When asked about the new collapse theory, a spokeswoman for Zurich told ENR in an emailed statement that the company "did not insure the crane involved in the case study and does not speculate on causes of accidents."

The new book, available from Haag for $249, is full of data. The study shows that "other field personnel," or workers not involved in a lift, are killed more often than any other workers, including those doing the lift. Injuries for non-lift workers are also high, second only to riggers (see chart). Wiethorn says crews can reduce danger by taping off the lifting zone and restricting it to authorized personnel.

Another finding is that booms collapse more frequently on large cranes than on small ones. You would think that heavier lifts are planned more carefully, but Wiethorn says that "those lifts are getting more complicated, and now the little things catch you." Ground preparation is critical to maintain proper lift radius, a boom deflects as it grows longer, and distance magnifies problems, he explains. Project controls, such as a restriction on lift radius, can prevent accidents, he adds.

Ray A. King, an associate engineer at Haag who gathered data for the book, says he hopes it will inform future industry standards. "To make those standards more applicable, it really helps to understand why an accident happens in the first place," he says. "That's ultimately the main objective."

Wiethorn is regarded by many as a go-to accident expert, but his theories often draw criticism. In the 2012 criminal trial of crane owner James Lomma in New York City, he testified that a fatal collapse in May 2008 resulted from operator error, not a faulty weld, as prosecutors contended. Lomma was acquitted but still faces a civil trial this fall. In both cases, Wiethorn was hired as an expert witness.

"I anticipate that his credibility will be seriously challenged at the upcoming civil trial," says Bernadette Panzella, an attorney representing one of the victims. Another attorney involved in a separate matter told ENR that Wiethorn "is not pro-worker. He is pro-business."

Wiethorn says his conclusions are based on engineering principles and an analysis of the facts. "When I tell a client they don't have a case, it drops," he says. "We're done."