How Vulnerable Are Bay Area Bridges to a Major Earthquake? - NBC Bay Area
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How Vulnerable Are Bay Area Bridges to a Major Earthquake?

While California’s Department of Transportation has retrofitted thousands of bridges across the state to better withstand a major earthquake, NBC Bay Area’s investigation found thousands more at potential risk should the earth start shaking significantly

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    How Vulnerable Are Bay Area Bridges to a Major Earthquake?

    Over years of research and investigation, NBC Bay Area found hundreds of bridges (which includes overpasses) in California and the San Francisco Bay Area that are vulnerable and need repair, even without an earthquake. These bridges are old, deteriorating or in need of critical immediate repairs. In other words they are bridges most at risk of failure if an earthquake strikes.

    (Published Friday, Nov. 30, 2018)

    It’s an area famous for its earthquakes and its bridges.

    San Francisco and surrounding areas.

    But while the iconic Golden Gate bridge as well as the newly refurbished Bay Bridge connecting the city to Oakland have both been retrofitted and brought up to standards that experts expect will allow each bridge to withstand an earthquake of magnitude 8.3 or even higher, Federal National Bridge Inventory Safety data shows hundreds of other bridges aren’t in as good a shape.

    During an earthquake, one of the biggest concerns is bridges and overpasses might collapse if they’re not designed properly, built strong enough or maintained well enough to withstand the natural forces.

    Over years of research and investigation, NBC Bay Area found hundreds of bridges (which includes overpasses) in California and the San Francisco Bay Area that are vulnerable and need repair, even without an earthquake. These bridges are old, deteriorating or in need of critical immediate repairs. In other words they are bridges most at risk of failure if an earthquake strikes.

    Laurie Berman is Director of California’s Department of Transportation (Caltrans), which oversees and maintains most of the state’s 25,657 bridges. Director Berman admits maintenance needs to be completed on more bridges throughout the state. For California, as well as the rest of the United States, it all comes down to money. Or rather lack of it.

    “For years we have not properly funded our transportation system and we're seeing a lot of the effects of that,” says Berman.

    She says the longer any critical maintenance is deferred, the more risk that bridge is exposed to during a catastrophic event such as a major earthquake.

    But Berman stresses Caltrans officials aren’t taking chances with infrastructure safety.

    “I want to emphasize that if a bridge is not safe we shut it down,” Berman said.

    According to NBC Bay Area’s analysis of National Bridge Inventory data, 336 different bridges in the Bay Area are classified by expert engineers as structurally deficient, meaning they should be repaired immediately.

    NBC Bay Area’s investigation found 135 of those structurally deficient bridges have been on that critical list for five years or longer.

    “We do need a new paradigm shift in how we look at our infrastructure,” Caltrans Director Berman told NBC Bay Area. “It's a lot like when i put a new roof on my house. It's necessary but it's nothing I go and show off to my neighbors. But it's necessary and we need to recognize that our infrastructure is really necessary to maintain it.”

    According to structural engineers bridges are most vulnerable to earthquake damage in three different ways:

    First—the soil in which a bridge and its supports are located could soften from the shaking and give way, collapsing a bridge.

    Second—vertical and horizontal forces put on the suspension during the quake, making the bridge sway or twist beyond its capabilities.

    Third—the quality and type of material used in bridge supports, and its age, could make it vulnerable.

    Bridges built before 1971 were believed by their designers to be able to withstand earthquakes just by their design.

    But they’re not.

    In fact, according to available public engineering research at several universities and the American Society of Civil Engineers, most bridges designed before 1971 were assumed to have greater loads and forces put on them by cars, trucks and pedestrians than by earthquakes.

    But the ASCE says subsequent natural disasters and science have disproved that assumption, changing the way modern bridges are designed and built.

    According to a 2003 report to Caltrans by its Seismic Advisory Board titled The Race to Seismic Safety, bridge design has changed significantly since 1971 and the San Fernando Earthquake, which collapsed the I-5 Golden State Freeway Interchange. Bridge design and construction was further refined after the Loma Prieta earthquake in 1989 which damaged several bridges, including a section of the Bay Bridge, and collapsed the two-level elevated Cypress Street Viaduct in Oakland. And bridge design was refined and improved even more following the 1994 Northridge earthquake in Southern California.

    Even with the changes, NBC Bay Area’s investigation discovered 15,205 bridges in California, including 2,215 in the San Francisco Bay Area, were built *before* 1971. Many of those bridges have yet to be retrofitted to meet the new standards. In fact, of those 15,205 bridges in California built before 1971, 1,430 are listed on the latest National Bridge Inventory system as “structurally deficient” In San Francisco Bay Area 195 of those pre-1971 bridges are listed by structural engineers as “structurally deficient.”

    Since the Loma Prieta earthquake in 1989 Caltrans has worked to retrofit and update more than 2,000 bridges statewide. But Caltrans officials admit there are still thousands more that need attention and remain at risk during a major quake.

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