Vibration isolators decouple equipment from the structure for noise and vibration control. In a seismic event, that same decoupling becomes a liability — the isolated equipment can rock, lift, and re-impact the supporting structure with damaging amplification. ASCE 7-22 §13.6.4 governs how to keep isolated equipment from walking away or shattering itself in an earthquake.

The two seismic problems with isolators

  • Lift-off: the isolator is in compression under static dead load; under seismic uplift it goes into tension and can lose contact, then re-impact.
  • Lateral displacement: the isolator stiffness is low (intentionally) so the equipment can shift several inches laterally before any restraint engages.

What ASCE 7-22 §13.6.4 requires

Per §13.6.4: vibration-isolated equipment must have a positive attachment that prevents lateral and vertical motion in excess of prescribed limits during the design earthquake. In practice this means snubbers (lateral) and tie-down restraints (vertical) sized for Fp and Fv.

  • The seismic restraint must be sized for the same Fp as a hard-mounted component.
  • Where the natural air gap between equipment and snubber exceeds ¼″, an amplification factor of 2.0 is applied to Fp per the §13.6.4 prescriptive provisions.
  • Restraints must be detailed to engage at the design displacement, not at the at-rest position.

Snubber design — what the calc shows

  1. Compute Fp per ASCE 7-22 §13.3 (see worked example).
  2. Determine air gap. If > ¼″, multiply by 2.0 per §13.6.4.
  3. Distribute Fp to each snubber per the snubber pattern.
  4. Check the snubber bracket for the lateral load.
  5. Check the snubber's anchorage to the housekeeping pad or slab per ACI 318-19 Chapter 17.
  6. Apply Ω0p on concrete-controlled limit states per ASCE 7-22 §13.4.2.

SSI-Series isolators with built-in seismic restraint

Some isolators (the SSI-Series we offer is one example) integrate the snubber into the isolator housing — the spring carries the gravity load and the housing engages laterally and vertically at the seismic limit. This eliminates the separate snubber bracket and the air-gap amplification. See our SSI-Series Isolators page.

OSHPD/HCAI rules for isolated equipment in hospitals

  • Active equipment (Ip = 1.5) on isolators still requires special seismic certification per AC156, with the isolator/snubber system as part of the test mounting (HCAI PIN 55 §6.4).
  • Per HCAI PIN 55 §6.5, rigid base-mounted components with neoprene pads ≥ ¾″ must be tested with the pads in place — the test cannot be performed hard-mounted and "extrapolated."
  • Cast-in-place anchorage of the snubber/isolator to the pad must be designed for the Fp demand including any §13.6.4 amplification.

Worked example — isolated 5,000 lb air handler on 4 spring isolators

  • Fp = 4,000 lb (from §13.3).
  • Air gap between equipment skid and snubber bumper = ⅜″ → §13.6.4 amplification factor 2.0 → Fp,design = 8,000 lb.
  • 4 snubbers → 2,000 lb lateral per snubber.
  • Snubber bracket designed for 2,000 lb · 1.0 = 2,000 lb (steel design per AISC).
  • Anchorage of snubber to slab: 2,000 lb shear per snubber, plus a tension component from Fv uplift. Apply Ω0p = 2.0 per ASCE 7-22 §13.4.2 → design demand 4,000 lb shear per anchor → typically 2 ½″ anchors per snubber bracket.

Common mistakes

  • Forgetting the §13.6.4 air-gap amplification when the snubber is loose-fit.
  • Designing the snubber bracket but not the anchorage of the snubber to the structure.
  • Using a hard-mounted AC156 test report for an isolated installation.
  • Sizing the spring for gravity but ignoring the seismic up-acting load on the spring (springs in tension fail very differently than in compression).
  • Treating SSI-Series-style integrated isolators as if they require external snubbers — they do not.

How PANACHE ENGINEERING handles isolated equipment

We design the snubber/isolator as a system: spring rate, snubber engagement, and anchorage. For SSI-Series specifications, see our isolator page; for project-specific snubber design, contact us.