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Portland bridges might not withstand quakes

By Fred Leeson
The Oregonian

A major earthquake in Portland could raise havoc with the network of bridges knitting the city's east and west sides.

Only two of 10 bridges spanning the Willamette River – the Marquam and portions of the Burnside – have been fitted with the easiest and cheapest seismic bracing.

State and county engineers have identified deficiencies that could prove crippling in a major earthquake. But so far, there is no timetable or budget for modifications that would run into tens of millions of dollars.

"I think the risk is great," said Franz Rad, a Portland State University civil engineering professor who has spent a decade studying potential earthquake impacts on buildings in the Portland region.

Although his work focuses on buildings, Rad has been concerned enough about bridges to discuss seismic bracing with the Oregon Department of Transportation as the state launches a major bridge repair program. "I think we are not moving fast enough," Rad said.

Improving the bridges involves the complex interplay of money, geology and antiquated bridge designs. "The problems are huge," said Sharon Wood Wortman, a Portland bridge historian and author who has paid close attention to earthquake risks.

Wortman said the nature of soil in the heart of Portland, laid down over centuries by the Willamette River, is a key danger. "Essentially, we have very large bridges sitting on an alluvial bog," she said.

Five of the city's 10 bridges are owned and maintained by Multnomah County: the Sellwood, Hawthorne, Morrison, Burnside and Broadway. "All our bridges were designed before seismic was an issue," said Ian Cannon, Multnomah County's engineering services manager.

As a result, he said it is difficult to estimate how powerful a jolt they could survive.

The picture is similar with four state-owned bridges. No seismic retrofits have been applied to the Fremont, Ross Island or St. Johns bridges. Nor is there any earthquake bracing on the Steel Bridge, which is owned by Union Pacific Railroad and is the only east-west line for Portland's growing light-rail transit system.

Whether a major quake will occur in Portland seems to be a question of when, not if. But scientists cannot predict the timing or severity of earthquakes. "It could be tomorrow," Rad said.

The city's newest river crossing, the Fremont Bridge completed in 1973, has a span that is expected to survive a major quake, but the bridge approaches at both ends are considered vulnerable.

State and county officials have approved improvements ranging from $12.5 million to $33 million each in recent years for the Ross Island, Hawthorne, Broadway and St. Johns bridges. But none of that work involves earthquake strengthening.

Cannon said the first priority is keeping the bridges functional for users ranging from heavier trucks to more pedestrians and bicyclists.

"California has a different philosophy," he added. "Seismic is an imminent risk. Oregon doesn't see the problem the same way."

California got the message in 1971 and again in 1989 after quakes in the San Fernando Valley and the San Francisco vicinity, respectively, severely damaged freeway bridges and overpasses. The 1989 quake persuaded the Oregon Transportation Department to apply "phase one" bracing techniques to the Marquam Bridge.

Phase one improvements tie bridge decks to vertical supports to keep decks from falling and protect the lives of people on bridges when a quake occurs. But damage to columns could still render a bridge useless. More expensive phase two repairs – not yet applied to any Portland bridges – add bracing to bridge piers under water.

The Oregon Legislature last year passed a $2.46 billion statewide transportation package that includes $1.6 billion to repair state bridges.

But it appears that only a small portion of that money will be used for earthquake bracing. That's because earthquake protection is required for new bridges, but not for the repair of existing structures.

State transportation officials still are deciding which bridges to repair and which to replace. "The money will fix some seismically challenged bridges," said Dave Thompson, a Transportation Department spokesman. "But it's a lucky side effect, not the main issue."

Rad, the civil engineering professor, said the state is "looking more at maintenance and crack control. They're not really worrying about earthquakes."

The state funding package also includes $361 million to be allocated to city and county bridges. Multnomah County hopes to tap that fund to complete a $30 million Sauvie Island Bridge replacement. That bridge also is considered highly vulnerable to earthquakes.

County Commissioner Maria Rojo de Steffey, who has been lobbying for the Sauvie Island project, also hopes some state money will be available for replacing the Sellwood Bridge, a lightly built structure that suffers from cracking prompted by shifting earth at its west end.

"The cost for seismic work is just exorbitant," Rojo de Steffey said. "Unfortunately, the funding is really hard to find. The issue is really about funding."

James Roddey, community education coordinator for the Oregon Department of Geology, said Portland is vulnerable to comparatively shallow crustal earthquakes, as well as to much stronger quakes from the Cascadia subduction zone that runs for about 800 miles off the Pacific Northwest coast.

Roddey said Portland has experienced a few crustal earthquakes of magnitude 5.5 in the past 50 years without suffering bridge damage. Cannon said some of the older bridges, such as the Steel, Broadway and Hawthorne, appear to have been more sturdily built because they originally carried heavy streetcar traffic.

But a subduction quake could prove far more severe than any recent quakes, perhaps hitting magnitude 8 or 9. On the logarithmic severity scale, each full number represents a-tenfold increase in earthquake strength.

A magnitude 7.2 subduction quake hit Kobe, Japan, in 1995, killing 5,400 people, leaving 235,000 homeless and creating losses estimated at $95 billion to $150 billion. Like Portland, much of Kobe was built on alluvial soils.
"We will get one of those in Oregon sooner or later," said Rad, the PSU engineer.

The most recent subduction earthquake in the Portland region is believed to have occurred in 1700. But the same type of quake rocked the Alaska coast as recently as 1964.