SAN FRANCISCO — Defects that occurred during fabrication, along with holes cut into the steel, are likely what caused two structural steel beams to crack just six weeks after the $2.2 billion Salesforce Transit Center opened to the public, officials said Thursday.
“It occurred very rapidly, and a lot of energy was released,” said Robert Vecchio, the president of New York-based LPI, Inc., which conducted a series of tests on samples taken from the steel.
The center closed in September after workers found the first crack in a four-inch-thick steel beam while installing ceiling panels. Authorities closed the center several hours later out of an “abundance of caution,” they said. A subsequent investigation revealed the second crack in an adjacent beam, both of which are in a section of the building above Fremont Street.
The cracks originated in an area where crews had cut “weld access” or “weld termination” holes. It’s unclear which type of hole the fabricators cut into the steel because they are not drawn into the shop designs. And the difference is critical.
A weld access hole has much more stringent requirements in the building code, said Robert Hazleton, the president of Herrick Corporation, the Stockton-based company responsible for fabricating the steel beams that cracked. He’d like the holes referred to as “weld termination” holes.
“It may sound like a small thing, but it does change how you finish the inside of the hole,” Hazleton said. “There are less specific requirements for a weld termination hole.”
Weld access holes allow workers access to the beam so they can complete the weld, said Ashwani Dhalwala, a principal of AEC Solutions who has worked extensively on the issue of fractures in steel. Weld termination holes are used in areas where girders are joined together with a perpendicular piece of steel, called the web, in order to reduce stress, which is concentrated where the pieces interest. It’s a way to provide continuity between the pieces and reduce stress, he said.
“If you have a sudden discontinuity, then you have very high stressors,” Dhalwala said.
The holes were added after shop designs were submitted for approval. So, Herrick crews fist built a set of girders without the holes and then had to build a new set of girders with them included, he said.
“Why they were added, that’s more of a design issue than a fabrication issue,” Hazleton said. A representative from Thornton Tomasetti, the design firm, declined to explain the purpose of the holes.
Flaws in the steel developed during fabrication and then were exacerbated when the steel was reheated as crews cut into the steel, Vecchio said. Tiny cracks formed in the brittle metal. And then, when the building opened and buses began rolling and people began flocking to the roofdeck garden above, the added movement and weight further stressed the steel, causing it to crack, he said.
But, Dhalwala said it’s important to consider the potential for fractures in the design itself. The building code doesn’t mandate a complete fracture analyses, though it does address some conditions as part of the design specifications. For large buildings, especially ones in earthquake-prone zones, fracture analysis is even more important, he said.
“Very few structural engineering professionals outside academic institutions are familiar with fracture mechanics and analysis,” Dhalwala said. “There is a fundamental gap in our design profession and in the engineering of our buildings that does not take this into account.”
There’s no estimation yet how long the repairs will take, and there is still more testing to do. The nature of the failure is prompting the contractors, as well as a peer review panel, to investigate other parts of the building that might be susceptible to cracking, as well, said Michael Engelhardt, a professor at the University of Texas at Austin, who is chairing the peer review panel.
But, the Transbay Joint Powers Authority, the public agency in charge of building and maintaining the terminal, expects to have a plan for the repairs and an estimated reopening date in January, said Mark Zabaneh, the authority’s executive director. That plan will include bolting steel plates onto both sides of the girder to reinforce it.
The center has remained closed as crews first worked to shore the structure with a series of jacks to take pressure off the weakened beams. Riders since then have been using the temporary terminal at Main and Howard streets established during the 10-year construction of the new transit center.
Skanska USA, a subcontractor of Webcor-Obayashi that was responsible for overseeing the procurement of steel at the transit center, filed a lawsuit in May against the authority, alleging the authority mismanaged the project and provided flawed or incomplete documents, causing delays and resulting in extra work for which Skanska was not reimbursed.
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