(CNN) -- Repairs on the San Francisco-Oakland Bay Bridge should be complete by Friday morning, three days after pieces of steel from the bridge fell onto the structure's roadway, a top transportation official said Thursday.

Crews working on the California bridge are replacing four steel rods, one of which failed and caused two rods to fall onto the bridge's deck, said California's Secretary of Business, Transportation and Housing Agency Dale Bonner at a news conference.

Engineers also will make sure the rods are centered and strengthen the welds to ensure stability, he said. Vibrations in the rods, caused by strong winds, caused the breakage, said Bonner.

The pieces that fell, which include a cross beam, came from the same section that was repaired over Labor Day weekend, when crews worked almost round the clock to fix a crack in the span.

The current repairs could be complete by 10 a.m. Friday, Bonner said. Afterward, federal inspectors will test the bridge for at least three hours and then determine whether the bridge can reopen to vehicular traffic, he said. But he shied away from stating when the bridge would reopen.

The 73-year-old bridge spans the San Francisco Bay and carries an average of 280,000 vehicles daily, according to the state's Transportation Department.

In the wake of the bridge's closure, commuters flocked to Bay Area Rapid Transit (BART) as an alternative. On Wednesday, BART carried the most passengers ever, according to the public transportation agency.

About 437,200 people rode BART, about 26 percent more than on an average Wednesday, the agency said in a news release.

When the Bay Bridge does reopen, Bonner said travelers should not worry over the safety of the span.

"The public need not fear that the bridge is going to fall down," he said. "There's no concern about that."

In 1989 during the Loma Prieta earthquake, a 50-foot section of the bridge collapsed, killing one person and prompting efforts to make it quake-tolerant.