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Report: Work Error Missed in CA Transbay Center

Monday, March 25, 2019

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San Francisco’s Transbay Joint Powers Authority Executive Director Mark Zabaneh revealed at a March 14 meeting that multiple inspections failed to miss a lapse in the building process in the Salesforce Transit Center.

Had the error been caught, the steel beams never would’ve cracked, Zabaneh said, resulting in the closer of the $2.2 billion structure.

What Happened Originally

On Sept. 25, 2018, 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

San Francisco’s Transbay Joint Powers Authority Executive Director Mark Zabaneh revealed at a March 14 meeting that the multiple inspections failed to miss a lapse in the building process in the Salesforce Transit Center.

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 Transbay Joint Powers Authority 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.

The Fix

In December, the report from LPI Inc., a metallurgical lab that investigated the issue, concluded that the problem started with weld access holes that were created in the beam during the joining process.

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

Also at that time, Tomasetti presented a fix that calls for bolting 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 said that it was in “general concurrence” with the proposed fix, and on Jan. 10 the authority confirmed that it is in the process of gathering materials for the approved scheme.

heyengel / Getty Images

In December, the report from LPI Inc., a metallurgical lab that investigated the issue, concluded that the problem started with weld access holes that were created in the beam during the joining process.

The repair is slated to be made not only to the girders with cracks, but to a pair of identical girders along First Street that have not fractured.

Work on the fix started in February 2019, when crews replaced hydraulic jacks with the shoring system to allow the TJPA to reinforce the girders on the bus deck.

The TJPA said in a press release that repairs are scheduled for completion the first week of June, though a reopening date has not been determined. The TJPA also noted that the review of documents is still ongoing.

What Now

Zabeneh reported that four layers of inspections—conducted by steel fabricator Herrick, steel installer Skanska, Webcor-Obayashi and the authority’s quality-assurance contractor Turner Construction—failed to detect a failure of a necessary part of the process.

Grinding the edges of the weld access holes smooth before welding can eliminate micro-cracking; Zabeneh said that had that failure been caught, the micro cracks never would have turned into the large fissures that compromised the integrity of the structure.

“The execution was not done properly, and that is something we are looking into,” Zabaneh said Thursday after an authority meeting. “It’s an area of great concern for us.”

In addition, the peer review of the tens of thousands of construction documents is reportedly about 85 percent complete as of March 14 and no additional problems have been identified.

   

Tagged categories: Good Technical Practice; Health and safety; Inspection; Mass transit; NA; North America; Ongoing projects; Public Transit; Safety

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