Base isolation with elastomeric bearings works, but it has limits: rubber creeps, stiffness depends on temperature, and the isolation period is locked in by geometry and durometer. Friction pendulum bearings (FPBs) solve the same problem with a completely different physics: a heavy articulated slider rides on a concave stainless steel dish coated with a low-friction polymer (typically PTFE-composite). When the ground shakes, the slider rises up the curve, converting horizontal seismic energy into potential energy and friction heat.

The elegance is that the isolation period depends only on the radius of curvature, not the building mass:

T = 2π√(R/g)

Where R is the dish radius and g is gravity. A 2.23 m radius dish gives a 3-second period — long enough to shift the building's response well below the dominant frequencies of most earthquakes (which peak around 0.2–1.0 s). Double the building's weight and the period stays exactly the same, because the restoring force (W·sin θ ≈ W·x/R) scales with weight just like the inertia does. Try that with a rubber bearing.

How the energy dissipation works: friction between the slider and dish (μ typically 0.05–0.15) burns kinetic energy as heat on every cycle. The equivalent damping ratio is roughly:

ζ ≈ (2/π) · (μ / (μ + D/R))

where D is displacement. For μ = 0.06 and modest displacements, you get 15–25% effective damping — several times what a bare structure provides.

Real-world example: the San Francisco International Airport International Terminal sits on 267 friction pendulum bearings with a 74-inch (1.88 m) radius, giving a ~2.75-second isolated period. During the 2014 South Napa earthquake, sensors recorded the isolators sliding while the terminal above barely noticed. The Benicia-Martinez Bridge and Apple Park's main ring building use the same technology.

Triple friction pendulum bearings (developed in the 2000s) stack three concave surfaces with different radii and friction coefficients inside one unit. Small quakes engage only the inner slider (short period, low displacement); larger events progressively engage outer surfaces (longer periods, more damping). This gives one bearing a piecewise-linear behavior tuned for multiple seismic hazard levels — a mechanical analog to gain scheduling.

Design rules of thumb:

• Displacement capacity typically 500–1000 mm for major infrastructure
• Vertical load capacity of a single large FPB: up to 30,000 kN (~3,000 tons)
• Self-centering: gravity pulls the slider back to the low point after shaking stops — no residual offset
• Life cycle: 50+ years with no maintenance if properly sealed against contamination