Base Isolation
This category is different from the typical structural foundations listed above. It represents systems that isolate the building from ground motion and are closely tied to the seismic isolation devices.
WHAT ARE THEY
Base isolation involves inserting engineered devices (like elastomeric bearings or slides) between the structure and its foundation to decouple ground motion from structural response. It is typically combined with other foundation types (spread footings, pile caps, or mat foundations).
HOW THEY TRANSFER LOADS
Base isolation decouples the superstructure while still transferring gravity and lateral loads; seismic forces are absorbed or attenuated before reaching the structure.
SEISMIC BEHAVIOR
Base isolation dramatically lowers seismic accelerations and forces transmitted to the superstructure, improving performance across seismic intensity levels. The building moves as a rigid body on the isolators while the ground moves beneath it.
| Pros | Cons |
| Excellent seismic performance | More expensive |
| Reduces structural and non‑structural damage | Requires careful detailing and expansion joints |
| Enhances functionality after earthquakes | Requires space for building to move (moat walls) |
They’re best for high seismic zones, critical facilities, and soft soil sites (when combined with other foundation systems).
DESIGN CONSIDERATIONS FOR SAN FRANCISCO CLAY
Base isolation is the best option for achieving immediate occupancy on soft clay. When paired with a pile foundation (to address settlement and lateral capacity in the soft soil), the combination provides excellent stability and seismic protection. This is the selected foundation approach for this project.