Seismic retrofits of California bridges

Since the 1989 Loma Prieta earthquake and the Northridge quake in 1994, the state of California has been trying to seismically retrofit bridges across the state. Given the significant earthquake yesterday, I thought it would be a good time to provide an update.

Also, to be frank, as a transportation reporter I've been meaning to get up to speed on the issue of bridge safety. So that's how I spent this afternoon -- reading and talking and trying to understand the basic issues.

The state of California owns more than 12,000 bridges. Of those, Caltrans says that it in the past 20 years it has retrofit 2,189 of the 2,194 bridges that needed updates -- with many of those bridges on the freeways and other major routes that serve as major corridors.

The remaining five bridges are the eastern span of the Oakland-Bay Bridge, the Ten Mile River Bridge on Highway 1 in Mendocino County, the High Street bridge on I-880 in Oakland, the 5th Street bridge on the 880 in Oakland and the Schuyler Heim Bridge that connects Long Beach to Terminal Island.

The Schuyler Heim will eventually be replaced, said David Anderson, a Caltrans spokesman. Like the eastern span of the Bay Bridge, it's less expensive to replace it than to rebuild it.

In addition, there are 479 bridges that are owned by cities and counties in California that need seismic upgrades and are eligible for funding from Prop 1B -- the $19.9-billion transportation bond approved by California voters in 2006.

Among these bridges that still need seismic updates are some noteworthy structures in the Southland. Eight cross the Los Angeles River in the city of Los Angeles, including the Sixth Street Bridge and bridges on streets such as Main Street, Glendale Avenue, Vanowen Street and Tampa Avenue. The Los Angeles Conservancy has expressed concerns over the river bridges, in particular, saying that while the need for public safety must be met, any fixes should also consider the bridges' historical architecture.

There are 40-plus bridges that Los Angeles County must fix, including the Foothill Boulevard bridge over the San Gabriel River in the San Gabriel Valley and an Imperial Highway bridge over the San Gabriel River. In Riverside County, the Van Buren Boulevard bridge over the Santa Ana River needs an upgrade. So does the MacArthur Boulevard bridge at John Wayne Airport in Orange County.

Why am I listing these bridges? Because they are some of the same ones mentioned by my colleague Sharon Bernstein when she wrote about bridges in need of seismic upgrades in March 2006.

The California Transportation Commission, or CTC, doled out about $21 million of the seismic funds last week, on top of $13 million previously allocated. Both the Vanowen and Sixth Street bridges got money in that round of funding.

That allows cities and counties to seek Caltrans' approval of their project, which then allows the cities and counties to apply to the Federal Highway Administration for 88.5% of the cost of the project. That's how the bureaucracy works.

The obvious question is what does the term "seismic upgrade" really mean? To put it simply, seismic upgrades are supposed to allow a bridge to withstand the maximum credible earthquake that could happen in any given area, said Anderson.

Of course, there are no guarantees. Intensive monitoring of earthquakes has revealed a lot of basic patterns about earthquake size, frequency and location (as the map at right shows) and scientists have mapped many of the region's faults. But there are a lot of variables when it comes to how earthquakes release energy. The bottom line is no one can say with certainty what kind of damage the next big quake may or may not cause.

"If you look at this whole planet of ours, the time window for which we have the [earthquake data is so short in geological terms," said Saiid Saiidi, a professor of civil engineering at the University of Nevada, Reno. "The science of seismology has progressed quite a bit in the last 30 years -- I may not know what the next big earthquake is going to be, but I can work on the structural side of it."

--Steve Hymon

Top photo: Steve Dykes / LAT

Bay Bridge photo: Ben Margot / AP

6th Street Bridge photo: Francine Orr / LAT

Earthquake map: U.S. Geological Survey