New Bay Bridge span designed to endure major quake

Published 15 October 2009

Twenty years ago a 250-ton section of the Bay Bridge fell into the water as a result of a 6.9 magnitude earthquake; the new bridge design will be able to withstand the largest plausible earthquake to occur within a 1,500-year period

Twenty years after the Loma Prieta earthquake shook loose a 250-ton section of the Bay Bridge, California transportation officials vowed Monday that the long-planned replacement span will be built to better withstand a major temblor. “When the bridge is complete, it’s going to be one of the most seismically advanced structures in the world,” Caltrans spokesman Bart Ney said.

San Francisco Chronicle’s Rachel Gordon writes that on Monday, Caltrans released a computer-generated simulation of how the new East Bay segment of the bridge — set to be completed in 2013 — is expected to respond when jolted by a large quake. 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 designed to absorb the earthquake’s energy and protect the main structure. The damaged beams then can be swapped out.

The soaring 525-foot-tall tower of the planned 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 independently. The legs will be connected by replaceable shock-absorbing beams.

The original 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 four inches of motion. The single-deck replacement span is designed 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 crashed 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 do not 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. 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 completed. New bolts and braces, extra steel, a new bearing system and other upgrades were put in place to make it less vulnerable to collapse.

The Bay Bridge project now is estimated to cost $6.3 billion and will not be finished until 2013 — two-and-a-half 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 materials. “What we’re building here,” Ney said, “is an icon - not something that’s built once in a generation, but once.”