Five thousand pounds of metal from the San Francisco-Oakland Bay Bridge fell on commuters Tuesday because vibrations, worsened by a windstorm, caused steel rods to break, a spokesman for Caltrans said Wednesday.
Bart Fey, the spokesman, said drivers should find "other routes around the Bay Area over the next day or so" while repairs and testing continue.
He said the bridge was expected to be closed this morning, and he did not know when it would reopen.
The part that failed was installed over the Labor Day weekend in an emergency repair after inspectors discovered a crack in a critical structural beam.
"Obviously there is an issue with the original design" of the repair of the fractured beam, Fey said.
He said Caltrans had been monitoring the repair and noticed problems during a recent inspection.
"We didn't get the modifications put in place in this for the latest windstorm," Fey said. Gusts of more than 50 mph Tuesday were a "contributing factor," increasing vibrations in the metal rods, he said.
Fey said the new repair would be "fairly similar" to the original design installed over Labor Day but would include safety "enhancements."
A spokeswoman for the Federal Highway Administration said the agency sent engineers to assist Caltrans on Wednesday.
Three vehicles were damaged but no one was seriously injured when the rupture occurred at 5:30 p.m. Tuesday. The bridge was then cleared and closed.
The closure of the span, traveled daily by 280,000 motorists, produced gridlock on other bridges spanning the bay and huge ridership on BART, the Bay Area's subway system. BART added more cars to trains to accommodate the crowds.
Engineers expressed concerns Wednesday about the safety of the bridge, which collapsed during the Loma Prieta earthquake in 1989.
The rods that snapped held a clamp that was supposed to keep the fractured beam, or "eyebar," together, said UC Berkeley civil engineering professor Abolhassan Astaneh-Asl.
He called the recent repair "a Band-Aid" and said a new solution should be devised.
"This was not designed correctly," he said.
"The combination of the weight of the traffic and the force of the wind exceeded the capacity of this temporary repair."
In an earthquake, the kind of beam that cracked could "completely fracture," he said. "As soon as that one member fractures, the entire bridge collapses."
He said the bridge should be shut down for a few days until all relevant beams can be inspected for hairline cracks with X-ray equipment.
USC civil engineering professor James C. Anderson agreed that the Bay Bridge "was susceptible to fatigue failure." Steel members and connections wear out, he said.
As did Astaneh-Asl, Anderson cited commercial tractor trailers crossing the 73-year-old bridge as a "particular concern."
"The size of modern tractor-trailers are probably twice the size of older models, thereby increasing the amount of loading on the bridge," Anderson said.
"While I do not have an opinion on the danger of collapse, these recent occurrences of pieces falling off the bridge are definitely a warning sign that the bridge may be wearing out," Anderson said.
A new bridge is under construction but it is not expected to open until 2013.
Times staff writer Jessica Garrison contributed to this report.