A flawed foundation design was the cause of the Bellevue, WA tower crane collapse in November 2006, according to a Department of Labor and Industries (L&I) investigation. One man was killed as a result of the accident.

The engineered foundation designed by Seattle-based Magnusson Klemencic Associates was designed to withstand only about one-fourth of the pressure that the 210 foot tower crane actually required, according to a statement from L&I.

Operator error was not a factor, with the L&I reporting that its investigation revealed that the crane operator was well experienced and that he was operating the crane properly.

“The inadequate design of the tower crane base clearly led to the collapse of the structure,” said Steve Cant, assistant director for L&I's division of Occupational Safety and Health, in a press report. “The problem was compounded by the failure of the general contractor to maintain and inspect the crane base in accordance with the manufacturer's specifications.”

The results of the six month investigation into crane collapse were shared with the companies involved in the project last month. The investigation was reported to cover all aspects of the crane's construction and operation, including the possibility that strong winds may have weakened the crane. L&I determined that the cause of the collapse was metal fatigue of the steel base frame, which was not strong enough to support the working crane.

In April, the Washington legislature enacted new laws establishing a construction crane certification program and a crane operator certification program, which will be administered by L&I.

According to the L&I press information, the Bellevue crane's foundation design was unusual in that a typical standard construction crane base would be a poured concrete pad in which crane anchors would be embedded. The base structure designed for this crane consisted of a configuration of steel I-beams welded and bolted together as an H-frame and attached through a set of collars to four concrete pillars, the L&I reports.

As a result of L&I's investigation, two firms are being cited for workplace safety violations, including Magnusson Klemencic Associates and Lease Crutcher Lewis, the general contractor.

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