How are those toothpick skyscrapers in Manhattan earthquake proof?
They aren’t. Especially the older ones. Newer skyscrapers are built to be earthquake *resistant* because of newer building codes but there’s no such thing as an earthquake proof structure.
Source – I literally studied earthquake resistant construction in school.
It works like this, the two most important factors in a building surviving an earthquake are the building’s height, and the type of shaking involved in the quake. Some earthquakes make small, fast shaking, others long, slow shaking.
Have you ever seen the thing where a certain frequency of sound can make a glass shatter? Imagine that but with buildings, the right frequency earthquake, combined with the right building height will determine if they fall. Believe it or not, some earthquakes will topple small 1-5 story buildings and leave the flimsy skyscrapers unharmed, others will knock down mid-height buildings, others only the tallest buildings.
So what they’ve invented is basically a big heavy weight they install in the top of the building. Sort of a massive pendulum the size of a small building itself. They call this a “tuned mass damper” and it basically makes the tall building act like a small one if it needs to. TMD’s are in almost all the super structures you see in the modern world and they are the heros in your example.
Hooray for tuned mass dampers! I can never hope to explain it as well as [Practical Engineering ](http://youtube.com/watch?v=f1U4SAgy60c&ab) did, but here goes anyway.
The tuning part is easy. Every pendulum has a frequency that it likes to oscillate at given by a fairly simple function using only length and gravity as variables. Gravity tends not to change noticably, so changing the length changes the frequency. A building is also a pendulum, just inverted. It’s well-anchored at the bottom, but wind (or earthquakes) can cause the top to move because steel and concrete are not perfectly rigid materials. All materials have some amount of flex. So you tune the pendulum to the same frequency as the building. This makes it fairly easy for the building to give some of its energy to the pendulum through a mechanism called resonance. The same reason armies must break step when marching over a bridge. And the same reason pushing a kid on a swing should happen at the same frequency as the swing happens.
The damping part is still not the worst, but it’s more complex than the resonance part. Once we’ve transfered energy to the pendulum, we need to bleed it off slowly so as not to cause too much deceleration (just as destructive as too much acceleration. As they say, it’s not the fall that kills you, but the sudden stop at the end). So using any number of methods from friction brakes to hydraulic dampers, they add resistance to motion which turns the energy into heat. Under damped systems can’t get rid of energy as quickly as the earthquake can add it, meaning the oscillations get worse until it bends beyond the yield point. Overdamped systems brake too hard, basically causing whiplash for the building which is also bad. So right in the middle is a sweet spot called critically damped. And that’s what they’re going for.
A lot of other people have already gone into the technical answers and special system that exist.
But I feel like another important point is to mention that they are not earthquake proof. The codes that builders have to follow change depending on where the building is located. Because places like Japan or California are way way way more likely to have a big earthquake than New York City.
So the codes are just different.
Because of that, if a Huge earthquake hit NYC, chances are many of the buildings won’t be able to manage it.
But NYC doesn’t get major earthquakes, so it’s not a huge problem.
Think of a tall tower made of blocks. If you try to push the tower, it might fall down because the blocks are not strong enough to hold each other up. But if you use really strong and heavy blocks, and if you stack them carefully, the tower will be able to withstand a lot of pushing and shaking.
The same idea applies to skyscrapers. They are built using very strong materials like steel and concrete, and the design is carefully planned to make sure the building can sway back and forth during an earthquake without breaking.
In addition, engineers also make sure that the building’s foundation is strong and secure. This is like the roots of a tree – they hold the tree firmly in the ground and prevent it from falling over. By making sure the foundation is strong, the engineers can make sure that the skyscraper won’t fall over during an earthquake.