The "dangling crane" that partially collapsed nearly 1,000 ft over Manhattan during Superstorm Sandy was caused by a worker who unintentionally tied down the rig and kept it from weather-vaning—or freely rotating—according to a forensic engineer who investigated the accident.

"He had no idea what he was doing," Jim D. Wiethorn, chairman and principal of Haag Engineering, tells ENR. Wiethorn discusses the collapse in the new book, "Crane Accidents: A Study of Causes & Trends To Create a Safer Work Environment," published last month. The book is the result of seven years of compiling more than 500 investigations of crane accidents that Haag's crane group, based in Sugar Land, Texas, has conducted since 1983.

Until now, few have pointed to a cause for the 2012 accident. Officials with the building's construction manager, Lend Lease, told ENR the crane operator put the unit in weather-vane mode, as standards and best practices dictate; city officials concurred. Further, the engineer that designed the crane's tie-ins characterized the incident as an "act of God."

Wiethorn does not dispute the contractor's claim but says one or more workers later grabbed two fall-protection lanyards attached to the climbing frame, on the machine's tower structure, and hooked the loose ends around the crane's slewing-gear bolts on the rotating upper structure in an effort to secure the straps prior to the storm.

As wind speeds increased, "the lanyards on both sides of the turntable prevented rotation," says the author. The boom had nowhere to go, so it flipped backward. Wiethorn, who visited the wreckage one day after the storm, took close-up photos but did not provide them to ENR.

Officials at Lend Lease would not confirm Wiethorn's theory. "As with many natural disasters, the aftermath of the damage caused by Superstorm Sandy will be forever the subject of speculation," it says in a statement. "Lend Lease stands by its prior statements related to the crane incident and declines to comment further given the pending insurance claims."

Messages left with the building developer, Extell, and the crane operator, Pinnacle Industries, were not returned. A spokesman for the city buildings department declined to comment, as well, saying the agency has not finalized its report on the failure. The building opened in late May, about six months behind schedule.

Whether the crane was covered under the project's insurance policy is the subject of a lawsuit in New York State Supreme Court. Last October, Lend Lease and Extell sued Zurich and others that underwrote the job. According to court filings, the 74-floor luxury condominium, called One57, was valued at $700 million, for which Zurich had insured $350 million, or half the job. Ace, XL, Travelers, Surplus Lines and Axis Surplus are also defendants named in the suit.

After the storm, Extell and Lend Lease submitted nearly $11 million in claims. Zurich refused to pay, arguing that the crane was not covered. Lend Lease says the rig was included in an $89-million concrete contract, which Pinnacle was to perform using the rented crane. In Zurich's filings, it argues that contractor equipment is not covered unless endorsed as part of the builder's risk policy.

Scaffolding was insured because it was endorsed as part of a $15-million hoist and sidewalk-bridge contract. "The same cannot be said for the crane," says Miles Wemken, a Zurich underwriter, in an affidavit filed on April 29.