NBSIR 82-2465

Investigation of the Kansas City

Hyatt Regency Walkways Collapse

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, DC 20234

Feburary 1 982

—QC

100

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'ARTMENT OF COMMERCE

82-2^65 bureau OF STANDARDS

1982

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NBSIR 82-2465

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C.ZINVESTIGATION OF THE KANSAS CITY

HYATT REGENCY WALKWAYS COLLAPSE

R. D. Marshall

E. 0. 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

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, DC 20234

February 1 982

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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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 v;as 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 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 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
Iv
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 Bolling 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 f 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).

VI
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 connections

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

beam-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.

vil

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viii
TABLE OF CONTENTS

Page

ABSTRACT ill

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 Summary 10

3. DESCRIPTION OF THE WALKWAY SYSTEM PRIOR TO COLLAPSE 11

3.1 Introduction 11

3.2 General Layout of the Hyatt Regency Atrium 11

3.3 Structural and Architectural Features of the Walkways 12

3.4 Probable Sequence of Erection 13

3.5 Designation of Walkway Components 14

3.6 Summary 14

4. DESCRIPTION OF THE WALKWAY COLLAPSE 31

4.1 Introduction 31

4.2 Summary of Events Preceding and Following the Collapse 31

4.3 Eyewitness Accounts of Collapse 32

4.4 Estimate of Walkway Occupancy at Time of Collapse 33

4.5 Removal and Storage of Walkway Debris 35

4.6 Summary 35

5. SITE INVESTIGATION 43

5.1 Introduction 43

5.2 Survey of the Hyatt Regency Atrium 43

5.2.1 Embedded Plates and Bearing Seats 44

5.2.2 Fourth Floor to Ceiling Hanger Rods 44

5.2.3 Impact Points on Atrium Floor 45

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TABLE OF CONTENTS (Continued)

Page

5.2.4 Alignment of Walkways Prior to Collapse 45

5.2.5 Other Observations Relevant to the Investigation 46

5.3 Observations of Walkway Debris 46

5.3.1 General 47

5.3.2 Hanger Rod Pull-through 47

5.3.3 Box Beam Longitudinal Welds 48

5.3.4 Box Beam to Clip Angle Fillet Welds 49

5.3.5 Other Observations Relevant to the Investigation 50

5.3.6 Determination of Span Weights 51

5.4 Removal of Specimens 52

5.4.1 Criteria for Specimen Selection 52

5.4.2 Specimen Removal 52

5.4.3 Specimen Cataloging and Preparation for Shipment 53

5.4.4 Specimen Storage and Handling 54

5.5 Replication of Walkway Weld Fracture Surfaces 54

5.6 Summary 55

6. NBS STRUCTURAL TESTING PROGRAM 95

6.1 Introduction 95

6.2 Background 95

6.3 Overview 95

6.4 Test Specimen Description 96

6.4.1 Full Length Box Beam 96

6.4.2 Short Box Beam 96

6.4.3 Fabrication 96

6.5 Test Setup 98

6.6 Loading Equipment 99

6.7 Instrumentation 100

6.8 Data Acquisition and Reduction 101

6.9 Loading Sequence 101

6.10 NBS Short Box Beam Tests 103

6.10.1 Parameters 103

6.10.2 Short Box Beam Specimens 103

6.10.3 Welding Process 105

6.10.4 Weld Area 107

6.10.5 Other Observations 108

6.10.6 Summary 110

X
TABLE OF CONTENTS (Continued)

Page

6.11 NBS Full-Length Box Beam Tests Ill

6.12 Tests on Walkway Box Beam Ill

6.12.1 Full-Length Box Beam Test Ill

6.12.2 Short Box Beam Tests 112

6.13 Box Beam-Hanger Rod Connection Capacity 114

6.14 Conclusions 113

7. NBS MATERIALS EVALUATION PROGRAM 155

7.1 Introduction 155

7.2 Initial Inspection 155

7.2.1 Visual 155

7.2.2 Radiography 156

7.3 Mechanical Properties of Structural Steel and Weldments 156

7.3.1 NBS Channel Specimens 157

7.3.2 Walkway Box Beams 158

7.3.3 Walkway Hanger Rods 158

7.3.4 Walkway Box Beam Longitudinal Welds 159

7.4 Metallography and Hardness Measurements 159

7.4.1 Box Beam Weldments 159

7.4.2 Walkway Hanger Rods 161

7.4.3 Washers 161

7.5 Chemical Analysis 162

7 .6 Properties of Concrete 162

7.7 Conclusions 163

8. FRACTOGRAPHIC ANALYSIS 193

8.1 Introduction 193

8.2 Specimen Preparation and NBS Fracture Results 193

8.3 Walkway Fracture Replicas 194
TABLE OF CONTENTS (Continued)

Page

8.3.1 General Observations 194

8.3.2 Box Beam Longitudinal Welds 194

8.3.3 Washer/Nut Contact Area 195

8.3.4 Washers 195

8.3.5 Clip Angle Fillet Welds 196

8.4 Direct Analysis of Walkway Box Beam Fractures 196

8.4.1 Procedures 196

8.4.2 Results 197

8.5 Summary . 198

9. STRUCTURAL ANALYSIS 219

9.1 Introduction 219

9.2 Loads 219

9.2.1 Dead Load 219

9.2.2 Live Loads 221

9.3 Analysis of Walkway Structural Components 222

9.3.1 General 222

9.3.2 Box Beam-Hanger Rod Connections 222

9.3.3 Hanger Rods 223

9.3.4 Summary of Analysis 223

9.4 Forces Acting on the Hanger Rods at Time of Collapse 224

9.5 Walkway Deflections 224

9.6 Effects of Dynamic Excitation 225

9.6.1 Dynamic Characteristics of the Walkways 225

9.6.2 Sources of Dynamic Excitation 226

9.7 Summary 228

10. INTERPRETATION OF TESTS, ANALYSES AND OBSERVATIONS 239

10.1 Introduction 239

10.2 Comparison of Loads and Box Beam-Hanger Rod Connection

Capacities 239

10.3 Ultimate Capacity Based on Code Requirements 240

10.4 Probable Sequence of Collapse 241

10.5 Effect of Change in Hanger Rod Arrangement 241

Xll
TABLE OF CONTENTS (Continued)

Page

10.6 Quality of Materials and Workmanship 242

10.7 Summary 243

11. SUMMARY AND CONCLUSIONS 249

12. REFERENCES 253

APPENDIX A-1

A6.10 - A6.11 A-2

A7.2 - A7.5 A-19

A7.6 A-83

A9.2 A-92

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XIV
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 terms 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 vzas 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 Bolling endorsed the Mayor's request for NBS to conduct an impartial

investigation.

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

1
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

beam-hanger rod connection capacity are discussed in detail. The chapter con- cludes with an assessment of the capacities of the box beam-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 walkway 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 weldments, metallographic 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

2
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
4
2.1 2. DOCUMENTS REVIEWED BY THE NBS INVESTIGATIVE TEAM

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

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:

5
® AISC* "Code of Standard Practice for Steel Buildings and Bridges"

(No date)

® AISC "Specification for the Design, Fabrication and Erection of

Structural Steel for Buildings" (No date)

o 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.

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

S302 Framing Plan, Level 106 North March 30, 1978

S303 Framing Plan, Level 121 North March 30, 1978

S304 Framing Plan, Level 136 North March 30, 1978

S305 Framing Plan, Level 151 North March 30, 1978

S306 Framing Plan, Level 166 North March 30, 1978

S405.1 Steel Details May 121, 1978

S601 General Notes March 10, 1978

* American Institute of Steel Construction

** American Welding Society

6
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:

Shop Drawing Number Subject

E3 Framing plan at Kiev. 121 ’-0" and elevation at column lines

8, 9, and 10

E4 Framing plan at Elev. 136 '-0"

E5 Framing plan at Elev. 151 ’-0" and expansion bearing detail

30 Box beams, cross beams, and hanger rods

31 Stringers, slide bearing plates, and bearing seats

23 Beams at Elev. 167 ’-1" and hanger rod brackets

E2 Framing plan at Elev. 167 '-1"

El Truss elevation

8 Truss T3

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.

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

Regency 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
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.

#344 October 30, 1979

Mentions report "... that fitters were refusing to work on the 151 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 Lombardi, 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 Investl-

* A member firm of PBNDML.

8
gators 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

Kansas City Fire Department

Kansas City Police Department

Crown 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

9
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 Works 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

of the Hyatt Regency walkways and that provide information on events immediately

preceding and following the collapse:

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

of the walkway structural system were referenced in the project

specifications

2. 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.

3. Drawings obtained for review and reference included architectural and

structural drawings and the fabricator’s shop drawings.

4. 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.

5. Efforts to obtain copies of the structural design calculations were unsuccessful

6. Gypsum board encasement of the walkway structural steel was added during

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

7. A videotape containing footage of activities in the atrium immediately

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

team.

8. 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.

10
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

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. A412) ISSUED FOR CONSTRUCTION ON AUGUST 28, 1978, AND DOES NOT ACCURATELY

11
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 m) 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 concrete 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 walkway spans apparently were designed to act as composite beams and were constructed without shores (l.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). 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

* American Society for Testing and Materials.

12
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 W18 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 T1 , also shown in figure 3.12. A detail of the hanger rod to beam

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

to connect the third floor walkway hanger rods to truss T1

* 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.

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

13
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 was 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

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 SUMflARY

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.

14
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 (l.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.

15
Figure 3.

i

1 I TOWER

J I

ATRIUM

FUNCTION

BLOCK

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

16
o 0 0

N

29r-0" 29T-0” 29’-0"

Figure 3.2 Plan view of atrium main floor (first floor) at level 106.

17
Figure 3.3 Elevation of atrium between column lines 8 and 9, looking north.

18
Figure 3.4 Schematic of walkways as viewed from north wall of atrium.

19
Figure 3.5 Second floor walkway framing plan - level 121.

20
Figure 3.6 Third floor walkway framing plan - level 136.

21
Figure 3.7 Fourth floor walkway framing plan - level 151.

22
Figure 3.8 Details of walkway stringers.

23
OQX

00oLlJ c/5

CO

00u

LU

00

ir> X

C*J 4

24

Figure

3.9

Typical

transverse

section

of

walkway

deck.
9’-4 3/8” 12MCe«8.5|

1 5/16" DIA HLS ON

A SECOND FLOOR BOX BEAM a ^

NOTE: WHEN OIFFERENT. DIMENSIONS FOR THIRD FLOOR BOX BEAM ARE SHOWN IN BRACKETS | |.

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

Figure 3.10 Details of walkway box beams and cross beams.

25
EL. 105'-9”v

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

26
Figure 3.12 Framing plan for atrium roof.

27
Figure 3.13 Detail of walkway expansion bearing at column line 11.

Figure 3.14 Simplified detail of walkway structural system and

architectural treatment.

28
I

©

Figure 3.15 Unified designation of walkway components (fourth floor walkway

shown)

29

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.

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.

31
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 removed.

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 displacement 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

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 t.o 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

32
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 v/alkway.

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.

o 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

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 KMBC 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

33
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

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.

34
4 . 5 REMOVAL AND STORAGE OF WALKWAY DEBRIS ... 1 nf Hphris from the atrium area involved materials such as

with a specitic waxi^ y v f„„rth floor walkway spans, along with segments of

wer^moved by the Crown Center Redevelopment Corpora-

?o a*:areruse loc:^ed at 28th an^d WarwlcU Streets, Kansas City, on the

night of July 21—22»

T 1-17 99 two NBS engineers were given access to the Hyatt Regency atrium

H“%srr"

b r,:n.”Ln“n

remained on the atrium floor*

p •rvVri- pf T,ilv 92-23 the third floor walkway was removed by the Crown

measurements were obtained hy the Crown Center Redevelopment Corporatio .

These measurements are referred to later in this report.

Based on subsequent Inspections of the third floor walkway ‘^e

warehouse, the spans apparently were removed by the ?wfLcesaround the W8 x 10 crossbeams at the quarter points of each span. This nec tlr.red 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 a

U ^ of oVinrt- nf the Stringer to box beam connection on each end of the

span.'"\emoval then progressed each way from span M9-10 with each span retaining

ore"ox heTand remLnL of the hanger rods which were cut just above che

level of the handrail.

on August 13, the walkway hanger rods between the third floor and celling and

between the fourth floor and celling, embedded plates

3e„t

the hearing seats at column line 11 also were removed from the atrium and

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.

35
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

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

Span Number No. of Persons Standing

at East Handrail

No. of Persons Standing

Near Walkway Center Line

Fourth Floor Walkway

U7-8 3 0

U8-9 2 0

U9-10 1 0

UlO-11 1 0

Second Floor Walkway

L7-8 12 8

L8-9 12 6

L9-10 10 4

LlO-11 4 0

36
37

Figure

4.1

Walkways

shortly

after

the

collapse.

View

is

to

the

west-northwest

with

span

U7-8

sloping

down

and

to

the

right.

Spans

U8-9

and

U9-10

are

visible

behind

the

trees.

Photo

by

Greg

Smith,

reprinted

by

permission

of

The

Kansas

City

Times

, ©

1981,

The

Kansas

City

Star

Company
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 coining from broken

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

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

38
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.

39
Figure 4.4 Scene from third floor balcony on north side of atrium,

the southwest showing spans L7-8 and L8-9 of second floor Videotape courtesy of KMBC TV.

View to

walkway

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.

40
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.

41
42
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 sununarizes

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.

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).

43
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

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 celling 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).

44
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

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 com45
bined 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 8UE 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

rods 8UE, 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.

46
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 celling 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).

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 8UE and 9UE. However, part of the washer at location 9UW was

47
also missing (figure 5.13) j suggesting that the missing washers fractured and were lost during the collapse. Figure 5.10 provides clear evidence that a washer

vjas 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 (480 mm) from the beam ends except for box beam 8M 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

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

48
f had been provided. The faying surfaces were observed to be

flange to the point fflare-V-groove weld as defined In

could, in most cases es section 3.3, the welding symbol shown on ^hfsh:; d^wlngs called for a preguallfled partial iolnt penetration groove

weld. This Is further discussed In section 10.6.

Tn general observed variability In penetration along a given weld (figure 5.15)

reLrry-orrfiareirofyffw'i

oriented in t g c Resul*-s of field measurements of top and

the weld penetration (figure 5.16). Resul.s or tie

bottom longitudinal and Interior ack weld

of the

floor box beams are presented in table 5.7. More uetaiie p

report,

box beam longitudinal welds are presented in chapters 7 and 8 of this rep

weld fractures occurred predominantly along the center line of

the upper^longltudlnal welds between the hanger rod holes at location

and 9ur Here the weld failure appeared to have occurred predominantly at the

? iine fflvure 5.17). In some cases the flange tips were curled up at

welds

What appeared to be a crack was noted In the "f^„red at

location 8MW (figure 5.18). Similarly, an apparent small crack was ^oted

the lower extremity of 8ME (figure 5.18), apparently having

^ r^lorof Ltal flLhing left over from the beam cutting operatron. There was

no interior tack weld at this location.

5.3.4 Box Beam to Clip Angle Fillet Welds

t^e detail shown In figure 3.10. Clip angles had aeparated rom -e srde or

the other of each box beam

Tnd^Ercracks were observed

^rthfbottom fmet weldHf the clip Ingles that remained attached (see

49
I

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 8 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 8,

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

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

50
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 llUW 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 considered 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.

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 midspan 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.

51
5o4 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 shoxra 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

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

52
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 24 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

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

53
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 ractographlc 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

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

54
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.

3. 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.

4. 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.

5. With one exception, damage done to the walkway spans during the rescue operation was superficial.

* 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.

55
6. All of the fourth floor to ceiling hanger rods pulled through both the

lower and upper flanges of the fourth floor box beams.

7. Evidence indicates that all hanger rod washers were installed.

8. 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.

9. Observed distortions strongly suggest an initial failure at box beamhanger rod connection 9UE.

10. The actual box beam longitudinal welds can best be described as

f lare-V-groove welds.

11. Variability in penetration for a given box beam longitudinal weld was

not excessive, but penetration varied considerably from weld to weld.

12. Interior tack welds were highly variable in length, location, and

penetration. In some cases there was no interior tack weld.

13. In general, the clip angle to box beam fillet welds were of good

quality

14. 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.

15. The shear connections at column line 7 experienced large rotational

distortion prior to failure of the fillet welds.

56
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

8W 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 ceil

1 in = 25.4 mm

Table 5.2 Elevations of

(Referenced to

floor level)

Fourth Floor to Ceiling Hanger Rods

Elev. 0.00 assumed datum at first

Hanger Rod Intersection with Celling 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

57
Table 5.3 Coordinates of Major Impact Points on Atrium Floor

Hanger 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

58
Table 5.4 Alignment of Upper Hanger Rods and Fourth Floor

Walkway Relative to Second Floor Walkway

Upper Hanger Rods at Ceiling Fourth Floor Walkway

8E 0.09’ E E at South end of Walkway 0.03’ W

0.15’ S (Col. Line 7) 0.15’ s

8W 0.07’ E E at Col. Line 8 0.02’ w

0.09’ S 0.16’ s

9E 0.08’ E E at Col. Line 9 0.02’ w

0.14’ S 0.17’ s

9W 0.05’ E E at Col. Line 10 0.01’ w

0.10’ S 0.19’ s

lOE 0.08’ E E at North end of Stringers 0.00’

0.14’ S (Col. Line 11) 0.21’ s

low 0.01’ E

0.13’ S

1 in = 25.4 mm

59
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

8E* 0.87’ W

3.01’ s

8W* 0.96’ w

3.09’ s

9E 0.87’ w

0.82’ s

9W 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

60
Table 5.6 Length and Position of Box Beam Interior Tack Welds

Connection

Number Bottom Flange Top Flange

Net Length

(inches)

Dimension

A B

( inches)

Net Length

( inches

Dimension

A B

( 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

9UW 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 ~

9LW 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

PLAN VIEW

(Definition Sketch]

61
Table 5.7 Field Measurements of Weld Penetration at Ends

of Fourth Floor Walkway Box Beams

Weld Penetration (mm)

Bottom Flange Top Flange

Connection

Number Exterior Interior Exterior Interior

SUE 1.0 - 1.5 0 - 2.0 1.0 - 1.5 no weld

8UW Complete Peiletration(^) 2.0 no weld

9UE 1.5 - 2.0 2.5 1.5 2.0

9UW 1.5 - 2 . 0 (b) 1.5 - 2 . 5 (b) 1.0 - 2.0 1.0 - 2.0

lOUE 2.0 2.0 - 2.5 1.5 - 2.0 2.0 - 3.0

lOUW 1.5 2.0 - 3.0 Complete Peinetration(^)

Note: (^) Exterior and the interior welds merged, leading to

complete penetration.

Edge of hole badly damaged by nut and washer.

1 in = 25.4 mm

62
Table 5.8 Measured Lengths of Stringers and Stringer Connections

Connection

Number

Span

Number

Distance From End of

Stringer to End of

Adjoining Stringer

(inches)

Length of Stringer

(feet)

East West East West

U7

U7-8

0.47 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

UlO

UlO-11

* 4.57

29.39 29.38

Ull** 2.25 2.25

L7

L7-8

0.51 0.51

* 29.33

L8

L8-9

4.65 4.76

* *

L9

L9-10

4.84 4.76

28.88 28.85

LIO

LlO-11

4.72 4.69

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

63
Table 5.9 Measured Weights of Damaged Spans

Span

No.

Weight in lbs at Indicated Load Cell

Total

(lbs)

No.l No. 2 No. 3 No. 4

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 mm) north of

normal positions due to inaccessibility.

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

oO

Cross Beam Cross Beam

uieag SSOJQ ©

PUN VIEW © Column

(Definition Sketch)

North

(As-built configuration)

64
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

specification.

3. Box beam 9M. The full box beam

will be obtained by removing the bolts

connecting the stringer 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 (Ji 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

Scanning and transmission electron

microscopy. Assessment of weld failure mode.

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.

Replication and direct scanning

electron microscopy.

Validation of replication technique

for f ractographlc analysis.

1 In = 25.4 mm

65
Table 5.11 Locations of Concrete Cores

Core Number Span Number E-W Coordinate (feet) N-S Coordinate (feet)

(Dist. from toe (Dist. from

of east handrail north end of east

support angle) stringer)

4-01 UlO-11 2.85 10.3

02 •• 2.95 10.3

03 2.85 16.8

04 " 1.85 17.1

4-05 U9-10 5.85 9.3

06 2.80 9.0

07 " 2.20 14.5

4-08 LlO-11 2.90 11.5

09 2.35 13.0

10 2.90 15.2

(Dist. from toe (Dist. from

of west handrail north end of west

support angle) stringer)

4-11 L9-10 2.35 14.4

12 2.30 16.5

13 • 1 2.85 18.0

(Dist. from (Dist. from

inside face of north end of

west handrail

glass panel)

west stringer)

4-14 M8-9 2.60 14.0

15 *' 2.60 10.9

16 ft 2.60 8.9

(Dist. from (Dist. from

inside face of centerline of 1st

east handrail crossbeam from

glass panel) south end)

4-17 M9-10 3.00 3.5

18 •* 2.55 3.5

19 ’ 2.00 6.9

1 in = 25.4 mm

66
Table 5.12 Thickness of Topping Material

Span

Number

Thickness of Topping (inches)

Number of

Average Maximum Minimum Observations

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 nnn

67
Table 5.13 Specimens Removed From Walkway Debris

NBS Specimen

Number 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 8UE, complete with nut and washer.

5B 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 8UE, 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.

llA 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 8MW.

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 68
Table 5.14 Replicas of Fracture and Washer Contact Surfaces

Location Number of

SUE - B - Cl

- 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)

z

i

OUTER

HOLE

<LU

COXo

COo

a

zUi X7777777777}

1 F “ \

B \WASHER CONTACT

SURFACE

1 C

1 E

'' C2 Cl

' / . 1

PLAN VIEW

(Definition Sketch)

Replications

4

3

3

3

5

56

53

3

54

4

3

1

4

3

2

2

2

2

3

(BOX BEAM

69
FOURTH FLOOR

NORTH

tHALL

FOURTH FLOOR

SOUTH

SECOND FLOOR

NORTH

iHALL

T.L 2

SECOND FLOOR

SOUTH

Figure 5.1 Locations of embedded plates and bearing seats
DETAILS OF EMBEDDED PLATES - COL. LINE 7

t ^33/4 ”-^ 2 11/16”

XXXXXXX XXXXX^I^

X X

X X

X X

X X

X X

>

1

(T

1

5”

yp)

<

-t 1 ” y|-- ^ xxxxxxx-^xxxx

X X

X X

^

X

X X ^1/8” X X

X ^

X X

X X

X X

X X

X X

X X

X X

X X —T~ XXXXXXX x:

15/8'' J 1^4'/,”-*

1

(T

)”

yp)

^

X

5

X

X X

X X

X X

X X

X X

X X

X X —T— XXXXXXX X

1 1/8”

1 /I ” (TwbI iiypj

7UE 7UW

-4”-

1 3/8”,,

X

X

X

X

X

X

X

X

X

k-3 7/8”- —XXX

X

X

X

X

X

X

X

X

X

X"

X

X

X

V

X

X

3/16”

X

X

X

X

X

X T

2 %'

_jL

•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.

71
Figure 5.3 Tile damage caused by second floor hanger rod lOE.

Figure 5.4 Fourth floor to celling hanger rods. View from fourth floor

looking south.

72
11M

N

i

10M

10M

11U 10U 9U 8U

9M

9M

10U 9U 8U 7U 8M

11L 10L 9L 8L 8M

7M

Figure 5.5 Layout of walkway spans in warehouse.

73
74

Figure

5.6

General

view

showing

spans

of

fourth

floor

walkwav
Figure 5.7 Hanger rod pull-through at location 9UE.

Figure 5.8 End view of box beam at location 9UE.

75
76

Figure

5.9

Evidence

of

permanent

distortion

in

box

beam

ends

of

third

floor

walkway.
Figure 5.10 Deformation at underside, outer hole, location 9UE

pull-through of washer/nut assembly. Note Imprint

(A) and evidence of slip-stick progressive failure

due to

of washer

(B).

1 .

77
Figure 5.11 Box beam at location lOUW (Span U9 10).

Figure 5.12 End view of 1/4" x 6" edging plate at location lOUW

(Span UlO-11)

78
79
t300

cnCo

•H

4-> 03OO

•U

o3 0^

03B

03

03Xo

rO

U

0-1 03 ° B C 03 O 03 •H ^

4-1

3-1

X

O

O

4-1 ,13 03

•t-l 03 03 ,C3 P

0-1o OP

3-1 03 •P >

03 •r4 B P S 03

>—1 03 03

3-1 03

03 C3 > O

3J 1-M 0303 Td

,33 c O cd m

03

3-1 3

60

•H

Pm

80
Figure 5.16 Fracture through the weld at location 9UE, lower flange.

Weld porosity denoted by arrows.

81
Figure 5.17 Fusion line weld fractures between hanger rod holes

weld, locations 9UE(a) and 8UE(b)

upper flange

82
b) Location 8ME. End crack.

Figure 5.18 Apparent cracks (arrows) in lower welds of box beams in third

floor walkway.

83
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.

84
Figure 5.21 Pull-out failure of top fillet weld on box beam web, location 9UE,

north side.

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.

85
Figure 5.23 Tearing of top end of north clip angle at location SUE

Figure 5.24 Tear in box beam web at location SUE.

86
Figure 5.25 Embedded plate at connection 7UE.

Figure 5.26 Embedded plate at connection 7UW.

87
88
b) After removal.

Figure 5.28 Specimen no. 1 before and after removal.

89
a) Before removal.

b) After removal.

Figure 5.29 Specimen no. 2 before and after removal.

90
a) Before removal.

b) After removal.

Figure 5.30 Specimen no. 3 before and after removal.

91
1

Figure 5.31 Specimen no. 5A and 5B after removal.

92
Figure 5.32 Concrete core drilling.

Figure 5.33 Concrete cores.

93
94
6.1 6. NBS STRUCTURAL TESTING PROGRAM

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 discusses 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 9M.

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.

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

95
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 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

96
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 ram), 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 mm) 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.

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

97
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 ram) 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.

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

98
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.

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.

99
6 . 7 INSTRUMENTAT ION

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

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.

100
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-digltal 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).

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

101
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.

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 maximum load

capacity was achieved in all but one test prior to reaching the stroke

limit

102
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

o Washer Presence

• Washer Hardness

• Washer Thickness

• Nut Shape

• Nut Orientation

• Loading Configuration

• Initial Upper Rod Load

• Stringer-Box Beam Clip Angle Presence

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

103
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.

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

104
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 J1 in figure 6.14.

Type II 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

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

105
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 m^/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

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 A1 , A2 , and B1 , 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 Cl, C3, II, and 12, table 6.1) the average peak

106
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 K1 , K2, G1 , 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, FI, and F2, table 6.1) and unhardened washers

(specimens M2, N1 , 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 Weld 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) , Cl and C3 having

a shallow penetration weld, and A1 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.

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

107
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 Other 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 fall 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 speci- mens 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

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)

108
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

that washers were present prior to the collapse. Thus these data are not used

as part of the data base in estimating capacity.

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 were tested using hardened steel

washers. These were followed by a series of short box beams tests using

unhardened washers.

109
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 (R1 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.

6.10.6 Summary

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

110
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 r2 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 dimensionally 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 (LI

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

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.

Ill
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 nonlinearity.

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.

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

112
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

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

113
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

8UE, 9UE, 9UW, and lOUE. The weld lengths and depths were measured, the

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 8UE 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

114
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.

4. 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.

5. 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.

115
Connection Mean Ultimate Capacity 95% Confidence Range

kips (kN) kips (kN)

8UE 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)

These capacities ranged from 18.2 kips (81 kN) to 19.3 kips (86 kN)

with an average of 18.6 kips (83 kN)

6. 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)

116
Table

6.1

MBS

Short

Box

Beam

Series

Test

Data,

Hardened

Washers

K3WlD3dS

111

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0) £

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ujqi

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33p3

<<<<<<<<< < < < zzzzzzzzz«««*zzz

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• • • • • • < <

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PI an o;

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• • • • • < <

z z

;3KI33dS

’ ^ n ui '-0 to -y: H r- H

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117

fiotfs;

1.

Process

I

-

Gas

Metal

Arc

Weldlnf*

Prores.s

2

-

Flux

Cored

Arc

Weldlnp,

Process

1

-

Shielded

Metal

Arc

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n70l^

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U

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Metal

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Welding,

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spoa Z poa I

0*6 SV

jexnsxpusdjsj

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punoa

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< < < <

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c c

CiO u u

c u u

•^ < <

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a. a. 0. a.

118

Interior

weld

not

counted

due

to

apparent

lack

of

fusion

to

base

metal
sC

<0 H

N3WI33dS -9

OQ U U

Failure Mode

III

11

1 • • •

sdTH

‘pcoi

r>. m CM

Average

Weld

Dimensions

and

Area

‘eaav

aA O • <M •

• 1 r«* <y 1

Interior Weld

QnD ‘mdap

1 1 1

1 1 1

nns ‘i(33uaj

1 1 1 1 1 1

Exterior Weld

cnD ‘mdap ^ 1 CM

• 1 • <r 1 CM

‘ijjauax

35 35

Clip

Angle

Presence 3uasqv auasaaa

• • •

No.

of

Rods spoa 1 poa \

• e •

Initial

Rod

Load

kips

0‘6

s-i • • •

Nut Orientation jejns^puadjaa

XaiTBjea • • •

Nut Bevel Shape punoH

ajenbs • • •

Washer

Thickness uim

< < <

z z z

Interior Weld Position 31 OH

33 P3 < < <

z z z

Nominal

Interior

Weld

Length

„8/E 1

-I

..Z/I

0 • • •

Exterior

Penetrat

ion d33Q "OITOMS

PI3M ON

•

• •

Weld

Process^ Z

1

•

• •

N3;.'133dS cn CM ^

a u u

119

Notes:

1.

Process

I

-

Cas

Metal

Arc

Welding

Process

2

-

Flux

Cored

Welding

Process

3

-

Shielded

Metal

Arc

Welding,

E7014

Electrode

Process

U

-

Shielded

Metal

Arc

Welding,

E7018

Electrode
QO

QDVH

sO

<0 H

N3WI33dS J J J J ^ J ^

1

Fal

lure

Mode

III

II

I

•

• •

• • • •

sdT^

‘peoT ^ead

r*» fsi O

^ O' O ^ ^ ^ rsj ^ ^ ^

Average

Weld

Dimensions

and

Area

^ona

* eajV

oO O O •/*' O vO

r>. in \0 rg O -"y >0 in r-» es <M r- o >0

Interior Weld

*L|adap

CM eg O O eg eg eg II • • 1 *1

1 1 eg eg 1 eg 1

nnn

‘ ijjSuax

eg eg eg eg

1 1 m in 1 eg 1

I 1 m 1 ^ 1

Exterior Weld

QJXD ‘M^dap

e*. O nD O e^ O' ^ e^ ^ eg eg

ono

‘ mSuax

in in in in -j in

<n eg e^ CO fn m fo

Clip

Angle

Presence ^uasqv

3uasajd

No

.

of

Rods spog z pog X

Initial

Rod

Load

kips

0-6

S'Z

Nut Orientation aeX"^TP*^3<*-*®d

XaxXEJBg

Nut Bevel Shape punoH

aaanbs

Washer

Thickness uxm

’laim

Interior Weld Position

ax OH

a3p3

Nominal

Interior

Weld

Length „8/e I

-1 „Z/I

0

• •

Exterior

Penetration

daac

ftoxieqs

PI art ON

Weld

Process^ Z

1

N3W133dS ^ eg in ^ r*» ^ J ^ ^ ^ ^ j

0) 4; u u

o c

U3 UJ

*o 00 u u ^ c u u ^ < 3 TS^ ^

U 4» <« 00 b 3 4J 4J < 4> 4) T3 s: r ^ 4;

flj U *o O 0) 4(

41 O TJ *0 S ^

X 4> 41 « 3 *4 ^

<q *H £ j; o b. c/) (/)

—• fNJ PO

4) 4/ 4) 4» U U U U

O O O O

u u u u

a. a. a. 0.

4>3

*tJ TJ

4) 4Jc

3O

U

120
Table

6.5

Walkway

Box

Beam

Test

Series

Data

gN3WI D3dS ^ ^ (M r*t U O U U

O ^

Fa1

Lure

Mode

III

III • • • • • •

sdjn

*peoT Head

19.4

20.0/23.6 17.0 16.3 14.7 15.2

Average

Weld

Dimensions

and

Area

‘eajV ^ O

X

X 00OX <s| CD 9^ 00

Interior Weld

nna •qjdap

® 1 1 1 1 • •till ^ 1 1 1 1

nnn * qaSuax

17 35

Exterior Weld

nnu ‘qidap

XX 9' ^ ^ Cvl (M

Qnn ‘ qjBuax

lA iTN

X tP3 iTi ^ ^ ^ ^

Clip

Angle

Presence ^uasqv

luasajd • • • • • •

No.

of

Rods spoa z poa 1 ! Initial

Rod

Load

kips

0-6

s-z

Nut

Orlentat

Ion jexnoxpuadjad

XauEjed

V

Nut Bevel Shape

puno)i

djpnbs

<f *9 -9 • • • •

r>. • •

Washer

Thickness uxqx

>i3iqi ^ lA

X fA

Interior Weld Position aioH

38 P3 • •

N.A. N.A. N.A. N.A.

Norn

Inal

Interior

Weld

Length -8/E 1

-I

..Z/I0 • • • • • •

Exterior

Penet

rat

Ion daaa noxxpqs

Plan ON

<sj r>i CM rsj fsi <<<<<• z z z z z

Weld

Process^

VEZt

A4 fM P^ fsj <N < < < < < z z z z z •

gNSUIDSdS j ^ rsj U U U

U

O ^ ^

*o 00uu

U 4» <0 U 3 4-i ^ < 0> 0) -o r r ^ 0*

egU

w o & 0(

Cl o ^ ^

S' ^ X 01 01 « 3 ^ —

<0 ^ £ o t*- tn c/j — fM m <r U U U (J o o o o

k. iw

U k* a. a. b. a.

m8«

01 XiXo

Xiu0X«

01 u

01»89

if XXoX

00

121

MBS

unhardened

washer.
Table 6.6 Cross-sectional Dimensions

Sec. Property

Nominal

Walkway Specimens

NBS Section

Weight d bf tw(in)

Ib/ft (in) (in) Max Min

8.5 8.00 1.874 0.179

8.84 8.050 1.860 0.194 0.185

8.43 7.975 1.833 0.186 0.182

bf

>

d

' -

122
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

Average

Depth, 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 2.4 130.2

11* 3.4

lOUE 35 2.4 35 1.9 150.5

lOUW 33 1.5 33 2.5 132.0

* A portion of 9UW was deeply penetrated.

123
1.0 in. = 25.4 mm

Figure 6.1 Short box beam test specimen.

124
Figure 6.2 Exterior weld penetration.

N

PARTIAL PENETRATION

EXTERIOR WELD

< NO WELD

TACK

mo

8

”

Figure 6.3 No weld specimen.

125
Figure 6.4 Interior weld location.

126

FRAME

Figure 6.6 Detail of full length box beam test setup drawing.

128
Figure 6.8 Clip angle strap on full length box beam.

129
Figure 6.9 Pin and clevis arrangement on full length box beam.

130
Figure 6.10 Short box beam test

REACTION BUTTRESS

1.0 in. = 25.4 mm

Figure 6.11 Short box beam test setup.

131
NOTE: LP - Linear Potentiometer

RP - Rotary Potentiometer

LVDT - Linear Variable Differential Transformer

Figure 6.12 Full length box beam instrumentation.

132
|4| (5) 161

ir— ^ {

5 »- 1- 1—

VD (41 VD QA

IS) I9l —

j

U i 1 ) TRANSVERSE LVDTs

Figure 6.13 Short box beam instrumentation.

133
134
o

Figure 6.15 In-plane load deflection curves for specimens C-3, F-2,

and T-3.

o

TRANSVERSE WEB DISPLACEMENT IN INCHES

Figure 6.16 Out-of-plane web load displacement curves for specimens

C-3, F-2, and T-3.

135
25

4 3 12 3 1 3 1 2

J

PROCESS

1

GMAW

2-

FCAW

3 • SMAW-E7014

4 • SMAW-E7018

SHALLOW penetration" • Hard washer

Unhardened

washer

DEEP PENETRATION

o Hard washer

0

No interior weld Vi” interior weld 1 3/8” interior weld

Figure 6.17 Effect of weld process.

136
Figure 6.18 Effect of exterior weld penetration.

Figure 6.19 Effect of interior weld length on peak load.

137
PEAK

UPPER

ROD

LOAD

(kips

Figure 6.20 Effect of weld area on peak load, hardened washers.

138
Figure 6.21 Effect of clip angles.

139
Figure 6.22 Effect of initial upper rod load.

140
Figure 6.23 Effect of loading configuration.
Figure 6.25 Effect of nut shape.

142
Figure 6.26 Effect of washer presence.

143
Figure 6.27 Effect of unhardened washers.

144
NUT FACE PARALLEL TO BEAM

LONGITUDINAL AXIS

Figure 6.28 Nut orientation.

145
PEAK

UPPER

ROD

LOAD

(kips;

Figure 6.29 Effect of weld area on pullout load for short box beams.

146
Figure 6.30 Comparison of full-length box beams with short box beam results.

147
Figure 6.31 Effect of weld area on pullout load for NBS tests.

148
Figure 6.32 In-plane load vs deflection curve for full length box beam

specimen taken from third floor walkway.

O

Figure 6.33 In-plane load vs displacement curves for specimens KC1-KC4

and I3.

149
Figure 6.34 In-plane load vs deflection curve for specimen KCl.

Figure 6.35 Deformed shape of walkway specimen KCl.

150
151

Figure

6.36

Walkway

deformed

shapes.
Figure 6.37 Comparison of walkway test series with NBS test results.

152
Figure 6.38 Estimated ultimate capacity for walkway box beam-hanger rod

connections.

153
20.3

18.3 16.3

Figure 6.39 Summary of ultimate capacities.

154
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 weldments.

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.

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

155
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). Two 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-f ilm 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 milliampere-mlnutes , 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.

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 which

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.

156
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 extensometer 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.

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 psi (490 MPa) with a range of 70,500 to 71,500 psi (486 to 493 MPa). The average yield strength

for all specimens was 44,800 psi (309 MPa) with a range of 43,700 to 45,500 psi

(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 psi (400 to 552 MPa), a minimum yield strength of 36,000 psi

(248 MPa), and a minimum elongation of 21 percent in 2 in (50.8 mm). All values

157
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 (0.180 to 0.189

in (4.57 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 psi (289 to 325 MPa) with an average of 44,400 psi (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

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.

158
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 psl (431 MPa) to 69,000 psl

(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 psl (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 psl (690 KPa) of the specified minimum and is considered to have met the

criterion in view of the estimated + 360 psl (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 weldment tested considering the small weldment area

sampled by each specimen. Figure 7.9 shows 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.

7.4 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.

159
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

rod holes at locations 8UE, 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

160
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 metallographically and were tested for

hardness. The microstructure, consisting of ferrite and pearlite in a normalized

condition, is considered typical of A36 structural steel. The microstructure

in NBS 5B was slightly coarser (ASTfi 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 SUE, 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 9MW that had been used in an NBS test on

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 decarburlzed 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.

161
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

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 ASTM C642-75 (Standard Test Method for

162
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,!) for 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/m^) 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/m^).

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.

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

chemical compositions between NBS and walkway box beam channel

material

163
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-ferrite 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 weldment 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.

164
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165
Table 7.2 Walkway Box Beam Tensile Specimen Identification

Specimen Code Specimen Orientation

A. Box Beam 9U (NBS 1)

1-LSl longitudinal

1-LS2

1-LNl

1-LN2

1-TSl

1-TS2

1-TNl

1-TN2

transverse

1-TFS longitudinal

1-TFN

B. Box Beam 8L (NBS 2)

2-LSl longitudinal

2-LS2

2-LNl

2-LN2

2-TSl

2-TS2

2-TNl

2-TN2

transverse

ft

2-TFS longitudinal

2-TFN

C. Box Beam 9M (NBS 3)

3-LSl

3-LS2

3-LNl

3-LN2

3-TSl

3-TS2

3-TNl

3-TN2

3-TFS

3-TFN

longitudinal

transverse

longitudinal

tt

Location

web south face

south face

" north face

north face

web south face

south face

" north face

" north face

flange top south

” top north

web south face

south face

" north face

" north face

web south face

" south face

" north face

’* north face

flange top south

top north

web south face

" south face

" north face

" north face

web south face

" south face

" north face

" north face

flange top south

" top north

166
Table 7.3 Tensile Test Results - Walkway Box Beam Channel

Specimen

Ultimate Tensile

Strength, psi(^)

Yield Strength

0.2% Offset, psi(^) Elongation, %(c)

Reduction o Area,

A. Box Beam 9U (NBS 1)

1-LSl 69,000 44,200 36.0 56

1-LS2 70,000 43,700 32.6 55

1-LNl 70,000 44,400 32.4 54

1-LN2 71,000 45,300 33.4 54

1-TSl 69,000 44,600 31.0 47

1-TS2 69,500 45,800 30.4 48

1-TNl 71,000 47,100 29.6 48

1-TN2 71,000 44,700 33.8 47

1-TFS 69,500 43,400 36.6 57

1-TFN 70,000 43,200 35.2 61

B . Box Beam 8L (NBS 2)

2-LSl 71,000 45,900 28.4 54

2-LS2 69,500 43,700 33.0 53

2-LNl 71,000 46,800 33.6 54

2-LN2 70,500 44,300 34.8 56

2-TSl 71,500 45,600 29.6 48

2-TS2 70,500 45,200 27.6 46

2-TNl 71,000 44,500 29.8 48

2-TN2 71,000 46,000 29.0 48

2-TFS 70,000 45,400 38.0 59

2-TFN 70,000 43,200 37.0 60

C. Box Beam 9M (NBS 3)

3-LSl 70,000 43,200 31.4 54

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 E8-81.

Rounded to nearest 100 psi in accordance with ASTM Standard E8-81.

Rounded to nearest 0.2 percent in accordance with ASTM Standard E8-81.

Rounded to nearest 1.0 percent.

1000 psi = 6.9 MPa

167
Table

7.4

Results

of

Tensile

Tests

on

Walkway

Hanger

Rods

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168
Table 7.5 Results of Tensile Tests on Walkway Box Beam Weldments

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

169
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 Percent Weld Area

SUE 63

9UE 55

9UW 58

lOUE 92

Penetration to 2.2 mm 48

LONGITUDINAL TACK WELD

170
Table 7.7 Results of Washer Hardness Tests

Hardness

Washer Number of HRC (surface) HRC (core)

Identification Measurements Average Range Average Range

SUE Top 8 40.2 39.0-41.0

8LW Top 8 43.0 40.0-45.0

9ME Top 8 37.5 34.5-40.5

9MW Top 8 44.0 42.5-45.0

8 MW 5/5* 40.0 39.0-41.0 40.3 39.7-40.7

9UE 5/5 43.6 42.5-45.0 43.3 40.7-44.2

lOMW 5/5 40.2 39.0-41.5 42.7 42.7

NBS Hardened (1) 5 39.6 38.5-41.9

NBS hardened (2) 4/5 39.0 38.5-39.5 39.8 37.7-40.7

NBS hardened (3) 5/5 37.5 36.0-38.5 39.2 37.7-39.7

NBS hardened (4) 4/5 40.5 40.0-41.0 41.9 41.7-42.2

NBS Regular Grade 5 [60.7]** [58.9-61.3]**

* surface/core

** Rockwell Hardness B (HRB)

Microhardness Survey

Washer

Identification Average HK5 oo(HRC) Range HK5 oo(HRC)

8MW 456 (44.3) 395-476 (39.5-45.5)

9UE 516 (48.4) 485-532 (46.5-49.5)

lOMW 456 (44.5) 450-457 (44.0-44.5)

NBS hardened (1) 457 (44.5) —

NBS hardened (2) 444 (43.5) —

NBS hardened (3) 442 (43.1) 323-465 (32.0-45.0)

171
Table 7.8 Chemical Composition Range of Base Materials

Element

NBS Box(a)

Beam Channel

Percent by Weight*

Walkway Box^b)

Beam Channel

Walkway(^)

Hanger Rod

ASTM A3

Specification

Carbon 0.179 - 0.183 0.181 - 0.188 0.159 - 0.200 Max 0.26

Phosphorus 0.012 - 0.013 0.016 - 0.017 0.009 - 0.038 Max 0.04

Sulfur 0.022 - 0.023 0.016 - 0.017 0.033 - 0.046 Max 0.05

Notes

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).

172
Table

7.9

Chemical

Composition

Range

of

Weld

Metal

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Table

7.10

Condition

of

Concrete

Cores

Prior

to

Testing

Wire

Reinforcement

o

13

<S

Yes

Visible

Cracks o s> s> S<f^ l>^

Yes X X X

General

Condition

Poor

or

Damaged

X X X

Fair X X

Good S<^ Sf^ *s<^ s«^ s<^ s<^ »r«^ 1*^ 1*^

Excellent X X X X X

Span Number

UlO-11

tt If If

U9-10

ti If

LlO-11

It ft

L9-10

If If

M8-9

ft M

M9-10

tf If

Core Number i-^cMcn-d*inv£)r^oooNOf-tcsico<rLnor^ooaN O O O O O O O O O ^ i~l f—f ^ 1—< rH r-^ i—J rH

1

174
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 106 5,450

02 ~ 0.60 114.0 109.0 128.3 125.8 — —

03 3.43 0.53 113.4 107.8 127.8 125.8 — —

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

^ o » ' ' Secant modulus at 40 percent of f^

Reduced to nearest 10 psi in accordance with ANSI/ASTM Standard C 42-74.

1 in = 25.4 mm

1 pcf = 16.02 kg/m^

1000 psi = 6.9 MPa

175
Figure 7. 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.

176
0009

spunod ‘avOH

o

o

(U (U

a)

)-i 3

W)

•H

177

0.01

0.02

0.03

0.04

STRAIN,

inches

per

inch
5000

spunod ‘avon

178

0.01

0.02

0.03

0.04

STRAIN,

inches

per

inch
spunod ‘avOH

179

Figure

7.4

Representative

load-strain

plot

for

NBS

channel.

Longitudinal

flange

specimen

BFl.
5000 ot o 0

Q.00

CM SI O O 6 .E<

GC

h"

CO

180

Figure

7.5

Representative

load-strain

plot

for

walkway

box

beam

8L

Longitudinal

web

specimen

2-LNl.
5000

spunod ‘avon

181

0.01

0.02

0.03

0.04

STRAIN,

inches

per

inch
5000

182

Figure

7.7

Representative

load-strain

plot

for

walkway

box

beam

9U

Longitudinal

flange

specimen

1-TFN.
10,000

Tl- od

COod

Oc

V.0)a

COo C\J ^ 9 o o c<

DCI

CO od

183

Figure

7.8

Representative

load-strain

plot

for

walkway

hanger

rod

specimen

5AT1.
z V

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

184
a

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

185
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

186
Figure 7.12 Cross sections through weld regions of box beam 8L. Sections were separated about 7.5 in (190 mm) along the beam length.

Microhardness indentations traverse the surfaces. X8 1/2

187
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

188
IS IS

IM —

U1IM

EB

ni

i/t s

U1a

m

1 UJ

:

IS

n

ut

AlaIM

Ut

Ut

m

I UJ

ZD

m

m

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CL

Un

UtIMarvi Ut

uta

I U

Z3

m

m

r3m

m

z

o

Vs^ UJ

m

m) Hid30 NDIiBdi3N3d

189

Figure

7.14

Penetration

depth

of

walkway

box

beam

longitudinal

welds.

Locations

9U,

8L,

9M

measured

on

cross

sections

of

beam

interior.

Locations

SUE,

9UE,

9UW

and

lOUE

measured

from

fractures

with

data

points

reflecting

indicated

millimeter

distances

from

beam

ends.
m pj ~

(WM) Hid3<l NQim3N3d £

U1fM

ca

njInaan

in

rvi

IMina

inaan

inAlaIM

in

inan

tn

rvj aIM

in

tna

m1 u

ZD

ISI 3

ZDmmj u

ZDmI LJ

ZDm

190

Figure

7.15

Overall

depth

of

interior

tack

welds

measured

from

fractures

with

data

points

reflecting

equally

spaced

measurements

from

beam

end

(left)

to

hanger

rod

hole

(right).

NONE

indicates

no

tack

weld

at

that

location.
a

b

Figure 7.16 Microstructures of washers from 9MW (NBS specimen 3),

hardened washer used in NBS tests, (b). Whiter layer

surface (top of illustrations) is decarburized layer,

structure is tempered martensite. Etchant: Picral.

(a), and of near Interior

X200

191
192
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 field

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). Failure

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

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

193
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 8UE, 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

through the porosity (figure 8.3). While many of the replicated sections provided reasonable resolution of the fractographic 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.

194
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 strlations 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.

Washers at locations 9UE and 8UE 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.

195
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 IIB,

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)

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 fractographic analysis:

8UE - north half

9UE - south half

9UW - north half

lOUE - south half

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

196
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 containing the fracture surfaces from the beam end to the outer hanger rod hole

were then prepared for fractographic 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

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, 8UE 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.

197
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 SUE 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

8. The fracture characteristics of the walkway and NBS washers varied from

predominantly ductile to predominantly brittle. However, the walkway

washers displayed a higher percentage of Intergranular fracture.

198
199

Figure

8.1

Comparison

of

quality

of

image

by

direct

SEM

(a)

and

by

replication

(b)

where

replica

contact

is

ideal.

Fracture

surface

is

located

at

root

of

upper

exterior

longitudinal

weld

of

walkway

box

beam

at

location

8UW

(north).

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Figure

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appearance

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No.

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SIDE VIEW

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END VIEW

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fracture surfaces for f ractographic analysis.

213
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218
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

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 concrete 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

219
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/m^) 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

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 17,750 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 concrete 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

220
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 2.6 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 2.3 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

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.

221
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-Hanger 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

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

222
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

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 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 connection 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.

223
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 walkway 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)

at location SUE. The estimated maximum load acting on a second to fourth 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

224
handrail assembly would have Increased this to O.Al 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 would 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

o 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 IIME.

• 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.

225
• 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

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

226
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 7UW 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

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

227
(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.

3. 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)

4. 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.

5. The fourth floor to ceiling hanger rods and the third floor walkway

hanger rods do not comply with the AISC provisions for allowable

stresses

228
6. 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.

7. The maximum walkway deflection due to dead load was approximately

0.5 in (13 mm).

8. Dynamic loads induced by walking or dancing on the walkways would not

have been significant in comparison to the static loads.

229
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(^)

Concrete

Deck(^)

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 9,500 9,500 7,960 17,460

L8-9 Span weight not measured 10, 050(b) 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; Includes topping material, shear studs and welded wire fabric.

Deck weight based on average unit deck weight of all other spans

as Indicated in table 9.2.

1 lb = 0.4536 kg

230
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(^)

(see table 9.1)

Weight of

Topping Material

(Y = 116.8 pcf)

Weight of Deck

Without Topplng(^)

Calculated Deck

Welght(^)

(Y = 113.5 pcf)

U7-8 , 9,630 360 9,270 8,640

U8-9 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(^) 8,560

L9-10 9,380 450 8,930 8,560

LlO-11 8,700 90 8,610 8,650

Note: 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/m^

231
Table 9.3 Composition of Span Weight for Typical Walkway Interior Span

Walkway

Component

Weight

(lbs)

Percent of

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(^) 950 5.3

Total 17,960 100.0

Notes: Includes topping material, shear

studs and welded wire fabric.

Includes formed metal deck,

footlights, carpet, sprinkler

pipes and aluminum slats.

1 lb = 0.4536 kg

232
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

233
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

234
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

(lbs)

7UE 3,800 200 4,000

7UW 3,800 60 3,860

SUE 18,900 2,490 21,390

8UW 18,900 1,060 19,960

9UE 18,210 2,080 20,290

9UW 18,210 830 19,040

lOUE 18,600 1,190 19,790

lOUW 18,600 420 19,020

HUE 3,930 60 3,990

IIUW 3,930 20 3,950

7LE 3,850 950 4,800

7LW 3,850 450 4,300

8LE 9,270 2,200 11,470

8LW 9,270 1,000 10,270

9LE 9,040 1,860 10,900

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

235
Table 9.7 Walkway Vertical Displacements Due to Dead Load

Dead Load Component

Vertical Displacements (Inches)

Location

8U&10U 9U 8L&10L 9L

Concrete deck and topping .22 .23 .32 .33

Handrail assembly .06 .06 .08 .08

Gypsum board .05 .05 .08 .08

Other items .01 .01 .02 .02

1 in = 25.4 mm

236
Table 9.8 Calculated and Measured Characteristics of the Third Floor Walkway

Calculated Measured

Mode

No. Classification*

Period

(Sec .

)

Frequency

(Hz) Classification*

Frequency

(Hz)

1 Hor.266 3.76 Hor. 3-3/4

2 Vert.127 7.88 V ert

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 Hor12

* Predominant motion.

237
Table 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 V ert

.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

9 Vert/Tors .122 8.21 .137 7.30

10 Tors.122 8.21 .137 7.30

17 Hor.079 12.65 .089 11.25

* Predominant motion.

238
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

as reported in section 6.13. The estimated mean ultimate capacity of 18.2 kips

(81 kN) is also shown. The connection at location SUE was required to support

forces imposed by combined dead and live load actions. These forces are shown

in the bar diagram to the right 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 SUE 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.

239
Figure 10.1 presents similar information for the other fourth floor box beamhanger rod connections. This figure shows that, with the exception of locations SUE 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. When 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

ceiling 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

240
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 fail 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 fillet 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

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

241
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

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.

242
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 corresponding weld area is 105 mm^ . From the plot of estimated peak upper rod load

vs index weld area on figure 6.37, 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)

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.

243
2 . With the exception of locations SUE 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 celling 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)

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.

244
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.

245
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Figure 10.1 Comparison of fourth floor walkway box beam-hanger rod connection

capacities with loads believed to have been acting at time of

collapse

246
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247
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248
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.

(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.

2. 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

249
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)

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

(a) With the exception of locations 8UE 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 sum 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.

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 )

(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.

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,

(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

250
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.

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.

251
(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.

252
12 REFERENCES

2.1 "Specifications, Hyatt Regency, Kansas City," PBNDML, 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 Skybrldge 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.

253
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.

254
NBS-H4A (REV. 2-80

U.S. DEPT. OF COMM.

4. TITLE AND SUBTITLE

Investigation of the Kansas City Hyatt Regency Walkways Col lap

^

se

j

5. AUTHOR(S)[^

D. Marshall, E.O. Pfrang, E.V. Leyendecker, K.A. Woodward,

R.P. Reed, M.B. Kasen, and T. R. Shives

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

NBSIR 82-2465

2. Performing Organ. Report No. 3. PubI ication Date

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State. ZIP)

10. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography 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

wal kway.

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. -( co ntin u ed on attached) ^

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

building; collapse; connection; construction; failure, steel; walkway.

13. AVAILABILITY 14. NO. OF

Unlimited

1 1 For Official Distribution, Do Not Release to NTIS

Order From Superintendent of Documents, U.S, Government Printing Office, Washington, D.C.

20402.

PRINTED PAGES

15. Price

Order From National T echnical Information Service (NTIS), Springfield, VA. 22161

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 virturally 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.
NATIONAL BUREAU

OF

STANDARDS

UNITED STATES DEPARTMENT OF COMMERCE

W W WASHINGTON, D.C. 20234

HOLD FOR RELEASE Mat Heyman

9:30 a.m., February 25, 1982 301/921-3181

CONNECTIONS IN COLLAPSED WALKWAYS AT KANSAS

CITY HOTEL FAR FROM ADEQUATE, NBS REPORTS

Critical connections in two Kansas City Hyatt Regency Hotel walkways

which collapsed July 17, 1981, were capable of supporting less than onethird of the load expected to be carried by a connection designed under

the Kansas City, Mo., building code. That was one of the major conclusions

in a report issued today by the Commerce Department's National Bureau of

Standards (NBS).

Dr. Edward 0. Pfrang, who headed up an NBS investigation of the

nation's worst building collapse, today identified the two factors

related to those box beam-hanger rod connections which were critical in

causing the walkways' failure.

First, Pfrang said, the "connections as initially detailed and

approved for construction provided a capacity far below the capacity

that would have been required to satisfy the Kansas City Building Code."

Second, a change in the supporting hanger rod arrangement "essentially

doubled the load to be transferred" by the connections, "thus further

aggravating an already critical situation."

more
i

- 2 -

Shortly after the accident which killed 113 people and injured 186

others, Kansas City Mayor Richard L. Berkley asked NBS to investigate

the failure. The mayor's request was endorsed by members of the Missouri

congressional delegation. NBS, a federal science and engineering research

laboratory which traditionally has served in a third party role to

resolve technical controversies, agreed to determine the most probable

cause of the collapse.

The NBS investigation included a review of various constructionrelated documents, on-site and laboratory structural and materials

investigations--including testing of selected parts of the walkways'

debris and NBS replicas of important walkway components--and extensive

engineering analyses.

The bureau's findings are given in a 349~page report submitted

today to Mayor Berkley in Kansas City and reviewed by Pfrang, Chief of

the NBS Structures Division, at a news conference in Gaithersburg, Md.

Walkway's Structure

Three suspended walkways spanned the Hyatt Regency atrium, which

connected a high-rise section and a lower "function block." The second

floor walkway was suspended from the fourth floor walkway directly above

it by six steel hanger rods. In turn, this fourth floor walkway was suspended from the atrium roof framing by another six hanger rods.

The third floor walkway, which was not involved in the collapse,

was offset from the other two walkways and was independently suspended

from the roof framing by another set of hanger rods.

The hanger rods from the roof and from the second floor walkway

passed through the ends of the fourth floor walkway box beams. These

box beams were made up of pairs of 8- inch steel channels which were

welded together. The hanger rods were secured by washers and nuts after

passing through the box beams. Under this arrangement, the fourth floor

box beam-hanger rod connections transmitted the loads of both the second

and fourth floor walkways.

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

atrium floor after the fourth floor to ceiling hanger rods pulled through

at the walkway box beam connections.

The NBS Investigation

To meet requirements of the Kansas City Building Code, Pfrang said,

the fourth floor connections between the walkways' box beams and supporting

hanger rods each should have been designed to handle 40,700 pounds--

including the weight of the walkways and a specified design live load of

people. However, the Kansas City code also incorporates a margin of

safety for steel components. Walkway connections designed according to

the code to have a capacity of 40,700 pounds would actually be expected

to have an ultimate capacity of at least 68,000 pounds.
- 3

-

After weighing the damaged walkway spans and estimating the number

of people on the walkways at the time of the collapse, NBS researchers

concluded that the maximum load on any of the six connections at the

time of the collapse was 21,400 pounds. Thus, the load on the walkways

was far below that required to be handled under the Kansas City Building

Code.

After conducting extensive structural and materials tests on selected

parts of the walkways and NBS-fabricated replicas, bureau researchers

determined that the average ultimate capacity of each fourth floor

hanger rod-box beam connection was just 18,600 pounds compared to the

expected ultimate capacity of 68,000 for code-complying connections.

Thus, the connections were capable of supporting only 27 percent of the

load expected to be supported by connections designed under the Kansas

City Building Code.

Since the loads on the connections from the weight of the walkways

and people on the walkways at the time of the collapse were so near to or in excess of the estimated ultimate capacity of the actual connections,

failure could have begun at any of the fourth floor box beam-hanger rod

connections, NBS researchers determined. Progressive failure of the

other connections and the walkways' collapse was assured after any one of the connections failed and the load from that connection was distributed

to the other connections. Based on studies of the walkways debris, NBS

researchers were able to pinpoint a connection at the middle fourth

floor box-beam as the most likely to have failed first.

A key factor contributing to the collapse, the NBS report noted, was that the walkways were not constructed in accordance with the contract

drawings. As originally approved for construction by the Kansas City

Codes Administration Office, the plan for the walkways called for a single set of hanger rods (attached to the roof framing) which would

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

beams. Under this arrangement, each box beam would transfer its own

load directly into the hanger rods.

However, during construction the hanger rod arrangement was changed

to an "interrupted" hanger rod scheme so that the fourth floor box beam

connections were required to transfer the load of both the fourth and

the second floor walkways.

This change essentially doubled the load on those fourth floor con- nections. While NBS researchers concluded that this original hanger

rod-box beam connection detail still would not have satisfied the Kansas

City Building Code, they also determined that the original connections

as shown on the contract drawings would have had the capacity to resist

the loads estimated to have been acting on them at the time of the

collapse.

Based on a variety of materials tests of walkway components--

including the welds, nuts and washers, steel channels, and hanger rods--

NBS researchers determined that "Neither the quality of workmanship nor

the materials used in the walkway system played a significant role in

initiating the collapse."
4 -

The bureau's report also notes that in addition to the box beamhanger rod connections, the fourth floor to ceiling hanger rods and the

third floor walkway hanger rods violated the design provisions of the

Kansas City Building Code. According to NBS investigators, though, the

hanger rod code violations were not a factor in the collapse.

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