SAN FRANCISCO -- Bay Area drivers braced for slow going across the Bay Bridge starting Friday night as Caltrans work on a permanent fix to the troublesome cracked eyebar begins forcing nightly lane closures on both decks of the span.

The California Department of Transportation announced earlier this week that the repairs would require some lanes on the Bay Bridge to be closed, but it won't have to close the entire span.

The repair and related lane closures will last approximately three weeks, officials said.

The repair job will entail cutting and removing approximately 12 feet of the cracked eyebar, officials said, and replacing the removed section with new structural steel that will be spliced onto the remainder of the existing eyebar.

"We would like to thank motorists in advance for their patience during the upcoming lane closures," said Caltrans Director Randell Iwasaki. "This is the right fix and a long-term one that should keep the 73-year old bridge safer until it is replaced."

Plans call for the three left lanes on the westbound upper deck of the Bay Bridge to be closed from 10 p.m. to 5 a.m. on Sunday through Thursday nights, from 11 p.m. to 8 a.m. on Friday nights and from 11:30 p.m. to 9:30 a.m. on Saturday nights.

For safety reasons, Caltrans will also close one lane, the far right lane, of the eastbound lower deck of the span from 10 p.m. to 7 a.m. on Sunday through Thursday nights, from 11 p.m. to 9 a.m. on Friday nights and from 11:30 p.m. to 10 a.m. on Sunday nights.

Iwasaki guaranteed commuters that the planned work schedule would allow all lanes to be open for the morning rush hour.

At an afternoon press conference earlier this week, Caltrans spokesman Ney said it's possible that additional repair work will have to be done early next year that also would force some lanes to be closed but that decision won't be made until after the current work is completed.

Work crews first discovered a crack in an eyebar during Labor Day weekend, while the bridge was closed so that a new section could be put in place on the new eastern span of the Bay Bridge. It delayed the reopening of a span by a day so that a repair could be made.

But on Oct. 27, during busy commute hours, heavy winds caused the repair to snap and two tie rods and a crossbar fell onto traffic on the upper deck of the Bay Bridge. The bridge was closed for six days while additional repairs were made, causing gridlock for Bay Area commuters.

When the span reopened on Nov. 2, Caltrans chief engineer Richard Land said his agency wanted to develop a better solution for the repaired area that would be more efficient and wouldn't require daily monitoring but it might force the bridge to be completely closed again for at least several days.

But Ney said that Caltrans and outside experts it has consulted with have determined that the work can be done while the bridge stays open.

Ney said bad weather, such as heavy winds and rain, could delay both the start and the completion of the repair work.

He said the work will be done in the overnight hours to try to minimize traffic delays but Caltrans still expects delays of 30 to 60 minutes in westbound traffic.

Ney said there won't be any extra service by BART or by bus agencies, so drivers should consider using other bridges when the work is being done. Drivers may also cal 511 or visit 511.org for traffic updates and transit planning, he said.

The design of the long-term repair was done by Caltrans with input and help from independent experts and the contractor, the joint venture American Bridge-Fluor, Ney said.

If it's determined that more work needs to be done, the repair will be supplemented with additional structural steel at a later date, Ney said.