What an earthquake does to a building

An earthquake does not push a building directly. It shakes the ground, and the building's mass, trying to stay where it was, lags behind. The result is horizontal inertia forces at every floor, largest where the mass is greatest and the shaking amplified most.

How strongly a building responds depends on its natural period, the time for one free sway. Stiff low-rise buildings have short periods; tall flexible buildings have long ones. When the ground shakes near a building's period, the response grows. Soft soils amplify some periods and lengthen the shaking.

Codes do not ask buildings to stay elastic in a large earthquake; that would be uneconomic. Instead, the structure is designed for a reduced force and detailed to be ductile, so that chosen parts yield and absorb energy without collapse. Closely spaced ties at column ends, strong columns relative to beams, and well-anchored bars are what make that possible.

Irregular buildings concentrate damage. A soft open ground storey, a heavy water tank on the roof, or a sudden change in plan can lead to collapse even when the rest of the building is sound.