San Francisco's Tale of Two Bridges

One of San Francisco’s most famous landmarks is the Golden Gate Bridge, but when the city’s residents talk about “crossing the bridge” that’s not the one they mean. Instead they’re referring to the Bay Bridge, or to give it its full name the San Francisco-Oakland Bay Bridge. Spanning San Francisco Bay, this is a vital commuter link and carries an average of about 240,000 cars a day – more than twice the traffic load over the Golden Gate.

The Bay Bridge is actually two separate bridges; one runs from San Francisco and the other from Oakland, with both of them ending on Yerba Buena Island. A tunnel through the island connects the two, forming a single 4.46 mile (7.18km) link across the Bay.
The Bay Bridge opened in 1936 and served the Bay Area well for decades, but during the 1989 Loma Prieta earthquake its strength was severely tested and turned out to be inadequate. The eastern (Oakland-Yerba Buena) span was a double-deck cantilever structure and a section of the top deck collapsed during the quake. The damage was repaired and the bridge reopened a month later but it was obviously not robust enough to guarantee surviving another major earthquake and an upgrade was planned. The western span used a different design – it’s a suspension bridge – and was refitted to improve its seismic resistance, but strengthening the eastern span wasn’t practical. Instead it had to be replaced.

Building a new bridge wasn’t simple. It wasn’t practical to close the existing one for more than a few days, so the replacement had to be built alongside it. That created its own problems. The entrance ramp for the new bridge on Yerba Buena had to be built where the old one was standing; the solution was to build a two-deck S-shaped extension beside the ramp, then during a short closure cut away the ramp and jack the extension into its place. That left the line of the new bridge clear.
The replacement bridge is a unique structure. Its most prominent span is a self-anchored suspension bridge, which differs from a normal suspension design in having the cables attached to its own deck instead of anchored in the ground past the ends of the span. These bridges are rare but they do have advantages for earthquake resistance. Even more unusually the new Bay Bridge span is asymmetric and has a single tower; the shorter western end – which is less than half the length of the 385-metre eastern one – carries a huge concrete counterweight to balance out stresses. This unconventional design was chosen to leave a wide channel under the bridge so ships can reach the port of Oakland.

The bridge is designed to survive a magnitude 8.5 earthquake, calculated to occur once every 1,500 years. That means it has many more unique design features. The road beds aren’t directly connected to the single tower, so they can sway from side to side without shearing the tower. Sacrificial reinforcements are designed to crush or deform under extreme stress, absorbing energy that would have been transmitted to structural components. The bridge should be usable by emergency vehicles immediately after an earthquake, and destroyed sacrificial parts can be gradually replaced later.
The new bridge opened on 2 September 2013, but work hasn’t finished yet. Large parts of the old bridge still stand alongside it and are being steadily dismantled. There have also been large projects to correct some problems with the initial build, including water leaks into the 700-ton box sections that make up the roadways and the failure of some of the main mounting bolts. Some problems are to be expected on a project as large and complex as this one and it’s likely more issues will need remedied over the next few years. There is also a good chance of it being upgraded to include new developments in earthquake-proofing. Either way the new Bay Bridge itself, and other highly resistant structures inspired by it, are likely to provide a lot of work for contractors in the future.