How do buildings collapse in a downward direction during earthquakes?


Example: [](

I don’t get it. The Physics of it. And also how would one prevent this?

In: 6

>And also how would one prevent this?

They’re designed that way on purpose. Falling sideways would be worse since it might fall into a building that otherwise would have been fine.

So, maybe you’re thinking of a jenga tower, where all the blocks are stacked on each other, and pulling out one from the bottom makes the whole thing topple over on its side. The difference here is that “blocks” in this case are actually attached to each other by beams and nails and wires and plumbing.

If the blocks on the bottom get removed or crushed, the ones directly above them will start to fall down. But they are attached to the blocks all around them, and so get pulled inward as they fall. They can pull the other blocks down around them, but they don’t have the freedom of movement to fall way out to the sides.

Note how the bottom floor collapsed with the upper floors structurally stable until they slammed down. You want buildings to collapse downwards rather than sideways, but they shouldn’t have collapsed at all. Those open-floor buildings were not built to code and had upper floors extended over basically air.

The building have an internal structure of pillars, beams and load bearing walls. You can not see most of this structure at once as it is mostly hidden. When the ground moves and the innertia of the building cause it to stay still there is a lot of forces on the bottom pillars of the building bending them sideways. They are usually not designed to handle forces in this direction. At some point a few of the pillars might get damaged and are no longer able to support the weight of the building. Once these pillars bend or crumbles the weight of the building above is transfered through the beams between the floors into the adjasant pillars. This happens in milliseconds. The sudden increased weight on these pillars cause them to fail as well. You can see this in the very first frames of the collapse in the video as the first floor suddenly collapses and the other floors above it come crashing down on top. The collapse of the first floor is due to the pillars supporting it collapsing first.

We have lots of research showing how to prevent this. But it is expensive. When designing any structure the engineer will have to design it to handle any possible forces applied to it, including earthquakes that might occur at that site. If they had done it properly in this case they would have seen that the forces applied to the bottom pillars would have been too high. So they would have to change the design. Maybe they could have added some gussets in the form of load bearing walls supporting the pillars from horizontal forces, but then the pillars in the seconod story might experience more movement and also needed to be strengthened. And the pillars and walls themselves might not even support their own weight when shaking from side to side. Another option is to use more steel in the construction as it have different material properties then concrete. Either more rebar in the concrete or use steel pillars instead of concrete ones. Steel is more expensive then concrete to do the same work and the maintainence is a lot harder as steel rusts and even cause spauling in concrete. But the steel can actually bend and still support the load above them unlike concrete. This will allow the building to bend during an earthquake instead of breaking.

Let’s say you got 4 interior walls and 4 exterior walls on each floor. You break one, and now you got a 12% more weight on the remaining ones, and given that there are conditions present that caused one to fail while supporting the normal amount of weight, the remaining ones holding the extra are even more likely to fail. Second one goes, even more weight applied to the remaining ones. Then another, and another, and boom, jet fuel/steel beams and whatnot.