Twenty years past the Loma Prieta earthquake shook movable a 250-ton section of the Bay Bridge, state transport officials vowed Monday to the long-planned substitution span will be built to improve withstand a main temblor. "When the bridge is total, it's going to be one of the majority seismically higher structures in the world," Caltrans spokesman Bart Ney said.


On Monday, Caltrans at large a computer-generated simulation of how the new East Bay segment of the bridge - set to be completed in 2013 - is likely to respond when jolted by a big quake. The imitation can be view at tr.im/BAa2. In the depiction, the bridge sways and undulates, moving with the earth's rumbling rather than resisting it. Imagine an undersea kelp forest pushed and pulled by a strong tide. The beams connecting different sections of the bridge are planned to take up the earthquake energy and protect the main arrangement. The damaged beams then can be swapped out.

The soaring 525-foot-tall tower of the designed self-anchored suspension span on the East Bay portion of the Bay Bridge will be held up by four steel legs, each able to move separately. The legs will be linked by consumable shock-absorbing beams. The unique East Bay span, which is crossed by about 280,000 cars and trucks a day and has undergone a temporary retrofit, was built to handle about 4 inches of motion. The single-deck substitute span is planned to be able to move at least 39 inches, said Marwan Nader, senior associate of the firm T.Y. Lin International, which helped design the project.


When the 6.9 magnitude Loma Prieta earthquake struck 20 years ago this week, the shaking snapped off a 50-foot-long section of the upper deck of the Bay Bridge’s eastern span, and it not working onto the lower deck. One driver died. The bridge was closed for a month, choking off one of the Bay Area's busiest transportation arteries.

The new bridge is designed to keep it standing in the largest plausible earthquake to occur within a 1,500-year period. Those calculations don't specify a quake of a certain size. Rather, engineers factored in projected ground motion emanating from various epicenters. "It will be one of the safest places to be in an earthquake," Ney said.

Bridge officials said the roadway may buckle in a major earthquake and get knocked out of alignment, rendering it temporarily impassible. But if the overall structure remains intact, as expected, Caltrans crews would lay down steel plates right away to make the bridge usable for emergency vehicles before it is repaired for public use.

Retrofit work on the San Francisco portion of the bridge has been finished. New bolts and braces, extra steel, a new bearing system and other upgrades were put in place to make it less susceptible to collapse.

The Bay Bridge project now is estimated to cost $6.3 billion and won't be over until 2013 - 2 1/2 decades after the 1989 Loma Prieta earthquake. Ney said there is an upside to the long delay: Advances have been made in seismic engineering and bridge-building resources. "What we're building here," Ney said, "is an icon - not something that's built once in a generation, but once."