By Erin Sherbert
By Erin Sherbert
By Leif Haven
By Erin Sherbert
By Chris Roberts
By Kate Conger
By Brian Rinker
By Rachel Swan
"We didn't design for blast," he says.
There is little disagreement about one thing: Building the curved suspension span is a huge challenge, requiring engineering methods never before attempted in a seismic zone.
With a typical straight-decked suspension structure, such as the Golden Gate Bridge, towers are erected first, the main cables are hung between them, and the deck is attached to the cables. That process is reversed with a self-anchored suspension bridge. Because the suspension cables are anchored in the deck, rather than in the ground at either end of the bridge, the deck must be placed high above the water on a temporary edifice known as "falsework." Then the cables attaching it to the tower are connected.
This means building two bridges -- one temporary and the other permanent. Seismic issues aside, the method required to construct such a bridge makes it enormously costly compared to other design alternatives. That is why two engineers with international stature, who served on an advisory panel entrusted with recommending a winner among several competing designs deemed less aesthetic, opposed the self-anchored concept.
"[It] is simply not an efficient and rational structure," says Manabu Ito, a professor emeritus at Tokyo University who is among Japan's most renowned structural engineers. Ito was part of the 36-member advisory panel assembled by the Metropolitan Transportation Commission in 1997 to help decide what kind of bridge to build after political winds prompted a shift in Caltrans' original aim of merely retrofitting the existing east span.
Critics complained that the competition was hardly that at all, and describe the MTC's deliberations as rife with conflicts of interest, noting that several of the advisory panelists were connected to contractors whose designs they were responsible for judging.
Besides Ito, another notable dissenter was T.Y. Lin, a professor emeritus at UC Berkeley whose pioneering work in prestressed concrete had a profound influence on modern structural design. After the MTC's 1998 final approval of the self-anchored suspension span, Lin denigrated the choice as a "monument to engineering stupidity" and predicted that if built, the bridge would become a "laughingstock."
Lin, who died recently, said little publicly about the new bridge after those blistering remarks. His critique might have attracted more attention had he not been in the awkward position of having promoted his own design for the east span while a member of the MTC advisory group. In the end, the winning concept was submitted by a consortium that included the company Lin founded half a century ago and later sold, T.Y. Lin International of San Francisco.
Several people close to Lin during the 16 months that the advisory committee held its hearings say the legendary engineer harbored some of the same safety concerns about the self-anchored design that Astaneh expresses. "There's no question that he had misgivings about it," says Steve C. Thompson, a Mill Valley architect who also served on the panel.
R. Gary Black, a UC Berkeley professor of architecture (who, partnering with Astaneh, promoted his own bridge design for a time), agrees. "I talked to T.Y. extensively during that time, and he had two issues with [the chosen] design. One was cost. He just thought it was a needless waste to build a bridge that way. And the other was potential safety. He hadn't done the kind of work that Professor Astaneh had done, but he felt it didn't really make sense to build [that type of bridge] in a seismic zone."
Unlike regular suspension bridges, where the main cables are connected to large concrete anchor blocks firmly buried in solid ground, the self-anchored bridge's cables will be hooked directly to the deck. The cables will run from beneath the deck on one side, up to the tower and down to beneath the deck on the other side, before looping back up and over the tower in the opposite direction.
"Think of a tower holding up a tray," Astaneh says. "Instead of being anchored in the ground, as is traditionally the case, the bridge is essentially holding itself up. That means if there is a sufficient rupture, all or part of the span could unravel."
An aspect of the design that Astaneh finds particularly troublesome is the tonguelike connector joints linking the suspension portion of the bridge with the skyway. A quake powerful enough to cause the earth beneath the bridge to shift, a phenomenon known as permanent displacement, could make the suspension span and skyway break apart, he says.
In the Loma Prieta quake, connector joints were the culprit when ground motion triggered a partial collapse of the existing bridge. Land beneath the span moved about 12 inches, snapping bolts that held segments of the bridge together. The result: A 50-foot-long section of the upper roadway collapsed at one end and fell onto the lower roadway at the ninth pier from the East Bay shore.
Given the steel truss design of the existing span at the break point, even if the lower deck had given way the worst that would have happened is that the bridge would have been severed at Pier E-9, like a gap in a row of teeth, says Astaneh. "But with a self-anchored suspension span, permanent displacement could cause a completely different story," he says. "Because the suspension cables are anchored into the deck, should the deck give way and lose compression, there's nothing to hold the cables. The entire [suspension] span could unravel."