The damaged Transbay Transit Center — already shut down more than four months — won’t welcome back buses, passengers and parkgoers until at least June, officials told The Chronicle.

No reopening date has been scheduled for the $2.2 billion transit hub that was open all of six weeks, but the Transbay Joint Powers Authority on Friday will announce a schedule to complete repairs on two cracked steel girders that support the 3-block-long building where it crosses Fremont Street, as well as a matching, but unbroken, pair over First Street.

Repairs are expected to be completed the first week of June. But an independent review committee of engineers and welding experts could determine that inspections or repairs are needed elsewhere in the structure, which would further delay the reopening, officials said.

“We are very eager to reopen the transit center, but we want to make sure it is safe and that there are no other issues,” said Mark Zabaneh, executive director of the Transbay Joint Powers Authority. “It’s a prudent measure on our part, and we owe it to the public to investigate other possibilities before we reopen.”

The multibillion-dollar transit hub, retail center and rooftop park opened in August, nine years after the drafty 71-year-old Transbay Terminal was demolished and construction of the replacement began. A hurried evacuation was ordered just six weeks after the transit center opened, and the building heralded as the “Grand Central Station of the West” was shuttered as buses and passengers were rerouted back to a nearby temporary terminal used during construction.

The new transit center, ensconced in a white steel-mesh veil, has sat largely empty, hovering like a ghost in the South of Market neighborhood. Meanwhile, engineers and designers have been probing the cause of the cracks and devising a strategy to fix the damage.

Repair plans have been approved, steel is being fabricated to bolster the broken beams, and work at the site may begin this weekend with crews removing some of the hydraulic jacks beneath the center on First Street, said Christine Falvey, a spokeswoman for the Transbay Joint Powers Authority. First Street will be closed between 5 a.m. Saturday and 8 p.m. Sunday while the jacks are replaced with more permanent supports.

But the repair work will take months to complete.

To bolster the broken beams, workers will place 2-inch-thick steel plates on both sides of the flanges of each girder and affix them with 224 bolts. Drilling that many holes and attaching the plates is a time-consuming process, Zabaneh said.

Steel fabrication is being done in the Central Valley by Herrick Corp., the same company that fabricated the two cracked girders. Transbay engineers and the review panel have tentatively concluded that holes cut into the girders using a torch may have led to the cracking. The investigation into the details of who or what caused the cracks continues.

Zabaneh said that drilling holes in a girder is safer than cutting an opening with a torch, and it doesn’t create the potential for cracking.

At the same time repairs are beginning, engineers are poring over about 15,000 records, including construction drawings, to determine if other places in the transit center could also be subject to brittle fractures in large steel beams or girders. Nothing yet has been discovered, Zabaneh said.

Michael Engelhardt, a University of Texas structural engineering professor, said the review should take weeks instead of months, unless drawings reveal a potential weakness that requires a deeper look into material tests or a physical examination. Any likely repairs would not be lengthy, he added.

The review panel’s investigation has been thorough, Engelhardt said, and the hope is to wrap up the work soon and reopen the center.

“Everyone involved, especially me, are looking forward to this thing ending,” he said. “It’s been interesting, but at some point the charm wears off.”

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