system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the concrete not seem to be cracking, crumbling, or disintegrating when a skyscraper or a bridge swinging or wobbling in the event of an earthquake?
gpt
Concrete is famously quite weak in tension, i.e., it doesn't bend. But what can bend are the spaces between concrete panels and pillars. When you see video of a bridge bending and flexing, it's able to do that because it has been designed to be able to do that, with flexible joints between the concrete members. On the other hand, a building swinging or wobbling might well be vibrating beyond what its design can handle. In that case, you might not see visible cracks forming, but micro-fractures are likely forming throughout the material that could lead to collapse. This is one of the reasons we add steel rebar to concrete - steel is a lot better in tension and will hold the mass of concrete together even if some micro-fractures and cracks form in it.