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Portland’s Burnside Bridge, age 92, may not look so old.

But appearances don’t mean a lot when it comes to the ability to withstand an earthquake’s devastating forces that can flatten a bridge in a matter of seconds. That possibility is driving Multnomah County‘s plan to pursue either a seismically enhanced retrofit or full replacement of the Burnside Bridge, which connects West and East Burnside Street across the Willamette River.

Since 2016, county officials have studied more than 100 possible options for enhancing the bridge’s resiliency and safety. Last week, they revealed that options have been narrowed down to those final choices.

鈥淚t wouldn’t be a true retrofit, because there’s one (bridge) section over the (Interstate 5) freeway that would have to be new,鈥� Multnomah County Communications Coordinator Mike Pullen said. 鈥淭here’s no way to upgrade the old bridge over the freeway without closing the freeway, as well as the train tracks. In anything we do, we’d replace that with a new structure; otherwise we’d have the freeway closed for six months to a year and that’s just unacceptable.鈥�

The other option, or group of options, Pullen said, is to replace the bridge entirely. The type of bridge would be determined in the future, should this route be chosen.

鈥淪ome of the (options) we’d expect to advance would be a higher fixed bridge, meaning it doesn’t open for ships,鈥� he said. 鈥淎nd then there are movable bridges. They have different advantages. The higher bridge never has to open, so ships aren’t delayed.鈥�

The county has ruled out three other options that had been under consideration: doing nothing; performing a full seismic retrofit; or retrofitting a different Willamette River bridge.

This spring and summer, county officials will take on the third and final step in determining the Burnside Bridge’s future. Public input will be received during that period, both online and through a series of open house meetings to be held in the summer.

Thursday’s announcement came the same day that the Oregon Department of Geology and Mineral Industries released a study stating a projected magnitude 9.0 earthquake along the Cascadia subduction zone off the Oregon coast could injure 27,000 Portland residents and displace more than 85,000 others.

The two preferred groups of options will be evaluated based on seismic resiliency, ease of use by nonmotorized transportation, connectivity to existing streets and transportation modes, equity, environmental impact and financial stewardship.

Cost estimates for a new or refurbished bridge remain nebulous, given very little is known about the final shape of the project. However, a preliminary timeline allocates up to $415 million for construction alone over the next decade, when the project will be designed and built. Also, $80 million more could be spent on design and engineering work.

Burnside Street is a designated regional lifeline route, giving it priority for projects of this type. And with a recent Oregon State University study finding the chance of a Cascadia subduction zone quake occurring within the next 50 years at up to 40 percent, county and state officials are not waiting to take action.

County officials are currently accepting public input on the two remaining options and expect a final decision to be made by fall 2018. Then the exact type of bridge or retrofit will be examined and chosen.

鈥淲e’re still in the early planning period on this one and we still have a ways to go,鈥� Pullen said.

People can share their viewpoints and find out more about the plan by visiting .