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Ready for the Big One

Bridges: San Francisco

Sitting on the dock of the Bay now allows views of one of San Francisco's biggest construction sites, a bridge capable of coping with even the largest earthquakes.

Nina Lovelace reports.

San Franciscans are no strangers to earthquakes. As long ago as 1906 residents were forced to take refuge in Golden Gate Park as an earthquake measuring 7.8 on the Richter scale caused their houses to crumble into dust.

In 1989, the city was hit again by a 7.0 Richter quake. This time it was the city's 13km San Francisco-Oakland Bay Bridge that sustained much of the damage.

The shaking caused part of the 3.2km eastern double decked cantilever bridge between Oakland and Yerba Buena Island to collapse, taking the bridge - a key route for commuters into the city - out of service for a month.

The east span failed largely because it was a cantilever rather than a suspension bridge like its western sister, explains Californian Department of Transportation (Caltrans) supervising engineer Steve Hulsebus: 'Suspension bridges are a more flexible design, so are more capable of coping with seismic forces than the cantilevered span.'

A key fault was also the rigid joint between the old bridge's cantilevered section and its two anchor arms (see box), which failed to let the three structures move separately.

Repairs and retrofitting ensured that the bridge was better prepared to withstand future tremors. However, Caltrans engineers knew that in anything more than a medium-sized earthquake the bridge could easily come crashing down again. 'The existing east span would not withstand the maximum design earthquake, ' warns Hulsebus. 'If we got the Big One, it's not going to do much.'

And the Big One is a event worth planning for - seismologists expect a large earthquake to hit the area within the next 30 years. So in 1997 Caltrans set about finding out how a more permanent solution.

'Originally we planned to retrofit the east span again but the high cost, close to £598M, was almost as much as a new bridge, so we decided to replace it, ' says Hulsebus.

A new bridge would have to be built alongside the old, and commissioned as the old one was taken out of service and demolished. Caltrans, together with the Bay's Metropolitan Transport Commission (MTC) started investigating a variety of alignments and designs.

Finally an alignment to the north of the old bridge was chosen because it offered the least environmental disruption to both the Oakland and Yerba Buena Island shorelines, and also provided superior geological conditions on which to build a single tower bridge. This fitted with the MTC's desire to have a suspension element in the new bridge.

'We thought, if we're going to replace it - let's replace it with something special. After all, a new bridge hasn't been built in the Bay area for 60 years, ' says Hulsebus.

Driving across the old double deck eastern span had also denied drivers the best views across the Bay, so MTC wanted the new crossing to have parallel decks. Another addition was a designated pathway for pedestrians and cyclists.

The new bridge decks were also required to have hard shoulders - something the old bridge had never had, explains Caltrans landscape architect Clive Endress. 'Some 3,000 people run out of gas each year on the Bay Bridge, ' he says, 'When it happens, it causes significant congestion.'

However MTC and Caltrans knew the bridge's most important feature would be its ability to cope with the Big One.

The chosen design has four distinct parts. On the Oakland shore, the bridge joins to the existing highway via a post-tensioned concrete box girder 'transition' span. This span moves into a 2.4km long segmental concrete box girder construction known as the 'skyway', which in turn becomes the piÞce de résistance - a 565m self-anchoring suspension span. Another transition span connects the suspension bridge to the east portal of the Yerba Buena Island traffic tunnel. A TY Lin International/ Moffat & Nichols joint venture designed the bridge, and the construction contract is currently out to tender.

Side by side 25m wide decks, 17m apart, will each carry five lanes of traffic. At each transition span, however, the two decks are brought into one to tie in with existing highways either side.

The skyway decks will be constructed using precast concrete box girders lifted into place by a floating crane, and post-tensioned. The girders, varying from 5.5m depth mid-span to 9m above the piers, are carried on flexible bearings over twin 50m high prestressed concrete piers located 160m apart.

The piers are constructed over pile caps founded on six, 2.5m diameter raked steel shell piles driven 100m into the bay mud. Piles are filled with earth for their uppermost 55m, with concrete at their base.

Such a stiff foundation structure will minimise movement of the piles under earthquake conditions, but will also allow the structure itself to move with the lateral forces.

Piling by US contractor KFM has already begun for the 2.4km long skyway section. To minimise noise and vibration that could affect the Bay's wildlife, the piles are surrounded by compressed air pipes providing a circular air curtain.

The skyway deck will also incorporate several nesting areas for the cormorants that currently live on the old east Bay bridge. Endress hopes that when the bridge is complete and before the old one is demolished, the cormorants will begin to move of their own accord. 'We can't send them a letter, ' he says.

The suspension bridge element has a 385m long main span and a 180m back span towards Yerba Buena Island. The 160m tall tower, supported on steel shell piles driven into the bedrock, comprises four steel shafts connected with shear links. 'In an earthquake, the four legs can move independently, ' says Endress. This ensures that if one is damaged it can be replaced without having to replace the whole tower.

Unusually, the bridge is selfanchored. The 0.78m diameter cable is anchored to the twin decks at the suspension bridge's eastern piers and looped around the west piers through deviation saddles. The two concrete box girder transition spans, the skyway, and the suspension bridge are each linked by a hinge to allow each section to move independently in case of earthquake.

Pedestrians and cyclists will use a 4.8m wide steel pathway cantilevered off the length of the bridge. This eccentric load is balanced with a counterweight on the north side, Hulsebus says.

Ground preparation is currently under way around the shoreline to prepare the soft bay mud for the Oakland approach.

Fill is vibrated after placing to speed settlement and vertical drains inserted to ensure pore pressures remain stable even under earthquake conditions.

The new bridge is scheduled to open in 2007.

Cantilever challenges

The existing13km San FranciscoOakland Bay Bridge was opened in 1936, six months before the more famous Golden Gate bridge. The west span features four suspension towers, while the 3.2km east span is a cantilever design. Chief engineer Charles H Purcell decided on a cantilever span because at the time anchoring a suspension bridge on the steeply dropping bedrock proved too costly. The two 150m anchor arms were built using huge derricks to lift large sections of steel which were then riveted into place. One of the most anxious moments came as the giant cranes lifted the final 21,000t section. A cold wind on the northern side and warm sun on the southern side of the span caused a differential of four inches between the two sections. Riveting was completed some hours later when the temperatures evened out.

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