The San Francisco–Oakland Bay Bridge, a complex of structures spanning San Francisco Bay, has a history marked by significant engineering challenges, particularly concerning seismic resilience. The bridge's two major sections—the western suspension spans between San Francisco and Yerba Buena Island, and the structures connecting Yerba Buena Island to Oakland—have both seen extensive work to ensure their durability in an earthquake-prone region. The original
eastern section, composed of a double balanced cantilever span, five through-truss spans, and a truss causeway, became a focal point of concern after a section collapsed during the 1989 Loma Prieta earthquake.
The Impact of the Loma Prieta Earthquake
The Loma Prieta earthquake, which occurred on October 17, 1989, measured 6.9 on the moment magnitude scale. Despite its epicenter being distant from the bridge, the earthquake caused a 50-foot section of the upper deck of the eastern truss viaduct to collapse onto the deck below, indirectly leading to one fatality at the collapse point. This event highlighted the urgent need for enhanced earthquake resistance. The bridge was closed for just over a month for repairs, reopening on November 18, 1989, with a new, stronger retrofit in place. Analysis of the event indicated that the bridge was close to a more catastrophic failure, where either the through-truss or the causeway segment could have dropped from their common support structure.
Bridge design experts had been aware for over 30 years that a major earthquake on either the San Andreas or Hayward faults could destroy the main cantilever span. However, little action was taken until the 1989 earthquake. Estimates in 1999 placed a 70% probability of a major earthquake in the area within the next 30 years, further emphasizing the critical need for upgrades. This led to the decision to replace the entire eastern span, rather than merely retrofitting it, due to projected cost savings, minimized traffic impact, improved safety, and additional amenities compared to a retrofit.
Designing for the Future: The Eastern Span Replacement
The replacement span for the eastern section was engineered to withstand the largest earthquake expected over a 1500-year period, with an anticipated lifespan of at least 150 years with proper maintenance. The initial proposal for the eastern span involved substantial concrete columns and modifications to lattice beams, similar to those completed for the western suspension spans. However, this approach was criticized for retaining the original structure's high maintenance costs and for its questionable robustness, as highlighted by a critical report from the Army Corps of Engineers and the collapse of a retrofitted overpass in the 1994 Northridge earthquake.
The chosen design for the replacement was a self-anchored suspension (SAS) bridge, a seldom-built type that is unique for being both single-tower and asymmetrical, tailored specifically to the site. This design was selected for its superior visual effect and improved driving experience. Construction began on January 29, 2002, with the span finally opening on September 2, 2013. The new span incorporates extensive energy-absorbing techniques to ensure survivability and immediate access for emergency vehicles following a Maximum Credible Earthquake, estimated at 8.5 moment magnitude. Rather than rigidity, the design emphasizes flexibility, with resonant motion absorbed by the plastic shear of sacrificial, replaceable components, ensuring the bridge's long-term resilience.
