Grafting a new artery

News October 31, 2000
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The first time some people ever heard of the San Francisco-Oakland Bay Bridge was when part of it collapsed during the Loma Pri

The first time some people ever heard of the San Francisco-Oakland Bay Bridge was when part of it collapsed during the Loma Prieta earthquake that shook the area on October 17, 1989. The Bay Bridge, like the many other bridges in the Bay area, lives in the shadow of the Golden Gate Bridge. It only stepped out of that metaphorical shadow because of the strongest earthquake on the San Andreas fault since the San Francisco earthquake in April 1906.

The Loma Prieta earthquake killed 63 people and caused an estimated $6 billion in property damage. One person was killed on the Bay Bridge when bolts holding a section of the upper deck on the truss section sheared, causing a portion of the deck to unhinge and fall onto the lower deck.

After the Loma Prieta earthquake, it was decided that the west span of the Bay Bridge could be seismically retrofit but that the east span would be safer and more economical if it were rebuilt rather than retrofit.

The California Department of Transportation (Caltrans) has settled on a $1.5 billion preferred alternative for the east span. The plan is to build a new bridge located alongside and a little north of the existing bridge. Caltrans is the owner-operator of the Bay Bridge, as well as the toll plazas at each end, as part of I-80.

T.Y. Lin International, San Francisco, is taking the lead in a joint venture with Moffatt & Nichol Engineers, San Francisco, to design the new bridge.

The east span connects Yerba Buena Island on the west end to Oakland on the east. The preferred alternative calls for a single-tower asymmetric self-anchored suspension bridge over the navigation channel near Yerba Buena Island and a segmental concrete haunched girder skyway structure near the Oakland shore.

Traffic will travel on side-by-side decks, with five eastbound lanes of traffic on one and five westbound lanes on the other, with a provision for a future light rail line. The decks will have standard 10-ft shoulders and, on the south side of the eastbound structure, a pedestrian and bicycle path.

Unlike a regular suspension bridge, where the cables are anchored in rock at the ends of the bridge, the cables of the new Bay Bridge will be anchored in the steel orthotropic deck itself. The long side of the suspension bridge will stretch to the east 385 meters. On the other side of the tower, the suspension bridge will stretch 180 meters.

The suspension tower will stand 160 meters tall, anchored in bedrock below, with a vertical clearance of 152 meters between sea level and the underside of the bridge deck.

Building a self-anchored suspension bridge is somewhat more difficult to erect than a regular suspension bridge, according to Rafael Manzanarez, an engineer at T.Y. Lin and design manager on the Bay Bridge east span reconstruction project, "because the deck has to be in place before you erect your cable." He explained that temporary towers would support the deck while the cables and hangers were being assembled, and then the weight of the deck would be transferred to the cables.

The skyway structure, with 160-m spans and three or four piers per frame, will cover 2.4 km from the suspension bridge to the Oakland shore. The skyway will rest on steel tubular piles 90-100 meters long driven to the lower Alameda Formation. Including intermediate structures and shore approaches, the east span will cover a total of about 3.6 km.

For more on the story, read the November issue of ROADS & BRIDGES.

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