Golden Gate Bridge Chief Engineer Ewa Bauer calls it “the largest undertaking in the history of the bridge since its construction.” Bridge district spokeswoman Mary Currie, who has been with the district for 20 years and is one of its foremost historians, says the seismic retrofit is “the most important project” among a series of renovations dating back to the early 1950s.
It’s also the most expensive, by far. Approximately $400 million has been spent reinforcing the bridge since 1997, and another $200 million still needs to be secured to finish the final phase, which should begin within two years and wrap up between 2016 and 2018.
By those metrics, the 20-year, $660 million project surpasses even the original construction, which required only four years and $35 million ($564 million in today’s dollars).
The Golden Gate Bridge, Highway and Transportation District set out to retrofit the span in response to the 6.9-magnitude Loma Prieta earthquake of 1989, which collapsed a section of the upper deck of the Bay Bridge, yet left the Golden Gate Bridge unharmed.
The difference was due less to how they were built than where: The Bay Bridge sits on soft bay mud and the Golden Gate Bridge on solid rock.
Still, engineers determined the bridge needed additional lateral support to survive an even larger event and decided to retrofit the landmark to withstand what’s known as a “maximum credible earthquake.”
That translates to about 8.3 on the Richter scale, greater than the 1906 temblor that leveled The City. The bridge was originally designed to withstand about 7.5 percent of its weight in horizontal forces, Bauer said, and today, after retrofitting, the same measure for the south anchorage housing — a critical component of the bridge foundation — is 220 percent.
Phase 3 of the project, under way since 2008, involves strengthening the north anchorage housing and replacing a portion of the roadway deck with 78 new concrete-on-steel panels measuring 25-by-10 feet and weighing approximately 41,000 pounds each; they’ll help the underlying anchorage housing withstand massive quakes by sharing some of the load.
Once the final $200 million infusion is approved by Congress, the bridge’s iconic 1.2-mile suspension span will be strengthened over the course of three or four years. None of the work will alter the appearance of the bridge in any way, Bauer said, and it’s something she takes great pride in. Still, she’s itching to see it complete.
“In my mind,” she said, “the most important thing I have is to get this job done.”