When Portland’s Burnside Bridge closes for a seismic rebuild, probably in 2027, it’s likely to remain closed for five years.
Due to the instability of the soil on which it stands and the infrastructure on either side of the Willamette River, Multnomah County engineers will essentially be building three bridges. Each one will have to connect to the other in a way that ensures they remain standing and functional after a major earthquake.
“We expect that it will be fully closed for five years,” said Multnomah County construction manager Emily Miletich.
There are several reasons for the long closure. Among them is the Endangered Species Act, which means work in the river is constrained to times when endangered salmon are not running.
“That really drives the length of the schedule for construction,” Miletich said.
Additionally, the bridge has a complex downtown location and a complicated design.
The bridge’s environmental review has just been completed, so most of the big structural decisions have been made. But the soil on the east side of the river could potentially liquefy during a large earthquake, so the county is still deciding what type of bridge to use.
“We’ve just selected a contractor to help us make a decision between a cable stay bridge and a tied arch bridge on the east side of the river,” said Miletich.
That area of the bridge is extra delicate because it goes over some key transportation corridors like Interstate 5, Interstate 84 and a key north-south railroad line.
“What we’re hoping to do is be able to span across all of those different elements, so it makes the structure really long,” Miletich said. “We’re looking at either a 600- or 700-foot long span.”
There are a limited number of bridge designs that can stretch such a distance, so the county wants to consult the contractor.
On the west side, the decision has been made to build a traditional girder bridge, which is a similar structure to the current bridge. But modern technology means there will be fewer beams, which should open up the space.
When the bridge was first built in 1926, the two piers in the river were placed on 380 tree trunks, driven into the mud. They’ve held up remarkably well. But in an earthquake, the soil could liquefy and make the trunks fall over like pick-up sticks. The trunks will be replaced by several 10-foot-diameter concrete, steel-reinforced columns sunk into bedrock.
If a major earthquake strikes along the Cascadia Subduction Zone, violent shaking could last up to five minutes. Most of Portland’s old downtown bridges are not expected to remain functional.
But the Burnside corridor has been chosen as a key emergency route that needs to remain open. It links Washington County to Portland and Gresham.
The current cost of the bridge is up to $895 million. So far, the county has applied for some federal grants and received some state finances to pay for it. There’s also $300 million in vehicle registration fees available from Multnomah County.
“We won’t be advancing to construction without that full package put together,” Miletich said.
The bridge is expected to have a moveable deck so large ships can still sail underneath. There will be bike and pedestrian lanes on either side and two lanes of traffic each way.
No rail is planned, but the bridge will be designed to be streetcar-ready, in case that option is ever adopted.
At the moment, demolition of the bridge is scheduled to start in 2027, with a potential opening in 2031.