‘Zero evidence’ of flaw in transit center design
The investigation into the pair of cracked girders that forced the closure of San Francisco’s newly inaugurated, $2.2 billion Transbay Transit Center in late September is now focused on the composition of the steel and cuts or other rough spots that may have put stress on the beams.
But the building’s overall design does not appear to be an issue, according to a top expert called in to review the work.
That’s good news for the troubled transit hub. A design flaw might have meant the beams couldn’t support the weight of the building’s bus deck and three-block-long roof garden, and required a major redesign that could have shuttered the center indefinitely.
“There is zero evidence of that being the problem,” said Mike Engelhardt, a structural engineering professor at the University of Texas at Austin. “It seems to be a very fine design.”
Engelhardt chairs the fivemember peer review panel hired by the Metropolitan Transportation Commission to
look over the shoulder of the Transit Center’s own team of experts, which is analyzing the reason for the cracks.
In late September, just six weeks after the transit hub opened, workers detected the cracks on the ends of a pair of 85-foot-long I-beams that support the terminal where it passes over Fremont Street. No fissures, however, were found in a nearly identical pair of girders that cross over First Street.
The transbay center’s problems are reminiscent of the broken bolts that turned up on the new $6.2 billion eastern span of the Bay Bridge. Experts scrambled to determine whether there was an underlying design flaw before concluding that designers had relied on “off-the-shelf ” galvanized bolts that were too brittle for the job — a problem only made worse when they were exposed to seawater and other elements.
While Engelhardt said it’s too early to draw conclusions, he described the cracks as “brittle fractures” typically caused by either bad steel, a rough spot or other welding defect, combined with high stress levels in the affected area.
He likened the beam problem to a piece of glass with a line etched in it that snaps more easily when under stress.
Chunks of steel — some weighing several hundred pounds — have been removed from the two suspect beams and either flown or driven to a testing lab in New York for analysis. Those tests began Nov. 5 and could last two or three weeks.
There is also some focus on the 4-inch-by-2 inch-weld access holes — designed to make it easier for workers to access tight welding spots — that were cut into the 4-inch-thick steel at the foot of the I-beams.
The rectangular shape of the cuts themselves appears to be of particular concern.
Memos among the steel supplier, fabricator and project inspectors dating to 2015 appear to flag the weld holes as a source of confusion and possible concern during the fabrication process. The holes were not included in the original design specifications, but were added by the fabricator after the beams were manufactured.
In all, there are eight weld access holes — one notched into each end of the two girders spanning Fremont Street, as well as the two girders over First Street.
However, one engineering expert with firsthand knowledge of the review, but who wasn’t authorized to speak for the record, said a close visual inspection showed no evidence of “any fabrication or workmanship issues” involving the weld access holes. In other words, the cuts might not have triggered the cracking, but they still could have weakened the beams and made the problem worse.
Mark Zabaneh, executive director of the Transbay Joint Powers Authority, which built and oversees the center, was cautious not to point a finger in any direction just yet.
“It would not be responsible to speculate about a cause without a thorough review and results of comprehensive testing of the steel beams,” he said, saying the focus at this point is on examining the steel and the welds.
“I can’t rule out anything at this point,” Zabaneh said. “But testing is always the logical first step.”
And while options for fixing the cracked beams appear to be limited, transit authority spokeswoman Christine Falvey tells us staff designers have been asked to come up with five different repair scenarios that can be “ready to go” quickly.
Even then, according to the experts we’ve consulted, any of the proposed fixes are likely to keep the Transbay Center closed for months.
In the meantime, San Francisco Supervisor Aaron Peskin — a longtime critic of the center’s $2.2 billion price tag — said the job shouldn’t end there.
“Government shouldn’t do what it always does, which is to sweep this under the rug,” he said. “We should hold people accountable.”