Transit crews scrambled to repair an unexpected crack in the San Francisco-Oakland Bay Bridge and finish a long-planned seismic upgrade with just a day to go before the crush of morning commuters.

The 73-year-old structure that carries about 260,000 vehicles a day between the two sides of the bay remained closed for a fourth day on Monday, and authorities hoped to avoid a fifth on Tuesday.

A team of workers has received the blueprint and materials they needed to repair the crack in a steel link that holds up part of the span, said CalTrans spokesman Bart Ney. But fixing the problem by Tuesday, when the workweek begins and the bridge had been set to reopen, poses "quite a challenge," Ney said.

"People should be braced for another day of it being closed," he said.

The bridge was shut down Thursday night so a section of the eastern span could be cut out and replaced with a new double deck section. California transportation workers used the closure as an opportunity to inspect the bridge from top to bottom, and they discovered the crack in the link on Saturday.

The damaged link — part of a network of eight similar pieces — is about 2 inches thick and was cracked halfway through.

Caltrans construction manager Mike Forner said the fissure probably wasn't a danger to motorists because the other seven links assumed some of the cracked link's load, but the damage is serious enough to justify a closure.

Friday was the first time that the bridge was closed on a workday since part of it collapsed in the 1989 earthquake. It had been scheduled to reopen by 5 a.m. Tuesday.

Other Bay Area bridges and public transportation systems were able to accommodate extra riders Friday, but Tuesday's rush hours could prove more troublesome.

Ney said every effort would be made to reopen the bridge to traffic on time. A steel contractor in Arizona worked overnight to produce the welds needed to make the repairs and rushed the materials to the Bay Area on a chartered jet, he said.