Base isolation decouples a building from ground shaking by inserting a soft horizontal layer between the structure and its foundation. You've already seen elastomeric bearings and friction pendulums — the third major family is the flat sliding bearing, typically a polished stainless steel plate against a PTFE (Teflon) puck. Simple, cheap, and fundamentally different from the others: it has no restoring force.

The mechanics are pure Coulomb friction. Below a threshold horizontal force, the bearing is locked and the building moves with the ground. Above it, the PTFE slides on the steel and transmits only the friction force — no matter how hard the ground shakes. That transmitted force is:

F = μ · W

where μ is the sliding coefficient (typically 0.05–0.15 for lubricated PTFE, higher at low velocity and low temperature) and W is the vertical load on the bearing. A 5,000 kN column on a bearing with μ = 0.08 will never see more than 400 kN of lateral force transmitted to the superstructure, regardless of ground acceleration.

The catch: no restoring force. A flat slider that displaces 200 mm during an earthquake stays displaced. This is why pure flat sliders are almost never used alone — they're paired with elastomeric bearings or rubber springs elsewhere in the isolation plane to provide re-centering. The sliders carry gravity load and limit force; the elastomers pull the building back to center.

Real-world example: The Utah State Capitol seismic retrofit (completed 2008) uses 265 flat PTFE sliders under the perimeter columns paired with lead-rubber bearings under the interior columns. The sliders handle high vertical loads cheaply where re-centering isn't critical for that column, while the interior LRBs provide the restoring force and damping for the whole isolated mass. Split the job: some bearings carry load and limit shear, others re-center.

Design considerations:

• Stick-slip: Static μ can be 2–3× kinetic μ, causing jerky motion at initiation. Modern bearings use lubricated dimpled PTFE to flatten the μ-vs-velocity curve.
• Wear and contamination: Any grit on the stainless surface will score the PTFE. Bearings must be sealed with a wiper.
• Displacement capacity: Size the plate for the maximum expected earthquake displacement plus a margin — running off the edge is catastrophic.
• Vertical uplift: Flat sliders can't resist tension. Under overturning moments, a corner column might try to lift off; a separate uplift restraint may be required.

Rule of thumb: Design isolation displacement is roughly D ≈ S_D1 · T_eff / (4π² · B), where T_eff is the effective isolated period (typically 2.5–4 seconds) and B is a damping reduction factor. For a typical building, expect 150–400 mm of displacement in a design-level event — plan bearing plate size and moat clearance accordingly.