Weld process ‘very likely’ behind cracks in Transbay Transit Center

After Thursday’s meeting of the Transbay Joint Powers Association board, we may finally have an idea what’s behind the cracking in the faulty girders that have kept the new $2 billion-plus Transbay Transit Center closed since September—although the “root cause” remains under investigation.

According to a report delivered by Robert Vecchio—president of LPI Inc., the metallurgical lab tasked with investigating the building’s troubles—it’s likely that the welding process did most of the damage to the fractured girders.

Apparently the problem started with the “weld access holes” (also known as “rat holes” in industry jargon) created in the beam during the joining process.

“We identified very shallow microcracks due to the thermal cutting,” Vecchio told the board. “It’s very likely the subsequent welding process caused some of those micro cracks to pop into larger defects.

Courtesy LPI

“Once the structure went into service the residual stress and loads [from bus traffic] were enough to pop those defects into fractures that went across the entire girder.”

But when SFMTA Director Ed Reiskin asked whether this meant the weld holes were ultimately to blame for the cracking, Vecchio demurred, noting that while the holes were “a significant factor”,” LPI’s testing “haven’t concluded what the driving force was.”

For example, Vecchio pointed out that they still need to determine “where did all of those stresses come from?” and whether some flaw elsewhere might hypothetically have put undue pressure on the microcracks and caused the “popping.”

The city—and commuters—will have to wait for additional testing and then a review by a panel of experts overlooking LPI’s work before an official cause or potential fix are determined.

Michael Hursh, general manager of AC transit, asked if similarly treated beams elsewhere in the terminal might also be in danger.

Vecchio confirmed, “We are looking at that now.”

Tuesday’s meeting of the San Francisco County Transit Authority confirmed that inspections have yet to find fissures elsewhere in the structure.

TJPA Executive Director Mark Zabaneh noted that while “today marks a significant milestone” in the process of figuring out what went wrong, “we do not yet have a date to reopen the facility.”

• TJPA Board, 11.13.18 [SF Gov TV]
• LPI Presentation [TJPA]
• NY Bridge Maintenance Report
• Transit Center Answers In January [Curbed SF]