Base isolation: making the building slower
A building does not fall because the ground moves: it falls because it moves AT THE RHYTHM of the ground. Base isolators do not make it stronger — they make it slower, and that takes it out of tune.
What you are looking at
- The two buildings are identical and the ground under them is the same, moving the same way. The only difference is right at the bottom.
- The left one has its base fixed to the ground: it deforms all the way up and its top travels far further than the ground. The figure below says how far: eight times.
- On the right one, watch the golden layer at the foot. That is where all the movement is going; above it the building is almost still. It is not resisting more — it never found out.
01The problem is not force, it is rhythm
Every building has a natural period: how long it takes to lean over and come back if you push it and let go. A stiff six-storey one takes about half a second; a tall slender one, several.
An earthquake does not shake at just any rhythm either: its energy concentrates around a certain period, which depends heavily on the soil. Soft ground, slower; rock, faster.
When those two rhythms match, every shake arrives just in time to push harder, and they add up. This is exactly resonance — the physics of a playground swing, and the reason an earthquake flattens one building and leaves its neighbour standing.
02The counter-intuitive fix: soften it
Intuition says "make it stiffer". But stiffer means a shorter period, and that moves it towards the rhythm of earthquakes, which is exactly where the danger is.
Isolation does the opposite. Between the building and its foundations it puts a deliberately soft layer — rubber in sheets alternating with steel plates, or bearings that slide — and with that the period of the whole thing goes from half a second to two and a half.
At that rhythm the earthquake has almost no energy to hand over. Building and ground stop keeping time together.
03What moves is the isolator, not the building
Almost everyone imagines an isolated building does not move. What does not move is the top: down below, the isolator layer deforms by tens of centimetres every cycle. That is the work it is doing.
Which is why an isolated building needs an empty trench around it — a seismic gap — so it has somewhere to move. If that gap gets filled in, the isolation stops working, and it is a mistake made when someone remodels without knowing what the space was for.
Where this is used
Hospitals
A hospital has to keep working right after the earthquake, which is when it is needed most. Chile has hospitals and clinics built on isolators for that reason: the expensive thing to protect is not the structure, it is the operating theatres, the equipment and the pipework inside.
The contents, not the building
In an earthquake most of the economic damage is not collapsed structure: it is ceilings, partitions, ducts, servers and broken machinery. Isolation protects the contents, which are usually worth more than the building.
Bridges
The neoprene bearings a bridge deck rests on are the same idea under another name: they decouple the structure from the movement of its piers and, incidentally, absorb thermal expansion.
Chile's seismic code
Chile builds the way it does because it learned the hard way: 1960, 1985, 2010. Each earthquake left a stricter code behind, and isolation is the expensive, good version of that lesson — which is why it is reserved for what cannot be allowed to fail.
Explaining it to a child
The idea, in plain wordsIf you put a glass on a tablecloth and yank the cloth, the glass barely moves. A building on isolators is that: the ground shakes underneath and the building stays almost still.
- Put a plastic cup on a book, and the book on the table. Move the book quickly side to side: the cup wobbles or falls.
- Now put four barely-inflated balloons, or two soft sponges, under the book. Shake the table just as hard: the balloons deform and the book hardly notices.
- That is the whole idea. Real isolators are rubber and steel and weigh tonnes, but they do exactly what the balloons do.
The question to leave them withSo why do we not build every building on balloons?
For the classroom
Ten minutes with the animation, ready to take into a room. Free under CC BY 4.0, like everything else here.
- Put a book on the table and a plastic cup on top. Move the book quickly side to side: the cup wobbles or falls.
- Now put four barely-inflated balloons, or two soft sponges, under the book. Shake the table just as hard. The book hardly notices.
- Ask what changed. Nobody made the book stronger; something soft went underneath.
- Play the animation and look at the figure: the fixed building multiplies the ground motion, the isolated one nearly cancels it. And watch where the movement is happening in the second case.
An isolated building needs an empty trench around it. What for, if it "does not move"?
Answer: Because what does not move is the top. Down below, the isolators deform by tens of centimetres every cycle — that is exactly the work they do. If that trench gets filled in, the isolation stops working.
Intuition says "make it stiffer so it takes more". Why is that a bad idea?
Answer: Because stiffer means a shorter period, and that moves it towards the rhythm where earthquakes concentrate their energy. You would be pushing it into resonance instead of away from it.
Why are hospitals isolated, rather than, say, warehouses?
Answer: Because a hospital has to keep working right after the earthquake. And because the expensive thing to protect is not the structure: it is the operating theatres, the equipment and the pipework inside, which break even if the building stays up.
The instinctive reaction is that softening something weakens it. That is reasonable — in almost everything else, stiffer is stronger. What undoes it is not an argument but the balloons: there you see the problem was never strength.
FísicaEterna classroom guide · CC BY 4.0 · fisicaeterna.cl
Take it with you
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The code behind it
It is written in Remotion, which renders video by writing React. The bar and the number are not drawn next to the figure: they come out of the calculation on the same geometry you are watching. If the drawing lies, the number lies with it.
Licence
This animation is under CC BY 4.0: you may copy, redistribute, adapt and use it commercially, as long as you credit the source.
Suggested attributionBase isolation: making the building slower — FísicaEterna (fisicaeterna.cl), CC BY 4.0