Photo: Noah Berger / Special To The Chronicle Photo: Photos By Noah Berger / Special To The Chronicle Photo: Noah Berger / Special To The Chronicle Photo: Noah Berger / Special To The Chronicle Photo: Noah Berger, Special To The Chronicle

Repairs to the fractured steel girders that hold up the Transbay Transit Center are moving faster than expected, officials said Wednesday, but they still have no projected date for reopening the transit hub, which has been closed for more than six months.

The soonest the three-block-long center could reopen would be early June, when repairs are expected to be completed and approved, Christine Falvey, a Transbay spokeswoman, said during a media tour of the repair work Wednesday. But before a reopening can be announced, a comprehensive review of the structure needs to be completed and the building dusted off and put back in shape for public use.

“No one can commit to a reopening date,” said Dennis Turchon, senior construction manager for the Transbay Joint Powers Authority. The agency is responsible for the construction and operation of the $2 billion transit hub, which includes retail space and a rooftop park.

The center has been closed since Sept. 25, six weeks after it opened, when a construction worker installing ceiling panels noticed a crack in a girder. The structure was evacuated and shut down. Temporary supports were installed under the two cracked girders above Fremont Street and at First Street, where a pair of similar but undamaged beams span the street.

Since then, engineers have searched for the cause of the fractures, devised repairs and inspected thousands of construction documents. Preliminary reports blame the cracks on problems in the fabrication process related to cutting welding-access holes in the beams.

Repairs are well under way inside the white-shrouded structure off Mission Street. Four-inch thick steel plates to repair and reinforce the four beams have been delivered.

Construction crews have drilled hundreds of 1½-inch diameter holes in the plates and are making matching holes in the girders. The plates will be placed above and below the girders, sandwiching them, and the bolts will be inserted and tightened.

Turchon said the plates are made of the strongest steel available, and categorized the repair design as conservative.

“It’s designed to be the same or better than the original design,” he said. “But I’d say it’s stronger.”

Work over Fremont Street is about two weeks ahead of First Street, Turchon said, and the bolstered girders could be completed in about two weeks. Construction crews will then release the hydraulic jacks supporting the structure and remove the temporary supports. Then they’ll move on to First Street.

Turchon expects the repairs to be completed by June 1 — a couple of weeks ahead of the original schedule. But reviews of construction paperwork and photographs for the entire structure have to be completed — and approved by an independent committee of engineers — before the buses, passengers, shoppers and park visitors will be allowed to return.

Thornton-Tomasetti, the Transbay’s structural engineer, has finished its paperwork review of all the connections made during construction, Turchon said. There are 47 different types of connections, each of which might have been used hundreds or thousands of times. Now the engineers are reviewing photos taken during construction to see if anything requires physical inspection or testing.

“To date they’ve found nothing,” he said. “But they are continuing to ascertain whether further work is needed.”

An oversight committee, assembled by the Metropolitan Transportation Commission at the request of San Francisco Mayor London Breed and Oakland Mayor Libby Schaaf, must sign off on those reviews before the center can reopen.

John Goodwin, an MTC spokesman, said it’s not clear when that will happen.

“We feel quite confident that the June 1 date for completing repairs is eminently doable,” he said, “but there’s a lot of other work to be done.”

That includes cleaning up the center, retesting things like fire alarms and communications equipment, and making sure the lights, video screens, signs, elevators and escalators are ready to go back into service, Falvey said.

During the tour of the nearly empty station Wednesday, the video screens flickered with colorful ads for tech products and airlines interspersed with blank screens where transit information will be displayed. The bus deck, where most of the girder repair work is taking place, was covered in construction materials and detritus, and a light coat of dust obscured the once-shiny newness of the transit center.

“There’s the whole element of starting up a building again,” Falvey said.

How much this all will cost remains unknown, Turchon said, but the joint powers authority expects the contractor or subcontractors deemed responsible for the failure to eventually cover the costs.

The agency has not yet deemed anyone responsible, though it has said the girders failed to meet construction codes.

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