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Phases of seismic strengthening

How does one upgrade a bridge in danger of suffering damage from an earthquake? There are two identified phases of seismic strengthening, according to Multnomah County transportation spokesman Mike Pullen.

Phase 1: The first focus of an upgrade is to prevent loss of lives, not to save the structure. An example of a phase-one upgrade is under the Marquam Bridge, where cables strap the structure to its supporting column. This kind of upgrade costs millions of dollars 鈥� the Marquam project was $4 million. The Burnside Bridge also has received some phase-one upgrades.

Phase 2: This upgrade entails repairs that allow a bridge to survive a quake and be quickly reopened afterward. This kind of upgrade, which is roughly equivalent to bringing a bridge up to current seismic standards, is more difficult and more costly (usually tens of millions of dollars).

Many of Portland’s bridges are built on timber pilings, Pullen noted, and a phase-two upgrade would require replacing those columns with cement ones that would extend down to bedrock.

鈥淲e’re skeptical that we’ll ever be able to do a phase-two seismic upgrade on a bridge鈥� because they’re so expensive, Pullen said. CH2M HILL has estimated that a phase-two upgrade of the Sellwood Bridge would cost $40 million.

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What kind of earthquake is Portland in for?

Scientists say that the Pacific Northwest is due to experience a significant earthquake within the next few decades. Such an event could devastate Portland, particularly if its bridges were to fail.

鈥淲e have a very bridge-dependent city,鈥� Portland Office of Emergency Management Director Carmen Merlo said.

鈥淎 majority of the bridges 鈥� were built prior to scientists’ current understanding of the regional seismic threat and prior to the engineers’ current understanding of effective seismic design,鈥� said Peter Dusicka, a Portland State University civil and environmental engineering professor. 鈥淲e have learned a lot in the past several decades; unfortunately, a majority of our infrastructure is significantly older.鈥�

Even the relatively new Fremont Bridge, built in the 1970s, is not expected to be useful following a major quake.

鈥淓ven if the bridge survives, the ramps won’t hold up,鈥� Merlo said.

Expensive seismic upgrades are not in the foreseeable future for Portland’s bridges, so POEM is drafting an earthquake contingency framework 鈥� scheduled to be released publicly in November 鈥� for a potential 鈥渨orst-case scenario鈥� in which no bridges function.

POEM expects to use boats and aircraft to transport supplies, people and even vehicles across the Willamette River. Merlo said that the city also would hope to receive federal assistance to provide a temporary 鈥渇loating bridge.鈥� If one of the city’s more seismically sound bridges was to stay standing, it would likely be reserved for emergency use.

鈥淚f we’ve got a (magnitude) 9.0 earthquake that happens, people should not be able to expect to make it to their house if they live or work across the river,鈥� Merlo said. 鈥淭he likelihood of us being able to transport everyone where they need to go immediately is not realistic.鈥�

Merlo described 鈥減ockets of isolation鈥� where people might be stranded amid broken transportation infrastructure for 72 hours or longer. Because significant damage is expected, those people would be provided with food and supplies, but likely would be stuck for some period of time.

鈥淲ithout question 鈥� people are going to be stranded wherever they are for a while,鈥� Merlo said.

The possible lack of a connection across the river is a concern because all of the city’s emergency public works vehicles that would be used to clear debris after a quake or other disaster are stored on the east side of the river, under the Fremont Bridge.

鈥淲e had desperately been looking for a west-side location鈥� to stage emergency vehicles, Merlo said.

Fortunately for the city, it is due to receive the Jerome Sears facility on Multnomah Boulevard in Southwest Portland from the federal government at no cost. The building, now used by the U.S. Army, will hold emergency vehicles for the Portland Bureau of Transportation and POEM.

The city also is seeking an east-side location to store some vehicles in a less precarious position than under the potentially unsafe Fremont Bridge.

鈥淧eople get upset when they hear that there’s a risk to our bridges, because they’re going to be so necessary in an earthquake,鈥� Multnomah County spokesman Mike Pullen said. 鈥淏ut if we don’t take care of the bridges’ existing wear-and-tear problems, they won’t be able to stay in service 鈥� while an earthquake may or may not happen.鈥�

Some of the city’s bridges have received seismic upgrades. The Marquam and Burnside bridges both have received 鈥減hase one鈥� upgrades to protect life safety during an earthquake. 鈥淧hase two鈥� upgrades, which would bring the city’s bridges up to present seismic standards and maintain their use following a quake 鈥� are much more expensive and less likely.

鈥淲e’re underfunded for existing maintenance,鈥� Pullen said. 鈥淭he challenge has been to convince our outside funders to make the seismic work a priority also.鈥�

Only new Portland bridges may ever be earthquake-resistant. Since the mid-1990s, new bridges and buildings have taken into account the seismic risk to the region. Both the new Sellwood Bridge and the Portland-Milwaukie light-rail bridge are being built to modern standards, and should survive an earthquake.