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New Bay Bride designed to withstand strong earthquakes, engineers say

New Bay Bride designed to withstand strong earthquakes, engineers say

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News & Events - Engineering News

February 15, 2012

When completed, the new San Francisco-Oakland Bay Bridge will be one of the world's most technologically advanced structures, according to engineers.

Officials in California are overseeing the construction of the new Bay Bridge, which is replacing the antiquated structure connecting San Francisco and Oakland. The bridge's story is rather unconventional, as it was designed and largely assembled in China, and it was engineered to last far longer than the bridge it replaces, according to The New York Times.

In fact, the new Bay Bridge carries an expected lifespan of approximately 150 years. Engineers tasked with designing the bridge affirmed that one of the biggest challenges they faced in their work was ensuring the Bay Bridge could withstand the impact of one of the powerful earthquakes forecast to hit the Bay Area over the next 30 years.

The existing Bay Bridge was partly severed by an earthquake in 1989, but engineers noted the new structure should be unaffected by such seismic activity. The key to its future success is in its design, T.Y. Lin International vice president Marwan Nader said. For example, anchor blocks - known as deadmen - were incorporated into the new Bay Bridge as a means of bolstering its strength in the future when its concrete girders begin to lose their effectiveness. Crews will simply have to connect the deadmen with cables to restore the girders to their original alignment, according to the Times.

"We wanted to make this bridge flexible so that when the earthquake comes in, the flexibility of the system is such that it basically rides the earthquake," asserted Nader, who was the lead designer of the bridge.

Such a design differs from those employed in other bridges, University of California, San Diego, Jacobs School of Engineering dean Frieder Seible noted. He said that engineers could have created a structure so massive and heavy in scope that it would have simply resisted movement during an earthquake. Such a bridge "would look absolutely ugly and be very, very expensive," he said.

The design team instead opted to engineer the bridge to sway in the event of a powerful earthquake, much like how many skyscrapers are designed to withstand seismic blasts. The new structure features a 525-foot-tall suspension bridge tower that comprises four steel shafts. They could sway  as much as five feet in the event of an earthquake, according to Nader. Still, the shear links, the connecting plates installed between the shafts, would absorb the majority of the force, he said.

Moreover, the bridge's concrete piers are engineered to sway in the event of exceedingly strong winds or an earthquake, which would help prevent damage to portions of the bridge that have extra steel reinforcing. While such engineering tools would help limit damage during a quake, they would not safeguard it completely.

"At the seismic displacement that we anticipate, there will be damage, but the damage is repairable and the bridge can be serviceable with no problems," Nader told the Times.

San Francisco lies between two active faults, and the 6.9-magnitude 1989 earthquake caused such damage on the bridge that officials were forced to install a 2.2-mile-long replacement span. The existing Bay Bridge was constructed in the 1930s and was not designed to withstand such seismic activity.

Officials contend that there is a 2-in-3 chance that an earthquake registering 6.7 or larger will hit San Francisco by 2035, according to projections from the U.S. Geological Survey. As crews work to assemble the new  Bay Bridge - it is expected to be completed in 2013 - they are hopeful that engineers have successfully created a structure capable of withstanding an even stronger quake.




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