UW releases report detailing potential effects of ‘Big One’ on Western Washington bridges

A recently released report from the University of Washington, sponsored by the Washington Department of Transportation, details the potential impact on bridges if a magnitude 9 earthquake should happen to shake Western Washington.

“The real nice takeaway and the thing we can feel better about is that we’re predicting a lot less damage in the Puget Sound metro area than previous studies have suggested,” said Jeffrey Berman of the UW’s Department of Civil and Environmental Engineering.

The news is not as good closer to the coast, where fallen bridges could leave people cut off.

“One of the really big issues is the potential of islanding some of these communities,” Berman said.

The new bridge research is based on complex ground motion simulations that UW scientists developed in recent years to predict how a 9.0 earthquake might play out.

Models were developed to evaluate the impacts on Washington bridges that were built before 1976 and others that were built between 1976 and 2018.

The models then experienced ground motions from 30 simulated earthquakes and, as expected, older bridges did not fare well.

Bridges nearest to the Cascadia Subduction Zone sustained widespread damage, with a likelihood of buckling ranging from 10% to 30% and columns breaking into fragments of rock ranging from 35% to 70%.

About 75% of the bridges evaluated were built before 1976, after the 1971 San Fernando earthquake.

About 68% of the bridges were supported with at least one reinforcement, while another 12% had single spans.

The 182-page report also suggests the provided data could be used to prioritize retrofitting efforts.

The entire report can be found here.

This study focused on typical bridges like highway overpasses. What it doesn’t consider is an earthquake from the Seattle fault. In the coming years, scientists will start modeling the impacts of that type of earthquake.