How engineers design buildings to survive earthquakes

HostWe usually think of a good building as something rock solid that stays put no matter what. But in a big earthquake, being stiff and unmoving is actually the worst thing a building can be. It's strange to think about, but the more a skyscraper can act like a blade of grass in the wind, the safer it is. How do you even start to plan for the ground literally moving out from under a house?

GuestYou have to change how you think about strength. For a long time, the plan was just to make walls thicker and use more stone. But the earth is stronger than any wall we can build. If you try to fight that much power with something stiff, the building just snaps like a dry twig. Modern engineering is more about being clever than being tough. We try to find ways to soak up that energy or stay out of its way entirely.

HostSo you're saying we want the building to be a bit floppy? That sounds like it would be a nightmare to live in on a normal day.

GuestWell, not floppy like a noodle. But we want it to have some give. Think about a paperclip. You can bend it back and forth a few times before it breaks. We use steel in buildings because it can stretch and bend without falling apart. We design the parts that don't hold the weight to be the first things that bend. They take the hit so the main columns stay standing.

HostWait, so you're basically building parts of the house that are meant to break?

GuestNot break, but change shape. It's like the front of a car. You want that part to mash up during a crash because that soaking up of the force keeps the person inside safe. In a building, we want those bendy parts to soak up the earthquake waves so the frame stays up.

HostBut if the building bends and stays that way, is it not ruined anyway? If I come home and my apartment is leaning five degrees to the left, I'm probably not going back in.

GuestThat's a fair point. For a long time, our only goal was to make sure the building didn't fall on anyone. We didn't care if the building was trash afterward. But now, we're trying to make buildings that can heal themselves. We use these massive steel cables that run through the middle of the walls. When the building sways, the cables stretch, and then they pull the building back to its original spot. It's like a giant rubber band keeping everything centered.

HostOkay, that makes sense for the structure itself. But what about the whole ground moving thing? Is there a way to just not be attached to the ground when it starts shaking?

GuestThat's actually one of the best ways we have. We call it base isolation, but you can think of it like putting the whole building on a set of big, heavy-duty skates. We put these layers of rubber and lead between the building and its foundation. When an earthquake hits, the ground underneath can slide back and forth several feet, but the building just kind of floats on top of those pads. It barely feels a thing.

HostIf that works so well, why is every house in a shaky city not sitting on rubber pads? It sounds like the perfect fix.

GuestIt's expensive. You have to dig a huge hole, put in these massive pads, and make sure all the pipes and wires coming into the house can stretch too. You can't have a stiff water pipe if the house is going to move three feet away from the street. So we usually save that for really important buildings, like hospitals, where we need things to work the second the shaking stops.

HostI have also seen those giant weights at the top of tall buildings. I think there's a huge gold ball in a skyscraper in Taiwan. Is that just a fancy weight to keep it from tipping over?

GuestIt's a counter-weight. Imagine you're standing on a bus and it suddenly brakes. You lean forward, right? To stay upright, you have to shift your weight back. That giant ball does the same thing. When the wind or a shake pushes the building one way, that heavy ball swings the other way. It uses its own weight to pull the building back toward the middle. It stops the building from swaying too much and making everyone on the top floors feel sea-sick.

HostIt feels like a lot could go wrong there. Having a three-hundred-ton ball swinging around inside your roof seems like it would be its own kind of danger.

GuestIt's held in place by massive shock absorbers. They turn the motion of the swinging ball into heat. So the building isn't just swinging wildly; it's actually dumping all that earthquake energy into those absorbers. The whole thing is a giant machine for turning a disaster into a little bit of warmth.

HostSo we have bendy steel, buildings on skates, and giant swinging balls. Is there a limit to this? Could we build something that could survive any shake the earth throws at it?

GuestThere's always a limit because we can only plan for what we have seen before. But our tools are getting much better. We now use computers to test thousands of different types of shakes on a virtual building before we ever pour the concrete. We can see exactly where the weak spots are before the ground ever moves.

HostWe're even seeing new ideas where buildings use smart sensors to feel the shake and change how stiff they're in real time to match the waves.

GuestEven with all our math, we still test these things by putting whole houses on giant shake tables to see exactly where they crack.

HostThe ground is going to move no matter what we do, so we just have to make sure our walls know how to move with it.