How the New Bay Bridge Shakes Off a Quake

The new eastward span of the San Francisco-Oakland Bay Bridge is supposed to last at least 150 years, even in this earthquake-riddled region.

By Tim Newcomb

May 9, 2013

Earthquake-prone California is a far from ideal place to string metal over water and hope it stays put. But engineers of the new eastern span of the San Francisco–Oakland Bay Bridge say the structure should last at least 150 years. This fall, when the new portion opens, the Bay Bridge will stretch 2047 feet, becoming the world's longest self-anchored suspension bridge.

Unlike a conventional suspension bridge, in which cables anchor on shore, the Bay Bridge can't rely on the surrounding muddy ground—which amplifies seismic movement—for support. So the bridge is anchored to itself, with a single cable looping around the roadway and held high by a steel tower.

The steel tower bears the burden of supporting the bridge. To stay strong during foundation-shaking earthquakes, the tower contains four steel legs joined by plates called shear links. The plates stiffen when the wind gusts, keeping the 77.6-million-ton bridge stable. In an earthquake, however, the shear links absorb the earth's movement and even break if the quake is strong enough, preventing damage to the rest of the bridge.