Bay Bridge

Building Bridges in Earthquake Country

Cutaway view of Bay floor geology (see enlargement)

Profile of new east span (see enlargement)

Beyond the aesthetic considerations, a number of geologic, engineering and functional issues figure into the discussions about the eastern Bay Bridge.

First examine this stretch of Bay floor, which starts out at Yerba Buena Island as bedrock -- ideal for supporting the massive towers needed for a suspension or cable-stayed bridge -- but quickly transitions to a thick layer of unstable mud that can support only more modest designs.

Then consider this engineering tenet: When building bridges in earthquake country, you want to construct rigid structures over soft ground, and flexible structures over hard ground -- do it the other way around, and you're likely to magnify the ground motion.

Now factor in the need to preserve an unobstructed, 500-foot-long shipping channel close to Yerba Buena Island, and you have some strong arguments for a bifurcated bridge: a flexible and monumental suspension or cable-stayed span that is anchored at Yerba Buena Island and that traverses the shipping channel, joined to a more rigid causeway that continues to Oakland.