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. When the new portion opens later this year, the Bay Bridge will stretch 624 metres, 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 70,4 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. – TIM NEWCOMB
1. The main cable bends around a large concrete-and-steel beam at the western end of the road deck.
2. Cables suspend the road deck from the main cable.
3. Steel piles, driven at an angle to increase stability, extend more than 90 m through mud and rest on bedrock.
Know your bridges:
Self-anchored suspension bridge: Relies entirely on itself – not on the ground – for support.
Suspension bridge: Cables transfer the load to anchorage points at either end of the bridge.
Cable-stay bridge: The towers, not the points on land, bear the bridges load.