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The Bridge

Why a New Bridge?
Earthquakes and Bridges
East Span Design Basics
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Seismic Safety
Site Challenges
The Bridge and the Environment
Bridging by Numbers
Bridgespeak Glossary

Site Challenges

In creating a new East Span that would withstand earthquakes and other forces of nature, designers had to take into account the pronounced variations in the site's geology.

To begin with, the bridge's setting changes dramatically from one side to the other. The Suspension Span tower structure will be sited on relatively shallow, sloping bedrock. But the skyway will be founded on mud as deep as 300 feet.

In a severe, close-by earthquake that mud can quiver like Jell-O. And while the mud is shaking the foundations of the bridge, the portion founded on rock remains steady. In a strong enough earthquake, a bridge facing these conditions is not shaken apart, but pulled apart - possibly in a matter of seconds.

To prevent this, the Skyway columns holding up the roadway will be supported by steel shells driven up to 310 feet deep down at angle, or battered, for extra stability. They will reach nearly to bedrock and will be filled with earth up to 180 feet from the top, the rest with concrete. (In comparison, the existing bridge piles are made of wood and extend only 100 feet into the mud.) This system produces a relatively stiff foundation system for the piers, the columns supporting the bridge decks. In addition, the pile cap where the piles meet the piers have built-in safety features where they interconnect.

Wind was also a factor in the design of the suspension span. Although winds in the Bay Area seldom exceed 70 miles an hour, the bridge was designed to withstand a wind of up to 125 miles an hour before a critical-flutter-velocity threshold was met. At that point the bridge in theory would begin a sort of damaging, self-feeding motion, attracting more and more energy from the wind. The likelihood of that wind happening before the bridge reaches the end of its designed 150-year-life is minimal. Just to be sure, the bridge was subjected to rigorous wind tunnel tests.

Other constraints in the design include navigational requirements. The minimum width had to be 499 feet and the minimum height 138 feet above mean sea level to accommodate ship traffic.

-- Source: American Society of Civil Engineers, Caltrans

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