That’s a very good question… One would think that would have had to start showing signs of cracking before it let go. I’m not sure how often parks do in depth inspections like that but you would think someone should have caught it before it let go.

Reality is that no one was likely in any danger because the neighbouring supports would still be enough to handle it (in the video you can see very little movement) for a while but I’m still really surprised to see it get to that point.

Fracture. Usually embrittlement in the weld zone. There are a couple of ways it can be introduced/ manifest, but the cyclic nature of the loading will result in growth of a flaw. Once the geometry of the connection is altered, the entire joint mechanics can get thrown off.

Note- I’m a structural engineer, but not an expert in fracture mechanics or weld metallurgy. I’ve seen failures like this but I know just enough to know when it’s beyond my competency. The details here are something I lean on other specialists for. This is definitely a juicy one.