Oct. 28 (Bloomberg) -- The indefinite closing of the San Francisco-Oakland Bay Bridge after steel rods crashed from a repaired section onto its upper roadway is prudent as authorities assess the structure’s safety, engineers say.
“There’s maybe not a full understanding of what caused that failure in the first place or it wasn’t properly repaired,” said Henry Petroski, a professor of engineering at Duke University in Durham, North Carolina, and author of a 1995 book on the history of bridges. “It could also mean that other parts of the bridge are overstressed, as engineers would put it, or overloaded.”
The Bay Bridge was closed last night after a crossbar and two steel tie rods fell from a section repaired last month, damaging three vehicles and causing minor injuries to one driver. Structural engineers and inspectors are working to determine how long repairs will take, according to a statement from the California Transportation Department, or Caltrans.
“At this time, the bridge is closed until further notice,” Caltrans said in the statement.
About 280,000 vehicles cross the bridge on an average day, compared with 137,500 using New York’s Brooklyn Bridge, according to the Bay Bridge Web site.
The 8.4 mile (13.5-kilometer) span, built in 1936, was damaged during the magnitude 6.9 Loma Prieta earthquake that killed 63 people and caused $7.8 billion of damage around the Bay area in 1989. A $5.49 billion replacement of the eastern span of the bridge is under construction and due to be completed in four years.
Labor Day Fix
The bridge was shuttered during last month’s Labor Day holiday weekend so engineers could replace a 300-foot (91-meter) double-deck section, as part of larger plans to make the structure able to withstand earthquakes.
“Most of these types of problems don’t show up until a few years after they’re installed,” said Jack Moehle, a professor of structural engineering at the University of California, Berkeley. “It’s a little surprising in this case that it happened six weeks after they installed it.”
More than 26 percent of the 600,905 bridges in the U.S. are either structurally deficient or functionally obsolete, the American Society of Civil Engineers said in its quadrennial report on the nation’s infrastructure, released March 25.
The number of deficient and obsolete bridges in urban areas is increasing, and authorities would have to raise spending on bridge repair to $17 billion annually, from $10.5 billion now, to “substantially improve” the state of U.S. bridges, the group said.
‘More Problems’
“In my judgment, people were a little negligent to do a fix like that,” said Fari Barzegar, a civil engineering consultant and founding principal of Habitat Engineering & Forensics in Oakland, California. “The fix introduced more problems.”
While he doesn’t think a collapse of the bridge is likely, Barzegar said, he expects a thorough review of how the September repair was conducted. Caltrans said Sept. 8 that the bridge reopened after crews worked almost 70 hours to repair a damaged steel beam, about 22 hours faster than scheduled.
“It might be the speed at which they do these things -- sometimes they shortcut things because they want to minimize the delay,” Barzegar said. “The details are very sketchy at the moment.”
The average U.S. bridge is 43 years old and was built to last about 50 years, the engineering society said. As of December, 72,868, or 12 percent, were structurally deficient and 89,024, or 15 percent, were functionally obsolete, the group said, citing data from the U.S. Transportation Department.
Collapse Unlikely
Petroski, author of “Engineers of Dreams: Great Bridge Builders and The Spanning of America,” also said a collapse is unlikely. The bridge’s age shouldn’t be an issue if it has been properly maintained and inspected, he said. The Brooklyn Bridge is more than 125 years old, he said, while there are bridges in England that are more than 200.
Caltrans engineers probably will spend the next few days inspecting the bridge, and the shutdown allows them to do so without having to dodge traffic, Petroski said. Bridges have a “certain symmetry to them,” so Caltrans also may scrutinize parts on other areas of the bridge that correspond to the failed piece. If the bridge does reopen in a few days, Caltrans may decide to limit traffic to cars and light trucks to reduce the load, he said.
“There’s one positive aspect of this and that is apparently it’s the same part that broke,” Petroski said.
“What would have been worse is if another part broke that they didn’t expect,” he said. “They thought they had identified what the problem was -- this one weak link -- and they thought they replaced that link with a stronger link and clearly that’s not been the case.”
The cost to build the Bay Bridge, including the Transbay Transit Terminal, was $77 million at the time, according to the Bay Bridge Web site. In comparison, the Bay area’s other landmark span -- the Golden Gate Bridge -- cost $35 million, according to the GoldenGateBridge.org Web site.
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