7.3.5: Bridges and Overpasses

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Overview

Freeways and bridges are lifelines, and their failure can disrupt the economy and kill people on or beneath them during an earthquake (Figure 12-14). The television images of people sandwiched in their cars in the collapse of the double-decker Interstate 880 Cypress Viaduct in Oakland, California, the collapsed span of the Oakland-San Francisco Bay Bridge, and the pancaked freeway interchanges in Los Angeles after the Sylmar and Northridge earthquakes were dramatic reminders of the vulnerability to earthquakes of highways and railroads. Structural engineers in the Bridge Division of the Oregon Department of Transportation visited the collapsed freeway overpasses after the Northridge Earthquake, and their recommendations led to the first thorough appraisal of the earthquake potential of Oregon faults. However, most of the overpasses on Interstate 5 have not yet been repaired. As pointed out above, the 2015 Oregon Legislature failed to pass a transportation bill that would have begun retrofitting seismically-dangerous bridges that, if they failed, would cut off the Oregon coast from the Willamette Valley in a subduction-zone earthquake. The resilience survey pointed out the problem, but the legislature did nothing about it.

The double-decker Cypress Viaduct is reminiscent of the Marquam Bridge in Portland (since retrofitted) and the Alaskan Way Viaduct in Seattle, built in 1953 for $8 million on liquefiable soils. The Alaskan Way Viaduct was damaged in the 2001 Nisqually Earthquake and was closed for a time. Many people feared that if the shaking had lasted longer or had been of higher intensity, the viaduct would have collapsed. The Alaskan Way Viaduct is now being replaced. On the other hand, twenty-three bridges in Seattle had been retrofitted before the earthquake, and none of those were damaged.

Freeway collapses during the Northridge Earthquake caused great disruption to commuters traveling from northern and western suburbs to downtown Los Angeles. Failure of the Golden Gate Bridge and Bay Bridge could isolate San Francisco from counties north of the Bay and from East Bay cities. Bridge collapses on Highway 101 on the Oregon and Washington coast from the next subduction-zone earthquake could isolate coastal communities for an indefinite period of time, as concluded in the recent resilience surveys in both states.

Five bridges collapsed in the 1994 Northridge Earthquake. All were designed to pre-1974 standards, and none had been retrofitted. The Santa Monica Freeway had been targeted for seismic retrofit, but the earthquake got there first. In some cases, a collapsed bridge was adjacent to a recently retrofitted bridge that suffered little or no damage, even though it had been subjected to earthquake forces similar to those endured by the bridge that collapsed. Clearly, retrofit worked for bridges and overpasses.

The problem in the older bridges was in the columns supporting the freeway superstructure. There was inadequate column confinement, inadequate reinforcement connections between the columns and the footings on which they rested, and no top reinforcement in the footings themselves. When these problems were overcome in retrofitting, bridges rode through earthquakes fairly well.

California, through Caltrans, is the nation’s leader in the seismic retrofit of bridges. In 2000, Caltrans estimated that about seventeen hundred bridges in the state—about 10 percent of California’s bridges—required retrofit to prevent collapse during a future strong-motion earthquake. In the Pacific Northwest, bridges such as Interstate 5 and Interstate 205 across the Columbia River and the Tacoma Narrows bridge require special consideration, because a collapse could drop a large number of vehicles and passengers into the water. The cost of retrofitting all of these bridges is prohibitive if done in a very short period of time, but both Oregon and Washington have begun the process. The resilience surveys in both states revealed that a majority of bridges in Oregon and Washington are obsolete and are subject to collapse in the forthcoming Cascadia Subduction zone earthquake. This information has been transmitted to the legislatures that authorized the surveys, but funding to fix the problem has not yet been authorized.

Which bridges to retrofit first? Establish priorities based on the potential magnitude of the loss, both directly in damages and lives lost and in economic losses, then allocate the resources to do the job.