The Golden Gate Bridge is about to see 10 years of intensive construction

San Francisco Chronicle​

3 days ago

The Golden Gate Bridge is not expected to collapse in a major earthquake. But it could suffer enough damage to close for months or longer — and repairs to ensure that won't happen are slated to start next year.
The bridge's steel elements are designed to move with seismic forces — as much as 27 feet from side to side at the suspension span's midpoint. But the concrete roadway and pylons (which anchor the span to the ground at either end) are less flexible, a mismatch that could be disastrous during a 1906-level earthquake.
'They won't be moving in sync, and they'll be banging into each other,' said John Eberle, an engineer for the Golden Gate Bridge, Highway and Transportation District.
That violent grating could cause bridge elements to 'pop out of alignment,' Eberle said. For example, joints could poke up through the roadway, creating speed bump-like steel hurdles impassable to vehicles. Repairing that damage could take 'months if not years of full closure', said bridge district spokesperson Paolo Cosulich-Schwartz.
The seismic retrofit expected to begin in January would make the iconic span resilient enough to open to emergency vehicles within 24 hours and to the public within 72 hours of the Big One, Eberle said. The bridge district plans to award a contract by December for the first of the project's two phases, which in total will cost close to $1.8 billion and take about 10 years to complete.
'The Golden Gate Bridge is a lifeline structure, and we are committed to opening up the bridge after a large-scale event,' Eberle said at a seismic commission meeting earlier this year. 'That's why the extent of the retrofit is what it is.'
The bridge lies about six miles east of the San Andreas Fault, and would also see shaking from a major quake along the Hayward Fault in the East Bay. The U.S. Geological Survey estimates a 72% chance of a 6.7-magnitude quake, a 51% chance of a 7.0-magnitude quake and a 20% chance of a 7.5-magnitude quake hitting the Bay Area in the next 30 years.
The project is the third and final chapter of a larger retrofit effort that began in 1996 with the viaducts that link the bridge's suspension span to land on either side. This retrofit will focus on the bridge's two 746-foot towers, the two 1,125-foot side spans north and south of the towers, and the 4,200-foot main span between them.
A bevy of upgrades is planned to dissipate ground shaking, including 40-foot-tall steel plates to be affixed to the base of both towers; 38 giant shock absorbers, some weighing over 26,000 pounds; and a new, stronger steel lattice system undergirding the suspension span.
The upgrades will be mostly unnoticeable to the casual eye, though the white-sheathed temporary platform needed to perform the retrofit will be visible during construction and run underneath the entire span. A close observer peering over the bridge's edge might spot a few of the shock absorbers.
The bridge's south tower will also get a fresh coat of International Orange paint, part of the 1.5 million square feet of painting associated with the retrofit. (The north tower, which is subject to milder weather conditions, isn't due for repainting yet.)
The work will be split into two phases. The first will retrofit the towers and the spans at the bridge's north and south ends over an estimated five years. Bridge leaders have the $870 million needed to fund the phase, between a $400 million federal grant, $200 million from Caltrans' Highway Bridge Program, and $270 million from the district's reserves.
The bridge district has yet to begin seeking funding for the second, roughly $900 million phase, which will focus on the main span between the towers and take a projected four additional years. Officials plan to apply for federal and state grants and do not expect to increase tolls to pay for the work, Cosulich-Schwartz said.
Though the bridge will stay open throughout construction, some lanes will be closed starting about six months into the project and lasting for both phases' duration. Authorities expect the closures will mostly occur on weekdays between the hours of 9 p.m. and 5 a.m., similar to during the installation of the bridge's suicide prevention net.
Retrofitting the bridge while it withstands daily traffic loads and harsh weather conditions will require precise work.
Eberle compared the towers to giant cans of soda — under constant pressure from the immense cables draped over top, which bear the weight of over 100,000 vehicles a day.
'If you're standing on a Coke can and now you have to put little holes in it and put steel plates on it, you have to do that in a very, very controlled fashion so you don't collapse that Coke can,' he said.
The retrofit also involves installing a new lateral system, the X-patterned steel lattice undergirding the entire bridge. But workers can't remove the old laterals, which are essential to the bridge's integrity, until the new ones are fully in place.
'You have to put in new elements while the old elements remain, so it's intricate work,' Eberle said.
The 38 giant shock absorbers included in the retrofit are designed to redirect some of the force an earthquake would otherwise inflict on the already-burdened bridge. During an earthquake, some of the shaking force would be transferred away from the bridge and into a 'sacrificial bronze disk' inside the shock absorbers, Eberle said.
The shock absorbers are immense — some measure more than 26 feet long — and workers will have to add over 800,000 pounds of steel to the towers to support their weight. But without them, the retrofit would have to be even more extensive, Eberle said.
The retrofit's complexity in part explains its roughly $1.8 billion bill, but labor laws around lead paint exposure and steel prices driven up by uncertainty around tariffs have contributed, too.
Authorities say the retrofit is more than worth the spending.
'I know it's a big price tag,' Eberle said, 'but if you look at what it would take to rebuild the Golden Gate Bridge, you're talking billions and billions of dollars, and years of no good alternate route around the Bay.'