Chief architect of the new $2.26 billion Transbay Transit Center, Fred Clarke, far left, leads a press tour through the Grand Hall of the project in downtown San Francisco, Calif., on Wednesday, Aug. 8, 2018. The facility opens to the public next week. (Karl Mondon/Bay Area News Group via Getty Images)

SAN FRANCISCO – Even as they sought to assure the public that the cracked steel beam found Tuesday in the new Transbay Transit Center was an isolated incident, San Francisco officials acknowledged discovering a second compromised beam near the first one.

The revelation added to the city's embarrassment and increased concerns about the safety of a $2.2 billion transit hub that had to be closed a mere 45 days after its unveiling — and at a time when San Francisco is trying to display its finest wares.

An estimated 170,000 visitors are in town for the yearly mega-convention hosted by cloud-computing giant Salesforce, whose name is attached to the transit center and the recently opened 1,070-foot building next to it.

"The timing couldn't have been worse,'' said Josh Brown, who lives in the city and is attending the convention. "If you're visiting and you're like, 'Let's go check out the new Transbay terminal, the Salesforce area,' Oh, sorry, you can't. It's going to collapse.''"

Beyond the poorly timed civic black eye, there's the more serious matter of the safety implications of detecting faulty beams on the third floor of a structure built over more than three city blocks on seismically active terrain.

Mark Zabaneh, executive director of the Transbay Joint Powers Authority, issued repeated assurances during a Wednesday news conference that the problem with the beams is limited to the east portion of the three-section facility, on the side over Fremont Street. Nonetheless, the hub will be closed at least until late next week while engineers work to develop a repair plan.

"We will expand our investigation to make sure the transit center is sound and safe,'' Zabaneh said. "We have no reason to believe at this point whatsoever that this situation exists anywhere but on Fremont Street.''

The reasons for the defective beams – one with a crack across its full width of 2½ feet, the other one with less extensive damage – have yet to be determined, Zabaneh said. He mentioned issues of fabrication, welding, installation and design as possible culprits.

Engineers plan to review the terminal's design and determine whether those beams can support the roof and the public park above it. In the meantime, they will shore up the area of concern to relieve the load while seeking a permanent solution.

"At this point in time, we don't know what the cause of the crack is; we know the building is safe,'' Zabaneh said.

The transit center, envisioned as a "Grand Central Station of the West,'' has been beset by complications that included an $800 million cost overrun and extensive construction delays. A month into its Aug. 12 opening, the half-mile walkway around the rooftop park started to crumble.

In addition, the neighboring Millennium Tower has sunk 18 inches and tilted to the northwest since its debut in 2009, which its developers have blamed on groundwater pumping at the Transbay terminal's construction site.

Riders are now being diverted to the old Temporary Transbay Terminal a few blocks away.

Waiting for a bus home to Oakland after his work shift at nearby BlackRock, Joseph Cobbina said he was "shocked'' and "saddened'' when he heard the new hub would shut down for a while.

Sarah Larone, an auditor who works in Oakland, said the transit center's problems don't reflect well on her home city of San Francisco.

"It's just a shame that it's new and it's already falling apart,'' Larone said. "I guess it happens, issues with buildings and windows and whatnot. At least the buses are still running. That would be upsetting if they didn't.''