City officials are investigating why the cable of a crane carrying steel beams at the 4 World Trade Center construction site snapped just before 10 a.m. Feb. 16, causing the beams to fall about 40 stories onto the flatbed truck used to transport the load. No one was injured at the site, which is set for completion this fall, say city officials.

"The incident occurred in an enclosed section of the site, which is not accessible to the public," Tishman Construction, 4 WTC construction manager, said in a statement. The job site is partially shut down pending an investigation, it adds. The investigation team includes representatives from the New York City Fire Dept., Dept. of Buildings, and the Port Authority of New York and New Jersey, which owns the WTC land.

"The wires themselves will be the evidence for inspectors as to the mode of failures," says Mike Parnell, president of Industrial Training International, Woodland, Wash., which provides crane and rigging training and consulting as well as accident investigations. "If we did an analysis on the wire rope failure, we would first look at the potential for pure tensile failure, but this is really uncommon because the crane might not be able to pick up the load if it were over capacity," he says.

Inspectors would next consider the weight of the hoist line plus the weight of the crane block, rigging and the lifted beams themselves, Parnell says. Another consideration would be whether "they snagged the beams along the building or against an obstruction while lifting," he says. "If it's not a tensile failure, [the cause] would likely be one of the other preexisting conditions such fatigue breaks, corrosion, cut wires, heat damage or severe metal loss," he adds.

The 4 WTC tower, located on the southeast corner of the WTC site bounded by Greenwich, Church, Cortlandt and Liberty streets, will rise 977 ft above street level. The building was originally slated to be the shortest of the site's four planned towers when completed. However, the WTC site has had a series of setbacks in recent months, including a threat from developer Larry Silverstein to cap what was to be an 80-story 3 WTC tower to just seven stories if he was unable to secure at least one major tenant for the structure, according to media reports.