[Metropolis magazine / February 2013]
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 closing one of the busiest bridges in the world for a month—the new eastern span of the bridge is scheduled to open later this year. The bridge collapse was a wake-up call for the Bay Area—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—the cause of both the Loma Prieta quake and the great 1906 earthquake that decimated San Francisco—to the west. Seismologists estimate there is a 2 in 3 chance another major temblor will strike the Bay Area by 2036.
The Bay Bridge is divided into two sections. The 2-mile-long western span, 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, linking Yerba Buena to Oakland was a different matter. It was clear the span had to be made more earthquake resistant, but engineers, architects, and local officials wrangled for years over how to do it. The simplest solution would have been to build 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 types of bridges—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”
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, lead designer of the new bridge and vice president of the engineering firm T.Y. Lin International. 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 and stiff structures tend to be unlovely to look at and expensive to build.
The new philosophy of earthquake-proofing bridges is to build a structure that is flexible enough—ductile is the term engineers prefer—to absorb the seismic shock and keep on ticking. And that’s what the designers of the new Bay Bridge have done. “The idea is to build a structure that can stretch and deform without breaking,” says Nader. “It’s like the bumper on a car—it’s designed to take a large impact and still be drivable.” In essence, the bridge is designed to ride an earthquake rather than fight it.
Nader and his team devised several innovative variations in this ductile defense strategy. The SAS 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 steel girders intended to absorb the brunt of any damage and be relatively easy to replace. In a major earthquake, the top of the tower should sway up to five feet without suffering any damage.
Along the roadway, segments of the deck are joined by 60-foot sliding steel tubes. These giant dowel-like devices contain soft steel centers designed to crumple during quakes, like a replaceable fuse. Indeed, all the bridge’s shock-absorbing elements are designed to be replaceable within hours of a seismic event. Even the bridge’s 160 concrete piers are designed to move in an earthquake, limiting the damage to areas with extra steel reinforcing. As an added precaution, the piers are hammered into the bay’s muddy bottom at an angle to better deflect seismic forces.
Perhaps the biggest innovation in the new bridge is the self-anchored suspension system. Unlike a conventional suspension bridge, in which the roadway is hung from parallel cables 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 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.”
The SAS tower, which soars 525 feet above the bay, creates a striking profile. But it comes at a lofty price. In a conventional suspension bridge the road deck is added last, hung from suspenders attached to the main cables. But in an SAS design, where the compression of the suspension system is anchored in the road deck itself, the deck has to be built first. In order to do that, Caltrans had to first build a temporary bridge, called falsework, 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 bridge has been built to extraordinarily high specifications. If properly maintained, it is designed to last 150 year—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 a “lifeline” structure. What that means is that merely surviving a major tremor without falling into the bay is not enough. Within hours of a big quake, the bridge should be serviceable to emergency vehicles rushing to bring aid to stricken residents. “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 word and has been involved in the Bay Bridge project since 1989, is confident the new span can survive the biggest seismic punch the Bay Area is capable of delivering. “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.”
Top: Photo by Dllu.
