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Final Report Released on CA Transit Center Cracks

Tuesday, March 3, 2020

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The peer review that was tasked with investigating the girder fractures that closed San Francisco’s Salesforce Transit Center in 2018 is now complete, and was sent to authorities last week.

The report details what went wrong with the $2.2 billion structure and recommends certain code changes for the American Institute for Steel Construction and the American Welding Society.

Some Background

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.

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.

Fullmetal2887, CC-BY-SA 4.0, via Wikimedia Commons
The peer review that was tasked with investigating the girder fractures that closed San Francisco’s Salesforce Transit Center in 2018 is now complete, and was sent to authorities last week.

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.

The peer review was also initiated at this point, overseen by the Metropolitan Transportation Commission.

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.

Also at that time, the transit center’s engineer-of-record, Tomasetti, presented a fix that called 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, 2019, the authority confirmed that it is in the process of gathering materials for the approved scheme.

That repair, which is now completed, was reportedly made not only to the girders with cracks, but to a pair of identical girders along First Street that had not fractured.

In February, the TJPA put a June completion date on all the fixes.

In looking closer at the causes for the fissures, TJPA Executive Director Mark Zabaneh revealed at a March 14 meeting that multiple inspections failed to miss a lapse in the building process. Had the error been caught, the steel beams never would’ve cracked, Zabaneh said.

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.

Michael Pearce / Getty Images

The report details what went wrong with the $2.2 billion structure and recommends certain code changes for the American Institute for Steel Construction and the American Welding Society.

Skanska, along with Webcor and Herrick, disagreed with TJPA’s opinion that a construction error was the root cause of the fissures.

Regardless, the repairs were finished in early May and the Center was reopened last summer.

The Report

The peer review reiterated the cause for the fractures, which was covered in the third-party LPI review. The factors that led to the fractures are summarized in the report as follows:

  • Thermal cutting of the secondary slots in the girder bottom flange produced microcracks in the radii of the reentrant corners of the secondary slots at mid-thickness of the bottom flange plate near the bottom flange CJP groove weld.
  • The thermal cutting process resulted in microcracks in a thin layer of hard martensitic material on the surface of the secondary slot. The depth of the microcracks were several hundredths of an inch.
  • Some of the microcracks later became larger “pop-in” cracks at the mid-thickness of the girder’s bottom flange plate, with a depth of 3/8-inch.
  • The pop-in cracks likely occurred due to tensile stresses generated by weld-area shrinkage from production of the complete joint penetration groove weld in the bottom flange and they were facilitated by the close proximity of the secondary slot reentrant corners to the CJP groove weld on the west end of the secondary slots.
  • Sometime after the girders were erected, brittle facture of the bottom flange occurred, initiating at the pop-in crack.

According to the report, many Charpy V-notch (CPN) tests were conducted on the samples of the steel from the fractured girders.

Because of the studies, the review recommended multiple amendments to AISC 360 Specification for Structural Steel Buildings requirements for “Built-Up Heavy Shapes,” including:

  • That AISC 360 reevaluate the standard quarter-thickness sampling location for CVN testing, considering the low mid-thickness CVN values found in the fractured flanges of the TTC girders. “Serious consideration should be given to changing the require sampling location to mid-thickness for details that may be susceptible to brittle fracture”;
  • That AISC 360 reevaluate the required number of CVN samples; and
  • That AISC 360 provide guidance on establishing CVN test temperatures for structure exposed to ambient temperatures, either during construction or during the service life of the structure.

The authors also made several other recommendations, including a few to the AWS Structural Welding Code-Steel (AWS D1.1), including a clarification of the term “weld access hold” to include only those details required to accommodate welding.

“The authors recommend that AISC, working on conjunction with industry stakeholders, consider developing a risk assessment approach for brittle fracture,” the report concluded.

Finally, the authors made recommendations on dealing with brittle fractures in general:

“Such an approach could be presented in a design guide and/or incorporated into AISC 360 and other appropriate standards. AISC should consider “IIW Recommendations for Assessment of Risk of Fracture in Seismically Affected Moment Connections” as a potential model for a broader-based brittle fracture risk assessment approach”

   

Tagged categories: Completed projects; Cracks; Health and safety; Maintenance + Renovation; NA; North America; Safety; Steel; Terminals

Comment from Harry Gendel, (3/3/2020, 10:02 AM)

Consider quality control testing for typographical errors above.


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