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Originally Posted by Wamy Einehouse
Tacoma Narrows Bridge collapse. Always was amazed by how the road bends like it was made of rubber.
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It's a suspension bridge so it's mostly just hanging in place by gravity and it's own weight. You can see there is a long expanse of area in which it is not fixed from below. What happened was the winds came at the same interval of the bridges natural period of vibration, so the 2 worked in unison and forced the bridge to wobble until it broke. Similar to how we gain momentum on a swingset.
There's a great book called "Why Buildings Fall Down" by a legendary structural engineer named Salvatori. He talks about this bridge in detail. This book helped me a ton in passing my 2 structural exams for my architecture license.