Under construction: Engineering the Bay Bridge

For Part 1 of this story, see the March 2011 story Superstructure rising.

Slated to open in late 2013, the new eastern span of the San Francisco–Oakland Bay Bridge is meant to do what the old one didn’t: withstand a major Bay Area earthquake (7.0 or greater), sustain only limited damage and quickly admit emergency vehicles and traffic. It must deliver a performance to match its “lifeline” designation, adopted by the state legislature not long after the Loma Prieta earthquake in 1989.

A rendering of the signature design upon completion. More than 20 years in the making, Nader’s project will be the longest single-tower, self-anchored suspension bridge in the world. (Photos courtesy of Caltrans)

One of four shafts, made of high strength steel, that compose the bridge’s signature 525-foot tower. Each shaft is made of four lifts, and the weight of each shaft varies from 440 to 1,050 tons.

A shaft is hoisted into an upright position using strand jacks. It will be connected to previously erected shafts with shear link beams designed to absorb seismic movement and keep the tower elastic yet stiff and upright during an earthquake.

One of two deviation saddles on the SAS bridge. Made of high strength cast steel, the saddles are located at the west end of the SAS bridge where the main cable loops around the west piers. Each saddle secures the main cable and turns it 90 degrees.

The bridge’s roadways will consist of orthotropic box girders (OBG) made of high performance steel. The OBG consists of 28 segments varying in weight from 600 to 1,500 tons. Asphalt will be placed on the top deck where cars will drive.

A large floating crane lifts one of the OBG segments off a barge, to be placed onto a temporary truss. The floating crane has a lifting capacity of 1,800 tons — making it the strongest such crane in the United States.

SAS designs pose challenging construction problems. “If the deck is needed to hold the cable and the cable is needed to hold the deck, it’s like the chicken and the egg. Which one do you build first?” Nader explains. The answer: a steel truss bridge — falsework — to support the permanent bridge, which will be taken down after the cable is installed and bears the bridge load. The falsework (seen here underneath an OBG segment) is partly responsible for the bridge’s high cost, Nader explains, a required extra step for this type of design.

The main span will connect to the skyway via hinge pipe beams that will expand and contract during an earthquake, absorbing most of the seismic energy and elastically transferring loads between the two different bridge segments. The hinge pipe beams are made of high strength forged steel and high strength steel plates rolled into a circular pipe and are designed to fuse in shear in the event of an overload.

A section of the bridge’s main span as seen from the construction site on Yerba Buena Island. Construction on the bridge began in 2006 and is expected to be completed in 2013.

Nader’s seismic innovations for the span’s single tower were inspired by a professor’s research. Engineers and architects from around the world are eager to use the design for their own projects. (Photo by Susan Lohwasser)

It’s also a lifeline for Marwan Nader (M.S.’89, Ph.D.’92 CE)—because he’s bet his career on it. Beginning in 1997, when he first got involved in the project as lead design engineer at renowned structural engineering firm T.Y. Lin International, he’s defended the sometimes controversial self-anchored suspension (SAS) plan; weathered waves of criticism and cost overruns; survived political tugs-of-war, delays and project kills; and still is working on it, adjusting the design to support the contractors as they erect the structure.

After the Loma Prieta quake, the Bay Bridge was deemed a ticking time bomb. In the years after the historic 6.9 temblor, state transportation officials debated whether to retrofit or replace the bridge. The western span could be retrofitted, Caltrans engineers determined. In 1997, after a lengthy economic and structural analysis, then-Gov. Pete Wilson approved replacement of the eastern span, citing longevity of 100 years or more with a new structure, as opposed to 30 with the existing span.

Caltrans entertained bids and selected two semi-finalist designs from two separate teams in a joint venture between T.Y. Lin International and Moffat & Nichol Engineers to compete for the winning design.

Nader led the single-tower SAS team. The challenge? Marry the precise aesthetic specifications dictated by the Metropolitan Transportation Commission with the stringent seismic performance required by Caltrans. Specifically, find a way to stabilize a single tower in a major earthquake so it wouldn’t snap off like a light pole. Remembering his Ph.D. adviser Egor Popov’s research involving shear link beams in eccentric-braced building frames, Nader realized he could split the tower into four slender shafts and connect them up the height with two-meter-long shear links, maintaining a single tower look. Seismic performance analysis outcomes proved excellent.

Nader’s team built a physical model and took it to the final competition. Nader was on cloud nine when the single-tower SAS design won the contract. “Not only was my team’s design selected,” he said, “but I knew it was going to become a reality. It was a fantastic moment.”

In early 1999, Nader assembled his team of engineers to meet with Caltrans officials and agreed to deliver final plans by mid-2001. They logged 60- and 90-hour workweeks, sleeping in their offices and putting personal lives on hold.

But, outside of T.Y. Lin International, all did not proceed as planned. A political controversy erupted over the alignment of the new span. Questions over right-of-way access to Yerba Buena Island put the Navy in conflict with Caltrans. Oakland and San Francisco raised objections. A bike and pedestrian path was studied, debated and incorporated. And critics raised concerns over the bridge’s performance during a major earthquake.

But Nader has remained confident in the bridge’s seismic integrity. T.Y. Lin International used industry-standard ADINA general-purpose finite element software to run three different seismic performance analyses: time history, pushover and local detailed analyses. “This bridge has been scrutinized more than any other structure I’ve worked on,” the long-time bridge engineer says. “The testing and performance are sound.”

Nader’s team delivered final plans on schedule and continued to tweak the design through more delays and setbacks. In 2004, when a single construction bid came in, escalating construction costs had pushed the price tag far too high. Political controversy and public outrage temporarily halted the project again. Construction finally began in 2006, four years late.

“This project eats up human beings,” says Brian Maroney, deputy director of the Toll Bridge Program at Caltrans, who has worked on the Bay Bridge upgrade since 1995. “A lot of people have fallen by the wayside. But Marwan Nader has taken a few hits and is not a quitter. He has done some great things for us. UC Berkeley has reason to be proud of him.”

Nader is already working on new bridges in Africa and the Middle East but admits there will be a vacuum in his life after the new Bay Bridge opens in 2013. “No question about it,” he says, “It’s once in a lifetime.”