Superstructure rising
BIG, BOLD STRUCTURES: Passionate about math and physics from an early age, Marwan Nader built models of skyscrapers and bridges as a young boy. By high school, he knew he wanted to be a structural engineer. At 34, Nader became the lead design engineer for the signature span portion of the new Bay Bridge, responsible for taking a never-been-done-before design all the way to PSE (plans, specifications and estimates). (Photo by Brooke Duthie.)
Marwan Nader (M.S.’89, Ph.D.’92 CE) was walking outside Davis Hall when the earthquake struck. He felt a jarring sensation, as if someone were trying to nudge him over. Something significant has happened, the graduate student thought. It was 5:04 p.m. on October 17, 1989. Reports trickled in—6.9 magnitude—and rumors quickly spread. The Bay Bridge had collapsed!
Actually, the San Francisco–Oakland Bay Bridge hadn’t collapsed, everyone learned, but a 50-foot section of the upper deck had fallen onto the lower deck, crippling the eastern span that straddled Oakland and Yerba Buena Island. The entire bridge closed for a month.
Not long after the Loma Prieta quake struck, Nader gazed at the hole in the bridge as he stood a safe distance away, part of a Berkeley team inspecting the damage. Twenty-two years later, he’s still standing on the bridge, so to speak. As lead design engineer of the self-anchored suspension (SAS) bridge, he is responsible for the standout architectural feature of the new portion of the bridge that will replace the old eastern span.
More than 20 years in the making, at an estimated total cost of $6.3 billion (the original 1997 estimate was $2.6 billion), the new eastern span bears the weightiest of expectations. For Bay Area cities, as dictated by the Metropolitan Transportation Commission (MTC)’s 1997 design criteria, it must be distinguished by a unique aesthetic, one that would match the beauty of the existing western span and the nearby Golden Gate Bridge. One easily identifiable on a tourist’s postcard. For the public, it must prove a wise use of funds. And, most important for the 280,000 vehicles crossing the bridge each day, it must stand and deliver.
The whole project is ambitious, to put it mildly. The construction contract alone, worth $5.2 billion, is the largest public works contract in California history. When the bridge opens in 2013, it will be the longest single-tower, self-anchored suspension bridge in the world. “We are giving the community a safe structure,” Nader says. “We are building a bridge for the future.”
Once or twice a week, Nader drives from the construction worksite on the Oakland shoreline across the bridge to the San Francisco office of renowned structural engineering firm T.Y. Lin International, where he is vice president. “You have to keep your eye on the road, of course,” he says, “but I can’t help but glance over to see how things are going.”
HANGING BASKET: The Bay Bridge’s new self-anchored suspension segment, planned for completion in 2013, uses a single cable anchored into the deck and wrapped around the tower to create the compression needed to hold up the bridge. The design solves the problem of the bay’s geology: mud so soft it behaves like Jell-O and makes anchoring traditional suspension cables into it difficult. (Photo by CALTRANS)
On the north side, two side-by-side elevated roadbeds—the 1.2 mile-long skyway portion of the span—curve gracefully toward Yerba Buena Island. Almost complete, the skyway stands in sharp contrast to the old double-decker cantilever span sheathed in gray steel, which, when it first opened in 1936, carried commuter trains as well as cars and trucks.
Farther along are inklings of the signature design, rising in an unusual asymmetry that departs from standard suspension bridge design. Among the features: a single, 525-foot tower comprising four separate, slender shafts (a first for suspension bridges) interconnected with shear link beams (another first) designed to absorb seismic movement and keep the tower elastic yet stiff and upright during an earthquake.
The SAS span, built with high-strength steel, will connect to the skyway via hinge pipe beams that expand and contract during seismic shaking, absorbing most of the energy and elastically transferring loads between the two different bridge segments. The tower and deck “float” around each other, unconnected. Although suspension cables appear to drape down from the tower, they are an illusion. Unlike the Golden Gate and other traditional suspension bridges—where separate cables are put into tension by the resistance of anchorages fastened into the earth on either end—here a single cable fastened to the east anchorage of the deck and looping around the west piers acts like a giant rubber band to put the span into compression.
Nader is anticipating the day in 2013 when the project is finally finished. For 14 years, he’s poured himself into the design. How has he managed?
“Look at it,” he says, pointing to a rendering that hangs in his Oakland office. “It’s a piece of art. It will be there for my grandchildren to see, a piece of history. That’s how I do it.”