What went wrong with steel girder?
It’s too early to know why the structural steel above the Transbay Transit Center’s bus deck is cracked, but here are a few starting points on what might be involved. Q: Is such a discovery typical, like the usually innocuous cracks that emerge in a stucco wall? A: No. “A structural steel beam should never crack,” said Joe Maffei of Maffei Structural Engineering in San Francisco. “It should not crack.” Q: This is obviously bad news. How bad might it be? A: “Any kind of a crack in a structural steel beam has potential implications,” Maffei said. “You need to always be careful about cracks because they can grow. And their fail-
ure can be sudden.” Q: What might be the cause? A: Two possible culprits are a fabrication problem — something went wrong when the beam was manufactured — or the beam is supporting more weight than it’s designed to bear.
If it’s a manufacturing problem, there could be hidden stress in the beam. These giant slabs of metal often consist of plates of steel welded together, and one area inspectors are sure to examine is any hint of imperfections in the steel.
Mark Zabaneh, who heads the Transbay Joint Powers Authority, offered scant details at his news conference Tuesday evening on the problem, but he did say the crack had been found near a weld.
“The process of welding can cause stress,” Maffei said.
If the weight above the beam is the problem — remember, there’s a 5.4-acre park on the roof — one indication could be the configuration of the crack.
“If there’s a horizontal crack running the length of the beam, there could be too much weight on the beam,” said David Friedman, a senior principal at Forell/Elsesser Engineers. Q: Could the crack mean that the transit center is sinking and tilting, like the Millennium Tower next door? Or could the two problems somehow be linked? A: Not likely. The transit center’s foundation is a 5-footdeep, 60,000-ton concrete mat. Work on the foundation was completed in 2014, and there has been no evidence since then of ominous settling. Q: What about the cracks and erosion in the rooftop park’s walkway? Are they connected to the cracked beam? A: Again, not likely. The pathway consists of decomposed granite, and has nothing to do with the structure. Q: How will they fix the problem? A: It depends, of course, on the source of the problem. But, according to Friedman, “it’s easier to strengthen a steel beam than a concrete one.”
If the crack is an isolated problem, the beam can be reinforced by welding additional steel panels to its underside. Q: What will the inspections involve? A: There are any number of tests, such as taking samples of steel from the beam — without weakening it, of course — and seeing how much strain they can withstand. But there are also straightforward measurements of the crack’s length and depth to be done.
“If it’s a horizontal crack running the length of the beam, then the problem could be the load,” Friedman said.
And if that’s the case? “I’d start looking at similar beams in similar (stress) conditions” — which can take a lot of testing and time.