We were wondering if, when a bridge undergoes repairs of any kind, it’s ever standard practice to employ accelerometers, vibrational analysis, and strain gauges to measure the loads and vibrations that the bridge experiences before and during the work.
The more I learn about the bridge collapse the more sure I feel that some aspect of the construction project is going to be shown to be the triggering event — not necessarily the reason the bridge collapsed at all, but the reason the bridge collapsed this week instead of last year or next year.
They were resurfacing. That’s not supposed to affect the structural integrity because the top of the decking isn’t supposed to be load bearing. But perhaps it was bearing some load. Could there have been an unseen failure in the steel that had shifted a compressive load to the concrete decking — like wallpaper that holds together a wallful of rotten lath? There were piles of aggregate here and there on the bridge, I remember seeing them. How did that heavy, static load change the bridge? The workers were using jackhammers. Did the vibration propagate some crack, somewhere, far faster than it might otherwise have? Some of the lanes were closed. Had the shift in traffic imbalanced the bridge?
Ordinary people have a lot of questions right now about fracture mechanics and metallurgy. I heard a woman call into MPR’s Mid-Morning and ask: I’m not an engineer, but in my experience, things bend before they break. Why didn’t the bridge bend before it broke? I saw a question on buzz.mn — Stresses triple at the edge of a hole? Is that a little hole or just big holes? I could pull my brittle-frac-mech book off the shelf and start rambling, but the working bridge engineers are around to answer those questions, so I won’t. I was a little relieved to find an op-ed by Henry Petroski in the LA Times — I have long appreciated his several books on design and failure and knew he’d weigh in eventually.
Milo made a design with rubber bands on a peg board yesterday and told me, "It’s a bridge. Here’s where you launch the cars." Who knows what the kids will remember from all this.
When I first watched the video, apparently taken from a security camera at the lock and dam, I thought: I am watching something impossible. Not that the bridge was falling, not that the disaster itself was truly incredible, but I thought — that first viewing — that I was watching the central span drop straight and flat, as if both ends had been severed at exactly the same time. That can’t be right. I watched again more closely, and I could see that, from one frame to the next, the end closer to the camera had dropped a tiny bit farther. Okay then. No laws of physics broken here. Still, as I pondered the way the structure collapsed, it seemed marvelous how it failed, a lucky save. It did not fail on one side and roll, dumping the cars into the river before falling down on top of them. It did not fail at one end and make a ramp for the cars to pile up behind each other in a screaming slide into the water. It fell nearly flat and straight, and the steel understructure became a crumple zone, and the cars on top of the span seem not even to have bounced. Look at this picture taken by survivor Kristin West’s cell phone seconds after the collapse. The cars are still aligned in the lane. Look at the man standing next to his car gazing upwards. Can you imagine?
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