10.02.2007

Unusual foundation blamed for Seattle collapse

Preliminary reports suggest that a poorly designed foundation is likely to have been the primary cause of the tower crane collapse in Bellevue, Seattle last November. The accident sparked the recently introduced crane-safety legislation that ranks among the toughest in the USA.

A three-month investigation into the crash by the Department of Labor and Industries has provisionally found that the crane's steel foundation failed, and that the 210 foot crane would not have toppled if bolted into concrete. The final report has not been released.

Engineers have based their findings on structural calculations, before-and-after photographs and dozens of interviews with people familiar with the construction project.

The November collapse at a downtown Bellevue office-tower site killed a man in a nearby apartment and badly damaged three buildings.

See: Tower crane collapses in Seattle

Following the accident, the state advisors urged construction companies to inspect their tower cranes. As a result, structural problems were found - and fixed - in several cranes operating in the Seattle area.
State laws governing crane safety have been severely criticised resulting in stricter standards to be introduced.

Starting in 2010, it is thought the new laws would require multiple inspections of construction cranes by an independent state-certified third-party inspector. Under current law, crane safety is largely the responsibility of the construction companies and crane owners.

The bills, introduced in the House and Senate, would require both cranes and crane operators to be certified. Currently operator certification is optional, and the state does not certify or issue permits for the structures. To be considered qualified, operators also would have to pass a drug test and have 2,000 hours of experience or training.

One section of the proposed law echoes the preliminary findings by L&I, requiring that an independent professional engineer "review and approve the plans for any non-standard tower crane base."

Most tower cranes are bolted into a mass concrete foundation. However the collapsed crane sat on a cross-frame base made of steel I-beams. Expers said the design was very uncommon.
Please register to see all images

Standard tower crane base and the base used for the collapsed crane

In January, a new tower crane was erected on site using the more usual concrete base.
Please register to see all images

The new tower crane operating on site

It's still unclear what other factors may have contributed to the fall, and why it fell when it did. Other concerns raised since the collapse include whether the crane was allowed to swing freely in the wind ; whether it was installed according to specifications; and whether operator error and the size of the loads it handled could have been a factor.