Buses won’t return to the damaged Transbay Transit Center until its broken girders are repaired, a process that could take at least several weeks. The rooftop park, however, could reopen sooner, officials said Tuesday.

At a special meeting of the Transbay Joint Powers Authority, Executive Director Mark Zabaneh said the agency should know by Nov. 1 what caused two large support beams to crack.

The lush 5.4-acre rooftop park could reopen by then, he said, even as engineers and architects are drawing up plans for a permanent fix to the two cracked steel girders, which hold up the bus deck and brace the roof above Fremont Street.

But resuming bus service will have to wait until the permanent fix is completed, Zabaneh said. While the temporary bracing could support the weight of people on the park plus buses on the deck, he said, Transbay officials prefer to be cautious.

Construction crews will also be on the bus deck working, which would make it difficult, and possibly dangerous, for drivers.

“Obviously, we don’t feel it is safe to operate buses while the temporary shoring is there,” Zabaneh said. “But it might be possible to open the park.”

Once crews know what caused the cracking, they’ll devise a permanent fix, which will be reviewed by a committee of design, engineering and metallurgical experts before it’s installed, said Dennis Turchon, the authority’s senior construction manager.

That will take time, Zabaneh admitted, but added: “My goal is weeks, not months. I’m not saying it will take four weeks — it’s going to take more than four weeks obviously — but not six months.”

Just how long the work will take depends on what tests reveal, Zabaneh said. Bruce Gibbons, a structural engineer, said the tests will involve removing pieces of the damaged steel girders and scrutinizing their structural composition under a microscope.

“We can tell how the actual fracture occurred,” Gibbons said.

The cracks could stem from errors in the manufacturing or fabrication of the girder, its installation or a structural design that placed too much weight on the beams, engineers have said.

Whatever the cause, Gibbons said, it will likely involve adding material — more steel — to the beams — but not removing and replacing them.

“There’s no question they can be fixed in place,” he said.

The $2.2-billion transit center has been closed since the afternoon of Sept. 25, shortly after workers discovered a crack in a support beam. A crack in a second beam was found hours later in the same area. Fremont Street, which passes beneath the damaged part of the transit hub was shut down out of what officials called “an abundance of caution.”

Over the weekend, crews installed six hydraulic jacks that prop up the damaged area. By the end of the week, sturdier support towers, made of steel pipe, are expected to be installed on Fremont Street and on the transit center’s bus deck, said Turchon.

Fremont Street is expected to reopen by Oct. 12, probably with narrower lanes that skirt around the support towers. Zabaneh said there likely will be two lanes on the left side of the one-way street and one on the right. The 10-foot-wide temporary support towers will be surrounded with concrete safety barriers.

Transbay officials are focusing on stabilizing the building over Fremont Street but they’ve also inspected beams throughout the center, looking for cracks. They’ve found none, Turchon said, including over First Street, a section that has the same kind of structural girders. Engineers are monitoring the area.

Members of the Transbay authority’s board, which includes representatives from San Francisco, AC Transit and the Peninsula Corridor Joint Powers Board that owns and operates Caltrain, expressed concern about the new center’s safety. They said they were eager to return it to service but didn’t want repairs rushed.

“I think the team needs to take the right amount of time it needs to make sure it’s done right,” said Jeff Gee, who represents Caltrain.

Michael Cabanatuan is a San Francisco Chronicle staff writer. Email: mcabanatuan@sfchronicle.com Twitter: @ctuan