U.S. Department of Labor
Occupational Safety and Health Administration
Occupational Safety and Health Administration
Directorate of Construction
Russell B. Swanson, Director
Office of Engineering Services
This report was prepared by
Scott Jin, Ph.D., P.E.
On October 24, 2001, at approximately 4:00 PM, two 142 ft high scaffold towers collapsed, killing five workers and injuring ten others. The towers were erected to provide a working platform for the facade renovation of a 20-story steel framed masonry building. The 1914 era building is located at 215 Park Avenue South in New York City. On the day of the incident, the face bricks and steel window lintels had already been removed from the 6th to the 14th floors and the cement plaster work had just begun at the 14th floor of the west wall. At the time of the incident, two workers were manually hoisting a cement hag to the top of the scaffold tower.
Personnel from Manhattan Area Office of Occupational Safety and Health Administration (OSHA) arrived at the scene within hours of the incident. The Directorate of Construction, OSHA, Washington, DC, was requested to provide assistance in technical assessment of the collapse and in determining the cause of the incident.
The OSHA investigation began soon after the incident and included interviewing witnesses, observing failures, inspecting recovered steel frames and supporting members, taking photographs and related field measurements, and performing an engineering analysis. Based on the findings of the investigation, it is concluded that:
The following is a chronological list of major events during the investigation.
On October 24, 2001, with hours of the incident, personnel from the Manhattan Area Office of the Occupational Safety and Health Administration (OSHA) arrived at the scene to start the investigation by interviewing witnesses and taking photographs.
On October 26, 2001, at the request of OSHA Region 11, two engineers from OSHA's Directorate of Construction (DOC), in Washington, DC, accompanied by two compliance officers from the Area Office, visited the incident site to inspect the collapsed scaffold towers and take photographs and field measurements. OSHA personnel also recorded the condition of the facade, the windowsills, and the remaining lateral bracing points. The Area Office reviewed the project organization, the responsibility of contractors and subcontractors, and the sequence of events leading to the incident.
On November 2, 2001, DOC received the first package of documents from the Area Office, which included the following:
On November 14, 2001, DOC received a second package of documents from the Area Office, which included the following:
On November 16, 2001, an engineer from DOC, accompanied by two compliance officers from the Area Office, revisited the incident site to take additional photographs and detailed measurements of the collapsed scaffold members, and conduct a meeting with the owner of the scaffold erector (TSS). The following are the highlights:
During the November 16 trip, DOC received 50 color photographs and 21 black and white digital photo printouts taken by TSS prior, to the collapse. In addition, the evaluation of the actual load at the time of the collapse was also discussed with the Area Office.
On December 11, 2001, after the face bricks and debris were partially removed from the roof deck at the second floor level, the engineer from DOC, accompanied by the same personnel from the Area Office, re-visited the incident site, to measure the actual size and length of the newly exposed scaffold supporting members, inspect the failed condition of the east end of three dunnages, and weigh the scaffold planks. During this trip, DOC received a copy of the installation manual for the disposal chutes (Chutes International). On December 26, 2002, the Area Office provided the DOC with a copy of 5-page letter size documents of the 1991 plan, section, and detail of the design condition of the existing dunnages in the south courtyard.
On February 19, 2002, after all key structural members were recovered from the roof deck area, the engineer from DOC, accompanied by the OSHA principal compliance officer, visited the incident site to examine the deformed conditions of recovered members and take associated measurements. During the same trip, DOC presented the preliminary findings of the investigation to the Area Office and regional solicitors.
On February 28, 2002, DOC received the following information from the Area Office:
On February 7, 2003, the Manhattan Area Office provided the DOC with the following information:
Throughout the course of the investigation, DOC worked with personnel from the OSHA Manhattan Area Office. The principal compliance officer, Marc Vargas, and the compliance officer, Robert B. Stewart, made significant contributions to this investigation. Dinesh Shah, structural engineer of the Office of Engineering Services, DOC confirmed the calculations of the draft report.
The project involved the renovation of the facade at the south courtyard of a 20 story steel frame masonry building. The building, constructed in 1914, is located at 215 Park Avenue South in Manhattan, New York City (See the insert project location plan in Figure 3-1). The renovation work included restoration of existing steel shelf angles (window lintels) and replacement of face bricks at the south interior courtyard. The proposed work limits were from the 2nd to the 14th floors at all three sides of the courtyard, as shown in Figures 3-1 through 3-3. A project organization chart is included in Figure 3-4.
To provide an adequate work platform for the masonry contractor, the general contractor retained a scaffold erector to build two scaffold towers in the south courtyard. The existing condition at the base of the courtyard prior to the scaffold erection is presented in Figures 3-5 and 3-6. This condition was the result of installation of an HVAC system in 1991. As indicated, roof decks at the second floor and mezzanine levels covered the entire courtyard, but were not capable to support any scaffold leg loads. There were three steel beams (dunnages) just above the roof deck at the second floor level to support the four existing air-cooled-condensing-units (ACCU). These beams were anchored to the building wall (Figure 3-6). Thus, the scaffold erector chose to use these beams to support the majority of the scaffold loads without performing any structural analysis.
Twelve of the eighteen legs of both towers were supported on the three existing steel beams (Figures 3-7 and 3-8). The next three legs were supported on top of the parapet wall at the mezzanine level. The last three legs at the southeast corner were primarily supported on an offset scaffold, built from the first floor level (Figures 3-9 and 3-10). Pressure jacks were also used along with the regular horizontal ties for lateral bracings and for connections between the two scaffold towers (Figures 3-1 1 and 3-12). Both towers were fully planked and completed on Tuesday, October 16, 2001, and released to the general contractor. The masonry contractor began work shortly thereafter on the two towers. The scaffold erector visited the scaffold towers on October 17, 20, and 24, 2001. The October 20 visit was to repair the disposal chute. In each of these visits, the scaffold erector had to re-tighten about 10 to 15 pressure jacks, which had loosened as a result of brick removal by the masonry contractor during the demolition operation.
On the day of the incident, October 24, 2001, face bricks and window lintels were removed from the 14th through the 6th floors on all three sides of the facade. New cement plaster work had begun from 14th through 11th floors on the west wall. Based on witness accounts, demolition bricks had accumulated in piles up to four feet high on the upper levels of both towers. However, no noticeable settlements or deflections were reported prior to the collapse.
Immediately prior to the incident, around 4:00 PM, two workers were manually hoisting a 94-pound cement bag from the first floor with a well wheel, supported on the hoist arm at the top level of the east tower and connected to the west leg of the southern most frame. It was reported that the hoisting operation caused vibrations and sways of the towers. With no warning, both scaffold towers collapsed with a loud crashing sound. The building trembled as both towers fell.
As a result of the collapse, two 12-story high scaffold towers were reduced to a 3-story high pile of twisted metal and splintered wooden planks that filled the south courtyard. Many of the victims were buried in this pile of rubble. The incident caused the death of five workers and injured ten others.
At the collapse, many workers were buried in the pile of rubble. A feverish rescue operation began involving more than 250 policemen and firefighters. During the rescue operation, most of the scaffold frames were saw cut and removed along with scaffold planks from the courtyard through the window openings into rooms at the 3rd and 2nd floors of the building. These materials were transported directly to a disposal site on Staten Island, New York City.
Based on the photograph on the cover of this report, the green colored "Scaff-Trac" hoist frame and the white colored electrical hoist (shown in Figure 3-12), landed in the southwest corner of the courtyard. Thus, the scaffold towers fell in a southwest direction. This observation is consistent with the witness accounts.
The collapsed conditions of the three steel beams supporting twelve scaffold legs are shown in Figures 3-13 through 3-18. As indicated, the west ends of all three beams were still attached to the building wall, while their east ends were either twisted or separated. Thus, the east end of the beams failed first, followed by the mid portion of the beams landing on the roof deck. The east end of the south beam was totally separated from the building wall and landed on the roof deck. However, the east ends of the remaining two beams were twisted but still attached to the building wall. Thus, it is believed that the east end of the south beam failed first.
The failed condition of the pegleg (which supported scaffold leg L15) from top to bottom is presented in Figures 3-19 through 3-22. As indicated, the pegleg showed a typical buckling failure under the applied load (Figure 3-21). Bending at its upper bracing connection (Figure 3-19), but remaining straight at its lower connection to the handrail (Figure 3-20). Thus, the handrail deflected with the pegleg, providing little resistance against buckling.
The deflected conditions of the top horizontal members at the upper frames of the offset scaffold, built from the first floor level, are shown in Figure 3-23. As indicated, the north top horizontal member adjacent to the pegleg experienced more deflections than the south top horizontal member, although the south carried more loads than the north prior to the collapse. In addition, as a result of the collapse, the second level handrails close to the pegleg were separated and removed from the roof wall, while those further away remained intact, as shown in Figure 3-24. The rotational (to the south) failure of leg L17 above the south top horizontal member of the offset scaffold is shown in Figure 3-25. The improper bracing condition of the bottom frame, which supported scaffold legs L7 and L8 from the parapet wall at the mezzanine level is presented in Figure 3-26.
The condition of the recovered scaffold frames, lateral bracings, and bottom support members are presented in Figures 3-27 through 3-30. As indicated, among the three W8X13 beams (dunnages), the south beam had the greatest deflections consistent with the above description. The other deformed members included one I8X7.02 aluminum beam, the pegleg, all lateral braces, and all scaffold frames. However, four W4X13 steel beams connecting ACCUs to the three dunnages and three recovered I8X6.35 aluminum sills remained intact. This implies that the three W8X13 beams, four upper W4X13 beams, and four upper I8X6.35 aluminum sills in the perpendicular direction (Figures 3-5 and A-8) failed as a unit, with the failure starting from the south side. The two S6X12.5 steel beams also remained intact. Perhaps the applied loads on the two beams were within their yield limit.
The collapsed scaffold towers in the south courtyard were not designed by a PE and no construction drawings were prepared. The adequacy of the towers was evaluated based on their size and height, and the applied loads. The plan dimensions of both towers were ascertained based on the scaffold erector's statements and photographs taken during the erection, then verified with the manufacturer's catalog and OSHA's field measurements taken after the collapse (Figure A-1 of Appendix A). Scaffold frames and legs of the towers are identified in this figure. The configuration of the 22-level high scaffold towers was is shown in Figure A-5 using information derived from a series of photographs (Figures A-2 through A-4). For clarity, the tower is divided in two parts, one above the main base plate level and the other below. The main base plate level is the top elevation of the four aluminum sills. the referenced figures and sections with a letter "A" are contained in Appendix A.
The first step was to determine the leg loads of the scaffold based on weight (dead load of scaffold material plus planks). These are calculated in Section A-2, tabulated in Table A-1, and compared against allowable loads. The allowable leg load for a 6 ft 4 in high frame is 1.89 kips (Figure A-6). It was determined that the actual loads in all eighteen legs exceeded allowable leg loads. The weight of planks contributed 64% to the leg loads. However, the actual loads were still considered within the ultimate capacity of 7.56 kips, but without the required factor of safety of four. The ultimate capacity of the leg was calculated by multiplying the allowable load by four.
Next, the leg loads were computed due to the dead load as discussed above plus four times the additional loads of bricks to be removed from the building facade. The weight of bricks was established based on the written statements from the scaffold erector, i.e., "They (masonry contractor) could load no more than two levels of the scaffold" and "Two levels of brick are enough for the job." One purpose of the investigation was to determine whether the scaffold and support members could resist four times the intended load. Therefore, the weight of the bricks was multiplied by four to determine compliance with the OSHA standard. The leg loads, calculated in Section A-4 and tabulated in Table A-2, exceeded the allowable loads in all eighteen legs. In 13 of 18 legs, the loads exceeded even the ultimate capacity of the legs. Thus, Section 1926.451(a)(1) was violated.
Third, the leg loads were computed based on the weight of the scaffold and the actual brick loads placed on the scaffold before the incident. The actual load was provided by the Manhattan area office, based on the weight of bricks recovered after the incident. The actual load was found to be approximately 1.7 times the maximum load the erector intended to place. The leg loads in all eighteen legs, calculated in Section A-4 and tabulated in Table A-3, exceeded the allowable load of 1.89 kips. Six of the eighteen legs were overloaded about 14% beyond their ultimate capacities of 7.56 kips.
The scaffold towers were supported as described in Section A-5 and presented in Figures A-8 and A-9. The structural adequacy of the supporting members is examined in Sections A-5-1 through A-5-7 based on their ultimate capacities. Load factors and capacity reduction factors were taken as one. The loads over the support members were computed in the same order as above in Section 4.1.
First, loads due to the weight of the scaffold and the planks were calculated. It was determined that three W8X13 dunnage beams and the 12-foot long pegleg were stressed beyond their ultimate capacities. These structural members are discussed separately in the subsequent paragraph.
Second, the loads based on the weight of the scaffold and four times the weight of two levels of bricks were computed. The following support members were found to be stressed beyond their ultimate capacities:
Thus, Section 1926.451(a)(1) was violated.
Third, the loads of the weight of the scaffold plus the actual load of the bricks (earlier determined to be 1.7 times the load of the two levels of bricks) were calculated. It was again determined that three W8X13 dunnage beams and the 12-foot long pegleg were stressed beyond their ultimate capacities.
Three W8X13 Dunnage Beams
Of all factors, the unbraced length of compressive flange of the dunnage beam had the greatest impact on the load carrying capacity. Crossbeams welded to the dunnage beam provided an unknown degree of lateral and translational restraint, which complicates the determination of the exact amount of unbraced length. The unbraced length might be the full length of the beam or the distance from one end of the beam to the crossbeam. In either case, however, under the actual load placed at the time of the collapse, the beam was stressed beyond its ultimate capacity and failure was inevitable. The beam was overstressed by more than 400% under the full unbraced length and by 140% under the reduced unbraced length.
The 12-foot Long Pegleg
The maximum load that the 12-foot long pegleg could support, without buckling, was determined to be 3.74 kips. However, the loads placed on the pegleg due to (1) the weight of the scaffold (dead load), (2) four times the weight of two levels of bricks plus the dead load, and (3) the actual load of the bricks at the time of collapse plus the dead load were 5.37, 10.17 and 7.45 kips, respectively. As can be seen, the pegleg was on the verge of collapse under the weight of the scaffold. The pegleg was a critical member of the framing, based on its long unbraced length. The direction of the collapse combined with the extensive damage sustained by the structural members adjacent to the pegleg indicate that the tower collapse could have been triggered by the failure of the pegleg, though it is difficult to say which of the two potential causal factors, i.e., the three dunnage beams or the pegleg, performed the predominant role.
The following findings are based on the investigation:
A-1 Reconstruction of the Configuration of the Scaffold Towers
A-2 Calculation of the Weight (Dead Load) of the Scaffold Towers
A-3 Estimation of the Maximum Intended (Live) Load
A-4 Estimation of the Actual Load on the Scaffold Towers at the Time of the Collapse
A-5 Estimation of the Stress Level in the Main Support Members under the Scaffold Towers
A-5-1 Three 8-inch Steel Beams (Dunnage Nos. 1, 2 and 3) Supporting Legs 1 through 6 and 9 through 14
A-5-2 6-inch Steel Beam Supporting Leg 16
A-5-3 8-inch Aluminum Beam Supporting Legs 17 and 18
A-5-4 Lower 6-inch Steel Beam Supporting Both the Steel and the Aluminum Beams
A-5-5 4-inch Steel Beam (Dunnage No. 4) Supporting 8-inch Aluminum Beam
A-5-6 Offset Scaffold Supporting the Lower 6-inch Steel Beam
A-5-7 12-foot Long Pegleg Supporting Leg 15
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