And how can one be sure of the level of earthquake a building can withstand?
In: 2
Two ways –
Buildings themselves are built in a system of steel and concrete and the physics of those materials are very, very understood. Skyscrapers are surprising flexible, you might not see it but the tops of skyscrapers are constantly swaying back and forth and the building is deliberately designed to allow this. Next time you’re on the 60th floor of a building, watch the window blinds – you’d notice them swinging back and forth and slapping the glass, that’s because skyscrapers are naturally very flexible and this allows to them to be very robust and durable. If you want a flexible band, you make it out of rubber, not glass.
The biggest problem in a building isn’t the swaying from an Earthquake, it’s resonance. You know how when you swing on a swing you need to pump at *just* the right time, otherwise you won’t get anywhere. That’s a process called resonance, when an energy input adds on itself in a sort of perfect timing that makes a small force and REALLY BIG FORCE. Forget an earthquake, if you don’t consider resonance in your structure you’re going to have a really bad day – check this out – [Tacoma Narrows Bridge](https://www.youtube.com/watch?v=j-zczJXSxnw) this was caused by a generous breeze, imagine what resonance during an Earthquake would do to a skyscraper…
The biggest of buildings have something called a “TMD” or “tuned mass damper”. In physics-talk “damping” means to “reduce the energy of” as opposed to “dampen” which means “to make wet”. In short, a TMD is a gigantic pendulum suspended up in the top of the building. By gigantic I mean a ball of metal bigger than your house. TMDs are essentially “anti-resonators” preventing a building from shaking itself apart during an earthquake and all the worlds biggest structures use them, including cruise ships.
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