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29 October 2012

NYC crane collapse, at top of 90-story building, West 57th St and 7th Ave, 29 October 2012, 3PM


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Source Anna Holmes

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Source Jonathan Wald

The weight of the boom and its load is usually balanced by the counterweight. With both on the same side there is danger of breaking the connection to the shaft with the entire lifting rig falling onto surrounding buildings and bouncing to the street along with debris of the buildings hit.

The boom might fall separately leaving the rest of the rig in place, but the upper photo shows the collapsed boom is still attached to the lifting rig and the cables connecting the boom to the lifting machine are quite strong if not weakened by the collapse or by wind-induced abrasion.

There is also a chance the swaying rig will wrench and bend the shaft at a connector or pull the entire shaft away from the building, causing a wider swath of destruction than that of the rig falling, especially to occupied structures. This happened in NYC some years ago with several persons killed.

These accidents, while rare, are anticipated but require quick action to secure the damaged parts, made much more dangerous in this case with the strong storm's arrival in about two hours.

For damage control, the boom can be secured by a crane rigging team, another crane erected atop the building (parts raised in building elevators), the counterweight and boom disassembled in parts and the components lowered to the ground.

Crane design provides for considerable wracking of the equipment during use, shifting and swinging loads, stops and starts, so there is ample safety factor at crucial elements and connectors. This accident looks as though it can be safely managed if the storm does not make the situation worse. Crane riggers are highly trained, licensed and expert in handling crises. They are a worldwide band of high-rise daredevil safety wizards essential for the skyscraper boom. Meticulous after-accident reports tell what happened and how to minimize recurrence.

It is not known why the boom was not more securely lashed for the storm as customary for imminent hazard.

Below: the crane at the start of construction.

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