall length of truss irdnus

end bearings In feet (2 x 5" = 10"). Example: 59'9" overall

length, ciearspan L = 59*9" minus 10" or 58'H" cr 58.92'.

Length of meiBber, clear of attachinents =

Depth of Truss

Composite type "C" (Keasurea top of shear member to bottom

of lower chord .

)

Standard type "S" (Measured out to out of chord members.)

Total depth of composite section «= "^t"

TOLERANCES:

Overall length 1/4"+ or Depth 1/8"+ or 1/8"-,

LOADS

:

Total load = Live load + Dead load

Applicable for composite design.

Applicable for combined slab and top chord design and

bottom chord design.

Construction load = Applicable Dead load

Applicable for top and bottom chord steel design.

Dead load = Actual weight of structural system in pounds per

square foot.

Live load = Assigned live load for panel area In pounds per

square foot

.

Design load in pounds per square foot « "w"

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-17. Design method for floor trusses in WTC 1 and WTC 2.

40 NISTNCSTAR 1-1A. WTC Investigation
Provisions Used to Design and Construct the Buildings

ACLEDE Steel Company

Date

DlOO-2

February 6

.

1967

Appljcabie design load in pounds per foot equals design

load in pounds per square foot tines spacing of t-usses

in feet = . , "w" TOTAL MOMENT;

"M" (In inch pounds) = WL^ x 1.5.

END REACTION:

"R" (In pounds) = "W" x .5 (overall length of truss in feet).

SHEAR:

At first top chord panel in pounds = V = R- (KxED)

Load

i I Load

ED = Distance first top chord

panel point to truss end

V i vi 1 Shear at other points in pounds = V = fR- En x W)-W (distance to first top chord panel point in feet^

(In no case less than 50S of end reaction "R".)

DESIGNATION OF MEMBERS:

LOAD

End diagonal (long end) tension member

First panel vertical (long end) compression

Second panel compression diagonal

First panel member (short end) compression

End diagonal (short end) tension

Top chord critical compression member at nild span Bottom chord critical tension member at mid span End top chord compression member (long end)

MATERIALS:

A-36 steel . 35 ksi minimum yield strength

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-17. Design method for floor trusses in WTC 1 and WTC 2 (continued).

NISTNCSTAR 1-1A. WTC Investigation 41
Chapter 2

15100-3

ACLEDE Steel Company Date February 6. 1967

A-^Jfl modified 50 ksl minimuTn yield strength

low alloy "H" Series.

See sheets DlOl through D103 for Properties of Sections.

APPLICABLE FORMULAE: Stress Determination (Web or Chord Members)

A-36 steel

(35 ksi fy = Minlmuni Yield Strength 36 ,000 psl

minimum = Tension 22,000 psi yield) ^ Compression for //r ^12? 18,5^^0 - .57^ (//r)^ psi

for-£'r>127 1^19,000,000

Jt = clear distance between attachments

r = least radius of gyration

A-ij^l steel

(50 ksi f^ = Klnimum Yield Strength 50,000 ksl

minimum 30^,,,

fsc = Compression for //r <^108 25,750 - 1.108 C//r)^ psi

for ^/r^ 108^200 H49 000,000 .

Continuous members shall be designed for only axial compressive

stress when the panel length clear of attachments does not exceed

2^". When the panel length clear of attachments exceeds 2k^\ the

top chord shall be designed as a continuous member subject to

combined axial and bending stress and shall be of proportion that

the quantity -

£a ^ £b

30,000

does not exceed unity at the panel point, or that the quantity -

Fa Ffed- La)

does not exceed unity at the mid panel.

Permissible axial unit compressive stress » "Fa

Penrilsslble bending unit stress = "Fb

Permissible compressive stress factor (column equation

1^9,000,000 . .,p,„

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-17. Design method for floor trusses in WTC 1 and WTC 2 (continued).

42 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

ACLEDE Steel Company Date February 6, 1967

Actual axial unit compressive stress = Actual bending unit stress = f^^

MAXIMU?'^ SLENDERNESS RATIOS:

Top chord panels (interior) = 85 Top chord end panels = 120

Compression members other than top chord » 200

Tension members = 2A0

FILLERS OR TIES:

Members In compression composed of two components shall have

fillers or ties spaced so that the ratio of -£/r of each

component shall not exceed the ratio of-^/r of the whole

member. The minimum "r" shall be used in calculating the

critical ratio //r of any component.

SHEAR CONNECTORS.:

Shall be considered to provide a minimum 15 ksi horizontal

shear per web end connector Imbedded in the concrete. This

is for 3,000 psl concrete, (f^)

DEFLECTION:

Applicable deflection formula for uniform load.

Maximum deflection

^ = 25-88 (WL ) 29 ,000,0001

COMPOSITE SLAB AND JOIST DESIGN:

Design values

Total depth of combined slab and truss in inches = D. Effective width of concrete flange in inches equal to

t

2 X 8t " ....... Bgff

Distance from top of concrete flange to neutral axis of

concrete flange = ....... yi

Distance from top of concrete flange to neutral axis of

top chord angles « y2

Distance from top of concrete flange to neutral axis of

bottom chord angles = ^3

Distance from top of concrete flange to neutral axis of

composite section = ....... y Distance from neutral axis of composite section to neutral

axis of concrete flange = dj

Source: Reproduced w\Vn permission of The Port Authority of New York and New Jersey.

Figure 2-17. Design method for floor trusses in WTC 1 and WTC 2 (continued).

NISTNCSTAR 1-1A, WTC Investigation 43
Chapter 2

SKRltHP

Htlll cHaisnAN$ni

Stiuilurti C Ctvti fnstinan

DATE

J

Shoa> K*.

rRETjCltCO BY

.J.QAPjKrT C^^pAClTy-.Oe.JflU3j_'5^

fc.lA)

r*> f ILV

It. (C ) WclOiCjj

._, ... .. —

\o £6 7r 4-7

1

1

/o 6 jiA7 *

\^ 7^

"

13

lA- 141 20<i>

15 82 \^'£>

17 Sq- r S2 54 ^- J£

19 HO 96> IIO

"'5

2 V

20 «^ 6>4 * 72 51 ^ 11 52-*

<i.q 57

C«J|- 50

107 95

CX % fcl 7/

17 h

lb g? 67 n

29 \i o Wl \IO

^o ^5 9g

7^ 70 7^

32 7o 75* 267

{I}-'22i>A \6o Zoo

50

1-

§4-

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-1 8. Tabulation of component capacities of floor truss connections in WTC 1 andWTC2.

44 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

IJCllU»Ki

CHAlSTIAilStll ROXRISON

Sln»tilral & Civil EngifMan 3 h\ JhMt lU.

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-18. Tabulation of component capacities of floor truss connections in WTC 1 and WTC 2 (continued).

NISTNCSTAR 1-1A, WTC Investigation 45
Chapter 2

2.3.4 Composite Steel Beams

"Design standards" for the composite floor systems in the core area are given in the Design Criteria (WSHJ 1965a), and are summarized in Fig. 2-19. As seen on the first page in the figure, the provisions

for effective flange width of the concrete slab were modified from those given in Sec. 906(d)

(Requirements for T-beams) of the 1963 edition of ACI 318 (ACl 1963) to accommodate the case that is depicted in the figure. Design of the composite members followed Sec. 1.11 (Composite Construction) in the AISC Specification (AISC 1963b).

WORTHINGTON. SKILLING, HELLE & JACKSON

Consuiting Cwil end Stracmra! Enfinwrs • 230 Park Avenve, Nt^t- York. Y. 10027 • M«. S-8S74

/

wh&rs. C'CX<ST Slab ihicjcn&t,^ oov&ms

, i

i

^ "1

- —-i

Moo/r/a^/av of secvcn op

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-19. Design standard for composite sections in WTC 1 and WTC 2.

46 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

WORTHINGTON. SKILLINS, HEULE 4 JACKSON" 1

GwKtii-f Civii S^rwawa! 230 fmk Avnn*. Smu Yeri, N'. V. 10017 • Mu. >*37-t _____ 'p D £ s / ST/>A/c/=) /? a s I M

tc

uss f, • ?;"^'f- ror caico'a-fi'ori f/

h.c. t fl 1' ' J.

WORTH!N6TON, SKILLINQ, HELLE & JACKSON

Cimaisa% CaH ai Smitiwi Engmnn 23C fmk Mtma, iV«»' York. S. Y. 10017 • «». 9-«874 O £. S i O kJ S T/^ A/ O ^. R C C

n • same OS n for srcne concrete far some ft exc-o' OSS n "—-tar eiefkc.iion cotC-i^Ia^ior,s onaJ

section ^C"? or Ac/ 'SiS-OS

for cancre-fe (-fl -.'gaao'j'^/d/'^h'iri^ //.^ nt"? for a'! calculotions G>icspt for c^ffect-o/^

CC'cu!c•^f/ons ' £cx.',6/0 Jj'si, gnc' n - /6

Source: Reproduced with permission of Tine Port Authority of New

York and New Jersey.

Figure 2-19. Design standard for composite sections in WTC 1 and WTC 2 (continued).

NISTNCSTAR 1-1A, WTC Investigation 47
Chapter 2

WORTMJNSTON, SKILLINQ, HELLE * UACKSOM

Cmiukmg CM! md $tnicir<m'. Bngineers • 23D Park Avexas, SetsYork, \'.Y. ICOIV • Mu. »Mt4 OES/G/J S7^/J/DAeo ~S/-/£Af? COUiUECTa/22

DS

I 4

5

*Canhector

/^J/owah/e Hoi-lzonial

Shear Laaa! (<i

)

IZ'cilarn.X, ?* !onq headeo' sfoot 6.1 ^.^

S.o

11. e 12.6 /S.3

iS.O

par Cb~!nC'CTcp^ C^/pz)

fc -4:000

'7. S 5. CP 7.3

^/4"ta'Va.ii, X 'S ' iong h //.s

'i8' o'iam,X'i''/i! ' JOria '

L.^nc/tA at sroa'c> ore nyi/r/m'.jfr-i - /ana/zr s-Z-ua's

Shc^/- Cfr^ri/^, Cc.l-rsTC ci£sIo.nC:.a w:fh fr'^sts aHowah-iC

I'oacfs sholl have a-f /cazr 1

* conc^efe cio'y&z' in all a'ii cc.h'o.-is , '

'

Source: Reproduced with permission of Tlie Port Authority of New York

and New Jersey.

Figure 2-19. "Design standard" for composite sections In WTC 1 and WTC 2 (continued).

Allowable horizontal shear loads for the connectors in "stone concrete," which are shown on sheet

number 4 in the figure, are taken directly from Table 1.11. 4 in the AISC Specification (AISC 1963b).

According to Commentary Sec. 1.1 1.4 (Shear Connectors), the allowable shear loads for connectors in concrete with aggregates not confonning to ASTM C 33^' must be established by a suitable testing

program. Note that the allowable shear loads for the connectors in "lightweight concrete" used in design

are 85 percent of the values listed for "stone concrete" (see Fig. 2-19). The Port Authority requested tests to be performed (based on a test program established by Skilling, Helle, Christiansen, & Robertson) at the

Fritz Engineering Laboratory at Lehigh University to determine the shear capacity of 3/4 in. diameter by

4 1/2 in. long studs welded through the troughs of Roll Fonn Type "B" steel deck in lightweight

This specification defines the requirements for grading and quality of fine and coarse aggregate (other than lightweight or heavyweight aggregate) for use in concrete.

48 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

aggregate concrete with a compressive strength of 3,000 psi (see Sec. 3.5 of this report). Results from this testing program could not be located, and no evidence was found that this system was utilized in WTC 1 and WTC 2. The floor trusses outside of the core area did not use shear studs to make them composite

with concrete slab. Instead, truss diagonals were extended above the top chord; this "knuckle" acted like a shear stud (see Sec. 5.4 of this report).

2.3.5 Connections

General design standards for the A325 bolts used in the connections are given in the Design Criteria (WSHJ 1965a) and are shown in Fig. 2-20. Page numbers from the Manual ofSteel Construction

(AISC 1963a) (which also contains the AISC Specification) are given in the figure for bolt dimensions

and properties and for allowable loads.

WORTMINGTON. SKILLIN6, HELLE & JACKSON

CmKuhmg Crul av.d SOT<c:i>rat En^inem 230 fufi Atmtie, Niw Yck. \. V. lOO.V • Mil.

DE&/QU ST.^.UD^/^PS ~ A 326 BOLTS

>)fts!i Heodc - h'aav^ Nsi^aaa^ ~— -A.l.S.C. ^ -S(a

e)Uijis -Heovy h'sxa^or, f^.l.€.0 . 31

3)'WasJ-;ers —A.1.£.C. 5-112

''^}£.rdCT:of-7 Cki^rancCS i^.l.S.Q . 4 SO

') T^nsso,-> A. ! .S .C.'^-t

2)Sh3a>- ~ 'A./.S.C.^-'te)S£arin^ — —— S. C. 4-5

\f)J. S.C. ^-7

U€& 6sssr/h<^ -fype conneciianc where, passible.

Ust. WQshzr^ tjnol&r 4h& -hj.-nr.a' /sor-r.

C/s^S. 'yjasMei-i uiia'si' hoi'f hdoa ^/yvi -for ava.rsizca' t s'oH&d hc'ci.B'olf ciiarricie'' phe ^s' Is maximum --. cye-s/ice p&r.-nirsa' vvir-hout rSciDC-fior: if-'

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-20. Design standard for bolted connections in WTC 1 and WTC 2.

Specifications for welded connections depended on the structural members that were being connected. In

particular, the specifications in the contracts with the suppliers of the floor trusses, box core columns and

built-up beams, exterior wall, and rolled columns and beams each contained requirements that the welding conform as a minimum to the provisions in the then current edition of Codefor Welding in Building Construction, Dl.O, American Welding Society (see Chapters 5 and 6 of this report for more

information on the requirements in these contracts). For the exterior columns, the welding electrodes that were to be used depended on the lower yield strength of the plates that were joined (see Sheet 2-AB2-3 in WSHJ 1967c, which is reproduced here in Fig. 2-21).

NISTNCSTAR 1-1A. WTC Investigation 49
Chapter 2

A connection manual was assembled by WSHJ that contained tables and charts with allowable loads for the typical connections used in the project (WSHJ 1967d).

WORTHINGTON, SKILLING. HEILE & JACKSON Civil & Sfruclufal fnginecrs

• 3£'V£R.AL. A'O r£5

/.) a'SZM ^£S3 £.7::?Jt^ is/i^^^rodes

3£ " -

,'.) AST.'W -^£33 <r/i74i»- £7^a:/i £/£iiTr£>a^i:£

/) A^TAf AZ33 c/£>s^- £:70JiA ^/££-/r^4:Z^s

c".) So-^^er^ety Arc iVe/crynp "f

A) ASTM AS/<S c/as^ SSOx* ^/iSCr^rx^o'ei

2.) ^c/Arr7^-^^/i',ry' Arc H'"<s/W/hc^ *

/.) A£TA<f A3/^ 'c/4>j^ JzSOxjf <s:/4S.c-r/~^ciZ<L'j } £^'A'7>a:/-^isaf' Arc H's/c/irP'S^

SOS < 90

/JASTA^ A3-'a 'c/^^s £/y>O^A £/'ecry£:yci'e-£ S.) Siy6j^e'/-a^£i' Arc H^'^s/W/h'a

£.) Sc/^/Tj^er^eiS^ Arc l^e/cZ/rja *'

** >5<y/?.^if/~i?i?i?' ore n-^i'/a'/hiO ysAsr// r^r/hr/T/

**Lc>>^ /^a'ro-^^o £/^cTra^£'^ ^£

'^f^y /nd/ca.^ed /s the Jo^er rg oP f/je p/ares /o/ned

1 2-y^e2-3

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-21. Schedule of welding electrodes for connections in exterior columns in WTC 1 and WTC 2. A note on structural drawing sheet S-24 of WTC 7 references the "AISC Beam Tables" for connection

design of composite and non-composite beams (The Office of Irwin G. Cantor 1983). As noted in Sec. 2.1.2 of this report, the project specifications for WTC 7 (WTC 7 Project Specifications 1984)

required that the structural steel be designed in accordance with the then current New York City Building

Code and the latest edition of the AISC Specification for the Design, Fabrication, and Erection of

Structural Steel for Buildings (AISC 1 963a).

50 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

2.3.6 Concrete Floor Slabs

According to the first general note for structural concrete contained in Book 8 of the structural drawings

for WTC 1 and WTC 2 (WSHJ 1967c). all structural concrete was to conform to the 1963 edition of

ACI 318 (ACl 1963). except where specifically modified, supplemented, or superseded by the Specifications or specific notes in the drawings (see Fig. 2-22).

REVISIONS

1-22-69 12- 1-70

SKILLING, HELLE, CHRISTIANSEN, ROBERTSON Structural & CiviT Enginaari

THI WORLD TRADE CENTEE CONCRETE CEFERhL NOTES

STRDCTORAL CONCRETE GERERAL NOTES

All structural concrete sh^ll conform to "ACl Standard

Building Code Rwiuireoents for Reinforced Concrete"

(ACI 318-63), except vhere specifically nodifled,

supplesiented or superseded by the Specifice tions or specific notes in the Drawings.

Refer to Architectural Drawings for openings, architectural

treatctents and dimensions not shown. Refer to Mechanical, Electrical and Architectural Drawings for size and location

of all ducts, piping, conduits, reglets, inserts, etc.

Structural steel and floor panels are inrprinted on the concrete drawings for orientation only. For actual

diisensions and other data, refer to the applicable

Drawings •

Mixing and placing of all concrete and selection of

otaterial Eball conforsi to the Specifications. Refer

to the Specifications for concrete nix design

Stone concrete (Korrnal weight concrete) or lighrweight

concrete (Lightweight sructural concrete) to be used

in each floor is indicated in Drawings or in the table beiou, or both.

The ulciute strength aehtod has been used In dealf^n of the concrete work sbown in the Drawines.

The slab thicknesses and f inish elevations of con- crete ineediately after placement ehall conforc to screedtne diapraras supplied by the Engineer. See Specifications

.

Concrete which will bond with previously placed concrete at a construction .1oint shall not be placed

until the existinR concrete at the construction joint has been cured a mlnlmio of 'iO hours.

PRECAST COKCRETE

Refer to Architectural Drawinge for openings, details

and dimensions not shown.

Strength (f'c) shall conform to the strength shown in

the Drawings. Uhere no strength is shown in the

Sub level 5 6 4 f'c 3000 p£i stone concrete Drawings, f'c shall he 3000 psi, except chat facing

Sub level 3 f'c 4OO0 psi stone concrete concrete, where called for, shall be f'c 5000 osi.

Sub level 2 S 1 f'c 4000 psi stone concrete

Service level & Floor 1 3000 psi stone concrete 3. Contractor shall submit design mires for apprejval by Intermediate level f'c 3000 psi stone concrete the Engineer. Reinforcing steel has not been proviFloor 2 f'c 3000 psi lightveight concrete ded to resist stripping, handling or erection

Floor 3 thru 6 iDClusive f'c 3000 psi stone concrete stresses. Where required, Contractor shall provide

Floor 7 f'c 4000 psi stone concrete reinforcing steel, stlffeners, bracing, etc. to Floor 8 4 9 f'c 30O0 psi stone concrete resist erection stresses. Lifting inaerts, etc. Floor 10 thru 40 Ind. f'c 3000 psi lightveight concrete have not been shown. Where required, inserts shall Floor 41, 42 S 43 f'c 3000 psi stone concrete be lnscalled in locations where they will be covered Floor 44 thru 74 loci. f'c 3000 psi lightweight concrete by the construction. Floor 75, 76 i 77 f'c 3000 psi stone concrete

Floor 7!! thru lOo incl. f'c 3000 psi lightweight concrete

Floor. 107 f'c 40,00 psi stone concrete Floor 108 thru PH Roof incl f'c 3000 psi stone concrete

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-22. General notes for structural concrete in WTC 1 and WTC 2.

The uhimate strength method was used in concrete design (see general note 6 in Fig. 2-22). The basic

requirement for strength design may be expressed as follows:

Required Strength < Design Strength, or (2-5)

U<^ (Nominal Strength)

where the required strength (U) is determined from the load combinations given in Sec. 1506, the nominal

strength is determined in accordance with the pro\ isions in Chapters 15 through 19. and the capacity

NIST NCSTAR 1-1A. WTC Investigation 51
Chapter 2

reduction factors (([)) are obtained from Sec. 1504, where all section and chapter numbers are from

ACI 318 (ACI 1963). The load combinations in Sec. 1506 of ACl 318-63 are summarized as follows:

• U=\.5D+\.SL

• L/= 1.25(D + I + JV)

• U=0.9D+ lAW

where

D = effects of the dead loads

L = effects of the live loads

effects of the wind forces

Additional assumptions used in the design of the floor slabs are contained in the Design Criteria

(WSHJ 1965a), as shown in Fig. 2-23.

WOmUNOrON, SKIUtNC,

MINCRU Y*)-1ASAKI A ASSOC CMEHV ROTH a toNS ipRe^AR eo ^ A

>iO<««D HOT-. «I«;'<1«C'' (APPROVED r-^/e//4 FOR, pss/(^y^y

ASSU>gTIONS !

a) The ultimate strength design jiethod is usee.

b) Diaphragc loads xay act in asy dlreccion, depending on the direction of the wind

and/cr the crookedness. Loads are assuaied to act in the sane direction at all

colurans at any given cine,

c) The reaction to diaphragm loading is furnished by the etructurc of the exterior

walls, in z cirectlori parallel to each wall.

d) The diaphrsgn loading or. floors separated fro3\ the exterior vails, suih as Floor

8, Is carried by the floors liaiedlateiy above ar.d below.

e) Diaphragn loading on Floors 2 - 6 is carried lainly by structural steel and has marginal influeace an concrete slabs.

f) Diaphraga loading on Floor 2 and below is directly trartstrsitted to the large frtised areas outside the towers and proauces negligible stress in the concrete.

g) The load factor for gravity coBOined with diaphragm action is 1.00. (A msrgln ot safety is provided in the incut to the IKPCOL Progran-;; also, the full diaphra^ni forces would not ect together and in the saxe direction &t all columns, as afisuned for desigr. .)

h) Structural steel floor systecs, inclsiding spandrels and bridging between floor bcara or trusses, act together vith the concrete,

1) Parts of concrete slabs 1" thic'ic (over 3" electric headers) are taken Ki.tX\ no

structural value. Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-23, Design assumptions for concrete floor slabs in WTC 1 and WTC 2.

52 N1ST NCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

Calculations for the slab design on floors 10 through 87 (WSHJ 1967e) as well as concrete design tables (WSHJ 1967f) confin-n the use of the ultimate strength design method. Figure 2-24 shows sample

calculations for the one-w ay slab design based on this design method.

WORTHINOTON, 5K1U1N0, nlUC 4 JACK&ON 0*" ^'r:t'6-7

r.

Chrfl L }tnrtwr«1 trgln^tre Rjooj' Cl/<r ^. C^r^. r-Z

-P, - i r CD

f,'nJ- /it USr>

/2

'l- Zc

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-24. Reinforced concrete one-way slab design in WTC 1 and WTC 2.

NISTNCSTAR 1-1A. WTC Investigation 53
Chapter 2

Specifications for the reinforcing steel used in the concrete members are given on structural drawing

8-AB 1-2.2 (WSHJ 1967c), and are reproduced here in Fig. 2-25.

SKiLUNO-H£UE-CHRISTIANSEN-iiOB£RTSON Civil & Structural Enginsart

COA'Ci^£r£ OEf^ERAL A/OTSS 3-/-<Z3

REUtrORaKC STEEL

1. RalnforclnS bars shall coaform to ASTM A^32 (60 ksl>, uoleaK specificAlli? noted In the DraMltiRs. 91 saooth reJniorciag bar« shall conform to AST« A15, Int«r»*diate Grade

.

2. welded wire fabric ehalX Corvform to ASTM

unless specifically noted in the Drawings.

3- Detailing of relaforcement and accessories (such as diair*) uhall coofona to ''Manuol taf Staiuiard Practice for Detailing Reinforced Concrete Strocturee" (ACT 315-65).

ft. Subject -to the Engineer's approval of relfwaot

dSitalis and cotistruction procedure?, the Contractor

aay replace r«luforcia{i Vats- 1« vhoie or in part with velded vire Jabric as follows:

a) Wlitije the Contractor elects to exercise

the option to replace Tciiifarcing bars Mich welded wir?? fabric, t>ar« may

be replaced by an equal sectional area

of weldfd irflTf fabric coniplyltif! wlrh

the Specifications. Swooth welded wire fabric &ad defonn«?ti welded Mlr« fabric without certifying tests shall be con- sidered to have a yield point «f 60 ttsi after fabrication.

b) Wlitre defora*d welded vire fabric Is ehovn by cer tilled t«et to possess a strain of 0-003 or Ices at 70 ksi and exhibits a crack vidtb of O.OIS*' or less at 5i6 ksi. In conformancG with

ACl Code Section 1508 <b), deformttd welded wire fabric asy be regarded as »ore efficient than A632 bars in Che ratio of 70 to 60, and the required sectional areas for replaceaeat of Ai32 r«inforc*fliHfnt E»i>j-- b*- calculated

on this basis.

) In no case shall less than the ainlnuzc reinforcement requ:!.red In ACl Code Stnrtion 807 (a) be furnished. This requireoent applies to botb the longitudiasl and the transverse directions.

) Ar*a of reloforceaoent cut at "knuckles"

of floor tTuases or oclicr obsirocrioaB

shall bt restored by providing additional

reinfOTcemeot e<{mil to the area cut

.

5. Reinforcing Isars shall be spUced bv lapping. Splices shall be staggered, with centers of Adjacent epllc«--£ ionpi tudiitally separated «

lainimita of i4 bar diajn^iers. Minimtai length

of lap ehall be as shown in the OrasrLn|;s. Weldffd wire fabric shall be spliced In accord- ance with ACI Code Section 805 (f) i, wherever specific splicing provisions are not shown Jn the ^swings or Includ*?^ in the Specifications.

6. Welding of reinforcing bars, whtre permitted

by the Engineer, or shown in the ITrawings. shall confons to the provision of AUS D12.i and

«.hall be perfortwd by approved, certified welderr..

7. Mi«i«uffl concrete cover at all openings.^ sleeves,

"Icnucklcs" of floor trusses, structural steel .penetrating through or into slabs, and other obstructions shall conform to ACl 318-63. The

asiouQt of reinfcrceawnt placed in a given band or strip, at an obstruction, shall at .least eqvial to rliat calculated from the full band width and the o|W)cin^ Khown^ unless oth^^rwise

' noted.

8. Where different spacings arc shown for adjacent bands or strips of r<;inforcem'-at , the distwicc between the first bar in adjacent bands or strips

ehall not exceed the averaye of the spaclngB

In the adjacent bands or strips.

5. Reinforcftjnent parallel to P/T (Power/Telephone) cells shall be placed mtcstde the width of the P/T cell, except where approved by the EnglDecr.

10. Bar spaclngfi eliown in the Drawings (such as "#3 ^ 12*^) are ncminal and mlniwua rc^julreaneots. For specific required spaciofie, see the applicable

detallfi or sections in the Drawings.

11. Lengths of bars shown in the Drawings do not include additional length needed for hooks or

bendti, where required,

12. Hie symbol ^T" Bcans "top" and the symbol **B'' aeans '^ottoa*'. All bare shown In plan view

without 3 synbol "T" or "B" are bottoci bars, except where specifically noted or shown In details. Ttie designation "10^5 T t B", and

ftSffliJar designations, shall mean 10 top bars and 10 bottoE bars, not 10 bars total.

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 2-25. Specification for reinforcing steel used in WTC 1 and WTC 2.

2.3.7 Steel Deck

The design criteria for the steel deck used in the composite floor system are in the Design Criteria . (WSHJ 1965a) and are shown in Fig. 2-26.

2.3.8 Hat Trusses

A series of diagonal members together with the building columns and floor members formed hat trusses between the 107th floor and the roof in WTC 1 and WTC 2. Six trusses ran parallel to the long direction,

and eight trusses ran parallel to the short direction of the core.

54 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct the Buildings

inUI4 JACXtON THE WORLD TRADE CENTER N£(* vo«K AUrMoHirr 1 Na

MINORC YAMA&AKI • AfcSOC. CMEJtr NOTH e *on»

mHD^J TAMAmAKI ABCHITXCT ICMAW KOTH. A»04IT«CT AVmOVKD • / / CRITSR/A FOR DESIGN

STESL DS.C'<

s. a = ^9,ooo .^s/

^. Cc:>nefruct/on //ye faad ' 30psf

S.Max def/iscf/on wi^h construction

dead load -

dead p,'os cron'^fruc^fjon //ve /oad * f24a

Source: Reproduced with permission of The Port Authority of New Yorl< and New Jersey.

Figure 2-26. Design criteria for steel deck in WTC 1 and WTC 2.

According to the 1 995 Structural Integrity Inspection report that was written by LERA. "the hat

trusses. . .control individual column expansion and contraction due to uneven column temperatures"

(LERA 1995). Additionally, the hat trusses in WTC I provided stability for the 362 ft tall TV mast that was centered on the top of that tower. The hat trusses in both buildings were designed to support one large

mast or four smaller towers near the perimeter of the core region. The 1995 report also noted that the horizontal members of the hat trusses were composite with the concrete floor slabs, which made the concrete floor slabs a \ ital component of the hat trusses.

Design calculations for the different r\'pes of trusses that were used are contained in SHCR (1969).

Members in the trusses were designed for axial forces or axial forces plus bending moments due to the combined effects of gravity loads (including the w eight of the TV mast) and wind loads. Typical

calculations for a truss running in the north-south direction in WTC 1 are shown in Fig. 2-27. These

calculations are representative of the allowable stress methods used to proponion the members in the

trusses. As can be seen from the figure, the AISC Specification (AISC 1963b) was used to proportion the members for the design loads contained on the first page of the calculations. No calculations were found

that showed how the trusses controlled column expansion and contraction due to une\'en temperatures, as discussed in the 1995 report by LERA.

NISTNCSTAR 1-1A. WTC Investigation 55
Chapter 2

T * * 1

II ii'*'fllg'>iLlum /

9 -/Z9

* // /if

sf 7W

Source: Reproduced with permission of Tine Port Authority of New York and New Jersey.

Figure 2-27. Design method for hat trusses in WTC 1 and WTC 2.

56 NISTNCSTAR 1-1A. WTC Investigation
Provisions Used to Design and Construct the Buildings

timmom

mma Man

S tiS

^

//At

4^ ie^Mf *^f/

Source: Reproduced with permission of Tine Port Authority of New York and New Jersey.

Figure 2-27. Design method for hat trusses in WTC 1 and WTC 2 (continued).

NISTNCSTAR 1-1A, WTC Investigation 57
Chapter 2

2.4 REFERENCES .

ACI (American Concrete Institute). 1963. Building Code Requirements for Reinforced Concrete.

Detroit, MI.

ACI (American Concrete Institute). 1983. Building Code Requirements for Reinforced Concrete.

Detroit, MI.

AISC (American Institute of Steel Construction). 1963a. Manual ofSteel Construction. Sixth edition. New York, NY.

AISC (American Institute of Steel Construction). 1963b. Specification for the Design, Fabrication, and

Erection ofStructural Steelfor Buildings. New York, NY.

AISC (American Institute of Steel Construction). 1978. Specification for the Design. Fabrication, and

Erection ofStructural Steel for Buildings. Chicago, IL.

Isyumov, N., and M. Poole. 1983. A Study of Wind Effects for the No. 7 World Trade Center, New York, N.Y. BLWT-SS21-1983. Boundary Layer Wind Tunnel Laboratory, The University of Western

Ontario. London, Ontario, Canada.

Laclede Steel Company. 1967. World Trade Center Floor Grid Trusses Basic Design Data, structural calculations. February. (WTCI-75-1).

LERA (Leslie E. Robertson and Associates). 1 995. World Trade Center Structural Integrity Inspection -

Hat Truss between Floor 107 and the Roof. February, (part of WTCI-66-L).

SHCR (Skilling, Helle, Christiansen, & Robertson). 1969. Structural calculations for floors 107 to 110

(part of WTCI-271-L).

SHCR (Skilling, Helle, Christiansen, & Robertson). 1971. Truss seat connection capacities. (RD-4,

No. 0458, 233 Park Avenue).

The City of New York. 1 968. The City ofNew York Building Code.

The Office of Irwin G. Cantor. 1983. Structural drawings for WTC 7. (WTCI-25-S, Disk 1 of 3). WSHJ (Worthington, Skilling, Helle, & Jackson). 1965a. Design Criteria for WTC 1 and 2. (WTCI-2-L

and part of WTCI-50-L).

WSHJ (Worthington, Skilling, Helle, & Jackson). 1965b. The World Trade Center - Wind Program

Interim Report, Supplement No. 4. January. (WTCI-558-L).

WSHJ (Worthington, Skilling, Helle, & Jackson). 1966a. Part III, Final Chapter. WSHJ Wind Report.

July. (partofWTCI-68-L). . WSHJ (Worthington, Skilling, Helle, &. Jackson). 1966b. Part II, Final Chapter, WSHJ Wind Report.

July. (partofWTCI-68-L).

58 NISTNCSTAR 1-1A, WTC Investigation
Provisions Used to Design and Construct tlie Buildings

WSHJ (Worthington, Skilling, Helle, & Jackson). 1966c. Wind Loading on Exterior Walls oi' WTC

Towers. (WTCI-393-L).

WSHJ (Worthington, Skilling, Helle, & Jackson). 196 7a. Parf IV, Final Chapter, WSHJ Wind Report.

February, (part of WTC1-68-L).

WSHJ (Worthington, Skilling, Helle, & Jackson). 1967b. Design Calculations, Exterior Column

Segments, El. 318-363, structural calculations. February. (WTCI-368-L).

WSHJ (Worthington, Skilling, Helle, & Jackson). 1967c. Structural drawings of WTC 1 and 2. (WTC1-50-L).

WSHJ (Worthington, Skilling, Helle, & Jackson). 1967d. Connection Manual. (Box CCC 233 Park

Avenue).

WSHJ (Worthington, Skilling, Helle, & Jackson). 1967e. Slab design - 10th to 87th floor, structural calculations. (WTC1-468-L).

WSHJ (Worthington, Skilling, Helle, & Jackson). 1967f. Structural calculations for slab design.

(WTCI-631-L).

WSHJ (Worthington, Skilling, Helle, & Jackson). 1973. Structural Design Specification - Antenna

Masts on the North & South Tower ofthe World Trade Center, (part of WTC1-50-L).

WTC 7 Project Specifications. 1984. (WTCI-187-P).

NISTNCSTAR 1-1A, WTC Investigation 59
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60 NISTNCSTAR 1-1A, WTC Investigation
Chapter 3

Tests Performed to Support Design Innovations

3.1 exterior wall panel tests

Scaled model tests were perfonned at the University of Western Ontario to dctennine load-deflection

characteristics of typical exterior wall panel units along the height of the building (Gardner 1966). One of

the main goals of these tests was to detemiine how the overall stiffness of the wall panels changed as changes were made in the sizes of the members that made up the wall panels (i.e., columns, spandrels,

and stiffeners). According to the report, it was anticipated that the results from these tests would help in determining the "most effective construction" for the wall panels.

In lieu of testing a typical wall panel, which was comprised of three columns and three spandrels,

subassembly ABCD depicted in Fig. 3-1 was tested. According to the report, this subassembly was

chosen for its simplicity, flexibility, and low cost. Models were built to a scale of one-quarter of full size and were fabricated from sheets of thermoplastic. The following advantages of using thennoplastic sheet were listed in the report: (1) it has a low modulus of elasticity, which produced large deflections for comparatively small loads, (2) it possesses linear stress-strain characteristics, similar to structural steel, and (3) it is easily machined and can be easily joined.

T

Jy^—

Source: Gardner 1966. Reproduced with permission of The

Port Authority of New York and New Jersey.

Figure 3-1. Subassembly used for testing external wall panel in WTC 1 and WTC 2.

NISTNCSTAR 1-1A, WTC Investigation 61
Chapter 3

Models of structural units were fabricated that replicated the external wall panels at floor levels 20, 47,

and 74. In some cases, stiffeners of vaiying thicknesses were added to the test model as described below.

The forces that were applied to the test models to simulate the forces acting on a unit of the actual wall

section are shown in Fig. 3-2. The models were tested in the test rig depicted in Fig. 3-3. The load in the "v" direction was applied to the models via chains with attachments that were adjustable so that the line of

action of the load passed through the shear center of the model. Axial load was applied to the model by a

threaded bar. Also shown in this figure are the stiffeners that were added to some of the specimens in order to measure their effect on the overall behavior.

Flange

Source: Gardner 1966. Reproduced with permission of The Port Authority of New Yorl< and New Jersey.

Figure 3-2. Loads applied to model of exterior wall panel.

Fifteen different tests were run—four for the case of the model replicating the 20th floor exterior wall,

nine for the 47th floor, and two for the 74th floor. In some cases, diaphragms were present and in other

cases, they were not. The effects of stiffener thickness, spandrel thickness, spandrel flanges

(see Fig. 3-4), depth between webs, and removal of outer webs were also studied.

The deflections and rotations that were measured during the testing are depicted in Fig. 3-5. Variation of

story deflection ( Aj ) was plotted as a function of load ( Py ) for the cases described above. In all cases, a

linear relationship was found between applied load and story deflection. The shear stiffness of a unit was

determined by dividing the load ( Py ) by the deflection ( A] ).

62 NISTNCSTAR 1-1A. WTC Investigation
Tests Performed to Support Design innovations

NISTNCSTAR 1-1A, WTC Investigation 63
Chapter 3

Source: Gardner 1966. Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 3-5. Displacements and rotations measured during model test program of

exterior wall panels.

The following conclusions from these tests were reported:

• Stiffeners are necessary for enhanced performance, but the thickness of the stiffeners is not

critical.

• Axial loads do not appear to affect the shear stiffness of the model.

• Spandrel flanges do not contribute to the shear stiffness of the model.

• The distance between the webs of the column should be the longest possible.

• The thickness of the spandrel increases the shear stiffness of the unit.

• The depth of the spandrel increases the shear stiffness of the unit.

• Increasing the thicknesses of either the column webs and/or the column flanges increases the

torsional stiffness of the model.

• Distortion due to twisting can be reduced by using thicker stiffeners.

64 NISTNCSTAR 1-1A, WTC Investigation
Tests Performed to Support Design Innovations

3.2 WIND TUNNEL TESTS

Wind tunnel tests were part of a four-pronged wind program that was developed by Worthington,

Skilling, Helle & Jackson (WSHJ) for the design of the World Trade Center (WTC) (WSHJ 1964). The

elements of this program were:

• Meteorological Program. The purpose of this program was to determine mean wind speeds,

return periods, the magnitude of wind shear and gradient, the directional characteristics of the wind, and the energy spectra of wind gusts that were expected at the site of the WTC.

• Wind Tunnel Program. The goals of this program were to (a) develop a physical model of

lower Manhattan and subject the model to wind velocities obtained from the meteorological

program, (b) obtain static and dynamic responses of the WTC towers, (c) study construction

problems (no additional infonnation on this could be found in the documentation), and

(d) study the effect of the structural parameters on the integrity of the towers.

• Structure Damping Program. The main objectives of this program were to detemiine the

critical damping ratio of the structural system and to detemiine ways of increasing this ratio.

• Physiological Program. The objective of this program was to determine acceptable levels of

response to wind-induced excitations as measured by perception levels of a cross-section of

the population.

The meteorological and wind tunnel programs are discussed in an 8-volume set of reports written by the

structural engineer, WSHJ. These reports are referenced in the following sections of this report. No

documentation was found on the structure damping program or the physiological program.

3.2.1 Meteorological Program

One of the basic requirements of the meteorological program was the acquisition of data from sources that measured wind velocity (WSHJ 1965a). According to the WSHJ report, both the mean wind speed and

turbulence characteristics were key items that needed to be determined. Air density corresponding to the extreme wind, a statistical distribution of wind speeds, and changes of wind velocity with respect to direction were other parameters that were needed as well.

The report points out that earlier studies of extreme wind speeds, including those carried out for the American Society of Civil Engineers (ASCE 1961), were not adequate for the design of the WTC towers

for the following reasons (WSHJ 1965a):

• They were general studies that did not address the specific environment at the site of the WTC.

They did not consider surface roughness to have an influence on wind speeds.

They did not relate specifically to building heights comparable to the WTC.

NISTNCSTAR 1-1A. WTC Investigation 65
Chapter 3

• They used wind speed parameters, such as the fastest-mile wind, that were not completely

appropriate for the WTC.

• They did not consider variations of extreme wind speed with respect to direction.

In order to help in detennining the extreme average wind speed that was expected at the top of the towers,

data from the following sources were examined:

• Annual maximum hourly average wind speed (1912-1958), annual maximum 5 min average

wind speed (1912-1958), and fastest-mile wind speed (1912-1959) from the U.S. Weather

Bureau Station at the Whitehall Building in lower Manhattan, which was less than a half mile

from the WTC site.

• Annual maximum hourly average wind speed from the Brookhaven National Laboratory

(1954-1964). Included were data relating to wind profile and hurricanes.

• Annual maximum hourly average wind speeds from weather stations on the Atlantic seaboard

in the Maritime Provinces of Canada for all years of record.

• Annual fastest-mile wind speed for all U.S. Weather Bureau Stations on the eastern seaboard

from Atlantic City, New Jersey, to Eastport, Maine, for all years since 1912.

• Records of surface winds from balloons launched at John F. Kennedy (JFK) Airport

( 1 956-1 964). Balloons were released and observed every 6 h. A statistical model for estimating the extreme wind velocity was developed based on a Fisher-Tippet

Type I theoretical distribution. It was reported that the agreement between the observed distributions

based on the data from the above locations and the theoretical distribution was satisfactory.

A study was also performed to determine a suitable mean wind velocity profile as a function of surface

roughness. The following relationship was reported to adequately represent the distribution of wind speed

with respect to height and exposure based on the data from the Brookhaven National Laboratory, the balloon study at JFK Airport, and the results from the wind tunnel tests:

where:

V. = wind velocity at height z

Vq = gradient wind velocity at height zq

The constants zq and a that were used in the study, which depend on the exposure, are given in Table 3-1.

(3-1)

66 NISTNCSTAR 1-1A, WTC Investigation
Tests Performed to Support Design Innovations

Table 3-1. Constants used in wind study of WTC 1 a nd WTC 2.

Exposure a 'C

Southeast direction (over Brooklyn)

0.30 1,250

Southwest direction (over open water)0.17 1,100

One other conclusion that was reported was that the wind speed at the top of the WTC towers was

expected to be approximately 1 .65 times greater than the wind speed at 355 ft above ground measured at the Brookhaven National Laboratory, based on Eq. 3-1.

A suitable averaging period for the design wind speed was also studied. In lieu of using averaging periods

that were used in routine meteorological observations (5 min average, hourly average, fastest-mile), the

report concluded that an averaging period should be selected considering the aerodynamic behavior of the towers and the wind tunnel tests. A 20 min averaging period was selected based on the following

considerations:

• Based on wind tunnel observations, a 20 min averaging time allowed steady-state response of

the towers to develop.

• The sampling period used in the Colorado State University (CSU) wind tunnel tests generally

corresponded to approximately 20 min.

An empirical relationship was developed for maxiinum wind speeds averaged over different periods. It was shown that the 20 inin average wind speed was expected to be approximately 1 0 percent greater than

the hourly average wind speed.

Based on a comparison of estimates of actual wind speeds obtained from the five sources noted above

(i.e., Whitehall Building, Brookhaven National Laboratory, Canadian weather stations, U.S. weather

stations, and JFK Airport), the following equation is given for the design 20 min mean wind speed V,. in miles per hour at the top of the towers for any return period r in years (WSHJ 1965a):

A study on air density was performed at the Whitehall Building. This study suggested that an appropriate

design value was 0.0024 slugs/ft^ at the bottom of the towers and 0.0023 slugs/ft^ at the top of the towers. These values were used to correct the wind tunnel results that were carried out at CSU.

The directionality of wind speeds was estimated from the balloon data at JFK Airport. It was found that winds were stronger from westerly and northerly quadrants, and that those from the southeast were the weakest. It was also observed that the direction of the strongest winds changed with height. On average,

the wind direction changed approximately 15 degrees between the surface and the top of the towers for the westerly wind quadrants and about 25 degrees for the easterly quadrants. According to the report,

these results were significant in the estimation of wind pressures on the towers.

(3-2)

NISTNCSTAR 1-1A, WTC Investigation 67
Chapter 3

Part III of the Final Chapter of the WSHJ Wind Report (WSHJ 1966a) re-examined the design wind

velocity equation presented in Supplement #3 of the Wind Program Interim Report (see Eq. 3-2 above),

since it was evident from the wind turmel tests, which are discussed in the next section of this report, that the response of the towers was highly sensitive to wind direction. It was reported that wind velocities based on a Weibull probability distribution P{V) closely fit the observations recorded at John F. Kennedy

Airport for wind velocities F greater than 16 m/s (36 mph):

where the velocity F in Eq. 3-3 is in meters per second.

Wind velocities less than 16 m/s (36 mph) were reported to have had only a small influence on the

structural perfonnance of the towers. According to the report, the Weibull distribution produced slightly

conservative values for wind velocities at the top of the towers assuming that these velocities were

equally likely from all directions, even though from the observed data, there appeared to be a higher

probability of stronger winds from the northwest and a relatively lower probability of the same from the

southeast. Wind velocities based on a Weibull distribution were also reported to adequately predict the maximum static plus dynamic deflections at the tops of the towers in both principal directions, which

were obtained from the CSU wind tunnel tests. As discussed in Sec. 2.3.2 above, these deflections were

used to determine the forces in the exterior columns and spandrels.

In order to obtain representative measurements of wind in the neighborhood of the WTC, anemometers

were mounted on the New York Telephone Building and the 40 Wall Street Building, which were both in close proximity to the WTC site in lower Manhattan. These sites, as well as the wind directions used in the wind tunnel tests, are depicted in Fig. 3-6 (WSHJ 1966b). The information from these measurements was used to adjust the characteristics of air flow in the wind tunnel tests, especially with respect to turbulence. Wind tunnel tests indicated that the velocity of the wind at the New York Telephone Building was similar to that at the same elevation at the WTC site. More details on the results of this study are contained in WSHJ (1966b).

3.2.2 Wind Tunnel Program

Wind tunnel tests were conducted at CSU and the National Physical Laboratory (NPL), located in Teddington, Middlesex, United Kingdom. Tests were conducted on single-tower and twin-tower

configurations subject to unifoitn and turbulent flow conditions. A description of the tests conducted at both locations follows.

Tests Conducted at CSU

Over 2,000 tests were conducted at the CSU Microclimatological Wind Tunnel to study the behavior of

static and aeroelastic models (WSHJ 1964). All work took place in the long test section, which made it possible to develop a boundary layer in the tunnel (WSHJ 1965b). The directions chosen for the wind

tunnel testing of the models of lower Manhattan corresponded to the most turbulent (southeast direction over Brooklyn) and the least turbulent (southwest over open water) directions.

68 NIST NCSTAR 1-1A, WTC Investigation

1.7 (3-3)
Tests Performed to Support Design Innovations

» / ' ——i t

-1^

)m)-.\ r i,\uY /

• :^'-^if-.-pv.^c^#''r:'-''''t''' ^H^^ CENTER SHOWING

^ LOCATIONS OF

V'-^f 3ijJ'^GALE.-W£>50c7v. ANEMOMETERS

•-—r v-vVx \ • '"-lis- FIG. 5!

Source: WSHJ 1966b. Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 3-6. Location of anemometers in wind study for WTC 1 and WTC 2.

According to WSHJ ( 1 965b), one of the most important requirements in the modeling process was to achieve correct simulation of the wind velocity profile (considering both surface roughness and its influence on wind velocity with respect to height) as it approached the model of lower Manhattan. From

the southeast direction, wind traveled across Brooklyn to the site of the WTC, which was a relatively

NISTNCSTAR 1-1A, WTC Investigation 69
Chapter 3

rough urban area. From the southwest, wind traveled mainly across open water. To simulate these

conditions in the tunnel at CSU, the Brooklyn fetch was represented by a bed of 1/4 in. to 1/2 in. gravel,

while the open water fetch was simulated by coarse emery cloth. Also, as discussed in Sec. 3.2.1, the mean wind velocity profile defined by Eq. 3-1 above was used.

Aside from wind velocity, the principal variables in the wind tunnel tests were the following

(WSHJ 1964): ,

• Spacing of towers

• Number of towers

• Damping

• Wind direction

• Boundary layer characteristics

^

• Relative stiffnesses of the models

It was found that the models oscillated in the wind due to vortex shedding, gust buffeting, and wake

buffeting under certain combinations of the above variables.

Two hundred tests were run at CSU to study the effect of tower spacing on the response of the buildings.

It was concluded that the "as planned" spacing was satisfactory.

Aeroelastic tests and measurements of steady pressure for single-tower and twin-tower configurations in unifonn flow (i.e., insignificant level of turbulence) constituted a major portion of the tests that were run

at CSU (WSHJ 1965c). Part of the purpose of these tests was to provide a comparison between the perfonnance of the models at CSU and at the NPL (Whitbread and Scruton 1965). The report concluded

that the aeroelastic tests at the two locations were in good qualitative and quantitative agreement.

The aeroelastic tests were designed to determine the predominant sway motion (i.e., deflections or amplitudes) of the towers and to provide a check of the steady-state component of the overturning

moment at the base. To determine the pressure distribution on the towers, tests were conducted using

models with pressure points along a regular grid. From these tests, shear forces and overturning moments

were obtained along the height of the towers.

Three aeroelastic models of the towers were constructed at CSU using a scale of 1/500, which was

dictated by the size of the wind tunnel. The basic components of the models included: (1) a rigid exterior

shell fabricated from Sitka spruce (a wood having high stiffness to weight properties), (2) spring elements

at the base that provided stiffness ratios about the two horizontal axes that corresponded to the full-scale structures, and (3) a damping unit that provided levels of structural damping between about 0.8 and

100 percent of critical damping (WSHJ 1965c). The model was based on prehminary studies that indicated that the largest amplitudes of the buildings would be associated with the fundamental mode of

oscillation and that the shape of the fundamental mode corresponded approximately to a straight line. Deformations were measured by strain gauges mounted on the model. Wind velocities were gradually

increased during the tests. Readings were taken for wind velocities up to 200 mph in the case of the low70 NISTNCSTAR 1-1A, WTC Investigation
Tests Performed to Support Design Innovations

frequency models and up to 140 mph in the case of the high-frequency models at 1 5 degree azimuth

intervals, except when large amplitudes were encountered; in those situations, readings were taken at 5 degree intervals. A discussion on the low- and high-frequency models used in the study is given later in this section of the report.

Models used for the pressure tests at the CSU were constructed of clear acrylic plastic at a scale of 1/500, the same scale used in the aeroelastic tests (WSHJ 1965d). Approximately 75 pressure taps were mounted

on the pressure models, and test results were obtained for the single tower (0 degrees to 45 degrees) and

the twin towers (0 degrees to 180 degrees).

During the tests, pressure differences were detennined between pressures measured at points on the model and the datum ambient pressure in the tunnel. Local pressure coefficients Cp were defined by the following equation:

^ REF - P ^ REF

where P and P/jf/r are the absolute pressures on the model and at the reference point, respectively,

9^REF / 2 is the reference velocity pressure, and Vfi£f is the wind velocity in miles per hour applied on

the model. An averaging process was used to determine average pressure coefficients on the tower in the two principal directions (see Figs. 17a through 17e in WSHJ [1965c]). From these average pressure

coefficients, shear force and overturning moment coefficients were obtained with respect to height. A

comparison of aerodynamic coefficients of overturning moments derived from steady pressure tests and

from aeroelastic model tests is given in Fig. 18 of WSHJ (1965c). It was reported that the results from

these tests were in good agreement. The results from the CSU tests were also compared to those obtained

at the NPL, and as noted above, the report states that resuhs from these two sets of tests were in good

qualitative and quantitative agreement.

The tests also indicated that large lateral deflections at the top of the building occurred for wind velocities

in the range of 125 mph to 130 mph for angles of incidence within approximately 10 degrees of normal

(see Fig. 3-7). The results are plotted in Figs. 19 and 20 in WSHJ (1965c). The deflections showed a

consistent dependence on the degree of damping and were shown to be inversely proportional to the damping ratio.

NISTNCSTAR 1-1A, WTC Investigation 71
Chapter 3

1 o°rh'i 0°

Wind

t

y

Produced largest

displacement in x-direction

10°

Wind 10°

Produced largest

displacement in y-direction

Source: WSHJ 1965c. Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 3-7. Wind directions that produced the greatest displacements at the top of the

tower during the wind tunnel tests.

Tests were also conducted at CSU using the southeast and southwest models of lower Manhattan

subjected to turbulent flow conditions (WSHJ 1966c)'. Both single-tower and twin-tower configurations were considered. Definition of the grid system and tower configurations used in the tests is illustrated in Fig. 5 of WSHJ (1966c), which is reproduced here as Fig. 3-8. Also shown in the figure are the fundamental frequencies of the towers in the two principal directions in cycles per second (cps). Included

in these tests were measurements of the maximum deflections at the tops of the towers (aeroelastic tests; wood models) and pressures along the height of the towers (thermoplastic models).

Similar to the other tests described above, test results for the single-tower model indicated that the most

severe oscillations were transverse to the wind and occurred with the wind blowing within a small range

of angles on either side of the nonnal to a face (see Figs. 9 through 13 in WSHJ [1966c]). The results also showed that an increase in turbulence, which was characteristic of the southeast model of lower

Manhattan, appeared to suppress vortex shedding but gave rise to turbulence excitation with increased

wind speed. Finally, it was observed that greater levels of damping reduced the dynamic response of the

single tower in all cases, more so in unifonn flow conditions than in turbulent conditions.

As noted in Sec. 3.2.1 of this report, it was found that winds were stronger from westerly and northerly quadrants. Wind from

the southeast direction was chosen in the wind tunnel program not because the velocity from this direction was the greatest, but because winds from this direction were the most turbulent (wind in this direction traveled over Brooklyn, which is a relatively rough urban area). Turbulence plays an important part in the dynamic excitation of structures, especially tall, slender

structures. A fundamental discussion on turbulence and resulting aeroelastic phenomena can be found in Simiu and Scanlon

(1996).

72 NISTNCSTAR 1-1A, WTC Investigation
Tests Performed to Support Design Innovations

0°

Source: WSHJ 1966c. Reproduced with permission of The Port Authority of New York and

New Jersey.

Figure 3-8. Definition of grid system and tower configurations for wind tunnel tests

at CSU.

Test results for the twin-tower model are plotted in Figs. 14 through 29 in WSHJ (1966c). These graphs,

which also include results from the wind tunnel tests conducted at the NPL (Whitbread 1967), give peak

amplitudes of oscillation (deflections) at the tops of the towers for a range of wind velocities, wind

directions, and degrees of damping for both the southeast and southwest models of lower Manhattan. In order to determine whether different time scales had an influence on the response of the towers due to wind velocity, two different time scales were considered in these tests. The first time scale was set equal

to the model scale raised to the two-thirds power, i.e., (1/500)"''' = 1/60. This time scale was used in what was referred to as the low-frequency model tests. The second time scale, which was used in the highfrequency tests, was set equal to 1/200. According to the report, with this time scale, the maximum wind

velocity of the tunnel would coincide with the maximum wind velocity that could reasonably be expected.

It was reported that since the natural frequency of vibration of the full-scale tower in the fundamental

mode was close to 0.1 cps, the required frequency of vibration of the model corresponding to a time scale of 1/60 (i.e., low-frequency model) was 0.1/(1/60) = 6 cps. Similarly, the required frequency of vibration

of the high-frequency model was 20 cps. These model frequencies were obtained by using different

stiffnesses of the springs attached to the base of the models, as described previously.

NISTNCSTAR 1-1A, WTC Investigation 73
Chapter 3

The following conclusions were made in the report on the test results for the twin-tower model

(WSHJ 1966c):

• In all tests, deflections (peak amplitudes) at the tops of the towers increased monotonically

with increasing wind velocity without any apparent peaks.

• At wind velocities below 150 mph, deflections at the tops of the towers from the southeast

model of lower Manhattan tested at CSU and NPL were qualitatively similar and had about

the same magnitude. At wind velocities greater than 150 mph, the largest deflections came

from the NPL tests. At a wind velocity of approximately 175 mph, the NPL deflections were

significantly larger. Deflections from the southwest model of lower Manhattan were less than

those obtained from the southeast model of lower Manhattan tested at CSU and NPL, but were qualitatively similar.

• Comparison of the high-frequency and low-frequency tests conducted at CSU indicated that larger displacements occurred in the southwest model of lower Manhattan with the highfrequency models. Results from the southeast model of lower Manhattan indicated the opposite effect.

• The largest displacements in all tests were found to be with wind from the directions noted in Table 3-2 below.

Table 3-2. Wind directions that produced the largest displacements at the tops of the

towers from the twin-tower wind tunnel tests. WTC 1 WTC 2

Building axis E-W N-S E-W N-S

Wind direction" 0°, 150°. 180° 90° 0°, 180°, 330° 270°

a. See Figs. 3-7 and 3-8 for definition of wind direction angle, a. Source: WSHJ 1966c. A comparison of the test results for the displacements at the top of WTC 1 in the north-south direction for wind blowing in the east-west direction (a = 90°, most severe case) is given in Fig. 30 of WSHJ (1966c)

and is reproduced here in Fig. 3-9. Results were plotted for the southeast and southwest models of lower

Manhattan obtained from tests at CSU as well as for those obtained from tests at NPL.

Based on the results obtained from the twin-tower wind tunnel tests, it was concluded in WSHJ (1966c)

that the response of the WTC towers was governed by three aerodynamic factors: (1) Magnitude of the

effective turbulence forces induced by the wind flow, (2) Magnitude of the effective forces induced by

vortex shedding and turbulence in the structure's own wake, and (3) Effective aerodynamic damping and

coupling forces generated by the motion of the tower through the airflow. It was also noted that the

effective mass, effective stiffness, the mode of vibration, and the mechanical damping of the towers

influenced these factors. A theoretical method was derived and was used to predict the dynamic behavior of the towers (WSHJ 1966c). Results from the theoretical models were compared to the results from the wind turmel

tests. A comprehensive discussion on this comparison can be found in WSHJ (1966c).

74 NISTNCSTAR 1-1A, WTC Investigation
Tests Performed to Support Design Innovations
Chapter 3

The results from the wind tunnel tests were used in the design of the exterior columns and spandrels,

which is discussed in Sec. 2.3.2 of this report.

The extensive wind tunnel testing that was perfonned to establish the lateral wind loads used in the design

of WTC 1 and WTC 2 was state-of-the-art at that time.

Tests Conducted at NPL

Tests were perfomied on single-tower and twin-tower models at NPL to measure deflections at the tops of

the towers in both smooth (unifonn) flow and turbulent flow conditions (Whitbread and Scruton 1965).

The models were constructed of light timber framework supported on diaphragms at 6 in. intervals from a

central 2 in. diameter aluminum tube. The models had an external covering of plywood.

Principal differences between the CSU and NPL models were (WSHJ 1965c): (1) the model scale was

1/400 at the NPL compared to 1/500 at the CSU, (2) displacements were determined from output of

accelerometers mounted near the tops of the models at NPL compared with strain gauges at CSU, and

(3) displacements were recorded on a resetting digital voltmeter at the NPL compared with chart records

at CSU. In the NPL tests, a grid of tubes in a plane normal to the wind stream was used to provide the required velocity profile over the height of the model. According to Whitbread and Scruton (1965), the velocity profile achieved in this manner was similar to that observed in the tests carried out at CSU on the model of lower Manhattan.

As noted previously, it was reported that the overall results obtained from the tests conducted at NPL

were in good qualitative and quantitative agreement with those obtained from the tests performed at CSU.

3.3 DAMPING UNIT TESTS

Two testing programs were carried out to test certain important properties of the damping units. These

programs were designed to help confirm the effectiveness and efficiency of the damping units in controlling building motion due to wind.

The Minnesota Mining and Manufacturing Company (3M) conducted the first set of tests in May 1967.^ Twenty-two full-size dampers were assembled and tested in accordance with the procedure outlined in Sec. Ill, paragraph b of the test report. The specimens, which were tested in a servo-controlled testing

machine, were subjected to cyclic axial defonnation in the fonn of a sine wave at 0.1 Hz frequency with a constant amplitude of 0.020 in. for 100 cycles. The specimens were also stretched or compressed

monotonically at a steady rate of 0.5 in. per minute until they were "physically broken." Although the number of tests that were run was insufficient for a rigorous statistical analysis, it was reported that the

results confirmed that the damper mechanical properties would meet or exceed the minimum

requirements prescribed in the specifications. The specifications for the damping units are given in Sec.

5.3.2 of this report.

- Letter dated June 22, 1967 and enclosure from Don Caldwell of 3M to Peter Chen of SHCR (WTCI-501-L; reproduced in Appendix B without appendices that are contained in WTCI-501-L).

76 NISTNCSTAR 1-1A, WTC Investigation
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Dr. S. H. Crandall of the Massachusetts Institute of Technology conducted the second test program during

1968 and 1969/ Thirty-nine prototypes, which consisted of the exterior column, the damping units, and

the floor truss system, were tested in a manner that simulated the in-place conditions of the damping

units."* Twenty units were tested according to the test procedures previously established for the first series of test that were perfomied by 3M, which, as noted above, consisted of cycling tests and monotonic

ultimate shear strength tests. Nineteen additional tests were performed to investigate the endurance

capabilities of the specimens under conditions that were different from the aforementioned tests. In particular, these tests included variations in (1) amplitude and frequency of the applied cyclic axial deformation, (2) ambient temperature, and (3) a static preload superimposed on the simple harmonic

loading. In all cases, the tests were performed in a specially built test frame, which was supposed to simulate the structural environment in which the damping units were to be placed (as noted above, the specimens were tested by 3M in a servo-controlled testing machine). In general, it was found that "...the energy absorbing capabilities of the elements are generally adequate to provide the expected damping

under design conditions and that the elements do perform satisfactorily under limited variations of loading

conditions, speed of oscillation, duration of oscillation, and ambient temperature." It was reported,

however, that specimens that were tested for ultimate shear strength would not meet the appropriate

acceptance requirements of the design specifications (see Sec. 5.3.2 of this report), due to a large standard

deviation.

A letter from Leslie Robertson of Skilling, Helle, Christiansen, & Robertson (SHCR) to Malcolm Levy of

the Port of New York Authority (Port Authority or PONYA) discussed deficiencies in the test equipment

used by Crandall, which may have had an effect on the test results." The possible influence of additional

bolt holes, which were made in the specimens in order for them to fit the test jig, on the ultimate strength

results obtained from this test program was noted in the SHCR review of the Crandall report (Crandall

and Wittig 1969).'' A response from Crandall to this review provided a more comprehensive description

of the testing machine that was used to deteraiine the ultimate shear strength, since the report contained a "somewhat abbreviated explanation."'' Additional testing of the damping units was also proposed by

Crandall after the dampers had been installed in the towers in order to compare those results to those that were performed previously in the laboratory. No evidence has been found that indicates whether these

tests were actually perfonned or not. A report was produced by SHCR that compared the two testing programs.^ Table 1 in the report contains

a summary of the methods employed in the two test programs, and Table 2 compares the results of the mechanical properties (dynamic stiffness, loss tangent, and ultimate strength) of the damping units. Major

differences in test results occurred with respect to uhimate strength: the tests performed by 3M indicated

that the ultimate strength of the units was satisfactory with respect to the design parameters (note: some of

' "Test Program for World Trade Center Viscoelastic Damping Units," by Stephen H. Crandall of MIT, May 20. 1968 {WTCI-501-L; see Appendix B).

" "Test of Viscoelastic Dampmg Units for World Trade Center Tower Buildings." S.H. Crandall and L.E. Wittig. April 23, 1969 (Box 9, 233 Park Ave.; see Appendix B).

^ Letter dated August 29, 1968 from Leslie E. Robertson of SHCR to Malcolm P. Levy of PONYA (WTCI-501-L; see Appendix B).

^ Letter dated May 22, 1969 from Leslie E. Robertson of SHCR to Malcolm P. Levy of PONYA (WTCI-501-L; see Appendix B).

' Letter dated June 2, 1969 from Stephan H. Crandall of MIT to John M. Kyle of PONYA (WTCI-501-L; see Appendix B).

* "World Trade Center Report No. DU-3, Viscoelastic Damping Units," by SHCR, June 2, 1 969 ( WTCI-50 1 -L; reproduced in Appendix B without appendices that are contained in WTCI-50 1-L).

NISTNCSTAR 1-1A, WTC Investigation 77
Chapter 3

the infonnation in the SHCR report, including design parameters, have been redacted), whereas, the tests perfonned by Crandall showed that about 20 percent of the damping units would be near or over the ultimate shear strength, which implies that they would fail in shear. According to the SHCR report, the reason for this discrepancy is not clear; however, the report goes on to state that perhaps this discrepancy

is due to differences in the test set up used in the two programs.

During construction of WTC 1, a number of damper units were installed in November of 1970 and

remained in place for almost a year, part of that time in unheated space. A request to test 12 of these

damper units for loss factor and stiffness, fatigue, and ultimate strength was made by Malcolm Levy of

the Port Authority to Don Caldwell of SM.*^ These tests were to help ascertain if cold temperatures during

the winter had any affect on the mechanical properties of the damper units. No results from these tests have been found in any of the documentation.

The damper units were periodically tested as part of the Structural Integrity Inspection program. Results

from that program are summarized in NIST NCSTAR 1-lC.

3.4 FLOOR TRUSS TESTS

3.4.1 Full-Scale Flexural Tests

According to Sec. 105.102 of the specification for the floor trusses, which was part of the contract between the Port Authority and Laclede Steel Company (PONYA 1967), full-scale load tests were to be

perfonned on completely fabricated floor truss components. A minimum of one load test was required for each of the 23 different types of floor trusses designated in the design drawings. During testing, two equal

concentrated test loads would be applied to the trusses in a test frame. Each load was to be applied at a panel point of the truss. For example. Fig. 3-10 shows the location of the concentrated loads that were

applied during testing of 32 in. deep short-span, long-span, and two-way floor trusses.'*^ In WTC 1 or WTC 2, a floor truss would be subjected primarily to a unifonnly distributed load on its top chord. Thus,

since the tests were conducted using concentrated loads instead of uniformly distributed loads, the unifoiTnly distributed loads had to be converted into equivalent concentrated loads (see footnote 10 for the reference that shows the details on this conversion). Included in Fig. 3-10 is the conversion factor (labeled "ECF" in the figure, which stands for "Elastic Conversion Factor") that was used to convert the bending moments obtained from the tests (based on concentrated loads) to bending moments based on

unifonnly distributed loads.

The floor trusses were to be cambered for a design load equal to the total dead load, which was specified

in the Design Criteria (see, for example, Fig. 2-4 in Sec. 2.2.1 of this report). Midspan deflections were measured for various target loads, including the design load, and were compared to the cambers that were

specified in structural drawing number 7-AB1-54. Results were found for the flexural tests for Shipment

No. 2 in May of 1969." Tabulated results (deflection vs. total applied load) from these tests are shown in Fig. 3-1 1, including the results for Test No. 27, which is depicted in Fig. 3-10. Also shown in Fig. 3-1

1

Letter dated November 5, 1971 from Malcolm P. Levy of PONYA to Don Caldwell of 3M (WTCI-513-L; see Appendix B). Letter dated April 3, 1969 from David B. Neptune of the Laclede Steel Company to W.C. Borland of PONYA (WTCI-503-L;

see Appendix B). " Internal Laclede Steel Company memo dated May 15, 1969 from David B. Neptune to R.D. Bay {part of WTCI-82-I; see Appendix B).

78 NIST NCSTAR 1-1A. WTC Investigation
Tests Performed to Support Design Innovations

are the design displacements (column 3), which are the cambers given in structural drawing number

7-AB1-54 for the various floor trusses. As noted above, the design loads (column 4) are the total dead

loads specified in the Design Criteria. The design load of 58 psf for the long-span trusses can be found in Fig. 2^ of this report. Maximum deflections at midspan as a function of total applied load were reported

for the 32 in. deep trusses in Shipment No. 2 and are shown in Fig. 3-12.

C32T 3 TRUSS MTX;: 12/19/68

3>. 58'3" - 58.250'

a-. l5'0-l/2" - 19.042'

i9'2-l/2" - 19.208'

Case H :a<b

cone END COLUMN ENO xav ^ ^ 4 a9.2Q8WS^.oa4^ - rj2

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 3-10. Location of concentrated loads in the full-scale testing of the floor trusses

in WTC 1 and WTC 2.

BOHr,D TRADE CEI.TBR May 15, 1^6s PLEXUHAL TESTS Page 1 of £

Coraparatlv« DeflectlonB in Inches for IncrenentBi 'far(;et Loads of 500 Pounds

f3) 0-)

I DESIGIJ DEFL

Xn.

DESIGN

LOAD

Pcf

TA.'torT 1.0/.D DS f51

1

1 ViW. NO. 1 TEST

'>00 i 1000 16 00 i'l >ao 1 "iOOO i 7500 'J 000 5000 i 2« a. 000 . _

l.55i . bos 060 i i.-^.v^ f 1.605 \ 1.910 2.170 ! 2.-l<J)7 2 751 t f.2 . ?07 i JioH 77*i i .QO" 1 l.'^W ! 1.7S2 1 -6? i 62 . s l^i 1.178 1 i.hiM 1 l.6J<6 1.888 1 2.120 2 367 20 j 0.750 o2 .370 i .708 1 . 0?9 1 ^•033 1 2.372

cv>r5 i 0.37=, =0; ,104 ! ,201 .209

.

_-m!, .602 ! ^.^ 0.<;39 d'd Una. .tiii6 I - i or, 6. l.-^io r.y- 1 1 .?57 1 l-ii'lO 1 5.675,.. r> 26 2.000 6? .?S6 1 .'-Al . 807 1 l.'iJ'l 1 ,v572j 1.8^9 2.122 1 2.«00 700 '

i.<;i8 6? .£^1 _l»Me_l 1.700 l.Oii-s i.Ji5 ! C32T10 ; 30 1,683 6;.' .^60 ^ .711 l.rAp 1 ?7X| 1 . 6>1<3 i 2.377 2.729 \ - ^1 1.3'i3 ('£ .boo .tU^ 1 i'Vi '. 1 .^i^fi i 2,7Ci- i ^? 62 .113 i .'i?8 i . '^70 ' - 1 - 1

^ii O.J138 6s . 1 09 i .IW .?f« Ji6'( i - 1 36 e.ooo '58 i .^^"^ ,&'-il I 1.377 j 2.226 i g.'iOO 2 ei2 i 3<? ».0(;3 6S 1 .511 , 7',a 977! 1.214 1 1.465 ! 1.716 1.9»7 i 2.2^21 2 51<? i ifO 1 . 3 ' 5 62 .i!l6 1 .765 lI.II? 1 461; 1 i.82';>l 2.165 1 2.575 2.017 i

i V J

1' i<fi .116 1 4^0! .6^8 1 - i 37 0.67=, 6? .07? i .1^4 2561 .387 1 .51 =

i 1

1 -— •

1 1

'—I'—\—

•

Source: Laclede Steel Company 1969. Figure 3-1 1 . Results from full-scale flexural tests of 32 in. deep floor trusses.

NISTNCSTAR 1-1A, WTC Investigation 79
Chapter 3

TOTAL LOAD (2P) IN POUNDS

Source: Laclede Steel Company 1969. Figure 3-12. Maximum midspan deflections from full-scale flexural tests of 32 in. deep floor trusses.

3.4.2 Shear Knuckle Tests

Composite action was achieved between the floor trusses and the concrete slab by extending diagonals

above the top chord (see Sec. 5.4.1 of this report). The "knuckle" acted like a shear connector, which

made the floor trusses and concrete slab act in a composite manner.

A test program was undertaken at Laclede's Madison plant to detennine the failure loads of the shear

knuckles. Failure loads were determined for specimens subjected to transverse and longitudinal loads. In

the transverse tests, shear knuckles were embedded in lightweight concrete (110 pcf) similar to the type

that was used in the WTC, while in the longitudinal tests, the shear knuckles were embedded in normal

weight concrete (152 pcf). It is not evident from the documentation why normal weight concrete was used

in the longitudinal tests.

Results were found for transverse and longitudinal shear knuckle tests conducted in September 1967 (see

Fig. 3-13 for the longitudinal test setup).'" Tabulated results from the longitudinal tests are given in Fig.

3-14. A summary of the shear knuckle tests that were completed to that date was reported to SHCR.'~^

According to the letter, shear strength of the knuckles determined from both transverse and longitudinal

testing were found to be well over the allowable values assumed in design.

Internal Laclede Steel Company memo dated September 7, 1967 from J.R. Paul to A.C. Weber (WTCI-85-I; see Appendix B). " Letter dated August 10, 1967 from A. Carl Weber of the Laclede Steel Company to Wayne Brewer of SHCR (WTCI-235-L;

see Appendix B).

80 NISTNCSTAR 1-1A, WTC Investigation
Tests Performed to Support Design Innovations

LO/tD

ELEVATION A

Source: Laclede Steel Company 1967. Figure 3-13. Test setup for longitudinal shear knuckle tests.

NISTNCSTAR 1-1A, WTC Investigation 81
Chapter 3

Spec

.

Cyl. pouwtd Date Tested

Curing

Time

Days

Tot a 3 Load

jiff Strcngti PSI Comments

A 5-U-67 6-I-fa7» 28 3891 —

B 5-i»-67 6-l-67*r! 28 i*687

C 8-8-67 96 80700 2850

1 5 -"-67 6-3-67 30 59850 At it5,000# faint popping noises heard

2 5-4-57 6-3-67 30 65660 At 60,000# faint popping noises heard

3 5-4-67 6-3-67 30 At i40,980# weld broke

Shear Test

Specimen 4 • * Specimen 5 • •

Deflection Deflection

Load #

Rl

In. R2m.

Load Rl

In, R2

in. 500 0 0 500 0 0

4,000 .003 .011 4,000 .001 .007

8,000

10,000

.017 .018 8,000 .003 .020

.018 .024 10,000 .005 .024

15.000 .022 .029 15.000 .006 .027

20,000 .023 .033 20,000 .007 .027

25,000 .026 .033 25,000 .0075 .027

30,000

Filler Weld Brone 60,550 Weld Broke

Main Weld EroM

NCTTES: Concrete supplied by:

Lyberger Supply, Granite City, 111. Max. Aggregate = 1" Approximate Slump = 5" Ave. Cylinder Wt. - 29.90#

Wt. Per Cu. Ft. = 152# Typical specimen as shown on Left. Cylinders used were standard 6" diameter

testing cylinders.

•Tested by Pittsburgh Testing Laboratory,

••poured frcraj same batch and same day as other specimens.

Source: Laclede Steel Company 1967. Figure 3-14. Results from longitudinal shear knuckle tests.

3.4.3 Interior Panel Connection Tests

A test program was established to verify the horizontal and vertical design loads for two connections

between the 32 in. deep floor trusses and the 24 in. deep bridging trusses (Laclede Steel Company 1968). Tests for 4C connections (5 kip connections of 24T bridging trusses to C32 trusses at center panel) were

run in the testing laboratoi^ at the Madison Plant of Laclede Steel Company. The test setup at the Madison Plant for the case of horizontal loads applied to the welds connecting the bridging trusses to the main floor trusses is depicted in Fig. 3-15. Load was applied raonotonically until failure, and the horizontal and vertical deflections of the transverse bridging truss with respect to the connection to the 32 in. floor truss were recorded. Resuhs from one of these tests are shown in Fig. 3-16.

82 NISTNCSTAR 1-1A, WTC Investigation
Tests Performed to Support Design Innovations

TKST MACHIHXj R1A1« B«m Typ* 200*^ CapMslty

IX8T COWDOCTID AT UCLKDl'S MADlflOH, ILL. PLAJiT T*SX SXT OP AS SHOra BKl^.

lead

I Te»t MMhtn« H«*d K , ~]

Oaflcotion

Loose 3

2 Welds i X 1^

cSf-—^- 1-^

C32T

Section A-A

Source: Laclede Steel Company 1968. Figure 3-15. Test setup for interior panel connection test - horizontal load on welds.

The test setup for vertical loads applied to the welds is depicted in Fig. 3-17. This test setup was

approved by SHCR, subject to the following additional requirements:

'"^

• The top chords of the C32T floor trusses were to be approximately 7 in. apart.

• The tests were to be conducted with the following weld sizes: 1/4 in. by 3 in., 5/16 in. by

3 in., and 3/8 in. by 3 in.

Two sets of tests were to be conducted: one set with the knuckle restrained and one set with the knuckle

unrestrained. According to the letter, the latter set of tests would allow evaluation of the joint strength

under construction loading conditions.

Similar horizontal and vertical tests for 5C connections (over 5 kip through 15 kip connections of 24T

bridging trusses to C32 trusses at center panel) were run at the Urbauer Laboratory at Washington

University, St. Louis, Missouri.

Average recorded failure loads for both 4C and 5C types of connections were equal to at least twice the design values (Laclede Steel Company 1968).

Letter dated April 19, 1968 from Wayne A. Brewer of SHCR to R.M. Monti of PONYA (WTCI-87-I; see Appendix B).

NISTNCSTAR 1-1A, WTC Investigation 83
Chapter 3

HKD 8Y DATE fAJlEL Cj^lWOTIoaS UKDM KCRIZO^^ I.QA1>

LOAD

_

Znsh*s

10 3 27/32 0

10.5 3 13/16 .0112

11.

0

3 25/32 .0625

11.5 3,2/3 .0687

12.0 3 23/32 .1250

*11.5 3 9/16 .2807

12.0 3 17/32 .3125

12.77 3 7/32 .6250

32" TOP CHORD DEP

Source: Laclede Steel Company 1968. Figure 3-16. Results from interior panel connection tests - horizontal load on welds.

Source: Laclede steel Company 1968. '

Figure 3-17. Test setup for interior panel connection test - vertical load on welds.

84 NISTNCSTAR 1-1A. WTC Investigation
Tests Performed to Support Design Innovations

3.4.4 Bearing Capacity Tests

Two types of tests were performed to determine the bearing capacity at the ends of the floor trusses.'^ The

first set of tests was designed to determine the bearing strength of the as-designed floor trusses. The test setup for these tests is depicted in Fig. 3-18 and the test results are shown in Fig. 3-19 (see reference

given in footnote number 15).

Figure 1 - TYPES OF BEARING CONDITTONS

Note

:

The particular truss configurations shown below are general and are not meant

to represent a particular type of truss or end condition (column end vs. core

end) . The sketches shown below do represent the actual bearing condition and

whether or not additional arc welding was used at the bottom end of the vertical

VI strut. Load

Machine head

Defl. Gauge

Load

\\-~>.:.\T. . \v"rf" Machine head

T^^^r^'^l" X 2" X 5" g,

•^—••^f^-t

.NNV

Load

TYPE A

2" Bearing

No arc welding on end

>r- Mactii^head^^

-vw'.W- .. S?^ 1" X 4" X 5" E.

26"

TYPE C 4" Bearing No arc welding on end

Befl. Gauge

o

Load

TYPE B 2" Bearing Arc welded end Machine head

7^

"^1^-1" X 4" X 5" a.

26'*

TYPE D

4" Bearing

Arc welded end

Defl. Gauge

>_,

Source: Laclede Steel Company 1969. Figure 3-18. Test setup for first set of bearing capacity tests on floor trusses.

Internal Laclede Steel Company memo dated March 18, 1969 from David B. Neptune to R.D. Bay (part of WTC1-82-I; see Appendix B).

NISTNCSTAR 1-1A, WTC Investigation 85
Chapter 3

GROt NO. 1 BEARIHG CAPACITY OF TRUSS ERDS

rest

Kc. C32T6

C32T6

C32T1

C32T1

2hT}M

tead at .06" Uitiinate

:st Deflection (Z\ Load

Soeciner. (1) KIPS •KIPS .'ations

- Type Coliann end 30 ''^8 Angles bent - v7eld failure

- Type A, Gore end 50 50 Test stepped •- no failure

- Type B, Cc3ut»n end 32 Angles iient - no weld failure

- T^rpe B, Ccliann eriii •^0 Aiigles bent - no weld failure

- T:;pe c

,

Coitimn end OO Angles bent - no v;ei£l failure

- T.vpe Gore end 38

52 Angles bent - no weld failure

- Type A, ColiKnn end 50 Angles bent - no weld failure

Notes:

(1) For speciJTsen type identification see drawing (Figure 1) on page 3-

(2) Load at .06" deflection as measured 22" froM esd of oearing is eqwivslent to a total

deflection at midspatn for a 6o foot span of .98" and to a total deflection at tr.idspan for a 35 foot span of .60", Source: Laclede Steel Company 1969. Figure 3-19. Results from the first set of bearing capacity tests on floor trusses.

The following is a summary of the test results:

• Only one test resulted in a broken weld and this was at a load greater than the load that caused the initial bending of the angles.

• Using a 2 in. bearing length (Types A and B in Fig. 3-18) resulted in a "more critical loading

condition" than using a 4 in. bearing length (Types C and D in Fig. 3-1 8). Deformation of the angles with a 2 in. bearing length occurred sooner than with a 4 in. bearing length.

• The weld failure load at the core end connection was found to be greater than that at the

column end.

• Arc welding the bottom of the vertical strut decreases the possibility of a weld failure.

In all of the cases tested in the first set of tests, the ultimate load of the bearing capacity of the floor truss ends was shown to be greater than the design loads.

The purpose of the second set of tests was to deteraiine the strength of repaired bearing ends that would

be welded onto floor trusses at the jobsite. According to the report on these tests (Laclede Steel Company 1969), it was sometimes necessary to perform such modifications after the resistance welding was completed. Two types of tests were performed. The first type of test, which is depicted in "Figure 2-A" in Fig. 3-20, tested the capacity of the end as a unit (see reference given in footnote 15). In

the second type of test, the strength of each joint in the bearing end was tested (see "Figure 2-B" in Fig. 3-20). The load capacities of the arc welded bearing ends obtained from these tests are shown in Fig. 3-21. The report concluded that the floor truss bearing ends, repaired in accordance with the procedure outlined in that report, were capable of carrying a load "substantially higher" than the design

end reaction (Laclede Steel Company 1969).

86 NISTNCSTAR 1-1A, WTC Investigation
Tests Performed to Support Design Innovations

LoacI

e!» CM 1" X 2" X 5" Plate

I J 1/4" Weld

A" Figure 2-A-BEARING TEST

Load

0

Load Load

0

1/4" Weld

Figure 2-E-WELD SHEAR TEST

Source: Laclede Steel Company 1969. Figure 3-20. Test setup for second set of bearing capacity tests on floor trusses.

GROUP NO. 2

LOAD CAPACITY" OF ARC WELDED BEARING ECTS

Test

1

2

Bearing Test (Figure 2-A)

Angle

Size

2" X 1 .37"

.37"

Web

Size

1 111"

Failure

Load

27K

25K

Type of

Failure

Angle bend

Angle bend

Other

Cominents

55K - No weld fail\jre

59K - No weld failure

Weld Shear Test (Figure 2-B)

:

Test

No.

3

Angle

Size

2" X It" X .37"

2" X It" X .37"

Web

Size

1.14"

l.lV'

Load at Weld Failure (lbs.]

Point 1 Point 2 Point 3

1^830

44180

56100

61000

33250

29250

Source: Laclede Steel Company 1969. Figure 3-21 . Results from the second set of bearing capacity tests on floor trusses.

NISTNCSTAR 1-1A, WTC Investigation 87
Chapter 3

3 5 STUD SHEAR CONNECTOR TESTS

A testing program was established to determine the horizontal shear capacity of 3/4 in. diameter by

4 1/2 in. long stud shear connectors welded through the troughs of Roll Form Type "B" steel deck and

embedded in a lightweight concrete slab. These tests were needed, since, as noted in Sec. 2.3.4 of this report, the allowable shear load for such connectors in concrete with aggregates not conforming to ASTM

International C 33 (i.e., the specification for nonnal weight aggregate) was to be established by a suitable

testing program (AISC 1963). Requirements for the test program were outlined in a letter from SHCR to Bethlehem Fabricators.'^' A work order was sent from the Port Authority to the Fritz Engineering

Laboratory at Lehigh University to perforai the tests on the specimens.'^

It has not been possible to locate any results from this testing program.

AISC (American Institute of Steel Construction). 1963. Specification for the Design, Fabrication, and

Erection of Structural Steel for Buildings. New York, NY.

ASCE (American Society of Civil Engineers). 1961 . Wind forces on structures-ASCE Final Report of

Task Committee on Wind Forces. Transactions ASCE, vol. 126, Part II.

Crandall, S.H., and Wittig, L.E. 1969. Test ofViscoelastic Damping Units for World Trade Center

Tower Buildings. (Box 9, 233 Park Avenue).

Gardner, N.J. 1966. Report on Tests on Models ofthe Structural Units ofthe World Trade Center. The

University of Western Ontario, (part of WTCI-67-L).

Laclede Steel Company. 1968. Tests to Verify the Design Strength ofInterior Panel Connections of24T

Bridging Trusses to C32 Trusses. (WTCI-68-I).

Laclede Steel Company. 1969. Report on Load Tests to Compare the Capacity' ofNormal Production

Truss Bearing Ends with Trusses Having Repaired Bearing Ends. (WTCI-505-1).

PONYA (The Port of New York Authority). 1967. Fabricated Steel Floor Trusses, Bridging, Beams and

Bracing for Prefabricated Floor Units for North and South Towers. World Trade Center Contract

WTC-221.00 (WTCI-71-I).

Simiu, E., and Scanlon, R.H. 1 996. Wind Effects on Structures - Fundamentals and Application to Design. Third Edition. John Wiley & Sons, Inc., New York, NY.

Letter dated November 3, 1969 from James White of SHCR to Lester S. Feld of PONYA (part of WTCI-253-L; see Appendix B). Contract dated January 6, 1970 from Guy F. Tozzoli of PONYA to Roger G. Slutter of the Fritz Engineering Laboratory,

Lehigh University (part of WTCI-253-L; see Appendix B).

3.6 REFERENCES

88 NISTNCSTAR 1-1A, WTC Investigation
Tests Performed to Support Design Innovations

Whitbread, R.E. 1 967. An Investigation of the Aerodynamic Stability ofa Model of the Proposed Tower

Blocks for the World Trade Center, New York, Part II - Further wind-tunnel studies relating mainly

to the response ofthe tower blocks to turbulent winds. National Physical Laboratory, (part of

WTC1-68-L).

Whitbread, R.E., and Scruton, C. 1965. An Investigation of the Aerodynamic Stability ofa Model of the Proposed Tower Blocks for the World Trade Center, New York, Part I - An investigation ofthe amplitudes ofoscillation in the fundamental modefor a range ofwind conditions. National Physical

Laboratory, (part of WTCI-271-L).

WSHJ (Worthington, Skilling, Helle, & Jackson). 1964. The World Trade Center - Wind Program

Interim Report. December. (WTC1-559-L).

WSHJ (Worthington, Skilling, Helle, & Jackson). 1965a. The World Trade Center - Wind Program

Interim Report. Supplement #3 with Appendix. June, (part of WTCI-68-L).

WSHJ (Worthington, Skilling, Helle, & Jackson). 1965b. The World Trade Center - Wind Program

Interim Report, Supplement #1 . January. (WTCI-561-L).

WSHJ (Worthington, Skilling, Helle, & Jackson). 1965c. The World Trade Center - Wind Program

Interim Report. Supplement #2. 1965. (part of WTCI-68-L).

WSHJ (Worthington, Skilling, Helle, & Jackson). 1965d. The World Trade Center - Wind Program

Interim Report. Supplement #4. January. (WTC1-558-L).

WSHJ (Worthington, Skilling, Helle, & Jackson). 1966a. Part lU, Final Chapter, WSHJ Wind Report.

July. (partofWTCl-68-L).

WSHJ (Worthington, SkiUing, Helle, & Jackson). 1966b. Part II, Final Chapter, WSHJ Wind Report.

July. (partofWTCI-68-L).

WSHJ (Worthington, Skilling, Helle, & Jackson). 1966c. Part I. Final Chapter. WSHJ Wind Report.

June. (partofWTCI-68-L).

NISTNCSTAR 1-1A. WTC Investigation 89
Chapter 3

This page intentionally left blank.

90 NISTNCSTAR 1-1A, WTC Investigation
Chapter 4

Port Authority Policies and Agreements with New York City

Department of Buildings

A memorandum of understanding between the Port Authority of New York and New Jersey (Port

Authority or PANYNJ) and the New York City Department of Buildings was established in 1993.' Even

though it was a "long-standing policy" of the Port Authority that its facilities meet or exceed New York

City Building Code requirements, the purpose of this document was to fonnally restate that policy.

Specific commitments were made by the Port Authority to the Buildings Department that would ensure

that any building construction project undertaken by the Port Authority or by any of its tenants at the buildings owned and operated by the Port Authority that were located within the Department of

Buildings' jurisdiction would confomi to the New York City Building Code.

A summary of this agreement follows:

• The Port Authority was to thoroughly review and examine all plans for conformance with the requirements of the then current New York City Building Code. Such reviews were to be

conducted by New York State licensed professional engineers or architects retained or employed by the Port Authority. Plans for projects undertaken by Port Authority tenants were

to be prepared and sealed by a New York State licensed professional engineer or architect retained or employed by the tenant. Similarly, for projects undertaken by the Port Authority,

plans were to be prepared and sealed by a New York State licensed professional engineer or

architect retained or employed by the Port Authority.

• The Port Authority was to maintain a file containing the most recent drawings, plans, and

other documents required in connection with the review of the project for code conforaiance.

• The Port Authority was required to obtain the certification of a New York State licensed

professional engineer or architect that any tenant project undertaken at any of its facilities was constructed in accordance with the approved plans and specifications for the project.

Such certification was to be kept in the project file described above.

• The Port Authority was required to provide copies of any project files to the Department of

Buildings at any time.

• The Port Authority was to promptly advise the Department of Buildings of any variances

from code requirements that were proposed on a project. In cases where the Department of

Buildings believed that such variances were unacceptable, further review by the Port Authority Board of Commissioners was required.

Memorandum of Understanding Between the New York City Department of Buildings and the PANYNJ, 1993 (WTCI-160-P;

see Appendix C).

NISTNCSTAR 1-1 A, WTC Investigation 91
Chapter 4

• The Port Authority was required to perfonn building inspections and structural integrity

inspections on a cyclical basis for all of its structures located in New York City.

• The Port Authority was responsible for life safety in buildings at its facilities. The

Department of Buildings was not responsible for any type of inspection or review.

• Personnel from the Port Authority and the Department of Buildings were not to be held

personally responsible under any provision of this agreement.

A supplement to this agreement was executed in 1995." The supplement added that the design

professional responsible for performing the review and certification of plans for World Trade Center

tenants must not be the same design professional providing certification that the project had been

constructed in accordance with the plans and specifications.

' Supplement to Memorandum of Understanding Between the New York City Department of Buildings and the PANYNJ, 1995 (WTCI-113-P; see Appendix C).

92 NISTNCSTAR 1-1A. WTC Investigation
Chapter 5

Innovative Systems, Technologies and Materials, and

Acceptance Procedures Used by the Port Authority

5.1 innovative features of the structural system

The structural system, comprising the lateral-force-resisting as well as the gravity-load-carrying systems,

of World Trade Center (WTC) 1 and WTC 2 towers incorporated several innovative features including

the following:

1 . The towers represented one of the earliest applications of the framed-tube lateral-force- resisting system to super high-rise buildings (see Sec. 5.2).

2. Unifonn perimeter column geometry (14 in. by 14 in. cross-section) was maintained over most of the height of the 1 10-story buildings.

3. Fourteen different specified grades of steel were used to allow the perimeter column

geometry to remain uniform throughout the heights of the buildings.

4. Deep spandrel plates were used as beam elements connecting perimeter columns, enabling

framed tube action by strapping around the structure.

5. Prefabrication of steel construction was extensively used, through using 3 -column-wide by

3-stories-high panels, bolted butt-plate column splices, and high-strength bolted shear

connections of the spandrel plates.

6. Specially designed comer panels with chamfered edges were used to facilitate force transfer around the comers of the framed-tubes.

7. Long-span floor trusses were used for the floor systems. Composite action was achieved

between the floor trusses and the concrete floor slab by extending the truss diagonals above

the top chord into the slab. The concrete floor slab acted as a rigid diaphragm, which

distributed the lateral forces to the elements of the tube according to their stiffnesses.

8. Viscoelastic dampers connecting the floor trusses to the perimeter framed tube system were

used in each tower to control dynamic response, as discussed in Sec. 5.4.

9. Extensive wind tunnel testing was perfonned to establish the lateral wind loads used in the design of the towers.

It is important to note that except for Items 7 and 8 above, the innovative features were not appraised by

acceptance procedures. Such procedures for Items 7 and 8 are discussed in Sees. 5.3 and 5.4, respectively. Tests to support the design innovations were done for Items 5, 7, 8, and 9.

NISTNCSTAR 1-1A, WTC Investigation 93
Chapter 5

5.2 LATERAL-FORCE-RESISTING SYSTEM OF WTC 1 AND WTC 2

The exterior walls of WTC 1 and WTC 2, comprised of steel columns and spandrel plates, were designed

to resist the lateral forces and a portion of the gravity forces. Above the 7th floor, the columns were welded steel plate box columns, spaced 3 ft 4 in. on center. The columns and spandrels were shopassembled and welded into 36 ft high by 10 ft wide panels, which consisted of three columns and three spandrels as shown in Fig. 5-1 (WSHJ 1967a). These panels were erected on site by bolting the base plate

of an upper column to a cap plate of a lower column. Such splices were staggered so that only one-third

of the panels were spliced at each story level, except at the base of the building and at the mechanical

floors where all of the panels were spliced at the same level. In such cases, supplemental welds were employed to improve connection capacity. Spandrels were connected at midspan with high-strength

bolted shear connections.

o

1

1

1

1

. -.^

1:

1

1

1

fi

i

. , _i

SpandreS^

Column^^

Exterior Co;.In* ""j^ i-L^ <m~

f ,/|„^...„U„',„t ^...,4,

rj«Ke ^ --^ _ ..... Section A-A

Source: WSHJ 1967a. Reproduced with permission of The Port Authority of New York and

New Jersey.

Figure 5-1. Exterior wall panels in WTC 1 and WTC 2.

94 NISTNCSTAR 1-1A. WTC Investigation
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Below the 7th floor, the columns were typically spaced 10 ft 0 in. apart. The transition from three columns to one column occurred just below the 7th floor level as illustrated in Fig. 5-2.' Below the 7ih

floor, where there were fewer perimeter columns, bracing was used in the core area to increase lateral stiffness, and the core columns were designed to resist a portion of the lateral forces.

WORTHIN«TOW. tXIUINO. Nfllf « lACKSOH Civil « }1t

Section AA

Source: WSHJ 1967b. Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 5-2. Exterior wall panel transition in WTC 1 and WTC 2.

This structural system is considered to be a framed-tube system (closely spaced columns and deep

spandrel members) (Khan 1983). In such systems, the frames parallel to the applied lateral forces act as the webs of the tube and resist the shear from the lateral forces through bending of the beams and

columns in the frames. The floor system is considered a rigid diaphragm and is typically assumed to distribute the lateral forces to the elements of the tube according to their stiffness (although in the case of

WTC 1 and WTC 2, no evidence was found from the calculations that diaphragm action was explicitly

Structural drawing 2-AB2-2 (WSHJ 1967b).

NISTNCSTAR 1-1A, WTC Investigation 95
Chapter 5

considered in the design). Portions of the nornial frames close to the comers of the tube act as flanges of

the parallel frames. When subjected to lateral forces, the columns in the windward wall (flange) are subjected to tensile forces, while those on the leeward wall (flange) are subjected to compressive forces. Framed-tube systems do not behave as a true cantilever when subjected to lateral forces. The flexibility of

the spandrel beams produces a shear lag that increases the axial forces in the comer columns and reduces

the axial forces in the inner columns of both the flanges and the webs. A representative stmctural framing

plan of a typical floor in WTC 1 or WTC 2 is shown in Fig. 5-3.

Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 5-3. Representative structural framing plan on a typical floor of WTC 1 or WTC 2. WTC 1 and WTC 2 are early examples of super high-rise buildings that were designed based on the framed-tube concept. The first application of a framed-tube system was the 43 -story DeWitt-Chestnut

apartment building (later renamed The Plaza on DeWitt) in Chicago, which was completed in 1965.

Designed by Skidmore, Owings & Merrill, this 395 ft tall building used reinforced concrete for the

96 NISTNCSTAR 1-1A, WTC Investigation
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structural framing system. Since then, many variations of this structural system were used in a number of

buildings, which were constructed between the mid-1960s through the early 1970s. A number of major

buildings that have incorporated the framed-tube concepts in the United States include:

• Brunswick Building, Chicago, Illinois. Completed in 1965, this 38 story, 550 ft tall reinforced concrete office building designed by Fazlur Khan of Skidmore, Owings & Merrill utilizes a tube-in-tube system. In this system, the shear walls in the core area form an inner tube and the closely spaced columns with deep spandrel beams at the perimeter of the building form the outer tube.

• John Hancock Center, Chicago, Illinois. Diagonal braces supplement the steel framed-tube

system in this lOO-stoiy, 1,127 ft tall mixed-use building, which was completed in 1969.

Skidmore, Owings & Menill designed this building as well.

• One Shell Plaza, Houston, Texas. Skidmore, Owings & Merrill, also designed this 50 story,

714 ft tall building. Completed in 1971, it uses a tube-in-tube structural system of reinforced

concrete.

• Aon Center, Chicago, Illinois. At 1,136 ft tall, this 83-story steel office building, which was

formerly known as the Amoco Building and before that as the Standard Oil Building, was

completed in 1973. This steel office building utilizes a framed-tube system. Perkins & Will was the structural engineer for this project.

• Sears Tower, Chicago, Illinois. A bundled tube system is used in this 108-story, 1,450 ft tall steel building designed by Skidmore, Owings & Merrill, which as completed in 1974. A

series of tubes are interconnected to fonn the lateral-force-resisting system. In this system,

wider column spacing than would be possible for only an exterior framed-tube was used.

Viscoelastic damping units were part of the structural system in WTC 1 and WTC 2 to supplement the tubular steel frame in limiting wind-induced building oscillations to levels below human perception.

According to Mahmoodi (1987), "The selection, quantity, shape, and location of the dampers was based on the dynamic analysis of the towers (computer modeling, wind tunnel, etc.), and of the damping

required to achieve performance standards." This may have been the first application of damping units for

this purpose in tall building structures, and would certainly qualify it as an innovative system at that time.

The damping units were uniformly distributed throughout both of the buildings. Approximately 100 were

used on each floor from the 7th to the 107th floor. The exact number and planned locations of damping

units on the various floors of the buildings are contained in structural drawings D-ABl-2 through

D-AB 1-14.2 (WSHJ 1967b). As the buildings oscillated from the wind, part of the energy of oscillation was dissipated by shear deformations in the viscoelastic part of the damping units.

Two different types of damping units were used in WTC 1 and WTC 2. Type A damping units were used on floors with trusses spanning between the core and the outside wall, and were located between the

5.3 DAMPING UNITS

5.3.1 Overview

NISTNCSTAR 1-1A, WTC Investigation 97
Chapter 5

bottom chords of the floor trusses and the columns of the outside wall (Fig.- 5^). Type B damping units were used on floors that had wide-flange beams spanning between the core and the outside walls

(i.e., floors 7, 9, 41, 43, 75, 77, and 107). This type of damping unit was located between the bottom

flanges of the floor beams and the outside wall, as shown in Fig. 5-5. The details of a damping unit are

illustrated in Fig. 5-6.

-Exterior Waff

Detail A

\A/1

IJBtaii A - Exterior Wall End Detail

Centerlirve of Exterior Column

3/8° Gus&el Plate Welded to Column and Top Chord

Two 5/8" Oiarneter Bolts

in Slotted Holes 3'-4'' (Typical)

Rod Diagonal j liameter Varies)

Two r Diameter ""^-^.09" Diameter

Two 7/8" Diameter Boils Damping unit extension

Source: McAllister 2002. Figure 5-4. Floor truss member with Type A damping units.

98 NISTNCSTAR 1-1A, WTC Investigation
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Source: WSHJ 1967b. Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 5-5. Wide-flange beam floor member with Type B damping units.

Type B damping units were slightly longer than Type A damping units. Also, the connections between

Type A damping units and the floor trusses were different than those between Type B damping units and

the wide-flange beams. Sheet DA-3 in the structural drawings shows specific details for each type of

damping unit (WSHJ 1967b).

Worthington, Skilling, Helle & Jackson (WSHJ) initially inquired about different types of viscoelastic damping materials in a letter to Minnesota Mining and Manufacturing Company (3M) in 1964.' A followup letter from them to 3M contained the physical and mechanical properties required for the viscoelastic material, based on calculations they had performed.'' Additional correspondence on various aspects of the damping units, including the results of tests that were run at 3M that measured the properties of the damper material and the strength of an assembled damping unit prototype, was exchanged subsequent to these letters.'* In particular, it was noted that testing of an assembled truss damping unit by 3M was

completed and that the results agreed with the theoretical predictions."

' Letter dated July 16, 1964 from Alan G. Davenport of WSH.T to Carl A. Dahlquist of 3M (WTCI-450-L; see Appendix D).

' Letter dated November 23, 1 964 from Richard D. Steyert of WSHJ to Carl A. Dahlquist of 3M (WTCI-450-L; see Appendix D). Various memos and letters in WTCI-450-L.

^ Internal correspondence dated February 1966 by Richard D. Steyert of WSHJ {WTCI-450-L; see Appendix D).

NISTNCSTAR 1-1A, WTC Investigation 99
Chapter 5

at

^Ho/es for "^HJ> high tensile bo/fs

Holes for

hi^h fensile bolts

T

1 +

1 + ^

1

j rl —

1 +

ST4B <2.S

'^^^^^ Machined or finished

surfaces

WJscoelasfic slat>s

0.05" X 4" X 10" SECTION a -a

Source: WSHJ 1967c. Reproduced with permission of Tine Port Authority of New Yorl< and

New Jersey.

Figure 5-6. Damping unit details - Types A and B.

5.3.2 Specifications

A draft specification for the damping units was written by WSHJ in mid-1966^\ and comments and

additions to the specification were supphed by 3M to WSHJ in late October of that year/

In addition to the specifications, Skilling, Helle, Christiansen, & Roberton (SHCR) proposed to Port of

New York Authority (PONYA) in 1967 a prototype test program for the damping units. ^ The report that was submitted to PONYA states the uniqueness of the proposed damping system and points out the value

of having independent testing (i.e., tests in addition to those perfonned by 3M) to measure the perfonnance of the damping units.

Undated intemal memo by R. Taylor of WSHJ. Includes draft of specification (WTCI-450-L).

^ Letter dated October 31, 1966 from Don Caldwell of 3M to James White of WSHJ (WTCI-501-L; see Appendix D).

Letter dated October 30. 1967 and enclosure from Leslie E. Robertson of SHCR to .Tohn H. Kyle (Chief Engineer), PONYA

(WTCI-501-L; see Appendix D).

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Included in the report were the test parameters that were needed for evaluating the effectiveness of the damping units, which included dynamic stiffness, loss factor, and temperature changes. These parameters

are defined in Fig. 5-7. The hysteresis loop that is shown in this figure represents the results obtained

from the tests that were perfonned on the damping units (see Sec. 3.3 of this report for a description of

these tests).

Displacement

Compression

Tension

F = stiffness = one-haif of the double amplilude of the axial force in the damper subjected to a

sinusoidal displacement with an amplilude of 0 020 in at 0 1 Hz (lbs) A = area of hysteresis loop (m -lb)

d = e)ctension (in ) U = ultimate strength = axial compressive force at the ends of the damper necessary to cause

shear failure of the viscoelastic bonded area when the force is applied at a rale of 0.5 in./minute (lbs)

L = bonded length of viscoelastic slab = 10.0 in. W = width of viscoelastic slab = 40 in.

T = thickness of viscoelastic slab = 0 050 in.

Ave = viscoelastic shear area = 2WL = 80 in.^

S = shear stress = FIKe ~ F/80 (psi)

y = maximum shear strain = 0.4 in./in

V = volume of viscoelastic material = 2WLT = 4 in

G* = complex shear modulus = S/ y (psi)

G" = loss shear modulus = (A x scale factors)/!ty'V (psi)

G' = elastic shear modulus = {(G*)^ - (G")^)"^ (psi)

Loss factor = G"/G' Source: Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 5-7. Parameters related to mechanical properties of damping units.

NISTNCSTAR 1-1A. WTC Investigation 101
Chapter 5

The draft contract between 3M and PONYA, dated November 1, 1968, contained the technical

specifications for the damping units (Appendix A of the contract).^ In general, these specifications

covered the manufacture and testing of the units. SHCR supplied comments on the draft contract to the PONYA.'" Other adjustments were subsequently made to the specifications, and the final draft of the

specifications was issued on November 6, 1969."

The specifications were to prevail in the event that there was a conflict between any requirements in the

specifications and the requirements on the contract drawings (Sec. 0.01 in the contract). No existing

standards (such as ASTM International) covered the damping units that were used in this project.

Damping units were accepted or rejected based on the requirements given in the specifications.

According to Sec. 21 of the contract, 3M was to confonn to all orders, directions, and requirements of the Chief of the Planning and Construction Division of the World Trade Center of the World Trade

Department of PONYA (referred hereafter, as in the contract, as the "Engineer"), and was to perform the requirements in the contract to the satisfaction of that person. The Engineer also had the power to alter the

contract drawings and specifications.

The following is a summary of the requirements in Chapters 1 and 2 of the November 6, 1969 edition of

the technical specifications. Unless otherwise noted, referenced section numbers are from the contract (PONYA 1969).

Chapter 1 - General Conditions

The materials and workmanship that went into the damping units were to conform to "the best modem

practice" (Sec. 0.02). If the contract drawings, specifications, or directions of the Engineer left any doubt

as to what was pennissible or failed to note the quality of any construction, the interpretation that called

for the best quality of construction was to be followed. Any errors or discrepancies in the contract drawings or specifications were to be reported to the Engineer as soon as possible (Sec. 0.04).

According to Sec. 0.06, Inspections, testing and storage operations were subject to inspection at any time

by the Engineer or by inspectors acting as agents of the Engineer. 3M was required to give the Engineer at least 10 days notice prior to any testing required in accordance with the specifications.

The contract drawings were considered part of the specification (Sec. 0.08). Revised drawings of the

structural tees (DA-1), structural bars (DA-2), and viscoelastic damping units (DA-3) were finalized on May 21, 1970. These drawings did not show all of the details of the components that made up the damping units, and were intended only to illustrate the character and extent of such units.

The responsibilities of 3IVI with respect to this contract are outlined in Sec. 0.09. They were responsible

for ( 1 ) machining the structural tees and bars that were to be supplied by others, (2) applying the

protective aprons to the viscoelastic material, bonding adhesives, and viscoelastic materials to the tee flange face and both sides of the bar, (3) assembling two tees and one bar into a damping unit,

(4) shipping and bundling the completed units according to type (Type A or B), and (5) testing the units

Draft contract WTC-224.00 for damper units dated November 1, 1968 between PONYA and 3M (WTCI-500-L).

Letter dated April 4, 1969 from Leslie Robertson of SHCR to Malcolm P. Levy of PONYA (WTCI-501-L; see Appendix D). " "Specification for Viscoelastic Damping Units" dated November 6, 1969 (PONYA 1969) (WTCI-501-L; see Appendix D).

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in accordance with the requirements contained in the contract. 3M was not responsible for furnishing the

structural tees or bars, painting the damping units, or instaUing them in the towers. Installation instructions were contained on structural drawing D-ABl-1.3 (WSHJ 1967b).

The structural tees and bars were fabricated from steel conforming to ASTM A 36-63T or ASTM A 572,

Grade 42 (Sec. 0.10). Fabrication tolerances were to confonn to the AISC Specifications for the Design,

Fabrication, and Erection ofStructural Steelfor Buildings dated April 17, 1963 (AISC 1963), and to the requirements contained in the contract drawings and paragraphs C, D, and E in Sec. 0.10 of the

specifications. Sections C and D contained the special requirements for the structural tees and structural bars, respectively. Section E required that certification be provided for all components that were supplied

by others.

Chapter 2 - Technical Requirements

Approved materials to be used in the manufacture of the damping units are contained in Sec. 2.0 of the

specifications and are summarized in Table 5-1 (PONYA 1969). The shop drawings for the structural

steel tees and bars that were used in the damping units were considered to be part of the material

specifications, even though 3M was not responsible for the manufacture of these members.

Table 5-1. Material specifications for damping units per WTC Contract WTC-224.0.^

Material Specification

Viscoelastic material 3M Brand Vibration Damping Elastomer, #¥-9274*'

Steel ASTM A 36-63T or ASTM A 572 Grade 42

Assembly bolts 1/4 in. diameter bolts confonning to ASTM A 307 Standard

Specification for Low-Carbon Steel Externally and

Internally Threaded Standard Fasteners Bonding adhesive 3M Scotchweld Brand Stmctural Adhesives EC 1614 and

3520

Protective aprons 3M Scotch Brand Pressure Sensitive Tape #465

a. Shop drawings for structural tees and bars were considered to be part of the material specifications.

b. Other viscoelastic materials could be used subject to approval of PONYA. Request for approval was to be accompanied by full technical data on the material including documentation of performance characteristics of the damping unit proposed for the work.

Quality Assurance Program— Section 5.0 contains the quality assurance program that was created for the damping units. This program included requirements for both initial and long-term (5 year) acceptance.

It also included the test methods that were to be used to determine whether damping units met these requirements. A brief summary of each of the elements that made up the quality assurance program is given below.

• Acceptance. A lot of dampers would be deemed acceptable by PONYA after sampled

dampers from that lot were tested in accordance with the procedures in Sec. 5.3 of the technical specification and were shown to meet the requirements in Sec. 4.1. An acceptance

lot consisted of all dampers made in each calendar week from the same lot of viscoelastic material by the same process and submitted for acceptance testing at one time.

NISTNCSTAR 1-1A, WTC Investigation 103
Chapter 5

The acceptance requirements of Sec. 4.1 are summarized in Table 5-2 (PONYA 1969).

Detailed test procedures for loss factor and stiffness, fatigue strength, and uhimate strength

are given in Sees. 5.3.6.1, 5.3.6.2, and 5.3.6.3 of the technical specification, respectively.

Methods on how to select a sample size for loss factor, stiffness, and fatigue tests are given in Sec. 5.1.3.1. Sample size for ultimate strength tests are provided in Sec. 5.1.3.2. In short, a

single lot of dampers is accepted if the predetennined sample meets all of the criteria contained in Table 5-2.

Sampled dampers in an accepted lot that were not damaged during testing were to be

delivered to PONYA. All dampers were to be labeled in accordance with the identification codes in Sec. 5.1.4. Dampers that were subjected to acceptance testing were labeled

differently from those that were not subjected to testing.

Table 5-2. Acceptance requirements for damping units per WTC Contract WTC-224.0.

Item (units)''

Number of Dampers in Sample Acceptance Requirement**

Loss Factor

(dimensionless)

5 Requirement average = 0.7 + 0.948ct,'^

10 Requirement average = 0.7 + 0.6700,

15 Requirement average = 0.7 + 0.5470,

Stiffness

(lb)

5

10 6,000 + 1 .25a, < Requirement average < 20,000 - 1 .25ct,

15

Ultimate Strength

(lb)

5 For an individual damper, ultimate strength > 40,000 lb at 75° F

If 0 or 1 damper fails, the lot is accepted.

If 2 fail, take a second sample of 5 dampers. All must pass.

Fatigue

(lb)

5

10 5,400 + 1 .25a, < Requirement average < 22,000 - 1 .25a,

15

a. See Fig. 5-5 for definition of terms.

b. Requirement average = limiting average value of the specified parameter determined from a given sample as set forth in the equations for each parameter,

c. Qi = standard deviation computed from Eq. 3-1 or Eq. 3-2 in Sec. 3.2.

• Five-Year Testing. Unused (or virgin) dampers were also to be tested not less than 5 years nor more than 5 years and 3 months after all the dampers in a given 5 year lot were

manufactured. In short, a number of dampers were to be set aside and tested within the time

frame described above to deteraiine whether any changes had occurred in stiffness, loss

factor, or ultimate strength. Unlike in the acceptance requirements, fatigue tests were not required for the damping units in the 5 year lots.

Damping units to be used in the 5 year tests were to be stored by 3 M in conformance with the conditions outlined in Sec. 5.3 of the specifications.

104 NISTNCSTAR 1-1A, WTC Investigation
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After the samples from a 5 year lot were tested in accordance with Sec. 5.3 and ihc requirements in Sec. 4.2 were met, the lot was deemed to have passed the 5 year test. The

requirements of Sec. 4.2 of the specifications are summarized in Table 5-3 (PONYA 1969).

Table 5-3. Five-year acceptance requirements for damping units per

WTC Contract WTC-224.0.

Item (units)"

Number of Dampers in Sample Acceptance Requirement''

Loss Factor

(dimensionless)

10 Requirement average = 0.63 + 0.9480,*^ 20 Requirement average = 0.63 + 0.670CTi 30 Requirement average = 0.63 + 0.547a,

Stiffness

(lb)

1020 5,400 + 1 .25(7, < Requirement average < 22,000 - 1 .25a,

30

Ultimate

Strength

(lb)

13 For an individual damper, ultimate strength > 36,000 lb at 75° F

If 0, 1, 2, or 3 damper fail, the lot is accepted.

If 4 fail, take a second sample of 1 3 dampers. All must pass.

a. See Fig. 5-5 for definition of tenns.

b. Requirement average = limiting average value of the specified parameter determined from a given sample as set I'orth in the equations for each parameter,

c. aj = standard deviation computed from Eq. 3-1 or Eq. 3-2 in Sec. 3.2. A 5 year lot was one-fourth of the total number of dampers specified in the contract

(Sec. 5.2.2). The number of dampers that were to be tested for loss factor and stiffness was

detennined in accordance with Sec. 5.2.3.1, while Sec. 5.2.3.2 of the contract contained the number of dampers that were to be tested for ultimate strength.

Similar to the acceptance testing, sampled dampers in an accepted lot that were not damaged

during testing were to be delivered to PONYA. Dampers subjected to 5 year tests were to be

labeled in accordance with the requirements in Sec. 5.2.4.

5.4 FLOOR TRUSSES

5.4.1 Overview

Outside of the central core area, floor construction of WTC 1 and WTC 2 typically consisted of 4 in. of

lightweight concrete on 1 1/2 in., 22-gauge fluted metal deck supported by a series of composite floor

trusses that spanned between the core and the exterior walls (see Fig. 5-8). A pair of main floor trusses, spaced 6 ft 8 in. apart on center, spanned either approximately 60 ft or 35 ft from the core to the exterior

walls, where they were supported on every other column. At the core, floor trusses were supported on

channels that were supported by the core columns. The metal deck spanned parallel to the main floor

trusses and was supported on transverse (bridging) floor trusses that were spaced at 1 3 ft 4 in. on center and on deck support angles that were spaced at 6 ft 8 in. on center from the transverse (bridging) floor

trusses. Pairs of flat bars (straps) extended diagonally from the top chord of the floor trtisses to the perimeter columns (see Fig. 5-3). Figure 5-8 shows a typical 20 ft by 60 ft prefabricated floor unit that was used in the towers (PONYA 1967). As shown in this figure, the floor trusses consisted of double

NISTNCSTAR 1-1A. WTC Investigation 105
Chapter 5

angles that were used for the top and bottom chords and round bars that were used for the diagonals. A

section through the main double trusses is shown in Fig. 5-9.

What made the floor system in WTC 1 and WTC 2 innovative from a structural standpoint was the way

that composite action was achieved between the floor trusses and the concrete slab. Truss diagonals were

extended above the top chord, as shown in Figs. 5^ and 5-8. This "knuckle" acted like a shear stud,

which made the floor truss and concrete slab act in a composite manner.

Source: PONYA 1967. Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 5-8. Prefabricated floor unit used in WTC 1 and WTC 2.

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4 in. slab on 1-1/2 in. metal deck

Priman^ Truss Members Bridging Truss

Source: PONYA 1 967. Reproduced with permission of The Port Authority of New York and New Jersey.

Figure 5-9. Section through the main double trusses in the floor system of WTC 1 and WTC 2.

The first recorded tests on composite open-web steel joists were conducted under a project jointly

sponsored by Granco Steel Products and Laclede Steel Company (who manufactured the trusses for WTC 1 and WTC 2) in September of 1964.'' In this study, the overall perfomiance of non-composite

joists was compared with composite joists. The joists were manufactured with their webs projecting

above the top chord. The tests revealed that the composite joists had greater moment capacities and

smaller deflections than the non-composite joists.

Additional tests on open-web joists were perfonned at Washington University (Tide and Galambos 1968).

The findings, which were reported in February of 1968, were similar to those reported from the previous

tests. In particular, the specimens with extended web diagonals into the concrete slab serving as shear connectors were shown to be strong and stiff, and failure was due to crushing of the concrete near the connectors. Further tests conducted at Washington University are reported in Sen and Galambos (1968).

In summary, the findings from this study confirmed those obtained from earlier research programs that are summarized in that report.

The composite floor trusses used in the WTC towers were similar to those that were tested only in the sense that the webs were used as shear connectors. Other than that, they were different in all other

aspects, including member sizes and overall lengths. It may have been the first time that this type of floor construction was used in a high-rise building, especially of this size.

See Sec. 1.1 of Sen and Galambos (1968).

NISTNCSTAR 1-1A, WTC Investigation 107
Chapter 5

5.4.2 Specifications

The contract between the Laclede Steel Company and PONYA, dated October 1967, contained the

technical specifications for the trusses (PONYA 1967). The floor trusses, bridging, beams, and bracing

supplied by Laclede were to confonn to these specifications, and according to Sec. 15 of the contract,

PONYA was to inspect these members at Laclede's plant prior to shipment.

According to Sec. 19 of the contract, Laclede was to confonn to all orders, directions, and requirements

of the Chief of the Planning and Construction Division of the WTC of the World Trade Department of

PONYA (referred hereafter, as in the contract, as the "Engineer"), and was to perform the requirements in the contract to the satisfaction of that person. The Engineer also had the power to alter the contract drawings and specifications.

The following is a summary of the requirements in the technical specifications. Unless otherwise noted,

referenced section numbers are from the contract (PONYA 1967).

Chapter 0 - General Requirements

The specifications were to prevail in the event that there was a conflict between any requirements in the

specifications and the requirements on the contract drawings (Sec. 0.001).

The materials and workmanship that went into the floor trusses and other supplied members were to confonn to "the best modem practice" (Sec. 0.003). If the contract drawings, specifications, or directions

of the Engineer left any doubt as to what was pennissible or failed to note the quality of any construction,

the interpretation that called for the best quality of construction was to be followed. Any enors or discrepancies in the contract drawings or specifications were to be reported to the Engineer as soon as possible (Sec. 0.005).

According to Sec. 0.006, Laclede was to comply with all provisions of federal, state, municipal, local, and

departmental laws, ordinances, rules, regulations, and orders that would affect the contract.

The contract drawings, as well as the structural details and design sheets, were considered part of the

specification (Sec. 0.009).

As a substitute for the design shown in the contract drawings (Sec. 0.009B), which can also be found in Laclede Steel Company (1967), Laclede was allowed to detail and fabricate the floor members in accordance with the design criteria prepared by WSHJ in 1965 (WSHJ 1965) (Sec. 0.009A). These

criteria were appended to the contract.

Items to be included and excluded from the contract are contained in Sec. 0.010. Laclede was responsible

for the following items:

• Floor trusses

• Bridging trusses

• Transverse beams or angles to support steel deck and power/telephone cells or angles

108 NISTNCSTAR 1-1A, WTC Investigation
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• Horizontal wind bracing at exterior end of prefabricated lloor unit

• Closure strips at top chord of floor trusses and bridging trusses

• Clips and patch plates required by the steel erector to assemble individual components into prefabricated panels

• End bearing connection material for floor truss seats at the exterior column and the core end

of the floor trusses

• Connection material at the exterior end for damping units.

Field bolts, assembly of the floor trusses, connections, damping units, and welding electrodes were

excluded from the contract.

Chapter 1 - General Provisions

The codes, standards, and specifications cited in the specification are contained in Sec. 101.300. Where

specific dates are not cited, the latest edition or revision as of September 1, 1966 was to be used in accordance with Sec. 101.100. Where codes, standards, and specifications given in Sec. 101.300 cite other codes, standards, or specifications, the edition or revision cited shall be used (Sec. 101.200). In cases where specific editions or revisions are not cited, the Engineer had final say over the appropriate

edition or revision to use.

The following codes and specifications are listed in Sec. 101.300:

• Specification foi' the Design, Fabrication and Erection ofStructural Steelfor Buildings,

American Institute of Steel Construction, April 1963 (AISC 1963).

• Code ofStandard Practice for Steel Buildings and Bridges, American Institute ofSteel Construction, February 1963.

• Codefor Welding in Building Construction, D1.0-66, American Welding Society, 1966.

• Specifications for Welded Highway and Railway Bridges, D2.0-66, American Welding

Society, 1966 (only where specifically noted in the drawings).

• Standard Specificationsfor Open Web Steel Joists and Longspan Steel Joists, Steel Joist

Institute and the American Institute of Steel Construction, 1965.

Requirements for the shop drawings are also contained in this chapter of the specifications.

Quality control and inspection requirements are given in Sec. 105. All fabrication and welding of the floor

trusses was subject to continual visual inspection, surveillance, and supervision by qualified personnel of

Laclede. Details of this quality control plan, which included fiill-scale load tests on completely fabricated

truss components, are given in Chapter 6 of this report.

NISTNCSTAR 1-1A, WTC Investigation 109
Chapter 5

Chapter 2 - Materials

Steels confonning to the specifications listed in Sec. 201 were approved to be used in the manufacture of

the floor trusses. Steels confonning to the ASTM grades A302, A441, ASM, and A533 with the specific

modifications listed in Sec. 202.100 were also allowed, as were the proprietary grades listed in Sec. 203

with the approval of the Engineer.

Specifications for bolts, welding materials, and structural steel pipe are contained in Sees. 204, 205, and

206, respectively.

Chapter 3 - Fabrication of Structural Steel

Structural steel was to be fabricated as shown in the contract drawings. Fabrication tolerances were to conform to the requirements of the American Institute of Steel Construction (AISC) Specification and

American Welding Society (AWS) Dl.O as well as to the requirements in Sec. 304.100. Additional details on the fabrication requirements are contained in Sec. 6.3.1 of this report.

Chapter 4 - Welding of Structural Steel

According to Sec. 401.100, welding was to conform to the requirements of the AISC Specification and

AWS Dl.O, except where the requirements in these documents were modified or supplemented by

information in the contract drawings or the specification.

Welders and welding operators had to pass the applicable AWS qualification tests prescribed in AWS Dl .0, Appendix D, Parts II and III. Such tests were to be supervised and witnessed by an outside

agency approved by the Engineer. This agency would issue certification papers for the welders based on

the results of the tests.

Specific requirements for the welding operations are contained in Sees. 403, 404, and 405.

Chapter 5 - Bolted Structural Joints

All bolts and washers for apphcable structural joints were to conform to ASTM A325, except in locations

where ASTM A307 or ASTM A490 bolts and washers were specifically called for in the structural drawings (Sec. 501.100).

High-strength bolts and washers were to be installed in confonnance with Specificationsfor Structural

Joints Using ASTMA325 or A490 Bolts, Research Council on Riveted and Bolted Joints of the Engineering Foundation, 1966.

Chapter 6 - Painting

According to Sec. 601.100, all floor trusses, bridging angles, and incidental structural items in the floor system were to receive a uniform shop coat of protective paint applied within one year or less of the delivery date in accordance to the requirements in this chapter. The protective paint was to be applied by

the electro-phoresces process involving a direct current through a deionized water paint bath, which was

110 NISTNCSTAR 1-1A, WTC Investigation
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to provide an average dry film of 1 mil thickness. Chord angles for trusses were to be cleaned by shot blasting prior to painting (Sec. 602.100).

The shop paint was to be in accordance with Pittsburgh Plate Glass (PPG) Company Standard RF-2] 84

initial tank charging material with PPG red power primer RF-2 184 replenishing material or Laclede

Standard Red Chromate Steel Primer, Specification LREP 10001. The red shop paint was to withstand

150 hours of 5 percent sah fog (equivalent to a nornial exposure of 18 months) when applied to a clean

rolled steel panel at 1 mil dry film thickness. It was to be tested in accordance to ASTM B 1 17-64 Salt Fog Test, and the maximum failure allowed was to be in accordance with ASTM D 714-56. Other

requirements for the painting system and painting of erection marks are contained in Sees. 604 and 605,

respectively.

5.5 REFERENCES

AISC (American Institute of Steel Construction). 1963. Specification for the Design, Fabrication, and

Erection ofStructural Steelfor Buildings. New York, NY.

Khan, F.R. et. al. 1983. Developments in Tall Buildings 1983. Council on Tall Buildings and Urban

Habitat. Hutchinson Ross Publishing Company. Stroudsburg, PA.

Laclede Steel Company. 1967. World Trade Center Floor Grid Trusses Basic Design Data. Structural Calculations. February. (WTC1-75-I).

Mahmoodi, P. et. al. 1987. Performance of Viscoelastic Dampers in World Trade Center Towers. ASCE

Conference. Orlando, FL. (WTC1-231-W&C).

McAllister, T., ed. 2002. World Trade Center Building Performance Study: Data Collection,

Preliminaiy Observations, and Recommendations. FEMA 403. Federal Emergency Management

Agency. Washington, DC, May.

PONYA (Port of New York Authority). 1967. Fabricated Steel Floor Trusses, Bridging, Beams and

Bracing for Prefabricated Floor Units for North and South Towers. World Trade Center Contract WTC-221.00. (WTCI-71-I).

PONYA (Port of New York Authority). 1969. Viscoelastic Damping Units for North and South Towers. World Trade Center Contract WTC-224.00. (WTCI-501 -L).

Sen, S.K. and Galambos, T.V. 1968. Composite Open Web SteelJoists with Extended Webs. Research

Report No. 8, Structural Division, Civil and Environmental Engineering Department, Sever Institute ofTechnology, Washington University. St. Louis, MO. (WTCI-504-L).

Tide, R.H.R. and Galambos, T.V. 1968. Composite Open-Web SteelJoists. Research Report No. 4, Structural Division, Civil and Environmental Engineering Department, Sever Institute of Technology,

Washington University. St. Louis, MO.

WSHJ (Worthington, Skilling, Helle & Jackson). 1965. Design Criteria for WTC 1 and 2. (WTCI-2-L

and part ofWTCI-50-L).

NISTNCSTAR 1-1A, WTC Investigation 111
Chapter 5

WSHJ (Worthington, SkiUing, Helle & Jackson). 1967a. Structural drawings of WTC 1 and 2, Book 4. (WTCI-17-L).

WSHJ (Worthington, Skilling, Helle & Jackson). 1967b. Structural drawings of WTC 1 and 2. (WTC1-50-L).

WSHJ (Worthington, Skilling, Helle & Jackson). 1967c. The World Trade Center - Viscoelastic

Damping Units. Report No. DU-1. July (WTCI-17-L).

112 NISTNCSTAR 1-1A, WTC Investigation
Chapter 6

Fabrication and Inspection Requirements at the

Fabrication Yard

6.1 overview

This section contains the fabrication and inspection requirements at the fabrication yard for the structural members in World Trade Center (WTC) 1, 2, and 7.

The discussion in Sec. 2.1.1 of this report points out that the Port of New York Authority (Port Authority

or PONYA) instructed the consultants to revise their designs for WTC 1 and WTC 2 to comply with the second and third drafts of the new New York City Building Code (the Code) and to undertake any

revisions necessary to comply with such provisions. The Code contains provisions that govern the

fabrication and inspection of materials used in buildings. Section 6.2 of this report contains summaries of

these provisions as they relate to WTC 1 and WTC 2. Section 6.3 contains summaries of fabrication and

inspection requirements obtained from contracts between the Port Authority and the steel fabricators for the towers. Unless otherwise noted, all referenced article and section numbers are from the 1968 New

York City Code. Fabrication and inspection requirements pertaining to WTC 7 are contained in Sec. 6.4.

6.2 summary of code requirements for fabrication and

inspection

Section C26-1000.7, Materials and methods of construction, gives the requirements for inspection of

materials and assembhes in Table 10-1. According to the table, all structural elements and connections of

structural steel are not subject to controlled inspection. Footnote c to the table states that mill,

manufacturer's, and supplier's inspection and test reports are accepted as evidence of comphance with the provisions in the Code for all structural materials and assemblies not subject to controlled inspection.

Therefore, this footnote is applicable to structural steel. Additional information on inspection is provided

in Sec. 6.2.2 of this report.

Section C26-1000.7 also requires steel to confonn to the provisions in Sub-Article 1005.0, Steel. According to C26-1005.1, structural steel must meet the requirements in Reference Standard RS 10-5,

which is the 1963 AISC Specification for the Design. Fabrication, and Erection ofStructural Steel for

Buildings (AISC 1963). Reference Standard RS 10-5 also contains modifications that were made to the American Institute of Steel Construction (AISC) Specification. The following sections give summaries of

the fabrication and inspection requirements in the AISC Specification, and include the modifications to the requirements as set forth in Reference Standard RS 10-5.

NISTNCSTAR 1-1A, WTC Investigation 113
Chapter 6

6.2.1 Fabrication Requirements

Section 1.23 of the AISC Specification contains minimum fabrication requirements for the following:

• Straightening material

• Gas cutting

• Planing of edges

• Riveted and bolted construction - holes

• Riveted and high strength bolted construction - assembling

• Welded construction

• Finishing \

• Tolerances

One minor modification was made to these requirements, which has to do with the reference made to American Welding Society (AWS) Dl.O (AWS 1964) in Sec. 1.23.6 , Welded Construction.

6.2.2 Inspection Requirements

Section 1.26 in the AISC Specification contains the inspection requirements for structural steel. Reference

Standard RS 10-5 deletes this entire section of the AISC Specification.

One of the main requirements given in Sec. 1.26 of the AISC Specification is that "Materials and

workmanship at all times shall be subject to the inspection of experienced engineers representing the purchaser." As noted above in this report, C26- 1000.7 does not require controlled inspection for structural

steel.

Also, Sec. 1 .26 of the AISC Specification gives minimum requirements for inspection of welding, which was to be perforaied in accordance with Sec. 6 of the Standardfor Welding in Building Construction of

the AWS. Table 10-2 in C26- 1000.7, which would have governed in the case of WTC 1, 2, and 7, lists the inspection methods for welded and bolted construction, which is based on the ratio of the calculated

stresses in the welds or bolts to the allowable stresses.

6.3 SUMMARY OF FABRICATION AND INSPECTION REQUIREMENTS AT

THE FABRICATION YARD FOR WTC 1 AND WTC 2

The following sections of this report summarize the fabrication and inspection requirements that were

used at the fabrication yard, which were obtained from the major contracts between the Port Authority

and the steel fabricators for WTC 1 and WTC 2. In general, the requirements from the specifications in the various contracts are at a minimum equivalent to those in the Code, and in many cases they are more

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comprehensive and stringent than the corresponding provisions in the Code. The details of these requirements are summarized in the next sections.

6.3.1 Floor Trusses

As discussed above in Sec. 5.3.2 of this report, the contract between the Laclede Steel Company and the Port Authority contained the specification for the manufacture of the floor trusses that were used in WTC 1 and WTC 2 (PONYA 1967a). Included in these specifications were requirements for fabrication

(Chapter 3) and a quality control and inspection program (Sec. 105). General requirements for welding of

the structural steel are given in Chapter 4 of the specifications. Applicable sections from the contract are reproduced in Appendix E of this report, starting on page 266.

6.3.2 Box Core Columns and Built-up Beams

The contract between the Stanray Pacific Corporation and the Port Authority (PONYA 1967b) contains

the specifications for the box core columns and built-up beams from the 9th story to the penthouse roof

Requirements for fabrication and welding of structural steel are in Chapters 3 and 4 of the specifications,

respectively, and inspection and quality control requirements are in Sec. 1 05 of the contract. These

requirements can be found in Appendix E of this report, starting on page 276.

In addition to the inspection requirements in the contract, requirements were also stipulated for inspection, testing, coordination, and supervision by an independent testing agency at Stanray Pacific's fabrication plant. According to Skilling, Helle, Christiansen, & Robertson (SHCR), these additional

requirements were necessary because the Port Authority was required by the contract to inspect and

accept the members before they left the fabrication yard and because a major portion of the steel used for the members was to be produced in Japan and England.' A comprehensive program for "supervision,

coordination, inspection, and testing based on the use of the personnel and facilities of a local independent

testing agency supervised by a Resident Engineer (a professional engineer employed full time by SHCR)"

was attached to the letter sent from Leslie Robertson of SHCR to Malcolm P. Levy of PONYA (see

footnote 1). The scope of this program was two-fold:

• To provide PONYA assurance through adequate documentation that fabricated steel conformed to the contract documents and to assure on-time delivery of fabricated steel.

• To provide detailed inspection by checklist and by non-destructive testing prior to final acceptance of the members.

The details of this program can be found in Appendix E, starting on page 301. In particular, the Resident

Engineer was responsible for the following items related to supervision:

• Prior to fabrication, performing a complete study of the fabricator's quality control procedures, proposed fabrication procedures, provisions for storage of incoming material, and

provisions for loading and shipping of completed building components.

' Letter dated June 5, 1967 from Leslie E. Robertson of SHCR to Malcolm P. Levy of PONYA (WTCI-491-L; see Appendix E).

NISTNCSTAR 1-1A, WTC Investigation 115
Chapter 6

• Acting as liaison between the Port Authority and SHCR with respect to preparation and

approval of shop drawings. -

• Ensuring proper interpretation of the contract drawings and specifications.

• Directing the work performed by the independent testing agency and its inspectors.

• Perfonning surveillance of the quality of work on a continuous basis.

With respect to coordination, the Resident Engineer was responsible for the following:

• Examining the approved progress schedule.

• Checking and accepting each unit from the beginning of fabrication through loading for shipment.

The duties of the independent testing agency, which was the U.S. Testing Company of New Jersey,

appeared in Appendix I of the draft contract of the United States Testing Company." The duties of the inspectors as outlined in that document were as follows:

• Assist the Resident Engineer in analyzing and cross-checking advance bills of material and

certified mill test reports.

• Check each plate upon arrival at the receiving and storage yard for (1) heat number and

specification conformance and (2) condition (edge defects, surface defects, and damage).

• Check each buih-up member during fabrication for (1) conformance to dimensional and

tolerance requirements, (2) defects, (3) conformance to welding specifications, and (4)

finishing.

• Final check of built-up members for (1 ) confonnance to dimensional and tolerance

requirements, (2) defects, (3) protection of iTiilled surfaces, and (4) accurate and clear marking.

The structural engineer (SHCR) also recommended that an independent testing agency be hired for mill

inspection of Japanese steel. ^ The main responsibility of the testing agency was to verify the accuracy of

the certified mill testing reports by witnessing tests at the manufacturing mill. Procedures were

established for witnessing the tests at both Stanray Pacific and Pacific Car and Foundry (see Sec. 6.3.3 of

this report for Pacific Car and Foundry) in the United States. The Port Authority subsequently contracted

with Superintendence Inc., an international inspection agency with affiliate firms in Japan and Great

Britain who provided the mill inspections in both countries.''

^ Draft contract between United States Testing Company and PONYA dated August 25, 1 967 (WTCI-493-L; see Appendix E

for the first page of the contract and Appendix I of this document).

Letter dated April 5, 1967 from Leslie E. Robertson of SHCR to Malcolm P. Levy of PONYA (WTCI-489-L; see Appendix E).

'* Letter dated September 21, 1967 from R. M. Monti of PONYA to R. E. Morris of the Stanray Pacific Corporation

(WTCI-490-L; see Appendix E).

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The Port Authority set forth requirements for the independent testing portion of the mill inspection

program."^ The requirements, which were part of PONYA's overall quality control program on fabricated

steel for the WTC, depended on whether the steel was from a domestic source or from a foreign source. For steel obtained from domestic sources, the independent testing portion of the mill inspection program

consisted of the following:

• For steel with yield points less than 50,000 pounds per inch (psi), one tensile test and one

check analysis on samples selected at random from 1 out of 10 heats.

• For steel with yield points of 50,000 psi and higher, one tensile test, one bend test, and a check analysis on samples selected at random from 1 out of 10 heats.

For steel obtained from foreign sources:

• For steel with yield points less than 50,000 psi, one tensile test and one check analysis on

samples selected at random from 1 out of 10 heats to perfonned abroad. In addition, one

sample suitable for a tensile test from 1 out of 4 heats was to be shipped by the inspection

agency to a laboratory in the United States for tensile testing and check analysis.

• For steel with yield points of 50,000 psi and higher, one tensile test, one bend test, and a check analysis on samples selected at random from 1 out of 10 heats to be performed abroad.

In addition, one set of samples suitable for machining into a tensile specimen and a bending

specimen was to be selected at random from 1 out of 4 heats and shipped by the inspection

agency to a laboratory in the United States for testing.

6.3.3 Exterior Wall from Elevation 363 ft to the 9th Floor Splice

The Pittsburgh-Des Moines Steel Company (PDM) fabricated the column trees, as depicted in Fig. 5-2 of

this report, from elevation 363 ft to the 9th floor splice. Specifications were established for both quality control and welding procedures.

The initial quality control and testing program was submitted to PONYA on October 21, 1966.^ Three

subsequent amendments were made to the original program (see Appendix E, page 326) based on comments made by SHCR. The final draft of the quality control program was submitted to PONYA on

September 28. 1967 and was subsequently approved by SHCR.

Requirements were also developed by PDM for the welding procedures. Different specifications were

written by PDM for the different types of welds that were to be used in the manufacture of the column

trees. These specifications were reviewed and approved by SHCR, usually after modifications were made

by SHCR. The Port Authority gave final approval on the use of the specifications, based on the recommendations from SHCR.^

^ Letter dated November 1 3, 1 967 from R. M. Monti of PONYA to R. E. Morris of Stanray Pacific Corp. (WTCI-498-L. see Appendix E). Letter dated October 2 L 1966 from PDM to James R. Endler of Tishman Realty and Construction Company Inc. (part of WTCI-745-L [second page and enclosure appear to be missing]; see Appendix E).

^ Examples of the welding specifications and subsequent approvals that are in WTCI-741-L can be found in Appendix E.

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Chapter 6

The Port Authority hired the Pittsburg Testing Laboratory, an independent inspection company, in 1967,

for mill inspection at PDM's suppliers' plants and for fabrication inspection at PDM's shop.^

6.3.4 Exterior Wall Above 9th Floor Splice

The contract between the Pacific Car and Foundry Co. and the Port Authority (PONYA 1967c) contains

the specifications for the exterior walls (box columns and spandrel plates as shown in Fig. 5-1 of this report) from the 9th story splice to the roof. Requirements for fabrication and welding of structural steel are in Chapters 3 and 4 of the specification, respectively, and inspection and quality control requirements

are in Sec. 105 of the contract. These requirements can be found in Appendix E, starting on page 356.

Based on comments from the Port Authority and from SHCR, the quality control and welding procedures

of the contract were revised.'' These revisions were subsequently approved by SHCR, subject to the following conditions:"*

• The weld numbers and designations used in the drawings that were attached to this letter were

to be used.

• The first three full penetration spandrel butt welds (Weld #10 in drawing attached to letter) performed by each new welding machine operator or welder was to be subjected to ultrasonic

testing.

• Where a spandrel weld was rejected, all welds made by the same welder or welding machine

were to be tested by the ultrasonic testing technique for the spandrel in question, as well as

for the spandrels produced immediately before and after the subject spandrel.

• Approval of the Pacific Car and Foundry Co. quality control and testing program does not

include approval of any welding process or procedure subject to AWS qualification tests.

• Visual inspection was to be carried out by certified Pacific Car and Foundry Co. inspection

personnel on 100 percent of all types of welds included in the work.

Weekly inspection reports were submitted by the SHCR resident engineer at the Pacific Car and Foundry

plant in Seattle, Washington, to the SHCR home office in New York." These reports reference a test jig that was built by Pacific Car and Foundry. Fabricated wall panels were checked for compliance with

required tolerances on the jig before they were approved for shipment.

6.3.5 Rolled Columns and Beams

The contract between the Montague-Betts Company, Inc. and the Port Authority (PONYA 1967d)

contains the specification for the rolled core columns, interior columns, louver wall struts, and rolled

Letter dated October 4, 1967 from R. M. Monti of PONYA to H. M. Fish of PDM (WTCI-745-L: see Appendix E). Letter dated July 8, 1967 from R. C. Symes of Pacific Car and Foundry to R. M. Monti of PONYA (part of WTCI-748-L; see Appendix E).

'° Letter dated July 13, 167 from James White of SHCR to R. M. Monti of PONYA (part of WTC1-748-L; see Appendix E). " Weekly inspection reports contained in WTCI-749-L.

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beams that were to be used at the locations in both towers specified in Sec. 0.008 of the contract. Requirements for fabrication and welding of structural steel are in Chapters 3 and 4 of the specification,

respectively, and inspection and quality control requirements are in Sec. 105 of the contract. These

requirements can be found in Appendix E, starting on page 369. It is important to note that the quality control and testing program was revised based on the infonnation in the letter from SHCR to PONYA

dated June 23, 1967. The revisions and the letter became part of the contract (see Appendix E, page 377).

In particular, the comments in the letter were as follows:

• Receiving: Material received should be checked against the certified mill test reports for size,

grade, heat number, and color code. One copy of each certified mill report should be

submitted to PONYA and SHCR.

• Fabrication: Overhangs, gross laminations, excessive slag inclusions, and similar defects should be defined and repair procedures for these defects should be outlined.

• Welding: Certification papers for each welder and welding machine operator should be

submitted to PONYA and SHCR. Welding procedures must be prepared and the fabricator

must perfonn qualification tests where applicable. All welds should receive 100 percent

visual inspection. Non-destructive testing of welds needs to be described.

• Inspection: The amount of periodic inspection of work in progress and the persons

perfomiing this inspection should be described. The inspection of finished work should be

documented in reports submitted to PONYA and SHCR.

6.3.6 Other Requirements

Where problems arose in the fabrication yards, particularly when it came to fabrication tolerances,

specific requirements that addressed the specific problems were adopted. The typical method used to remedy a problem was for the fabricator to submit a procedure for correction to the Port Authority. The

procedure was subsequently accepted or rejected by SHCR, and final approval from the Port Authority was contingent upon the fabricator satisfying the requirements set forth by SHCR. These variances from

the original specifications are in Chapter 8 of this report.

6.4 SUMMARY OF FABRICATION AND INSPECTION REQUIREMENTS AT

THE FABRICATION YARD FOR WTC 7

The following sections contain the fabrication and inspection requirements for WTC 7, as outlined in the

specifications for WTC 7 (WTC 7 Project Specifications 1984). No other documents pertaining to these requirements were found.

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Chapter 6

6.4.1 Fabrication

According to Sec. 5 A. 9.1 of the specifications (WTC 7 Project Specifications 1984), structural steel for WTC 7 was to be fabricated in accordance with the applicable requirements in the following codes and

standards:

• New York City Building Code (1968)

• Specification for the Design, Fabrication, and Erection ofStructural Steel for Buildings,

AISC

• Specifications for Structural Joints using ASTMHigh Strength Bolts, ASTMA 141 Rivets,

and ASTMA 307 Unfinished Bolts, Research Council on Riveted and Bolted Structural Joints

• Specifcations for Structural Joints using ASTMA 325 or A 490 Bolts, AISC

• Code ofStandard Practice, AISC (except that the first sentence of Sec. 4, paragraph d shall not apply)

• Code ofArc and Gas Welding in Building Construction, AWS Standard Code D 1 . 1

,

American Welding Society

• Steel Structures Painting Manual, Vols. 1 and 2, Steel Structures Painting Council

• Handbook ofBolts, Nut and Rivet Standards, Industrial Fasteners Institute

Work was to be of "highest quality" performed by mechanics skilled in the type of work required.

Structural steel was to be fabricated and assembled in the shop to the "greatest extent possible."

Mill test reports were to be furnished by the contractor (Sec. 5A.5 of the specification). These reports were to cover the chemical and physical properties of the steel. Also, mechanical and chemical tests were

to be performed for all materials regardless of thickness or use. Specifics on these tests are not given in the specifications.

Section 5A.12.14 of the specification contains the following modifications that were made to AWS Dl.l:

• The words "except as amended by these Specifications" was added to paragraph 6.7.4.

• A paragraph was added after paragraph 6. 1 9.5.2 that contained additional requirements for evaluation of discontinuities. The ultrasonic testing method was to be used to determine the extent of the discontinuity.

• A paragraph was added after paragraph 6.19.7.1 that contained additional acceptability

requirements for weld discontinuities.

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6.4.2 Inspection

Section 5A.2.2 notes that there was a separate contract for testing and inspection. This contract was not

found. However, specific requirements for inspection of shop and field welds by a testing agency are found in Sec. 5A.12.13 of the specification:

• Examination of welds: All welds shall be visually inspected. All groove welds, except only

25 percent of those at moment connections, shall be examined by the ultrasonic method for 100 percent of their length.

• Lamination testing: Ends of plates, 2 in. or more in thickness, which were to be butt welded,

shall be tested for lamination by the ultrasonic method prior to welding.

• Joints in which material is 2 in. or more in thickness shall not have the weld interrupted after operation has started, unless at least two-thirds of its length, or its full depth, has been

completed without an interruption of more than one hour. Welding was allowed to be

interrupted for longer periods, provided the preheat temperature was maintained for the full length of the joint for the entire time welding was interrupted.

Additional inspection was required when defects were found or suspected (Sec. 5A.12.15). The inspection

method to be used was at the discretion of the testing agency. Additional inspection of welds was required

when either the structural engineer or the testing agency had reason to question the quality of the weld.

6.5 REFERENCES

AISC (American Institute of Steel Construction). 1963. Specification for the Design, Fabrication, and

Erection ofStructural Steelfor Buildings. New York, NY.

AWS (American Welding Society) 1964. AWS Building Codefchanged to Structural Welding Code).

New York, NY

PONYA (Port of New York Authority). 1967a. Fabricated Steel Floor Trusses, Bridging, Beams and

Bracing for Prefabricated Floor Units for North and South Towers. World Trade Center Contract WTC-221.00. (WTC1-71-I).

PONYA (Port of New York Authority). 1967b. Fabricated Steel Box Core Columns and Built-Up

Beams From the 9th Stoiy Splice to the Penthouse Rooffor North and South Towers. World Trade

Center Contract WTC-2 1 7.00. (WTC1-244-L).

PONYA (Port of New York Authority). 1 967c. Fabricated Steel Exterior Wall From the 9th Stoiy Splice

to Rooffor North and South Towers. World Trade Center Contract WTC-2 14.00. (WTCI-242-L).

PONYA (Port of New York Authority). 1967d. Fabricated Steel Rolled Core Columns, Interior

Columns, Louver walls Struts and Rolled Beamsfor North and South Towers. World Trade Center

Contract WTC-226.00. (WTC1-243-L).

WTC 7 Project Specifications. 1984. (WTC1-187-P).

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NISTNCSTAR 1-1A, WTC Investigation
Chapter 7

Inspection Protocol During Construction

7.1 overview

Construction of World Trade Center (WTC) 1 and WTC 2 was overseen and managed by the Tishman

Realty & Construction Company (TRCC), acting as the construction manager. In that role, TRCC as the general contractor coordinated the scheduling of the various activities required on the project, including

the day-to-day construction activities at the site. The Port of New York Authority (Port Authority)

required that all correspondence pertaining to administration of a prime contractor's contract, including

contract changes, matters pertaining to field problems, job progress, and schedule be submitted to TRCC.'

Karl Koch Erecting Co. (KKE) performed structural steel erection work (WTC Contract 230.00).

Section 5A.14 of the WTC 7 specifications (WTC 7 Project Specifications 1984) contains general

erection requirements for fasteners, anchor bolts, column bases, installation, and bracing. No inspection

requirements during construction are given in the specifications.

To facilitate steel erection, a marking system for structural steel in WTC 1 and WTC 2 was developed by

the Port Authority and Nassau Bridge Detailers. This system was to be used by the fabricators to properly

identify the different steel members/pieces that went into the towers."

A quality control and inspection program was developed by KKE and submitted to the Port Authority for approval. The Port Authority requested that Skilling, Helle, Christiansen, & Robertson review and submit

comments on this program.^

The quality control and inspection program included information on the following:

• Survey control

• Control of construction and erection loads

• Field welding

• Bohing of structural steel

' General instructions from Malcolm P. Levy of PONYA to prime contractors for WTC contracts (WTCI-239-P; see Appendix F).

" General instructions on erection marks and marking system for structural steel from the Port Authority to steel fabricators/suppliers for WTC 1 and WTC 2 (WTCI-495-L; see Appendix F).

' Memo dated July 26, 1968 from David L. Brown of PONYA to James White of SHCR (WTCI-515-L; see Appendix F).

7.2 erection marks and marking system

7.3 QUALITY control AND INSPECTION PROGRAM

NISTNCSTAR 1-1A, WTC Investigation 12?
Chapter 7

• Control of stud welding operations

• Erection procedures

• Control of workmanship

• Control of erection tolerances

• As-built drawings

• Safety programs

A number of problems were encountered during the erection of WTC 1 and WTC 2. These problems

typically were due to structural members that did not fit or were not aligned properly. A number of these cases are cited in Chapter 8 of this report.

7.4 REFERENCE '

WTC 7 Project Specifications. 1984. (WTC1-187-P).

124 NISTNCSTAR 1-1A, WTC Investigation
Chapter 8

Deviations Granted by the Port Authority

8.1 overview

The Port of New York Authority (Port Authority or PONYA) approved numerous deviations to contract drawings and specifications in the fabrication and erection of structural members in World Trade Center

(WTC) 1 and WTC 2. The general procedure for deviation requests was as follows. In general, deviations were submitted by the fabricators or erector to the Office of the Construction Manager of the PONYA as a result of difficuhies encountered in complying with the contractual requirements for fabrication or

erection. Deviations were also requested when, in the opinion of a fabricator or erector, an alternative

detail or procedure was warranted. Such requests were usually submitted at the same time to the structural engineer (SkiUing, Helle, Christiansen, & Robertson [SHCR]).

Typically, the Office of the Construction Manager approved deviations after SHCR reviewed the details of the deviation and granted their approval. In many cases, SHCR submitted alternative methods, which

were incorporated into the deviation.

The deviations that were granted may be categorized into the following groups:

• Fabrication/erection tolerances

• Defective (cracked, laminated, misfit) components

• Fabricator/erector-preferred procedure

• Material substitutions

• Frequency/rate of weld inspections

No variance requests related to the New York City Building Code were found.

8.2 DEVIATIONS RELATING TO FABRICATION/ERECTION TOLERANCES

The following is a list of specific requests relating to deviations for fabrication and erection tolerances of

box beams, box columns, and floor trusses.

• SHCR notified the United States Testing Company that the deviation of the end tolerances of

column 604-9 was approved.' This permitted one flange to be offset 3/16 in. instead of 18 in.

as specified on page 3-04 of the Stanray Pacific contract (PONYA 1967).

' Letter dated December 27, 1967 from Richard Chauner of SHCR to Robert Dempsey of United States Testing Company

(WTCI-499-L; see Appendix G).

NISTNCSTAR 1-1A, WTC Investigation
Chapter 8

• SHCR notified the Port Authority that tolerances recommended by Mosher Steel Company

(WTC Contract 215.00) for box beams were approved." Approval was also granted for a maximum 1/4 in. twist in the fabrication of box columns.

• SHCR notified Laclede Steel Company that their request for the "hold exacf' dimension on

the top seat connection at the core end of 20 trusses to less than 4.5 in. was approved, as long

as this dimension was not less than 4 in. (see the figure on page 407 in Appendix G)^ This

approval was subject to Laclede's acceptance of rectifying any possible problems with the Karl Koch Erecting Company during erection.

• SHCR notified the Port Authority that the schedule for the maximum allowable tolerances

required to set floor truss seats was approved."* This was in response to the letter from Karl

Koch Erecting Company to Tishman Realty and Construction Company, Inc. outlining their inability to place truss seats in accordance with the contract drawings for type "G" panels on

floors 10 through 51 in WTC 1. The letter claimed that Laclede was fabricating C32T6 floor trusses at tolerances that did not pennit truss seats to be placed in a plumb position and

accurate location. The letter further stated that these discrepancies caused numerous field problems as well as "criticism" from inspection personnel. Approval was also granted for the

repair details submitted by Karl Koch Erecting Company for the vertical struts near the ends

of 64 of the C32T6 floor trusses fabricated by Laclede.

• SHCR notified the Port Authority that the request by Laclede to change the tolerances for the height above the top chord of the end stiffeners V3 and V4 in floor trusses from 3 in., ±1/8 in. to 3 in., +1/8 in., -3/8 in. was approved.'' This was done to speed up the fabrication process.

• SHCR notified the Port Authority that the request by Laclede for a tolerance of 3/8 in. for the

2 7/8 in. or 1 3/4 in. dimension at the top chord intersection of the inclined strut of 24T-type

floor trusses only was approved.*"

• The Port Authority notified Laclede Steel Company of numerous changes that were made in the field welding of connections for bridging trusses and bridging angles at panel joints.^

These changes were instituted after on-site difficulties in field welding were observed in WTC 1 due to misalignment and the addition of erection tolerances in the field. Laclede was

also infonned of changes that were to be made in their fabrication process to avoid these problems in the future.

- Letter dated December 22, 1967 from James White of SHCR to R. Monti of PONYA (WTCI-499-L; see Appendix G).

' Letter dated June 20. 1969 from James White of SHCR to R. Bay from Laclede Steel Company (WTCI-506-L; see Appendix G).

" Letter dated November 17, 1969 from James McGuiness of SHCR to R. Monti of PONYA (WTCI-506-L; see Appendix G).

Letter dated October 16, 1969 from James White of SHCR to R. Monti of PONYA (WTCI-506-L; see Appendix G).

^ Letter dated October 20, 1969 from James White of SHCR to R. Monti of PONYA (WTCI-506-L; see Appendix G).

^ Letter dated June 16, 1969 from Malcolm Levy of PONYA to Carl Weber of Laclede Steel Company (WTCI-506-L; see Appendix G).

126 NISTNCSTAR 1-1A, WTC Investigation
Variances Granted by PANYNJ

8.3 DEVIATIONS RELATING TO DEFECTIVE COMPONENTS

The following is a list of specific requests relating to deviations for defective components of column trees and floor trusses:

• SHCR notified the Port Authority that the 22 plates that were fabricated for truss connectors

that were 1/4 in. narrower than the required width were approved.**

• SHCR notified the Port Authority that 1 60 of the C32T1 1 floor trusses that were fabricated

by Laclede with fillers at the core end of the trusses located approximately 1 in. (2 in. in three cases) closer to the center of the truss than shown in the contract drawings was approved.'^

These floor trusses were originally approved by the inspection company PTL subject to approval by SHCR.

• SHCR notified the Port Authority that the repair procedure submitted by Laclede for the

vertical struts of the 32 in. floor trusses was approved.'" Repair welds were to be made as required after fabrication to adjust the top end of the vertical struts, which had a tolerance of

± 1/4 in.

• SHCR notified the Port Authority that the repair procedure submitted by Laclede for the floor

truss bearing ends was approved." Repair welds were to be made to adjust the bearing depth

of the seats, which had a tolerance of ± 1/8 in.

• SHCR notified the Port Authority that the method submitted by Laclede for the repair of 24

of the C32T1 A floor trusses by double-strutting the diagonal strut on the column end with a 3/4 in. diameter bar was approved.'' These floor trusses were originally fabricated with

L09 in. web stock instead of 1.14 in. web stock as shown in the contract drawings.

• SHCR notified the Port Authority that the repair method submitted by Pittsburgh-Des Moines

Steel Company (PDM) for laminations in Plate "d" in Panel 230B (part of column tree) was

approved.'^

• SHCR notified the Port Authority that the repair procedure submitted by PDM for a crack

that developed in Plate "b" of Panel 300B (tree column) was accepted.''*

• SHCR notified PDM that the sub-assembly for Column 3, Panel 200B was acceptable as fabricated and may be incorporated into Panel 200B.'"'* No other information was found

concerning the condition of this sub-assembly.

* Letter dated June 20, 1969 from James White of SHCR to R. Monti of PONYA (WTCI-506-L: see Appendix G).

Letter dated December 15, 1969 from James White of SHCR to R. Monti of PONYA (WTCI-506-L; see Appendix G).

Letter dated July 7, 1969 from James White of SHCR to R. Monti of PONYA (WTC1-506-L; see Appendix G). " Letter dated July 3, 1969 from James White of SHCR to R. Monti of PONYA (WTCI-506-L; see Appendix G).

Letter dated March 31, 1969 from James White of SHCR to R. Monti of PONYA (WTCI-506-L; see Appendix G).

Letter dated June 6, 1969 from James White of SHCR to R. Monti of PONYA (WTCI-736-L; see Appendix G).

Letter dated May 19. 1969 from James White of SHCR to R. Monti of PONYA (WTCI-736-L; see Appendix G).

Letter dated May 5, 1969 from R. Monti of PONYA to H. Fish of PDM (WTCI-735-L; see Appendix G).

NISTNCSTAR 1-1A, WTC Investigation 127
Chapter 8

• SHCR notified the Port Authority that the repair method submitted for Plate "b" of

Panel 339D was approved.'^' No other information was found on the condition of the

originally fabricated plates.

• SHCR notified the Port Authority that the repairs proposed by PDM to Panels 227B and

230B (column trees) were approved.'^ Both repairs required the addition of 2 by 1/4 in. bars

welded to the original fabricated plates.

• SHCR notified the Port Authority that the repair method proposed by PDM for a crack that developed in Plate "v"^" of Panel 224B was approved.'^

• SHCR notified the Port Authority that the repair method for laminations in Plate "Ur^",

Panel 130B and Plate "V^" of Panel 139B was approved, based on the ultrasonic tests perforaied by PDM.'^

• SHCR notified the Port Authority that the repair method of Plate "af^" of Panel 412B

submitted by PDM was approved."" No other information was found on the reasons why

repairs were required on this plate.

• SHCR notified the Port Authority that the repair method for Plate "b" in Panel 339b

submitted by PDM was approved, based on non-destructive testing of the repaired plate."'

• SHCR notified the Port Authority that the repair method developed by SHCR for a 6 ft long

crack in the weld between Plates "a" and "b" in Column 327B (column tree) at elevation

+372 ft 6 in. to elevation +378 ft 6 in., which was fabricated by PDM, was successful."" A

probable triggering mechanism that initiated the crack was the lower ductility of submerged

arc weld metal subjected to an undercut notch and possible metallurgical notch along the weld line, coupled with cold weather. Freezing of water in the column was not totally

discounted as a possible triggering mechanism, although, according to SHCR, its contribution was believed to be small.

• SHCR notified the Port Authority that the repair procedure for laminations in Plate "d"

shown in PDM shop drawing MP506 was approved.'" These laminations were discovered

after the plates were welded into a complete column tree assembly.

• SHCR notified the Port Authority that the weld repair procedure for Plate "V^" of

Panel 209A developed by PDM was approved."'^ The plate was inadvertently cut 6 in. too

short when originally fabricated.

Letter dated March 20, 1969 from James White of SHCR to R. Monti of PONYA (WTCI-738-L; see Appendix G). Letter dated June 6, 1969 from James White of SHCR to R. Monti of PONYA (WTCI-736-L; see Appendix G). " Letter dated May 16, 1969 from James White of SHCR to R. Monti of PONYA (WTCI-735-L; see Appendix G). " Letter dated June 9, 1969 from James White of SHCR to R. Monti of PONYA (WTCL736-L; see Appendix G).

^" Letter dated May 16, 1969 from James White of SHCR to R. Monti of PONYA (WTCL735-L; see Appendix G).

"' Letter dated May 16, 1969 from James White of SHCR to R. Monti of PONYA (WTCI-735-L; see Appendix G). ~ Letter dated July 15, 1971 from James White of SHCR to R. Monti of PONYA (WTCI-736-L; see Appendix G).

Letter dated August 21, 1968 from James White of SHCR to R. Monti of PONYA (WTCI-740-L; see Appendix G).

128 NISTNCSTAR 1-1A, WTC Investigation
Variances Granted by PANYNJ

• SHCR notified the Port Authority that the repair method for the butt welds at 1 8 separate

locations in comer panels lOOA, 200A, 300A, and 400A proposed by PDM was approved."^

Twenty-three additional deviations, which from a structural point of view would be considered less significant than those covered above, were found in documents labeled as WTC1-490-L, WTC1-506-L,

WTCI-735-L, WTCI-748-L, WTCI-748-L, WTCI-756-L, WTCI-759-L, and WTCI-736-L through

WTCI-739-L.

8.4 DEVIATIONS RELATING TO ALTERNATE FABRICATION/ERECTION

PROCEDURES

The following is a list of specific requests relating to deviations for alternate fabrication and erection

procedures of core columns, floor trusses, exterior wall columns, and beam seats:

• The Port Authority notified the Stanray Pacific Corporation that their request to splice core columns every 18 ft was approved."''

• The Port Authority notified the Laclede Steel Company that their request to use Hobart

automatic CO2 welding equipment and procedure was approved, provided that the requirements of the contract documents were met.'^

• SHCR notified the Port Authority that the elimination of clipped comers of stiffener plates in the exterior wall columns, as proposed by Pacific Car & Foundry, was approved.'^

• SHCR notified the Port Authority that 8 by 6 by 1 in. angles were approved to be used for beam seat types 7440 through 7494 instead of 8 by 6 by 7/8 in. angles, which were originally

required in the contract drawings for Pacific Car & Foundry."^

8.5 DEVIATIONS RELATING TO PRODUCT SUBSTITUTIONS

The following is a list of specific requests relating to deviations for product substitutions in the exterior

wall:

• SHCR notified the Port Authority that 24 steel plates with yield strengths ranging from 42 ksi to 100 ksi were allowed to be substituted for specific plates that were originally fabricated by

Pacific Car & Foundry for use in the exterior wall.^*'

Letter dated October 7. 1968 from James White of SHCR to R. Monti of PONYA (WTCI-738-L; see Appendix G).

Letter dated October 18, 1968 from James White of SHCR to R. Monti of PONYA (WTCI-739-L: see Appendix G).

-'' Letter dated September 21, 1969 from R. Monti of PONYA to W. Gibson of Stanray Pacific Corporation (WTCI-490-L: see Appendix G). " Letter dated October 16. 1969 from R. Monti of PONYA to Robert Bay of Laclede Steel Company (WTCI-506-L: see Appendix G). Letter dated December 15, 1967 from James White of SHCR to R. Monti of PONYA (WTCI-748-L: see Appendix G).

-^ Letter dated May 26, 1969 from James White of SHCR to R. Monti of PONYA (WTCI-756-L; see Appendix G).

^° Letter dated May 2. 1969 from James White of SHCR to R. Monti of PONYA (WTCI-756-L: see Appendix G).

NISTNCSTAR 1-1A. WTC Investigation 129
Chapter 8

• SHCR notified the Port Authority that 3/4 in. thick plates may. be substituted for 5/8 in. and

1/2 in. thick plates shown on the drawings for Plate TD7 of the top spandrels at reference

level D (7th floor level) fabricated by PDM.^'

• The Port Authority notified PDM that they were granted approval to increase the plate

thickness for certain "E-1" plates for 1 1 specified columns.

• The Port Authority notified PDM that they were allowed to use Lukens American Society for Testing and Materials (now ASTM International) A 441 Modified steel for 36 plates in lieu of

the steel originally specified.

8.6 DEVIATIONS RELATING TO INSPECTION PRACTICE

The following is a list of specific requests relating to deviations in inspection practice for the exterior wall

and welds:

• SHCR notified the Port Authority that the PDM request to revise the radiographic inspection

provisions that were included in the PDM control program as they relate to the full- penetration butt weld joining of spandrel plate D4 and E3 was not approved. Instead, SHCR

suggested an alternate program to be followed.

• SHCR notified the Port Authority that the Stanray Pacific request to revise their quality

control program with respect to the minimum inspection rate for welds was approved.^^

8.7 REFERENCE

PONYA (Port of New York Authority). 1967. Fabricated Steel Box Core Columns and Built-Up Beams

From the 9th Stoiy Splice to the Penthouse Rooffor North and South Towers. World Trade Center

Contract WTC-2 17.00. (WTCI-244-L).

" Letter dated June 11, 1968 from James White of SHCR to R. Monti of PONYA (WTCI-739-L; see Appendix G). " Letter dated December 18, 1967 from R. Monti of PONYA to H. Fish of PDM (WTCI-745-L; see Appendix G).

Letter dated December 18, 1967 from R. Monti of PONYA to H. Fish of PDM (WTCI-745-L; Appendix G).

Letter dated May 3, 1968 from James White of SHCR to R. Monti of PONYA (WTCI-742-L; see Appendix G).

Letter dated April 18, 1968 from James White of SHCR to R. Monti of PONYA (WTCI-483-L; see Appendix G).

130 NISTNCSTAR 1-1A, WTC Investigation