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Investigation of October 17, 1991
Roof Cable Structure Accident at Georgia Dome Construction Site,
Atlanta, Georgia

U.S. Department of Labor
Occupational Safety and Health Administration

March 1992

Investigation of October 17, 1991 Roof Cable Structure Accident at Georgia Dome Construction Site, Atlanta, Georgia - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Investigation of October 17, 1991
Roof Cable Structure Accident at Georgia Dome Construction Site,
Atlanta, Georgia


DOL Seal

U.S. Department of Labor
Lynn Martin, Secretary

Occupational Safety and Health Administration
Dorothy L. Strunk, Acting Assistant Secretary

Office of Construction and Engineering

March 1992

This report was written by
Mohammad Ayub
Fragrance Liu

Table of Contents
  1. Introduction
  2. Description of the Offshore Erection Procedure and Accident
  3. Conduct of the Investigation
  4. Interview Statements
  5. Laboratory Testing
  6. Discussion, Evaluation and Conclusion
  7. Appendix

Executive Summary:

A construction worker was killed and two workers were injured on October 17, 1991, at 2:40 p.m. when their work platform was struck by a collapsing steel post, 47 feet long, after a cable attachment plate fractured. The collision of the steel post with the work platform, 230 feet above ground, resulted in a worker falling to the ground and being killed. At the time of the accident, the workers were in the process of hoisting the center truss of the roof cable system during the construction of the Georgia Dome project in Atlanta, Georgia. There were six construction workers, employed by the roof erection contractor, on the work platform or on the bottom chord of the center truss when the accident occurred. The workers were using hydraulic pumps to apply loads to the temporary jacking strands in order to make the permanent diagonal cable connection to the center truss.

Representatives from the Occupational Safety and Health Administration's (OSHA) Atlanta West Area Office arrived at the scene of the accident site within four hours of the accident. The Office of Construction and Engineering, OSHA National Office, Washington, D.C., was requested to provided assistance in determining the cause of the accident. Personnel from the Office of Construction and Engineering visited the construction site to gather relevant information and documents for the investigation.

Based on eyewitness accounts, laboratory test results, observation of the fractured attachment plate and engineering calculations based on the estimated forces provided by the roof erection company in the cables immediately prior to the accident, the Occupational Safety and Health Administration concludes that:

  1. The accident occurred due to the collapse of a steel post following the fracture of an attachment plate located at the top of a hoop "A" post. The attachment plate was loaded in excess of its capacity. The overloading was the result of the bending moment created by the eccentrically applied tensioning force of the temporary jacking strands to the attachment plate.
  2. The fractured attachment plate was not adequately proportioned for the tensioning force applied during the roof erection immediately prior to the accident.
  3. Engineering calculations were not done, by the roof erection company, to determine and verify the structural adequacy of the attachment plate assemblies for the erection loads to which these plate assemblies were subjected during the erection of the roof cable system, as per contract document requirements.
  4. Workers, who were on the platform and involved in the specific construction activity preceding the accident, were not clearly instructed regarding the jacking sequence and the amount of tensioning force to be applied to the temporary jacking strands to make the final connection of the diagonal cables and the roof center truss. Written instruction or the erection drawings indicating the magnitude of tensioning force to be applied to the temporary jacking strands were not available to the erection crew.
1.0 Introduction

An accident occurred on October 17, 1991, at approximately 2:40 p.m. during the erection of the roof cable structure at the construction site of Georgia Dome in Atlanta, GA., resulting in one death and injuries to two construction workers. At the time of the accident the workers were located on top of a work platform, about 230 feet above ground. They were in the process of hoisting the center truss in position to make the final connection of a diagonal cable to the bottom joint of the center truss. While the temporary jacking strands were tensioned and the center truss was raised to about 12 inches within its final location, an attachment plate at the top a hoop "A" post fractured. Following the plate fracture, the post rotated and struck the work platform suspended below the bottom of the center truss, resulting in a worker falling to the ground and being killed. Two other workers on the platform sustained injuries to varying degrees.

A compliance officer from the OSHA Atlanta West Area Office arrived at the scene of the accident on 10-17-1991 at about 6.00 p.m. to gather information relating to the activities preceding the accident. The compliance officer took photographs and collected evidence and documents relating to the accident. The Office of Construction and Engineering, OSHA National Office in Washington, D.C., was requested by the Area Office to provide assistance in the investigation to determine the cause of the accident.

The OSHA investigation included the eyewitness accounts, interviews of the parties involved in construction, observations of fractured attachment plate assemblies, review of the cable roof erection procedure, construction documents and laboratory test report and engineering calculations. The engineering calculations were based upon the estimated magnitude of forces in cables connected to the failed attachment plate, immediately prior to the accident. The estimated cable forces were furnished by the roof erection contractor.

Throughout the course of this investigation, the Office of Construction and Engineering worked together with personnel of the OSHA Regional Office and OSHA Atlanta West Area Office.

2.0 Description of the Erection Procedure and Accident

The roof of the 70,500 seat Georgia Dome Stadium in Atlanta, Georgia is a cable supported structure of oval configuration, approximately 770 x 610 feet. The dome, in plan, consists of three elliptical configurations of decreasing dimensions at varying elevations, supported by a network of ridges cables, hoop cables, posts and diagonal cables. See Figures 2.01 and 2.02 for roof plans showing lay-out of ridge cable, hoop, and diagonal cable. Figure 2.03 indicates the cross-section of the roof structure along minor and major axes. The entire roof cable structure is supported at equally spaced connections by a concrete compression ring beam on the perimeter of the Dome. At the center of the Dome, a center truss, 35 feet deep and 184 feet long, spanning in the east west direction, connects the centers of the two circular segments.

The ridge cables consist of wire ropes of diameters varying from 3 inches to 1 inch. The diagonals consist of structural strands also of varying sizes of 3-58 inch to 1-3/8 inch in diameter. The vertical posts, are steel pipes of different heights spanning between ridge cables and the elliptically placed cables. A steel gusset plate at the top of the post is connected to an attachment plate which connects the ridge cables and diagonal cable. At bottom of the post is also a steel gusset plate which is connected to an attachment plate connecting the diagonal cables and the elliptically placed cables, called hoops. Figures 2.04 and 2.05 indicate typical vertical post details showing the gusset plates at both ends of the post. At some locations, as many as four ridge cables and two diagonals cables are connected to the attachment plate. The intersecting points of all the cables and steel post are named as "node" in the engineering drawings.

There are three cable hoops of elliptical configuration, identified as hoops "A", "B" and "C", consisting of two 3-18 inch diameter, four 4 inch diameter and four 3-58 inch diameter structural strands respectively. Each hoop is connected to the bottom of the steel posts and the lower end of the diagonal cables at 26 node points. The posts at the hoop "A" (identified as PA) are steel pipes of 16 inch diameter x 0.5 inch in thickness. The posts at the hoop "B" and "C" (PB and PC respectively) are 24 inch diameter x 0.562 inch thick. The top and bottom chords and the diagonals of the center truss are tension members consisting of structural strands. The vertical members of the center truss are compression members consisting of steel pipes. Ten ridge cables are spaced at equal angles and connected to the attachment plates of the end post members of the center truss. At the bottom of the end post of the center truss, 10 diagonals are connected to the plate with a similar detail. See figure 2.06 and 2.07 for center truss end post attachment plate details.

The accident occurred during the process of connecting the diagonal cable at node 1308 to the bottom of the east end vertical post of the center truss.

The construction of the Georgia Dome is administered by a team of four companies, hereafter called the construction managers. The fabrication and erection of the cable roof structure and the roof fabric are performed by a construction company, hereafter referred to as the erection company. The erection company was provided with a set of contract design drawings by the structural engineer of record to develop the erection procedure and prepare detail and erection drawings of the cable roof structure (shop drawings). According to the statement of the project engineer of the erection company, the structural engineer of record also provided a computer model of the roof structure to the erection company. The computer model indicated the final forces in all members of the roof cable structure under the application of dead load at the completion of roof cable assembly.

The method and sequence of erection of the cable system for the roof structure was not specified by the structural engineer of record. It was left to the steel erection company to select the system best suited for the job based upon the available resources of the erection company to deliver the finished assembled structure. The erection company prepared shop drawings for review and approval of the structural engineer of record through the construction managers. The submittal included a set of erection drawings and erection procedures. The erection drawings included details of the attachment plates, temporary weldment brackets, jacking cable layouts etc. The erection drawing also indicated the forces of all ridge cables and diagonals and the elevation of nodes of the roof structure at various stages of erection. No computations were submitted to the structural engineer of record for review and approval.

The erection procedure adopted and approved for use at the construction site consisted of assembling the entire cable ridge net on the ground and then lifting it in stages to the desired elevation. The approved erection procedure furnished by the erection company is attached in Appendix A. First of all, the entire ridge cable is laid out on the ground and with the help of a crane, the top attachment plates for the ridge cables were connected to the ridge cables at the appropriate locations. At this time, the ridge cables between the attachment plates were in a slack position. The center truss, temporarily supported by guys, was also assembled on the ground directly below its final location and placed vertically. Ridge cables were then connected to the center truss top chord joints. The lifting of the ridge net was then undertaken.

Lifting cables were placed from the top attachment plate of hoop C to the compression rings and to the. center truss. Fig. 2.08 is the layout of the ridge net lifting cable arrangement. Ridge net was then hydraulically lifted by jacking at selected locations. Ridge net was raised and the connections to the ring beam at the perimeter of the dome was completed. At this time, the center truss was also off the ground but distance away from its final position. Following the connection of the ridge net to the compression ring, hoop C cables were laid out in the elliptical configuration on the ground and jacked to position after which diagonal cables from the compression ring were connected to the "C" hoop. The PC posts consisting of steel pipes were raised by a crane and connections made with the attachment plates of the ridge cables at the top and with the hoop cables at the bottom. Thereafter, the "B" hoop cables were laid out on the ground and jacked to approximate location of the bottom of "B" posts suspended from the ridge net and the PB posts were lifted and attached to the ridge cables and hoop "B". Following this, diagonals were connected. The same steps were repeated for the "A" hoop. At the completion of hoop "A" installation, the PA posts were jacked up, the top and bottom connections were made in a similar manner. The installation of the diagonal cables to the top of PB posts were then completed. The final step was to raise the center truss to its final elevation and complete all the connections of the diagonal cable to the bottom chord of the center truss. At the time of the accident, the workers were in the process of applying tension force to the diagonal jacking strands in order to make the connection of the diagonal cable between the bottom of the east end post of the center truss and top of the PA post at node 1308.

As discussed earlier and shown in Figures 2.06 and 2.07, at each of the two end posts of the center truss, 10 ridge cables were to be connected at the top attachment plates of the end post and 10 diagonal cables were to be connected to the bottom attachment plates of the end posts. The 10 ridge cables had been connected to the top of each end posts prior to the accident. The truss was being raised to make the final connection of the diagonal cables to the bottom of the truss end post. Eight temporary jacking strands were connected to the top of the gusset plates of posts PA at nodal points 1303 through 1312 at the east end, and at nodal points 1316 through 1325 at the west end of the center truss, see Fig. 2.01. The other end of these eight temporary jacking strands from the nodal points mentioned above were connected to the bottom attachment plate of the center end posts. Two work platforms were suspended from the center truss, one on the east end and the other on the west end. The construction workers and the hydraulic pumps used to jack the temporary jacking strands were on the work platform. As per the erection company, only four of the eight temporary strands were supposed to be jacked, and the other four strands were provided as a means of "safety", which were to be used only in the final stage of the center truss hoisting. When the erection would approach near the final stage, all the 8 strands were to be tensioned for a force of 22.5 kips each, Le., a total of 180 kips.

The intent of the erection procedure as described by the steel erection company, in the interview after the accident, called for a force of 4 x 35 = 140 kips to be initially applied to a set of four strands at the diagonal cables at nodal joints mentioned above. A final tension force of 22.5 kips at each of the 8 strands, total of 180 kips (8 x 22.5 = 180 kips) was to be applied to the complete set of eight strands to bring the truss at the proper elevation, following which the diagonal cables would be permanently connected. This procedure, as explained above, was intended to be followed. However, on the day of the accident, all eight strands were jacked by the crew to a force of 30 to 35 kips each at the nodal point 1308. A force of 240 to 280 kips was therefore applied to the temporary jacking strands at nodal point 1308 resulting in fracture of the attachment plate at the top of "PA" post. Figure 2.11 shows the mating of the fractured sections of the attachment plate ( Photo was taken by the private testing laboratory, discussed in Section 5). The higher force in the jacking cables caused the fracture of the top attachment of "PA" post at node 1308 location. Subsequent to the fracture of the plate, the post rotated towards the work platform and struck the platform.

The top attachment plate which fractured is identified as TA-7 plate on the shop drawing. It is located at nodal point 1308 on the drawing prepared by the structural engineer of record. Figure 2.10 shows the elevation and plan of the plate assembly. The plate assembly is composed of four 1-14 inch thick steel plates identified as Pa, Pb. Pc and Pd. Plate Pd connects one ridge cable to the top and one diagonal cable to the bottom of the center truss end member. Plates Pc and Pd are the plates which connect the two cables from the top of post to the top of PB post of hoop "B" at nodes 1207 and 1208. The three plates Pd, Pc, and Pb are groove welded to a horizontal plate Pa, as shown in Figure 2.10. The assembly of all the four plates is then connected to the post top gusset plate with a 4 inch diameter bolt through a hole in the plate Pd, see Figs. 2.09 and 2.10. Fig. 2.09 also shows the temporary jacking strands connected to the top gusset plate of the post PA at the bracket with a 2" diameter high strength bolt. Plate Pa fractured along a horizontal plane adjacent to the end of plates Pb and Pc, plate Pd also fractured along a vertical plane at the end of plates Pb and Pc, and along a horizontal plane near the bottom side of the plate Pa, see Fig. 2.18 for fractured locations of the plate. Figures 2.12, 2.13, 2.14 and 2.15 are the east portion of the fractured plate TA-7. The west portion of the fractured plate was still attached to the top of the post in the rotated position, see. Figs. 2.16 and 2.17. Both portions of the fractured plate assembly were retrieved for examination by an independent testing laboratory by the construction managers.

 

Figure 2.01. Ridge Cable Layout Plan. For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.01. Ridge Cable Layout Plan

Figure 2.02. Hoop and Diagonal Cable Layout - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.02. Hoop and Diagonal Cable Layout

Figure 2.03. Roof Sections. Section Along Major and Minor Axis Depicted. For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.03. Roof Sections

Figure 2.04. Steel Post (PA) Details - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.04. Steel Post (PA) Details

Figure 2.05. Steel Post (PA) Details - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.05. Steel Post (PA) Details

Figure 2.06. Center Truss End Post Attachment Plate Detail - Top of the Post - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.06. Center Truss End Post Attachment Plate Detail - Top of the Post

Figure 2.07. Center Truss End Post Attachment Plate Detail - Bottom of the Post - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.07. Center Truss End Post Attachment Plate Detail - Bottom of the Post

Figure 2.08. Ridge Net Liftline Cable Arrangement - Bottom of the Post - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.08. Ridge Net Liftline Cable Arrangement

Figure 2.09. PA Post Attachment Plate and Bracket of the Jacking Strands - Bottom of the Post - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.09. PA Post Attachment Plate and Bracket of the Jacking Strands

Figure 2.10. Elevation and Section of the PA Post Attachment Plate - Bottom of the Post - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.10. Elevation and Section of the PA Post Attachment Plate

Figure 2.11. PA Attachment Plate After the Accident Showing the Mating Sections - Bottom of the Post - Post Hole, Pb, Pc and Pd - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.11. PA Attachment Plate After the Accident Showing the Mating Sections

Figure 2.12. East Portion of the Fractured Attachment PA-7 Plate - Bottom of the Post - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.12. East Portion of the Fractured Attachment PA-7 Plate

Figure 2.13a. East Portion of the Fractured Plate (another view) - Bottom of the Post - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.13a. East Portion of the Fractured Plate (another view)

Figure 2.13b. East Portion of the Fractured Plate (another view) - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.13b. East Portion of the Fractured Plate (another view)

Figure 2.14. East Portion of the Plate after the Fracture in an upside down position - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.14. East Portion of the Plate after the Fracture in an upside down position

Figure 2.15. East Portion of the Fractured Plate - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.15. East Portion of the Fractured Plate

Figure 2.16. Post PA in the Rotated Position after the Plate Fracture Post Hit the work Platform at the Left Side - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.16. Post PA in the Rotated Position after the Plate Fracture Post Hit the work Platform at the Left Side

Figure 2.17. Post PA and the West Portion of the Fractured Plate Rotated after the Accident - Work Platform at the east end of the center truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.17. Post PA and the West Portion of the Fractured Plate Rotated after the Accident

Figure 2.18. Elevation View of the Attachment Plate and Post Top Plate - Line of the Plate pa Fractured shown in plan of the attachment plate and Line of the plate Pd fractured shown in evaluation view of the attachment plate and post top plate - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2.18. Elevation View of the Attachment Plate and Post Top Plate

3.0 Conduct of the Investigation

The following construction documents were made available to the OSHA investigation team through the project manager of the Dome construction management group:

  1. Cablenet Erection Procedure prepared by the roof erection contractor, reviewed and approved by the structural engineer of record. (Appendix A)
  2. Structural Drawings relating to the structural notes, roof geometry, compression ring beam, ring beam cable anchor details, roof cable layout, center truss, cable connection detail etc. These drawings were prepared by the structural engineer of the record.
  3. Project Specifications, prepared by the structural engineer of record consisting of Section 700-Fabric Roofing System and Section 71 a-Fabric Roof Structural Steel. These specifications were furnished to the roof erection company for fabrication and erection of the fabric roof structure. (Appendix B-portion of the Specifications)
  4. Shop Drawings related to the roof erection sequences, attachment plates fabrication details, weldment lifting assembly, forces in the roof cables at various stages of roof erection etc. These drawings were prepared by the roof erection contractor and subsequently reviewed and approved by the project architect, structural engineer of the record and the construction management team of the project.
  5. Sketches of the revised top attachment plate assembly for the posts PA after the accident. The plates were redesigned by the structural engineer of record. However there were no structural calculations available associated with the redesign. (Appendix C)
  6. Outlines of "A Weldment Repair Procedure" proposed by the roof erection contractor to secure the stability of the roof cable after the accident. (Appendix D).
  7. Tabulations showing - "Jacking loads on temporary jacking stands - between "A" hoop and center truss" prior to De-jacking for the attachment plate reinforcements and the "Maximum Loads to Final Jack" after the accident. (Appendix E)
  8. Computer print out showing "Approximately nodal coordinates and cable force at the time of accident" from the erection company. (Appendix F)

Both portions of the fractured attachment plate TA-7 assembly were retrieved by the construction managers of the project and forwarded to an independent laboratory for testing. The tests was conducted to verify the mechanical and compositional properties of the fractured steel plate. The metallurgical examination was also conducted.

Interviews of construction workers and engineers were conducted to obtain accounts of the collapse, to determine the construction activities preceding the collapse and the design and erection procedures of the roof cable systems.

The structural calculations were made to compute the approximate steel plate stresses at its critical locations due to the jacking loads immediate preceding the collapse, based on the estimated forces provided by the roof erection company.

The conclusion regarding the cause of the failure was based on all the above information.

4.0 Interview Statements

Five construction workers were interviewed by personnel of the OSHA Atlanta West Area Office immediately following the accident. Among those interviewed were four workers who were located on the temporary work platform, 230 feet above stadium floor level and suspended under the bottom of the center truss. Another worker interviewed was at the concourse level at the time of the accident. The OSHA investigating team also interviewed the construction managers, the project engineer of the roof erection company and the structural engineer of record. The following are the summary of the highlights of their interview statements.

This table is best viewed on tablets, notebooks, or desktop computer screens.

Highlights of Interview Statements

Witness Number (Job Title)

Highlights of Interview

Superintendent Erection Co.

  • Were jacking the center truss.
  • Heard a pop very loud, then the mast broke loose.
  • An Ironworker directly hit by the falling mast.

Ironworker Erection Co.

  • Helping jacking crew.
  • The crew had pulled up with two pumps. One pump had come up to 35k and other was coming up about 30k.
  • Heard a sound like a loud bang. Nothing unusual noticed.
  • Went back pumping when heard another loud bang followed by the accident.

Rigger Erection Co.

  • Was running pump at time of the accident. Had 35k on pump.
  • Has started reacting pump.
  • Heard loud noise followed by accident.

Superintendent Structural Conc. Frame

  • Was on the main concourse level; at 33 A line.
  • Heard a loud noise. Saw a man going over the edge of the platform to the ground.
  • Saw another man hanging on the east side of the platform.

Rigger Erection Co.

  • On the work platform at the time of the accident.
  • Were working on hydraulic pumps which pulled up the cables to raise the structure
  • Heard a cracking sound on "A" hoop.
  • Wondered what was wrong, then started retracting my pump. "I was facing away from everybody. Accident occurred. Something hit me."

Project Engineer Erection Co.

  • Eight temporary cables were jacked, four with a tensile force of 35k each, four with a force of 30k each at the connection plate which fractured, immediately prior to the Erection Co. accident. The total force imposed on the fractured plate, prior to the accident, was in the order of 260 to 280 kips.
  • Erection procedure and details were reviewed and approved by the structural engineer of record. No calculations were made to verify the integrity of the connection plates for the construction loads. Since the erection forces in the cables were lower than the ultimate strength of the cables therefore, it was concluded there was no need to check the adequacy of the plate.
  • The workers were in error in applying this magnitude of force. They were instructed to apply a tensile force of only 35k in each of the four temporary jacks (total force = 140k) till the final lift in which case a force of 22.5k was to be applied in eight cables (total force = 180k).
  • The erection drawings indicated a force of 117k to be applied to the diagonal jacking cables at this connection plate. A decision was made at the site to increase the load of 117k to 140k.
  • Since the accident, it has been determined that a force of 180k, as estimated earlier, would not have been sufficient to make the final connection with the center truss. Instead, a force of 228k was needed to complete the erection.
  • The erection company provided a set of erection loads along with the estimated eccentricity to the structural engineer of record to redesign the connection plates after the accident.
  • The bolt used for the connection of the temporary jacking cable was 2" dia ASTM A354.
  • The center truss was approximately 12" to 16" away from final position prior to the accident.
  • The ultimate capacity of the jacking cable was 58 kips.
5.0 Laboratory Testing

The fractured portion of the top attachment plate at node point 1308 containing the plates marked Pb, Pc and a part of Pa and Pd was secured and delivered to a private testing laboratory by the construction management team for conducting metallurgical examination to determine the nature and origin of fracture. Tests were also conducted to verify mechanical and compositional properties of the steel plates. The following is a summary of the test results of the private testing laboratory based on a report dated, January 29, 1992, obtained from the construction managers.

  1. The plates Pa and Pd conformed to the compositional and tensile strength requirements of ASTM A572 Grade 50.
  2. The plates Pa and Pd satisfied the minimum toughness requirement of 15 ft-lbs at 70°F.
  3. The groove welds between plates Pa and Pd were satisfactory and no evidence of defects like porosity, entrapped slag or cracks were found.
  4. There were three origins of the fracture which were all located at the underside of plate Pa near the intersection of plates Pb, Pc and Pd. The examination of the fracture origins indicated that the failure was the result of overload tearing in a tough material. Inelastic (permanent) deformations had occurred at the surface of plate Pa adjacent to the fracture origins, and on the outboard surfaces of plates Pb and Pc adjacent to the end of plate Pa. The occurrence of inelastic deformation indicates that the connection had been subjected to loads, prior to fracture, which generated localized stresses in excess of the plate material's yield strength. Overload (overstress) condition precipitated the connection fracture.
6.0 Discussion, Evaluation and Conclusion
Erection of Roof Cable System

The erection of the dome roof system consisted of the "design, supply, fabrication, shipment and erection of these principal items - Fabric, acoustic liner, clamping system, cables, structural steels and all their fittings" as specified in the project specifications (Appendix B). The specifications were prepared by the structural engineer of record and furnished to the erection company as part of the contract documents.

In specification Section 700, paragraph 1.4 General Requirement, it is stated that:

  • General Requirements
  1. Continuously monitor the installation and the erection of the structural steel, steel cables, wire rope and the fabric membrane to insure that the structure is constructed in accordance with the engineering design and to insure that no construction loading either damages or over-stresses any part of the roof."

The specification Section 710, paragraph 1.4.A.8 also stated:

  1. The Roof Contractor shall be solely responsible for the determination of the erection sequencing and procedures. The Roof Contractor shall analyze the cable and steel structure at various stages in his erection sequence to ensure stability of the structure as well as to ensure that none of the temporary nor permanent components of the structure are overstressed."

Interview statements of the construction managers and project engineer of the erection company had revealed that there were no engineering design calculations made, by the roof erection company, to verify the adequacy of the steel attachment plate assemblies for the application of the temporary jacking load during the entire erection process. As per the project engineer of the erection company, "a comparison of the temporary jacking loads to the ultimate capacity of the cables were made to ensure that during no stage of the jacking operation, the jacking load would exceed the ultimate strength of the cables. Based on this premise, it was assumed that the attachment plate assemblies would be satisfactory to resist the temporary jacking loads during various stage of cablenet lifting, as long as the cables were not stressed in excess of their ultimate capacity."

The above specifications Section 710 paragraph 1.4.A also required:

  1. The above analysis results as well as pertinent calculations shall be submitted for review by the Engineer with the complete erection procedures and drawings. The Engineer's review shall in no way or means diminish the Roof Contractor's obligations and responsibilities to erect the structure in a safe and proper fashion." and
  2. The Roof Contractor's erection analysis, procedures and drawings shall be prepared under the supervision of a Professional Engineer registered in the Georgia and signed and sealed by him."

Shop drawings related to the roof erection sequences, details of attachments plates etc. (Item #4 in Section 3 of this report), were not signed and sealed by a Professional Engineering registrated in State of Georgia.

Construction Activities at the time of the accident

The lifting of ridge net assembly was underway and was close to the final stage at the time of the accident, as per the project engineer of roof erection company. The workers had completed all the permanent cables and posts connections with the exception of D1 cables. D1 cables are the diagonal cables connecting the bottom chords of center truss to the top attachment plates at PA posts. See Figures 6.01, 6.02 and 6.03. The D1 cables were in slack position and all the temporary jacking strands of the diagonal cables were tensioned to some initial loads to maintain the center truss in a position which was only a short distance away from its final elevation. The Jacking Sequence "D1 Cable Installation" as outlined in the "Cablenet Erection Procedure" (Appendix A) was in progress.

As per the erection company, immediately preceding the accident, the workers on the east work platform were using two hydraulic pumps and applying load to each of the 8 jacking strands, that were attached to the top of the post PA at node 1308. The east bottom joint of the center truss was only few inches (12 to 16") away from its final connection locations. According to the workers on the east work platform, while the strands were being jacked, they heard a loud bang sound at which time they stopped the pumps, but did not notice anything unusual. Soon thereafter they resumed pumping, then they heard another loud noise, following which they saw the PA post collapse. The jacking loads were approximately 30 kips in one pump and 35 kip in the other pump. A total of 240 kips to 280 kips force were applied to the jacking strands, when the accident occurred.

The intent of the erection procedure as described by the steel erection company, in the interview after the accident, called for a force of 4 X 35 = 140 kips to be initially applied to a set of four strands at the diagonal cables. A final tension force of 22.5 kips at each of the 8 strands, total of 180 kips (8 X 22.5 = 180 kips) was to be applied to the complete set of eight stands to bring the truss at the proper elevation, following which the diagonal cables would be permanently connected. This procedure was intended to be followed. However, on the day of the accident immediately prior to the accident, all eight stands were erroneously jacked by the crew to a force of 30 to 35 kips each at the nodal point 1308. A force of 240 to 280 kips was therefore applied to the temporary jacking strands at nodal point 1308. This force was higher than the intended force of 180 kips, as per the roof erection company.

Loads in The Jacking Strands of PA Posts:

Appendix E of this report contains the tabulations showing "Jacking Loads on Temporary Jacking Strands Between A Hoop and the Center Truss". Column 1 of the tabulations are loads that existed in all the 26 nodal joints of hoop A. These jacking loads were recorded by the roof erection company prior to the "De-jacking" of the strands for the repair and/or reinforcement of the PA plates. With the exception of "zero" force in node point 8 (which is node 1308 of the fractured plate) , the range of jacking loads that existed immediately prior to the accident was 20,000# to 132,000#. These loads were the initial jacking forces in the temporary strands between the A hoop and the center truss. Column 2 of the tabulation are the "Maximum Load to Final Jack". These are the forces actually applied to the D1 diagonal jacking strands to make the final cable connections after the repair/replacement of attachment plates were completed. These forces were also recorded by the roof erection company. The range of loads was 80,000# to 228,000#. The jacking load applied at node 1308 to make the final permanent connection with the redesigned attachment plate was 228 kips which was approximately 27% higher than the original intended loads of 180 kips.

The Attachment Plate TA-7 of Node 1308

The attachment plate TA-7 of node 1308 consists of four 1-14 inch thick steel plates identified as Pa, Pb, Pc and Pd in the shop fabrication drawings, prepared by the roof erection company. Plate Pb, Pc and Pd were groove welded to the Horizontal plate Pa as shown in Fig. 2.10. Plate Pb and Pc each has one 5-18 inch diameter hole where the ridge cable connects to the top posts of hoop B. Plate Pd has two 5-18 inch diameter and one 4-18 inch diameter holes, the 5-18 " holes were for the ridge cable to center truss top joint and the diagonal cable to the center truss bottom joint connections, the 4-18" hole was for the post PA top plate connection.

As per the structural engineer of record, at the completion of the roof structure, the forces in all four cables (3 ridge and 1 diagonal) and the steel post will be acting toward the W.P. (Working Point) concentrically. The W.P. is located 2 inches from the left side of the plate Pa center line as indicated in the detail of Fig. 2.10.

The method of the roof cable erection adapted by the contractor as described earlier, called for the center truss to be raised to its final location by jacking of the temporary diagonal strands between top of hoop A post and the bottom of the center truss. The diagonal jacking strands were attached to a welded bracket at the post PA top steel gusset plate and the welded bracket was connected to the post gusset plate by a 2 inch diameter high strength bolt, see Figures 6.04 and 6.05. During the jacking process, the jacking load applied to the strands was transmitted to the attachment plate through this 2" diameter bolt. Due to the offset position of the bolt hole with respect to the attachment plate assembly, the application of the jacking loads would not be concentric with the other cable forces during cablenet lifting operation. Hence, a bending moment was introduced to the attachment plate assembly by the eccentricity of the jacking loads.

Following the accident, the roof erection company proceeded securing all the PA posts by lashing all the attachment plates together and then visually inspected all the plates. All the DI temporary strands were then de-jacked and the PA top attachment plate assemblies were reinforced. Appendix C and Appendix D indicate revised plate assembly details and an outline of the repair procedure. The redesign was performed by the structural engineer of record and was based on the maximum eccentric force to be applied to the jacking strands, during the final erection after the accident.

Location and Forces of Node 1308 at The Time of The Accident

Appendix-F of this report contains the computer print-out, which identifies the coordinates of all the nodal joints and the forces which existed in all the cablenet members at the time of the accident. This information was provided by the roof erection company to the OSHA investigation team after the accident. From the above computer print-out and the contractor's shop drawings (drawing # 9080D1010 & 9080D1011), it was determined that the location of node 1308 was approximately 11 "-10" lower than its final elevation at the time of the accident. Nodal joints 1207 and 1208, at top of Hoop B, were about one foot lower than node 1308 and were approximately 1'-5" higher than their final location. The center truss east top joint was about 18 feet higher than node 1308, and was approximately 12 feet lower than its final elevation. See Fig. 6.06 for positions of the cables at the time of the accident and their final positions at the completion of the roof erection.

Forces in the two ridge cables between node 1308 to the nodes 1207 and 1208, at the top of hoop B post, were approximately 99 and 100 kips. Force in ridge cable between node 1308 and the center truss was approximately 89 kip and force in PA post was approximately 19 kips, as per the same computer print-out. The diagonal D1 cable was not connected to the bottom of the center truss, and therefore had no force.

Based on the eccentric forces of 240 to 280 kips in the diagonal jacking cables and the relative position of the nodal joints discussed above, the tensile stress at the groove welded joint of the plates Pa/Pd adjacent to the end of plate Pd plate was determined to be 62 to 72 ksi, and the flexural tensile stress at the bottom of plate Pa adjacent to the left end of the plates Pb and Pc determined to be 76 to 88 ksi. These stresses were higher than the yield strength of the plate material. As per the lab test results, the average yield strength of the plate Pa and Pd was 59 ksi and 55.5 ksi respectively and the average ultimate tensile strength of the plate material was 85.5 ksi, see Fig. 6.07. (test results taken from the lab report). At the time of the accident, the stresses in the critical locations of the attachment plate assembly due to the eccentrically applied jacking forces were determined to be either approaching or exceeding the ultimate tensile strength of the material. The high stresses caused the plate to fracture.

The above stresses were calculated based an the theoretical locations of the nodal joints 1308, 1207, 1208 and the center truss, and were also based on the farces in the ridge cables and the PA post, furnished by the roof erection contractor in their computer printout. Though it must be recognized that the position of the nodal joints and the magnitude of other member forces could be slightly different from the computer print-out values due to the application of higher than intended loads to the temporary diagonal jacking strands. Due to the structural complexity of the entire roof cable system and the ever changing position of the joints during the erection process, the exact locations of nodal joints and magnitude of the forces at node 1308 could not be ascertained with a high degree of accuracy at the precise moment of the accident. However, it was determined that regardless of the slight variations in the nodal position and the member forces, the tensile stresses in the critical locations of the plate assembly would remain higher than the yield strength of the material due to the eccentrically applied jacking loads in the diagonal strands.

Conclusions

Based on the above discussion and evaluation, the following conclusions are drawn.

  1. The accident occurred due to the collapse of a steel post following the fracture of an attachment plate located at the top of a hoop "A" post. The attachment plate was loaded in excess of its capacity. The overloading was the result of the bending moment created by the eccentrically applied tensioning force of the temporary jacking strands to the attachment plate.
  2. The fractured attachment plate was not adequately proportioned for the tensioning force applied during the roof erection immediately prior to the accident.
  3. Engineering calculations were not done, by the roof erection company, to determine and verify the structural adequacy of the attachment plate assemblies for the erection loads to which these plate assemblies were subjected during the erection of the roof cable system, as per contract document requirements.
  4. Workers, who were on the platform and involved in the specific construction activity preceding the accident, were not clearly instructed regarding the jacking sequence and the amount of tensioning force to be applied to the temporary jacking strands to make the final connection of the diagonal cables and the roof center truss. Written instruction or the erection drawings indicating the magnitude of tensioning force to be applied to the temporary jacking strands were not available to the erection crew.
Figure 6.01. The Completed Roof Cable Structure at the time of the Accident. Three Hoop Cables, the posts, and Ridge Cables. For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 6.01. The Completed Roof Cable Structure at the time of the Accident.

Figure 6.02. Top of the PA Posts Showing Three Ridge Cables, Steel Post, Temporary Jacking Strands. The Diagonal Cables were in a slack position. Three Hoop Cables, the posts, and Ridge Cables. For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 6.02. Top of the PA Posts Showing Three Ridge Cables, Steel Post, Temporary Jacking Strands. The Diagonal Cables were in a slack position.

Figure 6.03. Close up of the Top of PA Post. For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 6.03. Close up of the Top of PA Post.

Figure 6.04. Attachment Plate at top of PA Post and the Temporary Jacking Cable Connection Details. For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 6.04. Attachment Plate at top of PA Post and the Temporary Jacking Cable Connection Details.

Figure 6.05. PA Plate Assembly with Jacking Strand Bracket. For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 6.05. PA Plate Assembly with Jacking Strand Bracket.

Figure 6.06. Location of all Cables and Post at the Time of the Accident-As per computer print out (Appendix-F) For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 6.06. Location of all Cables and Post at the Time of the Accident-As per computer print out (Appendix-F)

This table is best viewed on tablets, notebooks, or desktop computer screens.

Results of the tensile tests are given in Table 3, as follows:

Table 3
Conncetion Plate Tensile Test Results

 

Plate - Pa
No. 1

Plate - Pa
No. 2

Plate - Pd
No. 1

Plate - Pd
No. 2

Specified per ASTM:A572
Grade 50

Yield Strength, ksi (0.2% offset)

60.0

58.5

56.0

55.0

50.0

Ultimate Tensile Strength, ksi

85.5

86.0

85.0

85.5

65.0

Elongation, % (2-in. gage)

29.5

29.5

26.5

27.0

21.0

Reduction in Area, %

57.0

58.0

65.0

64.0

 

Appendix (for the complete appendix, see PDF*)

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All other documents, that are not PDF materials or formatted for the web, are available as Microsoft Office® formats and videos and are noted accordingly. If additional assistance is needed with reading, reviewing or accessing these documents or any figures and illustrations, please also contact OSHA's Directorate of Construction at (202) 693-2020.

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