Transportation officials still have no firm estimate of when the bridge will reopen, but said the crossing would likely remain closed for the evening commute.
Commuters faced a second day without one of the region's most important traffic arteries, but officials reported that traffic was much lighter than Wednesday as people chose to take public transportation or work from home.
"Caltrans is doing everything it can to return the bridge back to a safe state," Bart Ney, a spokesman for the state transportation department said at a news conference Thursday morning. Three cars were damaged and one motorist had minor injuries when the chunk of metal fell onto westbound lanes during Tuesday evening's rush-hour commute.
The pieces that failed were parts of major repairs done last month after state inspectors discovered a crack in an "eyebar," an important structural beam. The rods that broke were holding a saddle-like cap that had been installed to strengthen the cracked eyebar.
Ney said Thursday the crack in the eyebar has not gotten any bigger as a result of Tuesday's failure. Engineers thought they had fixed the problem over Labor Day weekend, but the repairs did not hold.
Officials say strong winds likely played a role in the bridge failure, heightening concerns by some experts about the integrity of the repair and the bridge's safety in an earthquake. The 1989 San Francisco earthquake caused a 50-foot section of the bridge's upper deck to collapse onto the deck below.
Scientists in 2008 said there is a 63 percent probability of a quake similar to the 1989 temblor in the Bay area over the next 30 years.
Asked if the bridge will be safe when it reopens, Ney said, "We have some of the best design engineers working on the bridge. It should be safe when we reopen it," according to CBS station KPIX.
Engineers believe the rods snapped after vibrating against the edge of a flat metal plate that was used to bolt stress rods to the structure, said Ed Puchi, a spokesman for MCM Construction, Inc., the firm conducting the repairs.
Puchi said the flat bolts are now being replaced with rounded ones, which are less likely to shift when high winds cause the rods to vibrate. Also, Puchi said crews are installing straps on the rods to help dampen vibrations.
"If the rod fails again, the straps would prevent it from falling," Puchi said.
On the bridge, repair crews used cranes to thread the new rods into place. Workers hoisted 100 feet over the roadway also worked on fortifying the new saddle.
The bridge closure made for a rough commute Wednesday, with heavy traffic and crowded trains. Other bridges that provide access to San Francisco were especially congested, as some of the estimated 280,000 commuters who use the Bay Bridge each day looked for alternate routes. Traffic conditions were lighter on Thursday.
The main contractor on the repairs that failed, C.C. Meyers, Inc., stood by the work, but deferred to Caltrans to determine why the pieces failed, spokeswoman Beth Ruyak said.
Meanwhile, the Federal Highway Administration sent engineers on Wednesday to help Caltrans investigate. The federal agency said it had not inspected the Labor Day weekend repairs made to the heavily used span, instead relying on state inspection reports to ensure safety guidelines were met.