Twenty years after the Loma Prieta earthquake caused a section of the Bay Bridge to collapse, more than 200,000 vehicles cross the same span daily while a replacement is being built.
The 6.9-magnitude quake that shook the Bay Area on Oct. 17, 1989, caused a 250-ton, 50-foot section of the upper deck of the eastern span to fall onto the lower deck, killing a woman and causing a one-month closure of the bridge.
In the aftermath of the earthquake, state officials made the decision to retrofit the western span of the bridge, which runs between San Francisco and Yerba Buena Island. It was also decided to build a new span to run between Yerba Buena Island and Oakland.
Following years of political wrangling about the proposed replacement span’s design, officials in the late 1990s opted for a radical solution: The eastern span would be rebuilt as the world’s longest self-anchored suspension bridge.
The retrofit of the western span was completed in 2004, but the rebuild of the eastern section was plagued by controversy and cost.
The new eastern span will include four distinct components: A ramp connected to Yerba Buena Island; a suspension bridge; a skyway; and a touchdown in Oakland.
The skyway, which is mostly completed, will connect with Yerba Buena Island via an unconventional new suspension bridge.
Most suspension bridges hang from cables that are slung over multiple towers and anchored at both ends, but the new Bay Bridge section, which is under construction, will have a single tower, and the cables will not be anchored.
But the design of the section, with a $1.4 billion price, will be nearly twice what Caltrans originally predicted.
Questions have also arisen about whether the new bridge would withstand a major earthquake.
Studies using computer simulations of Bay Area ground movements during earthquakes suggest the suspension bridge could fail during a temblor and snap away from the skyway, according to UC Berkeley engineering professor Abdolhassan Astaneh-Asl.
Caltrans officials have dismissed the alarms sounded for years by Astaneh-Asl.
But how strong an earthquake the span could withstand is unclear.
Unlike BART, which is reinforcing its rail network to withstand the strongest hypothetical quake expected to strike the system, known as the “maximum credible earthquake,” Caltrans is rebuilding the bridge to a lower seismic standard.
“It would be difficult to determine what [a maximum credible earthquake] is for a bridge that lies between the two biggest faults in a region that has numerous others,” Caltrans spokesman Bart Ney said.
The new span is scheduled to open in 2013 and last for 150 years. The cost of the retrofit and rebuilding effort is expected to cost
$7.2 billion to $8.6 billion — triple the 1997 estimates of $2.6 billion.
Underwater BART tunnel attaining upgrade
The underwater tube that carries BART trains between the sides of the Bay was largely undamaged by the Loma Prieta earthquake, but the system is still receiving upgrades.
Voters in 2004 approved a bond sale to seismically reinforce the main elements of the network that runs between San Francisco and Oakland, which were built in the 1960s and 1970.
“The system was very robustly built in its day, but we’re bringing it up to modern standards,” BART spokeswoman Molly McArthur said.
Much of the seismic work involves strengthening 1,918 towers that support elevated train tracks and stations, most of which are in the East Bay.
But BART’s first priority is strengthening the tunnel beneath Oakland and San Francisco.
That stretch of the network was prioritized for seismic reinforcement based on public feedback, according to McArthur.
“The thought of being stuck under the Bay in an earthquake is not a pretty thought,” McArthur said.
Seismic work on the eastern half of the tunnel is complete, and work on the western half is expected to be finished by the spring, according to McArthur.
BART’s $1.3 billion retrofit project, which is designed to protect the entire network from the strongest hypothetical quake expected to strike, is scheduled to finish in 2013.
— John Upton
Along with roadways, major water pipes could be prone to damage during an earthquake.
$370 million Seismic improvements to the local and regional water system since 1989
$2.4 billion Additional seismic improvements planned to the local and regional water system by 2015
13 percent Expected water rate increase to fund seismic work between now and 2015
280 miles Pipes being seismically improved
11 Reservoirs being improved
5 Pump stations being improved
2 Water treatment plants being seismically improved
Source: San Francisco Public Utilities Commission
Electricity and natural gas are shipped into The City from other areas, leaving transmission lines vulnerable during earthquakes.
$2 billion Seismic improvements to natural gas transmission, distribution network
$304 million Seismic improvements to PG&E buildings, including offices and substations
$55 million Seismic improvements to PG&E power plants
Source: Pacific Gas & Electric Corp.