To the undiscerning eye, the Golden Gate Bridge, one of the most iconic structures in the world, looks nearly unchanged from the day it opened 75 years ago.
But visual markers of the seismic retrofits made in recent years, by design, may elude even those looking to find flaw, Golden Gate Bridge, Highway and Transportation District spokeswoman Mary Currie said today.
Because the bridge is eligible to be listed on the national registry of historic places, the structural upgrades heavily emphasize aesthetics, Currie said.
For example, the original structural elements were built using the best technology at the time -- such as rivets -- technology that is largely obsolete by today's engineering standards.
To mimic the look of those elements, new pieces were laser cut to match the appearance of the originals, Currie said.
"We had to develop the means to replicate how the bridge looks using today's construction techniques," she said.
Such measures were taken when retrofitting the bridge's concrete pylons -- the columns bookending the bridge's anchorages -- by enveloping them in steel and additional concrete.
"We had to stamp the concrete to make it look like it was treated with wooden forms, which is the way concrete was poured back then," Currie said.
After the 1989 Loma Prieta heavily damaged the neighboring Bay Bridge, the district assessed the Golden Gate Bridge's vulnerability and drafted plans to retrofit, rather than replace, the bridge in three phases that would allow the bridge to carry traffic uninterrupted.
With the goal of achieving a structure that can withstand a magnitude-8.3 quake, the bridge has been in a nearly continuous state of retrofit since 1997, when construction began on a retrofit of the approach to the bridge from Marin.
Since then, most of the structure has been upgraded, with exception of the main suspension span, main towers, south tower pier and fender, which are part of a $260 million contract that is expected to go to bid in 2013.
Currie said the district is still seeking $200 million in federal funding to be able to complete that subphase of the retrofits.
In all, the retrofits cost $660 million and are expected to be completed by 2018, nearly 30 years after the Loma Prieta earthquake.
"It's definitely been a long process," Currie said.
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