SACRAMENTO -- If the Loma Prieta earthquake were to strike again, the new eastern span of the Bay Bridge would be substantially safer than the 1936 cantilevered truss span motorists use today, according to new analysis presented to Bay Area legislators Thursday.
The 1989 Loma Prieta quake, a 7.1 temblor centered 60 miles southeast of the Bay Bridge, exceeded the original bridge's capacity to withstand the shaking, but it wouldn't have come close to taxing the new span, according to a line graph prepared by Caltrans bridge engineer Brian Maroney.
"Eighty years ago, engineers didn't understand or fully respect vibrations generated by an earthquake," Maroney said. "When you look at this line graph, it is easy to see why the eastern span failed in 1989."
Although bridge engineers have repeatedly assured the public that the new span is much safer than the old bridge, this comparison is the first side-by-side analysis Bay Area legislators have seen that shows how the old one fared during Loma Prieta and how the new one would handle a quake of similar magnitude and location.
The three agencies overseeing the Bay Bridge construction presented the new seismic comparison data to the Bay Area Caucus. Along with Senate Transportation Committee Chairman Mark DeSaulnier, D-Concord, the Bay Area lawmakers requested the briefing in the wake of mounting public concerns about the $6.4 billion span's seismic safety.
Much of the worry stems from the failure of 32 high-strength steel bolts embedded in seismic stabilizers. The bolts snapped in early March, forcing Caltrans and the contractor, American Bridge/Fluor Joint Venture, to scramble to repair the damage and reassure the public that the other steel components are sound.
The bridge is scheduled to open Sept. 3, but that date could be postponed if Caltrans, the Bay Area Toll Authority and the California Transportation Commission cannot complete the bolt retrofit, or if ongoing tests reveal problems with other steel fasteners on the span.
But the longer it takes to open the new bridge, the higher the chances motorists will still be using the vulnerable old span when the Big One hits. The Bay Bridge carries 280,000 vehicles a day, making it one of the busiest spans in the nation.
Engineers designed the 1936 bridge to withstand maximum ground accelerations of 10 percent of 1 g, a measurement of the force of gravity on a moving object, within a 5-second period. The rate of 10 percent of 1 g was roughly the same earthquake safety standard applied to new building construction at the time. The line graph Maroney prepared for lawmakers at the request of the Toll Bridge Program Oversight Committee shows that the Loma Prieta earthquake shook the bridge on Yerba Buena Island at G-forces of up to 20 percent, or double what the span was designed to handle. Compare that with the new bridge, which has been designed to survive equivalent Gs of up to 180 percent.
Picture what happens to the end of a whip, Maroney explained.
Crack the whip handle just a few inches and the energy travels its length, amplifies and sends the end violently snapping back and forth.
Just like the whip, when the Loma Prieta quake jolted the ground, the energy waves surged through the rock beneath Yerba Buena Island and jerked anything sitting on or attached to the island.
Once a structure like a bridge or a building starts moving, it will continue to oscillate depending on its size and materials. And it will break if engineers fail to design the structure in such a way that it moves with the vibrations like a flexible willow tree rather than a stiff oak.
At 5:04 p.m. on Oct. 17, 1989, ground motions that began 60 miles southeast on the San Andreas Fault roared through the Bay Area and shook the Bay Bridge. Bolts that held a portion of the upper deck sheared off, and a 50-foot-long, 250-ton section of the upper deck collapsed like a trap door. A young Berkeley woman's car plunged into the gap and she died.
"Back in the 1930s, the engineers thought one-tenth of a G-force was plenty," Maroney said. "They blamed the destruction after the 1906 earthquake in San Francisco on poor construction and the fire. They believed that a structure built with quality materials and skilled workers wouldn't be vulnerable in an earthquake."
Even though seismic experts first recorded a California earthquake in 1933, engineers didn't begin factoring vibration theory into bridge design until the mid-1970s, Maroney said.
James Gates, former Caltrans chief of structural and seismic analysis, brought vibration theory into bridge design following the 1971 San Fernando earthquake. Gates had been a student of geotechnical engineer I.M. Idress, a UC Davis professor emeritus and researcher who served on the 1990 Governor's Board of Inquiry into the impacts of Loma Prieta on Bay Area roads and bridges. Idress currently sits on Caltrans' Toll Bridge Program Peer Review Panel.