San Francisco’s new $2.2 billion public-transport hub has been closed until at least the end of next week after a second crack in a supporting beam was discovered during an overnight safety inspection. The city’s new Transbay Transit Center opened last month but was closed Tuesday afternoon after a tear was spotted in a beam that helps hold up the complex. The initial tear is roughly 2½ feet long and 4 inches deep through the bottom of a 60-foot-long beam; the second is reportedly slightly smaller. The Transbay Joint Powers Authority, which built and operates the transit center, said Wednesday that it didn’t know the causes of the cracks. “We will not open the transit center or Fremont Street until we are certain the issue is 100 percent rectified,” said Mark Zabaneh, TJPA executive director.