Yesterday, New York City Buildings Commissioner Robert LiMandri released the official findings of a yearlong investigation into the March 15, 2008 crane collapse on East 51st Street that killed 7 people. Conducted by Arup, the investigation combined structural laboratory testing and computer analyses with the review of photographs and eyewitness testimony to produce a 339-page report [PDF] that placed blame firmly on poor rigging practices.
In brief, the report states that the jumping crew disregarded a number of the crane manufacturer’s recommendations in their use of polyester web slings to hoist and hold an 11,000-pound steel collar in place. They used four slings as opposed to eight, connected them not at the recommended collar points but in the crook of the tower’s legs, and did not pad them against the sharp edges of the steel. In addition, one of the slings had suffered previous damage, greatly reducing its bearing capacity. The slings broke, the collar fell from the 18th floor—severing lower connections to the building—and the crane toppled. In January, a grand jury indicted William Rapetti, the master rigger in charge of the jumping operation, on charges of manslaughter.
Arup’s investigation also found that the DOB crane inspection and permitting protocols in place at the time would not have identified the rigging errors that caused the collapse, highlighting the need for more vigilant construction oversight. “This investigation shows the consequences of taking shortcuts on the job site,” said Commissioner LiMandri. “In the coming days, we will convene a series of meetings with the construction industry to review the report’s findings and identify ways to prevent tragedies like this from happening again.”
The DOB has already taken several steps to tighten safety regulations at construction sites. Since the collapse, the department has worked with Mayor Michael R. Bloomberg and Council Speaker Christine Quinn to enact 12 new laws to this effect. Among these are laws that mandate the presence of more safety managers on site, the drafting of specific safety procedures for jobs, an increased amount of training for rigging crews, and the restricted use of nylon or polyester slings unless specifically indicated by crane manufacturers. There is also a law that requires notification to New York State of disciplinary actions that DOB takes against licensed architects and engineers, so that the state can determine whether those disciplined need either remedial education or further disciplinary action.
Curiously, though, no law has been passed that addresses one other important factor that, while it did not cause the crane collapse, could very well have prevented it: the crane’s foundation. The base of the crane sat on dunnage beams, which in turn rested on concrete foundation walls. But the beams were only secured laterally by friction from the vertical load. According to the report, Dale Curtis, a member of the Crane Certification Association of America who was brought on as a reviewer, “observed that such a base frame is preferred to be positively restrained at the concrete below to prevent lateral movement during disassembly of the tower sections when only the lowest tie is attached to the completed building.”
During the collapse, only one tie remained attached to the building—the lowest, at the third floor. The base of the tower and the dunnage beams, however, suddenly exposed to the increased lateral loads, slid out from under the crane and allowed it to fall across 51st Street. As an industry insider who preferred to remain unnamed told AN in March 2008, “In this foundation there was no ability to withstand the lateral loads. When the ties failed it couldn’t handle it. There was no redundancy.”
DOB is aware of the issue, however. Among the 41 recommendations that resulted from the department’s recent $4 million dollar study of crane, hoist, excavation, and concrete operations was one that mentioned crane foundations. It recommended that the city require a New York State–licensed engineer to provide design drawings for a tower crane foundation and require notification to DOB prior to pouring the foundation. It also suggested requiring the engineer of record to conduct a special inspection of the tower crane foundation before and after the concrete is poured.