Base isolation is a seismic protection strategy that decouples a structure from ground motion by inserting flexible isolators between the superstructure and its foundation. This shifts the structure's fundamental period to a long value, reducing seismic demands and protecting both the structure and its contents.
Base isolation is a seismic design strategy in which a structure is decoupled from the ground by inserting a layer of flexible bearings β called isolators β between the superstructure and its foundation. By dramatically increasing the flexibility of the structure-foundation interface, isolation shifts the fundamental period of the structure from its original (typically 0.3β1.0 seconds) to a much longer value (typically 2β4 seconds), well beyond the dominant period range of most earthquake ground motions. This reduces seismic demand by a factor of 3 to 10 compared to a conventionally fixed-base structure, while simultaneously adding damping through the energy dissipation mechanisms built into the isolators.
The concept dates back to the late 19th century, but practical implementation only became widespread after the 1970s with the development of reliable elastomeric and sliding bearings. Two main families of isolators dominate modern practice. Elastomeric bearings use layers of natural or synthetic rubber alternating with steel shims to provide horizontal flexibility and vertical stiffness; high-damping rubber (HDR) or lead-rubber bearings (LRB) add energy dissipation through material damping or a lead core. Sliding bearings (friction pendulum systems, FPS) use a curved sliding surface and a polished slider; the restoring force comes from gravity acting on the displaced mass, and energy is dissipated through friction. Hybrid systems combine both approaches. Modern base isolation systems routinely achieve equivalent damping of 15β30%, far higher than conventional structures.
Base isolation is most effective for stiff, short-period structures β buildings of low to mid-rise height, nuclear power plants, bridges, and critical facilities. For very tall or very flexible structures, the long-period range is already beyond the dominant ground motion frequencies, and isolation provides less benefit. Isolated structures require careful design of the isolation layer, the superstructure (which must accommodate large horizontal displacements at the isolation plane, typically 200β500 mm), and the "moat" or clearance zone around the building to prevent pounding. The isolators must be designed for vertical loads, horizontal flexibility, and adequate displacement capacity under the maximum credible earthquake. Base isolation has been applied to hundreds of buildings worldwide β including hospitals, emergency centers, data centers, and heritage structures β and has demonstrated excellent performance in major earthquakes such as Northridge (1994), Kobe (1995), and Tohoku (2011). In Iran, base isolation is used in some new hospitals and critical facilities, though adoption remains more limited than in the United States, Japan, or New Zealand.