Conventional seismic design tries to make buildings strong enough to survive earthquake forces. Base isolation flips the problem: instead of resisting the shake, let the ground move underneath the building. A layer of flexible bearings between the foundation and the superstructure decouples the two, so ground acceleration doesn't fully transmit upward.

The physics is elegant. A rigid building has a short natural period (0.1–0.5 seconds), which sits right in the frequency band where earthquake energy is strongest. By inserting flexible isolators, you deliberately lengthen the building's natural period to 2–3 seconds — far above the dominant seismic frequencies. The building becomes a low-pass filter: it "sees" only the slow, gentle motion, while high-frequency ground shaking passes through the isolators as displacement rather than acceleration.

The three main isolator types:

• Lead-rubber bearings (LRBs): Alternating layers of rubber and steel plates around a central lead core. Rubber provides lateral flexibility; steel plates prevent vertical squish; lead core dissipates energy through plastic deformation.
• Friction pendulum bearings: A concave stainless steel dish with a sliding articulated puck. The building literally swings like a pendulum, with natural period set by the dish radius: T = 2π√(R/g).
• High-damping rubber bearings: Rubber compounds engineered with internal damping — simpler than LRBs, no lead required.

Real-world example: The Los Angeles City Hall (1928) was retrofitted with 526 base isolators after the 1994 Northridge earthquake. The 32-story building now sits on rubber-and-lead bearings that allow up to 20 inches of lateral movement. Utilities crossing the isolation plane use flexible loops. Cost was about $300 million — but the building survives design-level earthquakes with contents intact and remains operational, whereas a fixed-base retrofit would have merely prevented collapse.

Rule of thumb: For friction pendulum bearings, target period T ≈ 3 seconds. Since T = 2π√(R/g), that means R ≈ gT²/(4π²) ≈ 88 inches (2.2 m) of pendulum radius. That's why isolator dishes are big — they're literally sized so the building oscillates slowly enough to dodge the earthquake's frequency content.

What it costs you: Isolation is expensive (5–10% construction premium), requires a "moat" around the building for lateral displacement, complicates every utility crossing, and works poorly for tall buildings where wind loads dominate. It's typically justified for hospitals, data centers, museums, and emergency-response facilities — places where staying functional after the quake matters more than merely not collapsing.