Seismic bridge engineering used to be more of an “art” and less of a science—but it’s making progress as the latter, says Frieder Seible, dean of the Jacobs School of Engineering at the University of California, San Diego. Through lessons learned from real quakes and advancements in computer modeling, “we’re developing a scientific approach” to retrofits.
The approach informs continuing research on seismic behavior of bridges. “The most recent seismic events...have influenced code changes that are still being implemented today,” says Mark Christensen, engineer with TRC Imbsen, Sacramento. Colleague Shin-Tai Song adds that evaluation of recent quakes “indicates that the majority of damage can be related to the elastic design philosophy widely adopted prior to 1992, which specified a relatively low design lateral force level.”
Since then, “we have found that structures can survive earthquakes if columns are ductile—which allows for inelastic deformation without a significant loss of strength,” he adds. “The goal now is to provide more resilience through better detailing, capacity-protected members, isolation and increased damping.”
|California Dreams. New Bay bridge is designed for a 1,500-year seismic event, but revised national codes will pr obably not go that far. (Rendering for courtesy of newbaybridge.org)|
As a result of lessons learned in less than 30 years, bridge columns in California are regularly wrapped in steel or fiber jackets that allow but confine movement. Expansion joints have cable restrainers. Shear force transfer designs allow for both horizontal and vertical movement.
Retrofits or new construction provide for ductility or for isolation—as in bearings that allow the deck to move independently. Bridges may have a “sacrificial” bearing designed to break in a quake and become an isolation bearing, to be easily replaced afterward, says Khaled Mahmoud, president of Bridge Technology Consulting & Engineering, New York City.
What’s next? “Foundation retrofits are the next step,” says Seible. “We’re still learning about soil-foundation interaction.” Much remains to be discovered about best practices for liquefaction, ductility in foundations, performance of segmental bridge joints and the geographical variations of ground movements for bridges (see p. 26). Researchers also are looking at self-diagnostics for bridges during quakes and ways to combine seismic solutions with those for other hazards, says Michel Bruneau, director of the Multidisciplinary Center for Earthquake Engineering Research, at the State University of New York at Buffalo.
The American Association of State Highway and Transportation Officials continues work on a revision of its official seismic highway codes, and may be finished by next year. It will find a medium between a National Cooperative Highway Research Program report calling for designs to a 2,500-year event and current codes for a 500-year event. Designing for a 2,500-year return period is generally deemed too costly and complicated to apply nationwide, says Michael Abrahams, senior vice president with Parsons Brinckerhoff, New York City. “Not every bridge is a major bridge, and not every state is California,” he says.
The revision will stay true to the performance-based approach that evolved since the 1971 San Fernando earthquake. Performance standards require “lifeline” crossings to be open to emergency vehicles within two days. Seismic resilience of complete networks will be a focus of future research, says Ian Buckle, professor at the University of Nevada, Reno.
One lifeline, the new Oakland-San Francisco Bay Bridge, features the latest in seismic advancements, plus another new performance-based concept—seismic considerations during construction, says Ron Crockett, technical project manager with American Bridge, Coraopolis, Pa. (see sidebar). With Fluor Corp. it submitted the low bid of $1.4 billion for the self-anchored suspension span. The team will use falsework that must support work atop 100 ft of mud and 60 ft of water. Bracing will be designed to accommodate 12 in. of displacement.
Predicting displacement is improving as “we get better information from our seismologists about expected ground movements at certain locations,” notes Seible, who has worked with the California Dept. of Transportation on $10-billion worth of seismic bridge retrofits. But modeling must be used judiciously. “When we do full-scale tests, they are one-of-a-kind,” says Seible. “It’s too expensive to build ten.”
Peter Buckland, principal with Buckland & Taylor Ltd.,Vancouver, British Columbia, warns against too much analysis driving bridge design. “We have fantastic analysis tools and computer programs,” he says. “But analysis tends to drive the design instead of being a servant to it.”
“There’s much to be learned [about] non-linear analysis,” says Greg Orsolini, vice president in the San Francisco office of Parsons Corp. “For complex structures it’s very helpful, but it’s not done on a regular basis. It’s not necessary for normal structures.”
Normal structures include bridges along the Bay Area Rapid Transit system’s 74-mile system. Parsons Transportation Group is one of four designers for bridge retrofits in BART’s $1.3-billion seismic program. BART began awarding design contracts this year, including upgrades to the Transbay Tube tunnel. Built in 1960, the 3.6-mile-long immersed tube features the first use of seismic joints, designed by Parsons Brinckerhoff, “that allow the tube to move relative to the rigid ventilation buildings at the ends,” says Joe Wang, PB geotechnical engineer.
Wang is now involved in a federal study to provide seismic specifications for tunnels. Phase 1 is complete and Phase 2 will start this year. “It is the first attempt to incorporate tunnel seismic design codes for AASHTO,” Wang says.