A cut sling was the cause of the dropped load accident in Manhattan last month, according to a confidential source familiar with the investigation.
The roughly 13-ton air conditioner fell as it was being lifted into place at the top of 261 Madison Ave., falling 30 stories to the street below early on Sunday morning, May 31. The accident resulted in only minor injuries.
The load fell because the synthetic sling used to lift it in place was cut, most likely because the softener used to protect the sling was cut by a sharp edge on the bottom of the load, the source said. The softener has yet to be found.
The New York Dept. of Buildings had no comment on the cause of the accident and, through a spokesman, said that the incident is still under investigation.
Softeners or sling protectors are commonly used to protect slings at the points where it comes into contact with the load. Heavy loads generate extreme pressure at the point of contact and, if the load shifts, that pressure can generate heat that can melt or burn slings made of synthetic material.
When a sling makes a 90-degree bend around the corner of a load, there is 40% more pressure created at the corner, according to Mike Parnell, a rigging expert and CEO of Industrial Training International.
Cut and worn slings were among the leading causes of the worst crane accident in U.S. history.
Seven people died on March 15, 2008, when a tower crane collapsed in New York City during a jumping procedure that turned disastrous when synthetic slings supporting tie-in components failed.
William Rapetti of Rapetti Rigging Services Inc. of Massapequa Park, N.Y., was acquitted of multiple charges of manslaughter, criminally negligent homicide, assault and reckless endangerment in connection with the accident.
Skylift Contractor Corp. of Brooklyn, N.Y., was the crane operator at the Madison Ave. job. Requests for comment from Skylift were not returned by press time.