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Preliminary Fix Proposed for Transbay Center

Tuesday, December 18, 2018

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Preliminary findings as to the cause of the cracks in San Francisco’s $2.2 billion Transbay Transit Center have been released, along with a suggested fix.

Last week, the findings were presented to the board of the Transbay Powers Authority by Robert Vecchio, president of LPI Inc., a metallurgical lab that investigated the problem. A repair was also proposed to the board, by the transit center’s engineer-of-record, Thornton Tomasetti.

What Happened Originally

On Sept. 25, workers discovered a cracked steel beam in the third-level bus deck of the Transit Center, just six weeks after the structure opened to the public. The next day, another fissure was found on an adjacent beam.

Fullmetal2887, CC-BY-SA 4.0, via Wikimedia Commons

Preliminary findings as to the cause of the cracks in San Francisco’s $2.2 billion Transbay Transit Center have been released, along with a suggested fix.

The beams are part of the support system for the 5.4-acre rooftop garden and park, which includes a 5-foot layer of soil.

According to the Engineering News-Record, the bottom-flange cracks are near the 8-foot-deep midspan of each shop-welded girder. In the structure, the hanger plate slots through the bottom flange.

General contractors Webcor and Obayashi managed the center’s construction, which lasted from August 2010 to August 2018. The architect was Pelli Clarke Pelli, with Thornton Tomasetti serving as structural engineer.

In early October, the TJPA confirmed that the shoring systems for the cracked sections were complete, making way for technicians to get in to take samples for analysis at LPI. Testing included scanning electronic microscopy, Charpy V toughness tests, Rockwell hardness tests, tensile tests, fractographic analysis and metallographic analysis.

A peer review was also initiated, overseen by regional transportation agency MTC.

What Now

At Thursday’s meeting, Vecchio presented the preliminary report, which conclude that the problem started with “weld access holes” created in the beam during the joining process.

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

“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.”

The report states that four girder flanges were sampled, three of which contained full flange-width fractures. Analysis and testing suggests the probable cause of the fractures were cracks in the girder weld access hold radii prior to service.

The report says:

  • Initially, shallow surface cracks developed during thermal cutting of the weld access holes in the highly hardened and brittle martensitic surface layer;
  • Thereafter, larger pop-in cracks formed in two of the four flanges, potentially during butt welding of the flange plates;
  • Black, tenacious, high-temperature oxide was present on both the shallow surface cracks and the larger pop-in cracks, confirming that both crack types formed at elevated temperatures;
  • The fracture origins were located in the mid-thickness of the flange where low-fracture toughness, as confirmed by CVN toughness testing, provided little resistance to rapid, low-energy, brittle fracture;
  • CVN testing was performed on all flange samples at the top, 1/4 depth, mid-thickness, 3/5 depth, and bottom; 1/4 depth CVN results were found to be consistent with the project specification and girder plate mill certifications; and
  • Further material testing and stress analyses are currently underway and will be considered in the final root-cause assessment.

Vecchio pointed out to the board that they still need to find out where all the stresses came from and whether or not a flaw elsewhere might have put more pressure on the microcracks.

The San Francisco Examiner reports that Robert Hazleton, president of Herrick Corporation, the company that cut the holes, said that the holes were not in the initial design plans but added later on an approval and subsequent RFI.

When asked if the proper protocols were followed, Mark Zabaneh, TJPA’s executive director said, that his understanding is that the specifications complied with the code.

"There are a lot of unanswered questions,” Zabaneh said.

In addition to the preliminary cause findings, an early fix proposal was brought to the board as well.

ENR reports that the fix calls for bolding 20-inch-wide steel cover plates above and below the area around each fractured bottom flange, similar to a “14-foot-long double splint.” The independent peer review panel is in “general concurrence” with the proposed fix, which came from Tomasetti.

The panel is still completing its detailed review and is slated to make its official recommendation soon.

   

Tagged categories: Health and safety; Maintenance + Renovation; North America; Safety; Structural steel; Terminals

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