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Seismic upgrades are under way for two of five Portland-area bridges that engineers fear could sustain serious damage during a large-scale earthquake.

The other three projects are set to begin later this year as part of a $10 million phase of the I-5 Seismic Retrofit Project, Oregon Department of Transportation spokesman Don Hamilton said.

“The Oregon transportation system has some significant seismic issues,” he said. “This is a first step to get a handle on the different elements. The intention is to strengthen the most significant corridors that would be affected in a significant seismic event.”

Vancouver, Wash.-based Cascade Bridge LLC has the contract for all five bridges; it started work on the Southwest Hood Avenue and Southwest Lower Boones Ferry Road bridges in March, and will start on the one over the Tualatin River this summer, according to ODOT.

Two other I-5 projects – at Southwest Barbur Avenue and over Portland & Western Railroad in unincorporated Washington County – are slated to begin in the fall, Hamilton said.

Crews will strengthen piers, pilings and bridge abutments with modern seismic designs, according to ODOT. Work will vary based on each bridge’s structure and the surrounding terrain.

These bridges were selected because each one is a little different from the others, Hamilton said. ODOT is expecting to learn lessons it can apply later.

“We’re trying to address a variety of problems that each bridge will present to make sure it’s strengthened seismically,” he said. “What we learn on each of these bridges will help shape what we do on bridges in the future as part of the larger effort to strengthen I-5.”

The 2012-2015 Statewide Transportation Improvement Program is providing money for the projects.