PORTLAND, Ore. — In a major earthquake, most of Portland’s bridges wouldn’t stand a chance. They would shift, crumble and even collapse.
“For a significant number of months after a major event, a lot of these bridges would not be able to be used,” said Bruce Johnson, state bridge engineer for the Oregon Department of Transportation.
Most of the bridges crossing the Willamette and Columbia rivers are old and were never designed for earthquakes.
“When the Hawthorne Bridge was built, there were way more horse and buggies than there were cars,” said Ian Cannon, transportation director for Multnomah County.
The Interstate Bridge carries roughly 130,000 cars and trucks daily over the Columbia River along Interstate 5. In a magnitude 8 or 9 Cascadia Subduction Zone quake, this critical connection between Oregon and Washington would fail.
“I’m confident that it would collapse under a 9.0 earthquake,” said Johnson.
In a major quake, the drawbridge towers holding the counterweights would buckle, sending giant blocks of concrete into a free fall.
“That counterweight would come down just like a hammer, smashing through the bridge deck,” explained Johnson.
The middle spans of the Interstate Bridge would be severed. There would be no way to cross. The bridge would be broken into pieces, blocking ships.
The approaches on both the Oregon and Washington side would collapse because they have no support. The timber holding them up stands on a riverbed of dirt and sand. Those piles would shift and slide into the river.
“When the earthquake happens and shaking occurs that sandy saturated material just basically turns to a liquid,” said Johnson.
The thin blue lines represent an existing water main. The thick blue line is a proposed tunnel.
Replacement of Sellwood Bridge is nearly complete. When finished, it will be Portland’s most earthquake-resilient bridge for car and truck traffic.
The old Sellwood Bridge stood on concrete piers with hardly any rebar for support. Piers on the new bridge are filled with a forest of steel.
“Instead of cracking and crumbling and falling apart during an earthquake, they can flex some and bend a little bit and absorb the energy from the earthquake without collapsing,” said Cannon.
The foundations for the new Sellwood Bridge reach the bedrock. They replaced old timber that didn’t go down that far.
Construction crews also worked to reinforce the west bank of the Willamette River, which helps prevent a landslide at the bridge ramp.
Government records indicate the Ross Island, Hawthorne, Steel and Broadway Bridges would likely collapse during a major earthquake.
The Morrison and Burnside bridges could remain standing, but with extensive damage.
The Marquam and Freemont bridges would survive but they would have problems. The middle spans would be okay, but the approaches would collapse.
Engineers did not design bridges for major earthquake until the mid-1990s.
Newer bridges, like the Tilikum Crossing over the Willamette River, must be built to tougher standards.
Engineers tried to make some bridges more resilient by adding cable and restraints, like those visible beneath the Marquam Bridge.
Retrofits are expensive.
“Part of what makes this so expensive is the age of our bridges,” explained Paul Mather, administrator of ODOT highway division.
“They’re nearing the end of their life. They have a lot of serious structural problems, so it doesn’t make economic sense to retrofit those,” said state bridge engineer Johnson.
In November 2015, Multnomah County Commissioners approved $3 million for a study to figure out how to make the Burnside Bridge more resilient to earthquake. It is considered a critical east-west connector across the Willamette River.
As it stands now, if the big one hits, there will be catastrophic damage. Traffic will be paralyzed. There will be few options to cross the rivers that separate our city
“It’s not going to be like a snow storm, where you plow the snow and few days or weeks later it melts goes away,” warned Mather. “Life is going to be very, very different for many, many years.”