San Francisco News

A sneak peek at the new Bay Bridge

Monday, October 12, 2009

Caltrans says when it's done, the new Bay Bridge will be one of the safest places to be during the next major earthquake and here is why.

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The 70-year-old Bay Bridge was built when engineers thought structures should be rigid to survive an earthquake and now they know better.

A Caltrans animation shows how the new eastern side of the Bay Bridge is expected to sway and rock in a major quake.

"This new bridge is designed to withstand a quake with a 1,500 year return," said Bart Ney from Caltrains.

Caltrans says the simulation is not based on a richter scale measurement, but on the movement expected in an earthquake that would occur only once every 1,500 years -- one worse than the 1989 Loma Prieta quake that caused a section of the bridge to collapse.

Innovations on the new bridge include hinge pipe beams allowing movement back and forth but not side to side with a fuse that can bend and contort.

Beams on the suspension bridge tower are designed to absorb most of the energy and allow the four legs of the tower to move independently and the steel piles are driven 300-feet down at an angle, like the wide stance of a baseball batter, giving the bridge more stability.

"It goes beyond the critera of not failing and falling down. What it is supposed to do is be immediately available for emergency services right after an earthquake," said Ney.

The new bridge will also have a couple hundred motion sensors inside and at the base of the piles. they send digital data to recorders, which in turn send it to the Geological Survey in Sacramento.

"The designers say in a 1,500 quake we expect this to move left to right 10 inches, so the sensors say yeah verily it moved 9 inches or it only moved two or it moved 15," said Anthony Shakal from the California Geological Survey.

That information will help caltrans assess the conditoin of a quake-damaged bridge.

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