And how can one be sure of the level of earthquake a building can withstand?
In: 2
In practice: Most of our earthquake design practices come from going around after a major earthquake and looking at what worked well and what failed, then enacting policies to do the things that worked and don’t do the things that didn’t.
In the US, for example, there was a major rewrite of seismic design practices following the 1994 Northridge earthquake because a lot of things they thought would work turned out to not work.
In high seismic zones a lot of the strategies for ensuring buildings will survive earthquakes are based around:
1. Provide designs that allow some level of movement and flexibility so that forces from the earthquake are lowered due to the building being able to “roll with the punches” as it were.
2. Provide designs that allow ductile failure. That is you make sure that things that will “break” are over-strengthened by enough that something else will “bend” first. For example a beam – if the connection to the column fails that’s bad because the beam will fall and possibly collapse a lot of the rest of the building too, but if it bends then it’s not such a big deal. A bent beam can be repaired later on.
3. Similar to above – you make sure beams will fail (in a bendy way) before the columns fail, because a column failing can be maximally catastrophic for a building.
4. Provide dampers. These are basically pistons filled with goo, or some similar system, that are tied into the building stability system and can absorb energy from an earthquake in a controlled way rather than have the building take the energy into things that can break.
5. You can do things like “base isolation” where you have the building built on a foundation that is then supported by a secondary foundation system, with the ability to move a little. This lets the ground move underneath the building without moving the building much itself. If you think this sounds expensive you’d be right. By the time you’re into this you’re looking at very expensive work in very highly seismic areas.
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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