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SAN FRANCISCO (CBS 5 / KCBS / AP / BCN) ― State transportation officials said they couldn't guarantee that the San Francisco-Oakland Bay Bridge would be open in time for the Friday morning commute, even though they indicated that crews expected to finish emergency repairs to the span late Thursday night.

But California Department of Transportation spokesman Bart Ney said there was a "possibility" the bridge could reopen in time for the Friday morning commute if necessary testing could be completed overnight.

Engineers worked into the night for a second full day on Thursday fortifying the bridge after a 5,000-pound piece of steel came crashing down onto westbound lanes earlier in the week.

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.

Ney explained that Caltrans' work would be completed by about 10 p.m. Thursday, but Dale Bonner, Secretary of the state Business, Transportation and Housing Agency, said even after the repair work was completed that additional testing would have to be conducted before the bridge could reopen to traffic.

If the tests revealed any problems, the bridge could remain closed for another day or two, officials indicated.

Federal Highway Administration officials and other inspectors were also monitoring the work being done and would examine the fix when it was completed, Bonner said.

Caltrans Toll Bridge Program Manager Tony Anziano told CBS 5 that welding was completed in an area where two rods and a crossbar fell onto the upper deck and that workers on Thursday night were installing modifications to make sure the repair doesn't break again.

The design enhancements included drilling out old welds on crossbars and installing deeper, stronger welds to connect crossbars and saddles into a single unit to avoid the "metal-on-metal fatigue" believed to have caused the failure on Tuesday, Anziano said. 

As commuters Thursday dealt with a second-straight day without one of the region's most important traffic arteries, officials reported that traffic was lighter than Wednesday as many people chose to take public transportation — setting transit ridership records — or work from home.

Other bridges that provide access to San Francisco — the Richmond-San Rafael, San Mateo and Golden Gate bridges — experienced some delays despite extra staffing at toll plazas, as the estimated 280,000 commuters who use the Bay Bridge each day again looked for alternate routes.

On Thursday, ridership on the Bay Area Rapid Transit system, which travels in a tube beneath the San Francisco Bay, was up 60 percent as many of those commuters sought to steer clear of the freeway congestion.

That followed a busy Wednesday in which more than 437,000 people rode the system, more than any other single day in the system's 37-year history. The previous record for the system was set on Sept. 8, 2008, during Oakland Raiders and San Francisco Giants games.

The agency was "on pace to blow out that record" again on Thursday, BART spokesman Linton Johnson said.

He indicated that BART would continue to run longer trains during commute hours and more trains during non-commute hours to handle the additional passengers.

Although he had previously discussed the possibility of running overnight service, Johnson said Thursday that there were no plans to have trains running overnight, even if the bridge closure lasts into the weekend.

The overnight closure "is critical to make sure our trackways and trains are inspected correctly," he said. "So the decision is to use those few hours that we have overnight to keep those trains working."

Meantime, Ney maintained at a news conference Thursday night that "Caltrans is doing everything it can to return the bridge back to a safe state." Three cars were damaged and one motorist had minor injuries when the chunk of metal fell onto upper deck lanes during Tuesday evening's rush-hour commute.

The rods that broke were holding a saddle-like cap that had been installed to strengthen the cracked eyebar.

Ney said Thursday that the crack in the eyebar had 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.

Transportation officials claimed 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 Loma Prieta 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.

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., of Sacramento, the firm conducting the repairs.

Puchi said the flat bolts were 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 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.

Federal engineers said they 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.

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