Diagnosing the steel failure in the SF Transit Center

From the article:

The investigation focused on the 2-by-4-inch “welding access holes” that had been thermally cut into the beams. Vecchio displayed a photo showing the red oxidized color of the metal around the holes, indicating that microscopic cracks formed due to the intense heat generated by an acetylene welding cutting torch. He pointed out the buildup of martensite, a brittle substance with a crystalline structure, which formed around the cuts as they cooled.

Vecchio explained the high hardness of the structural steel made it prone to microcracks. But at the same time, he emphasized, the metal had been tested prior to welding and met all specifications and requirements. The problem was that the martensite deposit around the cuts hadn’t been ground smooth and polished after the welds had cooled. The martensite produced microcracks, which eventually grew into brittle fractures.

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For the 1st Street girders, which did not fracture, the thermally cut holes were made after the main welds were made. There were no small cracks when the buses started to roll. This minor detail proved to be critical.

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Four levels of inspection [...] had missed the detail of the unpolished microcracks. After a March board meeting of the TJPA, Zabaneh said, the holes “were not installed to code in both dimensions and treatment, [meaning] they were not ground to bright metal finish… Had the weld access holes been ground per code, fissures would not have taken place and the girder’s bottom flange would not have been cracked.”