Seismic Strategy

LEAD DESIGNERS
T.Y. Lin International Group
www.tylin.com

PROJECT
San Francisco–Oakland Bay Bridge

Oakland, California

Nearly a quarter-century after a 50-foot section of the San Francisco–Oakland Bay Bridge collapsed in the 1989 Loma Prieta earthquake—sending one motorist hurtling to her death and shutting down one of the busiest bridges in the world for a month—its new eastern span is scheduled to open later this year. The 1989 collapse was a jarring reminder of the vulnerability of one of the region’s most important transportation corridors. The bridge sits between two major faults—the Hayward to the east and the San Andreas to the west. Its two-mile-long western section, a suspension bridge that runs from downtown San Francisco to Yerba Buena Island, escaped serious damage in the 1989 quake. The 2.2-mile eastern span, which runs on to Oakland and is now being replaced, is a different matter. Seismologists estimate there is a two-in-three chance that another major temblor will strike the Bay Area by 2036.

Engineers, architects, and local officials wrangled for years over how to make the replacement bridge more quake-resistant. The simplest solution would have been a more robust version of the existing structure—an uninspiring series of low trusses supported by piers. But East Bay residents were determined to have a signature design to rival the more celebrated Golden Gate Bridge across the bay. “The region was pretty adamant that they wanted an iconic design,” says Bart Ney, a spokesman for the California Department of Transportation, better known as Caltrans. The solution Caltrans came up with is an elegant marriage of two designs—a 1.3-mile skyway linked to a single-towered self-anchored suspension (SAS) bridge.

Since the original Bay Bridge was built in the mid-1930s, there has been a seismic shift in the way engineers design structures to survive earthquakes. “The old philosophy was to calculate the maximum force of an earthquake and build the structure to resist that force,” says Marwan Nader, the lead designer of the new bridge and the vice president of the engineering firm T.Y. Lin International Group. One problem with that approach, Nader notes, is that if the force exceeds that maximum it could lead to the collapse of the entire structure. Another concern: massive, stiff structures tend to be both unlovely and expensive.

The new approach is to build a structure that is flexible enough—“ductile” is the term engineers prefer—to absorb the shock. “The idea is to build a structure that can stretch and deform without breaking,” says Nader. Along the bridge’s roadway, segments of the deck are joined by 60-foot sliding steel tubes. These giant dowels contain soft steel centers designed to yield during earthquakes, like replaceable fuses. Indeed, all the bridge’s shock-absorbing elements are designed to be restorable within hours of a seismic event.

Perhaps the biggest innovation in the new bridge is the SAS system. Unlike a conventional suspension bridge, in which the roadway is hung from cables that are slung like a hammock across two or more towers, the SAS works more like a sling. A single cable loops from the eastern end of the bridge over the tower to the western end and back again. While other SAS bridges have been built, the Bay Bridge will be the longest and the only one to feature a single, asymmetrical tower. The tower is divided into four shafts that function as a single support system but can move independently to dissipate seismic forces. The shafts are connected by shock-absorbing steel beams that can be replaced easily. In a major earthquake, the top of the tower can sway up to five feet without suffering any damage.

Soaring 525 feet above the bay, the tower creates a striking profile. But it comes at a lofty price. In a conventional suspension bridge the road deck is added last, but in an SAS design, where the compression of the suspension system is anchored in the road deck itself, it has to be built first. To do that, Caltrans had to construct a temporary bridge to hold up the deck until the suspension system was completed. That process added considerably to the cost and timeline of the project. Indeed, the span’s $7.2-billion price tag makes it the most expensive bridge in U.S. history and one of the costliest structures ever built.

Given the vital importance of the Bay Bridge to the region, the new structure has been built to extraordinarily high specifications. If properly maintained, it is designed to last 150 years—three times longer than a conventional bridge. Its seismic defenses were calculated to withstand the largest ground motions projected in the region over the next 1,500 years. The state of California has designated the bridge as a “lifeline” structure. What this means is that shortly after a big quake, the bridge needs to be used by emergency vehicles. “There will be damage [in a major quake],” says Nader, “but the damage will be repairable, and the bridge should be quickly returned to service.”

Nader, who has worked on major bridges around the world and has been involved in the Bay Bridge project since 1989, is confident that the new span can survive even the biggest seismic punch. “This is a unique structure,” he says. “I have never seen a structure as [thoroughly] engineered and peer reviewed as this one.” Asked what his biggest fear was, Nader, like the bridge he designed, deflected deftly: “I sometimes joke that if a quake does hit I want to be at the top of the tower because that will be the safest place to be.”