How can a building move less while the ground shakes?

A building attached rigidly to its foundation must respond when the ground shakes. Its floors can accelerate, bend and sway. Base isolation introduces carefully designed flexible bearings between the foundation and the building above, allowing substantial horizontal movement at that level.

The building still needs vertical support. Many isolators combine a capacity to carry large vertical loads with much greater horizontal flexibility. That flexibility lengthens the structure’s vibration period: the time associated with one cycle of its natural back-and-forth motion. Some isolation systems also dissipate energy through rubber, lead or other damping mechanisms.

This change can reduce how sharply the upper building accelerates during many faster parts of earthquake shaking. The ground and the foundation move, while much of the relative movement occurs across the isolators. Equipment and contents above them can therefore experience gentler shaking. The isolators themselves need enough room to move.

Isolation is not a promise that every kind of shaking becomes smaller. Some long-period shaking can produce a larger response. Real earthquakes contain many changing frequencies, and real buildings have damping, several vibration modes and limited movement space. Engineers evaluate all of those effects. The central idea is to change how shaking reaches a structure, while preserving its ability to carry its loads.