By Roland Li

San Francisco's Transbay Transit Center closed on Tuesday during the afternoon rush hour after officials found a cracked six-and-a-half-foot steel beam that supports the rooftop park.

What is the Transbay Transit Center? The $2.2 billion publicly funded structure opened just a month ago with bus service, a rooftop park and potentially future connections to Caltrain and high-speed rail. It stretches from Beale Street nearly to Second Street, between Mission and Howard streets. The structure is officially called the Salesforce Transit Center after the tech giant, which has its headquarters next to the center, bought naming rights.

What's the cause of the crack? Unknown. Officials are still investigating and are unsure of the severity of the problem. "While this appears to be a localized issue and we have no information that suggests it is widespread, it is our duty to confirm this before we allow public access to the facility," Mark Zabaneh, executive director of the Transbay Joint Powers Authority, the public agency that owns the center, said in a statement.

What is the beam made of? The beam's steel is American-made, Zabaneh said at a press conference, but he didn't immediately have information on the manufacturer.

Is the crack related to the sinking, tilting Millennium Tower, which is next to the center and has a crack of its own? It's unclear if the transit center's crack has any relationship to the problems at Millennium Tower.

How does this affect my commute? Muni, AC Transit, WestCAT and Golden Gate Transit are redirecting their routes to the Temporary Transbay Terminal, which is bounded by Folsom, Howard, Beale and Main Streets.

Westbound traffic exiting at Fremont Street from the Bay Bridge will likely encounter massive delays on Wednesday morning. That street between Mission and Howard streets will be closed until at least noon Wednesday because the crack is near Fremont Street. This also is the exit that buses and carpools generally use to get to the carpool dropoff and Temporary Transit Terminal.

Who built the center? General contractors Webcor and Obayashi managed construction of the center, which lasted from August 2010 to August 2018. The architect was Pelli Clarke Pelli, the same firm that designed the adjacent Salesforce Tower. Thornton Tomasetti is the structural engineer.

Is this the first problem at the center? No, the center's grappling with a number of problems. The rooftop concrete walkway is already crumbling just a few weeks after opening. The center cost $800 million more than expected, in part because its security features were upgraded as construction started. A judge ruled that the Transbay Joint Powers Authority must also pay the legal fees for the developers of Millennium Tower. The developer, Millennium Partners, is being sued by homeowners and the city, and has blamed the Transbay authority for allegingly causing the sinking during construction of the center.

What's next? Zabaneh said officials are still investigating and there will be another scheduled update at noon on Wednesday.

(c)2018 the San Francisco Chronicle