How do you build a bridge in a region that is bordered by some of the most dangerous fault lines in the world? If you're the California Department of Transportation, you pull out every engineering trick in the book.
The agency, known locally as Caltrans, is in the midst of what is being called the most ambitious public works project in California's history: the replacement of the eastern half of the historic San Francisco-Oakland Bay Bridge. In its place, Caltrans is erecting a modern concrete span consisting of a pair of 1.2-mile-long viaducts followed by the world's first single-tower, self-anchored suspension bridge. The project will be completed by 2010 at the earliest and is expected to cost a whopping $5 billion.
The result, according to Caltrans, will be a bridge capable of functioning even after a so-called 1,500-year Seismic Evaluation Earthquake -- a rare magnitude 8.0 on the San Andreas Fault or a 7.25 on the Hayward Fault, the two major fault lines crossing the region.
"This bridge was labeled a lifeline structure," said Marwan Nader, a principal at T.Y. Lin International, the architectural firm that designed the new bridge. "You'll still be able to drive over it after a major earthquake."
The lifeline designation is an important one, as it means that the bridge must be designed so that emergency vehicles and suppliers can use it to reach the San Francisco peninsula after the worst of disasters. Few structures have that designation, according to Caltrans' Douglas Coe, resident engineer for the "skyway" portion of the project.
"Following an earthquake, this might be the safest place to be -- even more so than hospitals and other buildings," he said.
The movement to turn the Bay Bridge into a lifeline structure got its start soon after the 1989 Loma Prieta earthquake. The magnitude 7.0 temblor resulted in 63 deaths and 13,757 injuries around Central and Northern California, according to the National Information Service for Earthquake Engineering. And though it was centered 60 miles away, the quake knocked a 50-foot segment of roadway off the top level of the Bay Bridge and sheared the bridge's anchor bolts.
The month-long closure that followed snarled traffic around the area and cost regional businesses millions of dollars in lost productivity. It also moved civic planners on both sides of the bay into action. A major earthquake on a nearby fault line could be twice as powerful as the Loma Prieta quake, they figured. If the region was to survive such a calamity, the bridge would need to not only be repaired, but also updated, they argued. Furthermore, it would have to be done as fast as possible.
"It's a race against the next big earthquake," said Coe. "The big one could happen at any time."
Unfortunately for area residents, a quick retrofit was not in the cards. Research into the bridge's weaknesses and possible repair scenarios took several years and revealed that the piers underneath the bridge's eastern span were not as well anchored as the ones under the western span. To truly shore up the bridge, engineers recommended extending the eastern piers nearly 200 feet deeper into the bay mud -- a costly and time-consuming process.
Caltrans had little choice, said Coe. "Unless you can anchor the bridge down, it doesn't matter what you do to the superstructure."
What followed is still a matter of great controversy. One independent estimate put the cost of a major retrofit at $300 million, but Caltrans said its own studies showed that the cost would be $900 million. Building a new bridge to replace the eastern span altogether would not only be cheaper but less disruptive for bridge commuters, and it would cost less to maintain in the long run, the agency argued.
Caltrans architects went to work producing several designs and, in 1997, presented what it thought was the best option to the public: two basic side-by-side viaducts, one for each direction of traffic, stretching from Oakland to Yerba Buena Island, part of the way across the bay. (The western span, from the island to San Francisco, would maintain its suspension design, but would be retrofitted.)
Though the price of this replacement option was more than the original estimate, what bothered many community members the most was the design. Some felt it should be more iconic, like the Golden Gate Bridge. Some wanted a bike lane; others a railway line. San Francisco's mayor at the time, Willie Brown, thought the bridge's touchdown on Yerba Buena Island would ruin the city's chances to develop the area. Oakland's outgoing mayor Elihu Harris thought the design was plain ugly.
What was once considered to be a race against time turned into a seemingly endless process, stuck in committee meetings and bureaucracy.
The design committee never managed to please all its critics. But in 1998 -- nine years after the Loma Prieta quake first shook up the Bay Area -- it announced that it had settled on a final design. The eastern span would be replaced with a skyway similar to the original design, followed by a so-called signature span: the world's first single-tower, self-anchored suspension bridge. The suspension bridge, a 525-foot-tall white tower with cables that drape the roadway, would connect the skyway to the existing tunnel on Yerba Buena Island and onward to the bridge's retrofitted western span. Together, the two new sections would not only have five lanes and one emergency lane in each direction, but also a bike lane on the south side.
And so the new bridge was almost born.
After further setbacks, almost all of which contributed to the rising cost of the bridge, construction crews finally began work on the skyway portion of the new span in 2002.
To ensure that the skyway is strong enough to be worthy of lifeline status, the crews are using just about every engineering trick in the book.
The first of these is the use of 160 "battered" steel piles. Driven 300 feet into the bay mud at a slant, the 8.5-foot diameter tubes create a more stable foundation for the bridge than would completely vertical piles, according to Caltrans engineers.
"Milking a cow on a stool with diagonal legs is a lot sturdier than milking a cow on a stool with straight legs," explained Coe. During tests, the design proved to be stronger than expected, he added. "We were very happy about that."
The second safety measure in the new bridge is the use of loops to hold together the rebar, or steel bar, columns in each of the piers that hold up the bridge roadway. Under this design, each of the thick groups of rebar at the core of each column -- (there are four columns in each of the 42 piers) -- is bound together with loops of steel to ensure that the individual bars don't bend and come apart in an earthquake.
The tactic is not exactly new. Engineers have been stringing loops around rebar ever since examining damage from the 1971 Sylmar earthquake in Southern California, according to Coe. Still, it is a welcome innovation in the Bay Area, where the lack of such reinforcements led to the collapse of some of the region's highways during the 1989 Loma Prieta earthquake.
Once two corresponding piers are in place, a short segment of the concrete roadway is cast atop them. These are the only segments of the skyway that are cast in place. The remaining 452 sections, each one weighing anywhere from 480 to 780 tons, are barged in from one of the world's largest precast concrete factories in nearby Stockton.
The assembly of these giant building blocks requires not only precision but patience. Because concrete shrinks as it dries, dropping almost 7 percent in size, the blocks must sit at the precast yard for anywhere from two to 18 months before they can be lifted into place at the bridge site.
Even with this precaution, bridge engineers are taking no chances. They have ordered the piers to be built so that they lean slightly away from the center of the bridge. As the concrete roadway dries even further, it will eventually pull the piers upright, relieving stress on the bridge rather than adding to it.
Other stresses on the bridge, like heat and moisture -- and earthquakes, of course -- are being accounted for with the installation of special expansion joints at four different places in the roadway. The joints consist of 60-foot lengths of 6-foot-diameter steel pipes fitted inside stainless steel sleeves.
Unlike traditional expansion joints that only move a few inches before breaking -- leading to structural failures like the collapse of the bridge deck in 1989 -- the new joints can slip up to a meter in a worst-case scenario. They are also designed to bend but not break if an earthquake pulls the roadway on each side of the joint in different directions.
"It's designed so that the bridge remains in service even if it deforms," said Coe. "We can weld a new piece in there after an earthquake. At worst, we put some steel plates in the roadway while we work."
As of mid-August, the crew had already hauled the first roadway segments into place and construction was proceeding on schedule, according to Coe. If all continues according to plan, the full skyway is expected to be completed by spring 2007.
But that doesn't mean that Bay Area drivers will get a chance to zip across the new bridge any time that year, or the next, or the next. A call for bids to complete the tower and roadway for the suspension portion of the project returned one lonely offer that was $1.1 billion over Caltrans' $700 million estimate for the project. Now the agency is working with state and federal lawmakers to determine if it can lower the total cost by using foreign steel. It is expected to have a decision in September.
Meanwhile, the clock continues to tick down for the region's next big quake -- and not a single driver of the 280,000 vehicles that cross the old Bay Bridge each day knows just when the alarm will go off.
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