2nd crack found at SF Transbay Transit Center - to stay closed through next week

San Francisco’s new Transbay Transit Center will remain closed at least through the end of next week, transportation officials said Wednesday, after a second cracked steel beam was discovered during an overnight safety inspection.

The $2.2 billion hub for buses and eventually trains, which opened just last month, was abruptly closed Tuesday afternoon after workers installing ceiling tiles on a third-floor bus deck saw a crack in a 6½-foot structural beam that runs on the east-west axis over Fremont Street.

Officials with the Transbay Joint Powers Authority, which built and operates the transit center, did not know the cause of the initial crack, nor the cause of the second crack, which was described as being in an adjacent beam and less severe. But they were worried about the potential of failure of both.

Fremont Street between Mission and Howard streets, which runs underneath the center, also will remain shut down until at least the end of next week.

Mark Zabaneh, executive director of the Transbay Joint Powers Authority, said Wednesday that the problem was isolated to the two beams above Fremont Street. But inspectors were planning to evaluate dozens of structural beams to make sure there were no other flaws.

“The beam is cracked, so the behavior of the beam is unpredictable,” Zabaneh said. “It’s a safety issue, and we take it seriously and we can’t take any chances.”

City officials have advised motorists to avoid downtown as traffic is expected to back-up with the closure of the transit center and Fremont Street. This week’s Dreamforce conference, hosted by tech titan Salesforce, is also contributing to congestion in the area — 170,00 people are attending.

AC Transit, SamTrans and Muni buses that serve the transit center have resumed using the Temporary Transbay Terminal at Howard and Main streets, where they operated during construction of the new building.

The American-made steel in the cracked beam was produced as part of a $189 million contract that Skanska USA of New York had with the Transbay Joint Powers Authority, records show. The beam was fabricated by Herrick steel in the Central Valley, Zabaneh said.

The pillar is among more than 22,000 tons of steel that make up the skeleton of the building, according to construction documents.

While officials provided little information on the crack, Zabaneh said it was near a weld, which construction experts told The Chronicle can create stress or imperfections in the steel. If the steel is found to be sound, experts said there might simply be too much weight on the beam.

San Francisco Mayor London Breed Wednesday pledged to get to the bottom of the problem as swiftly as possible.

“The Transbay Transit Center is too important for our city and our regional transportation system not to act quickly to have definitive answers for the public, and someone needs to be held accountable once the cause is determined,” she said in a statement.

The transit center is next door to Millennium Tower, a 58-story residential complex that was also found to be having construction problems when surveyors learned that it had sunk 18 inches since its opening in 2009. Transit officials said issues in the neighboring buildings were not linked.

The cracked beam was not the first problem for the transportation center, a hulking structure with a lacy facade and roomy, well-lit foyers inside, and officially known as Salesforce Transit Center. The walkway that loops around the rooftop park has recently begun to crumble. The center and park have been open since Aug. 12.

The publicly funded building has been in the works for nearly two decades, with groundbreaking in 2010. Envisioned as the “Grand Central station of the West,” its full potential won’t be realized until planned connections with Caltrain and high-speed rail service are made. It could be a decade or more before those ties are completed.

Kurtis Alexander and Evan Sernoffsky are San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com esernoffsky@sfchronicle.com Twitter: @kurtisalexander @EvanSernoffsky