No reopening date has been set, but the steel is on order to repair the cracked girders that have kept the Transbay Transit Center closed for more than three months.

Four sets of 20-inch-wide, 14-foot-long steel plates will be fabricated and then sent to San Francisco, where they’ll be attached to the four immense girders that brace the $2.16 billion structure where it spans Fremont and First streets. Cracks were found Sept. 20 in the two above Fremont Street, and the gleaming new facility and its popular rooftop park have been closed ever since.

No cracks were found in the two girders that span First Street, but they will be reinforced for protection as well.

The independent peer review committee appointed to analyze the cracked girders has signed off on the proposed repairs, which essentially would bolt plates above and below the vulnerable section of the girders.

“We’ve given directions to our contractor to start looking for steel” and finding plants where it can be manufactured quickly, said Mark Zabaneh, executive director of the Transbay Joint Powers Authority. The agency built the transit center, which includes a bridge with direct access for commuter buses to and from the Bay Bridge. “As soon as they locate material, we can set a schedule to fix the girders.”

A better idea of what lies ahead could emerge Thursday at the monthly meeting of the Transbay Joint Powers Authority’s board. There will be an update from the construction team, as well as a presentation by the initial peer review committee to explain the original evaluation process for the design of the center and changes made in the field during construction.

Whatever the timeline for repairs, the center won’t reopen until the addition of the plates to the girders is scrutinized and tested by the Transbay Joint Powers Authority and other agencies.

The authority has also asked the current peer review committee to recommend whether other sections of the structure should be inspected.

“It’s insurance,” Zabaneh said. “Knowing what took place, and the results of the tests that we’ve done, it’s important to determine if there are other areas we should look at.”

John King is a San Francisco Chronicle staff writer. Email: jking@sfchronicle.com