It's the rhythm that does it. I posted a video maybe 3 weeks ago about a pedestrian bridge that started to sway when people were walking it across it on opening day.

They had to close the bridge because they were afraid it would collapse.


As people would walk together on it..they fell into the same rhythm side to side. They eventually figured out how they engineered it wrong and fixed it to great expense.

But no casualties.

I can't recall enough about the video or post to pull it up from memory.


That grandstand was OBVIOUSLY overloaded and the swaying in rhythm is most likely why that happened.