News Column

Bay Bridge Made to Withstand Major Earthquake

Sept. 1, 2013
bay bridge, renovations, oakland, san francisco

Sept. 01--When the new east span of the Bay Bridge opens, drivers, bicyclists and pedestrians will undoubtedly be taken with its gleaming white paint job, its rocket-like tower and its spider web of suspension cables, not to mention its sweeping views.

But as esthetically impressive as the $6.4 billion new span may be, its graceful looks overshadow its main mission: to not only remain standing after a devastating earthquake but to be able to carry emergency vehicles and supplies across the bay soon after the ground stops shaking.

Seismic engineers who designed the span used a variety of innovative techniques -- some borrowed from offshore construction -- to help make the bridge strong, yet flexible enough to withstand the greatest seismic forces imaginable.

Brian Maroney, Caltrans' chief engineer, said the spirit of innovation started well before the design. The work started with research -- "understanding the problem," he said. Working with earthquake experts, engineers studied the Bay Area's seismology, including a variety of "rupture scenarios" in which quakes would hit in various parts of Northern California, sending seismic waves racing toward the bridge from different directions.

People often ask how big an earthquake the new span is designed to withstand. It's not, Maroney said: "Engineers don't use magnitude any more."

A 1,500-year quake

Instead, they analyze ground forces and design for the strongest ones likely to occur over a certain period, much the way flood control experts plan. Most modern Bay Area buildings other than schools and hospitals design for a 500-year time frame, Maroney said. The Bay Bridge is built "for those motions we expect to occur once every 1,500 years," he said.

When engineers began designing the new span's quake protection system, Maroney instructed them to assume that damage would occur -- and to decide where it should occur, not unlike crumple zones in cars that absorb the impact of collisions.

"It is naive to assume that the bridge is not going to take any damage," he said. "You need to decide where the bridge is going to take the damage."

Two of the span's seismic innovations rely on that principle. The four legs of the 525-foot tower are connected with a series of shear link beams, which are designed to move independently during an earthquake, absorbing its energy and impact and becoming bent or distorted. The shock-absorbing beams can be replaced if necessary.

Key stress points

Similarly, Caltrans installed hinge pipe beams at key stress points in the span. The beams are deck joints that resemble long, large pipes. They're anchored on one side and can slide back and forth during an earthquake. The center of the beam will absorb energy like a fuse, deforming during a major earthquake. Damaged fuses then can be replaced.

Battered piles are another key seismic feature not typically used on bridges. The piles are steel pipes 8 feet in diameter, 4 inches thick and pounded at a slight angle down 300 feet through the soft bay mud into harder, more solid soils, then filled with concrete. That makes them stiffer and sturdier than traditional piles that are driven straight into the earth, said Sajid Abbas, an engineer from T.Y. Lin International, who worked on the span's seismic design.

"The foundations are what really do it," said Mike Whiteside, a Caltrans engineer who worked on the span's seismic system.

Before installing any of the innovations on the span, pieces were built and tested in laboratories from Reno to San Diego, Maroney said.

"We did a tremendous amount of testing," he said. "That's not done on most bridges."

Engineers admire the old Bay Bridge, saying it was meticulously designed and well-assembled. But it is riddled with seismic deficiencies because when the span was built, seismic engineering was neither common nor sophisticated.

The old east span sits atop treated Douglas fir trees, 85 to 120 feet long. They were pushed into soft bay mud that a strong quake could cause to liquefy, subjecting the bridge to wobbling and intense shaking. The steel supports holding up the road decks also aren't wide enough to withstand motions generated by a strong earthquake. The section that fell in the Loma Prieta quake fell off its seat, or support, and seismic engineers say more segments would have collapsed if the quake had lasted a few more seconds. The old bridge is too flexible, which makes it subject to being shaken to pieces, they say.

Engineers not worried

Well-publicized troubles with the new span's construction -- from concerns over welds in the skyway foundations, the suspension span's steel deck boxes and the base of the tower to broken bolts or rods holding down seismic devices -- have caused some skeptics to question the safety of the new bridge. But the engineers who worked on the east span say those problems have either been repaired, disproved or are in the process of being fixed.

The new east span is safe for the daily commute and as a lifeline span that will survive the major earthquake geologists have long warned is coming.

"I absolutely have every confidence in this bridge," Whiteside said. "When there is a major earthquake, I want to be there."

Michael Cabanatuan is a San Francisco Chronicle staff writer. E-mail: mcabanatuan@sfchronicle.com Twitter: @ctuan

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Source: Copyright 1SF 2013


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