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.”

Pop-Ins Become Fractures

A construction worker could have used a grinder on those corners to essentially buff out the cracks. But no one did. And then the welding started. “When you’re making a weld, you’re making steel. You’re melting the base metal, the electrode, and it’s a liquid that has to cool and solidify,” Frank says. “When it does that, it shrinks.”

Or at least, it wants to. But as the metal solidifies, it’s stuck to surfaces that can’t shrink. The weld pulls on the metal around it. Here, that extra stress put tension on the microcracks, creating “pop-in fractures” about an inch long and ⅜-inch deep. “They’re cracks that just jump in and then stop,” Frank says.

Those pop-ins turned into all-out fractures. In one girder, the stress was so great that both sides of the flange fractured. “It’s a big noise, a big bang. You release a lot of energy,” Frank says. On the other girder, one side of the flange cracked but the other side stayed intact—and essentially bore the entire load of the roof above. It dropped about an inch.

Steel doesn’t usually break this way. When it fails, it usually deforms; despite its apparent solidity, steel is actually ductile, which means it sort of squishes, like when you bend wire. In this case, after the pop-in cracks formed, the crack seems to have moved a little, and then it tore open. The tear then became what’s called a brittle fracture, a break that Vecchio, using a scanning electron microscope, says he could see along the surfaces of the crystals that comprise the metal. Brittle fractures typically require little energy to spread; once they start, they keep going.