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51st Street Crane Investigation 51st Street Crane Investigation Report

12

Conclusions

Following the collapse of an external self-climbing tower crane at 303 East 51st Street, New York, New York, on March 15, 2008, Ove Arup & Partners, PC was hired by the New York City Department of Buildings to provide engineering and investigative services. Based upon a review of the tower components, pertinent codes and standards, the OSHA report, computer analyses of the tower structure, photographs and other documentation as described in the foregoing report, the following is our professional opinion to a reasonable degree of engineering certainty. 1) The collapse of the tower was initiated when the four polyester web slings supporting a steel collar at the 18th floor level failed; allowing the collar to fall. 2) Improper usage of the polyester web slings resulted in the failure of the slings. a) One of the four polyester web slings was in a deteriorated condition prior to use and should not have been installed. Specifically, i) ii) UV degradation was evident on the sling; and, physical damage to the sling was evident.

b) The number of supports provided by the polyester web slings did not meet the tower crane manufacturer's requirements; i) ii) Only four support points were used in contrast to the manufacturer recommended specified eight support points. With only four slings present, analysis indicates that failure of one sling may have been sufficient to cause failure of the remaining three slings.

c) The positioning of the polyester web slings as installed was inconsistent with manufacturer's instructions. d) The method of attaching the polyester web slings to the tower was not in accordance with accepted industry practice and standards. Specifically, i) ii) sharp bearing edges were not padded; and, the slings were bunched and edge loaded.

3) As demonstrated by structural analysis, the unanticipated loads arising from the dynamics of the falling 18th floor level collar caused failure of the collar connection at the 9th floor level. a) The tie-back system for horizontal restraint had limited vertical capacity (1) The system was held up initially by polyester web slings from the collar to the tower and then by cables. In either case, the system had little ductility to resist the effects of falling debris. (2) It is noted that this approach is commonly used, and not in violation of any standards to our knowledge. 4) The tie-beam assembly at the 9th floor level was not welded as specified. a) Baseplate fillet welds did not meet the design specifications.

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Ove Arup & Partners Consulting Engineers PC Issue 0 March 4, 2009

51st Street Crane Investigation 51st Street Crane Investigation Report

(1) Baseplate fillet welds were insufficient to meet the minimum weld size as specified by code. b) Pin end plate connections did not meet the design specifications. c) It is noted that computer analysis indicates that the tie-beam welds would still have failed if the welds had been properly fabricated per the design. Further, site observations indicated that all three connections of the tie-beams to the collar also failed. 5) The 3rd and 9th floor wire ropes used to provide vertical support to the collars were not installed in accordance with accepted rigging standards nor in accordance the tower crane manufacturer's requirements. a) It is noted that structural analysis indicates that the wire rope slings would still have failed if installed per industry standards. 6) As demonstrated by structural analysis, after the loss of the Level 9 collar connection, the remaining supports at level 3 and the base were not sufficient to restrain the tower. The tower rotated about the level 3 collar, with its base sliding towards the building and its top falling away from the building. 7) The dynamics of the 18th and 9th floor level collars impacting the 3rd floor level collar caused slippage of the dunnage steel at the base of the crane resulting in the overturning of the tower. a) Horizontal sliding of the base was restrained only by friction while uplift of the tower legs was not restrained at all. i) It is noted that this approach is commonly used, and not in violation of any standards to our knowledge.

8) A review of the crane engineer's submissions (by Stroh Engineering) found that the tower and tower supports were generally well-engineered and designed to industry standards. 9) Permitting procedures were properly followed and completed. In summary, based upon the information supplied and work completed, it is our professional opinion to a reasonable degree of engineering certainty that the cause of collapse of the tower crane on March 15, 2008 was the failure of the polyester web slings due to improper usage. These conclusions and recommendations are subject to alteration should additional information be forthcoming, whether as additional documents, document review or site investigation.

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