# Eli5: how can a building be on rollers when everywhere I look it’s connected to the ground?

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We had a medium earthquake a few weeks
Ago but it felt pretty strong. I was at work and someone sent out an email that said “the earthquake felt stronger here because the building is on rollers” but everywhere I look the building is connected to the ground and there is a center courtyard inside our triangle shaped building that looks like a mound of earth. How can my building be on rollers?

In: Engineering

They’re not actual rollers, but the name for a flexible rubber and/or spring layer of the foundation that flexes with applied lateral forces. The flexible nature dampens major movement but can prolong the effect, sort of like a slinky bobbing up and down, except laterally.

[Seismic base isolation – Wikipedia](https://en.wikipedia.org/wiki/Seismic_base_isolation)

Just because the ground is touching the building, that doesn’t mean that the frame of the building is supported by that ground.

You’d have to look at the foundation and how it connects the frame to the ground below to see how the building is supported.

If I build a Lego tower on a table and then cover the table with an inch of sand, the table is still what the tower is on, not sand.

Somwhere there is a area were there can be movement. Exacty how it is done depend on the building.

Look at a insulation system like that at [https://civildigital.com/wp-content/uploads/2016/03/Base-Isolation-for-a-Building-1024×649.jpg](https://civildigital.com/wp-content/uploads/2016/03/Base-Isolation-for-a-Building-1024×649.jpg) Look at the wooden bridge into the building and consider what happen if you would build it so it is only attached to the building but float just above the ground on the stationary part outside. That would be very had to spot.

Compare that idea to [https://imgur.com/a/LQTIWoT](https://imgur.com/a/LQTIWoT) that is the steps into San Francisco City Hall that is on a insulation system like that. It is quite clear on the magnification part.

So you can hide the gap. The building you looked at might have the foundation that is on contact with the ground stationary and the gap is where the building part stars, the clading might extend down so you can see the gap direct from the side. Flexible material can alos be used to cover the gap like rubber,

I’ve studied this a bit when I was in engineering school. I’d provide three important answers to your question.

First off – there is a relationship between the *type* of Earthquake and the *height* of the building that’s based on resonance – like the idea that a force accumulates over time. If you imagine each Earthquake as ‘humming a unique note’, certain buildings are uniquely sensitive to that specific note and other’s less so. So in your example, small buildings might all have collapsed to rubble but the sky scrapers barely swayed at all, or vice versa.

Secondly – I’m not familiar with the latest in tech but a lot of buildings in seismic areas are built where the structural backbone attaches to the ground with connections that can move. Either literally on a form of roller or a sort of rubber tube like a pencil eraser. It’s not that the whole building is rolly or wiggley, but the really important connections that are sensitive to ‘snapping’ in an Earthquake are given some flex.

Finally – many super tall buildings have something called a TMD – a “tuned mass damper” which you imagine as being an absolutely massive boulder housed up at the top of the building. If we go back to our music example, if you imagine an earthquake as plucking a guitar string and the shape of the building being the resonant body of the guitar, it’s bad news when you strum a perfect making the body vibrate along with the note, that equals bye bye building. You can thing of a TMD as a device that no matter what, “randomizes” the guitar body so that never responds to any guitar note or string you can pluck.