NATL INST OF STANDARDS & TECH R.I.C.

A1 1102151996

/NBS building science series TA435 .U58 V143;1982 C.2 NBS-PUB-C 1974

NBS BUILDING SCIENCE SERIES 14 3

Investigation of the

Kansas City Hyatt Regency

Ikways Collapse TA

435

.U58

m

1982

C.2

'ARTMENT OF COMMERCE • NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

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the Institute for Computer Sciences and Technology.

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'Headquarters and Laboratories at Gaithersburg, M D, unless otherwise noted; mailing address Washington, DC 20234. ^Some divisions within the center are located at Boulder, CO 80303.
NBS BUILDING SCIENCE SERIES 143

Investigation of the

Kansas City Hyatt Regency

Walkways Collapse

R. D. Marshall

E. O. Pfrang

E. V. Leyendecker

K. A. Woodward

Center for Building Technology

National Engineering Laboratory

R. p. Reed

M. B. Kasen

T. R. Shives

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

Washington, DC 20234

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued May 1982
Library of Congress Catalog Card Number: 81-600538

National Bureau of Standards Building Science Series 143

Nat. Bur. Stand. (U.S.), Bldg. Sci. Ser.143, 3bO pages (May 1982)

CODEN: BSSNBV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1Q82

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402

Price (Add 25 percent for other than U.S. mailing)
ERRATA SHEET

NBS Building Science Series 143

Investigation of the Kansas City Hyatt Regency

Walkways Collapse

R. D. Marshall, E. 0. Pfrang, E. V. Leyendecker , K. A. Woodward

page iii Delete last paragraph of Abstract.

Page ix Section 1.2 begins on page 2.

Section 2.3 begins on page 6.

Page 6 Second paragraph, replace £ sign with • in front of each code and standard listed.

Page 10 Fourth paragraph and tenth line, delete

conuna after Kansas City.

Page 25 Figure 3.8 (at top of page), replace 1 5/8"

with 2 1/8" for dimension of distance from

left end of stringer to center line of holes

in clip angle.

Page 35 First paragraph of Section 4.3, replace E

sign with • in front of each eyewitness

account.

Page 46 Fourth line of Section 5.2.1, replace El. 45.00

with Elev. 45.00.

Page 227 First paragraph of Section 9.3.3, the equation

should read:

0.7854

Page A-98 Delete last paragraph of Abstract.

ABSTRACT

An investigation into the collapse of two suspended walkways within the atrium

area of the Hyatt Regency Hotel in Kansas City, Mo., is presented in this

report. The investigation included on-site inspections, laboratory tests and

analytical studies.

Three suspended walkways spanned the atrium at the second, third, and fourth

floor levels. The second floor walkway was suspended from the fourth floor

walkway which was directly above it. In turn, this fourth floor walkway was suspended from the atrium roof framing by a set of six hanger rods. The third

floor walkway was offset from the other two and was independently suspended

from the roof framing by another set of hanger rods. In the collapse, the

second and fourth floor walkways fell to the atrium floor, with the fourth

floor walkway coming to rest on top of the lower walkway.

Based on the results of this investigation, it is concluded that the most

probable cause of failure was insufficient load capacity of the box beam-hanger

rod connections. Observed distortions of structural components strongly suggest

that the failure of the walkway system initiated in the box beam-hanger rod

connection on the east end of the fourth floor walkway's middle box beam.

Two factors contributed to the collapse: inadequacy of the original design for

the box beam-hanger rod connection, which was identical for all three walkways,

and a change in hanger rod arrangement during construction that essentially

doubled the load on the box beam-hanger rod connections at the fourth floor

walkway. As originally approved for construction, the contract drawings called

for a set of continuous hanger rods which vjould attach to the roof framing and

pass through the fourth floor box beams and on through the second floor box

beams. As actually constructed, two sets of hanger rods were used, one set

extending from the fourth floor box beams to the roof framing and another set

from the second floor box beams to the fourth floor box beams.

Based on measured weights of damaged walkway spans and on a videotape showing

occupancy of the second floor walkway just before the collapse, it is concluded

that the maximum load on a fourth floor box beam-hanger rod connection at the

time of collapse was only 31 percent of the ultimate capacity expected of a connection designed under the Kansas City Building Code. It is also concluded

that had the original hanger rod arrangement not been changed, the ultimate

capacity would have been approximately 60 percent of that expected under the

Kansas City Building Code. With this change in hanger rod arrangement, the

ultimate capacity of the walkways was so significantly reduced that, from the

day of construction, they had only minimal capacity to resist their own weight

and had virtually no capacity to resist additional loads imposed by people.

Supplementary material is contained in an appendix to this report and

published as NBSIR 82-2465A.

Key Words: building; collapse; connection; construction; failure; steel;

walkway

.

iii

EXECUTIVE SUMMARY

On July 17, 1981, two suspended walkways within the atrium area of the Hyatt

Regency Hotel in Kansas City, Mo., collapsed, leaving 113 people dead and 186

injured. In terms of loss of life and injuries, this was the most devastating

structural collapse ever to take place in the United States.

On July 20, 1981, Senator Thomas F. Eagleton's office contacted the National

Bureau of Standards and requested that technical assistance be provided to

Kansas City. Shortly thereafter Kansas City Mayor Richard L. Berkley asked

NBS to determine the most probable cause of the walkways' collapse. Senators

Thomas F. Eagleton and John C. Danforth and Congressman Richard Boiling endorsed

the Mayor's request for the NBS to conduct an independent investigation. NBS

researchers first arrived in Kansas City on July 21, 1981. During the course

of the investigation, the NBS team of engineers and scientists inspected the

Hyatt Regency atrium area and the warehouse where the walkway debris was stored.

In the early phases of the investigation, NBS involvement was limited by court

order to visual and photographic observations and measurements. NBS requested

permission to weigh selected walkway spans and to remove for additional study

and destructive examination certain portions of the walkway box beams, hanger

rods, concrete decks, and nuts and washers. The Bureau's requests were eventually granted under court orders after agreement was reached with litigants

involved in various legal actions stemming from the collapse of the walkways.

Documents such as drawings, specifications, inspection reports, test reports,

and construction logs, as well as photographs and videotapes became available

to NBS from a number of sources. These were useful in gaining information

about the walkways as originally approved for construction and as modified

during the construction process.

NBS conducted extensive laboratory tests on Bureau-fabricated mockups of parts

of the walkways. Tests were also conducted by NBS on portions of the debris.

Analytical models were developed to predict the response of the walkways to

various loading conditions. Also, tests were conducted to determine material

properties and weld and fracture characteristics.

The Hyatt Regency consists of three main sections: a high-rise section, a "function block," and a connecting atrium area. As built, three suspended

walkways spanned the atrium at the second, third, and fourth floor levels and

connected the high-rise and function sections. The second floor walkway was suspended from the fourth floor walkway which was directly above it. In turn,

this fourth floor walkway was suspended from the atrium roof framing by a set

of six hanger rods. The third floor walkway was offset from the other two and was independently suspended from the roof framing by another set of hanger rods.

In the collapse, the second and fourth floor walkways fell to the atrium floor,

with the fourth floor walkway coming to rest on top of the lower walkway. Most

of those killed or injured were either on the first floor level of the atrium

or on the second floor walkway. The third floor walkway was not involved in

the collapse.

V
As originally approved for construction by the Kansas City Codes Administration

Office, the plans for the walkways called for a single set of hanger rods

(attached to the roof framing) which would pass through the fourth floor box

beams and on through the second floor box beams. The box beams—made up of a pair of 8-inch steel channels with the flanges welded toe to toe—were to rest on hanger-rod washers and nuts below each set of beams. Under this arrangement

each box beam would separately transfer its load directly into the hanger rods.

However, during construction, shop drawings were prepared by the steel

fabricator which called for the use of two sets of hanger rods rather than a single set. One set of hanger rods extended from the fourth floor box beams

to the roof framing and another set extended from the second floor box beams

to the fourth floor box beams. Under this arrangement all of the second floor

walkway load was first transferred to the fourth floor box beams, where both

that load and the fourth floor walkway load were transmitted through the box

beam-hanger rod connections to the ceiling hanger rods. As indicated by their

stamps, these shop drawings were reviewed by the contractor, structural engineer^

and architect.

Efforts were made to establish as accurately as possible the loads on the

walkways at the time of the collapse. Weighing of selected walkway debris

along with measurements taken on concrete cores removed from the walkway decks

permitted NBS to determine that the actual walkway dead load (self-weight) was approximately 8 percent higher than the nominal dead load. (The nominal dead

load is an estimate based solely on the project's contract drawings.) Estimates

of the loads due to people present on the walkways at the time of collapse (live

load) were much more difficult to make. Witnesses made a number of conflicting

statements regarding the numbers and activities of people on the walkways just

prior to the collapse. However, a television crew had videotaped parts of the

walkway minutes before the collapse. By studying this videotape, NBS concluded

that 63 people represented a credible estimate of combined second and fourth

floor walkway occupancy at the time of collapse.

Based on field, laboratory, and analytical investigations, NBS concluded that:

1 . Collapse of the walkways occurred under the action of loads that were substantially less than the design loads specified by the Kansas City

Building Code.

2. The ultimate capacity of box beam-hanger rod connections can be predicted

on the basis of laboratory test results.

3. Under the action of the loads estimated to have been present on the

walkways at the time of collapse , all fourth floor box beam-hanger rod

connections were candidates for initiation of walkway collapse

.

4. Observed distortions of structural components strongly suggest that failure

of the walkway system initiated in the box beam-hanger rod connection at

location 9UE (east end of middle box beam in fourth floor walkway).
5. As constructed, the box beam-hanger rod connections, the fourth floor to

ceiling hanger rods, and the third floor walkway hanger rods did not

satisfy the design provisions of the Kansas City Building Code.

6. The change in hanger rod arrangement from a continuous rod to interrupted

rods essentially doubled the load to be transferred by the fourth floor

box beam-hanger rod connect ions

,

7. The box beam-hanger rod connection would not have satisfied the Kansas

City Building Code under the original hanger rod detail (continuous rod),

8. Under the original hanger rod arrangement (continuous rod) the box

bean-hanger rod connections as shown on the contract drawings would have

had the capacity to resist the loads estimated to have been acting at the

time of collapse

.

9. Neither the quality of workmanship nor the materials used in the walkway

system played a significant role in initiating the collapse

.

vii

TABLE OF CONTENTS

Page

ABSTRACT iii

EXECUTIVE SUMMARY v

1. INTRODUCTION 1

1.1 Background 1 1.2 Objective and Scope of the Investigation 1 1.3 Organization of the Report 2

2. DOCUMENTS REVIEWED BY THE NBS INVESTIGATIVE TEAM 5

2.1 Introduction 5 2.2 Access to Documents 5 2.3 Project Specifications and Relevant Code Provisions 5 2.4 Architectural and Structural Drawings 6 2.5 Shop Drawings 7 2.6 Daily Logs and Inspection Reports 8

2.7 Other Documents and Materials Reviewed 9 2.8 Sumiaary 10

3. DESCRIPTION OF THE WALKWAY SYSTEM PRIOR TO COLLAPSE 13

3.1 Introduction 13

3.2 General Layout of the Hyatt Regency Atrium 13

3.3 Structural and Architectural Features of the Walkways 14

3.4 Probable Sequence of Erection 16

3.5 Designation of Walkway Components 16

3.6 Summary 17

4. DESCRIPTION OF THE WALKWAY COLLAPSE 33

4.1 Introduction 33

4.2 Summary of Events Preceding and Following the Collapse 33

4.3 Eyewitness Accounts of Collapse 35

4.4 Estimate of Walkway Occupancy at Time of Collapse 36

4.5 Removal and Storage of Walkway Debris 37

4.6 Summary 38

5. SITE INVESTIGATION 45

5.1 Introduction 45

5.2 Survey of the Hyatt Regency Atrium 45

5.2.1 Embedded Plates and Bearing Seats 46

5.2.2 Fourth Floor to Ceiling Hanger Rods 46

5.2.3 Impact Points on Atrium Floor 47

ix
TABLE OF CONTENTS (Continued)

Page

5.2.4 Alignment of Walkways Prior to Collapse 47

5.2.5 Other Observations Relevant to the Investigation 48

5.3 Observations of Walkway Debris 49

5.3.1 General 49

5.3.2 Hanger Rod Pull-through 49

5.3.3 Box Beam Longitudinal Welds 50

5.3.4 Box Beam to Clip Angle Fillet Welds 52

5.3.5 Other Observations Relevant to the Investigation 52

5.3.6 Determination of Span Weights 53

5.4 Removal of Specimens 54

5.4.1 Criteria for Specimen Selection 54

5.4.2 Specimen Removal 54

5.4.3 Specimen Cataloging and Preparation for Shipment 55

5.4.4 Specimen Storage and Handling 56

5.5 Replication of Walkway Weld Fracture Surfaces 56

5.6 Summary 57

6 . NBS STRUCTURAL TESTING PROGRAM 97

6.1 Introduction 97

6.2 Background • 97

6.3 Overview 98

6.4 Test Specimen Description 98

6.4.1 Full Length Box Beam 98

6.4.2 Short Box Beam 98

6.4.3 Fabrication 99

6.5 Test Setup 100

6.6 Loading Equipment 101

6.7 Instrumentation 102

6.8 Data Acquisition and Reduction 103

6.9 Loading Sequence 104

6.10 NBS Short Box Beam Tests 105

6.10.1 Parameters 105

6.10.2 Short Box Beam Specimens 106

6.10.3 Welding Process 107

6.10.4 Weld Area 109

6.10.5 Other Observations 110

6.10.6 Summary 113

X
TABLE OF CONTENTS (Continued)

Page

6.11 NBS Full-Leagth Box Beam Tests 113

6.12 Tests on Walkway Box Beam 113

6.12.1 Full-Length Box Beam Test 113

6.12.2 Short Box Beam Tests 114

6.13 Box Beam-Hanger Rod Connection Capacity 116

6.14 Conclusions 117

7. NBS MATERIALS EVALUATION PROGRAM 157

7.1 Introduction 157

7.2 Initial Inspection 158

7.2.1 Visual 158

7.2.2 Radiography 158

7.3 Mechanical Properties of Structural Steel and Weldments 159

7.3.1 NBS Channel Specimens 159

7.3.2 Walkway Box Beams 160

7.3.3 Walkway Hanger Rods 161

7.3.4 Walkway Box Beam Longitudinal Welds 161

7.4 Metallography and Hardness Measurements 162

7.4.1 Box Beam Weldments 162

7.4.2 Walkway Hanger Rods 163

7.4.3 Washers 163

7.5 Chemical Analysis 164

7.6 Properties of Concrete 164

7.7 Conclusions 166

8. FRACTOGRAPHIC ANALYSIS 195

8.1 Introduction 195

8.2 Specimen Preparation and NBS Fracture Results 195

8.3 Walkway Fracture Replicas 196

xi
TABLE OF CONTENTS (Continued)

Page

8.3.1 General Observations 196

-8.3.2 Box Beam Longitudinal Welds 197

8.3.3 Washer/Nut Contact Area 197

8.3.4 Washers 198

8.3.5 Clip Angle Fillet Welds 198

8.4 Direct Analysis of Walkway Box Beam Fractures 199

8.4.1 Procedures 199

8.4.2 Results 199

8.5 Summary 200

9. STRUCTURAL ANALYSIS 223

9.1 Introduction 223

9.2 Loads 223

9.2.1 Dead Load 223

9.2.2 Live Loads 225

9.3 Analysis of Walkway Structural Components 226

9.3.1 General 226

9.3.2 Box Beam-Hanger Rod Connections 226

9.3.3 Hanger Rods 227

9.3.4 Summary of Analysis 227

9.4 Forces Acting on the Hanger Rods at Time of Collapse 228

9.5 Walkway Deflections 229

9.6 Effects of Dynamic Excitation 229

9.6.1 Dynamic Characteristics of the Walkways 229

9.6.2 Sources of Dynamic Excitation 231

9.7 Summary 232

10. INTERPRETATION OF TESTS, ANAI.YSES AND OBSERVATIONS 243

10.1 Introduction 243

10.2 Comparison of Loads and Box Beam-Hanger Rod Connection

Capacities 243

10.3 Ultimate Capacity Based on Code Requirements 244

10.4 Probable Sequence of Collapse 245

10.5 Effect of Change in Hanger Rod Arrangement 246

xii
TABLE OF CONTENTS (Continued)

Page

10.6 Quality of Materials and Workmanship 2A6

10.7 Summary 248

11. SUMMARY km CONCLUSIONS 253

12. REFERENCES 2 57

APPENDIX A-1

A6.10 - A6.11 A-2

A7.2 - A7.5 A-1

9

A7.6 A-83

A9.2 A-92

xiii

1. INTRODUCTION

1.1 BACKGROUND

On July 17, 1981, at approximately 7:05 p.m., two suspended walkways within

the atrium area of the Hyatt Regency Hotel in Kansas City, Mo., collapsed,

killing 111 people and injuring 188. Two of the injured subsequently died.

In terras of loss of life and injuries, this was the most devastating structural

collapse ever to take place in the United States.

At the time of the collapse, the hotel had been in service for approximately

1 year. The Hyatt Regency consists of three main sections; a 40-story tower

section, a function block, and a connecting atrium area. The atrium is a large

open area approximately 117 ft (36 m) by 145 ft (44 m) in plan and 50 ft (15 m)

high. Three suspended walkways spanned the atrium at the second, third, and

fourth floor levels. These walkways connected the tower section and the function block. The third floor walkway was independently suspended from the atrium

roof trusses while the second floor walkway was suspended from the fourth floor

walkway, which in turn was suspended from the roof framing.

In the collapse, the second and fourth floor walkways fell to the atrium first

floor, with the fourth floor walkway coming to rest on top of the second. Most

of those killed or injured were either on the atrium first floor level or on

the second floor walkway.

On July 20, 1981, Senator Thomas F. Eagleton's office contacted the National

Bureau of Standards (NBS) and requested that technical assistance be provided

to the city of Kansas City. This was followed later that day by a request to

NBS from Mayor Richard L. Berkley for technical advice regarding the tragedy

and its cause. Accordingly, two NBS structural research engineers arrived in

Kansas City on July 21 and met with the Mayor and other city officials. On

July 22, Mayor Berkley formally requested that the NBS independently ascertain

the most probable cause of the collapse of the Hyatt Regency walkways. On

July 24 in a letter to Mayor Berkley, Senators Eagleton and Danforth and

Congressman Boiling endorsed the Mayor's request for NBS to conduct an impartial

investigation.

1
1.2 OBJECTIVE AND SCOPE OF THE INVESTIGATION

The objective of this investigation was to determine the most probable cause

of the collapse. As part of this investigation, it was necessary to reconstruct

the events that preceded the collapse and the condition of the walkways at the

time of the collapse. To carry out its study, the NBS investigative team used

data obtained from on-site inspections; drawings, specifications and other

records maintained by the city of Kansas City; documents and other information

from parties involved in the construction and operation of the Hyatt Regency

Hotel; information from those who participated in the rescue operation and from

eyewitnesses; and photographs, videotapes, and other records available from the

media. NBS also conducted inspections and took measurements in the atrium area

and on the debris from the collapse. NBS conducted extensive laboratory studies

on materials and raockups representative of materials and assemblies used in

the walkway construction. Extensive laboratory studies were also conducted

on specimens obtained from the walkway debris. NBS acquired these specimens

under the terms of several court orders. These studies were in turn supported

by analytical studies aimed at predicting the structural performance of the

walkways

.

1.3 ORGANIZATION OF THE REPORT

This report is organized in 12 chapters:

Chapter 2 describes the various documents reviewed during the course of this

investigation and the procedures followed by the NBS investigative team in

gaining access to them. Documents described include project specifications,

relevant codes and standards, contract and shop drawings, daily logs, and

Inspection reports.

Chapter 3 describes the general layout of the Hyatt Regency atrium, the

structural and architectural details of the walkways, their probable sequence

of erection, and the system established to designate walkway components.

Chapter 4 summarizes events preceding and following the collapse and eyewitness

accounts of the collapse. This chapter also discusses the walkway occupancy

prior to the collapse and presents what is believed to be a credible estimate

of walkway occupancy at the time of collapse. The chapter closes with a description of the removal and storage of walkway debris.

Chapter 5 addresses the site investigation which included detailed observations

of the walkway debris and a survey of that part of the atrium occupied by the

walkways. The damage observed is related to probable mode of failure. Also

described are the procedures followed in weighing selected walkway spans,

removing specimens from the debris, and obtaining replicas of the weld fracture

surfaces

.

Chapter 6 describes the NBS structural testing program which involved the

testing of box beams fabricated at the NBS and a box beam removed from the walkway debris. Test specimens, instrumentation, loading systems, and loading

sequence are described. Various parameters studied and their effects on box

2
beam-hanger rod connection capacity are discussed in detail. The chapter con- cludes with an assessment of the capacities of the box beara-hanger rod

connections in the fourth floor walkway.

Chapter 7 describes the NBS materials testing program and the results of tests

carried out on materials used to fabricate NBS test specimens and on materials

removed from the walkv^ay debris. Included in this chapter are results of a radiographic analysis of longitudinal welds on a walkway box beam, mechanical

properties of structural steel and weldraents, raetallographic examinations,

hardness measurements, chemical analyses of structural steel and weld material,

and mechanical properties of concrete used in the walkway decks.

Chapter 8 addresses the f ractographic analysis of weld fracture surfaces in

selected NBS box beams and in the walkway box beams. Results obtained from

direct optical and scanning electron microscopy using replicas and actual

fracture surfaces are discussed. Also addressed are washer fractures and

washer/nut contact areas.

Chapter 9 addresses structural analysis. Included are dead and live loads,

forces acting on the hanger rods, walkway deflections, analysis of the walkway

structural components and their compliance with codes and standards, and the

significance of dynamic excitation.

Chapter 10 interprets the results of structural tests and analyses and discusses

the probable sequence of the walkway collapse. Also discussed is the significance of replacing the continuous hanger rod arrangement as shown on the contract drawings with interrupted rods in the second and fourth floor walkway

system. The chapter closes with an assessment of the quality of materials and

workmanship

.

Chapter 11 summarizes the findings of the investigation and presents conclusions

reached by the NBS investigative team.

Chapter 12 lists the references cited in the text.

3

2. DOCUMENTS REVIEWED BY THE NBS INVESTIGATIVE TEAM

2.1 INTRODUCTION

Section 2.2 of this chapter describes the procedure followed in obtaining

access to documents.

Sections 2.3, 2.4 and 2.5 discuss documents prepared during the design stage,

prior to actual construction of the walkways. Section 2.6 deals with daily

logs and inspection reports filed during the construction phase, while section

2.7 in large part addresses documents relating to the collapse and the rescue operation. Section 2.8 provides a summary of the findings reported in this

chapter.

2.2 ACCESS TO DOCUMENTS

This chapter describes the various documents reviewed and the procedures

followed in gaining access to them. In arranging for access to documents, the

Bureau submitted its requests to the Office of the Mayor which in turn contacted

the appropriate firms and agencies, requesting their cooperation and assistance.

Access by NBS to the requested documents was therefore voluntary. Firms contacted by the Office of the Mayor and their roles in the design, construction,

and operation of the Hyatt Regency Hotel were as follows:

Crown Center Redevelopment Corporation - Owner

Hyatt Hotels Corporation - Operator

Patty Berkebile Nelson Duncan Monroe Lefebvre (PBNDML)

,

Architects Planners, Incorporated - Architect

Gillum-Colaco , Consulting Structural Engineers - Structural engineer

Concordia Project Management Limited - Construction manager

Eldridge & Son Construction Company - General contractor

Havens Steel Company - Steel fabricator

5
2.3 PROJECT SPECIFICATIONS AND RELEVANT CODE PROVISIONS

Shortly after the collapse, the Kansas City Public Works Department assembled

documents in its possession related to the design, construction, and inspection of the Hyatt Regency Hotel. The Office of the City Attorney made these

documents available for public review beginning July 21, 1981.

Included in these documents was a set of project specifications for the Hyatt

Regency Hotel. They had been prepared by Patty Berkebile Nelson Duncan Monroe

Lefebvre Architects Planners, Inc. (PBNDML), the project architects, and were

dated August 28, 1978 [2.1]. Section 0510 of the project specifications covers structural steel and, among other things, refers to applicable standards, quality assurance, fabrication, erection, and field quality control. Codes and

standards referenced in the project specifications include, but are not limited

to, the following:

i. AISC* "Code of Standard Practice for Steel Buildings and Bridges"

(No date)

i. AISC "Specification for the Design, Fabrication and Erection of

Structural Steel for Buildings" (No date)

f AWS**D1.1 "Structural Welding Code" (No date).

Also included in the documents provided by the Office of the City Attorney was

a copy of the Kansas City Building Code [2.2] in effect at the time the Hyatt

Regency project specifications and design drawings were reviewed by the Public

Works Department. This issue of the Kansas City Building Code is, with minor

exceptions, an adoption of the Uniform Building Code, 1976 Edition [2.3].

The AISC Specification for the Design, Fabrication and Erection of Structural

Steel for Buildings [2.4] forms the basis for the steel design provisions of

the Uniform Building Code and the Kansas City Building Code. Certain provisions

of the Kansas City Building Code and the codes referenced in the project

specifications are referred to later in this report.

2.4 ARCHITECTURAL AND STRUCTURAL DRAWINGS

In addition to the project specifications just described, the Office of the

City Attorney also made available sets of architectural and structural

drawings. The drawings listed below were "issued for revision" or "issued for

construction "during the period August 1-28, 1978, and were referred to during

the course of this investigation.

* American Institute of Steel Construction

** American Welding Society

6
Architectural

A302 Floor Plan, Level 106 North

A303 Floor Plan, Level 121 North

A304 Floor Plan, Level 136 North

A305 Floor Plan, Level 151 North

A412 Building Section

A508 Details, Expansion Joints, Bridge

Structural Date prepared

5302 Framing Plan, Level 106

5303 Framing Plan, Level 121

5304 Framing Plan, Level 136

5305 Framing Plan, Level 151

5306 Framing Plan, Level 166

S405.1 Steel Details

S601 General Notes

North March 30, 1978

North March 30, 1978

North March 30, 1978

North March 30, 1978

North March 30, 1978

May 12, 1978

March 10, 1978

Simplified versions of the framing plans at levels 106 to 166* and a sectional

elevation of the atrium were developed by NBS as part of this investigation and are included in sections 3.2 and 3.3 of this report. The General Notes

(Dwg. S601) set out the design criteria, including a design live load of 100 psf

(4.8 kPa) for hotel corridors and lobby areas. These are interpreted by NBS

to include the walkways in the atrium.

2.5 SHOP DRAWINGS

Shop drawings detailing the walkway structural system were made available to

the NBS investigative team by the Crown Center Redevelopment Corporation,

the hotel's owner. Initially, review of the drawings was by appointment at

the Crown Center corporate offices with visits for such purpose occurring on August 7 and 10, 1981. Arrangements were subsequently made for NBS to take

custody of the drawings until issuance of its report. The following shop drawings, prepared by the steel fabricator, Havens Steel Company, dated between

January 7, 1979, and February 9, 1979, and reviewed, as indicated by their

stamps on the drawings, by the contractor, structural engineer and architect, were received from the Crown Center Redevelopment Corporation on September 2,

1981:

* The contract and shop drawings used during the construction of the Hyatt

Pvegency Hotel use this notation to designate the various floors within the

atrium. For convenience this same system is adopted for use in this report.

The second floor walkway was level 121, the third floor walkway was level 136,

and the fourth floor walkway was level 151.

7
Shop Drawing Number Subject

Framing plan at Elev« 121 '~0 and elevation at column lines

o , y , and iu

rraming p±an at tiiev. ijo ~\j

h J Framing plan at Elev. 151 '~0" and expansion bearing detail

'\r\ Dox ueams } crosfa ucdLUS , aiiu udiigcL Lutiis T 1 J i Stringers, slide bearing plates, and bearing seats

0 1 Beams at Elev. 167'~1' and hanger rod brackets

nZ rraming plan at biev. lo/ —

i

C 1 Cii Truss elevation

Qo i ruH s 1

J

12 Truss T3

8A Truss T3 member layout

12A Truss parts (T3)

The first five of the above listed drawings were referred to extensively during

the course of this investigation. Field observations and measurements show

that, in general, the structural details, member sizes, and dimensions of the

as-built structure faithfully reflected that which was called for on the drawings. No attempt was made to check or confirm the details of the structural

work above the upper hanger rod brackets at Elev. 165 '-7", this being deemed

to be beyond the scope of the investigation.

2.6 DAILY LOGS AND INSPECTION REPORTS

Portions of the daily logs maintained by the architect during construction of

the Hyatt Regency Hotel were made accessible to NBS and were reviewed at the

offices of Duncan Associates, Inc.* on two occasions—September 4 and 25, 1981.

These portions contained limited reference to construction of the walkways. In

a number of logs, reference is made to construction work in the atrium along

column lines 7 and 11 and at levels 136 and 151. However, there is very little

specific Information on the walkways. Those logs that did refer to the walkways

can be summarized as follows:

#312 September 14, 1979

Notes that Havens Steel Company completed rewelding of composite deck

studs at the walkways.

#320 September 26, 1979

Mentions placement of concrete for bridges 151, 136, and 121 from column

lines 7 to 11.

* A member firm of PBNDML.

8
#344 October 30, 1979

Mentions report "... that fitters were refusing to work on the 15L bridge

because of a comment made by the inspection team now looking at the

atrium connections."

#372 December 7, 1979

Notes problem on 151 bridge where piping and granite hangers were in

conflict

.

On September 4, 1981, a visit was made to the offices of the owner's inspection

agency, General Testing Laboratories, Inc., where an inspection report was reviewed which mentioned the inspection of bolted connections on August 17 and

20, 1979. The report noted that the hanging walkways were inaccessible due to

metal decking or "difficulty of position" and that the connections were not

checked by calibrated torque wrench. Representatives of General Testing Laboratories indicated that unless the specific dates of walkway construction were

known, it would be very difficult for them to retrieve reports of interest to

the NBS team.

Also on September 4, 1981, copies of inspection reports related to construction

in the atrium area were reviewed in the offices of Blackwell, Sanders, Matheny,

Weary, and Lorabardi, legal counsel to Concordia Project Management Ltd. These

reports, filed by General Testing Laboratories, did not provide any additional

information on the walkways. In response to an earlier request, NBS investigators were shown a handwritten note indicating the following schedule for

walkway construction;

August 8-15, 1979 Erect bridges.

September 7-12, 1979 Pour bridge concrete on fourth level.

September 10-14, 1979 Pour bridge concrete on third level.

January 31-Feb. 7, 1980 Pour bridge concrete on second level.

It appears that only the erection of structural steel sometime before

August 17, 1979, is consistent with dates mentioned in the various daily logs

and inspection reports reviewed by the NBS team. Given the limited quantity

of concrete in the walkway decks, the single-day placement indicated in the

architect's daily logs seems more credible.

2.7 OTHER DOCUMENTS AND MATERIALS REVIEWED

In addition to the specifications, drawings, logs and reports described above,

the NBS investigative team also reviewed several hundred photographs taken

immediately after the collapse, during the rescue operation and on the day

following the collapse. The following agencies and establishments allowed

members of the NBS team to inspect and, in some cases, retain copies of their

photographs

:

9
Kansas City Fire Department

Kansas City Police Department

Crovm Center Redevelopment Corporation

The Kansas City Star

The Kansas City Times .

Videotapes from various news services were also reviewed. Of particular

importance was a videotape obtained from KMBC TV, Kansas City, Mo., which

contained footage taken immediately before and after the collapse. Analysis

of information contained on this tape is described in chapter 4.

The NBS team also reviewed photographs and documents related to an investigation

conducted by the U.S. Occupational Safety and Health Administration (OSHA) following a fatal construction accident at the Hyatt Regency Hotel in October 1979.

As the OSHA investigation was not concerned with or related to the walkway

construction, only limited portions of the walkways could be identified in the

photographs

.

With regard to construction materials, the firm of Miller and Glynn, legal

counsel for Havens Steel Company, allowed the NBS team to review mill reports

related to the walkway hanger rods and the MC8 x 8.5 shapes used to fabricate

the walkway box beams. Information was also provided on welding procedures and

electrodes believed by the steel fabricator to have been used in fabricating

structural components of the walkways.

Of particular interest to the NBS investigative team were the original

structural design calculations and any change orders involving the walkway

structural system. However, efforts by the Office of the Mayor to obtain

these documents were not successful. With regard to review and modification of

the walkway design, the Office of the City Attorney provided copies of two

documents that are relevant to this investigation. The first document, dated

March 10 and March 16, 1978, contains six pages of handwritten notes addressing

the issue of fire endurance of the walkway structural steel. It was prepared

in the course of a routine design review by the Codes Administration Office,

Kansas City, Public VJorks Department. The second document, dated April 4, 1978,

is a two-page summary of a meeting between representatives of the Codes Administration Office and the architects, PBNDML. These documents, which resulted in an agreement to encase the walkway structural steel with gypsum board, are referred to in section 9.2 of this report.

One other document of considerable value to this investigation is a report

describing dynamic response measurements carried out on the third floor walkway

just prior to its removal during the night of July 22-23, 1981 [2.5]. These

measurements made it possible to validate the dynamic modeling of the walkways

as described in section 9.6 of this report. Access to this material was arranged by the Crown Center Redevelopment Corporation.

2 .8 SUMMARY

The following summarizes the results of efforts by NBS to obtain and review

documents and other materials that are relevant to the design and construction

10
the Hyatt Regency walkways and that provide Information on events immediately

eceding and following the collapse:

Codes and specifications applicable to the design, fabrication, and erection

of the walkway structural system were referenced in the project

specifications

.

The AISC Specification for the Design, Fabrication and Erection of

Structural Steel for Buildings forms the basis for the steel design

provisions of the Kansas City Building Code.

Drawings obtained for review and reference included architectural and

structural drawings and the fabricator's shop drawj_ngs.

The project design criteria specify a design live load of 100 psf (4.8 kPa)

for hotel corridors and lobby areas. This is interpreted by NBS to include

the walkways

.

Efforts to obtain copies of the structural design calculations were unsuccessful

.

Gypsum board encasement of the walkway structural steel was added during

the design review stage to meet code requirements for fire endurance.

A videotape containing footage of activities in the atrium immediately

before and after the walkway collapse was reviewed by the NBS investigative

team.

Results of vibration testing of the third floor walkway were provided to

NBS and were subsequently used to check analytical models of the walkway

dynamic response.

11

3. DESCRIPTION OF THE WALKWAY SYSTEM PRIOR TO COLLAPSE

3.1 INTRODUCTION

This chapter describes the structural features of the walkway system and the

probable sequence of construction with emphasis on those structural aspects

that are believed to be of significance to the investigation of the collapse.

Extensive use is made of the contract and shop drawings referred to in

chapter 2. In the remainder of this report all drawings other than shop drawings will be referred to collectively as "contract drawings." Only where a specific drawing is referred to will the terms "architectural" and "structural"

be used.

Section 3.2 describes the Hyatt Regency atrium and location of the walkways.

Also described are the areas served by the walkways and means of access from

the atrium main floor, the tower section, and the hotel function block.

Section 3.3 describes the structural and architectural features of the walkways,

again with major emphasis on those features that are believed to be of

importance in identifying the most probable cause of collapse.

Section 3.4 discusses the probable sequence of construction. Where possible,

this is related to documents referred to in chapter 2. However, very little of

the construction sequence could actually be documented, and this section is by

necessity based on what is considered to be a logical progression of construction. Section 3.5 describes a unified system used in this report for designating walkway components. Findings reported in this chapter are summarized in

section 3.6.

3.2 GENERAL LAYOUT OF THE HYATT REGENCY ATRIUM

The atrium is centrally located between the 40-story tower section to the north

and the function block directly to the south (figure 3.1). Plan dimensions of

the atrium are approximately 117 ft (36 m) north-south by 145 ft (44 m) eastwest. Clear ceiling height is approximately 50 ft (15 m) . The west portion

of the atrium between column lines F and I and part of the tower section to the

north of column line 11 are shown in plan view on figure 3.2. As can be seen

13
from figure 3.2, the main entrance is located on column line I, between column

lines 10 and 11. The registration and cashier desks are located to the east

of column line F, directly under a second floor terrace which extends over the

entire east side of the atrium. Direct access to the terrace and to the shops

and restaurants on the second floor of the adjoining function block (level 121)

is by stairway or escalator located in the southeast corner of the atrium. An

adjacent escalator provides access to the ballroom area on the third floor of

the function block (level 136). A focal point of the atrium was a sunken lounge area on the main floor between column lines 8 and 10 (shown cross-hatched in

figure 3.2).

Spanning the atrium in the north-south direction were three walkways as shown

in figure 3.3. NOTE THAT THIS FIGURE IS BASED UPON AN ARCHITECTURAL DRAWING

(DWG. AA12) ISSUED FOR CONSTRUCTION ON AUGUST 28, 1978, AND DOES NOT ACCURATELY

REFLECT THE SECOND AND FOURTH FLOOR HANGER ROD DETAIL ACTUALLY USED IN THE CONSTRUCTION OF THE WALKWAYS. The hanger rod detail actually used will be

explained in the discussion of figures 3.9-3.11.

In addition to providing a direct traffic way between the tower section and the

function block, the walkways were a dominant feature of the atrium and afforded

a view of the atrium and terrace. Of the three walkways, the second floor walkway was most easily accessible from the atrium floor—by stairway or elevator

(between column lines G and H) in the tower section and by stairway or escalator

in the atrium. The third floor walkway, which served the ballroom area, evidently was intended to carry larger volumes of traffic since it was slightly

wider than the other two walkways. The fourth floor walkway (level 151) served

a health club and sports area in the upper portion of the function block and was accessible only by elevator or stairway in the tower section. Of the

three walkways, the fourth floor walkway quite likely carried the least traffic.

A schematic view of the walkways from the north end of the atrium is shown in

figure 3.4.

3.3 STRUCTURAL AND ARCHITECTURAL FEATURES OF THE WALKWAYS

Structural framing plans for the three walkways are shown in figures 3.5 to 3.7.

Unless noted otherwise, all structural steel is ASTM* A36. Aside from the fact

that the third floor walkway is l'-3 5/8" (0.397 ra) wider than the second and

fourth floor walkways, structural details are identical. Each walkway consisted

of four spans made up of W16 x 26 stringers placed 7'-6 3/8" (2.295 m) center

to center (third floor = 8'-10" (2.692 m)) with W8 x 10 cross beams on 7 '-3 3/4"

(2.229 m) centers. Details of the stringers, including a 1/4 x 6 in (6 x

152 mm) bent plate used to form up the edge of the 3 1/4 in (83 mm) lightweight concrete deck are shown in figure 3.8. As shown in figure 3.9, the con- crete deck was placed on a 20 ga (1 mm) formed steel deck with 6 in (152 mm)

pitch and 1 1/2 in (38 mm) rib height. The v^alkway spans apparently were designed to act as composite beams and were constructed without shores (i.e.,

it is assumed that the concrete deck does not participate in resisting forces

American Society for Testing and Materials.

14
and moments due to self-weight of the concrete and structural steel). Shear

studs are 3/4 in (19 mm) diameter x 3 1/2 in (89 mm) long. Reinforcement is

W2.1 X W2.1 welded wire fabric with 6 in (152 mm) spacing each way.

The W16 X 26 stringers were bolted to clip angles welded to box beams located

on column lines 8, 9 and 10. High strength 3/4 in (19 mm) bolts (ASTM A325) were used to fasten the stringer to clip angle connections. Details of the

box beams and box beam to stringer clip angles are shown in figure 3.10.

The box beams were fabricated from MC8 x 8.5 shapes joined toe to toe by

continuous longitudinal welds. The associated welding symbols included in

figure 3.10 are as shown on the shop drawing (Havens Steel Company Dwg. 30).

In the absence of a backing strip and with complete joint penetration indicated, this welding symbol is interpreted to mean a prequalified partial joint

penetration groove weld as defined in section 2.10 of AWS Dl.1-79, Structural

Welding Code-Steel [3.1]. This welding symbol and its applicability to the

box beam longitudinal welds are discussed further in chapters 5 and 10 of this

report

.

The walkway hangers were 1 1/4 in (32 mm) diameter rods threaded top and bottom

to receive a nut and washer. Layout of the holes for the box beam-hanger rod

connections is shown in figure 3.10. Layout of the box beams and hanger rods

for the three walkways is shown in figure 3.11. Note that two hanger rods with

a 4 in (102 mm) offset at the fourth floor walkway were substituted for the

single continuous hanger rod arrangement shown in figure 3.3 and detailed on contract drawings A508 and S405.1. The significance of this change is discussed

in chapter 10 of this report. The second and fourth floor walkways were sus- pended from the WIS x 40 beams shown on the atrium ceiling framing plan in

figure 3.12 while the third floor walkway was suspended from the bottom chord

of truss Tl , also shown in figure 3.12. A detail of the hanger rod to beam

connection at Elev. 165'-7" is shown in figure 3.11. A similar detail was used

to connect the third floor walkway hanger rods to truss Tl.

The south end (column line 7) of each walkway was supported by field welding

the clip angles to 1/2 in (13 mm) plates embedded in the perimeter floor beams

of the function block. The north end (column line 11) was supported by

expansion bearings as detailed in figure 3.13.

The walkway handrail assembly consisted of 40 x 57 1/2 in (1.016 x 1.460 m)

panels of 1/2 in (13 mm) tempered plate glass set in extruded aluminum grips

supported by 4 x 3 x 5/16 angles running along each edge of the concrete deck

as indicated in figure 3.9. Adjustment of the supporting angle was achieved

by shim packs field welded to the concrete deck edging plate described previously. An extruded brass section capped the top edge of the plate glass

panels and a continuous wood handhold completed the handrail assembly.

Documents on file with the Kansas City Public Works Department indicate that

to satisfy requirements for fire endurance as specified in the Kansas City

Building Code, the sides and bottom of each walkway were encased with two

layers of 5/8 in (16 mm) gypsum board, supported by 22 ga (0.8 mm) metal studs

and nailing strips attached to the structural steel by power driven fasteners.

15
A simplified detail of the structural system, handrail, and gypsum board encasement is shown in figure 3.14. Not shown is a lighting strip along the base

of the handrail assembly and a system of 20 ga (1 mm) aluminum slats suspended

from the bottom face of the walkways. The 1 1/4 in (32 mm) diameter hanger

rods were also encased with insulating material, resulting in an overall diameter of 3 1/2 in (89 mm). A sprinkler system was suspended from the W8 x 10 cross beams of each walkway.

3.4 PROBABLE SEQUENCE OF ERECTION

Although a number of documents relating to construction of the walkways were reviewed during the course of this investigation, the actual construction

sequence could not be clearly established. With regard to the second and fourth

floor walkways, it is likely that all structural steel was erected and bolted

prior to placement of metal decking. The next step would likely have been the

arc spot welding (puddle welding) of the formed steel deck and placement of

shear studs on the W8 x 10 cross beams, followed by placement of concrete by

pumping. The daily construction log described in section 2.6 indicates that

placement of concrete on all three walkways v/as carried out on September 26,

1979.

Since the concrete deck was unshored during construction and the W16 x 26

stringers were supplied without camber, the fraction of dead load due to the

structural steel, formed steel deck, and the concrete produced midspan deflections of the order of 1/2 in (13 mm). To correct for this, a sand/cement

topping course was added later. This is discussed further in chapter 5. The

above procedure meets the provisions contained in the general notes of

Dwg. S601.

Upon completion of the deck, the next logical step in the construction

sequence would have been the installation of the handrail system. The midspan

deflections described in the previous paragraph are apparent when the vertical

alignment of the handrail supporting angle is compared relative to the top

flange of the W16 x 26 stringers. All increments of dead load added during

this and subsequent phases of construction were resisted by composite action

of the stringer/deck system. Completion of the walkways involved the installation of the nailing strips and gypsum board, sprinkler system, aluminum slats,

footlights, and walkway carpeting with underlying pad.

3.5 DESIGNATION OF WALKWAY COMPONENTS

Various components of the three walkways will be referred to rather extensively

in the remainder of this report. So that these components can be clearly and

concisely identified in the text, a unified system for designating walkway

components will be adopted. In this system column lines 7 through 11 will be

used to designate walkway components located on or between those lines while

the letters U, M, and L (for upper, middle, and lower) will be used to designate

the fourth, third, and second floor walkways, respectively. The letters E and

W will be used to differentiate between the east and west sides of the walkways.

For example, the designation "span U8-9" refers to the second span from the

south end of the fourth floor walkway while "clip angle lOLW" refers to a clip

16
angle on the west end of the box beam in the second floor walkway at column

line 10. Where it is clear that a particular level is intended, the U, M, or

L designations are not included as, for example, in discussing the fourth floor

to ceiling hanger rod at location 8E (column line 8, east side). This unified

system applied to the fourth floor walkway is illustrated in figure 3.15.

3.6 SUMMARY

The following summarizes information available to the NBS investigative team

regarding the layout of the Hyatt Regency atrium, location of the walkways and

their accessibility, structural and architectural features of the walkways,

and the probable sequence of their erection.

1. As its name implies, the atrium is central to hotel activities. The

walkways were the dominant feature of the atrium.

2. Of the three walkways, the second floor walkway was most easily accessible

from the atrium main floor. The third floor walkway served the hotel

ballroom and, because of its greater width, was evidently intended to

carry larger volumes of traffic than the second or fourth floor walkways.

The fourth floor walkway was the least accessible and is likely to have

carried less traffic than the other two walkways.

3. The walkway spans were apparently designed to act as composite beams and were constructed without shores (i.e., it is assumed that the concrete deck

does not participate in resisting forces and moments due to self-weight of

the concrete and structural steel).

4. In preparing the shop drawings, two hanger rods with a 4 in (102 mm) offset

at the fourth floor walkway were substituted for the single continuous

hanger rod arrangement detailed on the contract drawings.

17
N

i

FUNCTION

BLOCK

Figure 3.1 Plan showing outline of tower section, atrium, and function block.

18
19
Figure 3.3 Elevation of atrium between column lines 8 and 9, looking north.

20
Figure 3.4 Schematic of walkways as viewed from north wall of atrium.

21
7'-6-3/8" —15'-0"- O © 0

Figure 3.5 Second floor walkway framing plan - level 121.

22
Figure 3.6 Third floor walkway framing plan - level 136.

23
Figure 3.7 Fourth floor walkway framing plan - level 151.

24
j>-8"-|- 6'-11}4"

29'-4 5/8" |W16»26|

-r-3V," Itypl- —-|- 7'-3?4" Itypi -|. -7'-l 3/8" Ityp)

1 5/8"

EL, t50'-7V," 135'-7y,"

120'-7i/," ^

2'/," 3"

3"

SCi" ItypI

PL: %>6" Iflent)

— —2 5/16"|T»pl -1 *H'/." |t»Pl

ISOUTHI

—

I I

5" ItypI 5" ItypI

^2iLs4«3»5/16>0'-814" Section A-A

-28'-10%" IW18-26)- ^2 3/8"

J

2 3/8" - 29'-4 5/8" |W16"26| -

-6'-11K"-

-3"T

-t fISOUTHl 3/8"•y t 2 13/16"

•-SLIDE BEARING |4'/i">4'//'

5'/,"

Figure 3.8 Details of walkway stringers.

25
CO

X .

26
1 5/16" DIA HLSONt

EL. ISO'-"/'/,"

9'-4 3/8" |2MCe-8,5|-

7'-6r/8"-

-Z^3x3x5/16xr-0!/2"|typ|

FOURTH FLOOR BOX BEAM

5 I/ell 1/8" 1-

-1

4" 2'

-15/16" DIA

' HLSONt 3g^g„

1 7/8"

3 5/8"

1 7 IV

Section A-A

1 5/16" DIA HLSONit

EL. 120'-7i/,"

lEL. 135'-7'/4")\

11 1/8"

51/8" |91/8"|

-9'-4 3/8" |10'-8"||2MC 8-8.51-

7'-61/8" |8'-9y,"|

2'/2"

5"

111/8"

51/8" 191/8"Ah

L^fl SECOND FLOOR BOX BEAM

NOTE: WHEN DIFFERENT, DIMENSIONS FOR THIRD FLOOR BOX BEAM ARE SHOWN IN BRACKETS I |.

1 5/16" DIA HLSON <t

EL. 150'-7y,"

|EL. 135'-7%"

EL. 120'-7i/,"

31/2" C/C

-2-^s3x2x3/8x0'-5i/;" |Typ| CROSSBEAM

NOTE: WHEN DIFFERENT. DIMENSIONS FOR THIRD FLOOR CROSS BEAM ARE SHOWN IN BRACKETS I )

3 1/8M" B.O.

31/2" C /C

Figure 3.10 Details of walkway box beams and cross beams.

27
11/4" DIA. ROD

EL. 120'-7y4" 2-MC8-8.5

2-MC8-8.5 Z

EL. 135'-7'/4"

EL. 105-9"

EL. 167'-1'

o o

o o

o o

o o WEST

3'-l 3/8"

W18x40x14'-9'/2"

V/2" DIA, HLS. — 9 -0 3/8 '- 2'-8 3/16"

BAR 3x'/2''0'-41/2"

0'-4'/2" (TypI

4" (Typl BAR 3'l'<0'-4"^

Section A-A

Detail A

Figure 3.11 Elevation of typical box beam-hanger rod assembly.

28
Figure 3.12 Framing plan for atrium roof,

29
Figure 3.14 Simplified detail of walkway structural system and

architectural treatment.

30
Figure 3.15 Unified designation of walkway components (fourth floor walkway

shown)

.

31

4. DESCRIPTION OF THE WALKWAY COLLAPSE

4.1 INTRODUCTION

This chapter addresses the major events related to the collapse of the walkways,

the rescue operation, and the removal and storage of the debris. Walkway

occupancy at the time of collapse is also discussed.

Section 4.2 gives a chronology of events during the night of July 17-18, and

presents photographs showing the walkway spans immediately after the collapse

and after completion of the rescue operation. Section 4.3 summarizes eyewitness

accounts of the collapse as reported in local newspapers.

Section 4.4 describes the analysis of videotape footage taken just prior to the

collapse and presents an estimate, based in large part on this analysis, of

walkway occupancy at the time of collapse.

Section 4.5 describes the removal and storage of the walkway debris, inspection

of the atrium by NBS engineers, and removal of the third floor walkway.

Findings reported in this chapter are summarized in section 4.6.

4.2 SUMMARY OF EVENTS PRECEDING AND FOLLOWING THE COLLAPSE

The following is a summary of events related to the walkway collapse and to

the rescue operation as reported in The Kansas City Times, Metropolitan Edition,

Saturday, July 18, 1981, and in The Kansas City Star , Extra, Sunday, July 19,

1981.

Friday, July 17

3:00 PM - People begin to arrive for dance event.

4:30 PM - Seating on first floor of atrium is fully occupied; people begin

to occupy seating on atrium terrace and to assemble on walkways.

7:00 PM - Crowd in atrium area is estimated at 1500 to 2000.

33
7:04 PM - Band returns from break and begins to play for dance contest.

7:05 PM - Second and fourth floor walkways collapse.

7:08 PM - Kansas City Fire Department (KCFD) dispatcher receives first

call for help.

7:09 PM - Kansas City Police Department (KCPD) dispatcher called.

7:12 PM - KCFD rescue squad at scene calls for additional help.

7:18 PM - Seven city ambulances at scene.

7:19 PM - Call goes out for cutting tools.

7:23 PM - Call goes out for forklift.

7:52 PM - More than 100 firefighters and emergency workers involved in rescue operation.

8:30 PM - Heavy crane arrives.

10:30 PM - Cranes moved into position near west wall of atrium.

Saturday, July 18

3:15 AM - First walkway section (span U8-9) lifted by cranes.

4:30 AM - Last survivor removed from debris.

6:00 AM - Span U7-8 lifted and set aside.

7:45 AM - Last span (presumably L7-8) is lifted and 31 bodies are remove

As of Sunday, 'July 19, the reported death toll was 111. The number of injured

was reported to be 188. (Two of the injured subsequently died.)

Figures 4.1 and 4.2 show the walkways shortly after the collapse, just as the rescue operation was getting under way. The photograph in figure 4.1 appears

to have been taken from the stairway landing at the south side of the atrium

between the second and third floors. The view is to the west-northwest and

shows part of span U7-8 which lodged in the second-floor portal originally

occupied by span L7-8. Spans U8-9 and U9-10 can be seen behind the trees in

the middle of the picture. The photograph in figure 4.2 was taken from the

third floor with the view directly to the north along the axis of the walkways

Span U7-8 is in the foreground and the third floor walkway can be seen to the

upper right. Span UlO-11 is visible at the far (north) side of the atrium.

The north end of this span appears to have experienced an appreciable displace

ment to the west during the collapse. Water is coming from walkway sprinkler

pipes severed when the north walkway spans were pulled off of their bearing

seats

.

34
A general view of the walkway debris after all victims had been removed is

shown in figure 4.3. The view is to the south and shows span L9-10 resting on span U9-10 in the foreground. Directly to the south is span U8-9 (left) and

span L8-9 (right). At the upper left is the south end of span U7-8. Span

L7-8 is at the top of the picture, directly below its original position in the

second floor walkway. Much of the debris is timber cribbing used to support

the spans during the rescue operation. Span LlO-11 is out of view in the left

foreground and span UlO-11 has been moved through the main entrance (to the

right) and into the hotel driveway.

4.3 EYEWITNESS ACCOUNTS OF COLLAPSE

In the same newspapers cited in section 4.2, eyewitness accounts of the

collapse are reported as well as estimates of the number of people on the

walkways and on the atrium floor directly below. These accounts can be

summarized as follows:

£ Individual standing at end of walkway (level unknown) saw upper walkway

collapse at center first and then fall onto second floor walkway.

' Estimated about 50 people standing on fourth floor walkway, 100 people

on second floor walkway and 200 people on atrium floor beneath the

walkways

.

£. Individual (vantage point unknown) observed at least 40 persons on fourth

floor walkway dancing or looking at scene below. Fourth floor began to

buckle and collapsed. Area under walkway in vicinity of bar was jammed.

Glass exploded, hitting people 100 to 150 ft away with shards.

f Individual on bandstand observed second floor walkway split in middle

and begin to fall.

£ Individual was "... sitting on steps in front part of lobby after walking

across the catwalk moments before." Saw top walkway come straight down

and hit second walkway which then fell. Estimated about a dozen people

on fourth floor walkway, 75 to 100 on the second floor walkway, and

100 to 150 people below the walkways.

In view of the conflicting nature of eyewitness accounts and the availability

of videotape showing parts of the walkways a few minutes before the collapse,

this investigation did not include any organized effort to interview the injured

or to solicit eyewitness accounts of the collapse. Attempts to obtain information from rescue workers regarding the numbers of dead and injured removed from

the various sections of the walkways were largely unsuccessful, not because of

any unwillingness to cooperate, but primarily because of great difficulty in

recalling and discussing the event.

As will be discussed in chapters 5 and 10, it is possible to partially

reconstruct the mode of collapse based on physical evidence contained in the

walkway debris. However, the number of people and their location on the

walkways at the time of collapse can only be estimated and will never be known

35
with certainty. Efforts to establish such an estimate are described In the

following section.

4.4 ESTIMATE OF WALKWAY OCCUPANCY AT TIME OF COLLAPSE

The most significant and helpful Information regarding walkway occupancy Is

contained on a videotape shot by KMBC TV, channel 9, Kansas City, Mo. The news team was In the Hyatt Regency Hotel to film activities at the dance and was in the process of changing batteries when the collapse occurred. The tape

contains scenes showing portions of the second and third floor walkways a few

minutes before the collapse and scenes Immediately following the collapse.

Three segments of the tape provided by Kt-IBC TV were suitable for analysis.

The scene in figure 4.4 is from the third floor balcony on the north side of

the atrium with the view to the southwest. All of span L7-8 and most of span

L8-9 of the second floor walkway are visible. A portion of the third floor

walkway is visible at the upper right. The time bracket is believed to be

6:50 to 7:00 PM, based on the scheduled break of the band and their absence

from the bandstand which is located directly below the banner on the south wall

of the atrium. There appear to be 11 people on span L7-8 and 16 people on span L8-9 . Of the 11 people on span L7-8, 10 appear to be standing at the

handrail. Of the 16 people on span L8-9 , 12 appear to be standing at the

handrail

.

The scene in figure 4.5, believed to have been taken a few minutes after the

frame shown in figure 4.4, appears to be from the landing of the main stairway

in the southeast corner of the atrium, looking northwest. Part of span L8-9

and all of spans L9-10 and LlO-11 of the second floor walkway are visible.

Eight or nine people are occupying the south portion of span L9-10 along the

handrail and span LlO-11 is not occupied.

The scene in figure 4.6, taken a few frames later, appears to be from the same location as that in figure 4.5, but shows more of span L8-9 . Judging from

their relative heights, the people on span L9-10 have not moved from their

positions shown in figure 4.5. It appears that the count on span L9-10 is now

11, but blurring makes an accurate count difficult.

Figure 4.7 is a close-up of the southernmost span (span M7-8) in the third floor

walkway. The view is from the same location as figure 4.4. Again, it is difficult to make an accurate count, but at least 11 persons can be observed.

Based on information obtained from the KMBC TV videotape, it is likely that the

second floor walkway was occupied by approximately 40 people shortly before the

collapse. The loading is taking place from south to north which is consistent

with the location of the stairway and escalator in the southeast corner of the

atrium. It is also consistent with the location of the bandstand and the dance

contest in progress at the time of collapse.

In view of the relative inaccessibility of the fourth floor walkway, as described In section 3.2 of this report and the number of persons observed on

the more accessible second and third floor walkways, the eyewitness accounts of

36
from 40 to 50 people on the fourth floor walkway would appear to be an exaggeration.

Based on information obtained from the KMBC TV videotape, an occupancy of the

order of 16 to 20 people with 10 to 12 of them standing along the handrail is

believed to be a reasonable upper bound for a second floor walkway span. It is

also believed that three to four people per span, substantially less than

the number observed on the second or third floor spans, is a reasonable upper

bound for a fourth floor walkway span. Given these bounds and the fact that

the bandstand and dance contest could best be observed from the south spans of

the walkways, the distribution indicated in table 4.1 is considered to be a credible upper-bound estimate of walkway occupancy at the time of the collapse.

The combined occupancy of the second and fourth floor walkways as indicated in

this table is 63. However, it must be emphasized that the actual number of

walkway occupants and their distribution never will be known with certainty.

4.5 REMOVAL AND STORAGE OF WALKWAY DEBRIS

The initial removal of debris from the atrium area involved materials such as broken glass, gypsum board and aluminum slats that could not be identified

with a specific walkway span. After marking each span with a code to identify

its origin, the second and fourth floor walkway spans, along with segments of

handrail and hanger rods, were moved by the Crown Center Redevelopment Corporation to a warehouse located at 28th and Warwick Streets, Kansas City, on the

night of July 21-22.

On July 22, two NBS engineers were given access to the Hyatt Regency atrium

and were allowed to take photographs. This visit was limited to making very

general observations. The condition of the fourth floor to ceiling hanger

rods was noted, including the absence of washers at locations 8E and 9E. The

third floor walkway was still in place and small pieces of walkway debris

remained on the atrium floor.

On the night of July 22-23, the third floor walkway was removed by the Crown

Center Redevelopment Corporation and transported to the warehouse for storage

with the spans of the other two walkways. Just prior to its removal, vibration

measurements were obtained by the Crown Center Redevelopment Corporation.

These measurements are referred to later in this report.

Based on subsequent inspections of the third floor walkway spans in the

warehouse, the spans apparently were removed by the use of lifting slings placed

around the W8 x 10 crossbeams at the quarter points of each span. This necessitated four cuts through the deck of each span to expose the ends of the cross

beams. Span M9-10 appears to have been removed first since the stringers had

been cut just short of the stringer to box beam connection on each end of the

span. Removal then progressed each way from span M9-10 with each span retaining

one box beam and remnants of the hanger rods which were cut just above the

level of the handrail.

On August 13, the walkway hanger rods between the third floor and ceiling and

between the fourth floor and ceiling, embedded plates at column line 7, and

37
the bearing seats at column line 11 also were removed from the atrium and sent

to the warehouse for storage.

4.6 SUMMARY

The following summarizes information and findings presented in this chapter:

1. The collapse of the second and fourth floor walkways is believed to have

occurred at approximately 7:05 PM, Friday, July 17, 1981.

2. Analysis of the KMBC TV videotape indicates approximately 40 people on

the second floor walkway shortly before the collapse.

3. A reasonable upper bound for single-span occupancy is believed to be 16 to

20 people.

4. It is concluded that a total of 63 people represents a credible upper-bound

combined occupancy of the second and fourth floor walkways at the time of

collapse

.

38
Table 4.1 Estimated Upper Bound on Walkway Occupancy at Time of Collapse

Span Number

Fourth Floor Walkway

U7-8

U8-9

U9-10

UlO-11

No. of Persons Standing

at East Handrail

No. of Persons Standing

Near Walkway Center Line

Second Floor Walkway

L7-8

L8-9

L9-10

LlO-11

12

12

10

4

39
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40
Figure 4.2 Walkways shortly after the collapse. View is directly to the north

along the axis of the walkways. Span U7-8 is in foreground and

third floor walkway is at upper right. Water is coming from broken

sprinkler pipes. Photo by Greg Smith, reprinted by permission of

The Kansas City Times, © 1981, The Kansas City Star Company.

41
Figure 4.3 Aftermath. View is to the south with span L9-10 resting on span

U9-10 in foreground. Next are spans U8-9 (left) and L8-9 (right).

Span U7-8 is at upper left and span L7-8 is at upper right.

Photo courtesy of Kansas City Police Department.

42
Figure 4.4 Scene from third floor balcony on north side of atrium. View to

the southwest showing spans L7-8 and L8-9 of second floor walkway.

Videotape courtesy of KMBC TV.

Figure 4.5 Scene from landing of main stairway in southeast corner of atrium.

View to northwest shows part of span L8-9 and all of spans L9-10

and LlO-11 of second floor walkway. Videotape courtesy of KMBC TV.

43
Figure 4.6 Same view as figure 4.5, but a few frames later. Videotape

courtesy of KMBC TV.

Figure 4.7 View from third floor balcony on north side of atrium. View to

southwest showing southernmost span (span M7-8) of third floor

walkway. Videotape courtesy of KMBC TV.

44
5. SITE INVESTIGATION

5.1 ' INTRODUCTION

This chapter deals exclusively with measurements and observations made in the course of several visits to the Hyatt Regency atrium and to the warehouse

during the period July 28 to December 17, 1981.

Section 5.2 describes the survey of the Hyatt Regency atrium and the probable

alignment of the walkways prior to collapse. Initial movement of the walkway

spans and their general drift during the collapse are related to observations

of hanger rod distortion and points of impact on the atrium floor.

Section 5.3 describes the layout of the walkway spans in the warehouse and

their general condition. Emphasis is on the box beams of the fourth floor walkway. Distortions of the box beams due to hanger rod pull-through and fractures

along the longitudinal box beam welds are described in detail. Also described

in this section is the procedure used to estimate the original weights of the

walkway spans

.

Section 5.4 describes the criteria used for selection of physical specimens,

procedures followed in marking and removing the specimens, and their cataloging

and handling after arrival at NBS. Section 5.5 describes the procedure used in

obtaining replicas of fracture surfaces and the subsequent removal of selected

fracture surfaces for scanning electron microscopy. Section 5.6 summarizes

the conclusions drawn in this chapter.

5.2 SURVEY OF THE HYATT REGENCY ATRIUM

To establish the actual alignment of the second and fourth floor walkways

prior to collapse, NBS commissioned a survey of the west portion of the Hyatt

Regency atrium [5.1]. This survey was carried out on August 5, 1981, after

removal of the third floor walkway but prior to removal of the fourth floor to

ceiling hanger rods and the embedded plates and bearing seats at column lines

7 and 11, respectively. No measurements were made with respect to the third

floor walkway.

45
For convenience, a benchmark was established on the atrium floor at a point

0.15 ft (45 mm) west and 0.10 ft (30 mm) south of the southwest corner of the

column located at the intersection of column lines 11 and H. For the purposes

of this survey, the benchmark elevation was assumed to be 0.00. To establish

relative horizontal position, three transit lines (T.L.) were located on the

atrium floor; (1) T.L. 1 parallel to column line 11 on the north side of the

atrium. (2) T.L. 2 on a north-south line centered with the first-floor

passageways directly under the walkway at column lines 7 and 11. (3) T.L. 3 parallel to column line 7 on the south side of the atrium (see figure 5.1).

Positions of hanger rods, bearing seats, and points of impact on the atrium

floor are subsequently described as north-south and east-west departures from

the intersection of T.L. 1 and T.L. 2. All distances and elevations were determined to the nearest 0.01 ft (3 mm).

5.2.1 Embedded Plates and Bearing Seats

Locations of the embedded plates and wall openings at column line 7 (south) and

the expansion bearing seats and wall openings at column line 11 (north) are shown in figure 5.1. It is apparent from figure 5.1 that the embedded plates

at column line 7 on the fourth floor level (El. 45. 00 with respect to the datum

of this survey) are approximately 0.15 ft (45 mm) south of the corresponding

plates on the second floor level. As will be discussed subsequently, this

misalignment required a retrofit of the bearing pads on the stringers of the

north span of the fourth floor walkway (span UlO-11). North-south alignment

of the bearing seats at column line 11 is far better, the fourth floor bearing

seats being 0.02 ft (5 mm) north of those at the second floor level.

Each embedded plate at column line 7 is made up of two 1/2 x 6 x 14 in (13 x

152 X 356 mm) plates placed as shown in figure 5.2. Also shown in figure 5.2 are the locations of the fillet welds which made up the shear connection with

the walkway stringers. Based on these dimensions, the east-west alignment of

the walkways with respect to T.L. 2 at column line 7 was as follows:

Fourth floor walkway = 0.01 ft (3 mm) E

Second floor walkway = 0.04 ft (10 mm) E

The expansion bearing seats at column line 11 have already been described in

section 3.3 (see figure 3.13). The gusset plates, in addition to increasing

the stiffness of the bearing seats, also served as lateral guides for the

stringer bearing pads. Assuming these pads were centered between the gusset

plates, the east-west alignment of the walkways with respect to T.L. 2 at

column line 11 was as follows:

Fourth floor walkway = 0.05 ft (15 mm) E

Second floor walkway = 0.05 ft (15 mm) E

5.2.2 Fourth Floor to Ceiling Hanger Rods

In table 5.1 are listed the north-south and east-west coordinates of the fourth

floor to ceiling hanger rods which remained in place during the collapse. As

previously noted, these coordinates are referenced to the point of intersection

46
of T.L. 1 and T.L. 2. Two sets of coordinates are presented in table 5.1 —one

set for the hanger rod positions at a point approximately 18 in (460 mm) below

the plane of the ceiling and one set for the lower end of the hanger rods.

Ideally, the coordinates of the hanger rods at the upper hanger fixture should

have been established, but this was not possible with the ceiling panels in

place. The upper coordinates are based on the assumption that the hanger rods are centered in the 3 1/2 in (89 mm) diameter fire resisting encasement.

The elevations of the lower rod tips and the elevations of the points at which

the hanger rods pass through the ceiling are presented in table 5.2. Again,

these elevations are referenced to Elev. 0.00 (assumed datum) at the first

floor level. The overall length of the fourth floor to ceiling hanger rods was subsequently determined to be 15'-11 1/8" (4.85 m).

5.2.3 Impact Points on Atrium Floor

When the second floor walkway impacted the atrium floor, the second to fourth

floor hanger rods and the second floor walkway stringers punched through or otherwise damaged the floor tiles at column lines 8, 9 and 10. At column

lines 9 and 10 these hanger rod impact points were clearly identifiable. The

corresponding impact points at column line 8 are believed to have been caused

by the stringers in the south span of the second-floor walkway (L7-8). At

column line 10 there are two sets of hanger rod impact points (see figure 5.3),

indicating rebound of the walkway and possibly the effect of the fourth floor

walkway Impacting. Coordinates of major impact points are listed in table 5.3.

5.2.4 Alignment of Walkways Prior to Collapse

With the survey data just presented and with measurements obtained from the

walkway debris (see section 5.3.5), it is possible to establish the general

alignment of the walkways prior to collapse. In doing this it is convenient to use the second floor walkway as a base line because of its apparent good fit

with the embedded plates and bearing seats and because it is directly related

to the observed impact points on the atrium floor. It is assumed here that

both walkways had no horizontal curvature and that the hanger rod holes were

located in the box beams according to the dimensions indicated in figure 3.10.

Alignment of the upper hanger rods at their intersection with the ceiling and

the alignment of the fourth floor walkway, all relative to the second floor

walkway, are presented in table 5.4. Other than the problem with the embedded

plates on the fourth floor at column line 7 as previously described, the horizontal misalignment of the walkway system was less than 0.10 ft (30 mm). It

should be noted that the upper ends of the fourth floor to ceiling hanger rods were more closely aligned with the fourth floor walkway than with the second.

The survey data and measurements from the walkway debris (table 5.8) also make

it possible to check the positioning of the walkway stringers on the beam seats

at column line 11 prior to collapse. Typically, the 4 in x 4 in (102 x 102 mm)

slide bearing plates are positioned 3/8 in (10 mm) from the south edge of the

bearing seats. Based on measurements of actual member lengths as described in

section 5.3.5 and accounting for the fact that retrofit of the fourth floor

47
bearing pads extended the stringers by 2 1/4 in (57 mm), the calculated overlap

of the slide bearing plates is 4.2 in (107 mm) and 3.7 in (94 mm) at the second

and fourth levels, respectively. Scratch marks observed on the slide bearing

plates indicate that the actual overlap was very close to 4 in (102 mm) for

both walkways.

With regard to vertical alignment of the fourth floor walkway, measurements

taken on the embedded plates and bearing seats in the atrium and on the debris

subsequent to its removal and storage indicate the top flanges of the W16 x 26

stringers were positioned 0.44 to 0.48 ft (135 to 145 mm) below the level of

the finished floor (Elev. 45.00). Assuming the structural steel was properly

leveled during erection, the minimum protrusion of the fourth floor to ceiling

hanger rods through the bottom face of the box beams would have been 0.14 ft

(45 mm) at hanger rod lOE. Subsequent measurements indicated a maximum combined thickness of nut and washer equal to 0.115 ft (35 mm). Therefore, with

the fourth floor properly leveled, there would have been adequate clearance to

install a nut and washer on each fourth floor to ceiling hanger rod.

5.2.5 Other Observations Relevant to the Investigation

In table 5.5 the locations of the impact points are listed in terms of

departures from the lower hanger rod positions at the second floor walkway

prior to collapse. All four spans in the second floor walkway experienced a consistent drift to the west and south during the collapse. The westward drift

suggests initial loss of support along the east side of the walkway system

while the fixity at column line 7 would be expected to promote a southward

drift of both walkways. There was no visible sign of the walkway spans con- tacting the north wall of the atrium, suggesting that they continued to move

to the south after the stringer bearing pads cleared the bearing seats. The

locations of the impact points in the vicinity of column line 8 strongly suggest

that span L7-8 rotated vertically around its supports at column line 7 and

that the north end of the span drifted to the west before the bottom flanges

of the stringers impacted the atrium floor.

There was extensive damage to the granite paneling on the south wall of the

atrium at the second floor level. It is likely that most of this damage

resulted from impact of the south span of the fourth floor walkway which came

to rest with its stringers positioned on the second floor edge beam and slightly

west of the original alignment.

As has been noted in section 4.5, washers were missing from the lower ends

of the fourth floor to ceiling hanger rods at locations SUE and 9UE. It was

also observed during the course of the atrium survey that the washer at location 9UW had fractured along two radial lines, resulting in the loss of a short

segment. This will be discussed further in section 5.3.2.

One final observation of importance is the pronounced bend to the west at the

bottom end of hanger rod lOUW and a lesser bend at the bottom of hanger rod

8UW as can be seen in figure 5.4. Not apparent in figure 5.4 is a slight bend

at the bottom of hanger rod 9UW which was detected after the hanger rods had

been removed to the warehouse. The absence of appreciable distortions in hanger

48
rods SUE, 9UE, and lOUE strongly suggests that failure initiated along the east

line of hanger rods.

5 . 3 OBSERVATIONS OF WALKWAY DEBRIS

On July 29, 1981, the NBS investigative team was allowed its first access to

the walkway debris. Under the terms of a court order dated July 24, 1981,

issued by the Circuit Court of Jackson County, Mo., and covering protective

custody of the debris, the taking of impressions, marking, or otherwise disturbing any of the debris was expressly forbidden. Until permission was eventually

obtained to weigh the walkway spans and to remove certain physical specimens,

on-site activities of the investigative team were limited to visual observations,

taking of photographs, and dimensional measurements.

The layout, orientation and identification of the walkway spans in the warehouse

at 28th and Warwick Streets in Kansas City are indicated on figure 5.5. Also

indicated in this figure are the locations of the box beams which remained

attached to one or the other adjacent spans during the collapse. A general

view of the fourth floor walkway spans is shown in figure 5.6.

5.3.1 General

It was evident that the moving and storage of the debris had been carried out

with considerable planning and great care. The NBS team found no reason to

believe that any of the spans suffered additional damage or were otherwise

altered during the move from the hotel. The NBS team also found all spans to

be reasonably accessible and properly Identified.

In general, the rescue operation at the hotel caused only superficial damage to

the spans as can be seen from photographs taken immediately after the collapse.

This included loss of some glass panels in the handrail assembly which was necessary for positioning of the lifting slings, penetration of the concrete

deck by jackhammering , and various cuts in the second to fourth floor hanger

rods. The one exception was span L8-9 where extensive portions of the deck had

been removed and the stringers had been flame cut near the north end of the

span. It also appeared that the bolts in the clip angle to stringer connections

at the south end of span U7-8 had been removed and reinstalled.

5.3.2 Hanger Rod Pull-Through

The fourth floor to ceiling hanger rods had pulled through both the bottom and

top flanges of each box beam in the fourth floor walkway (figure 5.7). The mode

of failure involved the upward rotation of the bottom flanges of the MC8 x 8.5

shapes and extensive localized yielding in each web directly above the fillet

line (figure 5.8). This was accompanied by fracture of the lower longitudinal

weld back to the inner hanger rod hole and, in some cases, well beyond the inner

hole. Pull-through in the upper flange generally resulted in less distortion

of the flanges and the localized yielding in the webs much closer to the fillet

line. Similar plastic deformation of the box beam ends was also observed at

several locations in the third floor walkway (figure 5.9).

49
Hanger rod pull-through was marked by substantial scraping and gouging damage

to the flange surfaces adjacent to the bottom outer hanger rod holes in the

fourth floor box beams. Steps in the abraded region indicated that slippage of

the washer/nut combination was periodically arrested during the failure process.

Figure 5.10 illustrates this "slip-stick" type of failure at location 9UE. The

absence of impact marks on the inner flange surfaces around the upper hole at

location 8UW indicates the upper longitudinal weld seam was widely separated at

the time of pull-through. The mangled condition of the box beam at location

lOUW is believed to have been caused by the 1/4 x 6 in (6 x 152 mm) edging

plate on span UlO-11 running past the box beam on impact (figures 5.11 and

5.12). A similar effect was observed at location 8UW.

Washers were missing from the lower ends of the fourth floor to ceiling hanger

rods at locations SUE and 9UE. However, part of the washer at location 9UW was

also missing (figure 5.13), suggesting that the missing washers fractured and

were lost during the collapse. Figure 5.10 provides clear evidence that a washer

was originally installed at location 9UE. There was also visual evidence which

indicated that a washer was present at location SUE. This was later confirmed by

detailed examination of the washer/nut contact area as described in chapter 8.

There is a symmetry to the deformation and distortion of the box beam ends in

the fourth floor walkway. As illustrated by figure 5.14, there is no evidence

of rotation in the north-south direction of box beam 9U during the collapse.

Conversely, the beam ends at SUE and lOUE display rotational distortions having

a mirror symmetry around 9UE. Damage to the beam ends at locations 8UW and lOUW

was much more severe as has been pointed out. It is possible that rotation of

the fourth floor walkway about the west line of hanger rods, as evidenced by the

previously described hanger rod distortions and impact points, contributed to

this damage. Again, the distortions clearly indicate a symmetry about box

beam 9U. These combined rotations strongly suggest an initial failure at box

beam-hanger rod connection 9UE.

5.3.3 Box Beam Longitudinal Welds

Box beam longitudinal welds had been made in a single pass from the exterior by

either a manual or semi-automatic process as evidenced by the rough contour of

the weld reinforcement (weld cap) . The reinforcements had been removed from the

top surface of all box beams as called for on the shop drawings. Reinforcement

had also been removed from the bottom surface of all box beams for a distance

of approximately 19 in (4S0 mm) from the beam ends except for box beam SM in

the third floor walkway where the weld reinforcement had been entirely removed.

Presumably the reinforcement was removed to provide a flat surface for arc spot

welding of the formed steel deck on the top surface of the box beam and for

hanger rod washer contact on the bottom surface.

In addition to the exterior welds just described, interior tack welds were observed along the longitudinal seam in the region between the outermost hanger

rod hole and the end of the beam. At most locations these welds were placed

along the seam in both the top and bottom flange. As these welds are not called

out on the fabricator's drawings, it is assumed that their purpose was to facilitate alignment of the MC8 x 8.5 shapes preparatory to placing the exterior

50
longitudinal welds. As will be shown in chapter 6, these interior tack welds

have a significant effect on resistance to hanger rod pull-through. A summary

of the length and position of these welds is presented in table 5.6. Because

of gross distortions of the flange material caused by hanger rod pull-through,

true longitudinal dimensions can be expected to differ from the listed values

by as much as + 0.1 in (2.5 mm). In the case of location 8MW, saw marks are visible at the outer ends of the welds, indicating the box beam was trimmed to

final length after placement of the welds. In other instances it was evident

that no final trimming was performed.

In the case of the fourth floor box beams where the longitudinal welds had been

fractured during hanger rod pull-through, it was possible to make a visual

assessment of weld quantity and quality. There was no evidence of joint preparation and no evidence to indicate that any root opening (separation of members

at root of joint) had been provided. The faying surfaces were observed to be

continuously curved over the entire depth of the flange toes. In general,

joint penetration did not exceed the distance from the outside face of the

flange to the point of tangency of the faying surfaces. The resulting weld

could, in most cases, best be described as a f lare-V-groove weld as defined in

reference 5.2. However, as noted in section 3.3, the welding symbol shown on

the shop drawings called for a prequalified partial joint penetration groove

weld. This is further discussed in section 10.6.

In general, observed variability in penetration along a given weld (figure 5.15) was not considered excessive for a manual or semi-automatic weld, given the

geometry of the flange toes of the MC8 x 8.5 shapes. More variability in depth

of penetration was noted from weld to weld. For example, penetration at the

top longitudinal weld between the hanger rod holes at location 9UW appeared to

be about 1.5 mm as compared to a depth of penetration of about 2.8 mm for the

bottom longitudinal weld at that same location. The longitudinal welds appeared

to contain a substantial amount of porosity in the shape of elongated pores

oriented in the through-wall direction and of dimensions approximating that of

the weld penetration (figure 5.16). Results of field measurements of top and

bottom longitudinal and interior tack weld penetration at the ends of the fourth

floor box beams are presented in table 5.7. More detailed descriptions of the

box beam longitudinal welds are presented in chapters 7 and 8 of this report.

Weld fractures occurred predominantly along the center line of the welds as indicated in figure 5.16. In several cases a shear lip caused by fracture

transition from the weld center line to the fusion line was observed near the

inner hanger rod hole. A marked exception to this failure mode was noted at

the upper longitudinal welds between the hanger rod holes at locations SUE

and 9UE. Here the weld failure appeared to have occurred predominantly at the

fusion line (figure 5.17). In some cases the flange tips were curled up at

the weld line, suggesting plastic weld behavior. In other cases the flanges were perfectly straight, suggesting less ductile behavior of the longitudinal

welds

.

What appeared to be a crack was noted in the lower interior tack weld at

location 8MW (figure 5.18). Similarly, an apparent small crack was noted at

the lower extremity of 8ME (figure 5.18), apparently having developed in a

51
region of metal flashing left over from the beam cutting operation. There was

no interior tack weld at this location.

5.3.4 Box Beam to Clip Angle Fillet Welds

The clip angles, which served as shear connections between the W16 x 26

stringers and the box beams, were welded to the box beams in accordance with

the detail shown in figure 3.10. Clip angles had separated from one side or the other of each box beam by fracture through the weld metal or through the

heat-affected zone. In some cases (SUE, 9UE, lOUW and 9LE) cracks were observed

in the bottom fillet welds of the clip angles that remained attached (see

figure 5.19). Evidence of a pull-out or tensile type failure was observed at

the top fillet weld on 11 out of the 13 box beam to clip angle weld failures

in the second and fourth floor walkways. This suggests that the fractures were,

in most cases, initiated by positive bending moment at the ends of the spans

causing shearing failure of the bottom fillet weld, followed by a tension

failure (or mixed shear-tension failure) progressing upward and across the top

fillet weld. Figures 5.20 and 5.21 illustrate this failure mode for location

9UE. Exceptions were noted at locations 9UW and 9LW where failure of the

fillet welds on the box beam web appeared to be entirely of the shear type

(figure 5.22). No cracks were in evidence in the bottom fillet welds of the

clip angles that remained attached at those locations.

Figure 5.23 clearly shows a tear in the upper end of the north clip angle at

location SUE, suggesting that the connection experienced a negative bending

moment at some time during the collapse. However, the south clip angle at this same location appears to have separated from the box beam under the action of

positive bending as is evidenced by the shape of the tear in the box beam web

shown in figure 5.24. Based on these observations, it is likely that the upper

spans at column line S first experienced severe negative bending, followed by

positive bending prior to final separation of the clip angles from the south

web of the box beam. It is also likely that the concrete deck developed a pronounced transverse tension crack, accompanied by failure of the wire reinforcement, during the negative moment phase. With moment reversal, the crack

then closed and span rotation was centered about a point near mid-depth of the

deck.

There is no evidence of moment reversal in the fourth floor walkway at column

line 9, the connection apparently failing under continuously increasing positive

moment. The situation at column line 10 is similar to that at column line S,

but the indicators of moment reversal are not as pronounced in the clip angle

distortions and fillet weld failures.

Visual examination of the intact box beam to clip angle fillet welds and the

fracture surfaces of separated welds indicated welds of good quality. Leg

lengths of the fillet welds were typically 5-8 mm.

5.3.5 Other Observations Relevant to the Investigation

Other welds having structural significance and not previously described include

the clip angle to embedded plate welds at the south ends of spans U7-8 and

52
L7-8, and the stringer bearing pad welds at the north ends of spans UlO-11 and

LlO-11. Attachment of the clip angles to the embedded plates at column line 7 was by field welding. Inspection of the embedded plates after their removal

from the atrium showed lack of penetration into the clip angles at the bottom

of connection 7UE and at the top of 7UW as can be seen from figures 5.25 and

5.26. However, distortion of the clip angles, which remained attached to the

W16 X 26 stringers during the collapse, clearly indicates large vertical rotations of the stringers before failure of the clip angle fillet welds. Leg

lengths of these fillet welds were typically 5-7 mm except for location 8UE

where leg lengths of 6-9 mm were noted.

As indicated in section 5.2.1, misalignment of the embedded plates at column

line 7, fourth floor level, required a retrofit of the stringer bearing pads at

the north end of the fourth floor walkway. This was accomplished by cutting out

the original bearing pad and welding on a pair of 6 x 3 1/2 x 3/8 angles with

the short legs machined down to clear the gusset plates in the slide bearing

seats. This effectively extended the fourth floor stringers by 2 1/4 in

(57 mm). The modification at location IIUW is shown in figure 5.27.

Certain dimensional checks were made to confirm the sizes of structural shapes

shown on the fabricator's shop drawings. In addition, length measurements of

the stringers and stringer gaps at the box beams were made to check the longitudinal alignment of the walkways prior to collapse as discussed in section

5.2.4. These measurements are summarized in table 5.8.

A final observation is that the stringer to clip angle bolted connections

performed very well. The only cases where bolt slip was evident occurred when

the connection was badly distorted by direct impact.

5.3.6 Determination of Span Weights

Preliminary estimates of span weights prior to collapse Indicated that the

weight of the concrete deck could account for as much as 50 percent of the total

weight, thus making such estimates particularly sensitive to measurements of

deck thickness. The variability in thickness of concrete and topping material,

which in some cases could only be measured at the ends of the spans, was con- sidered too high to place any confidence in this approach, and permission was therefore requested of Crown Center Redevelopment Corporation by NBS to weigh

selected walkway spans. This operation was carried out on September 3, 1981.

The weighing procedure consisted of temporarily supporting an individual span

on four electronic load cells whose output signals are proportional to applied

force. Transfer of support was accomplished by raising the span from its

supporting timber cribs with hydraulic jacks and inserting the load cells

between the bottom flange of the W16 x 26 stringers and the floor. Typically,

the spans were raised 1/8 to 1/4 in (3 to 6 mm) during this operation. To

prevent damage to the stringer flanges, the jacks and load cells were topped

with jacking pads consisting of a 1/2 x 5 1/2 x 5 1/2 in (13 x 140 x 140 mm)

steel plate faced with a 1/4 in (6 mm) neoprene sheet. Since some stringers were severely distorted, hardwood wedges were used as required to ensure the

level positioning of the jacking pads.

53
Unless otherwise indicated, the load cells were positioned directly under

the center of the bolt groups for those cross beams located a nominal distance

of 7 '-3 3/4" (2.229 m) north and south of the raidspan cross beam (see definition

sketch, table 5.9). Tolerances on the horizontal positioning of the load cells were +1/4 in (6 mm) in the north-south direction and + 1/8 in (3 mm) in the

east-west direction. Prior to recording load cell outputs, the timber cribs

were checked for freedom of movement to ensure complete load transfer.

Results of the load cell measurements are summarized in table 5.9. Regression

analysis of the load cell calibration data indicates a standard error of estimate of 20 lbs (89 N) for individual load cell readouts over the range of

weights encountered.

5.4 REMOVAL OF SPECIMENS

Shortly after the start of the investigation, comprehensive structural and

materials testing programs were undertaken as described in chapters 6 and 7 of

this report. Tests were carried out on structural assemblies and materials

believed to be faithful representations of those assemblies and materials used

in the actual walkway construction. To confirm this, a formal request for

permission to remove and test certain specimens from the walkway debris was submitted by NBS to the Crown Center Redevelopment Corporation on September 18,

1981. This was followed by a request to the Liaison Committee of Plaintiffs'

Counsel and the Liaison Committee of Defendants' Counsel on October 27, 1981.

Criteria for specimen selection and details of the request are described in

the following section.

5.4.1 Criteria for Specimen Selection

Criteria for choice of specimens to be removed from the walkway debris were as follows

:

1. No truly unique specimens to be removed from the debris.

2. Specimen location and method of removal to be chosen so as to minimize

disturbance of the walkway debris.

3. Amount of material to be removed from a given component or assembly

to be limited so that, if necessary, similar specimens can be obtained

and independently tested by others.

4. Specimens to be representative of materials and fabrication techniques

used in the construction of the walkways.

Based on these criteria, a detailed list of specimens was developed, including

the type of test or examination and purpose, as shown in table 5.10.

5.4.2 Specimen Removal

Permission to remove specimens from the walkway debris was granted in the form

of a modified order issued by both the Jackson County Circuit Court and the

54
United States District Court for the Western District of Missouri on October 28,

1981. Actual removal of specimens was carried out over the period November 3-5,

1981. The first step in removing structural steel specimens was to mark the

specimens in place with cut lines and specimen identification numbers. After

photographing, the specimens were then removed and engraved by representatives

of NBS and the Crown Center Redevelopment Corporation. The specimens were again photographed to show the specimen numbers and engraving. Although flame

cutting would have been faster and more convenient, an abrasive saw was used

to minimize heat effects near the cut lines. With the exception of the box

beam at location 9M, all structural steel specimens were easily accessible.

To remove box beam 9M, the remnants of the third floor to ceiling hanger rods were first removed, followed by the bolts in each of the four clip angle to

stringer connections. Concrete around the shear studs at each end of the box

beams was then removed by impact hammer, followed by separation of the arc spot

welds attaching the formed steel deck to the top flange of the box beam.

Hanger rod nuts and washers were retained with the box beam for subsequent

use in structural tests. Photographs of specimens 1, 2 and 3 before and after

removal are shown in figures 5.28 - 5.30. Specimens 5A and 5B are shown in

figure 5.31.

Cores were removed from the concrete decks of 6 of the 12 walkway spans using a

2 in (51 mm) I.D. diamond coring bit as shown in figure 5.32. To obtain homogeneous cores with flat ends for subsequent density and compression tests, it was necessary to center the bit over the flat portion of the formed steel deck

and, at the same time, avoid the welded wire reinforcement. Where possible,

cores were removed at a point directly over a cross beam and at the quarterand mid-points in the cross beam spacing to obtain some measure of the variation

in slab thickness. Considerable difficulty was encountered in obtaining com- plete cores from the second and fourth floor walkways because of fractures in

the concrete decks. A total of 2A cores were removed, but only 19 of these were considered to be of sufficient quality for testing. Each core was assigned

a specimen number on removal and the same specimen number was marked on the

deck adjacent to the core hole. Table 5.11 lists the cores and their locations.

The cores selected for testing and actually removed from the warehouse are shown in figure 5.33.

At the same time the concrete cores were removed, measurements of topping

thickness for each span in the second and fourth floor walkway were obtained.

For those spans having most of the carpeting and footlights in place, measurements were extremely limited. Topping removed with the cores was included in

the measurements to obtain the average thicknesses listed in table 5.12.

5.4.3 Specimen Cataloging and Preparation for Shipment

Upon removal from the debris, each specimen was measured and all identifying

marks were noted. All specimens were then wrapped in clear plastic sheet to

await final inspection by designated representatives of the Liaison Committee

of the Plaintiffs' Counsel and the Liaison Committee of the Defendants' Counsel.

Prior to this inspection a listing of all specimens, complete with descriptions

55
of origin and physical features, was jointly prepared by representatives of

NBS and the Crown Center Redevelopment Corporation.

On November 5, 1981, after representatives of the Liaison Committees had

inspected and certified the specimens, custody was transferred to NBS and all

specimens were then wrapped for shipment and wax seals were affixed. NBS

personnel transported the specimens by van to the NBS laboratories at

Gaithersburg, Maryland, arriving on November 6.

Because of problems encountered with the replication of fracture surfaces (see

section 5.5), four fracture surfaces from the bottom flanges of the fourth floor

box beams were removed on December 16-17, 1981. Five additional hanger rod nuts

and washers were also removed at this time for the purpose of obtaining hardness

measurements. A description of all specimens removed from the walkway debris

and their corresponding NBS identification numbers are presented in table 5.13.

5.4.4 Specimen Storage and Handling

Upon arrival at NBS, the specimens were stored in a secure area to await

inspection of the seals by NBS personnel and photographic documentation. After

this was completed on November 9, 1981, a detailed physical description of each

specimen was prepared.

Because of the many tests and operations which had to be carried out

simultaneously, a chain of possession was maintained for each specimen. In the case of box beam 9M (specimen No. 3), physical descriptions were prepared and

chains of possession were maintained for the five subspecimens cut from the beam

for use in structural tests.

The procedure followed in subdividing a specimen was to mark clearly the cut

lines and the subspecimen identification numbers on the specimen. The specimen

was then photographed and the subspecimens were removed and machined to final

shape. Each subspecimen identification number was unique and was coded to provide information on both the origin and orientation of the subspecimen. For

example, the number "2-LNl" designates a tensile coupon removed from specimen

no. 2. It is oriented in the longitudinal direction on the north face of the

beam and is the first in a series of such specimens.

5.5 REPLICATION OF WALKWAY WELD FRACTURE SURFACES

In addition to specimens of construction materials, the request of October 27,

1981, also addressed the field replication of certain fracture and washer contact surfaces for the purpose of f ractographic analysis. This work was carried

out over the period November 2-4, 1981.

The replication procedure involved the application of cellulose acetate (C.A.)

tape to the fracture surface. A 1 in (25.4 mm) piece of tape with a thickness of 0.001 in (0.03 mm) was softened by applying several drops of acetone to one surface. One to two drops of acetone were applied to the fracture surface

and the tape was then worked into place with light finger pressure. Entrapped

air bubbles were worked out by smoothing pressure, allowing the softened tape to

56
conform with the fracture surface. After drying for approximately 1 hour, the

replica was removed by carefully lifting the edges until free.

The first replica removed from each fracture surface usually contained most of

the surface corrosion products and dust. In each case this replica was retained

as the archival sample. Additional replicas were stripped until no significant

amount of rust was observed. Each successive replica was edge-notched to

indicate the side oriented with the outside surface of the box beam.

In some cases a plastic tape (butyrate) with 0.010 in (0.25 mm) thickness and

a longer drying time was applied. The details of the fracture topography

replicated with butyrate and C.A. were essentially identical. After the final

replication, each fracture and washer contact surface replicated was given two

spray coats of protective acrylic (Krylon).* Table 5.14 lists the locations of

fracture and washer contact surfaces and the number of replications of each

surface.

Subsequent examination of these replicas by scanning electron microscope (SEM)

revealed that attempts to replicate the lower fracture surfaces at the ends of

the fourth floor box beams were only partially successful. This prompted an additional request to the Liaison Committees by NBS on November 30, 1981, for

permission to remove portions of certain fracture surfaces for direct scanning

electron microscopy.

On December 16-17, 1981, four fracture surfaces from the bottom flanges of the

fourth floor box beams were removed and transported to NBS for SEM examination.

The fracture surfaces, which are listed in table 5.13, were removed by cutting

longitudinally along the fillet-web tangent line and then directly across the

flange to a point midway between the two hanger rod holes. Details of these

specimens are described in chapter 8. Transfer of custody involved the same inspection and certification procedures described in section 5.4.3.

5.6 SUMMARY

Based on measurements and observations made during the site investigation,

the following conclusions were drawn:

1. The walkways were properly aligned on their bearing seats prior to

the collapse.

2. There was sufficient room to install a nut and washer on the lower

end of each fourth floor to ceiling hanger rod with the fourth floor

walkway properly leveled.

Certain commercial equipment, instruments or materials are identified in this

report in order to adequately specify the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau

of Standards, nor does it imply that the materials or equipment identified

are necessarily the best available for the purpose.

57
Observed distortions in the west line of fourth floor to ceiling

hanger rods and the absence of similar distortions in the east line of

fourth floor to ceiling hanger rods suggest that the collapse

initiated along the east line of hanger rods.

Impact points on the atrium floor indicate a general drift of the

walkway spans to the south and west during the collapse. This drift

was approximately 1.0 ft (300 mm) south and 0.9 ft (270 mm) west.

With one exception, damage done to the walkway spans during the rescue operation was superficial.

All of the fourth floor to ceiling hanger rods pulled through both the

lower and upper flanges of the fourth floor box beams.

Evidence indicates that all hanger rod washers were installed.

There is a mirror symmetry to the deformation and distortion of the

box beam ends with respect to box beam 9U in the fourth floor walkway.

Observed distortions strongly suggest an initial failure at box beamhanger rod connection 9UE.

The actual box beam longitudinal welds can best be described as

f lare-V-groove welds.

Variability in penetration for a given box beam longitudinal weld was

not excessive, but penetration varied considerably from weld to weld.

Interior tack welds were highly variable in length, location, and

penetration. In some cases there was no Interior tack weld.

In general, the clip angle to box beam fillet welds were of good

quality.

Observed clip angle failure modes strongly suggest moment reversal

at column line 8 and possibly column line 10. No signs of moment

reversal at column line 9 were observed.

The shear connections at column line 7 experienced large rotational

distortion prior to failure of the fillet welds.

58
Table 5.1 Coordinates of Fourth Floor to Ceiling Hanger Rods

Hanger Rod Upper End* Lower End

8E 4.65' E 4.62' E

83.90' S 83.81' S

BW 4.40' W 4.48' W

83.84' S 83.81' S

9E 4.64' E 4.68' E 54.64' S 54.68' S

9W 4.42' W 4.47' W

54.60' S 54.63' S

lOE 4.64' E 4.73' E

25.39' S 25.38' S

low 4.42' W 4.46' W

25.37' S 25.37' S

* Approximately 18 in (460 mm) below the plane of the ceiling.

1 in = 25.4 mm

Table 5.2 Elevations of Fourth Floor to Ceiling Hanger Rods

(Referenced to Elev. 0.00 assumed datum at first

floor level)

Hanger Rod Intersection with Ceiling Lower End

8E 52.48 43.59

8W 52.47 43.55

9E 52.44 43.70

9W 52.45 43.55

lOE 52.44 43.71

low 52.43 43.53

59
Table 5.3 Coordinates of Major Impact Points on Atrium Floor

Rod Impact Point Coordinates

8E* 3.36' E

86.76' S

8W* 5.10' W

86.84' S

9E 3.36' E

55.32' S

9W 5.01' W

55.60' S

lOE (primary impact) 3.41' E

26.29' S

(secondary impact) 3.17' E

26.30 S

low 5.01' W

26.24' S

* Observed tile damage in vicinity of column line 8 was probably due to

impact of north end of W16 x 26 stringers in span L7-8.

1 in = 25.4 mm
Table 5.4 Alignment of Upper Hanger Rods and FourtVi Floor

Walkway Relative to Second Floor Walkway

Upper Hanger Rods at Ceiling

8E 0.09' E

0.15' S

8W 0.07' E

0.09' S

9E 0.08' E

0.14' S

9W 0.05' E

0.10' S

lOE 0.08' E

0.14' S

low 0.01' E

0.13' S

Fourth Floor Walkway

E at South end of Walkway

(Col. Line 7)

t at Col. Line 8

t at Col. Line 9

E at Col. Line 10

t at North end of Stringers

(Col. Line 11)

1 in = 25.4 mm

61
Table 5.5 Locations of Impact Points on Atrium Floor Relative to

Original Hanger Rod Positions at Second Floor Walkway

Hanger Rod Impact Point Relative Location

0 .87

w

3.01' s

8W* 0.96' w

3.09' s

9E 0.87

w

0.82' s

y w 0 87 ' w

1.10' s

lOE (Primary impact) 0.82' w

1.05' s

(Secondary impact) 1.06' w

1.06' s

low 0.87' w

1.00' s

* Observed tile damage in vicinity of column line 8 was probably due to

impact of north end of W16 x 26 stringers in span L7-8.

1 in = 25.4 mm
Table 5.6 Length and Position of Box Beam Interior Tack Welds

Bottom Flange Top Flange

Number

Net Dimension Net Dimension

Length A B Length A B (inches

)

(inches

)

( inches

)

(inches

)

SUE 1.42 0 0 No weld

8UW 1 .38 0 0 No weld

9UE 0.98 0 0.47 0.71 0.28 0.31

yUvi 1 .06 0.20 0 1.18 0 0

lOUE 1.18 0.20 0 0.87 0.28 0.24

lOUW 1 .30 0 0 1.30 0 0

8LE No weld 1 .97 0 —

8LW 1 .73 0 3.43 2.24 0 2.90

9LE 1 .77 0 — 1 .57 0 —

^ Li W 2.56 0.20 — 1 .50 0.08 —

lOLE 1.54 0.08 — 1.73 0 —

lOLW 1.69 0.12 — 1 .57 0.12 —

8ME No weld 1.10 0.35

8MW 0.39 0 0.75 0.12

9ME 1.26 0 0 No weld

9MW 0.75 0 0.67 No weld

lOME 1 .06 0.08 No weld

lOMW 1.14 0.08 No weld

1 in = 25.4 mm

i BOX BEAM

PLAN VIEW

(Definition Sketch]

63
Table 5.7 Field Measurements of Weld Penetration at Ends

of Fourth Floor Walkway Box Beams

Weld Penetration (mm)

Connection

Number

Bottom Flange Top Flange

Exterior Interior Exterior Interior

8UE

8UW

9UE

9UW

lOUE

lOUW

1.0 - 1.5

Complete Per

1.5 - 2.0

1.5 - 2.0(b)

2.0

1.5

0 - 2.0

letration(^)

2.5

1.5 - 2.5(b)

2.0 - 2.5

2.0 - 3.0

1.0 - 1.5

2.0

1.5

1.0 - 2.0

1.5 - 2.0

Complete Pf

no weld

no weld

2.0

1.0 - 2.0

2.0 - 3.0

jnetration(^)

Note: (^) Exterior and the interior welds merged, leading to

complete penetration.

(b) Edge of hole badly damaged by nut and washer.

1 in = 25.4 mm

64
Table 5.8 Measured Lengths of Stringers and Stringer Connections

Distance From End of

Connection Span Stringer to End of

Number Number Adjoining Stringer

( inches)

Length of Stringer

(feet)

East West East West

U7

U7-8

0.43 0.35

29.38

U8

U8-9

4.65 4.61

28.85 28.85

U9

U9-10

4.72 4.65

28.84 28.84

UIO * UlO-11

UU** 2.25

4.57

2.25

29.39 29.38

L7 0.51 0.51

L7-8 * 29.33

LB 4.65 4.76

L8-9 * *

L9 4.84 4.76

L9-10 28.88 28.85

LIO 4.72 4.69

LlO-11 29.40 29.38

Notes:

* Could not be accurately measured due to severe damage. Nominal values used

in calculations.

** Represents extension of stringer bearing pad.

1 in = 25.4 mm

65
Table 5.9 Measured Weights of Damaged Spans

Span

1a7o W 1 cr h t" tr JL ^ 1 1 L in lbs at Indicated Load Cell

Total

No.

No.l No. 2 No. 3 No. 4 (lbs)

U7-8(l) 4,450 3,410 3,690 4,210 15,760

U8-9 4,610 3,250 4,030 4,060 15,950

U9-10 4,030 3,370 4,160 3,040 14,600

UlO-11 3,570 3,610 4,230 2,590 14,000

L7-8 4,760 3,030 4,060 3,280 15,130

L8-9(2)

L9-10 3,320 3,790 3,500 3,550 14,160

LlO-11 3,420 3,230 3,610 2,910 13,170

M8-9(3) 5,020 4,190 5,890 3,720 18,820

Notes

:

Load cells 2 and 3 were positioned 12 in (305 ram) north of

normal positions due to inaccessibility.

(2) Span L8-9 not weighed because of excessive damage.

(3) Load cells 1 and 4 were positioned 5'-6'" (1.68 m) south and

load cells 2 and 3 were positioned 7'-0 1/8" (2.14 m) north of

normal positions due to inaccessibility.

1 lb = 0.4536 kg

E

1 © ®

a>

l_i

Cross Beam Cross Beam Cross Beam North

(As-built configuration)

© ® c:E

PLAN VIEW

(Definition Sketch)

66
Table 5.10 Request for Physical Specimens from Hyatt Regency Walkway Debris

MATERIALS TESTING

Specimen Description Type of Examination Purpose

1. Segment of box beam 9U. A 24 in longitudinal segment beginning at a point 30 in from the west end of 9U and extending inward.

2. Segment of box beam 8L. A 36 in longitudinal segment beginning at the west end of 8L.

Tensile tests per ASTM E8

.

a) Transverse weld tensile tests.

b) Metallography and hardness of weld, heat-affected zone, parent

metal

.

c) Chemical analysis, weld metal,

and parent metal.

Conformance to mechanical property

specifications

.

Weldment mechanical property

variability

.

Assessment of microstructure phases

contributing to low weldment

toughness

.

Conformance to composition

specif icat ion

.

3. Box beam 9M. The full box beam

will be obtained by removing the bolts

connecting the strif^ger to the clip

angles and removal of limited concrete

to allow separation of the box beam

from the deck.

Destructive structural load tests. Determine resistance of box beam to hanger rod pull-through load.

4. 3 in diameter cores of concrete

deck. Up to 30 cores taken at random locations on walkway decks.

Length of core and unit weight

determination.

Establish average weight of concrete

deck per unit of area.

5. 36 in lengths of 1 1/4 in (j> hanger

rod. One specimen to be removed from

straight segment of 4th floor to 2nd floor hanger rod, and one specimen to be removed from ceiling to 4th floor hanger rod.

Tensile tests to determine yield

strength, tensile strength,

elongation, and reduction of area

in accordance with ASTM E8.

To clearly establish grade and yield

strength of steel used in hanger rod assemblies

.

FRACTOGRAPHY

1. Access for field replication of:

a) lower fracture surfaces of box beams 8U, 9U, and lOU

b) fracture surface of clip angle

at box beam 9UE

c) tack welds at level M.

2. A 2 in section of the upper fracture at connection 8UW (north

surface) centered at a point 9 in from

the west end. Depth of cut to extend

1/2 in from fracture surface.

Scanning and transmission electron

microscopy

.

Replication and direct scanning

electron microscopy.

Assessment of weld failure mode.

Validation of replication technique

for f ractographic analysis.

1 in = 25.4 mm

67
Table 5.11 Locations of Concrete Cores

Core Number Span Number E-W Coordinate (feet) N-S Coordinate (feet)

(Dist. from toe

of east handrail

support angle)

(Dist. from

north end of east

stringer)

4-01

02

03

04

UlO-11 2.85

2.95

2.85

1.85

10.3

10.3

16.8

17.1

-05

06

07

U9-10 5.85

2.80

2.20

9.3

9.0

14.5

4-08

09

10

LlO-11 2.90

2.35

2.90

11.5

13.0

15.2

(Dist. from toe

of west handrail

support angle)

(Dist. from

north end of west

stringer)

4-11

12

13

L9-10 2.35

2.30

2.85

14.4

16.5

18.0

(Dist. from

inside face of

west handrail

glass panel)

(Dist. from

north end of

west stringer)

4-14

15

16

M8-9 2.60

2.60

2.60

14.0

10.9

8.9

(Dist. from

inside face of

east handrail

glass panel)

(Dist. from

centerline of 1st

crossbeam from

south end)

4-17

18

19

M9-10 3.00

2.55

2.00

3.5

3.5

6.9

1 in = 25.4 imn

68
Table 5.12 Thickness of Topping Material

Thickness of Topping ( inches)

Span

Number Average Maximum Minimum

Number of

Observat ions

U7-8 0.16 0.47 0 6

U8-9 0.20 0.39 0 7

U9-10 0.38 0.68 0 13

UlO-11 0.47 0.75 0 13

L7-8 0.17 0.47 0 9

L8-9 0.44 0.67 0.08 10

L9-10 0.20 0.43 0 15

LlO-11 0.04 0.39 0 14

1 in = 25.4 mm

69
NBS Specimen

Number

Table 5.13 Specimens Removed From Walkway Debris

Specimen Description and Origin

Specimens Removed November 3-5, 1981

1 A 23.2 in longitudinal segment of box beam 9U beginning at a point 30 in from west end of 9U and extending inward.

2 A 35.7 in longitudinal segment from west end of box beam 8L.

Includes bent segment of hanger rod, complete with nut and washer.

3 Box beam 9M, complete with hanger rod nuts and washers.

4-01 to 4-19 Concrete cores located as shown in table 5.11. Nominal

diameter = 2 in.

5A A 35.7 in segment from upper end of fourth floor to ceiling hanger

rod at location SUE, complete with nut and washer.

53 A 35.8 in segment from second floor to fourth floor hanger rod.

Installed location unknown.

6 A 2 in section of upper longitudinal weld fracture at location 8UW

(north surface) centered 9 in from west end of box beam.

Specimens Removed December 16-17, 1981

7 Fracture surface, location SUE, bottom flange, north surface.

8 Fracture surface, location 9UE, bottom flange, south surface.

9 Fracture surface, location 9UW, bottom flange, north surface.

10 Fracture surface, location lOUE, bottom flange, south surface.

IIA Nut from upper end of fourth floor to ceiling hanger rod at location 9UE.

IIB Washer from upper end of fourth floor to ceiling hanger rod at location 9UE.

12A Nut from upper end of fourth floor to ceiling hanger rod at location lOUW.

12B Washer from upper end of fourth floor to ceiling hanger rod at location lOUW.

13A Nut from upper end of third floor to ceiling hanger rod at location 8MW.

13B Washer from upper end of third floor to ceiling hanger rod at location SMW.

14A Nut from upper end of third floor to ceiling hanger rod at location 9ME.

14B Washer from upper end of third floor to ceiling hanger rod at

location 9ME.

15A Nut from upper end of third floor to ceiling hanger rod at location lOMW.

15B Washer from upper end of third floor to ceiling hanger rod at location lOMW.

1 in = 25.4 mm 70
Table 5.14 Replicas of Fracture and Washer Contact Surfaces

Location Number of Replications

SUE - B - CI - Washer contact surface

(south)

- Washer contact surface

(north)

9UE - A

- B - D

- E - Washer contact surface

( south)

- Washer contact surface

(north)

9UW - A

- C

- F

- Washer contact surface

( south)

- Washer contact surface

(north)

lOUE - A

- C2

- Washer contact surface

( south)

9UE - North clip angle weld

fracture (top)

- North clip angle weld

fracture (middle)

- North clip angle weld

fracture (bottom)

8MW - Interior tack weld (bottom)

s

<t UJ

Ul

00 o3C Xa

CO 0£

UJ

HZDO

CDa

zUJ r///////////

F D V B

WASHER CONTACT

1

C SURFACE / . Y/////////A

PLAN VIEW

(Definition Sketch]

71
FOURTH FLOOR

NORTH

iHALL

GRANITE FACE

SECOND FLOOR

NORTH

4 HALL

GRANITE FACE

T.L 1

T.L. 2 T.L. 2

108.98'S 90°01'02"

iHALL

FOURTH FLOOR

SOUTH

4.32'-

T.L 3

L35

4.34'TOP 3.15" BOnOM

112'

4.36' TOP 3.25' BOTTOM

3.22'

^CONCRETE FACE WELD PLATE

-

435' GRANITE FACE

-438' 4.39'-

i HALL SECOND FLOOR

SOUTH

Figure 5.1 Locations of embedded plates and bearing seats.

72
DETAILS OF EMBEDDED PLATES - COL. LINE 7

•33/4"

1 5/8"

T

2 11/16"

xxxxxxx xxxxxX X

X X

X X

X X

X X ^ ^ X

X

X

X

X

X

X

X

xxxxxxx

X

x

X

X

X

X

X

x:

14" (Typ)

7UE

4-/4'

(Typ)

6"

ITyp)

4"-

1/8"

1 1/8"

xxxxxxxX

X

X

X

X

X

1xxxx

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

xxxxxxx---

7UW

X

X

X

X

X

X

X

X

X

X

4"- 1 3/8"

X

X

X

X

X

X

X

X

X

l-«-3 7/8"- —XXX

X

X

X

X

X

X

X

X

X

X

X

X

V

X

X

3/18"

X

X

X

X

X

X

T"

t

•XXXXXX) xxxxxx

7LE

NOTE: This portion of embedded

plate remained attached

to span L7-8

7LW

Figure 5.2 Details of embedded plates and fillet welds at column line 7.

73
Figure 5.3 Tile damage caused by second floor hanger rod lOE.
11M

N

J

10M

10M

11U 10U 9U 8U

9M

9M

10U 9U 8U 7U 8M

11L lOL 9L BL 8M

lOL 9L X 8L 7L

BOX BEAM |typ.

7M

Figure 5.5 Layout of walkway spans in warehouse.

75
76
Figure 5.7 Hanger rod pull-through at location 9UE.
78
79
Figure 5.11 Box beam at location lOUW (Span U9-10)

.
Figure 5.13 Washer at bottom end of fourth floor to ceiling hanger rod 9UW.

81
[2O

•HUooOo

•H

cdao do

•H

4-) COao

13COco

•H

4-1 Oo BO

4-1 0) o S

,nX o o

CO •H <U 13 ^4-1 4-1O OU

>^U (U u >

(U -H S C« >^ <-i

(f> 0) XI

0) 113 > O

^-1 r-l cu O CO

5-1

•H

82
Figure 5.15 Weld penetration at location 8UW, upper flange.
(a)

-

Figure 5.19 Incipient shear failure, bottom fillet weld of intact clip

angle, location 9UE, south side.

Figure 5.20 Shear failure at bottom and pull-out failure at top of clip

angle, location 9UE, north side.

86
Figure 5.22 Shear failure in clip angle fillet weld at location 9UW, north

side. This weld and the corresponding weld at location 9LW did

not show evidence of a pull-out (tensile) failure at the top

fillet weld.

87
Figure 5.23 Tearing of top end of north clip angle at location SUE.
igure 5.25 Embedded plate at connection 7UE.
gure 5.27 Modified bearing pad at location IIUW.

90

a) Before removal.

b) After removal.

Figure 5.29 Specimen no. 2 before and after removal

92

94
Figure 5.32 Concrete core drilling.

6. NBS STRUCTURAL TESTING PROGRAM

6.1 INTRODUCTION

This chapter describes the NBS structural testing program relating to the

walkway collapse, presents the test results, and discusses observations made

based on the test results. In addition, the estimated load capacity for each

of the upper walkway box beam-hanger rod connections is presented with a discussion of how the capacities were determined.

Section 6.2 presents a background to the test program and section 6.3 presents

an overview of the test program. Section 6.4 describes the geometry of the

test specimens and fabrication methods. Section 6.5 presents a detailed

description of the physical setup for testing the specimens, while section 6.6

describes the equipment used to load the specimens. Section 6.7 describes the

instrumentation used in monitoring specimen performance while section 6.8 presents the method of acquiring, storing, and processing data. Section 6.9 dis- cusses the sequence of applying loads to the specimens. Section 6.10 presents

a detailed description of the NBS-fabricated short box beam specimens and presents the pertinent test results. Section 6.11 similarly presents the NBSfabricated full-length box beam tests. Section 6.12 discusses the structural

tests (both full-length and short specimens) performed on walkway box beam 9ti. Section 6.13 presents estimates of the upper walkway box beam-hanger rod

connection load capacities. Section 6.14 presents the significant conclusions

reached in this chapter.

6.2 BACKGROUND

The purpose of the test program was to estimate the walkway box beam-hanger rod

connection load capacities. The load capacities could be defined adequately

only by examining the influence of a number of parameters. The limitations

imposed by the unavailability of as-built drawings, actual materials (in the

initial stages of the investigation), and design calculations made it necessary

to vary the parameters to take into account the range of variations that were likely for the actual walkway construction. The parameters were varied to

examine their influence on load capacity and in this way identify and emphasize

the parameters most affecting capacity.

97
6.3 OVERVIEW

The structural test program to examine the box beam-hanger rod connection

capacity used two types of test specimens: 1) short box beam and 2) fulllength box beam. The specimen types are described in more detail in

section 6.4. Two sources of material were used for the test specimens. A

supply of steel channel, all from a common heat, was purchased for the box

beams fabricated by NBS. Welding, using several different processes, was done

in the NBS shops. Additional tests, both full-length and short box beam types, were conducted on material obtained from the actual walkway structure. The

short box beam specimen was developed as a simple and cost-effective alternative

to the full-length box beam and was used as the main test specimen to permit

the examination of a number of variables. The full-length box beam tests were

used to validate the short box beam tests.

6.4 TEST SPECIMEN DESCRIPTION

Two types of specimens were tested in the investigation of box beam-hanger rod

connection capacity. The full-length box beam specimens were dimensionally similar to a typical fourth floor walkway box beam (figure 3.10). The short box

beams were essentially the end 22 in (560 mm) of a full-length box beam. Sixty

short box beams and seven full-length box beams were fabricated by NBS and

tested. Four short box beam specimens and one full-length box beam specimen

were prepared from an actual walkway box beam and tested.

6.4.1 Full-Length Box Beam

The full-length box beams were fabricated using the dimensions shown on the

walkway box beam shop drawing (figure 3.10). All material was nominally as specified on the shop drawing. A detailed description of the actual properties

of these materials is presented in chapter 7.

6.4.2 Short Box Beam

Examination of the walkway debris indicated that the zone of distress in the

box beam-hanger rod connection was generally limited to the portion of the

box beam between the end of the box beam and the stringer connection with the

box beam. The distress zone was primarily near the hanger rod locations.

Based on the visual examination, the short box beam could have been made by

cutting the full-length box beam at the stringer connection. However, a slightly longer test specimen was used to conform to the physical constraints

of the laboratory test facility. The comparability of full-length and short

box beam behavior is discussed in section 6.11.

The actual test specimen, figure 6.1, was approximately 22 in (560 mm) long.

The specimen was mounted to a test rig through two angles welded parallel to

the webs at one end of the specimen. The hole size and locations in the test

specimens were as specified on the shop drawings for the walkway box beams.

The first tests on specimens without stringer-box beam clip angles, using a whitewash coating on the specimen as a brittle coating, indicated that the

98
zone of distress did not extend to the clip angle position. It was thus con- cluded that the effect of the clip angles was minimal and for simplicity they were omitted in all but four tests. The validity of this assumption was confirmed by four tests that are described in section 6.10.5.

6.4.3 Fabrication

All test specimens were fabricated by the same experienced welder who used

different techniques until he could consistently produce a weld that was visually similar to the observed walkway box beam welds specified as part of

the testing program. A supply of channel the same size as that used in the

walkway was obtained in 20 ft (6.1 m) lengths from a single source and was specified as being from a single heat. Each box beam, identified in tables 6.1,

6.2, 6.3, and 6.4, was made from channel pieces taken from a single 20 ft

(6.1 m) length of channel.

The walkway box beams had been fabricated by laying a weld bead along the

channel flange tips from the outside of the box beam over the entire length of

the beam. For purposes of describing the structural tests, such a weld is

termed an exterior weld in this chapter. The depth of exterior weld penetration

(figure 6.2) estimated from visual examination of the damaged walkway box beams

was used as a guide in the fabrication of the short box beams. The welder

fabricating the short box beams was able to produce welds having a fairly uniform penetration along the beam. However, to examine the significance of

exterior weld penetration, specimens having penetration less than and greater

than the estimated penetration were fabricated. The majority of short box

beams were fabricated with a weld penetrating partially into the flange thickness and were termed "shallow penetration" welds. Several specimens were fabricated in which the welder was told to "fully penetrate" the flange thickness without using a backing strip. These welds were termed "deep penetration."

Two specimens were fabricated with no weld bead laid along the outer 8 in

(203 mm) of the short box beam except for a small tack weld at the free end of

the specimen (figure 6.3). These two specimens were termed "no weld" specimens

and were tested to determine a lower bound strength of the box beam-hanger rod

connection.

The walkway box beams had weld beads of varying lengths on the interior of the

flange joint outboard of the exterior hole (figure 6.4). These weld beads are termed interior welds in this chapter. Short box beams were fabricated having

several variations of the interior weld length. Insofar as possible, the penetration of the interior weld was kept constant. The penetration was such that

the interior weld and the exterior weld roots were separated by a small gap

(figure 6.4). The lengths of interior weld were nominally equal to zero, 1/2 in

(13 mm), 1 in (25 mm), and 1 3/8 in (35 mm). In the NBS beams the interior weld

was laid prior to boring the rod holes. The 1 3/8 in (35 mm) length resulted

from welding to the centerline of the exterior rod hole (2 in or 51 ram) and

removing the weld in the hole location during the hole boring operation to

obtain the actual length of 1 3/8 in (35 mm). In the NBS beams the interior

welds were laid after the exterior weld and were done by the same process as that used for the exterior weld.

99
All short box beams in series A through R, excluding 13 (listed in table 6.5) were cut to length prior to welding. The remaining short box beams (except

for the S series described below) were cut 1 in (25 mm) longer than their

final length and then welded. This was done in an effort to control possible

variation in welding due to startup of the welding process at the beam end.

After all welding was completed the extra length was removed at the ends and

the hanger rod holes bored. Full-length box beams (table 6.4) LI and L2 were

cut to length, while L3 through L7 were cut 2 in (51 mm) longer than their final

length. After welding, 1 in (25 mm) was cut off from each end of the beam.

The S-series short box beam with stringer-box beam clip angles were fabricated

by removing the unfailed ends of four full-length box beams as is discussed

further in section 6.10.5. Specimens L5, L4 , L6 and L3 were used to fabricate

Si through S4 respectively.

The sequence of fabrication for both NBS short and full-length box beams was as follows

:

• Tack weld the interior of the ends of the box beam, top and bottom

• Weld along the exterior of the box beam seam, top and bottom

• Weld along the interior of the box beam seam (if specified), top and

bottom

« Weld on the attachment clip angles used to fasten the specimen to a reaction buttress described in section 6.5

® Grind off the exterior weld bead flush with the surface completely

along the top and at least 12 in (305 mm) from the end(s) on the bottom

• Bore a pilot hole for each hanger rod hole. Finish the hole using a numerically controlled milling machine. Each hole was made in a separate operation.

6.5 TEST SETUP

Full-Length Box Beam. A photograph of the test setup simulating a fourth

floor box beam is shown in figure 6.5. A drawing with the various parts of the

test setup labeled is shown in figure 6.6. The full-length box beam test setup

was contained within a self-reacting structural steel test frame. The test

frame was bolted to a structural tie-down floor. The frame columns were 12 in

(305 mm) deep wide flange shapes and each reaction beam was made of two 36 in

(915 mm) deep wide flange sections placed side by side with a 1 1/2 in (38 mm)

gap between the flanges to permit the loading rods to pass through. The two

load transducers for the upper loading rods rested on a 1 in (25 mm) thick steel

plate which spanned the gap between the beam flanges. A hardened steel washer

and heavy hex nut terminated the upper loading rods and rested against the top

surface of the load transducer.

100
Each hydraulic cylinder was held in place against the lower surface of the lower

reaction beam by a frame bolted to the reaction frame. The load transducers

were separated from the hydraulic cylinder piston by a 1/2 in (13 mm) thick

steel plate. The lower end of the loading rod was terminated by a hardened

washer and a heavy hex nut.

The upper walkway loading system applied the simulated fourth floor dead load

to the box beam through the stringer-box beam clip angles (figure 6.7). The

system used a spreader beam with three pinned attachment points which enabled

the spreader beam to rotate while keeping the applied load acting vertically.

A plate was bolted between and below the clip angles to permit attachment of

the clevis to the box beam (figure 6.8). The middle pin was the connection

point for the loading rod. The attachment of the spreader beam to the clip

angle was through a pin and clevis arrangement as shown in figure 6.9.

Short Box Beam . The test setup, figures 6.10 and 6.11, used an existing

structural tie-down floor and reaction buttress plus two specially made vertical reaction supports. The vertical reaction supports provided the reaction

for the hydraulic cylinders applying load to the upper (fourth floor) and lower

(second floor) hanger rods. The reaction supports were bolted to the tie

down floor to resist both shear and overturning forces. The test specimen was bolted to the reaction buttress. Prior to each test the bolts holding the test

specimens were pretensioned using an impact wrench.

The assembled test specimen was rotated 90 degrees from its normal orientation.

The webs of the box beam were parallel to the floor. The hanger rods (loading

rods) were also parallel to the floor. Each loading rod passed through the

specimen, reaction support, center-hole hydraulic cylinder, and center-hole load

transducer and was terminated with a hardened washer and heavy hex nut.

6.6 LOADING EQUIPMENT

Full-Length Box Beam . The loading equipment included three 100 ton

(890 kN) , 10 in (254 mm) travel, double acting hydraulic cylinders; an electric

hydraulic pump; and a ten channel hydraulic pressure maintainer. The hydraulic

pump had a discharge rate of 0.26 gallons per minute (16.4 x 10"^ m-^/s).

The hydraulic cylinders were of the center hole type to permit the loading rod

to pass through the cylinder. The loading rod was centered in the cylinder by

a wooden sleeve. The lower walkway hydraulic cylinders were connected by manifolds to common pressure and return lines. Thus, while there were three hydraulic cylinders, there were only two pressure channels, one for the upper walkway

hydraulic cylinder and one for the two lower walkway hydraulic cylinders. The

hydraulic cylinders were supplied with oil by means of the hydraulic pressure

maintainer and the hydraulic pump. The hydraulic pump supplied oil to the

maintainer at a nominal pressure of 5000 psi (35 MPa). The maintainer con- trolled the hydraulic pressure actually applied to each cylinder. The operator

adjusted the pressure in each pressure channel independent of the other.

101
Short Box Beam . The loading equipment consisted of two 60 ton (534 kN)

,

3 in (76 mm) travel, single acting, spring return hydraulic cylinders; an electric hydraulic pump; and a ten-channel hydraulic pressure maintainer . The

hydraulic pump and pressure maintainer were the same as used for the fulllength box beam tests.

The hydraulic cylinders were of the center hole type to permit the loading rod

to pass through the cylinder. The loading rod was centered in the cylinder by

two teflon sleeves. The hydraulic cylinders rested on a wooden cradle fabricated so that the cylinder centerline was in line with the box beam-hanger rod

hole centerline.

6.7 INSTRUMENTATION

Full-Length Box Beams . The instrumentation included five load transducers,

one for each loading rod, two pressure transducers, four linear potentiometers

(LPs), two rotary potentiometers (RPs), and four linear variable differential

transformers (LVDTs). All devices were connected to a common DC power supply.

The load transducers were of a full bridge resistance strain gage type. The

load transducers were center holed with machined steel bearing surfaces fabricated to center the transducer on the loading rod.

The displacement measuring devices were placed as shown in figure 6.12. The

instrumentation was identical on each end of the beam so only one end is

described. Two mounting beams were bolted to each test frame column, one above and one below the box beam. These beams served as attachment points for

the displacement measuring LPs and RPs. The linear potentiometers had 10 in

(254 mm) travel. One LP was attached to the box beam about 1 in (25 mm) from

the end and to the upper mounting beam such that it was vertical. Similarly

the other LP was attached to the lower loading rod nut and the upper mounting

beam. The rotary potentiometer measured linear displacement through a cable

and spring mechanism. The RP had a total travel of over 20 in (508 mm). The

mechanism was attached to the lower mounting beam and the cable ran vertically

to the upper loading rod. The cable was attached to the rod by a hook screwed

into a hole tapped in the end of the rod. Two LVDTs measured the transverse

(out-of-plane) distortion of the box beam webs. The LVDTs were mounted on

one web of the box beam and spanned to plates mounted on the other web. One

LVDT was mounted near the middle of the box beam depth and the other LVDT was mounted near the bottom of the box beam.

One pressure transducer was placed in the common pressure line for the two lower

loading rod hydraulic cylinders and the other pressure transducer was placed

in the pressure line for the upper walkway loading rod.

Short Box Beam . The instrumentation consisted of two load transducers, one

for each loading rod, and up to six linear variable differential transformers

(LVDTs) for measuring displacements. All devices were connected to a common DC

power supply.

The load transducers were placed between the hydraulic cylinders and terminating

nut (figure 6.11) with a 1 in (25 mm) thick plate between the transducer and the

102
cylinder and a hardened washer placed between the transducer and the nut. The

load transducers were of a full bridge resistance strain gage type. The transducers were also center holed with machined steel bearing surfaces fabricated

to center the transducer on the loading rod.

The LVDTs were ordinarily placed as shown in figure 6.13. Three LVDTs measured

the out-of-plane or transverse distortion of the box beam webs and three LVDTs

measured the in-plane displacement of the loading rods and box beam free end.

All tests used the in-plane LVDTs. However, when photographs or videotapes

were taken of the tests in progress the specimen had either no transverse

LVDTs or only the middle transverse LVDT. The transverse LVDTs were mounted

on one web of the box beam and spanned to plates mounted on the other web.

The in-plane LVDTs were mounted on stands resting on the tie-down floor and

spanned to either the specimen end of the rods or the upper flange of the box

beam near its free end.

6.8 DATA ACQUISITION AND REDUCTION

Both data acquisition and reduction were computer based operations. The

hardware components of the system consisted of a minicomputer with a variety of

peripherals and a high speed amplifier/multiplexer analog-to-digital converter.

The software component was a test executive which interfaced the operator and

the hardware. The software performed the data conversion and reduction as well

as controlled the acquisition of the data.

The analog-to-digital converter had a sample rate of 50 kHz. The time

required to sample all data channels (maximum of 9) once (a scan) was about

180 microseconds. In most of the tests a scan was acquired every three seconds

except during breaks in loading when no scans were required. All data signals were amplified by the analog-to-digital converter such that the maximum expected

analog signal was at least half of the converter's full scale (10 volts) range.

The analog-to-digital converter transferred binary representations of the input

analog signal to the minicomputer through direct memory access. Software then

stored the data on disk and manipulated the data to produce converted and

reduced data.

The binary representation of the analog signal is converted to a decimal

number knowing the amplifier gains and the full scale range of the converter.

The decimal voltages are reduced by using a selected scan as a zero reference

and adjusting all subsequent values to reflect the zero reference value. The zero reference scan is a data scan acquired prior to activating the hydraulics.

The reduced values for the load transducers are corrected for excitation voltage

changes. The voltages are supplied by a regulated power source and do not vary

during a test. However, the load transducers are calibrated at a 6.00 volt

excitation level, but the test excitation level is generally slightly different

(e.g., 6.08 volt). The reduced values also include a multiplicative coefficient

which expresses the voltage in terms of any desired unit (e.g., kips).

103
6.9 LOADING SEQUENCE

The stress state in the box beam-hanger rod connection region is dictated by

the loads from both the lower and upper walkway hanger rods. The exact distribution of load between the two rods is influenced by a number of factors, but

it can be approximated by the assumption that all of the load from the lower

walkway hanger rod is transferred to the adjoining upper walkway hanger rod in

addition to the contribution due to upper walkway loads. The general loading

sequence used in most of the box beam tests is based on applying only dead load

to the upper walkway and dead load plus all live load to the lower walkway.

This loading is not an exact simulation of the actual situation but, as discussed in section 4.4, is reasonably consistent with the small number of people

believed to have been present on the fourth floor walkway at the time of

collapse. The test results also indicate that the load capacity is not

significantly affected by small variations in loading sequence, as pointed out

in section 6.10.5.

The hydraulic loading system used in the tests applies load as long as there

is resistance to the stroke of the hydraulic cyclinder. As failure begins to

occur, the applied load decreases until the movement of the cylinder can "catch

up" with the movement of the structural member. The actual walkway loading was

a gravity-type loading. That is, the loads remained in place regardless of the

deformation of the structure. Thus, as the structure begins to fail the gravity

load remains constant although the resistance of the structure is decreasing

with deformation. These two methods of loading, that is, hydraulic vs. gravity,

will produce little difference in structural response until deformations become

large. At that point, the gravity-type loading causes a more rapid failure of

the structure than the hydraulic loading. The ultimate capacity which would be

obtained by either method of loading would be expected to be the same. The

load-deformation curves would also be expected to be the same up until the

maximum load is reached.

Full-Length Box Beam . The upper walkway dead load was applied to the box

beam through the upper walkway loading system. The lower loading rods were

kept unloaded. After the upper walkway dead load was reached the hydraulic

pressure maintainer kept the load constant. The load in the lower rods was then

gradually increased until failure occurred. Using this loading procedure the

load carried by the upper rod was equal to dead load from the fourth floor and

dead load plus live load from the second floor. In most tests the loading was terminated after lower flange failure at the location of an upper walkway

hanger rod while in other tests loading was continued until failure also

occurred in the upper flange. The upper flange failure load was always smaller

than the lower flange failure load and the large deformations involved following

lower flange failure made additional distortion measurements of questionable

value

.

Short Box Beam . The loading sequence used in nearly all short box beam

tests was as follows:

1. Load the upper rod to a value representing the nominal fourth floor

dead load contribution keeping the lower rod unloaded.

104
2. Once the nominal dead load value was reached in the upper rod, load was added equally to both rods until the test was terminated when

full piston stroke for the upper rod was reached. The load carried

by the upper rod was then equal to dead load from the fourth floor

and dead load plus live load from the second floor. The raaximura load

capacity was achieved in all but one test prior to reaching the stroke

limit

.

6.10 NBS SHORT BOX BEAM TESTS

6.10.1 Parameters

A range of parameters was studied in order to define adequately the load

carrying capacity of the box beam-hanger rod connection. A number of parameters were required for reasons such as:

• variability of details among the walkway box beams (e.g., interior weld

length)

• uncertainty of conditions at the onset of failure (e.g., nut orientation,

washer presence), and

• unavailability of walkway box beam properties at early stages of the

test program (e.g., welding process, washer hardness).

The parameters studied in the short box beam tests were:

• Welding Process

• Exterior Weld Penetration

• Interior Weld Length

• Interior Weld Position

• Washer Presence

• Washer Hardness

• Washer Thickness

o Nut Shape

• Nut Orientation

o Loading Configuration

• Initial Upper Rod Load

• Stringer-Box Beam Clip Angle Presence

105
These are discussed in detail in the following subsections along with the test

results

.

6.10.2 Short Box Beam Specimens

The characteristics of each short box beam specimen along with the peak load

resistance for each specimen determined by the load in the upper rod are listed

in tables 6.1, 6.2, and 6.3. Data for specimens which used hardened washers

beneath the fourth floor rod connecting nut are listed in table 6.1 while

specimens which used unhardened washers are listed in table 6.2. Three

specimens tested without washers are listed in table 6.3.

Three different failure types, assigned the identifiers I, II, and III, were observed and are shown in figure 6.14. The bottom flanges of specimen K2

,

and others exhibiting type I behavior, fold back about a plastic hinge in the

web and do not show any discernible bending. The flanges in type III behavior,

illustrated by Jl , fold back about the plastic hinge in the web but also show

pronounced bending in the flange. Type II behavior exhibits slight bending in

the flange tips as illustrated by specimen Ml. None of the washers fractured

in these tests.

Three typical in-plane load-deflection curves for the short box beam specimens

are shown in figure 6.15. (A complete set of load-deflection curves is presented in appendix A. 6. 10.) Each curve is a plot of the load measured by the

upper rod load transducer versus the displacement of the rod measured by the

LVDT at the box beam terminus of the upper loading rod (LVDT no. 2 in

figure 6.13). The peak load achieved by each of the specimens is shown on

their respective curves. The asterisk on each curve indicates the end of

reliable measurement by the displacement device and does not necessarily

denote final rod pull-through.

The in-plane load-deflection curves in figure 6.15 illustrate specimen behavior

usually associated with failure types I, II, III and are again illustrated by

specimen C3 , specimen T3, and specimen F2 , respectively.

Type I behavior is characterized by a load-deflection curve which shows a relatively sharp peak at maximum load. In addition to general observations

which were made on the behavior of all specimens, several specimens were subjected to detailed observations during their entire loading sequence. Observation of the inside of the box beam, which was whitewashed. Indicates that the

exterior weld between the two rod holes begins to crack at an upper rod load of

between 5 to 7.5 kips (22 to 33 kN) . The interior weld (or exterior weld if no interior weld was present) between the outer hanger rod hole and the beam end

begins to crack adjacent to the outer hanger rod hole at an upper rod load of

from 8 to 10 kips (36 to 44 kN) . Web deformation became visible at 10 to

12 kips (44 to 53 kN) . The achievement of maximum load is accompanied by

fracture of the interior weld (or exterior weld if no interior weld is present)

between the exterior hole and the beam end. After the exterior weld breaks

the bottom flanges fold back about a plastic hinge in the web. The bottom

flanges exhibit no discernible bending.

106
Type III behavior Is usually characterized by a load-deflection curve which has

a relatively rounded shape (as compared to type I) at maximum load. Visual

observation of a few specimens indicates that the exterior weld between the

lower flange hanger rod holes also cracks between 6 to 8 kips (27 to 36 kN) of

upper rod load. Cracking of the interior weld begins adjacent to the outer

hanger rod hole at between 10 to 12 kips (44 to 53 kN) and progresses to the

end of the beam by about 15 kips (67 kN) . Shortly after this stage of cracking

the deformations become visible. At the maximum load the Interior weld has

not yet fractured between the exterior hole and the beam end. The resistance

to load decreases with increased deflection, but at a constant rate. The rate

is quite similar to the load loss rate following the peak load in the type I behavior. The gradual loss in resistance to load continues with progressive

cracking until the interior weld between the outer hole and the beam end fractures, at which point there is a sharp drop in load resistance. The bottom

flanges exhibit pronounced bending as illustrated by specimen Jl in figure 6.14.

Type 11 behavior is between the two extremes (figure 6.15). Initially it

exhibits the characteristics of type III behavior. However, the interior weld

between the exterior hanger rod hole and beam end fractures at or just after

the peak load, unlike the type III beams. The bottom flanges appear more like

type I but show a small amount of bending as for specimen T3 in figure 6.14.

The out-of-plane load-deflection curves show the same trends as exhibited by

the in-plane load-deflection curves. The out-of-plane load-deflection curves

for the above described specimens are shown in figure 6.16. Each curve is a plot of the same load described for figure 6.15 versus the displacement measured

by the middle transverse LVDT (figure 6.13). These data show that the web is

beginning to deform outward shortly after the application of dead load from

the second floor is started.

A complete discussion of the effects each parameter has on the ultimate capacity

is presented in the following subsections.

6.10.3 Welding Process

Since there was uncertainty regarding the welding process actually used in the

box beam fabrication, the influence of welding process was examined by using

several different procedures to fabricate the short box beams. The selection

was made by visually comparing the results of welds made using several types

of electrodes used in shielded metal arc welding (SMAW) [6.1] with the visual

observations of the walkway box beam welds. Specimens were fabricated with

AWS class E7014 and E7018 stick electrodes to give an indication of the effect

electrodes had on load carrying capacity. Both the E7014 and E7018 electrodes were 5/32 in (3.97 mm) in diameter.

After a number of specimens had been fabricated and tested, information was received from legal counsel (section 2.7) to the steel fabricator indicating

either SMAW, or "MIG welding" was used in the walkway fabrication. Legal

counsel further stated that if "MIG welding" were used, the electrode was flux

cored. "MIG welding" was taken to include flux cored arc welding (FCAW) or gas metal-arc welding (GMAW). Visually, the GMAW process with a mild steel

107
electrode most closely resembled the walkway welds. Therefore, a mild steel

electrode was used in fabricating the bulk of the remaining short box beams.

However, four short box beams were also fabricated using a flux cored electrode.

These four specimens provided additional indications as to the effect of different electrodes. The mild steel electrode, AWS class E70S-3, was manufactured

by Racco and was 0.045 in (1.14 mm) in diameter. The flux cored electrode,

AWS class E70T-1, was a Hobart Fabco 81 electrode also 0.045 in (1.14 mm) in

diameter

.

The four processes are summarized below:

Process 1

GMAW - Gas metal arc welding process using a 0.045 in (1 mm) diameter

mild steel electrode.

In all cases the gas was carbon dioxide delivered at 20 cfm

(0.009 m3/sec).

In all cases the voltage was 26 volts and the current was 150 amps.

Process 2

FCAW - Flux cored arc welding process using a 0.045 in (1 mm) diameter

flux cored electrode.

In all cases the gas was carbon dioxide delivered at 20 cfm

(0.009 m^/sec).

In all cases the voltage was 26 volts and the current was 150 amps.

Process 3

S14 - An E7014 stick electrode was used without preheat

In all cases the rod diameter was 5/32 in (4 mm)

In all cases DC straight polarity was used

In the C, F, and G series the current was 150 amps

In the A and E series the current was 185 amps

In the D series the current was 180 amps

Process 4

S18 - An E7018 stick electrode was used without preheat

In all cases the rod diameter was 5/32 in (4 mm)

In all cases DC reverse polarity was used

In all cases the current was 140 amps

Comparisons between specimens in which the only intentional difference is the

welding process appear in figure 6.17. Six sets of data, each consisting of

comparable specimens, are shown enclosed in the figure. Hardened washer data

are used where possible since it was found (section 6.10.5) that hardened

washers were used in the actual walkway construction. The specimens compared

for a particular process were fabricated at the same time. This selection

of data was done to maintain as much similarity as possible since there was variation in the amount of weld material deposited and, as discussed in

108
section 6.10.4, the peak upper rod load was later found to vary in proportion

to weld area.

Data for deep penetration welds (no interior weld) are used to compare process 3 and 4 (specimens Al , A2 , and Bl , table 6.1). There is no difference between

the average for process 4 data and the process 3 test specimen.

Two further data sets with no interior weld are compared. For specimens with

hardened washers (specimens CI, C3, II, and 12, table 6.1) the average peak

upper rod load for process 1 is only 500 lb (2.2 kN) more than process 3. For

unhardened washers (specimens LSI, LS2 , PI, and P2 , table 6.2) the maximum

difference in the average is 700 lb (3.1 kN) comparing processes 1 and 2. Even

this difference is only about five percent of the peak upper rod load.

The 1/2 in (13 mm) interior weld data set (specimens Kl , K2 , Gl , and G2

,

table 6.1) comparing process 1 and 3 differs by only an average of 300 lb

(1.3 kN) . The two data sets for a 1 3/8 in (35 mm) interior weld are for

hardened (specimens Jl , J2 , Fl , and F2 , table 6.1) and unhardened washers

(specimens M2 , Nl , and N2 , table 6.2). The average difference is zero for the

comparison of process 1 and 3 with hardened washers. The comparison of process

1 and 2 for unhardened washers differs in the average by only 400 lb (1.8 kN)

.

Because of these small differences it is concluded that the welding process

itself, within the range of variation considered by NBS, does not have a significant effect on the peak upper rod load for the short box beams. Based on

this result data comparisons are not differentiated by weld process.

6.10.4 V7eld Area

Exterior Weld Penetration . The exterior weld penetration depth for

specimens without an interior weld has a significant effect on the peak upper

rod load of the short box beams, as shown in figure 6.18. The average peak

loads for specimens El and E2 having no weld (no penetration) , CI and C3 having

a shallow penetration weld, and Al and A2 having a deep penetration weld are shown in the figure to illustrate that there is significant difference in load

resistance between the case of shallow penetration and deep penetration. The

difference is about 2.5 kips (11.1 kN)

.

Interior Weld Length . The length of interior weld has a significant effect

on load resistance of the short box beams. A limited comparison of test results

is shown in figure 6.19 to illustrate this point. Data shown include shallow

penetration weld specimens shown in table 6.1 except for variations due to .weld

position, washer thickness, nut shape, nut orientation, clip angle presence,

initial rod load, and number of rods. Although a regression analysis was not

done it is apparent that increasing the interior weld length increases the load

resistance. The increase in load resistance is roughly linearly proportional

to the increase in interior weld length. However length of interior weld

alone is insufficient to account for load variation since it does not consider

penetration of either the interior or exterior welds.

109
Weld Area. The effect of weld length and depth of penetration of the

exterior and interior weld was examined by determining the interior and exterior

weld areas between the beam end and the hole for the fourth floor rod. Fracture

surfaces between the exterior hole and beam end were examined under magnification for all short box beam specimens. Lengths and depths were measured.

Depths were measured at five locations and averaged. These average measurements

are summarized in tables 6.1, 6.2, and 6.3, and 6.4 for the NBS tests. Weld

area was determined as the sum of the products of the average depth and length

for the exterior and interior welds. The peak upper rod load is plotted in

figure 6.20 against the weld area. Weld area is used as a measure or index

for effectiveness of the weld material to resist load although it is realized

that this is a simplification. The trend of the data is as expected; an increase in capacity occurs with the addition of weld metal.

The data shown in figure 6.20 include all tests on short box beams with hardened

washers as listed in table 6.1 except for round nut bevel, initial rod load,

single rod and clip angle presence. These latter parameters are discussed in

section 6.10.5.

The correlation coefficient, r, is 0.90 for the equation for the mean peak upper

rod load [6.2]. An r value of 1.0 indicates perfect correlation. The r^ value

is 0.80, indicating that 80 percent of the total variation in the peak upper

rod load is accounted for by the regression line. The other 20 percent is

unaccounted for by the weld area alone. Thus the equation for the mean based

on the weld area is a reasonable representation of the load resistance within

the range of the test data. A 95 percent confidence band for the data is also

shown in figure 6.20. Ninety five percent of the test results for individual

specimens are expected to be within the limits of the band.

6.10.5 OtVier Observations

Stringer-Box Beam Clip Angle Presence . Four short box beam tests were conducted to examine the influence of stringer-box beam clip angles on load

resistance. These specimens were obtained by cutting off the outside 22 in

(560 mm) of full-length beams L3 through L6 that did not fail during the fulllength box beam tests described in section 6.11. Except for the presence of

clip angles and the fact that the specimens had been loaded during full-length

box beam tests, tests were conducted in the same manner as used for typical

short box beam specimens.

The presence of clip angles does not significantly affect the peak upper rod

load capacities of the short box beams as is apparent from the data for specimens S1-S4 shown in figure 6.21 compared to the mean peak upper rod load and

the 95 percent confidence band. This observation was expected based on the zone of distress observed on the white-washed box beams since the zone of

distress did not extend to the location of the stringer-box beam clip angle.

Initial Upper Rod Load . The short box beam tests began prior to the

weighing of the walkway spans. The nominal upper walkway dead load was calculated on the basis of the contract drawings and thus for this part of the

laboratory test program the dead load contribution to the upper hanger rod was

110
set at 7500 pounds (33 kN) in all but three of the short box beam tests. The

two remaining specimens were tested using an initial upper rod load of 9000

pounds (40 kN) to examine the effect of the initial rod load on the load resistance of the box beam-hanger rod connection. As evidenced by a comparison of

these two tests (figure 6.22) with the regression analysis from figure 6.20,

the influence of initial upper rod load on peak load capacity is insignificant

for the range of 7500 pounds (33 kN) to 9000 pounds (40 kN)

.

Loading Configuration . The bulk of the short box beam tests used a two

rod loading configuration as discussed in section 6.5. The two-rod configuration best modelled the actual upper walkway box beam loading. However, the

original design of the second and fourth floor walkways called for a continuous

hanger rod and the third floor walkway connections had a single-rod configuration. For purposes of comparison, four short box beams were loaded using only

a single hanger rod. All loads were applied through the upper loading rod.

The lower floor loading rod was omitted for the single rod tests although the

hole was bored.

A comparison of the single rod data for B2 , Hi, XI and X2 listed in table 6.1

is presented in figure 6.23. These data are consistently about 2.0 kips (9 kN)

higher than the mean for double rods for a given weld area. Thus the single

rod tests are considered to be outside the range of the regression analysis

and were not used as part of the data base in estimating load resistance of

connections loaded by two hanger rods.

Nut Shape . Based on the observed dimensions of the walkway hanger rod nuts

it was determined that they were 1 1/4 in (32 mm) heavy hex nuts. Such nuts

were procured locally and their dimensions were similar to the walkway hanger

rod nuts. However, the amount of bevel at the intersection of corners and

bearing surface was greater on the actual walkway hanger rod nuts (figure 6.24).

The effect of nut shape was explored by testing two specimens with nuts which

were altered to a geometry similar to the walkway hanger rod nuts. This is

referred to as a "round" nut bevel while the nut used on most tests has a "square" bevel.

The comparison of data for specimens VI and V2 is presented in figure 6.25 for

the hardened washer tests. The comparison indicates that nut shape may have

an effect on peak upper rod load, with the rounded nuts reducing the load resistance. Although the reduction is within the data scatter the two specimens are considered to be outside the range of the regression analysis and were not used

as part of the data base in estimating load resistance.

Washer Presence . Early in the investigation it was uncertain whether or not washers were present on all of the upper walkway box beam-hanger rod connections since two were missing from the debris.

The majority of the short box beams were tested with washers, but three

specimens were tested without a washer between the upper rod nut and the box

beam. Washers were always used between the second floor loading rod nut and

box beam. The absence of a washer on the upper floor rod reduces the peak

upper rod load by about 2.5 kips (11 kN) for a given weld area as shown in

figure 6.26. As noted in section 8.3.3 of this report, there is strong evidence

111
that washers were present prior to the

as part of the data base in estimating

collapse

.

capacity.

Thus these data are not used

Washer Hardness . Prior to obtaining material from the walkway debris it was not possible to determine whether the washers were hardened or unhardened.

Initially, the short box beam specimens v/ere tested using hardened steel

washers. These were followed by a series of short box beams tests using

unhardened washers.

Based on the data shown in figure 6.27 it is estimated that an unhardened washer

reduces the load resistance by approximately 2.0 kips (9 kN) below that of

the hardened washer. Since all of the tests on debris indicated hardened

washers were used, these data were not used in estimating load resistance.

Seven unhardened washer data points are identified separately in figure 6.27.

This was done for convenience in discussing subsequent parameters.

Interior Weld Position . In all but two short box beams with interior

welds, the weld began at the box beam free end and extended inwards (termed the

edge position). The two remaining specimens (Rl and R2) with unhardened

washers had the interior weld extending from the exterior hole edge outwards

(termed the hole position). The weld position was varied to explore the importance of weld material at the hole position as compared to weld material near

the free end.

The lengths of interior weld beginning at the edge of the hole are 1/4 in (6 mm)

and 5/16 in (8 mm). Comparison of these data with other unhardened washer data

in figure 6.27 indicates that the location of the interior weld within the

length between the edge of the exterior hole and the beam end is not a significant parameter within the range of variation studied.

Washer Thickness . All but one short box beam was tested with washers

having a thickness of about 0.15 in (3.8 mm). Specimen N6 (table 6.2) was

tested using an unhardened washer having a thickness of about 0.18 in (4.6 mm).

The data in figure 6.27 shows that the test result for N6 is within the overall

scatter of the data. Although a thicker washer should improve the load

resistance, the small variation tested appears to make no difference.

Nut Orientation . The orientation of the nuts on the walkway hanger rods

at the time of failure could not accurately be ascertained. For purposes of

the investigation a uniform orientation was used for most tests, but three

specimens (N3, N4 , and N5) with unhardened washers were tested using a different

orientation. The normal nut orientation was with two nut faces parallel to the

short box beam longitudinal axis. The alternate orientation had two nut faces

perpendicular to the short box beam longitudinal axis (figure 6.28).

The data points for nut orientation are shown in figure 6.27. Based on these

data nut orientation is not considered a significant parameter.

112
6.10.6 Suramary

The short box beam data base was discussed in sections 6.10.4 and 6.10.5. After

review of the parameters it is concluded that additional data may be used as part of the data base used to develop figure 6.20. Specifically, all of the

data for hardened washers should be used except that for single rod tests

(B2, HI, XI, and X2) and nut shape tests (VI and V2). All of the data in

tables 6.2 and 6.3 for tests using unhardened washers and no washers are also

excluded

.

A regression line is shown in figure 6.29 based on these 28 tests. This

regression line for the mean peak upper rod load is shown with a 95 percent

confidence band for the data and has a correlation coefficient, r, of 0.91 and

an r^ of 0.84. This line of best fit characterizes the NBS short box beam test

results

.

6.11 NBS FULL-LENGTH BOX BEAM TESTS

Seven full-length box beams fabricated by NBS were tested with the parameters

shown in table 6.4. All seven were diraensionally identical. All full-length

box beams were fabricated using the GMAW process and the same mild steel electrode used in the short box beam tests. The first two full-length box beams (Ll

and L2) had a 1/2 in (13 mm) interior weld extending from each end of the top

and bottom flanges while the remaining five full-length box beams (L3-L7) were constructed similarly except that the interior weld extended 1 3/8 in (35 mm).

Load-deflection curves are presented in appendix A6.11.

The full-length box beam data are compared to the regression analysis results

for the short box beam tests (from figure 6.29) in figure 6.30. Based on the

comparisons between the full-length and short box beam tests, it was concluded

that there is good agreement between the results obtained using either type of

test specimen. Therefore, a regression analysis was performed using the short

box beam data shown in figure 6.29 along with the full-length box beam data

points shown in figure 6.30. The results of this analysis, which incorporates

all of the applicable results of the NBS test program as indicated (35 tests)

,

are shown in figure 6.31. The correlation coefficient, r, is 0.87 and the r^

value is 0.76.

6.12 TESTS ON WALKWAY BOX BEAM

One full-length box beam was obtained from column line 9 of the third floor

walkway (figure 6.7). One full-length box beam test and four short box beam

tests were conducted on this beam. Specimen characteristics are shown in

table 6.5.

6.12.1 Full-Length Box Beam Test

The third floor walkway box beam, designated KCL, was longer than the second

and fourth floor walkway box beams and only had holes for a single hanger rod

at each end (figure 3.10). The holes for the lower loading rods were made at

NBS in the same way as holes in the NBS box beams were made. The nuts and

113
washers used on the exterior (upper) loading rods at the box beam were those

taken by NBS from the walkway box beam during removal. The interior (lower)

loading rods were fitted with NBS rounded nuts and hardened washers.

The extra beam length was located between the clip angles and was accommodated

in the test setup by repositioning the loading rods and hydraulic cylinders.

A longer spreader beam was used to span between the clip angles. The instrumentation was identical to that used in the NBS full-length box beam tests

except that for purposes of videotaping the transverse LVDTs were omitted.

The first failure occurred at the west lower flange exterior hole. The interior

weld on this flange did not extend to the exterior hole while the interior weld

on the east lower flange did extend fully to the exterior hole. The rod load

(upper west rod) at first failure was approximately 19.4 kips (86 kN). The

second failure occurred at the west upper flange exterior hole. No interior

weld was observed on this flange. The rod load at failure was approximately

16.2 kips (72 kN) . The upper rod loads versus channel tip displacements are shown in figure 6.32.

Specimen KCL was recorded on videotape, which included a record of load on the

videotape image. Thus, the load-deflection behavior in figure 6.32 may be

described by correlation with the videotape. Due to the test setup it was not

possible to observe directly the weld surfaces on the interior of the box beam.

However, based on observations of other NBS tests, cracking probably occurred

in the exterior welds between the two rod holes on the lower flange by a load

of 7.5 kips (33 kN) representing the dead load from the fourth floor walkway.

By approximately 11 kips (49 kN) cracking probably started in the interior

weld near the outer hanger rod hole and progressed towards the end of the box

beam. Observation of the videotape showed that at about 16 kips (71 kN) the

web bending was pronounced. At about that load level the load-displacement

curve is beginning to show increasing nonlinear ity.

The videotape which included an audio track indicated, based on noise emitted,

that considerable weld fracturing occurred between 18.1 and 15.5 kips (81 and

69 kN) on the descending portion of the load-displacement curve with complete

fracture (visible on the video portion of the tape) occurring at about 15.5 kips

(69 kN) with the load dropping rapidly to 12.8 kips (57 kN) . At about 8 kips

(36 kN) on the descending portion of the curve the washer fractured with the

load dropping almost immediately to 5.9 kips (26 kN)

.

6.12.2 Short Box Beam Tests

After the full-length box beam test, the box beam was sawn into four lengths

which were made into short box beams. The first short box beam included the

unfailed east end of the full-length box beam. The other three short box beams

were taken in succession along the length of the box beam. The longitudinal

welds were not modified. The first short box beam included the stringer connection clip angles. A fifth short box beam was included in this series and was fabricated using the NBS channel sections.

114
The first short box beam (KCl) was a continuation of the test of the full-length

box beam's east end. The next three short box beams were similar except for

the nut and washer used on the upper loading rod. The fifth box beam was an NBS box beam (13), but the nut and washer on the upper loading rod was from

the walkway debris.

The in-plane load deflection curves for the short box beam tests are shown in

figure 6.33. Each curve is a plot of the upper rod load transducer versus the

displacement of the rod measured by the LVDT at the box beam terminus of the

upper loading rod. The peak load achieved by each of the specimens is shown

on their respective curves. The asterisks on each curve indicate the end of

reliable measurement by the LVDT.

Specimen KCl exhibited a double peak load deflection curve as shown in

figure 6.34. This is a continuation of the data shown in figure 6.33. The

unloading shown in figure 6.34 at about 1.8 in (46 mm) of displacement was necessary since the stroke of the piston in the hydraulic ram had been reached.

After repositioning the piston, load was reapplied. After exhibiting typical

behavior through about the first 1.8 in (46 mm) of displacement, another resistance mechanism took effect which enabled the load to increase again. As

larger deformations occurred the web buckling progressed further into the

length of the box beam. Apparently the progression was stopped by the clip

angles which stiffened the web. The load increased again to a second peak of

23.6 kips (105 kN) . The weld between the outer rod hole and the beam end

cracked but never completely fractured. Pull-through was achieved only after

full separation of the flanges between the rod holes occurred. An end view

of the KCl deformed shape is shown in figure 6.35.

It is emphasized that the fourth floor walkway box beams that failed on July 17 did not exhibit the same behavior mechanism described for KCl. Their

behavior, as shown by the end views in figure 6.36 was much more typical of

the NBS specimens (figure 6.14) as were the other specimens sawn from specimen

KCL.

Specimens KCl, KC2 , and 13 also used washers obtained from the walkway debris.

The washers fractured in all three instances but only after the peak upper rod

load had been reached (figure 6.33). The KCl washer fractured at a load of

about 15 kips (67 kN) while the washers for KC2 and 13 fractured at about 7 kips

(31 kN) and 6 kips (26 kN) respectively. In 13 only a sector of the washer

fractured as compared to complete fractures for KCl and KC2 . Washers did not

fracture in the KC3 and KC4 tests which used NBS hardened and unhardened

washers respectively.

Specimen KC4, which consisted of a walkway short box beam tested with an NBS

unhardened washer, is shown plotted in figure 6.27. When compared with the

results of NBS box beams tested with unhardened washers, KC4 appears to give

consistent results.

The remainder of the box beam test results involving walkway materials are shown in figure 6.37. Since the weld of specimen KCl never fully fractured

the weld area recorded for this specimen is an estimate based on the area of

115
the weld at the center line of the longitudinal seam rather than the weld

area actually measured on the fracture surface. Since the regression analysis

was based on weld fracture area, direct comparison of the KCl test with other

data is difficult. Specimen 13, which consisted of an NBS short box beam tested

with a walkway washer, shows a load resistance which is consistent with the

other NBS box beam test data.

Specimen KC3 , which consisted of a walkway short beam tested with an NBS

hardened washer, similarly shows a load resistance which is consistent with the

NBS box beam data. Specimens KCL, KCl, and KC2 , which were the only tests

conducted on specimens with the box beam, nut, and washers of walkway origin,

fall within or near the 95 percent confidence band for the NBS test data.

However, two of the three specimens are either at or just outside the upper

boundary of the 95 percent confidence band.

The size of the data base for NBS box beams with NBS hardened washers is of

reasonable size (35 specimens shown in fig. 6.31), and thus, can be used with

reasonable confidence as a tool for predicting ultimate strength of box beam

hanger rod connections having similar characteristics. While the walkway test

results generally fall within the 95 percent confidence band for the NBS data

base, it must be recognized that the walkway data base is extremely limited

(three specimens with walkway box beams, nuts and washers) and does show a tendency toward the upper boundary of the 95 percent confidence band of the

NBS data base.

As noted earlier, and as shown in table 6.6, the walkway box beams had a somewhat heavier cross-section which might be expected to slightly increase

load resistance. Recognizing this limitation, the NBS data base is used as the basis for estimating the ultimate strengths of the fourth floor walkway

box beam-hanger rod connections. The validity of the ultimate strengths thus

derived is further discussed in chapter 10.

6.13 BOX BEAM-HANGER ROD CONNECTION CAPACITY

The regression equation shown in figure 6.38, along with the 95 percent

confidence band for single values of pullout load, is used to estimate ultimate

strengths for the fourth floor box beam-hanger rod connections. The figure is

the same as the one shown in figure 6.31, but with the data points omitted.

It was developed based on the short box beam data described in section 6.10.6

and the full-length box beam data described in section 6.11. Furthermore, the

regression analysis was validated by the use of actual walkway test specimens.

The meaning of figure 6.38 is amplified by discussing box beam-hanger rod

connection 9UE as an example. Simply described, if many specimens like 9UE

were tested, their average should fall close to the regression line. However,

for a single test one can be 95 percent confident that the test result would

fall within the 95 percent confidence band. Thus each connection should be

thought of in terms of the range of ultimate strength as well as the average.

Weld areas were determined by examination of the actual fracture surfaces for

SUE, 9UE, 9UW, and lOUE. The weld lengths and depths were measured, the

116
depths being measured at 5 locations along the weld length. Computed areas

are shown in table 6.7. Weld areas for fracture surfaces 8UW and LOUW are based on field measurements since the actual fracture surfaces were not

obtained. These weld areas are also shown in table 6.7,

These ultimate capacities are summarized in figure 6.39 on a layout of the six

box beam-hanger rod connections. A bar diagram Is shown adjacent to each

hanger rod. Location SUE is described to denote the meaning. The range from

16.3 to 20.2 kips (73 to 90 kN) is the 95 percent confidence interval estimate

for this particular box beam-hanger rod connection. The mean ultimate capacity

of 18.2 kips (81 kN) is also shown. The mean capacity for all connections is

18.6 kips (83 kN)

.

Although these comparisons indicate that the regression analysis based on the

data shown in figure 6.31 reflects the trend of the walkway data, the walkway

section is slightly heavier than the section used in the NBS tests, as shown

in table 6.6. Thus, somewhat greater ultimate strength might be anticipated for

the walkway cross-sections. This heavier cross-sectional area may explain

why the test data fall essentially between the average and the upper portion

of the confidence band with none below (recall that KC4 was tested with an unhardened washer).

The regression analyses were developed for the double or interrupted rod

connection that was actually used in the walkway. Based on the results in

section 6.10.5 the original continuous hanger rod arrangement would have been

about 2.0 kips (9 kN) higher than the arrangement actually used. Thus the

average ultimate capacity would have been about 20.5 kips (91 kN)

.

6.14 CONCLUSIONS

The following conclusions are made subject to the limitations on the range of

variables tested.

1. The welding process has little effect on ultimate capacity.

2. Three types of washers were used during the test series. Hardened

and unhardened washers were used in the NBS test program. A third

type of washer which exhibited a hardness similar to that of the NBS

hardened washer was obtained from the walkway debris. However,

this washer differed from the NBS washer in that it fractured during

testing. However, the fracture occurred after the peak load had been

reached and at a load smaller than the peak load. None of the washers

tested by NBS other than those from the debris fractured.

3. Ultimate capacity of the double rod connection varies linearly with

an index weld area consisting of the sum of the exterior and interior

weld area in the region between the outer hanger rod hole and the

beam end. The variation for hardened washers is P = 13.18 + 0.0391A

with a correlation coefficient of 0.87.

117
Test results for box beam samples from the walkway debris were within or near the 95 percent confidence interval estimate based on

the use of the linear equation developed using the NBS test data.

The ultimate capacity of box-beam hanger rod connections can be

predicted on the basis of laboratory test results. Estimates for the

mean box beam-hanger rod connection ultimate capacities and a 95 percent confidence interval estimate for the walkway box beam-hanger rod

connections are listed below.

Connection Mean Ultimate Capacity 95% Confidence Range

kips (kN) kips (kN)

SUE 18.2 (81) 16.3 to 20.2 (73 to 90)

8UW 19.3 (86) 17.3 to 21.4 (77 to 95)

9UE 18.2 (81) 16.2 to 20.2 (72 to 90)

9UW 18.3 (81) 16.3 to 20.3 (73 to 90)

lOUE 19.1 (85) 17.1 to 21.1 (76 to 94)

lOUW 18.3 (81) 16.3 to 20.3 (73 to 90)

; capacities ranged from 18.2 kips (81 kN) to 19.3 kips (86 kN) an average of 18.6 kips (83 kN).

Ultimate capacity for the single rod connection is about 2.0 kips

(9 kN) higher than for a double rod connection or an average value of

about 20.5 kips (91 kN).

118
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Table 6.6 Cross-sectional Dimensions

Sec. Property Weight

lb/ft

d

(in) (in)

tw(in)

Max Min

Nominal 8.5

Walkway Specimens 8.84

NBS Section 8.43

8.00 1.874 0.179

8.050 1.860 0.194 0.185

7.975 1.833 0.186 0.182

bf

tw

Ik

124
Table 6.7 Walkway Box Beam Weld Measurements

Box BeamHanger Rod

Connection

Exterior Weld Interior Weld

Total

Length, mm Area, mm^

Average

Depth, mm Length, mm AverageDepth, mm

SUE 36 1.8 36 1.8 129.6

8UW 35 4.5 157 .5

9UE 38 1.8 25 2.4 128.4

9UW 32 1.7 16

11*

2.4

3.4

130.2

lOUE 35 2.4 35 1.9 150.5

lOUW 33 1.5 33 2.5 132.0

* A portion of 9UVJ was deeply penetrated.

125
.1 5/16" DIA

1 7/8" DIA.

1.0 in. = 25.4 mm

O

o o

—8"— ^

Figure 6.1 Short box beam test specimen.

126
A

EXTERIOR WELD

DEPTH OF PENETRATION

EXTERIOR WELD BEAD

Figure 6.2 Exterior weld penetration.

PARTIAL PENETRATION

EXTERIOR WELD

< NO WELD

TACK

WELD

8"

Figure 6.3 No weld specimen,

127
Zone with

interior weld

Interior weld

Exterior weld Intentional gap

between roots

Figure 6.4 Interior weld location.

128
129
8'-6"

SELF-CONTAINED

TEST FRAME

9'-0"

4'-0"

3'-0"

FRAME

4'-6 3/16"- 4 -6 3/16"-

REACTION TRANSDUCER

RODS TO CEILING

UPPER WALKWAY BOX BEAM

UPPER WALKWAY LOADING SYSTEM

—3'-8 7/8" —

RODS TO LOWER

WALKWAY

3'-8 7/8"

LOWER REACTION BEAM

LOAD TRANSDUCER

UPPER ,WALKWAY

LOAD ACTUATOR

LOWER WALKWAY LOAD ACTUATORS

4'-2 3/16"- 4'-2 3/16"

7'-6" 7'-6"

1.0 in. - 25.4 mm

Figure 6.6 Detail of full length box beam test setup drawing.

130
Figure 6.8 Clip angle strap on full length box beam.

131
Figure 6.9 Pin and clevis arrangement on full length box beam.

132
Figure 6.10 Short box beam test.

TERMINATING HYDRAULIC NUT CYLINDER LOAD TRANSDUCER

/////.

REACTION BUTTRESS

/y

VERTICAL REACTION LOWER

SUPPORT LOADING ROD

1" STEEL

PLATE

TEST SPECIMEN

1-3 7/8"

r-7 7/8"

1" STEEL PLATE

^4'-3"

UPPER

LOADING ROD

:4'-3"-

1.0 in. = 25.4 mm

Figure 6.11 Short box beam test setup.

133
REACTION TRANSDUCER

UPPER REACTION BEAM

LP LVDTa

LVDT^

LP LP

BOX BEAM

RP

LP gLVDT

Plvdt

RP

LOWER LOADING BEAM

LOAD TRANSDUCERS

NOTE: LP - Linear Potentiometer

RP - Rotary Potentiometer

LVDT - Linear Variable Differential Transformer

Figure 6.12 Full length box beam instrumentation.

134
/////////////////////////////////////////

O LVDT

LVDT 13)

(6)

TRANSVERSE LVDTs

Figure 6.13 Short box beam instrumentation,

135
136
J9.8lt (F2)

... Type III

\ Type II

O.-IO 0.80 1.20 1.60 UPPER ROD DISPLACEHENT IN INCHES

2.00

Figure 6.15 In-plane load deflection curves for specimens C-3, F-2

,

and T-3.

.49.85 (F2)

TVPICSL SHORT BOX BEflPl LO<lD-D€FLECTION (TRftMSUERSE) CURUES

O.'IO 0.80 1.20 1.60 2.00

TRANSVERSE WEB DISPLACEMENT IN INCHES

Figure 6.16 Out-of-plane web load displacement curves for specimens

C-3, F-2, and T-3.

137
4 3 1 2 3 1 3 1 2 PROCESS

1 - GMAW

' o 1 2-FCAW

1Lo ol •J /

'-^

L.^ J

3 - SMAW-E7014

4 - SMAW-E7018

SHALLOW penetration" • Hard washer

^Data set for coinparison

Unhardened

washer

deep penetration

—

o Hard washer

—

No interior weld y{ interior weld 1 3/8" interior weld

Figure 6.17 Effect of weld process.

138
No weld

Shallow penetration

2Q Deep penetration

CD

Figure 6.18 Effect of exterior weld penetration,

0 '/2 1 V/l

INTERIOR WELD LENGTH |in|

Figure 6.19 Effect of interior weld length on peak load,

139
Figure 6.20 Effect of weld area on peak load, hardened washers.

140
25

20 95% CONFIDENCE BAND

CO

15 -

I 10 QP = 13.33 + 0.0377A

P = pullout load, kips

A = weld area, mm^

20 40 60 80 100 120

WELD AREA [mm^]

140 160 180

Figure 6.21 Effect of clip angles.

141
Figure 6.22 Effect of Initial upper rod load.

142
25

_ 20

CO

3 15

Oec^ 10

95% CONFIDENCE BAND

-P = 13.33 + 0.0377A

P = pullout load, kips

A = weld area, mm^

20 40 60 80 100 120 140 160 180

WELD AREA (mm^j

Figure 6.23 Effect of loading configuration.

1A3
Round Beve Square Bevei

Figure 6.25 Effect of nut shape.

144
WELD AREA |mm2|

Figure 6.26 Effect of washer presence.

145
25

_ 20

</> 95% CONFIDENCE BAND

15 : — ^-Q--«r

I 10

IxJ O- rP = 13.33 + 0.0377A

P = pullout load, kips

A = weld area, mm 2

R1, R2 - Weld position

N6 - Thick washer

N3, N4, N5 - Nut orientation KC4 - Walkway box beam

0 20 40 60 80 100

WELD AREA fmm2]

120 140 160 180

Figure 6.27 Effect of unhardened washers.

146
NUT FACE PARALLEL TO BEAM

LONGITUDINAL AXIS

NUT FACE PERPENDICULAR TO BEAM

LONGITUDINAL AXIS

Figure 6.28 Nut orientation.

147
P = 13.44 + 0.0363A

P = pullout load, kips

A = weld area, mm^

20 40 60 80 100

WELD AREA (mm^

120 160 180

Figure 6.29 Effect of weld area on pullout load for short box beams.

148
Figure 6.30 Comparison of full-length box beams with short box beam results.

149
Figure 6.31 Effect of weld area on pullout load for NBS tests.

150
SPECinEN KCL

UALKUAY BOX BEAD

o.-io ' o.'eo 1 .bo 1 .GO sToo CHANNEL DISPLACEMENT IN INCHES

Figure 6.32 In-plane load vs deflection curve for full length box beam

specimen taken from third floor walkway.

O.'lO 0.80 1.20 1.60 2.00

UPPER ROD DISPLACEMENT IN INCHES

Figure 6.33 In-plane load vs displacement curves for specimens KC1-KC4

and 13.

151
SPECIflEN KCl

UALKUAY SHORT BOX BEAfI

UPPER ROD DISPLACEnENT IN INCHES

Figure 6.34 In-plane load vs deflection curve for specimen KCl.

Figure 6.35 Deformed shape of walkway specimen KCl.

152
T3£

153
25

20

15

10

95% CONFIDENCE BAND

P = 13.18 + 0.0391A

5

- P = pullout load, kips

A = weld area, mm2

KCL - long walkway box beam with walkway nut and washer

KC1 - short walkway box beam with walkway nut and washer KC2 - short walkway box beam with walkway nut and washer KC3 - short walkway box beam with NBS nut and hardened washer

13 - short NBS box beam with walkway nut and washer

20 40 60 80 100

WELD AREA (mm2

120 140 160 180

Figure 6.37 Comparison of walkway test series with NBS test results.

154
25

20

00

g 15

Q.

O

10

95% CONFIDENCE BAND _ — — "S.^' Range ~

'jtL — —

Connection mmEB ^ «

P = 13.18 + 0.0391A

P = ultimate capacity, kips

A = weld area, mm^

20 40 60 80 100

WELD AREA Imm^

120 140 160 180

Figure 6.38 Estimated ultimate capacity for walkway box beam-hanger rod

connections.

155
156
7. NBS MATERIALS EVALUATION PROGRAM

7.1 INTRODUCTION

This chapter describes the procedures and results of various tests and

evaluations of NBS test materials and of materials obtained from the walkway

debris. The objective was to determine compliance with appropriate codes and

standards and to characterize the construction materials and the weldraents.

Section 7.2 describes visual inspection and the radiographic examination of

the longitudinal welds in a box beam removed from the walkway debris and assesses conformance of weld quality to AWS Dl.1-79, Structural Welding

Code-Steel [3.1].

Section 7.3 describes the tests to determine the mechanical properties of the

structural steel and weldments. These tests were performed to characterize the

materials and to determine whether the materials conformed to the requirements

of ASTM A36 (Standard Specification for Structural Steel) [7.1]. Properties

determined by these tests include ultimate tensile strength, yield strength,

elongation and reduction of area.

Section 7.4 describes the results of metallographic examinations and hardness

measurements carried out on parent material and on weld material and heat

affected zones associated with the box beam longitudinal welds. Also included

in this section are metallographic and hardness data obtained from hanger rod

specimens, from steel washers used in the walkway construction and from washers

used in the NBS structural testing program described in chapter 6.

Section 7.5 presents the results of chemical analyses to determine conformance

of the structural steel to the requirements of ASTM A36 and to determine, if

possible, the type of electrode used to fabricate the walkway box beams.

The results of physical tests carried out on concrete cores removed from the

walkway decks are described in section 7.6. Included here is the determination of bulk specific gravity, modulus of elasticity and compressive strength

of the concrete. Also included is the bulk specific gravity of the topping

material used to level the walkway decks.

157
Section 7.7 summarizes the conclusions drawn in this chapter.

7 .2 INITIAL INSPECTION

7.2.1 Visual

Section 0510 (Parts 1.4b and 3.3d) of the project specifications [2.1] allows

for the visual inspection and nondestructive spot testing of shop and field

welds by the owner's inspection agency. As noted in section 2.6 of this

report, inspection reports that were made available to the NBS investigative

team contained very little information that was relevant to the construction

of the walkways. Therefore, it is not known what, if any, nondestructive

testing was carried out on the walkway box beam welds. In the absence of

such information, the criterion used by NBS to determine weld quality was visual inspection as described in Par. 8.15.1 of AWS Dl.1-79. No cracks or piping porosity were observed. Weld profiles were normal. The weld quality

therefore conformed to the welding code requirements under visual inspection.

7.2.2 Radiography

A radiographic examination was made of the top and bottom longitudinal welds in

box beam 9M (NBS 3). The top longitudinal weld had been ground flush for the

entire length of the beam and the bottom weld had been ground flush for a short distance from each end as discussed in section 5.3.3.

The welds were marked off in 1 ft (300 mm) sections and numbered sequentially

beginning with the bottom weld at location 9MW (No. 1), traversing to 9ME

(No. 11), beginning again at the top weld at location 9MW (No. 12) and continuing along the beam to 9ME (No. 22). Tv/o exposures were made at each section;

one to achieve 2 percent radiographic sensitivity through the toe of the

flange adjacent to the center of the weld and another to achieve a readable

radiographic density at the center of the weld. Radiography was performed at

140 KVp using a 2 mm aluminum filter, a source-to-film distance of 1 meter

with focal spot of 0.4 mm x 0.4 mm. Two sheets of Kodak type M film were used

in flexible cassettes containing a 0.005 in (0.13 mm) thick front lead screen

and a 0.010 in (0.25 mm) thick back lead screen. Exposure conditions varied

from 8 to 36 milliarapere-minutes , depending on results desired for a particular

section. A reproduction of the radiographs of sections 1, 11, 12 and 22

encompassing the beam ends appears in figure A7.2.2-1 of the appendix.

The radiographs revealed porosity lying along the weld centerline in all

sections. Study of the fracture surface of a small length of weld at section

11 confirmed the presence of porosity. More porosity was evident between the

beam ends and the outer hanger rod holes than between the hanger rod holes.

Portions of the welds were free of porosity, suggesting that its development

coincided with the start of a weld run.

Linear indications on the radiographs apparently deviating from the weld

centerline at locations 7 in (178 mm) and 10 in (254 mm) from the end of beam

at section 1 had the appearance of cracks. These were not confirmed by

metallographic sectioning and fracturing the welds at these locations.

158
7.3 MECHANICAL PROPERTIES OF STRUCTURAL STEEL AND WELDMENTS

As has been described in chapter 6, all NBS box beam replicas were fabricated

from MC8 x 8.5 structural channel believed to have been produced from a single

heat. Tensile tests were performed to establish consistency within the lot and

to determine conformance to the requirements of ASTM A36 . Components from whic

specimens were machined included two walkway box beam segments, the full-length

walkway box beam, hanger rod segments, and NBS box beam channels. Tests were

also carried out to determine the tensile properties of the walkway box beam

longitudinal weldments.

7.3.1 NBS Channel Specimens

Specimens were machined from two NBS channel segments coded B and Y. Three

specimens transverse to the rolling direction and three longitudinal specimens

were machined from the web region and one longitudinal specimen was prepared

from each of the two flange regions of the B-coded channel. Two transverse and

two longitudinal specimens were machined from the web region and one longitudinal specimen was prepared from each of the two flange regions of the Y-coded

channel. All specimens conformed to ASTM E8-81 (Standard Methods of Tension

Testing of Metallic Materials) [7.1]. A representative specimen is shown in

figure 7.1, left side. These specimens were 8 in (200 mm) long, 0.500 +

0.010 in (12.70 + 0.25 mm) wide at the reduced section and had a 2.000 +

0.005 in (50.80 + 0.13 mm) gage length. The web specimens were tested full

thickness (ranging from 0.180 to 0.183 in (4.57 to 4.65 mm)) whereas the flange

specimens were machined to a uniform nominal thickness of 0.24 in (6.1 mm) to remove the taper.

Specimens were tested to failure in tension in one of three testing machines:

(A) a 25,000 kgf (245 kN) capacity Satec Systems machine (NBS No. 184093),

calibrated October 14, 1981, (B) a 50,000 Ibf (222 kN) capacity Gilmore testing

machine (NBS No. 172647), calibrated October 12, 1981, or (C) a 10,000 Ibf

(44.5 kN) capacity Gilmore testing machine (NBS No. 172648), calibrated

October 15, 1981. Estimated load inaccuracy is + 1 percent.

Strain data were recorded during the initial part of each test by an LVDT

extensometer attached to the reduced section of the specimen. Estimated strain

inaccuracy was + 3 percent. The extensometer was removed from the specimen

when the strain exceeded the yield point of the material. The testing machine

crosshead speed was maintained at 0.02 in (0.5 mm) per minute until the extenso

meter was removed from the specimen. Subsequent crosshead speed was 0.05 in

(1.3 mm) or 0.10 in (2.5 mm) per minute. Ultimate tensile strength, 0.2 percent offset yield strength, percent elongation in 2 in (50.8 mm), and reduction

of area at the fracture were calculated. Individual test results are listed

in table 7.1. Load-strain plots are given in section A7.3.1 of the appendix.

Representative load-strain curves for a longitudinal web specimen, a transverse

web specimen, and a longitudinal flange specimen are given in figures 7.2 - 7.4. Increasing load as a function of strain beyond 1.5-2 percent strain

noted on these curves reflects the onset of work hardening.

159
There were no significant variations of tensile properties between channels nor were there significant differences between longitudinal and transverse tensile

properties or between the web and flange region tensile properties. The average

ultimate tensile strength for all specimens was 71,000 psl (490 MPa) with a range of 70,500 to 71,500 psl (486 to 493 MPa). The average yield strength

for all specimens was 44,800 psl (309 MPa) with a range of 43,700 to 45,500 psl

(302 to 314 MPa). Average elongation was 33.5 percent with a range of 30.4 to

36.8 percent. Variation in strength was minor.

ASTM Standard A36 for structural steel requires an ultimate tensile strength

of 58,000 to 80,000 psl (400 to 552 MPa), a minimum yield strength of 36,000 psl

(248 MPa), and a minimum elongation of 21 percent in 2 in (50.8 ram). All values

for tensile strength, yield strength and elongation satisfy the requirements

of ASTM A36.

7.3.2 Walkway Box Beams

Two longitudinal and two transverse specimens were machined from the web regions

and one longitudinal specimen was machined from the flange region of each

channel piece of each of the two box beam segments (9U, 8L) and the full-length

box beam (9M) obtained from the walkway debris. The specific locations of the

specimens in the box beams are shown in section A7.3.2 of the appendix. Specimen identification, specimen orientation, and the face of the box beam from

which the specimen was machined are listed for each specimen in table 7.2.

Specimens were prepared in accordance with ASTM Standard E8-81 for rectangular

tension specimens and have the same dimensions as the specimens prepared from

the NBS channel. The web specimens were tested full thickness (0.147 to 0.189

in (3.73 to 4.80 mm)), whereas the flange specimens were machined to a uniform

nominal thickness of 0.24 in (6.1 mm) to remove the taper. Operation of the

LVDT extensometer and the machine crosshead speeds were as described in

section 7.3.1.

The results are presented in table 7.3. Load-strain plots are presented in

section A7.3.2 of the appendix. Representative load-strain curves are given

in figures 7.5 - 7.7 for a longitudinal web specimen, a transverse web

specimen, and a longitudinal flange specimen, respectively.

Neither the ultimate tensile strength nor the yield strength varied significantly

between the beam segments. In addition, there was no discernable variation

between longitudinal and transverse orientations in the web or between the web

and flange regions. Ultimate tensile strength values ranged from 69,000 to

71,500 psl (476-493 MPa) with an average of 70,100 psl (484 MPa). Yield

strength values ranged from 41,900 to 47,100 psl (289 to 325 MPa) with an average of 44,400 psl (306 MPa).

Average elongation for all tests was 32.3 percent, ranging from 27.6 to

38.0 percent. The average elongation for longitudinal specimens was about

10 percent greater than that for transverse specimens in the case of box beam

9U and about 18 percent greater in the case of box beam 8L. The difference in

160
average elongation between transverse and longitudinal specimens from beam 9M was less (about 4 percent).

The results from all tests fell well within the requirements of ASTM A36 as stated in section 7.3.1 of this report for tensile strength, yield strength and

elongation. The results from these tests agree very well with the results from

the NBS channel tests.

7.3.3 Walkway Hanger Rods

Two round test specimens were prepared from each of the two hanger rod segments

obtained from the walkway debris. These specimens were machined in accordance

with ASTM E8-81 for round tension specimens with a 5 in (127 mm) length,

0.500 + 0.010 in (12.70 + 0.25 mm) reduced section diameter and a gage length

of 2.000 + 0.005 in (50.80 + 0.13 mm). The locations of the specimens from

hanger rod segments NBS 5A and NBS 5B are shown in section A7.3.3 of the

appendix. A representative specimen is shown in figure 7.1. The testing

procedure was the same as that used for the NBS channel and the walkway box

beam specimens. The results of these tests are given in table 7.4 and a representative load-strain plot for one of the specimens is shown in figure 7.8.

Load-strain plots for all of the specimens are given in section A7.3.3 of the

appendix.

The ultimate tensile strength ranged from 62,500 psi (431 MPa) to 69,000 psi

(476 MPa), meeting the ASTM A36 requirement of strength specified as 58,000 psi

(400 MPa) to 80,000 psi (552 MPa). The 0.2 percent offset tensile yield

strength ranged from 35,900 psi (248 MPa) to 46,900 psi (324 MPa) with three of

the four specimens giving values over the ASTM A36 specified minimum of

36,000 psi (248 MPa). The yield strength of the remaining specimen was within

100 psi (690 KPa) of the specified minimum and is considered to have met the

criterion in view of the estimated + 360 psi (2.48 MPa) measurement inaccuracy.

Elongation values ranged from 36 to 42 percent, thereby conforming to the ASTM

A36 specification of 21 percent minimum.

It is concluded that the tensile properties of the walkway hanger rod material

satisfied the requirements of the ASTM A36 specification.

7.3.4 Walkway Box Beam Longitudinal Welds

Variation in the transverse strength of the walkway box beam longitudinal welds

was determined by testing 1.0 in (25.4 mm) wide tensile coupons to failure.

Weld caps were machined flush to simulate conditions near the beam ends. The

results, summarized in table 7.5, show a substantial variation in breaking load

among specimens taken from the same beam. The observed variability is not

unusual for the type of weldraent tested considering the small weldment area

sampled by each specimen. Figure 7.9 shov/s the variability in the amount of

weld metal from specimen to specimen at the fracture. This variability correlates reasonably well with the specimen breaking loads (table 7.5). Differences

in the amount of porosity found on the fracture surfaces of the specimens also

contribute to the ultimate strength variation.

161
7. A METALLOGRAPHY AND HARDNESS MEASUREMENTS

This section summarizes the results of metallographic examination and hardness

tests performed on box beam longitudinal welds, on walkway hanger rods and on washers used in the box beam-hanger rod connections. Details of these studies

appear in section A7.4 of the appendix.

7.4.1 Box Beam Weldments

Metallographic and hardness studies were performed on cross sections taken

from the longitudinal weld region of an NBS weldment and from walkway beams 9U,

8L and 9M. The regions examined included the channel flanges and portions of

the channel web as well as the weld and heat-affected zone.

Figure 7.10 illustrates the appearance of an NBS weldment made by the GMAW

process, while figures 7.11-7.13 illustrate the appearance of weld regions in

box beams 9U, 8L and 9M, respectively. Basic walkway weldment hardnesses were

normal. Maximum hardnesses were 92-95 Rockwell B (HRB) for the weld regions,

85-93 HRB for the heat-affected zones and 70-79 HRB for the unaffected parent

metal. Small zones of increased hardness (28-33 HRC) were noted in the heataffected zones of one section taken from location 9U and one section taken

from location 8L. The structure of the hard zones appeared to be tempered

martensite

.

Walkway weldment microstructure was normal and comparable to the NBS weldment

microstructure . The weld metal structure was primarily bainitic; the heataffected zones contained ferrite primarily as a network in prior austenite

grain boundaries plus bainite, partially spheroidized carbides and tempered

martensite; while the base metal structure contained ferrite and pearlite in a normalized structure.

Figures 7.10 to 7.13 illustrate a substantial variability in the contour of the

mating flange ends forming the faying surfaces for welding. The effect, coupled

with differences in weld penetration, creates regions of varying notch acuity in

the weld region. However, no cracks were observed to have initiated from such

notches in any of the sections studied.

A series of optical and metallographic measurements of longitudinal weld

penetration in the interiors of box beams 9U, 8L and 9M and at the ends of box

beams at locations 8UE, 9UE, 9UW and lOUE are summarized in figure 7.14. The

latter set was taken in the lower flange region between the beam end and the

outer hanger rod hole following weldment fracture. From figure 7.14 the weld

penetration data from the sectional specimens average about 2.2 mm. The data

from the fracture surface at location lOUE average about 2.4 mm, while the

average weld penetration at locations 8UE, 9UE, and 9UW is near 1.5 mm. All

weld penetration data from fractures represent minimum values, considering

that the surfaces examined represent final reductions in area at fracture.

It is known that the depth of weld penetration will be less at the beginning

of a weld run than it is after steady welding conditions have been attained.

Progressively increasing penetration from the beam ends toward the outer hanger

162
rod holes at locations SUE, 9UE and 9UW (figure 7.14) suggests that a "run-on"

tab was not used in walkway beam fabrication. Weld penetration at location

lOUE suggests that this beam was cut to length after the welding operation.

Evidence of such an operation was also noted at location 8UW during the site

investigation (section 5.3.3).

Interior tack welds were present at the bottom flanges of all beam ends

examined. The location of these welds relative to the beam ends and their

approximate overall depth as measured from photographs of the fractures are shown in figure 7.15. Estimates were made of the total area of interior tack

weld plus the longitudinal weld in the box beam length between the outer hanger

rod hole and the box beam end. The data in table 7.6 were compiled by measuring

the penetration depth of the longitudinal and tack welds along the fractures at

1 mm intervals on lOX photographic enlargements. Areas of both welds were calculated from the profiles. As illustrated by the sketch accompanying table 7.6,

the total cross-sectional area (T x B) was taken as the projected area of the

flange toes and the additional weld metal due to tack weld buildup beyond this area was not considered. Had this been considered, the percent weld area reported in table 7.6 would have been substantially increased.

In summary, typical box beam longitudinal weld penetration averages about

2.2 mm, approximately the depth from the flange surface to the point of tangency

of the mating sides. At some beam ends, due probably to weld start-up, the

depth of weld penetration is less than 2.2 mm. At these locations, however,

there is always an interior tack weld present and the cross-sectional area produced by a combination of longitudinal plus interior tack weld areas exceeds

the area achieved with only a longitudinal weld of penetration depth equal to

2 .2 mm.

7.4.2 Walkway Hanger Rods

Transverse and longitudinal sections removed from two walkway hanger rod

segments (NBS 5A and 5B) were examined raetallographically and were tested for

hardness. The raicrostructure , consisting of ferrite and pearlite in a normalized

condition, is considered typical of A36 structural steel. The microstructure

in NBS 5B was slightly coarser (ASTM grain size 7.5-8) than in NBS 5A (ASTM

grain size 9), but this variation is not significant. Hardness values were normal at 66-76 HRB.

7.4.3 Washers

Metallographic and hardness studies on washers from the walkway and washers used

in NBS tests proceeded in two stages. The initial effort was a general characterization of the walkway washers associated with the initial components of the

debris removed for analysis. These were identified as the upper washer from

the fourth floor to ceiling hanger rod at location 8UE, the lower washer from

location 8LW, and the two lower washers from box beam 9M. Two washers from

the NBS tests, one hardened and one unhardened, were included in this initial

study. Additional hardness tests were conducted on (1) three walkway washers

obtained as a result of a petition for access to additional debris material,

(2) a walkway washer identified as 9^W that had been used in an NBS test on

163
box beam 9M and had fractured during the test, and (3) three additional NBS

hardened washers. Results of the hardness tests, summarized in table 7.7,

confirmed that hardened washers had been used in the walkway construction.

Core hardness (obtained after removal of the soft decarburized layer) was also

measured on selected specimens and the data are included in table 7.7. The data

in table 7.7 indicate that the walkway washers were, on the average, slightly

harder than the NBS washers; walkway washer average was 41.6 HRC with a range of

34.5-45.0 HRC and NBS washer average was 39.1 HRC with a range of 36.0-41.9 HRC.

Little difference was noted between surface and core hardness.

Photomicrographs of the fractured washer (9MW) and of the NBS hardened washer

in figure 7.16 show a tempered martensite interior and a thin decarburized

layer on the surface. This is typical for hardened washers.

7.5 CHEMICAL ANALYSIS

Chemical analysis was performed on the base metal and on longitudinal welds of

box beams from the walkway and from box beams fabricated by NBS. An interior

tack weld from the walkway and specimens removed from the walkway hanger rods were also analyzed. Details of the locations analyzed and of analytical

procedures used are given in section A7 . 5 of the appendix.

Table 7.8 shows that the NBS and walkway structural channel and the walkway

hanger rod compositions are within the specifications of ASTM A36 for structural

steel

.

Table 7.9 compares the chemical composition of weld metal in NBS box beam

longitudinal welds made with various processes with the composition determined

for the walkway box beam longitudinal welds from 9U, 8L and 9M and the interior

tack weld from location 8LW. NBS welding procedures have been discussed in

section 6.10.2.

Similarity in composition between the walkway longitudinal and interior tack

welds suggests they were made by the same welding process. Similarity of com- position between the walkway welds and the NBS weld made with the flux-cored

electrode suggests this process was used in welding the walkway box beams. The

evidence is not conclusive, however, because solid welding wire giving the same chemical composition is available. The lower manganese and silicon contents

determined for weld metal deposited with the E7018 and E7014 stick electrodes

as compared to that of the walkway welds indicate that these electrodes were

not used in the fabrication of the walkway box beams.

7.6 PROPERTIES OF CONCRETE

Laboratory tests were carried out on the 19 cores removed from the concrete

decks to determine modulus of elasticity, compressive strength and bulk specific

gravity. The bulk specific gravity of the topping material recovered with the cores was also determined. Prior to conducting these tests, each core was photographed, weighed, and measured. In most cases the topping material had

separated from the main body of the core during the coring operation. In those

164
few cases where it remained attached, the topping was removed and the weight

and length measurements were repeated. After noting its general condition and

subjectively classifying its quality based upon surface smoothness, distribution of aggregate, and flatness of ends, each core was closely examined for the

presence of surface cracks or wire reinforcement. Results of this examination

are summarized in table 7.10.

Bulk specific gravity of the concrete and of the topping material was determined

for two conditions: (1) air dry as received from the warehouse and (2) after

oven drying for 24 hours. Sample preparation and weighing were carried out in

accordance with the provisions of A.STM C642-75 (Standard Test Method for

Specific Gravity, Absorption, and Voids in Hardened Concrete) [7.1] except for

the determination of saturated weight after boiling.

A total of nine cores was used to determine modulus of elasticity and

compressive strength. Each core was instrumented with two resistance strain

gages oriented in the lengthwise direction and positioned 180 degrees apart.

Gage length was 2 in (51 mm) and the gages were operated in a full-bridge

configuration. The cores were capped and tested in accordance with ASTM C617-76

(Standard Method for Capping Cylindrical Concrete Specimens), C469-79 (Standard

Test Method for Static Modulus of Elasticity and Poisson's Ratio for Concrete

in Compression) and C39-72 (Standard Test Method for Compressive Strength of

Cylindrical Concrete Specimens) [7.1].

Table 7.11 lists the unit weights, the modulus of elasticity (E) and the

compressive strength (f^) ior the various cores tested. The unit weights for

the concrete in the air dry condition are very consistent with an average of

113.5 pcf (1,820 kg/m3) and a range of 109.0 to 118.4 pcf (1,750 to 1,900 kg/m^)

based on results from 10 cores. For the oven dry condition the average was 108.1 pcf (1,730 kg/m3) and the range was 104.0 to 112.1 (1,670 to 1,800 kg/m"^).

The variation in unit weights obtained for the topping material is greater. For

the air dry condition the average of eight specimens was 116.8 pcf (1,870 kgm-^)

and the range was 101.5 to 135.2 pcf (1,630 to 2,170 kg/m^) . For the oven dry

conditions the average was 113.7 pcf (1,820 kg/m^) and the range was 99.1 to

133.9 pcf (1,590 to 2,150 kg/ra^).

Based on nine cores tested, the modulus of elasticity (secant modulus at

40 percent of compressive strength) averaged 1.86 x 10" psi (12.8 x 10^ MPa)

and ranged from 1.75 to 2.08 x 10^ psi (12.1 to 14.4 x 10^ MPa). The average

compressive strength based on these same nine cores and corrected for core aspect ratio was 5,030 psi (34.7 MPa) and ranged from 4,440 to 5,520 psi

(30.6 to 38.1 MPa). Measurements upon which the values listed in table 7.11

are based can be found in section A7.6 of the appendix.

The average unit weight for concrete of 113.5 pcf (1,820 kg/m^) compares with

a nominal value of 110 pcf (1,760 kg/m^) for lightweight concrete. The

average compressive strength of 5,030 psi (34.7 MPa) is well above the 3000 psi

(20.2 MPa) minimum for lightweight concrete fill on metal deck indicated in

the General Notes (Dwg. S601). There are no specific requirements stated for

modulus of elasticity.

165
7.7 CONCLUSIONS

The following conclusions are made subject to limitations of the materials

evaluated and the test variables selected:

1. Quality of the box beam longitudinal welds conformed to the visual

inspection requirements of AWS Dl.1-79 Structural Welding Code-Steel.

2. There were no significant differences in tensile properties and

chetnical compositions between NBS and walkway box beam channel

material.

3. Tensile properties and chemical compositions of NBS channels, walkway

box beam channels and hanger rod materials satisfy the requirements

of ASTM Standard A36 for structural steel.

4. Microstructure and hardness of the NBS channels, walkway box beam

channels and hanger rod materials were normal. Differences were noted

in grain size and in apparent pearlite-to-f errite ratio in the two

hanger rods examined; however, the differences are not considered

to be significant.

5. Microstructure and hardness of longitudinal and interior walkway box

beam welds were normal.

6. Longitudinal weld penetration in the box beam interior averaged about

2.2 mm, approximating the distance from the point of tangency of the

mating flange surfaces to the outside face of the channel flanges.

Some lesser penetration was noted near the beam ends in regions reinforced with interior tack welds; however, the presence of interior

tack welds considerably increased the total weldraent cross-sectional

area at these locations.

7. Hardened washers had been installed in the walkway. The hardness of

these washers was slightly higher than that of hardened washers used

in the NBS test series.

8. Compressive strength of lightweight concrete used in the walkway decks

exceeded the requirement of the project design criteria.

166
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Table 7.2 Walkway Box Beam Tensile Specimen Identification

Specimen Code

A. Box Beam 9U (NBS 1)

1-LSl

1-LS2

1-LNl

1-LN2

1-TSl

1-TS2

1-TNl

1-TN2

1-TFS

1-TFN

B. Box Beam 8L (NBS 2)

2-LSl

2-LS2

2-LNl

2-LN2

Specimen Orientation

longitudinal

Location

transverse

longitudinal

longitudinal

web

web

flange

web

south face

south face

north face

north face

south face

south face

north face

north face

top south

top north

south face

south face

north face

north face

2-TSl

2-TS2

2-TNl

2-TN2

2-TFS

2-TFN

C. Box Beam 9M (NBS 3)

3-LSl

3-LS2

3-LNl

3-LN2

transverse web

longitudinal

longitudinal

flange

web

south face

south face

north face

north face

top south

top north

south face

south face

north face

north face

3-TSl

3-TS2

3-TNl

3-TN2

3-TFS

3-TFN

transverse

longitudinal

web

flange

south face

south face

north face

north face

top south

top north

168
Table 7.3 Tensile Test Results - Walkway Box Beam Channel

Ultimate Tensile Yield Strength Reduction of

Specimen Strength, psi (^) 0.2% Offset, psi ^*^) Elongation, % (^) Area, % (^)

A. Box Beam 9U (NBS 1)

1 —T c:i oy , uuu JO . IJ JO

1-LS2 70,000 43,700 32.6 55

1-LNl 70,000 44,400 32.4 54

7 1 nno /i4 'lOO J , JUU ^A

, uuu A/i (inn J i . u A 7 1-TS2 69,500 45,800 30.4 43

1-TNl 71,000 47,100 29.6 48

1-TN2 71,000 44,700 33.8 47

J. i r o O 7 , J uu AT^ J AOO , t U U JO . 0 J 1 l-TFl'J 70,000 43,200 35.2 61

•

I. Box Beam 8L (NBS 2)

71 000 ZlS *-r y qoo 7 \J \J 9ft A J AH 2-LS2 69,500 43,700 33.0 53

2-LNl 71,000 46,800 33.6 54

9_T fj9 70/ u , Jsnnuu AAHH "^00 , JUU J H . o J O

<C> i. O -1. 7 1 SOO 45 600 99 6 48

2-TS2 70,500 45,200 21 .b 46

2-TNl 71,000 44,500 29.8 48

2-TN2 71,000 46,000 29.0 48

9c. —TTTQ Leo 70 000 AH_>S AOO , HUU JO1R . 0u J -/ 2-TFN 70,000 43,200 37.0 60

;. Box Beam 9M (NBS 3)

-J LiO J. 70/ 000 u , uuu A'X 900 nJ J, . AH JSAH 3-LS2 70,000 44,400 31.4 54

3-LNl 69,500 43,000 30.8 54

3-LN2 69,500 44,600 33.0 54

3-TSl 70,000 44,300 31.6 51

3-TS2 70,000 44,100 30.2 51

3-TNl 69,000 43,500 31.6 50

3-TN2 69,000 41,900 30.6 51

3-TFS 70,500 42,400 33.8 58

3-TFN 69,500 43,500 33.4 61

Rounded to nearest 500 psi in accordance with ASTM Standard E3-81.

(b) Rounded to nearest 100 psi in accordance with ASTM Standard E8-81

(c) Rounded to nearest 0.2 percent in accordance with ASTM Standard E8-81.

(d) Rounded to nearest 1.0 percent.

1000 psi =6.9 MPa

169
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*> >>h> r"

tl I <U Q)

tu •H o O

CO ,-1 CO c c ^ 1 cO QJ CO o o fM1 CO pi frt 4-1 in m *^

PS \j Vj in 00 CO CO Q

niw

(3 *

•rH

wJ •H c

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CO o O CM

O

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4J JJ 4-1 4J c CO CO CO CO

0) (U Q) (U 0) 0 I—1

I—1

<M S-i

u

u

•H

H

HH

H CO CO CO CO

O <: <: « CO <u <U

(U in in in c c c c o o o o

4J 4-1 4J 4J ^3 •o

0) (U 0) o c C! C

Pi o O o o u < pa Pi

dJ in to

W)

cn CO <~\

CO PQ pq CO IS

CO 04

CDCU Ooo

170
Table 7.5 Results of Tensile Tests on Walkway Box Beam Weldraents

Box Beam Specimen Maximum Load (lbs)

9U 1BX3W2 6740

1BX3W4 5890

8L 2BX3W2 4790

2BX3W4 2200

9M 3BW1 3920

3BW4 5100

1000 lbs = 4.448 kN

171
Table 7.6 Percent of Longitudinal Box Beam Weld Plus Interior Tack Weld

Area as a Function of Total Cross Sectional Area for Walkway

Beam Weldments

Beam End to Outer Hanger Rod Hole

Location

SUE

9UE

9UW

lOUE

Penetration to 2.2 mm

Percent Weld Area

63

55

58

92

48

LONGITUDINAL

WELD

TACK WELD

ZZZZZZ^

y y / / / /V y y y y y y

172
Table 7.7 Results of Washer Hardness Tests

Washer

Identification

8UE Top

8LW Top

9ME Top

9MW Top

8MW

9UE

lOMW

NBS Hardened (1)

NBS hardened (2)

NBS hardened (3)

NBS hardened (4)

NBS Regular Grade

Number of

Measurements

8

8

8

8

5/5*

5/5

5/5

5

4/5

5/5

4/5

5

Hardness

HRC (surface) HRC (core)

Average

40.2

43.0

37.5

44.0

40.0

43.6

40.2

39.6

39.0

37.5

40.5

Range

39.0-41.0

40.0-45.0

34.5-40.5

42.5-45.0

39.0-41.0

42.5-45.0

39.0-41.5

38.5-41.9

38.5-39.5

36.0-38.5

40.0-41.0

[60.7]** [58.9-61.3]**

Average Range

40.3

43.3

42.7

39.8

39.2

41.9

39.7-40.7

40.7-44.2

42.7

37.7-40.7

37.7-39.7

41.7-42.2

* surface/core

** Rockwell Hardness B (HRB)

Washer

Identification

8MW

9UE

lOMW

NBS hardened (1)

NBS hardened (2)

NBS hardened (3)

Microhardness Survey

Average HK5oo(HRC)

456 (44.3)

516 (48.4)

456 (44.5)

457 (44.5)

444 (43.5)

442 (43.1)

Range HK5oo(HRC)

395-476 (39.5-45.5)

485-532 (46.5-49.5)

450-457 (44.0-44.5)

323-465 (32.0-45.0)

173
Element

Carbon

Phosphorus

Sulfur

Table 7.8 Chemical Composition Range of Base Materials

Percent by Weight*

NBS BoxCa)

Beam Channel

0.179 - 0.183

0.012 - 0.013

0.022 - 0.023

Walkway Box(^)

Beam Channel

0.181 - 0.188

0.016 - 0.017

0.016 - 0.017

Walkway^ ^)

Hanger Rod

0.159 - 0.200

0.009 - 0.038

0.033 - 0.046

ASTM A3

6

Specification

Max 0.26

Max 0.04

Max 0.05

Notes

(a) Eight channels, two analyses per channel.

(b) Six channels, two or three analyses per channel.

('^) Two rods, one or two analyses per rod.

* Encompasses specimens 1BX4, 2BX1, 3BX1 and 2TFX4 (cf, table A7.5-1).

174
in o ON in O CO o o o

H • • • • o o 00 1—1 o >-< O 1 1 o 1 1 •H • I—

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o

<U 0^ oo <t

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o CO r-l CO O

r-H <r 00

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>^ TS

CO 3 1 1 1 1 1 5 4-1 ^ -H .—1 vO o

iH M t—

1—.—1 r-~- CO CJ 1—1 o o <Ti cn

rs o o o o O o

175
bOC

•H

4J

CO

<u EHo

4-1 u

o

CO

(U

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0) 4J

0)

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cou

HHoo

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u

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c

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o

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o

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2

0) cu o B O 3

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X x: X!

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k'S »^

x:

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I ON 00I 2

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176
Table 7.11 Properties of Concrete Cores

Core

Number

Unit Weight (pcf)

E

('^) (psi)

f'(b)

^psi)

Thickness (in) Concrete Topping

Concrete Topping Air Dry Oven Dry Air Dry Oven Dry

4-01 3.55 0.35 — — — — 2.08 X 10^ 5,450

02 — 0.60 114 .0 109.0 128.3 125.8

03 3.43 0.53 113.4 107.8 127 .8 12 5.8

04 3.44 0.44 114 .6 109.0 101.5 100.9

05 3.38 0.67 — — 1.78 4 ,940

06 3.43 0.53 114.0 109.6 128.3 122.7

07 3.43 0.68 — — — — 1.96 5,110

08 3.25 — 118.4 112 .1 — —

09 3.16 — 114.6 109 .0 — —

10 3.18 — — — — — 1.88 5 ,140

11 3.24 0.38 110.9 105.9 135 .2 133.9

12 3.35 0.41 — — — 1.81 4 ,790

13 3.45 0.38 — — — — 1.75 4,440

14 3.08 0.58 109 .0 104 .0 104 .0 99.7

15 2.92 0.73 113.4 107.8 103.4 99.1

16 2.91 0 . 68 1.75 4 ,550

17 3.59 0.72 112.8 107 .2 105.9 101.5

18 3.45 0.84 1.90 5,520

19 3.47 0.78 1.80 5,290

T Secant modulus at 40 percent of f

(b) Reduced to nearest 10 psi in accordance with ANSl/ASTM Standard C 42-74.

1 in = 25.4 mm

1 pcf = 16.02 kg/m3

1000 psi = 6.9 MPa

177
Figure 7.1 ASTM E8-81 tension test specimens. The rectangular specimen

on the left was obtained from channel material and the round

specimen on the right was obtained from hanger rod material.

178
spunod 'avon

179
spunod 'avon

180
o

d

CO

o

d

o

d

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181
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182
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183
184
COod oc0

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CO 9 o o c<

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u

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185
Figure 7.9 Fracture surfaces of walkway weldment tensile specimens. Both

halves of each fracture are shown. The bright regions represent

fracture and indicate the depth of weld penetration. Specimen

key is as follows. XI

1. 1BX3W2

2. 1BX3W4

3. 2BX3W2

4. 2BX3W4

5 . 3BW4

6. 3BW1

186
b

Figure 7.10 Transverse section through NBS weldment IC welded by the GMAW

process. Dark regions separating weld metal from flange base

metal are heat-affected portions of base metal. Aligned

indentations are microhardness measurements. a, XI; b, X8 1/2

187
Figure 7.11 Cross sections through weld regions of box beam 9U (NBS 1).

Sections were separated about 7 in (180 mm) along the beam

length. Microhardness indentations traverse the surfaces.

X 8 1/2

188
189
Figure 7.13 Cross sections through weld regions of box beam 9M. Sections

were separated about 7.5 in (190 mm) along the beam length.

Microhardness indentations traverse the surfaces. X8 1/2

190
1/1

fVI IB

1/1

1/1

1/1

t/1 n

inm

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cn

mI Ld

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EB

m

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191
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n

t/1

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rvj

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s-< 60

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h———I-———^ 1—-—H- —

SI IS] ISI

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(WW) Hidjd NDliyHi3N3d

192
Figure 7.16 Microstructures of washers from 9MW (NBS specimen 3), (a), and of

hardened washer used in NBS tests, (b) • Whiter layer near surface (top of illustrations) is decarburized layer. Interior

structure is tempered martensite. Etchant: Picral. X200

193

8. FRACTOGRAPHIC ANALYSIS

8.1 INTRODUCTION

This chapter describes the specimen preparation and application of optical and

scanning electron microscope (SEM) fractography to relate the microstructural

features of fractured or deformed portions of the walkway structure to the

mode of failure.

Section 8.2 reviews studies conducted on fractures of longitudinal welds

produced during testing of box beams fabricated in the NBS shops. Replica and

metal specimen preparation for fractography are discussed. Experience was gained in replicating fractures under conditions similar to those in the fields

and data were obtained which allowed comparison of the macroscopic aspects of

the fractures with those of the actual walkway structure.

Section 8.3 describes studies of fracture surface replicas from the walkway

debris

Section 8.4 reviews the results obtained from direct optical and scanning

electron microscopy on selected fractures removed from the walkway box beams.

Section 8.5 summarizes f ractographic analysis results.

8.2 SPECIMEN PREPARATION AND NBS FRACTURE RESULTS

Procedures used to replicate the fracture surfaces have been described in

section 5.5. The replication technique is capable of reproducing details of

the fracture surface with reasonable fidelity, provided that intimate contact

is achieved between the replication media and the surface (figure 8.1). Failur

to achieve contact produces artifacts in the replica that must be discounted

in the analysis. The replica is a mirror image of the original surface and

may have a different sense of image contrast—in one case the image contrast is

due to both topography and composition variation (8.1a) and in the other the

image contrast is due to topography alone (8.1b). Selection of a specific

replica from each surface for study was based on visual evaluation of the

195
largest Intact replica contact area and on the least amount of embedded rust

and other debris. The replicas were trimmed of excess plastic and attached to

a 1 in (25 mm) diameter SEM support stub by a small piece of doublesided adhesive tape. Three point contacts of either carbon or silver paint were then made

between the SEM stub and the replica corners to increase their support and to

provide electrical contact to the sputter coating later applied to the replicas.

A replica identification was also inscribed on the stub at this time.

Replicas were sputter coated by placing three replica-mounted stubs on the

pedestal of a DC sputter coating unit. The coater electrode was a circular

planar magnetron triode having a pulse mode deposition capability which allowed

a 5 second ON-OFF cycle. This maintained specimen temperature below 25° C. After reducing chamber pressure to 30 millitorr (40 kPa) , argon gas was bled

into the system to produce a stable pressure of 95 millitorr (127 kPa) . A

deposition time of 5 to 7 minutes with a 60 Au-40 Pd alloy at a target-tospecimen distance of 1 in (25 mm) and a plasma current of 15 mA was used. These

conditions produced an electrically conductive and thermally protective film

allowing examination and SEM photomicrography at magnifications ranging from

5 X to 2,000 X. Stereo pair views of the fracture surfaces and washer contact

areas were made by tilting the replicas approximately 5 to 6 degrees between

the first and second exposure from a nominal tilt angle of 0 degrees. These

were used for the analysis of the spatial relationships of the topographic

features

.

Metal specimens for direct SEM examination were cleaned in an ultrasonic tank

containing acetone or ethyl alcohol. The cleaning solution was changed as required to remove all water, oil, and loosely adhering debris. These specimens were then mounted on a spring-clip holder stub for SEM observation. All

residual magnetization was removed by passing the mounted specimens through a demagnetizing coil several times in different directions.

Figure 8.2 illustrates that the gross features of the fracture in the lower

longitudinal weld between the outer hanger rod hole and beam end of box beams

tested to failure were similar, regardless of whether the beam had been fabricated at NBS or had been obtained from the walkway debris. Rounded and piping

porosity are evident in both fractures. The sharply delineated weld region in

these figures indicates the absence of rust on the freshly fractured surfaces.

8.3 WALKWAY FRACTURE REPLICAS

Replicas from the walkway fracture and washer contact surfaces have been

described in section 5.5 and are listed in table 5.13.

8.3.1 General Observations

Rounded gas porosity (both large and fine) and piping (elongated) porosity

similar to that illustrated in figure 8.2 were evident in the fracture areas of

the interior tack and exterior longitudinal welds at locations SUE, 9UE, 9UW,

and lOUE. Fractographic examination indicated that these flaws did not serve preferentially as crack initiation sites. It appears that, in the case of the

lower exterior welds, cracks initiated at the root of the weld and propagated

196
through the porosity (figure 8.3). While many of the replicated sections provided reasonable resolution of the f ractographic features, figure 8.4 illustrates that in some cases a very tight oxide layer had modified the original

fracture surface.

8.3.2 Box Beam Longitudinal Welds

Different textures in the fracture topography provided evidence that cracks had

existed in the weld roots in the regions between the hanger rod holes and in

the regions between the outer hanger rod holes and beam ends prior to gross weld

failure at the time of collapse. Figures 8.5 (a-c) illustrate the appearance

of several such regions in the exterior longitudinal welds at locations 9UW,

9UE and 8UW.

As opposed to the lower exterior welds, fracture in the lower interior tack

welds initiated at the weld surface and propagated towards the weld root. An

example is shown for location 9UE in figure 8.6. Topographic features developed

by momentary crack front arrest and changes in crack propagation direction are helpful in defining the crack path (figure 8.3), while interpretation of microdimple patterns in the ductile fracture are useful in defining the point of

origin. These observations suggest that the crack observed in the tack weld

at location 8MW during the site investigation (figure 5.18a) indicated an initial stage of weldment failure. Fractures in the lower exterior longitudinal

welds were found to initiate at the root of the weld with the crack front propagating towards the outside surface of the box beam where the shear lip is

observed. These observations are consistent with deflection of the flange

regions upward during deformation prior to structural collapse.

No indications of fatigue failure were observed. The predominant failure mode

in the weld metal was ductile rupture, although small areas of cleavage were observed in the vicinity of the weld center line at locations 9UW and lOUE

(figure 8.7).

8.3.3 Washer/Nut Contact Area

Two regions of washer contact were typically observed on the lower surface of

the box beam flanges. The outermost region contains imprints of the washer

edge and shows relatively shallow striations reflecting slippage of the washer

in the initial deformation phase. Transition to a more intensely abraded region

was observed nearer the hanger rod hole (figure 8.8) for location 9UE.

Evidence for a slip-stick mode of failure in which the washer and/or nut

combination was periodically arrested was observed in the deeply abraded region

at all locations examined. One indication of this mode can be seen at location

9UE (figure 8.8), while multiple indications can be seen at location lOUE

(figure 8.9). This indicates that pull-through of the washer/nut assembly

proceeded in a slip-stick manner, with the hardened steel washer digging into

the comparatively soft steel of the flange.

197
Washers at locations 9UE and SUE were missing after the collapse. However,

figures 8.8 and 8.10 provide clear evidence that washers had been installed at

both locations.

8.3.4 Washers

As noted in chapter 6, some washers obtained from the walkway debris fractured

in the course of NBS beam tests while hardened washers purchased by NBS did not

fracture under similar test conditions. Examination of the fracture surface of

a walkway washer at location 9MW that failed during an NBS test conducted on walkway box beam 9M revealed that the washer had failed in a predominantly

brittle manner. This is evidenced by the dominant intergranular and cleavage

modes of failure illustrated in figure 8.11b. This is a magnified photomicrograph of the central region of the fracture surface shown in figure 8.11a.

By contrast, figure 8.11c shows predominantly ductile failure at the surface

decarburized layer of the washer.

Prominent elongated MnS inclusions can be observed on the fracture surfaces

in figures 8.11a and 8.11c. Hardened washers bent around a direction parallel

to such inclusions could be expected to display low ductility.

Other comparisons were made of the fracture characteristics of the walkway and

NBS washers. Walkway washers included for study were identified as NBS llB,

13b and 15B. Their original locations are described in table 5.13. Two ran- domly selected hardened washers purchased by NBS for in-house studies were

also included. These are designated NBS 2 and 3. These washers were initially

intact and were fractured by bending to provide surfaces for examination. In

addition, a section of a walkway washer that fractured into two pieces during

an NBS shortbeam test on walkway box beam 9M was also included. This latter

washer was designated KCl to correspond to the designation of the test in

which it fractured (chapter 6). Photomicrographs of the fractured surfaces of some of these washers are shown in figures 8.12 to 8.15. Macroscopic features

are illustrated by low magnification views while the microscopic features at

mid-thickness are detailed in higher magnification views.

These figures show that the fracture characteristics of the walkway and NBS

washers were basically similar. Both contained elongated stringers of MnS

(figures 8.12a and 8.14a). Fracture characteristics of the walkway and

NBS washers varied from predominantly ductile (figures 8.12 and 8.14) to

predominantly intergranular (figures 8.13 and 8.15). However, the walkway

washers displayed a higher percentage of intergranular fracture.

8.3.5 Clip Angle Fillet Welds

Replicas taken from the failed clip angle fillet weld surfaces at location 9UE

indicated ductile failure and that: a) the bottom fillet weld failed primarily

by shear normal to the box beam axis, b) the vertical side fillet weld failed

by shear with the top weld acting as a hinge, and c) the top fillet weld failed

primarily in tension. Figure 8.16 illustrates that failure of the top weld

initiated at the lower edge (location A), propagated upward, and terminated at

a shear lip at the upper edge (location C)

.

198
8.4 DIRECT ANALYSIS OF WALKWAY BOX BEAM FRACTURES

8.4.1 Procedures

As has been noted in chapter 5, one half of the lower fracture surfaces at the

following box beam-hanger rod connections were removed from the debris for

direct optical and SEM f ractographlc analysis:

Each specimen consisted of a section of the bottom flange cross cut to the

flange fracture surface, then cut 4 1/4 in (108 mm) inward from the beam end

along the web toe of the fillet as indicated on figure 8.17. Two layers of

duct tape were applied to the fracture surfaces for protection prior to cutting.

Upon delivery to the NBS laboratories the duct tape was removed, the specimens

were cleaned with paint thinner, rinsed in petroleum ether and photographed.

A third cut (figure 8.17) was then made with a jeweller's saw. Sections con- taining the fracture surfaces from the beam end to the outer hanger rod hole

were then prepared for f ractographlc analysis by ultrasonic cleaning under

acetone

8.4.2 Results

Figures 8.18 and 8.19 illustrate the overall appearance of the lower longitudinal

weld fractures between the outer hanger rod hole and beam end at locations 9UE,

9UW, 8UE and lOUE, respectively. Figures 8.20 and 8.21 illustrate the weld

fractures between the two hanger rod holes running from the outer hole to cut #2

(figure 8.17). The weld regions and other salient features are identified on these figures.

A much higher concentration of piping porosity is observed in the region from

the beam end to the outer hanger rod hole than between the two holes. This was also noted in the radiographic examination of walkway box beam 9M (section 7.2.2)

and suggests that the porosity is associated with the beginning of a weld run. There was no evidence of cracks preferentially initiating from this porosity.

Figure 8.18b suggests that the length of weld from the outer hanger rod hole

to the beam end was less at location 9UW than at other locations. However, this was not the case because the outer hole region at 9UW had been severely damaged

during pull-through of the hanger rod and some of the original metal at that

location had been fractured away or distorted out of the plane of the weld. The

fracture surface at lOUE (figure 8.19b) shows similar damage in the outer hole

region.

Evidence of cracks existing prior to final weld failure was observed at

location 9UE in both the lower exterior longitudinal and lower interior tack

weld regions between the outer hanger rod hole and beam end (figure 8.18a). A

very small area of preexisting crack was identified at location 8UE in the

SUE

9UE

9UW

lOUE

north half

south half

north half

south half

199
interior tack weld region (figure 8.19a). Areas of preexisting cracks were

noted in the longitudinal weld regions between the hanger rod holes at locations

9UE, SUE and lOUE (figures 8.20a and 8.21a,b). The difference in topography

in the preexisting crack region and in the region through which the final crack

propagated at location 9UE is illustrated on figure 8.22. The preexisting crack

region was heavily oxidized while the final fracture surface was relatively

clean and exhibits fine ductile fracture dimples. It is not certain whether

these preexisting cracks were formed during fabrication of the beams, during

construction of the walkways or after the walkways were put into service.

However, their locations in regions of maximum stress support the premise that

they were formed during construction or during service. While the final fracture generally initiated at the root of the lower exterior longitudinal weld

and propagated toward the exterior beam surface, and in the upper surface of

the interior tack weld and propagated toward its root, no specific evidence

was noted that preexisting cracks at these locations initiated the final

fracture preferentially.

8.5 SUMMARY

1. Gross features of the fractures of the lower longitudinal exterior

welds of both NBS fabricated box beams and walkway box beams were similar

.

2 . Rounded and piping porosity were observed in all lower longitudinal

exterior welds in the fourth floor walkway box beams, but no evidence

was noted of preferential crack initiation from such flaws.

3. The predominant failure mode in the lower longitudinal exterior welds was ductile rupture. Between the beam end and outer hole, crack initiation

appeared to be at the top of the interior tack weld with propagation

toward the weld root. Failure in the lower longitudinal exterior welds at

these locations initiated at the root and propagated toward the

exterior beam surface in a direction normal to the flange surface.

4. Evidence was obtained of cracks existing in the welds prior to final

weld failure during collapse. Their location supports the premise that

these cracks were formed during construction or during service. There

is no specific evidence that these preexisting cracks were the

preferential sites of final fracture initiation.

5. No evidence of a fatigue failure mode was observed.

6. Evidence indicates that washers at locations 8UE and 9UE, missing

after the collapse, had been initially installed.

7. Two regions of washer imprints were observed on the bottom surfaces

of the box beam flanges. The outermost region contains the imprint

of the washer and shows shallow striations. Nearer the hanger rod

hole, there was a transition to a more deeply abraded region. This

region showed evidence that the failure proceeded in a slip-stick

manner

.

200
8. The fracture characteristics of the wal.way and -S ushers va.ied^fro.

predominantly ductile to predominantly brittle.

washers displayed a higher percentage of intergranular fracture.

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9. STRUCTURAL ANALYSIS

9.1 INTRODUCTION

The walkway design loads and the loads believed to have been acting on the

walkways at the time of collapse are discussed in this chapter. Also discussed

are the adequacy of structural components and connections, walkway deflections

and the significance of dynamic loads.

Section 9.2 addresses dead loads, design live loads and the upper-bound live

loads associated with walkway occupancy at the time of collapse.

Section 9.3 presents the results of analyses carried out on walkway components

and connections to establish their compliance with the project specifications as set out in reference 2.1. Forces believed to have been acting on the box beamhanger rod connections at the time of collapse are addressed in section 9.4.

Section 9.5 describes the procedure used to determine walkway deflections at

the hanger rod locations due to dead load. Section 9.6 considers the matter

of dynamic excitation of the walkways and the significance of dynamic effects

when compared with the static loads discussed in section 9.2. Findings reported

in this chapter are summarized in section 9.7.

9.2 LOADS

In this section the walkway design loads and the loads believed to have been

acting on the walkway system at the time of collapse are discussed. While the

loads at the time of collapse will never be known exactly, it is possible to

place reasonable bounds on their intensity and distribution. For purposes of

structural analysis and comparisons with code-specified design loads, it is

convenient to differentiate between dead load and live load. Each of these

load categories is discussed separately in the following subsections.

9.2.1 Dead Load

The dead load includes the weight of all structural and architectural components

and materials that are permanently fastened to or supported by the walkways.

Normally the dead loads can be determined with sufficient accuracy simply

223
by preparing a list of quantities of materials from "as built" drawings and

then calculating total weights from nominal unit weights. As has been discussed in section 5.3.6, the observed variability in the thickness of the concrete deck and topping material, the limited number of representative thickness

measurements that could be made, and uncertainty with regard to unit weights

prompted the weighing of selected walkway spans.

The approach used to obtain dead load estimates was to take the total weights

of the damaged spans as listed in table 5.9 and deduct the estimated weights of

all walkway materials, including all debris on the decks, but excluding the

concrete deck with topping, shear studs and welded wire fabric. Weights of

the damaged concrete decks so calculated were then corrected for loss of con- crete and topping material during the collapse or during the rescue operation.

The total weight of each span prior to collapse was then estimated by adding on

the weights of all materials other than the deck that went into the "as built"

configuration. Details of these calculations are presented in section A9.2.1

of the appendix. It should be pointed out that any errors in estimating weights

of other materials attached to the damaged spans are reflected in the estimated

concrete deck weights.

Results of these calculations are presented as estimated weights of undamaged

spans in table 9.1. Allowing for the second to fourth floor hanger rods, the

upper walkway spans average some 550 lbs (249 kg) more than the lower spans.

This is consistent with the observation that more topping material was used to

level the upper decks. With regard to errors in the estimated span weights,

it is to be expected that the gypsum board could be a major contributor. Even

though selected gypsum board assemblies were weighed, it is likely that the

estimated weights are in error by as much as + 5 percent. This is also the case for other walkway components and architectural features such as the handrail assembly, footlights and carpeting. Weights of structural shapes could

be expected to vary from their nominal weights by + 2.5 percent. Based on

this, errors in the estimated span weights of the order of + 300 lbs (136 kg) are to be expected.

To provide an independent check on the above calculations, the estimated

concrete deck weights from table 9.1 are compared with calculated deck weights

based on a nominal deck thickness of 3 1/4 in (83 mm) and an air-dry unit

weight of 113.5 pcf (1,820 kg/ra-^) for the concrete as was determined in

section 7.6. An allowance of 200 lbs (91 kg) per span has been made to account

for the weight of shear studs and welded wire fabric. For a valid comparison,

it is necessary to deduct the weight of the topping material from the estimated

deck weight as is indicated in table 9.2. This adjustment is based on the

topping thicknesses listed in table 5.12 and on an air-dry unit weight of

116.8 pcf (1,870 kg/m^) for the topping material.

It is seen from table 9.2 that the estimated deck weights (excluding span L8-9)

average 500 lbs (227 kg) more than the calculated weights. In the case of

span U8-9 , this difference is 1,130 lbs (513 kg). If the average thickness of

the concrete cores (table 7.17) is substituted for the nominal deck thickness,

the calculated deck weights increase by approximately 150 lbs (68 kg), resulting

in an average difference between estimated and calculated deck weights of

224
350 lbs (159 kg). In view of the uncertainties involved in assessing the

weights of the damaged spans, the topping thickness, and the unit weight of the

topping material, this difference is not considered to be excessive. Based on

the average estimated deck weight of 39.0 psf (190 kg/m^) for seven spans, the

deck weight and total in-place weight of span L8-9 have been back-calculated as indicated in tables 9.1 and 9.2. From table 9.1, the average estimated in-place

weight of a walkway span is 17,7 50 lbs (8,050 kg).

In design calculations, the precise unit weights would not be known. If a nominal unit weight of 110 pcf (1,760 kg/m^) is assumed for lightweight con- crete and if no allowance is made for the weight of the topping material, the

calculated weight of the concrete deck with shear studs and welded wire fabric

averages 8,400 lbs (3,810 kg) per span. Assuming that the weight of all other

materials that make up a walkway span are as listed in table 9.1, the nominal

dead load due to each upper walkway span would be about 16,500 lbs (73 kN) , and

about 200 lbs (890 N) less for the lower walkway spans, as will be described

in further detail in section 9.3.1. Thus the actual span weights would, on average, exceed the nominal span weight by approximately 1,300 lbs (590 kg), or about 8 percent.

In section 2.7 it was noted that a modification of the walkways, involving the

addition of gypsum board to meet fire endurance requirements, was agreed to on April 4, 1978. It was also noted in section 2.4 that structural drawings S303

through S305 were issued March 30, 1978. Although these same drawings were issued for review on May 26 and June 30, 1978, it is not clear what, if any,

changes were made to the walkway structural details because of the addition of

gypsum board. Failure to account for this component of the dead load could be

significant as can be seen in table 9.3 which lists the principal contributions

to the dead load for a typical walkway interior span.

9.2.2 Live Loads

Live loads are those loads produced by the use and occupancy of the structure.

They include loads produced during maintenance and during the life of the

structure by movable objects and by people. The project design criteria listed

on structural drawing S601 imply a walkway design live load of 100 psf

(4.79 kPa). Section 2306 of the Uniform Building Code [2.3] allows for the

reduction of unit live loads based on the extent of floor area supported by a structural member. However, it specifically excludes live load reductions in

the case of places of public assembly or where the design live loads are in excess of 100 psf. The atrium and adjoining balconies in the hotel tower

section and function block are clearly places of public assembly. The walkways

are located within and connect places of public assembly and, therefore, it

would be inappropriate to apply a live load reduction factor.

In section 4.4 an assessment was made of the second floor walkway occupancy

based on the analysis of selected videotape frames. It was concluded that

approximately 40 people occupied this walkway shortly before the collapse. In

addition, it was concluded that 63 people, distributed as indicated in

table 4.1, represented a credible upper bound for the combined second and fourth

floor walkway occupancy at the time of collapse. To translate this occupancy

225
into an equivalent distributed live load, it is necessary to make certain

assumptions regarding body weight and centroidal position. Data presented in

reference 9.1 and summarized in reference 9.2 suggests a body weight of 159 lbs

(72 kg) for an average adult. Reference 9.3 presents data on centroidal locations (also summarized in reference 9.2) which suggest that for people standing

along the walkway railing, the distance from railing to body center of gravity

is approximately 16 in (406 mm). Given the box beam dimensions of figure 3.10,

it follows that approximately 80 percent of the live load distributed along

the east railing as described above is carried by the east line of walkway

hanger rods and end supports. Live loads due to people standing near the

center of the walkway were assumed to be distributed equally between the east

and west line of walkway hanger rods and end supports.

Using the average body weight and centroidal location indicated above, the

upper-bound walkway occupancy described in table 4.1 is translated into east and

west live load components acting in the plane of the fourth floor to ceiling

hanger rods. These loads, which in the following analysis are assumed to be

uniformly distributed over the length of a given span, are listed as total span

live loads in table 9.4.

9.3 ANALYSIS OF WALKWAY STRUCTURAL COMPONENTS

9.3.1 General

As a part of this investigation, the walkway structural components and their

connections were checked for compliance with the project specifications [2.1],

Specifically, this involved the checking of structural details and stresses

against the geometric limitations and working stress provisions of the AISC

Specification for the Design, Fabrication and Erection of Structural Steel for

Buildings [2.4] using the nominal dead load and specified design live load

discussed in section 9.2. In this analysis the walkway system was assumed to

be statically determinate, i.e., all stringer to box beam connections and the

shear connections at column line 7 were considered to be pinned. The nominal

dead loads are based on a unit weight of 110 pcf (1,760 kg/m^) for concrete,

a deck thickness of 3 1/4 in (83 mm) and the estimated weight of all other

materials for each span as listed in table 9.1. The area over which the 100 psf

(4.79 kPa) live load acts is based on the clear width of walkway (inside face

to inside face of railing glass) which is 8.25 ft (2.51 m) for the second and

fourth floor walkways. These loads are listed in table 9.5. Under the combined

action of nominal dead load and design live load, the maximum load acting on

a fourth floor box beam-hanger rod connection would be 40.7 kips (181 kN)

.

The maximum load on a second floor box beam-hanger rod connection would be

20.3 kips (90 kN) . From this analysis of the walkway system, it was concluded

that the box beam-hanger rod connections and the fourth floor to ceiling hanger

rods do not satisfy the requirements of the AISC Specification.

9.3.2 Box Beam-IIanger Rod Connections

The box beam-hanger rod connection detail used in the walkway construction is

not among the connection details suggested in the AISC Manual of Steel

Construction [9.4]. Without a properly validated analytical model or supporting

226
test data, there can be no reasonable certainty as to the load capacity of such

a connection.

The hanger rods apply concentrated loads to the flanges of the box beam. It is

well recognized that concentrated loads may cause a problem in steel structures

unless their effects are taken into account properly in design. Section 1.10

of the AISC Specification addresses this point. Section 1.10.5 indicates that

bearing stiffeners shall be provided, where required, at points of concentrated

load. Section 1.10.10 limits the compressive stress at the web toe of fillets

resulting from concentrated loads not supported by bearing stiffeners to

0.75 Fy; otherwise, bearing stiffeners shall be provided. It is common practice

to limit this compressive stress by providing a bearing plate for the concentrated load. The purpose of these provisions is to distribute the effects of

concentrated load over a larger area of the structural member and, by doing

so, prevent instability or local plastif ication of the web. It is implicit in

these provisions that the loads are applied in the plane of the web. These

provisions do not anticipate intentional and significant eccentricities of

applied load, as in the case of the box beam-hanger rod connections.

The absence of any stiffeners or bearing plates in the box beam-hanger rod

connections indicates that the effects of concentrated load were not given

proper attention. By depending solely on the channel flanges to transfer load

from the hanger rod to the webs, there can be no assurance that the webs are stable or that the web stresses are not critical. It is therefore concluded

that this connection detail does not satisfy the intent of the AISC Specification with regard to the proportioning of such connections.

9.3.3 Hanger Rods

With regard to hanger rod stresses, section 1.5.2 of the AISC Specification

allows a tensile stress equal to 0.60 Fy applied to a tensile stress area

equal to

0.7854 D

- Q-9743 ^ n

where D is the major thread diameter and n is the number of threads per inch.

For the 1 1/4 in (32 mm) diameter rods used in the walkway construction,

Fy = 36 ksi (250 MPa) and the allowable load per hanger rod is 21.3 kips

(95 kN) . The combined nominal dead loads and design live loads of table 9.5

call for a design capacity of 20.3 kips (90 kN) for the second to fourth floor

hanger rods and 40.7 kips (181 kN) for the fourth floor to ceiling hanger rods.

Under dead load alone, the fourth floor to ceiling hanger rods are loaded to

approximately 75 percent of allowable design load. The required design

capacity for the third floor walkway hanger rods would be approximately

23.1 kips (103 kN).

9.3.4 Summary of Analysis

To summarize the analysis of walkway components under the action of nominal

dead load plus specified design live load, it is concluded that without a

227
properly validated analytical model or supporting test data, there can be no reasonable certainty as to the load capacity of the box beam-hanger rod conne- ction used in the walkway construction. It is further concluded that the

connection detail does not satisfy the intent of the AISC provisions for the

proportioning of such connections.

With regard to hanger rods, it is concluded that the second to fourth floor

hanger rods complied with the the AISC provisions for allowable stresses, but

the fourth floor to ceiling hanger rods and the third floor walkway hanger

rods did not.

9.4 FORCES ACTING ON THE HANGER RODS AT TIME OF COLLAPSE

A two-dimensional finite element representation of the walkway system was used

to obtain estimates of forces acting on the hanger rods at the time of collapse.

At the box beams it was assumed that the length of channel flanges acting in

longitudinal walkway bending induced by the stringers was equal to 30 times the

average flange thickness. Both top and bottom box beam flanges were assumed to

be effective. This is consistent with the idealization used in the dynamic

analysis to be discussed in section 9.6.1. For the distribution of loads

applied after placement of the concrete deck, composite action was assumed.

The transformed section for one stringer was based on an effective width of

concrete deck equal to the sum of the stringer flange width and six times the

total depth of the deck [9.4]. The walkways were assumed to be pin-connected at

column line 7 and the hanger rod stiffnesses were modified to account for the

load/deformation characteristics of the box beam-hanger rod connections as described in chapter 6. The computer code used in the analysis is an in-house

version of GIFTS [9.5] .

•

Dead loads due to the span weights listed in table 9.1 were applied in three

stages. The first stage loading included the hanger rods, structural steel and

formed steel deck. Loads were distributed to the hanger rods with the assumption that the connections at the ends of each stringer were pinned. The second

stage loading consisted of the concrete deck weights without topping material

as listed in table 9.2. In this case the box beam flanges were considered to

be acting in longitudinal walkway bending as previously described. The final

stage included the topping material and all other v^alkway components with the

stringers and deck assumed to be acting as a composite.

The two-dimensional finite element model of the walkway system was also used to

distribute the upper-bound live loads of table 9.4 to the hanger rods and walkway end supports. Implicit in this approach is the assumption that the east

and west halves of the walkway system are uncoupled and compatibility between

east- and west-side vertical deflections can be ignored. In view of the small

load differential between east and west sides as indicated in table 9.4, this is

not considered to be a serious limitation. The resultant forces acting on the

walkway hanger rods and end supports due to combined dead load and upper-bound

live load are listed in table 9.6. It is seen from table 9.6 that the estimated

maximum load acting on a fourth floor to ceiling hanger rod is 21.4 kips (95 kN)

228
at location SUE. The estimated maximum load acting on a second to fourtli floor

hanger rod is 11.5 kips (51 kN) at location 8LE.

9.5 WALKWAY DEFLECTIONS

Walkway dead load deflections at the hanger rod positions are listed in

table 9.7 according to the composition of span weight listed in table 9.3.

Although little information is available regarding the walkway construction

sequence, it is reasonable to assume that the walkways were brought into final

vertical alignment after placement of the formed steel deck. Subsequent placement of the concrete and topping material would have caused an initial vertical

displacement of approximately 0.33 in (8.4 mm) at location 9L. Addition of the

handrail assembly would have increased this to 0.41 in (10.5 mm) and the addition of gypsum board and various finish materials would have resulted in a total

vertical displacement of approximately 0.5 in (13 mm) at location 9L. As has

been noted in section 3.4, the handrail assembly was adjusted to compensate for

midspan deflections, and it is reasonable to assume that a similar procedure was followed during installation of the gypsum board. Thus the cumulative displacements due to dead load V70uld be apparent only at the hanger rod locations.

Given the length of the walkways, it is doubtful that these displacements, if

detected, would have been cause for concern.

9.6 EFFECTS OF DYNAMIC EXCITATION

Thus far in this chapter the emphasis has been on statically applied loads and

their effect on various components of the walkway system. As has been noted in

chapter 4, there was at least one report of people dancing on the walkways at

the time of collapse. It is the purpose of this section to examine the dynamic

characteristics of the walkways, the manner in which activities such as dancing

could have excited the walkway system, and the significance of the resulting

forces when compared with the static load effects discussed earlier in this

chapter

9.6.1 Dynamic Characteristics of the Walkways

Preliminary to investigating the role of dynamic excitation in the collapse, the

walkway system, consisting of the second and fourth floor walkways, was modeled

analytically to determine the undamped natural frequencies. This work, which

involved the development of a three-dimensional discrete element analytical

model, is reported in reference 9.6.

Procedures used in the analysis were verified by modeling the third floor

walkway first. Results were then compared with field measurements reported in

reference 2.5. Essential features of the idealized third floor walkway are as follows

9 Walkway end supports : All supports are constrained in the vertical

direction. No rotational constraints are imposed. North-south translation is constrained at location 7ME. East-west translation is

constrained at locations 7ME and llME.

229
• Longitudinal beams : Assumed to be of continuous composite construction

(W16 X 26 stringer and portion of concrete deck). Longitudinal continuity is provided at each box beam by a short flexural element consisting

of portions of the deck and of the top and bottom flanges of the box

beam.

• Deck : Portions of composite concrete deck act with longitudinal beams in

resisting vertical and horizontal flexure. Deck also provides horizontal

shearing resistance and, for this purpose, is idealized as an equivalent

truss web of crossed diagonals.

• Masses : Masses are lumped at the nodes which correspond to the locations

of the stringer to box beam and stringer to cross beam connections.

Assumed weight per span is 19,000 lbs (8,620 kg). Live load is assumed

to be zero.

Calculated and measured dynamic characteristics of the third floor walkway are compared in table 9.8. For those modes that could be clearly identified in the

measured acceleration spectra of reference 2.3 the agreement is exceptionally

good.

With this confirmation of the analytical technique, the next step was to model

the second and fourth floor walkways as a combined system using essentially the same idealization outlined above. In the resulting model there were 610 static

and 120 dynamic degrees of freedom. Two cases were considered: (1) dead load

equal to 17,500 lbs (7,940 kg) per span and (2) dead load plus a superimposed

live load of 20 psf (958 Pa). Results of these analyses for the first 10

modes and the second horizontal mode (mode no. 17) are listed in table 9.9.

Note that the first two modes are essentially identical and correspond to

horizontal excitation of the second and fourth floor walkways, respectively.

Three other frequencies associated with the walkway system are of interest in

assessing dynamic effects:

• In addition to those constraints as described for the third floor

walkway end supports, constraints on north-south and east-west

translations at locations 7UW and 7LW.

- Dead load only, first horizontal mode, f = 4.98 Hz

- Dead load plus superimposed live load of 20 psf (960 Pa), first

horizontal mode, f = 4.43 Hz

• Simply supported walkway stringer with corresponding dead load,

vertical mode, f = 8.24 Hz

• Vertical vibration of hanger rod and masses due to dead load on associated second and fourth floor influence areas (2 degrees of

freedom), f = 8.11 Hz

230
9.6.2 Sources of Dynamic Excitation

In assessing the significance of dynamic excitation, the walkway system is

considered to be represented adequately by a single degree of freedom, viscously

damped system. The magnification factor, or ratio of the dynamic deflection

to the zero frequency deflection, is determined for the frequency of harmonic

excitation using the most critical walkway natural frequency. The ratio of the

induced force to the applied force is taken to be equal to the magnification

factor

Given the range of frequencies of the walkway system, both steady state harmonic

oscillations and transient vibrations need to be addressed. Considering first

the case of harmonic oscillations, the lowest natural frequency of the walkway

system in the vertical mode is 6.6 Hz for the case of a 20 psf (958 Pa) live

load. The corresponding frequency for the upper-bound live load, assumed to be

uniformly distributed over the two walkways, is approximately 7.1 Hz. Estimates

of the damping ratio, obtained from time histories of the third floor walkway

vertical mode accelerations presented in reference 2.5, ranged from 0.012 to

0.028. Considering the fact that these time histories were obtained at very

low levels of response, it is reasonable to select a damping ratio of at least

0.02 for this analysis.

Although no reference to actual measurements of dynamic excitation associated

with dancing could be found in the literature, reference 9.7 presents response

spectra for floor systems excited by a person treading at a fixed point. For

lack of a better model, this motion will be taken as an acceptable approximation of vertical motions due to dancing. A recording of the music being played

at the time of the collapse was analyzed and the tempo corresponds to a treading

frequency of about 1.1 Hz. For a natural frequency of 7.1 Hz and a damping

ratio of 0.02, the magnification factor from reference 9.7 is 0.6. Taking the

dynamic component of the treading load to be equal to body weight as suggested

by reference 9.7, the corresponding effective dynamic force is 0.6 times 159, or 95 lbs (422 N) per person. The force imposed on a hanger rod would depend

upon the mode shape of the walkway system and body location, but it can be

conservatively taken to be divided equally between a hanger rod pair, or 95 lbs

(422 N) per hanger rod for each couple dancing.

While the effects of horizontal vibrations on hanger rod forces would be

expected to be negligible, they are examined here for completeness. It is shown

in reference 9.6 that the walkways essentially respond as uncoupled structures.

From table 9.9, the lowest natural frequency for the horizontal mode is 2.87 Hz

for the case of a 20 psf (958 Pa) live load. The corresponding frequency for

the estimated fourth floor walkway dead load and upper-bound live load is

approximately 3.2 Hz. Taking the frequency of the horizontal excitation to be

one-half that of the vertical excitation, or 0.55 Hz, the corresponding magnification factor is 1.03. When the north-south and east-west translations at

location 7UT^ are constrained (pinned connections at column line 7), the natural

frequency is approximately 4.9 Hz and the magnification factor is 1.01. Assuming the higher harmonics of the horizontal component of forces applied by

dancing to be negligible, the walkway would be expected to respond in a quasistatic manner. Reference 9.8 presents data that suggest a horizontal component

231
of contact force equal to 32 percent of body weight for the case of right angle

turns while walking. Using this to approximate the horizontal component of

force due to dancing, the corresponding applied force would be approximately

100 lbs (445 N) for each couple dancing. Unlike the vertical forces, the

horizontal forces would tend to be randomly directed and would not be directly

additive

.

Based upon this brief analysis, it does not appear that dancing involving a reasonable number of walkway occupants would give rise to steady state

oscillations that would warrant a more detailed analysis.

Transient vibrations arise from impulse loads such as heel drop (sudden release

from tiptoe position, landing on heels). These forces are of short duration

(less than 0.060 sec) and their magnitude is strongly influenced by the hardness

of the contact surface. Reference 9.7 presents response spectra for heel drop

loading on floor systems and suggests a contact force equal to 3.75 times the

static contact force. The largest magnification factors occur at natural frequencies of approximately 16 Hz. In section 9.6.1 the vertical mode frequencies

for a simply supported walkway stringer and a hanger rod with tributary masses

are listed as 8.24 and 8.11 Hz, respectively. Taking a heel drop directly

adjacent to a hanger rod as the worst case, the corresponding magnification

factor from reference 9.7 is approximately 1.05. With body position taken to

be along the railing as described in section 9.2.2, the increase in hanger rod

loading due to heel drop would be approximately 0 . 8 [ ( 1 .05) ( 3 . 75) ( 159) - 159] = 375 lbs (1.7 kN) . Because of their short duration time, simultaneous heel

drops by two or more people would be dfficult to achieve.

Considering the fact that the fourth floor box beam-hanger rod connections

would respond inelastically at a load level equal to dead load plus upper-bound

live load and the fact that the walkway surface was covered with a commercial

grade carpet and underlying pad, the actual increase in hanger rod load would

be less than 375 lbs. With only a modest attenuation of -3dB, the increment of

hanger rod load due to heel drop would be reduced to approximately 0.71 x 375 = 265 lbs (1.2 kN) or about 1 percent of the average hanger rod load at the

fourth floor level under dead load plus upper-bound live load (see table 9.6).

This is within the range of uncertainty for estimates of span weights as pointed

out in section 9.2.1.

9.7 SUMMARY

Based on load data, relevant codes and standards, and analytical studies, the

following conclusions regarding the walkway system were drawn:

1. The average estimated in-place weight of a walkway span is 17,750 lbs

(8,050 kg).

2. Actual span weights would, on average, exceed the nominal span weights

by approximately 1,300 lbs (590 kg) or about 8 percent.

232
Under the combined action of nominal dead load and design live load,

the maximum load acting on a fourth floor box beam-hanger rod connection would be 40.7 kips (181 kN) . The maximum load on a second floor

box beam-hanger rod connection would be 20.3 kips (90 kN)

The box beam-hanger rod connection detail used in the walkway

construction does not satisfy the intent of the AISC provisions for

the proportioning of such connections.

The fourth floor to ceiling hanger rods and the third floor walkway

hanger rods do not comply with the AISC provisions for allowable

stresses

The maximum load believed to have been acting on a fourth floor to

ceiling hanger rod at the time of collapse is approximately 21.4 kips

(95 kN) at location SUE. For the second to fourth floor hanger rods,

this load is approximately 11.5 kips (51 kN) at location 8LE.

The maximum walkway deflection due to dead load was approximately

0.5 in (13 mm).

Dynamic loads induced by walking or dancing on the walkways would not

have been significant in comparison to the static loads.

233
Table 9.1 Summary of Span Weights

Weight in Damaged Condition

(lbs)

Estimated Weights of Undamaged Spans

(lbs)

Span

Number

Measured

Span Weight

Calculated Weights

of Materials Other

Than Deck

Concrete

Deck(a)

Concrete

Deck(a)

All Other

Materials

Total Weight

of Span

U7-8 15,760 6,130 9 ,630 9 ,630 8,090 17 ,720

U8-9 15,950 6,240 9,710 10,140 8,200 18,340

U9-10 14,600 4,850 9,750 9,950 8,210 18,160

UlO-11 14,000 4,530 9,470 10,150 8,120 18,270

L7-8 15,130 5,630 Q Kr\r\ y , jUU y , DUU / , you

L8-9 Span weight not measured 10,050(t>) 7,940 17,990

L9-10 14,160 4,800 9,360 9,380 7,950 17,330

LlO-11 13,170 4,570 8,600 8,700 7,990 16,690

Note: (a) Includes topping material, shear studs and welded wire fabric.

(b) Deck weight based on average unit deck weight of all other spans

as indicated in table 9.2.

1 lb = 0.4536 kg

234
Table 9.2 Comparison of Concrete Deck Weights Based on Measured Span Weights

and on Measured Unit Weights

Weight (lbs)

Span

Number

Estimated Weight

of Deck(a)

(see table 9.1)

Weight of

Topping Material

(Y = 116.8 pcf)

Weight of Deck

Without Topping(^)

Calculated Deck

Weight(^)

(y = 113.5 pcf)

U7-8 9,630 360 9,270 8,640

TTR-9 10,140 450 9,690 8,560

U9-10 9,950 860 9,090 8,560

UlO-11 10,150 1,080 9,070 8,650

L7-8 9,500 390 9,110 8,640

L8-9 10,050 990 9, 060(b) 8,560

L9-10 9,380 450 8,930 8,560

LlO-11 8,700 90 8,610 8,650

Note: (a) Includes shear studs and welded wire fabric.

(b) Based on average unit deck weight of 39.0 psf for seven other

spans

1 lb = 0.4536 kg

1 pcf = 16.02 kg/ra3

235
Table 9.3 Composition of Span Weight for Typical Walkway Interior Span

Walkway Weight Percent of

Component (lbs) Total

Walkway deck(^) 9,880 55.0

Handrail assembly 2,450 13.6

Structural steel 2 ,370 13.2

Gypsum board 2,310 12.9

Other(t)) 950 5.3

Total 17,960 100.0

Notes

:

(a) Includes topping material, shear

studs and welded wire fabric.

(b) Includes formed metal deck,

footlights, carpet, sprinkler

pipes and aluminum slats.

1 lb = 0.4536 kg

236
Table 9.4 Upper-Bound Live Loads for Second and Fourth Floor Walkway Spans

Resultant Live Load Acting in Plane of Hanger Rods

Span East Side West Side

Number (lbs) (lbs)

U7-8 382 95

U8-9 254 64

U9-10 127 32

UlO-11 127 32

L7-8 2,162 1,018

L8-9 2,003 859

L9-10 1,590 636

LlO-11 509 127

1000 lbs = 4.448 kN

237
Table 9.5 Span Loadings Based on Nominal Dead Load and Design Live Load

Span

Number

Nominal

Dead Load

(lbs)

Design

Live Load

(lbs)

Total

Design Load

(lbs)

U7-8 16,500 24,400 40,900

U8-9 16,500 24,100 40,600

U9-10 16,500 24,100 40,600

UlO-11 16,500 24,400 40,900

L7-8 16,300 24,400 40,700

L8-9 16,200 24,100 40,300

L9-10 16,300 24,100 40,400

LlO-11 16,400 24,400 40,800

1000 lbs = 4.448 kN

238
Table 9.6 Forces Acting on Walkway Hanger Rods and End Supports

Due to Combined Dead Load and Upper-Bound Live Load

Location Dead Load

Component

(lbs)

Upper-Bound Live

Load Component

(lbs)

Total

v-LDs;

7TTF / U Ci J , ouu 9 OA A nnn 7UW 3,800 60 3,860

OU Ci 9 AQO 9 1 "^on 8UW 18,900 1,060 19,960

9 n 9 Qn

9UW 18,210 830 19,040

1 otttt 1 ft Ann 11

1 QO , i 7U 1 Q 7Qn

lOUW 18,600 420 19,020

1 1 TIP 1 1 U Cj DU 1 QQn

IIUW 3,930 20 3,950

7t F J , o _)vj so Q SO H , O W W 7LW 3,850 450 4,300

OljEi Q 9 7n 9 9 no 1 1 A70

8LW 9,270 1,000 10,270

Qt p 1 RAn 1 0 QOO

9LW 9,040 770 9,810

lOLE 8,860 1,060 9,920

lOLW 8,860 380 9,240

IILE 3,690 180 3,870

IILW 3,690 40 3,730

1000 lbs = 4.448 kN

239
Table 9.7 Walkway Vertical Displacements Due to Dead Load

Dead Load Component

Vertical Displacements (inches)

Location

81I&10U 9U 8L&10T, 9T.

Concrete deck and topping .22 .23 .32 .33

Handrail asspmblv .06 .06 .08 .08

Gypsum board .05 .05 .08 .08

Other items .01 .01 .02 .02

1 in = 25.4 mm

240
Table 9.8 Calculated and Measured Characteristics of the Third Floor Walkway

Calculated Measured

Mode Period Frequency Frequency

NoClassification* (Sec.) (Hz) Classification* (Hz)

1 Hor

.266 3.76 Hor. 3-3/4

2 Vert. .127 7.88 Vert. 7

3 Tors

.127 7.88

4 Vert. .122 8.20 Vert. 8-1/4

5 Tors. .118 8.46

6 Vert. .112 8.91 Vert

10

7 Vert. .106 9.46

8 Tors

.103 9.70

9 Tors . .094 10.65

10 Hor. .068 14.64 Hor. 12

* Predominant motion.

241
9.9 Calculated Characteristics of the Second and Fourth Floor Walkways

Mode

No. Classification*

No Live Load 20 psf Live Load

Period

(Sec.)

Frequency

(Hz)

Period

(Sec.)

Frequency

(Hz)

1 Hor

.

.310 3.23 .349 2.87

2 Hor. .310 3.23 .349 2.87

3 Vert

.

.135 7.42 .152 6.60

4 Vert. .134 7.44 .151 6.62

5 Vert. .129 7.76 .145 6.90

6 Tors. .125 7.98 .141 7.10

7 Tors. .124 8.03 .141 7.14

8 Tors. .123 8.14 .138 7.23

QJ VcLC/ J.LII.O .122 8.21 137 7 30

10 Tors. .122 8.21 .137 7.30

17 Hor. .079 12.65 .089 11.25

* Predominant motion.

242
10. INTERPRETATION OF TESTS, ANALYSES AND OBSERVATIONS

10.1 INTRODUCTION

This chapter evaluates the results of structural tests and the analysis of the

walkway system for the purpose of determining the most probable cause of the

collapse. The probable sequence of collapse, effect of change in hanger rod

arrangement, and quality of materials and workmanship are also addressed.

Section 10.2 makes a direct comparison of estimated fourth floor box beam-hanger

rod connection capacities with loads believed to have been acting on the

connections at the time of collapse.

Section 10.3 compares the mean ultimate capacities of the box beam-hanger rod

connections with the ultimate capacity that would be expected of a connection

designed in accordance with the AISC Specification.

Section 10.4 combines observations of damage described in chapter 5 of this

report to establish the probable sequence of the collapse.

Section 10.5 addresses the significance of the change from a continuous hanger

rod to interrupted rods in the second and fourth floor walkway construction.

Quality of materials and workmanship is discussed in section 10.6. The findings

and conclusions of this chapter are summarized in section 10.7.

10.2 COMPARISON OF LOADS AND BOX BEAM-HANGER ROD CONNECTION CAPACITIES

A layout of the 6 fourth floor box beam-hanger rod connections is shown in

figure 10.1. Two bar diagrams, one for load capacity of the connection and

one for imposed load, are shown adjacent to each hanger rod. The bar diagram

on the left side of the figure will be discussed in section 10.3.

Location SUE is described in detail to illustrate the meaning of the diagrams.

The bar diagram to the left of the hanger rod denotes load capacity of the

connection. The range from 16.3 to 20.2 kips (73 to 90 kN) is the 95 percent

confidence interval estimate for the ultimate capacity of this particular box

beam-hanger rod connection based on tests conducted on NBS fabricated specimens

243
as reported in section 6.13. The estimated mean ultimate capacity of 18.2 kips

(81 kN) is also shown. The connection at location 8UE was required to support

forces imposed by combined dead and live load actions. These forces are shown

in the bar diagram to the rigVit of the hanger rod. Of the 6 fourth floor box

beam-hanger rod connections, the connection at location SUE was the most heavily

loaded. The estimated dead load action at location 8UE is 18.9 kips (84 kN)

and the contribution of the upper-bound live load at the time of collapse is

2.5 kips (11 kN), resulting in a total load of 21.4 kips (95 kN) as addressed

in section 9.4 and as listed in table 9.6. Under the action of nominal dead

load plus code-specified live load, this load would have been 40.7 kips

(181 kN). Thus the maximum load acting on a fourth floor box beam-hanger rod

connection at the time of collapse was 53 percent of what was required for

design under the Kansas City Building Code.

Figure 10.1 presents similar information for the other fourth floor box beamhanger rod connections. This figure shows that, with the exception of locations 8UE and lOUW, the estimated mean ultimate capacity of each fourth floor

box beam-hanger rod connection either equals or slightly exceeds the estimated

dead load effect. However, the estimated mean ultimate capacity is exceeded

by the sum of the estimated dead load and upper-bound live load in every case.

As noted in section 6.12.2, the data base for connections tested using box beam

specimens, nuts and washers removed from the walkway debris consists of only

three points. \{hen compared with the NBS box beam test series, all three points

are within or near the 95 percent confidence band, but there is a tendency

toward the upper limit of the band. The MC8 x 8.5 shapes used in the walkway

box beams are slightly heavier than those used to fabricate the NBS specimens

(see table 6.6). Of particular significance is the larger average web thickness

(0.190 vs 0.184 in (4.83 vs 4.67 mm)) which would increase the bending resistance of the webs. It is also possible that the actual live load on the walkways at the time of collapse was less than the upper-bound live load used to

arrive at the estimated hanger rod loads shown in figure 10.1. Recall from

chapter 4 that approximately 40 people were observed on the second floor walkway

shortly before the collapse and that 63 people are considered to be a credible

upper bound on combined second and fourth floor walkway occupancy.

Regardless of these qualifications on estimated loads and ultimate capacities,

it is clear that each of the 6 fourth floor box beam-hanger rod connections

had a high probability of failure; each connection was a candidate for initiation of walkway collapse. Failure of any one of the connections would have

been accompanied by a redistribution or transfer of load to other hanger rods.

With no apparent significant reserve capacity to accommodate this redistribution, progressive failure of the remaining connections was assured. Thus,

failure of any one connection would have led to complete collapse of the

walkway system.

10.3 ULTIMATE CAPACITY BASED ON CODE REQUIREMENTS

It is important to point out that the load capacities described in figure 10.1 are ultimate values. As noted in section 9.3.3, the design load (estimated

nominal dead load plus code-specified design live load) for a fourth floor to

244
celling hanger rod Is 40.7 kips (181 kN) . This Is the load that should be

used In a working stress approach to design in which the nominal strengths of

materials are reduced by an appropriate factor of safety.

Because of the safety factor, the resulting connection would be expected to have

an ultimate load capacity substantially higher than the working stress value of

40.7 kips (181 kN). Using a factor of safety of 1.67, which Is the lowest value

that would govern any part of the design of the box beam-hanger rod connections

under the applicable provisions of the AISC Specification, It would be expected

that the ultimate load capacity of the resulting connection would be at least

1.67 times 40.7, or 68 kips (302 kN) . Thus any given box beam-hanger rod con- nection In the fourth floor walkway should have been able to support an ultimate

load of at least 68 kips (302 kN) Instead of the average ultimate capacity of

18.6 kips (83 kN) estimated for the 6 fourth floor walkway connections. Since

the AISC Specification is the basis for the steel design provisions of the

Kansas City Building Code, this same ultimate load capacity would be expected

of a connection designed under the Kansas City Building Code. The hanger rod

load to be applied in working stress design under the Kansas City Building

Code and the expected ultimate load capacity are shown in the bar diagram on the left in figure 10.1.

10.4 PROBABLE SEQUENCE OF COLLAPSE

While the collapse could have initiated at any of the connections, the physical

evidence described in chapter 5 points to 9UE as the initiating point. To

summarize that evidence, the observed mirror symmetry of distortions about box

beam 9U is a compelling argument for initiation of collapse at column line 9.

As has been discussed in sections 5.2.5 and 5.3.2, observations made in the

atrium and on the debris at the warehouse strongly suggest that the west side

box beam-hanger rod connections of each of the three fourth floor box beams

failed after the corresponding east side connection.

The sequence of collapse is believed to have been loss of support at location

9UE with loads being transferred to locations SUE, 9UW, and lOUE. It is not

entirely certain which of these three connections failed next, but it is clear

that all three box beams rotated downward about the west line of hanger rods.

With the failure of connection 9UW, spans U8-9 and U9-10 would have rotated

downward about box beams 8U and lOU, respectively, and in the process, would

have pulled span UlO-11 off of its bearing seats at column line 11. The nature

of the clip-angle weld failures on box beam 9U suggests that spans U8-9 and

U9-10 separated during their downward rotation. At that same time the fourth

floor walkway would have experienced severe negative bending at column lines 8

and 10. However, the evidence suggests that the clip-angle welds at these

locations did not fall until after the box beam-hanger rod connections at

locations 8UW and lOUW had failed (see section 5.3.4). In both cases the clipangle flilet welds fractured from bottom to top, indicating positive bending

moments in the walkway at column lines 8 and 10. In the case of box beam 8U it

is likely that positive moment was induced by the north end of span U8-9 striking the second floor walkway while span U7-8 was rotating downward about the

shear connections at column line 7. The behavior of the second floor walkway

245
during the collapse is much less clear, but it is believed to have generally

followed the progression of failure in the fourth floor walkway.

10.5 EFFECT OF CHANGE IN HANGER ROD ARRANGEMENT

As has been noted in chapters 3 and 5, the hanger rod detail actually used in

the construction of the second and fourth floor walkways is a departure from

the detail shown on the contract drawings (Dwgs. A412 and A508) . In the original arrangement each hanger rod was to be continuous from the second floor

walkway to the hanger rod bracket attached to the atrium roof framing. The

design load to be transferred to each hanger rod at the second floor walkway

would have been one-half the sum of the dead load and the resultant live load

for a single span, or approximately 20.3 kips (90 kN) (see table 9.5). An

essentially identical load would have been transferred to each hanger rod at

the fourth floor walkway. Thus the design load acting on the upper portion of

a continuous hanger rod would have been twice that acting on the lower portion,

but the required ultimate capacity of the box beam-hanger rod connections would

have been the same for both walkways (20.3 kips).

As has been described in chapter 3, the hanger rod configuration actually used

consisted of two hanger rods: the fourth floor to ceiling hanger rod segment as originally detailed and the second to fourth floor segment offset 4 in (102 mm)

inward along the axis of the box beam (see figure 10.2). The washer and nut

arrangement called for in the original connection detail as shown on Dwg. S405.1

was retained in the actual construction. With this modification the design load

to be transferred by each second floor box beam-hanger rod connection was unchanged, as were the loads in the upper and lower hanger rod segments. However, the load to be transferred from the fourth floor box beam to the upper

hanger rod under this arrangement was essentially doubled, thus compounding an already critical condition. The design load for the fourth floor box beamhanger rod connection would be 40.7 kips (181 kN) for this configuration.

Had this change in hanger rod detail not been made, the ultimate capacity of

the box beam-hanger rod connection would still have been far short of that

expected of a connection designed in accordance with the AISC Specification.

In terms of ultimate load capacity of the connection, the minimum value should

have been 1.67 times 20.3 or 33.9 kips (151 kN) . Based on test results reported

in chapter 6, the mean ultimate capacity of a single-rod connection is approximately 20.5 kips (91 kN), depending on the weld area. Thus the ultimate capacity actually available using the original connection detail would have been

approximately 60 percent of that expected of a connection designed in accordance

with the AISC Specification. Note that, because of the greater dead load and

design live load, the third floor walkway connection would have had approximately 53 percent of the expected ultimate capacity. Had the change in hanger

rod arrangement not been made, the third floor walkway would have been the

most critical of the three.

10.6 QUALITY OF MATERIALS AND WORKMANSHIP

With the possible exception of the box beam longitudinal welds, there is no evidence to indicate that either quality of materials or quality of workmanship

246
played any significant role in initiating the collapse. As noted in chapter 7,

tensile properties and chemical compositions of the walkway box beams and

hanger rods were found to comply with the requirements of ASTM Standard A36

for structural steel which was specified. Microstructure and hardness of the

box beam welds and the box beam and hanger rod materials were normal. Compressive strength of the lightweight concrete used in the walkway decks exceeded

the requirement of the project design criteria.

Although misalignment of the embedded plates at column line 7 required a retrofit of the stringer bearing pads on the north span of the fourth floor

walkway, direct measurements and observed scratch marks indicate that the walkways were properly aligned with the bearing seats prior to collapse. There was sufficient room to install a nut and washer on the lower end of each fourth

floor to ceiling hanger rod, and there is clear evidence that all hanger rod

washers were installed.

In general, the structural welding exhibited good workmanship. Although certain

deficiencies were observed in the field welding of the clip angles to the

embedded plates at column line 7, these connections experienced large rotational

distortions prior to failure of the fillet welds, thus indicating good weld performance. Dimensions of fillet welds were, with few exceptions, in agreement

with what was called for on the shop drawings.

The welding symbol and field observations relevant to the box beam longitudinal

welds have been discussed in sections 3.3 and 5.3.3, respectively. As noted in

section 3.3, the welding symbol used on the shop drawings is interpreted to

require a prequalified partial joint penetration groove weld. However, the

geometry of the mating surfaces as discussed in section 5.3.3 and as illustrated

in figure 10.3 does not satisfy the dimensional tolerances for a prequalified

partial joint penetration groove weld as stated in section 2.10 of AWS Dl.1-79;

the root face is less than 3 mm (Item 2.10.2.4), and the required minimum

root opening was not provided.

For the case of an MC8 x 8.5 shape, flange thickness at the toe is approximately

3/16 in (5 mm). According to AWS Dl.1-79 (table 2.10.3), the corresponding

minimum effective throat for partial joint penetration groove welds is 3 mm.

As noted in section 7.4.1 of this report, joint penetration averaged 2.2 mm

at interior sections of the fourth floor walkway box beams. Data obtained

from fracture surfaces at the ends of the box beams indicate average joint

penetrations of from 1.5 to 2.4 mm in the region between the outer hanger rod

hole and the beam end. Because of reductions in area at fracture, the latter

represent minimum values of joint penetration.

To evaluate the significance of the weld actually provided, the effect on ultimate capacity of the box beam-hanger rod connections must be examined. For an effective throat of 3 mm and a weld length of 1 3/8 in (35 mm), the corre- sponding weld area is 105 mm^ . From the plot of estimated peak upper rod load

vs index weld area on figure 6.38, the expected ultimate capacity is approximately 17.3 kips (77 kN). With the contribution of the interior tack weld,

estimates for the mean ultimate capacities of the actual fourth floor box

beam-hanger rod connections range from 18.2 to 19.3 kips (81 to 86 kN)

247
Based on observations of the fracture surfaces and on the relationship between

connection load capacity and index weld area, it is concluded that the actual

connection load capacities were probably higher than the capacity that would

be expected for a partial joint penetration groove weld having an effective

throat of 3 mm and acting alone. It is therefore concluded that quality of

workmanship did not play any significant role in initiating the collapse.

10.7 SUMMARY

The following are the major points resulting from the structural tests,

analysis, and observations of damage.

1. The maximum load acting on a fourth floor box beam-hanger rod

connection at the time of collapse was 53 percent of what was required for design under the Kansas City Building Code.

2. With the exception of locations 8UE and lOUW, the estimated mean

ultimate capacity of each fourth floor box beam-hanger rod connection

either equals or slightly exceeds the estimated dead load effect.

However, the estimated ultimate capacity is exceeded by the sum of

the estimated dead load and upper-bound live load in every case.

3. Each of the 6 fourth floor box beam-hanger rod connections had a high probability of failure; each connection was a candidate for

initiation of walkway collapse.

4. Failure of any one of the fourth floor box beam-hanger rod connections

would have been accompanied by a redistribution or transfer of load to

other hangers. With no apparent significant reserve capacity to

accommodate this redistribution, progressive failure of the remaining

connections was assured. Thus, failure of any one connection would

have led to complete collapse of the walkway system.

5. The design of a fourth floor box beam-hanger rod connection under the

AISC Specification would be expected to result in a connection with

an ultimate capacity of at least 68 kips (302 kN) . The average ultimate capacity of the 6 fourth floor walkway connections is estimated

to have been 18.6 kips (83 kN)

.

6. Observed distortions of the fourth floor to ceiling hanger rods and

the fourth floor walkway box beams strongly support the initiation of

the collapse at box beam-hanger rod connection 9UE.

7. The change in hanger rod arrangement from a continuous rod to

interrupted rods essentially doubled the load to be transferred by

the fourth floor box beam-hanger rod connections.

8. For the continuous hanger rod arrangement, the design load to be

transferred to each hanger rod at the second and fourth floor levels

would have been approximately 20.3 kips (90 kN)

.

248
9. For the continuous hanger rod arrangement, a box beam-hanger rod

connection designed in accordance with the AISC Specification would

be expected to provide an ultimate capacity of at least 33.9 kips

(151 kN).

10. Had the change in hanger rod arrangement not been made, the third

floor walkway would have been the most critical of the three, the

ultimate capacity of the connections being approximately 53 percent of

that which would be expected of a connection designed in accordance

with the AISC Specification.

11. Average joint penetration for the box beam longitudinal welds was less

than that required by AWS Dl.1-79 for a prequalified partial joint

penetration groove weld. However, it is concluded that due to the

presence of tack welds the actual ultimate capacities of the box beamhanger rod connections probably were higher than would be expected for

a partial joint penetration groove weld with the prescribed minimum

effective throat acting alone.

12. There is no evidence to indicate that either quality of workmanship

or materials played any significant role in initiating the collapse.

249
250
I

As Built Original Detail

Figure 10.2 Comparison of interrupted and continuous hanger rod details.

251
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252
11. SUMMARY AND CONCLUSIONS

Following the collapse of two suspended walkways within the atrium area of the

Kansas City Hyatt Regency Hotel in Kansas City, Mo., Mayor Richard L. Berkley

formally requested the National Bureau of Standards (NBS) to independently

ascertain the most probable cause of the collapse. In response to this request,

NBS carried out an extensive field investigation to reconstruct events preceding

the collapse and to establish the condition of the walkways at the time of

collapse. This included review of documents related to the design and construction of the walkways and measurements and inspections of the debris remaining

after the collapse. NBS also conducted comprehensive laboratory studies of

materials and mockups representative of the walkway construction and laboratory

studies of specimens removed from the debris. These studies were supported by

structural analyses of the walkways under the loading condition believed to have

existed at the time of collapse. Conclusions drawn from this investigation are summarized in the following statements:

1. Collapse of the walkways occurred under the action of loads that were

substantially less than the design loads specified by the Kansas City

Building Code.

(a) It is concluded that a total of 63 people represents a credible

upper-bound combined occupancy of the second and fourth floor walkways

at the time of collapse.

(b) Based upon measured weights of damaged walkway spans, the dead load

prior to collapse averaged 17.8 kips (79 kN) per walkway span. This

is approximately 8 percent higher than the nominal dead load that

would be estimated on the basis of the contract drawings.

(c) Dynamic loads induced by walking or dancing on the walkways would not

have been significant in comparison to the static loads.

253
(d) The maximum load (estimated dead load plus upper-bound live load)

believed to have been acting on a fourth floor box beam-hanger rod

connection at the time of collapse is 21.4 kips (95 kN) . Under the

action of nominal dead load plus code-specified live load, this load

would have been 40.7 kips (181 kN) . Thus the maximum load acting on

a connection at the time of collapse was 53 percent of what was required for design under the Kansas City Building Code.

The ultimate capacity of box beam-hanger rod connections can be predicted

on the basis of laboratory test results

,

(a) Extensive structural tests show that ultimate capacity varies linearly

with an index weld area.

(b) Welding process has little effect on ultimate capacity.

(c) Mean ultimate capacities of the fourth floor box beam-hanger rod

connections were estimated on the basis of the NBS test series and

these capacities ranged from 18.2 kips (81 kN) to 19.3 kips (86 kN)

with an average value of 18.6 kips (83 kN)

.

Under the action of the loads estimated to have been present on the

walkways at the time of collapse f all fourth floor box beam-hanger rod

connections were candidates for initiation of walkway collapse

,

(a) With the exception of locations SUE and lOUW, the estimated ultimate

capacity of each connection either equals or slightly exceeds the

estimated dead load effect. However, the estimated ultimate capacity

is exceeded by the sura of the estimated dead load and upper-bound live

load in every case.

(b) Because of the small reserve load capacity of the connections and

because of load redistribution that would have accompanied initial

failure, progressive failure of the walkway system was assured.

Observed distortions of structural components strongly suggest that failure

of the walkway system initiated in the box beam-hanger rod connection at

location 9UE (east end of middle box beam in fourth floor walkway),

(a) The lower ends of the west side hanger rods were bent to the west,

while those along the east side showed no such distortion, indicating

that the box beam-hanger rod connections along the east side of the

fourth floor walkway failed first.

(b) Mirror symmetry of distortions about the longitudinal axis of the

middle box beam in the fourth floor walkway, coupled with the hanger

rod distortions, indicate initiation of failure at location 9UE.

As constructed , the box beam-hanger rod connections , the fourth floor to

ceiling hanger rods, and the third floor walkway hanger rods did not

satisfy the design provisions of the Kansas City Building Code.

254
(a) The AISC Specification for the Design, Fabrication and Erection of

Structural Steel for Buildings forms the basis for the steel design

provisions of the Kansas City Building Code.

(b) The box beam-hanger rod connection detail used in the walkway

construction does not satisfy the intent of the AISC provisions for

the proportioning of such connections.

(c) The design of a fourth floor box beam-hanger rod connection under the

AISC Specification would be expected to result in a connection with an ultimate capacity of at least 68 kips (302 kN) . The average ultimate

capacity of the six fourth floor walkway connections as constructed

is estimated to have been 18.6 kips (83 kN)

(d) The fourth floor to ceiling hanger rods and the third floor walkway

hanger rods do not comply with the AISC provisions for allowable

stresses

6. The change in hanger rod arrangement from a continuous rod to interrupted

rods essentially doubled the load to be transferred by the fourth floor

box beam-hanger rod connections

(a) For the continuous hanger rod arrangement, the design load to be

transferred to each hanger rod at the second and fourth floor levels

would have been approximately 20.3 kips (90 kN)

(b) For the interrupted hanger rod arrangement, the design load to be

transferred by a fourth floor box beam-hanger rod connection would

have been 40.7 kips (181 kN) (see item 1(d)).

7. The box beam-hanger rod connection would not have satisfied the Kansas

City Building Code under the original hanger rod detail (continuous rod).

(a) See items 5(a) and 5(b).

(b) Under the original hanger rod arrangement (continuous rod), a box

beam-hanger rod connection designed in accordance with the AISC

Specification would be expected to provide an ultimate capacity of

at least 33.9 kips (151 kN)

(c) Based on NBS structural tests, the mean ultimate capacity of a single-rod connection as detailed on the contract drawings is estimated

to be 20.5 kips (91 kN)

(d) Had the change in hanger rod arrangement not been made, the third

floor walkway would have been the most critical of the three, the

ultimate capacity of the connections being approximately 53 percent of

that which would be expected of a connection designed in accordance

with the AISC Specification.

255
8. Under the original hanger rod arrangement (continuous rod) the box beamhanger rod connections as shown on the contract drawings would have had

the capacity to resist the loads estimated to have been acting at the time

of collapse

.

(a) The maximum load (estimated dead load plus upper-bound live load)

believed to have been acting on a second floor box beam-hanger rod

connection at the time of collapse is 11.5 kips (51 kN)

.

(b) The ultimate capacity of a single-rod connection is estimated to be

20.5 kips (91 kN) (see item 7(c)).

9. Neither the quality of workmanship nor the materials used in the walkway

system played a significant role in initiating the collapse

.

(a) Tensile properties and chemical compositions of the walkway box beam

channels and hanger rod materials satisfy the requirements of ASTM

Standard A36 for structural steel.

(b) Microstructure and hardness of the walkway box beam channels, box beam

longitudinal welds, and hanger rod materials were normal.

(c) Average joint penetration for the box beam longitudinal welds was less

than that required by AWS Dl.1-79 for a prequalified partial joint

penetration groove weld. However, it is concluded that due to the

presence of tack welds the actual ultimate capacities of the box beamhanger rod connections probably were higher than would be expected for

a partial joint penetration groove weld with the prescribed minimum

effective throat acting alone.

(d) Evidence indicates that hanger rod washers at locations SUE and 9UE,

missing after the collapse, had been installed.

256
12. REFERENCES

2.1 "Specifications, Hyatt Regency, Kansas City," PBMDML, Kansas City, MO,

August 1978.

2.2 "Kansas City Building Code," Kansas City, MO, June 1, 1977.

2.3 "Uniform Building Code," 1976 Edition, International Conference of

Building Officials, Whittier, CA, 1976.

2.4 "Specification for the Design, Fabrication and Erection of Structural

Steel for Buildings," American Institute of Steel Construction, New York,

NY, February 1969.

2.5 "Summary of Data from Vibration Testing of the Third Level Skybridge in

the Kansas City Hyatt Regency Hotel," Wiss, Janney, Elstner and Associates

Confidential Report WTE No. 81553 (with supplement of data), September 28,

1981, Northbrook, IL.

3.1 "Structural Welding Code-Steel," Third Edition, AWS Dl.1-79, American

Welding Society, Miami, FL, 1978.

5.1 "Miscellaneous Locations, Hyatt Regency Hotel, Kansas City, Jackson County,

Missouri," The Tuttle-Ayers-Woodward Co., Project No. 220587, August 5,

1981, Kansas City, Missouri.

5.2 "Symbols for Welding and Nondestructive Testing," AWS A2.4-79, American

Welding Society, Miami, FL, 1979.

6.1 "Welding Terms and Definitions," AWS A3. 0-80, American Welding Society,

Miami, FL, 1980.

6.2 Lipson, C. and Sheth, W. J., Statistical Design and Analysis of Engineering

Experiments , McGraw Hill, New York, 1973.

7.1 Annual Book of ASTM Standards , Parts 4, 6, 10 and 14, American Society for

Testing and Materials, Philadelphia, PA, 1981.

9.1 Diffrient, N., Tilley, A. and Bardagjy, J., Humanscale 1/2/3 , MIT Press,

Massachusetts Institute of Technology, Cambridge, MA, 1974.

9.2 Fattal, S. G., Cattaneo, L. E., Turner, G. E. and Robinson, S. N., ~ "Personnel Guardrails for the Prevention of Occupational Accidents," NBSIR

76-1132, National Bureau of Standards, Washington, D.C., July 1976.

9.3 Swearingen, J. J., "Determination of Centers of Gravity of Man," Report

62-14, Aviation Medical Service, Federal Aviation Service, Oklahoma City,

OK, August 1962.

9.4 Manual of Steel Construction, Seventh Edition, American Institute of Steel

Construction, New York, NY, 1970.

257
9.5 Kamel, H. A. and McCabe, N. W., "Graphics-Oriented Interactive Finite

Element Time-Sharing System," Version 5.0 User's Manual, University of

Arizona, Tucson, AZ, May 1979.

9.6 McGuire, W., "Dynamic Response Characteristics of the Kansas City Hyatt

Regency Hotel Skybridges," Consultant's Report to the National Bureau of

Standards, December 3, 1981, Ithaca, NY.

9.7 Becker, R., "Simplified Investigation of Floors Under Foot Traffic,"

Journal of the Structural Division , ASCE, Vol. 106, No. STll, Proc. Paper

15804, Nov. 1980, pp. 2221-2234.

9.8 Harper, F. C. , Warlow, W. J. and Clarke, B. L., "The Forces Applied to the

Floor by the Foot in Walking," National Building Studies Research Paper 32,

Department of Scientific and Industrial Research, Building Research

Station, London, 1961.

258
APPENDIX

Presented here are additional

graphic, tabular, and photographic

data, referenced in the main body

of this report. These data are keyed to the report's sections by use of the section number preceded

by an "A" (for Appendix), e. g. A6.10.

A-1
Sections A6.10 and A6.11

Thirty-two load-deflection curve graphs,

30 for short box beam specimens (A6.10),

and two for full-length specimens (A6.11),

follow.
SPECIMENS Al AND A2

0.10 0.80 1.20 1.60 UPPER ROD DISPLACEHENT IN INCHES2.00

Figure A6.10-1 In-plane load vs displacement curves for specimens Al and A2

SPECIMEN Bl

0 . 40 0 . 80 1 . 20 1 . 60 2 . 00 UPPER ROD DISPLACEMENT IN INCHES

Figure A6.10-2 In-plane load vs displacement curve for specimens Bl

A-3
Figure A6.10-4 In-plane load vs displacement curve for specimens B3

.

A-4
SPECiriENS CI AND C3

0.40 0.80 L.20 1.60 UPPER ROD DISPLACEPIENT IN INCHES

2.00

Figure A6.10-5 In-plane load vs displacement curves for specimens CI and C3

SPECIMENS Ca AND C4

0.40 0.80 1.20 l.SO UPPER ROD DISPLACEflENT IN INCHES2.00

Figure A6.10-6 In-plane load vs displacement curves for specimens C2 and C4

A-5
SPECIHEN Dl

0.40 0.80 1.20 1.60 2.00 UPPER ROD DISPLACEMENT IN INCHES

Figure A6.10-7 In-plane load vs displacement curve for specimens Dl.

SPECIflENS El AND £2

UPPER ROD DISPLACEtlENT IN INCHES

gure A6.10-8 In-plane load vs displacement curves for specimens El and

A-6
SPECinENS Fl AND FE

0.10 0.80 1.20 l.GO UPPER ROD DISPLACEHENT IN INCHES

2.00

Figure A6.10-9 In-plane load vs displacement curves for specimens Fl and F2

SPECIMENS Gl AND G2

0.40 0.80 1.20 1.60 UPPER ROD DISPLACEPIENT IN INCHES

Figure A6. 10-10 In-plane load vs displacement curves for specimens Gl and G2

ASPECiriEN HI

O.-IO 0.80 1.20 l.GO UPPER ROD DISPLACEHENT IN INCHES

2.00

Figure A6. 10-11 In-plane load vs displacement curve for specimens HI

SPECIflENS II AND 12

0.10 0.80 1.20 1.60 UPPER ROD DISPLACEriENT IN INCHES

2.00

gure A6. 10-12 In-plane load vs displacement curves for specimens II and

A-8
Figure A6. 10-14 In-plane load vs displacement curves for specimens Kl and K2

ASPECIflENS K3 AMD K6

0.10 0.80 1.20 l.GO UPPER ROD DISPLACEHENT IN INCHES

2.00

Figure A6.10-15 In-plane load vs displacement curves for specimens K3 and K6

.

SPECinENS K4 AND KS

0.10 0.80 1.20 1.60 UPPER ROD DISPLACENENT IN INCHES2.00

Figure A6. 10-16 In-plane load vs displacement curves for specimens K4 and K5

.

A-10
SPECinENS LSI AND LS2

O.'lO 0.80 1.20 1.60 UPPER ROD DISPLACEriENT IfS INCHES2.00

Figure A6. 10-17 In-plane load vs displacement curves for specimens LSI and LS2

SPECIMENS ni AND PIS

0.10 0.80 1.20 l.GO UPPER ROD DISPLACEtlENT IN INCHES

Figure A6. 10-18 In-plane load vs displacement curves for specimens Ml and M2.

A-11
SPECiriENS Nl AND NE

0.10 0.80 1.20 l.GO UPPER ROD DISPLACEriENT IN INCHES

2.00

jure A6. 10-19 In-plane load vs displacement curves for specimens Nl and

SPECiriENS N3, N4, AND N5

0.40 0.80 1.20 1.60 UPPER ROD DISPLACEdENT IN INCHES2.00

Figure A6. 10-20 In-plane load vs displacement curves for specimens N3 , N and N5. A-12
SPECIflEN N6

O.-IO 0.80 1.20 1.60 UPPER ROD DISPLACEnENT IN INCHES2.00

Figure A6. 10-21 In-plane load vs displacement curve for specimens N6

SPECiriENS 01 AND 02

0.^0 0.80 1.20 l.GO UPPER ROD DISPLACEflENT IN INCHES

2.00

Figure A6. 10-22 In-plane load vs displacement curves for specimens 01 and 02.

A-13
o SPECIflEfSS PI AND P2

0.10 0.80 1.20 l.eo 2.00 UPPER ROD DISPLACEMENT IN INCHES

Figure A6. 10-23 In-plane load vs displacement curves for specimens PI and P2.

SPECIMENS Rl AND RE

1 ' 1 ' 1 —' r

' 1 0.40 0.80 1.20 l.SO 2.00 UPPER ROD DISPLACEMENT IN INCHES

Figure A6. 10-24 In-plane load vs displacement curves for specimens Rl and R2

.

A-IA
u

p

p

ER

R

0

D

L0A

D

IN

K

IP

S

SPECIPIENS S1,S2,S3,AND S4

/;<-s

0.40 0.80 1.20 l.GO UPPER ROD DISPLACEflENT IN INCHES

2.00

Figure A6. 10-25 In-plane load vs displacement curves for specimens SI, S2, S3,

and S4.

SPECIPIENS Tl,Ta,AND T3

0.10 O.eo 1.20 1.60 UPPER ROD DISPLACEflENT IN INCHES

2.00

Figure A6. 10-26 In-plane load vs displacement curves for specimens Tl , T2

and T3.

A-15
SPECIHENS Ul AND U2

0.40 0.80 1.20 i.eo UPPER ROD DISPLACEFIENT IN INCHES2.00

Figure A6. 10-27 In-plane load vs displacement curves for specimens Ul and U2

SPECiriENS Ul AND U2

0.40 0.80 1.20 l.GO UPPER ROD DISPLACEnENT IN INCHES

2.00

Figure A6. 10-28 In-plane load vs displacement curves for specimens VI and V2.

A-16
SPECIMENS U1,U2,AND U3

0.40 0.80 1.20 i.eo UPPER ROD DISPLACEriENT IN INCHES

2.00

Figure A6. 10-29 In-plane load vs displacement curves for specimens Wl , W2

and W3

SPECiriENS XI AND X2

0.40 0.80 1.20 1.60 UPPER ROD DISPLACEriENT IN INCHES

2.00

Figure A6. 10-30 In-plane load vs displacement curves for specimens XI and X2

A-17
SPECIflENS LI AND L2 FULL LENGTH BOX BEAPIS

0.40 0.80 1.20 l.GO CHANNEL DISPLACEHENT IN INCHES

2.00

Figure A6.11-1 In-plane load vs deflection curves for specimens LI and L2

SPECIflENS L3-L4-L5-L6-L7

FULL LENGTH BOX BEAMS

0.40 0.80 1.20 1.60 CHANNEL DISPLACEHENT IN INCHES

2.00

Figure A6.11-2 In-plane load vs deflection curves for specimens L3 , L4

,

L6. and L7
Sections A7 . 2 through A7 .

The photographs, tables, and graphs

that follow provide additional data

on the physical and chemical properties

of walkway materials.

A-19
A7. NBS MATERIALS TESTING PROGRAM

A7 .2 INITIAL INSPECTION

kl .1,1 Radiography

Radiographs of portions of box beam 9M used in the analysis described in

section 7 of the report are produced in figure A7.2.2-1.

A7 .3 MECHANICAL PROPERTIES OF STRUCTURAL STEEL AND WELDMENTS

A7.3.1 NBS Channel Specimens

Load-strain plots for tensile tests performed on NBS channel specimens are given in figures A7.3.1-1 to A7.3.1-3.

A7.3.2 Walkway Box Beams

Locations from which base metal tensile specimens were removed from box beams

9U, 8L and 9M are illustrated in figures A7.3.2-1 through A7.3.2-3. Loadstrain plots for the tensile tests performed on walkway box beam specimens

appear in figures A7.3.2-4 through A7.3.2-9.

A7.3.3 Walkway Hanger Rods

Locations from which tensile specimens were removed from hanger rods NBS 5A and

5B are illustrated in figure A7.3.3-1. Load-strain plots for the walkway hanger

rod tests appear in figure A7.3.3-2.

A7.3.4 Walkway Box Beam Longitudinal Welds

Locations from which tensile specimens were removed from box beams 9U , 8L and

9M are illustrated in figures A7.3.2-1 through A7.3.2-3.

A7 .4 METALLOGRAPHY AND HARDNESS MEASUREMEOTS

A7.4.1 Box Beam Weldments

A "U" shaped transverse section through NBS weldment IC was examined for

microstructure and hardness. Weldment IC was welded by the GMAW process using

a mild steel electrode. The section examined, shown at low magnification in

figure A7.4.1-1, included the welded flange region and part of the web region

of each box beam channel. The section was metallographically polished and

chemically etched to reveal the microstructure. The region of this section

containing the weld is shown at higher magnification in figure A7.4.1-2. In

this figure, the light region in the center represents weld metal; the darker

regions on either side of the weld are the heat affected zones associated with

the weld. The light regions at both sides of the figure, adjacent to the heat

affected zones, are flange base metal.

A-20
Knoop microhardness measurements at a load of 500 grams force (HK5QQ) were made

across the weld metal, across one of the heat affected zones, and into the base

material on this same transverse section through NBS weldment IC. The indentations produced by the microhardness tests appear as a line of small, dark,

elongated marks in figure A7.4.1-2. Approximately equivalent Rockwell B (HRB)

hardness values, converted in accordance with ASTM E140-79 (Standard Hardness

Conversion Tables for Metals) [7.1], are reported along with the HK5QQ values.

A representative region of the weld metal appears in figure A7.4.1-3. The

mlcrostructure consists primarily of bainite. Microhardness values based on

17 measurements average 205 HK500 (90.9 HRB) and range from 195 to 228 HK500

(88.5 to 95.5 HRB).

The transition region between the weld metal and the heat affected zone is

shown in figure A7.4.1-4. Nothing unusual was observed at the transition. The

microstructure within the heat affected zone is shown in figure A7.4.1-5. The

mlcrostructure consists of ferrite, primarily as a network in prior austenite

grain boundaries, bainite, partially spheroidized carbides, and tempered martensite. Average hardness of the heat affected zone based on five measurements

is 202 HK500 (90.2 HRB) with a range of 196 to 206 HK500 (89 to 91 HRB).

Figure A7.4.1-6 shows the transition from the heat affected zone to the base

material of the box beam. Hardness in this region averages 173 HK5QQ (83 HRB)

and the hardness range is 167 to 177 HK500 (81 to 84 HRB).

A region representative of the base material away from the heat affected zone

is shown in figure A7.4.1-7. The microstructure is relatively fine grained and

consists primarily of ferrite (light) and pearlite (dark) in a normalized condition. Within the base material, the average hardness is 152 HK5QQ (75.4 HRB)

and the range is 139 to 161 HK500 (70 to 70.9 HRB).

Two transverse sections similar to that described in the previous section on the

NBS weldment were examined from each of the two box beam segments (9U and 8L)

and the full length box beam (9M) obtained from the walkway debris. The locations of these sections in the box beams are indicated in figures A7.3.2-1

through A7.3.2-3. Each "U" shaped transverse section included the bottom of

the beam (channel flanges and weld region) and parts of the web region of each

box beam channel. To facilitate handling during metallographic preparation,

parts of the channel web regions were removed from each section. The metallographic examination was concentrated in the remaining shorter "U" shaped sections, but hardness measurements were made both in the shortened sections and

in the pieces that had been removed. The sections were polished metallographically and chemically etched to reveal microstructure. Knoop microhardness

measurements at a load of 500 grams force (HK5Q0) were taken in the weld, heat

affected zones, and adjacent base channel material of these sections. Rockwell

B (HRB) hardness measurements were taken in the flange and web regions of each

channel piece. Summaries of the hardness test results appear in tables A7.4.1-1

through A7.4.1-3 for sections from box beams 9U, 8L and 9M, respectively.

Approximately equivalent Rockwell B (HRB) hardness values, converted in

accordance with ASTM Standard E140-79, are reported along with the HK500 values.

A-21
The sections from box beam 9U (NBS 1), designated IBMl and 1BM2, are shown in

figures A7.4.1-8 and A7.4.1-10, respectively. These two sections were separated

by about 7 in (180 mm) as measured along the length of the beam. The weld

regions of sections IBMl and 1BM2 are shown at higher magnification in figures

A7.4.1-9 and A7.4.1-11, respectively. The depth of weld penetration is greater

in section 1BM2 than in section IBMl and there is some misalignment of the

channel flanges at the weld.

Representative regions of the weld metal from sections IBMl and 1BM2 are shown

in figures A7.4.1-12 and A7.4.1-13, respectively. In both cases, the microstructure appears to consist of bainite. A number of inclusions are evident in

section IBMl. The average hardness of the weld in section IBMl (95 HRB) is

slightly higher than in the weld of section 1BM2 (91 HRB) and the hardness

range in the weld of section IBMl is greater.

There were some patches of hard material at the transition region from the

weld material to the heat affected zone at both the north and south channels in

section IBMl. One such patch can be seen in figure A7.4.1-14. A similar region

is shown at higher magnification in figure A7.4.1-15. The microstructure in

this region appears to be fine-grained tempered martensite. Maximum hardness

measured in these patches was 336 HK5QQ which is approximately equivalent to a Rockwell C hardness of 33. This value compares to a hardness of 241 HK500

(98 HRB) in an adjacent location of the transition region.

Hard patches were not observed in the transition region from weld to heat

affected zone in section 1BM2. An area representative of this region in

section 1BM2 is shown in figure A7.4.1-16.

Regions representative of the heat affected zones (HAZ) of sections IBMl and

1BM2 are shown in figures A7.4.1-17 and A7.4.1-18, respectively. In both cases,

the microstructure appears to consist of ferrite, both as a network in prior

austenite grain boundaries and as needles, bainite, partially spheroidized

carbides, and tempered martensite. Hardness of the heat affected zones in

section IBMl is somewhat greater at the south channel than at the north. At

both heat affected zones, hardness decreased with distance from the weld.

Hardness values in the heat affected zones of section 1BM2 were similar for

both zones.

The transition regions from the heat affected zone to the base channel material

appeared similar for both sections. A region representative of the transition

zone in section 1BM2 is shown in figure A7.4.1-19.

Base material from both channel pieces appeared similar. A region representative

of section IBMl is shown in figure A7.4.1-20. The microstructure consists

primarily of ferrite (light) and pearlite (dark) in a normalized condition.

Rockwell B hardness measurements made on both sections indicated no significant

differences in hardness between the two channel pieces used to fabricate the

box beam. Regions of the channel adjacent to the heat affected zones and in

the web region about 0.6 to 1 in (15 to 25 mm) from the bottom of the beam were harder than elsewhere with IIRB hardness averaging 81.5. The hardness of the

A-22
channel material elsewhere in the sections averaged 74.4 HRB for section IBMl

and 74.3 HRB for section 1BM2.

The transverse sections examined from box beam 8L (NBS 2), designated 2BM1 and

2BM2, are shown in figures A7.4.1-21 and A7.4.1-23, respectively. These sec- tions were separated by about 7 1/2 in (190 mm) as measured along the length of

the beam. The regions of sections 2BM1 and 2BM2 containing the weld are shown

at higher magnification in figures A7.4.1-22 and A7.4.1-24, respectively. There

appears to be minor undercutting at the weld in section 2BM2 and the heat

affected zone is somewhat narrower than in section 2BM1 . In section 2BM1 there

is a void at the base of the weldraent adjacent to the gap between the channel

flanges

Regions representative of the weld metal in sections 2BM1 and 2BM2 are shown in

figures A7.4.1-25 and A7.4.1-26, respectively. Overall, the raicrostructure in

both cases appeared similar and consisted primarily of bainite. Inclusions

are evident in both sections. The average hardness values for weld metal in

the two sections do not differ significantly.

The transition from weld to the heat affected zone for section 2BM1 is shown

in figure A7.4.1-27. There are no unusual features.

As was the case in section IBMl from box beam 9U, there were some hard spots at

the transition from weld to the heat affected zone in section 2BM2 on the south

side. The transition in section 2BM2 is shown in figure A7.4.1-28 at 200 magnifications. One of the hard regions is shown at much higher magnification (X1200)

in figure A7.4.1-29. The microstructure of this region appears to be tempered

martensite. The highest hardness value obtained in this region was 298 tIK5Qo

which is approximately equivalent to 28 Rockwell C. Away from the hard regions,

hardness averaged 247 HK500 (99 HRB).

In the heat affected zones, the microstructure was similar in both sections

2BM1 and 2BM2. A representative region from section 2BM1 is shown in figure

A7.4.1-30. The microstructure appears to consist of ferrite, both as a network

in prior austenite grain boundaries and as needles, bainite, partially

spheroidlzed carbides and tempered martensite. Hardness was 5 to 6 HRB points

lower for the heat affected zones in section 2BM2 than in the weld metal. In

section 2BM1 , hardness was similar for both weld metal and the heat affected

zone

The transition regions from the heat affected zones to base channel material

appeared similar for both sections. A region representative of this transition

in section 2BM1 is shown in figure A7.4.1-31. A region representative of the

base channel material in both sections is shown for section 2BM1 in figure

A7.4.1-32. The microstructure of the channel material consists primarily of

ferrite and pearlite in a normalized condition.

There were no significant differences in hardness between the two channel

sections. As in the case of the section from box beam 9U , the channel material

was somewhat harder adjacent to the heat affected zones and in the web about

0.6 to 1 in (15 to 25 mm) from the flange base than elsewhere.

A-23
The sections examined for microstructure and hardness from box beam 9M (NBS 3) are designated 3BM1 and 3BM2 and are shown in figures A7.4.1-33 and A7.4.1-35,

respectively. These two sections were separated by about 7 1/2 in (190 mm) as measured along the length of the beam. The weld regions of sections 3BM1 and

3BM2 are shown at higher magnification in figures A7.4.1-34 and A7.4.1-36,

respectively. The weld configuration differs considerably between the two

sections. In section 3BM1 , the heat affected zones penetrate to a point

slightly greater than half the flange thickness, whereas the heat affected

zones in section 3BM2 penetrate all the way through the flange.

Representative regions of the weld from sections 3BM1 and 3BM2 are shown in

figures A7.4.1-37 and A7.4.1-38, respectively. In both cases, the microstructure appears to consist primarily of bainite. More inclusions are evident in

section 3BM1 than in section 3BM2. The average hardness of the weld was essentially the same for both sections, being somewhat lower than that of the welds

at the sections from the other two walkway box beams.

Nothing unusual was observed at the transition from the weld to the heat

affected zones in either section. None of the harder regions found in the

transition regions of sections IBMI and 2BM2 were in evidence. A region representative of these transition regions in section 3BM2 is shown in figure

A7.4.1-39.

The microstructure and hardness of the heat affected zones in both section 3BM1

and 3BM2 were similar. In both cases, the microstructure consists of ferrite

both as a network in prior austenite grain boundaries and as needles, bainite,

partially spheroidized carbides and some tempered martensite. Hardness in the

heat affected zones was similar to that of the weld. A region representative

of the heat affected zone in section 3BM1 is shown in figure A7.4.1-40.

The transition from the heat affected zone to the base channel material appeared

similar in both sections. A region representative of the transition zone in

section 3BM2 is shown in figure A7.4.1-41.

The base material of both channel pieces of the box beam appeared similar in

microstructure in both sections 3BM1 and 3BM2 . In both cases, the microstructure consists of ferrite and pearlite in a normalized condition. A region

representative of the channel material is shown in figure A7.4.1-42. Hardness

of both channels was also similar.

In summary, the walkway and NBS box beam structural steel weldment microstructure

and hardness are comparable and normal.

A7.4.2 Walkway Hanger Rods

Transverse and longitudinal sections from each of the walkway hanger rod

segments obtained from the debris (NBS 5A and 5B) were examined metallographically. The locations of these sections in the hanger rods are shown in

figure A7.3.3-1. A representative region showing the etched microstructure of

the transverse section from hanger rod NBS 5A is shown in figure A7.4.2-1. The

A-24
mlcrostructure consists of ferrite (light) and pearlite (dark) in a reasonably

fine grained (ASTM grain size 9) normalized structure.

The representative mlcrostructure of the transverse section from hanger rod

NBS 5B is shown in figure A7.4.2-2. The mlcrostructure again consists of

ferrite (light) and pearlite (dark) in the normalized condition. This microstructure is coarser (ASTM grain size 7-1/2 to 8) than that of hanger rod NBS

5A, and there appears to be more pearlite present than in hanger rod NBS 5A.

Longitudinal sections through each hanger rod segment revealed the material of

both to be fully normalized with equiaxed grains. Regions representative of

the longitudinal microstructures are shown in figure A7.4.2-3 for rod segment

NBS 5A and in figure A7.4.2-4 for rod segment NBS 5B.

Rockwell B (HRB) hardness measurements were made on each of the transverse

sections examined for mlcrostructure. Twelve measurements were made on each

section. The average hardness value for the finer grained hanger rod NBS 5A

material was IIRB 74.5 with a range of HRB 72-76. The average hardness value

for hanger rod NBS 5B material was HRB 68.5 with a range of HRB 66 to 71.5.

In the section from hanger rod NBS 5B, there was a slight hardness gradient

with hardness decreasing from the outside toward the center.

A7.4.3 Washers

Hardness measurements were made on NBS washers and on washers obtained from

the walkway debris. A metallographic examination was performed on sections

through a hanger rod washer obtained from the walkway debris at location 9MVJ

and through one of the NBS hardened washers.

Rockwell C (HRC) hardness measurements were made on the surfaces of four washers

obtained from the walkway debris. The results of these measurements are given

in table A7.4.3-1. It is apparent from these results that the washers obtained

from the walkway were hardened washers.

Rockwell C hardness measurements were made on the surface of one NBS hard

washer, and Rockwell B (HRB) hardness measurements were made on the surface of one unplated regular grade washer. The results of these measurements are also

given in table A7.4.3-1.

Knoop microhardness measurements at a load of 500 grams force (HK500) were made

on a cross section through a washer from the walkway debris at location 9MW

after it had been used in an NBS full length test on box beam 9M. During the

test, the washer fractured into five pieces. A microhardness traverse was made

through the thickness of the washer on the section examined. The washer thickness was approximately 0.075 in (1.80 mm). Average hardness from 16 measurements across the section was 459 HK500 which is approximately equivalent to

44.3 HRC and the hardness range was 422 to 514 HK500 (41.5 to 48.5 HRC). The

highest values were near the center of the section and the lowest values were

near the surface of the washer. The mlcrostructure of the washer from location

9MW is shown in figure A7.4.3-1. There is a very small apparent decarburized

A-25
layer at the surface (light area at the top in the figure). Tempered martensite

is the primary constituent of the microstructure

.

A similar microhardness traverse on a section through one of the NBS hard

washers was made. Hardness values in this section, excluding the decarburized

layer at the surface, averaged 431 HK500 (42.5 HRC) and ranged from 401 to

422 HK500 (^0 to 43.5 HRC). Measured hardness in the decarburized layer was

as low as 247 HK500 (99 HRB). The microstructure of the NBS hard washer is

shown in figure A7.4.3-2. The decarburized layer at the surface appears light

in the figure. Tempered martensite is the primary constituent in the

microstructure

.

A7.5 CHEMICAL ANALYSIS

Five specimens from the walkway box beam longitudinal welds (four exterior

welds and one interior tack weld) and weld specimens from four NBS box beam

replicas were analyzed for chemical composition by emission spectrometry. Also

analyzed for chemical composition were two samples of hanger rod material

removed from the walkway debris.

A 3/4 mm vacuum spectrometer with an adjustable waveform excitation source was used for the analysis. The instrumental analytical curves were constructed

using NBS SRMs 1261, 1262, 1263, 1264 and 1265 plus other selected reference

materials. All specimens were surfaced using 50 grit AI2O3 sanding disks. In

order to confine the analytical discharge to the weld area, a boron nitride

disk with a 0.157 in (4 mm) opening was used in the excitation stand.

The specimen identification numbers and origins are listed in table A7.5-1.

Actual locations of specimens within the walkway box beam and hanger rod segments are described in figures A7.3.2-1 through A7.3.2-3 and in figure A7.3.3-1.

Normal laboratory routines used for SRM homogeneity testing were used in the

analysis. Program outputs included averages, standard deviations and a pictorial presentation (box plots) of the individual values for those elements

normally considered the most important in ferrous analysis. Each value reported

in the tables of this section is an average of four exposures.

A7.5.1 Box Beam Longitudinal Welds

Results of the analysis of longitudinal weld specimens obtained from segments

of the walkway box beams are listed in table A7.5.1-1. In table A7.5.1-2, the

results obtained from the four weld specimens removed from NBS box beam replicas

are presented. In each case the tables include analytical values for the base

metal in the flange on either side of the longitudinal weld seam.

It is concluded that of the four NBS weld specimens analysed, specimen M2 (flux

cored electrode) most closely corresponds to the weld material in the walkway

box beam longitudinal welds. This is based primarily on the comparable values

for manganese, silicon and titanium. li-Jhile this comparison does not rule out

the possibility of other types of electrodes being used in the fabrication of

the walkway box beams, it is in agreement with information provided by the

A-26
fabricator's legal counsel regarding welding procedure as noted in chapter 6 of

this report.

A7.5.2 Walkway Box Beam Interior Tack Weld

Results of the analysis of the interior tack weld specimen removed from box beam

8L (NBS 2) are listed in table A7.5.2-1. The analysis of this weld is comparable to the four walkway box beam welds discussed in section A7.5.1, and it is

concluded that exterior and interior welds were made using the same type of

electrode

A7.5.3 Walkway Hanger Rod Specimens

The chemical analysis of two hanger rod specimens (one specimen from each of two

segments designated NBS 5A and NBS 5B) is presented in table A7.5.3-1. The

analysis clearly shows that the two hanger rod segments are from different

heats since nine elements have significantly different concentrations. In

addition, the sulfur value in specimen 5AC (NBS 5A) shows this element at or just above the limit specified for ASTM Grade A36 steel. Further analysis of

this material by a gasometric method showed the sulfur value to be within

specification with levels of 0.046 + 0.001 for rod 5A and 0.033 + 0.001 for

rod 5B as reported in table A7.5.3-1.

A-27
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A-30
Table A7.4.4-1 Results of Washer Hardness Tests

Hardness

Washer

Identification

SUE Top

8LW Top

9ME Top

9MW Top

NBS Hardened

NBS Regular Grade

Number of

Measurements

8

8

8

8

5

5

Average

40.2

43.0

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44.0

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Range

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Average Range

60.7 58.9-61.3

A-31
Table A7.5-1 Origin of Specimens Analyzed for Chemical Composition

Specimen Identification

1BX4

2BX1

3BX1

2TFX4

B2

03

M2

N6

2C

5AC

Origin

Bottom flange of walkway box beam 9U (NBS 1)

Bottom flange of walkway box beam 8L (NBS 2)

Bottom flange of walkway box beam 9M (NBS 3)

Top flange of walkway box beam 9M (NBS 3)

NBS box beam replica B2 . E7018 electrode

NBS box beam replica C3. E7014 electrode

NBS box beam replica M2 . Flux cored electrode

NBS box beam replica N6 . Mild steel electrode

Interior tack weld, bottom flange, walkway box

beam 8L (NBS 2)

Fourth floor to ceiling walkway hanger rod BE

(NBS 5A)

SAB Second to fourth floor walkway hanger rod (NBS 5B)

A- 32
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A-33
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A-34
Table A7.5.2-1 Chemical Analysis of Walkway Box Beam Interior Tack Weld

Analytical Values —Percent by VJeighi

Specimen Identification Number

Element

base

2C

weld base

Carbon .184 .091 .184

no/ .184 .095 .181

Phosphorus .017 .012 .016

. 018 .013 . 01 /

Sulfur .016 .018 .015

. 01

.018 .015

Manganese .711 1 .096 .719

. 722, 1 .086 .722

S ilicon .193 .452 .198

.195 .442 .197

Nickel .011 .010 .010

. U i i

r\r\Q

. uuy . U i U

Chromium .018 .023 .018

. U i 0 . U/ i . U i O

V anadium .002 .006 .002

. OOz . UUb . UUz

Titanium .001 .014 .001

. 001 • 014 . 001

Molybdenum .005 .004 .005

. 005 . OUh . UO J

Copper .012 .016 .012

.012 ,01b .013

Cobalt .005 .004 .005

• UUd . 00 J

All! Tn T 1 1 m i\ ±um mum • 001 \J\J L 001

.001 .001 .001

Niobium .001 .003 .001

.001 .003 .001

Zirconium .001 .002 .001

.001 .001 .001

A-35
Table A7.5.3-1 Chemical Analysis of Walkway Hanger Rod Specimens

Analytical Values —Percent by Weighc

Specimen Identification Number

Element

SAC 5BC 5AX1* 5BX*

Carbon .168

.168

.200

.197

.159 .200

Phosphorus .037

.038

.010

.009

Sulfur .050

.052

.035

.035

.046 .033

Manganese .510

.527

.672

.696

Silicon .187

.185

.038

.039

Nickel .094

.098

.053

.055

Chromium .106

.110

.115

.117

V anad ium .001

.001

.001

.001

Titanium .001

.001

.002

.002

Molybdenum .009

.009

.009

.009

Copper .330

.338

.112

.114

Cobalt .008

.008

.007

.007

Aluminum. .001

.001

.003

.005

Niobium .001

.002

.001

.001

Zirconium .001

.002

.001

.001

* Gasometric me thod

A-36

spimod 'avol

A-38
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A-39
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A-40
a. North face of box beam segment 9U (NBS 1).

Figure A7.3.2-1 Location of test specimens from box beam segment 9U obtained

from the walkway debris.

a. North face:

Longitudinal web tensile test specimens 1-LNl , 1-LN2

Transverse web tensile test specimens 1-TNl , 1-TN2

Parts of metallographic specimens 1-BMl , 1-BM2

b. Bottom:

Parts of metallographic specimens 1-BMl , 1-BM2

Transverse tensile test specimens across the

weld 1-BX3W2, 1-BX3W4

Chemical analysis specimen 1-BX4

c. South face:

Longitudinal web tensile test specimens 1-LSl , 1-LS2

Transverse web tensile test specimens 1-TSl , 1-TS2

Parts of metallographic specimens 1-BMl, 1-BM2

d. Top:

Longitudinal flange tensile test specimens 1-TFN, 1-TFS

A-41
d. Top of box beam segment 9U (NBS 1).

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A-42
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A-43
A-44

Figure A7.3.2-3 Location of test specimens from box beam segment NBS 3-5

from box beam 9M obtained from the walkway debris.

a. North face:

Longitudinal web tensile test specimens 3-LNl , 3-LN2

Transverse web tensile test specimens 3-TNl , 3-TN2

Parts of metallographic specimens 3-BMl , 3-BM2

b. Bottom:

b.l: Parts of metallographic specimens 3-BMl , 3-BM2

Transverse tensile test specimens across

the weld 3-BWl , 3-BW4

Chemical analysis specimen 3-BXl

Fractographic analysis specimen 3-DBB-2

b.2: Fractographic analysis specimen 3-DBB-l

c. South face:

Longitudinal web tensile test specimens 3-LSl , 3-LS2

Transverse web tensile test specimens 3-TSl , 3-TS2

Parts of metallographic specimens 3-BMl , 3-BM2

d. Top:

Longitudinal flange tensile test specimens 3-TFN, 3-TFS

A-46
A-47
A-48
A-49
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A-56
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bO •H

spunod 'avon

A-57
Figure A7.4.1-1 Transverse section through NBS weldment IC welded by the GMAW

process using mild steel welding rod. The weld is in the

center connecting flange parts of the two channel segments,

Parts of the web regions of each channel are also included in

the section. Etchant : two percent nital ~X1

Figure A7.4.1-2. Part of the transverse section shown above, but a higher

magnification. The light region in the center is weld metal.

The dark regions adjacent to the weld metal are the heat

affected zones. The light regions to the left and right in

the photograph represent base flange regions of the channel

segments. The line of small dark, elongated marks is the

result of microhardness measurements. Etchant: two percent

nital. X8.5

A-58
A.r.. -L- ST ft

Figure A7.4.1-3 Microstructure representative of the weld metal in transverse

section through NBS weldment IC. The microstructure appears

to consist primarily of bainite. Etchant: two percent nital.

X200

Figure A7.4.1-4 Transition region from weld metal (right) to heat affected

zone (left) in transverse section through NBS weldment IC

Etchant: two percent nital. X200

A-59
It

.-^r

Figure A7.4.1-5 Representative microstructure of the heat affected zone in

transverse section through NBS weldment IC . The microstructure consists of ferrite, primarily as a network in

prior austenite grain boundaries, bainite, partially

spheroidized carbides, and tempered martensite.

Etchant: two percent nital. X200

Figure A7.4.1-6 Microstructure representative of the transition region from

the heat affected zone (right) to base channel material (left)

in NBS weldment IC . Etchant: two percent nital. X200

A-60
^ . r ^ • '-^ ?n",.\ », »- , - ---5','

Figure A7.4.1-7 Microstructure representative of the base channel material

from NBS weldment IC. The microstructure consists of relatively fine grained ferrite (light) and pearlite (dark) in a normalized condition. Etchant: two percent nital. X200

A- 61
Figure A7.4.1-8 Transverse section IBMl through the bottom of walkway box

beam 9U (NBS 1). The north channel of the box beam is to

the left and the south channel is to the right. The weld

connecting the flanges of the two channels used to fabricate

the box beam is in the center. Parts of the web regions of

each channel had been removed before this photograph was

taken. The indentations appearing on the surface of the

section are the result of hardness measurements.

Etchant: two percent nital. ~X1

Figure A7.4.1-9 Part of the transverse section shown in figure A7.4.1-8, but

at higher magnification. The region at the bottom center is

weld metal. The dark gray regions on either side of the weld

metal are the heat affected zones associated with the weld.

The lighter regions at either side of the figure beyond the

heat affected zone are base channel material.

Etchant: two percent nital. X8.5

A-62
Figure A7.4.1-10 Transverse section 1BM2 through the bottom of walkway box beam

9U (NBS 1). The north channel of the box beam is to the left

and the south channel is to the right. The weld connecting

the flanges of the two channels used to fabricate the box beam

is in the center. Parts of the web regions of each channel

had been removed before this photograph was taken. The indentations appearing on the surface of the section are the result

of hardness measurements. Etchant: two percent nital. ~X1

Figure A7.4.1-11 Part of the transverse section shown in figure A7.4.1-10, but

at higher magnification. The region at the bottom center is

weld metal. The dark gray regions on either side of the weld

metal are the heat affected zones associated with the weld.

The lighter regions on either side of the photomacrograph are base channel material. Etchant: two percent nital. X8.5

A-63
'A,

Figure A7.4.1-12 Representative microstructure of the weld metal in transverse

section IBMl through box beam 9U (NBS 1). The microstructure

consists primarily of bainite. Some inclusions are evident.

Etchant: two percent nital. X200

Figure A7.4.1-13 Representative microstructure of the weld metal in transverse

section 1BM2 through box beam 9U (NBS 1). The microstructure

consists primarily of bainite. Etchant: two percent nital.

X200

A-64
Figure A7.4.1-14 Patch of hard material (dark streak) at the transition from

weld metal (top right) to the heat affected zone (bottom

left) in section IBMl . The elongated dark areas are hardness

impressions. Etchant: two percent nital. XlOO

Figure A7.4.1-15 Hard area similar to that shown above, but at higher

magnification. The hard area is in the center running from

side to side. The microstructure appears to consist of fine

grained tempered martensite. Parts of two hardness impressions can be seen at either side of the figure.

Etchant: two percent nital. X800

A-65
16 Transition zone from weld metal (right) to heat affected

zone (left) in section 1BM2. Some inclusions can be seen. Etchant : two percent nital. X200

17 Region representative of the heat affected zone in

section IBMl. The microstructure appears to consist of

ferrite, both as a network in prior austenite grains and

as needles, bainite, partially sheroidized carbides, and

tempered martensite. Etchant: two percent nital. X200

A-66
Figure 7.4.1-18 Region representative of the heat affected zone in

section 1BM2. The microstructure appears to consist of

ferrite, both as a network in prior austenite grains and

as needles, bainite, partially spheroidized carbides, and

tempered martensite. Etchant: two percent nital. X200

Figure A7.4.1-19 Transition from heat affected zone (right) to base channel

material (left) in section 1BM2. Etchant: two percent

nital. X200

A-67
Figure A7.4.1-20 Region representative of base channel material in

section IBMl . The microstructure consists primarily

of ferrite (light) and pearlite (dark) in the normalized

condition. Etchant: two percent nital. X200

A-68
Figure A7.4.1-21 Transverse section 2BM1 through bottom of walkway box beam

8L (NBS 2). The north channel of the box beam is to the

left and the south channel is to the right. The weld

connecting the flanges of the two channels used to fabricate

the box beam is in the center. Parts of the web regions of

each channel had been removed before this photograph was taken. The indentations appearing on the surface of the

section are the result of hardness measurements.

Etchant: two percent nital. ~X1

Figure A7.4.1-22 Part of the transverse section shown in figure A7.4.1-21, but

at higher magnification. The region at the bottom center is

weld metal. The dark gray regions on either side of the

weld metal are the heat affected zones associated with the

weld. The lighter regions on either side of the figure

beyond the heat affected zones are base channel material.

Etchant: two percent nital. X8.5

A-69
Figure A7.4.1-23 Transverse section 2BM2 through bottom of walkway box

beam 8L (NBS 2). The north channel of the box beam is

to the left and the south channel is to the right. The

weld connecting the flanges of the two channels used to

fabricate the box beam is in the center. Parts of the

web regions of each channel had been removed before this

photograph was taken. The indentations appearing on the

surface of the section are the result of hardness

measurements. Etchant: two percent nital. XI

Figure A7,4.1-24 Part of the transverse section shown in figure A7.4.1-23, but

at higher magnification. The region at the bottom center

is weld metal. The gray regions on either side of the

weld metal are the heat affected zones associated with the

. weld. The lighter regions on either side of the figure

beyond the heat affected zones are base channel material.

Etchant: two percent nital. X8.5

A-70
Figure A7.4.1-25 Representative microstructure of the weld metal in transverse

section 2BM1 through box beam 8L (NBS 2). The microstructure

appears to consist primarily of bainite. Some inclusions are evident. Etchant: two percent nital. X200

Figure A7.4.1-26 Representative microstructure of the weld metal in transverse

section 2BM2 through box beam 8L (NBS 2). The microstructure

appears to consist primarily of bainite. Some inclusions are evident. Etchant: two percent nital. X200

A-71
Figure kl .h .1-21 Transition zone from weld metal (right) to heat affected

zone (left) in section 2BM1 . Etchant: two percent nital.

X200

Figure A7.4.1-28 Transition zone from weld metal (right) to heat affected

zone (left) in section 2BM2. Etchant: two percent nital.

X200

A-72
Figure A7.4.1-29 Region of the transition zone from weld metal to the heat

affected zone in section 2BM2. This region is one of the

hard areas. The microstructure appears to consist of

tempered martensite. Part of a hardness test impression can

be seen at the left center. Etchant: two percent nital.

X1200

Figure A7.4.1-30 Representative microstructure of the heat affected zone in

section 2BM1 . The microstructure appears to consist of

ferrite, both as a network in prior austenite grain

boundaries and as needles, bainite, partially spheroidized

carbides, and tempered martensite. Etchant: two percent

nital. X200

A-73
Figure A7.4.1-31 Transition region from heat affected zone (right) to base

channel material (left) in section 2BM1

.

Etchant: two percent nital. X200

r.:.^v:-

^^^^ '^^nc.'^

Figure A7.4.1-32 Region of section 2BM1 representative of the base channel

material in both sections through box beam 8L. The microstructure consists primarily of ferrite (light) and pearlite

(dark) in the normalized condition. Etchant: two percent

nital. X200

A- 74
Figure A7.4.1-33 Transverse section 3BM1 through the bottom of walkway box

beam 9M (NBS 3). The south channel of the box beam is to

the left and the north channel is to the right. The weld

connecting the flanges of the two channels used to fabricate

the box beam is in the center. Parts of the web regions of

each channel had been removed before this photograph was

taken. The indentations appearing on the surface of the

section are the result of hardness measurements.

Etchant: two percent nital. ~X1

Figure A7.4.1-34 Part of the transverse section shown in figure A7.4.1-33, but

at higher magnification. The region at the bottom center

is weld metal. The gray regions on either side of the

weld metal are the heat affected zones associated with the

weld. The lighter regions at either side of the figure

beyond the heat affected zone are base channel material.

Etchant: two percent nital. X8 .

A-75
Figure A7.4.1-35 Transverse section 3BM2 through the bottom of walkway box

beam 9M (NBS 3). The south channel of the box beam is

to the left and the north channel is to the right. The

weld connecting the flanges of the two channels used to

fabricate the box beam is in the center. Parts of the

web regions of each channel had been removed before this

photograph was taken. The indentations appearing on the

surface of the section are the result of hardness

measurements. Etchant: two percent nital. ~X1

Figure A7.4.1-36 Part of the transverse section shown in figure A7.4.1-35, but

at higher magnification. The region at the bottom center is

weld metal. The gray regions on either side of the weld

metal are the heat affected zones associated with the weld.

The lighter regions at either side of the figure beyond the

heat affected zones are base channel material.

Etchant: two percent nital. X8 .

5

A-76
Figure A7.4.1-37 Representative microstructure

section 3BM1 through box beam

consists primarily of bainite.

Etchant : two percent nital.

of the weld metal in transverse

9M (NBS 3). The microstructure

Several inclusions are evident.

X200

'f-

"1 u

1'

Figure A7.4.1-38 Representative microstructure

section 3BM2 through box beam

consists primarily of bainite.

X200

of the weld metal in transverse

9M (NBS 3). The microstructure

Etchant: two percent nital.

A-77
J.

^ ^

«1 ' ' •.

Figure A7.4.1-39 Transition zone from weld metal (right) to heat affected

zone (left) in section 3BM2 . Some inclusions can be seen primarily in the weld metal. Etchant: two percent nital.

X200

Figure A7.4.1-40 Region representative of the heat affected zone in

section 3BM1 . The microstructure appears to consist of

ferrite both as a network in prior austenite grain boundaries and as needles, bainite, partially spheroidized

carbides, and tempered martensite. Etchant: two percent

nital. X200

A-78
Figure A7.4.1-41 Transition zone from heat affected zone (right) to base

channel material (left) in section 3BM2.

Etchant: two percent nital. X200

Figure A7.4.1-42 Representative region of base channel material in

section 3BM1 . The microstructure consists of ferrite

(light) and pearlite (dark) in the normalized condition.

Etchant: two percent nital. X200

A-79
W ^ *' - * 'Alt - ^ V ^ *

r ... • r

Figure A7.4.2-1 Transverse section through hanger rod NBS 5A showing the

representative microstructure . The microstructure consists

of relatively fine grained ferrite (light) and pearlite

(dark) in a normalized condition.

Etchant: two percent nital. XlOO

Figure A7.4.2-2 Transverse section through hanger rod NBS 5B showing the

representative microstructure. The microstructure consists

of ferrite (light) and pearlite (dark) in a normalized

condition. Etchant: two percent nital. XlOO

A-80
^r;->v^i3-,--^r--v-'S--^:v I'r.

^ - ^ fr \ yr *^

^ y >. j; -7 *v'*•-

Figure A7.4.2-3 Longitudinal section through hanger rod NBS 5A showing the

normalized microstructure consisting of ferrite and pearlite.

Etchant: two percent nital. XlOO

Figure A7.4.2-4 Longitudinal section through hanger rod NBS 5B showing the

normalized microstructure consisting of ferrite and pearlite.

Etchant: two percent nital. XlOO

A-81
Figure A7.4.3-1 Microstructure of washer from location 9MW In the walkway

debris. The thin, light layer at the surface (horizontal

near the top in the figure) is the decarburized layer at the

washer surface. The microstructure consists primarily of

tempered martensite. Etchant: Picral. X200

Figure A7.4.3-2 Microstructure of NBS hard washer. The light layer at

the surface (horizontal near the top in the figure) is

decarburized. The microstructure consists primarily of

tempered martensite. Etchant: Picral. X200

A-82
Section A7 .

The following tables and graphs

offer additional data on the

physical properties of walkway

concrete

A-83
Table A7.6-1 Bulk Specific Gravity of Drilled Concrete Cores

Core

Number

Weight (grams)

A (^) B (b)

A B C D E C-D

C-D C-D

4-02 215.4 206.2 222.8 105.1 222.7 117.7 1.83 1.75

03 238.3 227 .4 245.5 114.3 245.5 131.2 1 .82 1.73

04 282.3 269.4 291.3 137.5 291.3 153.8 1.84 1.75

06 299.2 286.4 310.1 147.0 310.0 163.1 1 .83 1.76

08 307.8 292.7 318.1 155.9 318.1 162.2 1.90 1 .80

09 287 .7 273.7 297 .1 140.6 297 .1 156.5 1.84 1.75

11 277 .5 264 .9 290 .9 134.7 290.7 156 .2 1.78 1.70

14 268.7 256.4 280.8 127.6 281 .4 153.3 1.75 1.67

15 265.5 253.6 276.5 130.3 276.4 146.2 1.82 1.73

17 323.9 308.9 336.2 157 .0 336.2 179.2 1.81 1.72

Notes: A = original weight

B = 24 hr oven dry weight

C = saturated surface-dry weight

D = weight immersed in water

E = saturated surface-dry weight after immersion

Bulk specific gravity, air-dry

(b) Bulk specific gravity, oven-dry

1 lb = 0.4536 kg

A-84
Table kl .6-1 Bulk Specific Gravity of Topping Material

Core

Number

Weight (grams)

A

(a) B (b)

A B C D E C-D

C-D C-D

4-02 CO "7 JO . / 5/ .

62 • 7 34 .

/" O "7 62.7 28 .

2.06 2.02

03 CI "7 51 . / 51 .0 C C "7 55 . / 30 .

c c c 55.5 z5 .

2.05 2.02

04 41.5 /I 1 41 .

An n 4y . y 24 .

49 • 9 25.4 1.63 1 .62

06 o o c 2o . D CO o OA O 2.06 1.97

11 38.6 38.2 41 .2 23.4 41.2 17.8 2.17 2.15

14 46.3 44.5 51.7 23.9 51.7 27.8 1.67 1 .60

15 58.3 55.8 64.3 29.1 64.3 35.2 1 .66 1 .59

17 58.7 56.3 65.6 31 .0 65.6 34.6 1 .70 1.63

Notes: A = original weight

B = 24 hr oven dry weight

C = saturated surface-dry weight

D = weight immersed in water

E = saturated surface-dry weight after immersion

Bulk specific gravity, air-dry

(b) Bulk specific gravity, oven-dry

1 lb = 0.4536 kg

A-85
CO

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4-1 4-1 r-~ 00 in

m

m o ao ^ <f m O CO 00 <r in CM 00

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0) 5-1 U O o to a C

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4-1 CO &0 tu C H

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A-86
o

o_o

O.OOl 0.002 0.003 0.004

STRAIN IN INCHES/INCH

Figure A7.6-1. Stress-strain plot for concrete core 4-01

o

o

0.001 0.002 0.003 O-OO-l

STRAIN IN INCHES/ INCH

Figure A7.6-2. Stress-strain plot for concrete core 4-05

A-87
0.001 0.002 0.003

STRAIN IN INCHES/INCH

0.00-1

Figure A7.6-3. Stress-strain plot for concrete core 4-07.

0.001 0.002 0.003

STRAIN IN INCHES/INCH

0.004

Figure A7.6-4. Stress-strain plot for concrete core 4-10.

A-88
o

0.001 0.002 0.003 0.004

STRAIN IN INCHES/INCH

Figure A7.6-5. Stress-strain plot for concrete core 4-

o

o

CD

0.001 0.002 0.003 O.OO-t STRAIN IN INCHES/INCH

Figure A7.6-6. Stress-strain plot for concrete core 4

A-89
0.001 0.002 0.003

STRAIN IN INCHES/INCH

O.OO'l

Figure kl .6-7 . Stress-strain plot for concrete core 4-16

0.001 0.002 0.003

STRAIN IN INCHES/ INCH

0.004

Figure A7.6-8. Stress-strain plot for concrete core 4-18

A-90
o

CD

o.dol 0.002 ' 0.003 oToi STRAIN IN INCHES/INCH

Figure A7.6-9. Stress-strain plot for concrete core

A- 91
Section A9.2

Additional data on the weights

of walkway materials and spans are provided by the following

tables

.

A- 92
Table A9.2.1-1 Unit Weights of Walkway Materials

Item Unit Weight

Qt-Y"iir*t-iiY'!a1 Q-hdi:i1 O L I UC L U. L d. ± OLcG-L Dot- a TQP Manual of Steel Construction

1 1/2" X 20 Ga. Formed Steel Deck 2.20 Ibs/f t2

5/8" Type "X" Gyp. Bd . & Framing

(a) 5/8" Gyp. Bd

2.56 Ibs/f t2

(b) 24 Ga. Steel Studs 0.39 lbs/ft

(c) 24 Ga. Steel Runners 0.31 lbs/ft

Wood Handrail & Brass Extrusion 5.30 lbs/ft

Top Glass Grip 1 .78 lbs/ft

1/2" Tempered Plate Glass 6.54 Ibs/f t2

Bottom Glass Grip 5.19 lbs/ft

Steel Support Angle (4 x 3 x 3/8) 8.50 lbs/ft

Footlights 2.26 lbs each

Footlight Base Plate & Cap 1.33 lbs/ft

Carpet 0.60 Ibs/f t2

P PI H H T n p' 0.25 lbs/ft2

Sprinkler Pipes and Fittings Per AISC Manual of Steel Construction

Aluminum Slats 70.4 lbs/span
Table A9.2.1-2 Weight Analysis of Damaged Walkway Spans

Item Weight/Span (lbs)

U7-8 U8-9 U9- LO UlO-11 L7-8 L8-9 L9- LO LIO- LI

Cross Beams & Stringers 2191 7 2057 . 1 2057 .1 2174.5 2191 7 2057 .1 2174 .5

Box Beams — 343 . 8 184 .6 — 184 .6

Clip Angles 12 7 — 12.7 12 7 12 .7

1 1/4" (f) Hanger Rods — 110. 0 —

1 1/2" X 20 Ga. Formed Steel Deck 498 9 493. 4 493 .4 498 .9 498 9 493 .4 498 .9

Gyp. Bd . Attached to Span 1369 2 976 . 5 968 .6 836 .8 985 9 932 .7 850 .1

24 Ga. Steel Framing for Gyp. Bd

.

67 6 67. 6 67 .6 67.6 67 6 67 .6 67 .6

Gyp. Bd . Scattered on Deck — 52. 0 39 .0 41.6 26 0 a

0) 7 .8

Handrail Assembly 79 5 297 . 4 106 .2 63 6 eigh 155 .0

Top Glass Grip 102 .5 — 52 1 3

4-J O 1/2" Tempered Plate Glass Panels 416 .5 520. 6 208 2 c

0^

1 Broken Plate Glass Panels 52 .1 156. 2 13.1 52 .1

00

c Bottom Glass Grip & Support Angle 795 8 790. 0 790 .0 800.4 795 .8

CX CO 7 90 .0 800 .4

Footlights & Baseplate 95 9 89. 3 45 .8 24.4 69 .0 26 .7 33 .0

Carpet & Padding 187 .3 187. 3 187 .3

Sprinkler Pipes — 97 .1 64.4 43 .7 86 .9 58 .0

Miscellaneous Metal Work 100. 0 100 .0

Miscellaneous Debris 100 .0 — — 20 .0

Timber Cribbing 156 .2 59 .4

Embedded Steel Plate 10 .2

Broken Granite Panels 262 .5

Total 6125 .9 6241 . 2 4849 .4 4534.4 5627 3 4801 .8 4574 .6

A- 94
Table A9.2.1-3 Weights of Undamaged Walkway Span

Excluding Concrete Deck and Topping

Weight/Span (lbs)

Component

Description U7-8

L7-8

U8-9

L8-9

U9-10

L9-10

UIOLIO11

11

W16 X 26 Stringers & 1/4 x 6 Bent Plate 1847 .0 1798.6 1798.6 1829 .8

W8 X 10 Cross Beams & Clip Angles 344 .7 258.5 258.5 344 .7

MCB X 8.5 Box Beams & Clp Angles 92 .3 184.6 184.6 92 .3

1 1/4" (]) Hanger Rods (Fourth Floor)

(Second Floor)

130 .5 261 .0 261.0 130 .5

1 1/2" X 20 Ga. Formed Steel Deck 498 .9 493.4 493.4 498 .9

5/8" Type "X" Gyp. Bd . & Framing 2309 .4 2309.4 2309.4 2309 .4

Handrail Assy & Top Glass Grip 408 .3 408.3 408.3 408 .3

1/2" Tempered Plate Glass 1249 .4 1249.4 1249.4 1249 .4

Bottom Glass Grip & Support Angle 795 .8 790.0 790.0 800 .4

Footlights & Baseplate 1 OA Q. O 104.8 104.8 104 .8

L.arpeu o raauxng 187 .3 187.3 187.3 187 .3

Sprinkler Pipes 51 .1 oo . z. Q7 1 Q1 «. o

Aluminum Slats 70 .4 70.4 70.4 70 .4

Total for Fourth Floor Spans 8089 .9 8203.9 8212.8 8118 .0

Total for Second Floor Spans 7959 .4 7942.9 7951.8 7987 .5

A-95

NBS-1 14A I REV. 2-8C) U.S. DEPT. OF COMM. 1. rUDLILAllUN UK 2. Performing Organ, Report No. Om r UDIit-dllOn L/aLC BIBLIOGRAPHIC DATA

REPORT NO. SHEET (See instructions) BSS-143 •Aay 1^8 2

4. TITLE AND SUBTITLE

Investigation of the Kansas City Hyatt Regency Walkways Collapse

5. AUTHOR(S) R. D. Marshall, E. 0. Pfrang, E. V. Leyendecker, K. A. Woodward,

R. P. Reed, M. B. Kasen, and T. R. Shives

6. PERFORMING ORGANIZATION (If joint or other thon NBS. see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234 Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Same as Item 6.

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 81-600538

Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most s i gn i fi cant in formation . If document includes a significant

bi bl io graphy or literature survey, mention it here) An investigation into the collapse of two suspended walkways within the atrium area of

the Hyatt Regency Hotel in Kansas City, Mo., is presented in this report. The investigation included on-site inspections, laboratory tests and analytical studies.

Three suspended walkways spanned the atrium at the second, third, and fourth floor

levels. The second floor walkway was suspended from the fourth floor walkway which

was directly above it. In turn, this fourth floor walkway was suspended from the

atrium roof framing by a set of six hanger rods. The third floor walkway was offset

from the other two and was independently suspended from the roof framing by another

set of hanger rods. In the collapse, the second and fourth floor walkways fell to the

atrium floor with the fourth floor walkway coming to rest on top of the lower ^^7alk\^7ay.

Based on the results of this investigation, it is concluded that the most probable

cause of failure was insufficient load capacity of the box beam-hanger rod connections.

Observed distortions of structural components strongly suggest that the failure of the

walkway system initiated in the box beam-hanger rod connection on the east end of the

fourth floor walkway's middle box beam.

(continued, next page)

12. KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) building; collapse; connection; construction; failure; steel; walkway.

13. AVAILABILITY

[x] Unl imited Q For Official Distribution. Do Not Release to NTIS

[X] Order From Superintendent of Documents, U.S. Government Printing Office, Washington. D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

360

15. Price

USCOMM-DC 6043-P80
Investigation of the Kansas City Hyatt Regency Walkways Collapse

ABSTRACT - (continued)

Two factors contributed to the collapse: inadequacy of the original design for the

box beam-hanger rod connection which was identical for all three walkways, and a change

in hanger rod arrangement during construction that essentially doubled the load on the

box beam-hanger rod connections at the fourth floor walkway. As originally approved

for construction, the contract drawings called for a set of continuous hanger rods which

would attach to the roof framing and pass through the fourth floor box beams and on through the second floor box beams. As actually constructed, two sets of hanger rods

were used, one set extending from the fourth floor box beams to the roof framing and

another set from the second floor box beams to the fourth floor box beams.

Based on measured weights of damaged walkway spans and on a videotape showing occupancy

of the second floor walkway just before the collapse, it is concluded that the maximum

load on a fourth floor box beam-hanger rod connection at the time of collapse was only

31 percent of the ultimate capacity expected of a connection designed under the Kansas

City Building Code. It is also concluded that had the original hanger rod arrangement

not been changed, the connection capacity would have been approximately 60 percent of

that expected under the Kansas City Building Code. With this change in hanger rod

arrangement, the ultimate capacity of the walkways was so significantly reduced that,

from the day of construction, they had only minimal capacity to resist their own weight

and had virtually no capacity to resist additional loads imposed by people.

Supplementary material is contained in an appendix to this report and published as NBSIR 82-2465A.

A-98

ft U.S. GOVERNMENT PRINTING OFFICE: 1982 - 373-655


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