The seismic design force Fp applied at the equipment's center of gravity (CG) creates an overturning moment that the anchor pattern must resist. On tall or narrow equipment — generators, tall cabinets, vertical tanks — this moment governs anchor tension. Plan reviewers always check the CG-based moment calculation because it is the most common shortcut in submitted calcs.

Anchor tension from overturning — the basic equation

For floor-mounted equipment with anchors at the base:

  • Fp · hcg = overturning moment about the base.
  • The anchor pattern resists this moment via tension on the anchors farthest from the rocking axis (the leeward edge).
  • Tmax per anchor = Fp · hcg / (n · d) — where n is the number of anchors carrying tension and d is the lever arm from the rocking axis to the anchor row.

Plus the static dead-load reduction: each anchor sees an additional compressive component from Wp/N (where N is the total number of anchors). The net tension is Tmax − Wp/N per anchor — but only if Wp/N is large enough to fully offset the overturning. Per ASCE 7-22 §2.3 load combinations, you also subtract 0.6D from the demand (or apply Fv upward).

Including the vertical seismic force Fv

From ASCE 7-22 §13.3.1.2: Fv = ±0.2 · SDS · Wp.

The up-acting case reduces the dead-load that resists overturning and tension. For unanchored or lightly-anchored equipment, the up-acting Fv + the Fp moment is the governing check. The combined effective tension per anchor is:

Tnet = (Fp · hcg) / (n · d) + Fv/N − (0.6 · Wp)/N

Worked example — 3,000 lb chiller, hcg = 18 in., 24 in. anchor spacing

  • Fp = 1,800 lb (from ASCE 7-22 §13.3 with SDS = 1.0, Ip = 1.0).
  • Fv = 0.2 · 1.0 · 3,000 = 600 lb up.
  • Overturning moment about leeward edge: 1,800 · 18 = 32,400 lb-in.
  • Anchor pattern: 4 anchors (2 windward, 2 leeward), spacing 24 in.
  • Tension on the 2 windward anchors: T = 32,400 / (2 · 24) = 675 lb each from overturning.
  • Add up-acting Fv: 600/4 = 150 lb each.
  • Subtract gravity restraint: 0.6 · 3,000 / 4 = 450 lb each.
  • Net per windward anchor: 675 + 150 − 450 = 375 lb tension. Plus shear from the Fp/N = 450 lb each.

Worked example — 800 lb tall electrical cabinet, hcg = 36 in., 12 in. anchor spacing

  • Fp = 800 lb.
  • Overturning: 800 · 36 = 28,800 lb-in.
  • 4 anchors, 12 in. spacing.
  • Tension on windward pair: T = 28,800 / (2 · 12) = 1,200 lb each.
  • Gravity restraint per anchor: 0.6 · 800 / 4 = 120 lb.
  • Net tension: 1,200 + 800·0.2/4 − 120 = 1,120 lb each. Now compare to anchor capacity — most ¼″ anchors will fail this on concrete breakout.

That is why narrow tall cabinets fail anchorage so often: the lever arm dominates and the lateral footprint is too small. Solutions are seismic clips that extend the lever arm or a base frame that widens the anchor pattern.

When the equipment is on isolators or housekeeping pads

  • Isolators with snubbers: the snubber lift-off limit defines the effective hcg; design the snubber for Fp · hcg directly.
  • Housekeeping pads: the pad becomes part of the load path; check the pad's anchorage to the slab as a separate calc.

Wind vs seismic — different but related

For tall outdoor equipment (cooling towers, dry coolers, exhaust stacks), the wind overturning calc uses the same CG moment-arm approach but with the wind pressure × projected area as the lateral force. The same anchor often governs under wind as under seismic. Always check both and report the governing.

Plan-check checklist

  1. State hcg, footprint dimensions, and anchor spacing.
  2. Compute the overturning moment from Fp.
  3. Add the up-acting Fv.
  4. Subtract the resisting dead load (per the governing load combination).
  5. Solve for T per windward anchor.
  6. Combine with shear from Fp per the §17.8 interaction (see tension vs shear).

Common mistakes

  • Using the equipment's geometric center as hcg when the manufacturer publishes a true CG (often higher).
  • Forgetting the up-acting Fv — overturning gets worse, not better, with vertical seismic.
  • Counting all anchors as resisting tension, including the leeward (compressed) row. Only the windward anchors resist tension.
  • Using "1.0D" gravity restraint instead of the seismic load combination's "0.6D − 0.7E" form.
  • Ignoring the housekeeping pad's own anchorage to the slab.

How PANACHE ENGINEERING handles this

Our equipment anchorage calcs always include the CG-based overturning check as a separate step on the cover sheet. See our Equipment Anchorage Design workflow or talk to an engineer for project-specific review.