Base isolation and dampers

Base isolation decouples a building from the ground. Flexible bearings, laminated rubber, lead-rubber or curved sliding bearings, are placed between the foundation and the superstructure. They lengthen the fundamental period, typically to 2 seconds or more, moving the building to the low-acceleration part of the response spectrum. The building above moves almost as a rigid block while the bearings take the displacement.

The trade-off is displacement. Lower acceleration comes with larger movement at the isolation level, often 200 to 400 mm in high zones, so services crossing the isolation plane need flexible connections and the building needs a clear moat around it. Damping, from lead cores, high-damping rubber or friction in sliding bearings, limits that movement. Isolation suits stiff, low to medium-rise buildings on firm ground; it suits tall slender buildings less, because they already have long periods.

Supplemental dampers take a different approach: they add energy dissipation within the structure. Viscous fluid dampers in braces, friction dampers, metallic yielding dampers and buckling-restrained braces reduce drift and damage. Tuned mass dampers, a large mass on springs near the top, counter wind sway in tall towers.

Isolated and damped structures are designed with nonlinear analysis and the devices are proof-tested. They are especially valuable for hospitals, emergency centres and buildings housing sensitive equipment, which must stay functional after an earthquake, and for retrofitting heritage buildings.