Monday, August 12, 2019

Up, up, and away

Yesterday they welcomed buses back to the TransBay Bus Terminal, aka Salesforce Park.

Here's a nice story in Wired with lots of pictures and fascinating details: How 'Microcracks' Undermined San Francisco's New Bus Terminal

the construction process involved cutting away 8 inches of the flange right where it needed to be strongest. “The problem was the geometry of the weld access hole,” Frank says. “It has this corner on it, and it acts as a stress concentrator.”

The holes weren’t circular—they were rectangular, with rounded edges. And those corners, probably cut with a plasma cutter, acquired “microcracks” just a few hundredths of an inch deep. The investigators know they were cut with something hot, because the surfaces of the cracks were coated with a colored deposit, an oxide that could only have resulted from exposure to high heat. “You can actually see it,” Vecchio says. “It’s a very deep red, as opposed to what regular rusting of steel looks like, which is going to be more orange in color.”

Much of this story has been told before, but the Wired article is well-written and has some great pictures from the forensic studies of the failed girder.

Some bits are quite new to me:

The ability of something—steel, in this case—to resist fracture after it cracks is called “fracture toughness.” It’s measured with what’s called a Charpy impact test, basically a very precise banging on the metal until it breaks. According to spec, the steel in the Transbay Terminal was supposed to absorb 20 foot-pounds of energy before it fractured at room temperature. It did, but testing by LPI showed lower toughness deeper inside the steel. That’s where the pop-in cracks formed

And this new information appears to suggest an explanation for one of the most tantalizing questions: why did the beams fail in one part of the structure, but not in another very similar part:

That may also explain why the girders over Fremont Street cracked, but the ones over First Street did not. “The difference was the sequence of construction,” Engelhardt says. “On First Street, the welds were made first, and the holes were made after. On Fremont Street, the holes were made first. That turned out to be the decisive difference.”

This morning, the buses were again running in the terminal.

Let's hope things continue to work well now.

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