Eight months after the discovery of cracked steel girders forced its closure, the Transbay transit center is safe to reopen, an independent panel of engineers and experts concluded Monday evening.

But the date when buses and people will be able to re-enter the $2.2 billion transit hub, retail center and rooftop park will have to be set by the Transbay Joint Powers Authority, which oversaw construction and manages operation of the three-block-long center.

A five-member peer-review committee appointed by the Metropolitan Transportation Commission at the request of San Francisco Mayor London Breed and Oakland Mayor Libby Schaaf approved the structure for reopening in a letter sent to the mayors and the authority, which oversaw construction and manages operation of the facility.

“The steel structure is sound for continued service,” wrote Therese McMillan, MTC’s executive director.

The authority’s board of directors, which meets Wednesday, has been told that it could take as long as four weeks after it got the go-ahead to prepare the center to be reoccupied. It was open just six weeks before the fractured beams were discovered by a worker on Sept. 25.

Work to prepare for a reopening has been under way for weeks, and Transbay officials said as recently as the beginning of June that the center might be able to open before the end of the month. Christine Falvey, a spokeswoman for the authority, said before the letter’s arrival that a specific opening date would be set soon after the receipt of the letter.

The review panel was assigned to look at results of the investigation into the cause of the fractures, the repairs and whether the incident had caused other structural damage. The panel also decided to review drawings and documents for other places that might be susceptible to similar fracturing. The review of thousands of documents plus a few on-site inspections took a couple of months.

Michael Engelhardt, a University of Texas structural engineer, said some minor welding flaws were found, repaired and examined but determined not to be of concern.

“This was chased down rather exhaustively,” he said. “But we concluded it is really not a safety issue at all.”

McMillan said that, based on the independent panel’s work, the MTC has concluded that the transit hub is ready to reopen.

“We can represent to you and the public alike confidence that the Transbay Transit Center’s girder problem was isolated and that the appropriate repairs have been performed,” she wrote.

Michael Cabanatuan is a San Francisco Chronicle staff writer. Email: mcabanatuan@

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