What is the function of a shear key in base isolated buildings?

The function of a shear key in base isolated buildings is to resist non-seismic lateral loads, such as wind loads or minor seismic events, while allowing the base isolation system to function as intended during a major earthquake. A shear key is a structural element designed to engage only when the lateral forces exceed a predetermined threshold. Under normal conditions, with small lateral loads, the shear key remains inactive, allowing the base isolators to freely accommodate thermal expansion, creep, and other minor movements. However, when a significant lateral force, such as from an earthquake, occurs, the shear key engages and provides lateral resistance, preventing excessive displacement of the superstructure relative to the foundation. The shear key is typically designed to fail or yield at a force level that is lower than the design force for the isolators and the superstructure. This controlled failure protects the isolators from being subjected to forces beyond their capacity and ensures that the isolation system can effectively reduce the seismic forces transmitted to the building. In essence, the shear key acts as a safety mechanism, preventing unwanted movement under normal conditions and protecting the base isolation system during extreme events.