Why buildings shake in an earthquake
An earthquake makes the ground move back and forth. The base of the building moves with the ground. The upper part wants to stay where it was, because of inertia (things resist a change of motion). So the building bends sideways and back, again and again. This is sway.
The taller the building, the more the top moves. Damage comes from big sway and from joints failing, and also from soft ground. A weak floor can fail first. The aim of seismic design is simple: people must not be hurt, even if the building is damaged.
Idea 1: seismic resistance (be strong)
This idea (Japanese 耐震, taishin) makes the building strong and stiff.
- Shear walls and braces: walls and slanting bars that stop the frame leaning.
- Tied joints: beams and columns joined firmly so they act together.
- Ductile parts: steel and well-detailed RC can bend without sudden breaking.
- Balance: a regular, even shape works better than an uneven one. No weak soft floor, e.g. an open parking floor.
The building takes the shaking, so it still gets some damage in a very big quake.
Idea 2: base isolation, Idea 3: dampers
Base isolation (免震, menshin): the building sits on laminated rubber bearings, layers of rubber and steel. The rubber is soft sideways, so the ground slides under the building while the building above stays almost still. Often there is also a gap around it so it can move.
Vibration control (制震, seishin): dampers inside the building take in the energy of the sway, just like a shock absorber in a vehicle. They may use oil, special rubber or metal that yields. The swaying dies down faster.
Compare: isolation protects best but costs most; dampers cost less and help tall buildings in wind too; a strong frame is the base of both.
Seismic reinforcement of houses
Many old houses were built before strong rules were made. They may have heavy roofs, thin walls and weak joints. Making them safer is called seismic reinforcement or retrofitting (耐震補強).
First an expert does a seismic check (survey of the walls, joints, foundation and ground). Then the common fixes:
- add braces or structural boards to walls,
- tie the frame to the foundation with metal connectors,
- strengthen weak joints,
- replace a heavy roof with a lighter one,
- fix a cracked or weak foundation.
This is called making houses quake-resistant (耐震化). It is cheaper than rebuilding and saves lives.
Try it
In the 3D: in the last step drag the strength slider to 80%. See which of the four buildings sways least. Which one hardly moves at all?
At home: stand a tall book on a table and shake the table gently. Then put the book on marbles under a tray: the table moves, the book stays steadier. That is base isolation. Now add a rubber band between two stacked books: they sway less because the band slows the shaking, like a damper.
Key formulas and definitions
- Inertia: the building wants to stay still, so it sways
- Resistance (taishin): stiff frame + braces + ductile joints
- Isolation (menshin): rubber bearings under the building
- Vibration control (seishin): dampers take energy out of the sway
- Retrofit = check, then braces, ties, light roof, strong foundation
Worked examples
1. Why does the top floor of a building move more than the ground floor in a quake?
The ground floor moves with the ground. The upper floors are held back by inertia and are far from the base, so they swing more.
2. Which idea stops the shaking from reaching the building at all: braces, rubber bearings or dampers?
Rubber bearings (base isolation). The rubber is soft sideways, so the ground slides while the building stays almost still.
3. An old wooden house has a heavy clay-tile roof and thin walls. Name two retrofit steps.
Add braces or boards to the walls, and replace the heavy roof with a lighter one. Tying the frame to the foundation also helps.
Common mistakes
- Thinking a stiffer building is always safe. Very stiff buildings can still be damaged, and isolation or dampers can do more.
- Mixing up isolation and dampers. Isolation separates the building from the ground; dampers soak up sway inside.
- Thinking earthquakes kill by shaking alone. Falling parts, weak joints and collapse do the harm.
- Believing old houses must be pulled down. Retrofitting can make many of them much safer.