Plan check rejections on nonstructural anchorage almost always trace back to the same root cause: the engineer skipped a step in the ASCE 7-22 force chain, or applied an ASCE 7-16 habit (the old 1 + 2·z/h term, an obsolete Rp, a forgotten Ω0) to a 2025 CBC project. This article walks the entire calculation, end to end, for a representative piece of mechanical equipment so you can see where every variable comes from and how the demand drops onto each anchor.

The example we'll solve

  • 1,200 lb floor-mounted air-cooled chiller (Wp = 1,200 lb).
  • Mounted on the 4th-floor mechanical mezzanine of a 5-story essential-facility hospital (z = 50 ft, h = 65 ft).
  • Building SFRS: special steel concentrically braced frame (Table 12.2-1).
  • Site: Los Angeles. SDS = 1.50 g.
  • Anchorage: four 5/8" post-installed expansion anchors at the corners of a 36" × 24" footprint.
  • Center of mass 30" above the base. Concrete: 4,000 psi normal-weight, cracked.

Step 1 — Component importance factor Ip

Per ASCE 7-22 §13.1.3, this is a Risk Category IV facility and the chiller serves life-safety HVAC for surgery — Designated Seismic System. Ip = 1.5. (See our deep dive on the component importance factor.)

Step 2 — Component coefficients CAR and Rpo

From Table 13.6-1 (mechanical and electrical components), an air-cooled chiller falls under "HVAC components — air handlers, chillers, cooling towers, condensers". Read across:

  • CAR = 1.0 (rigid, base-mounted)
  • Rpo = 1.5
  • Ω0p = 2.0 (used later for concrete-controlled anchor design)

Step 3 — Height amplification Hf and ductility Rμ

For a 5-story building with a special steel concentrically braced frame (R = 6 in Table 12.2-1), interpolate Hf from Table 13.3-1 with z/h = 50/65 = 0.77:

Hf=1 + a1·(z/h) + a2·(z/h)10
ASCE 7-22, §13.3.1

With a1 = 1/N = 0.20 (5 stories) and a2 = capped appropriately, Hf ≈ 1.20. The ductility modifier Rμ from Table 13.3-1 for a special concentric braced frame is 1.5.

Step 4 — Compute Fp

Fp=0.4 · SDS · Ip · Wp·HfRμ·CARRpo
ASCE 7-22, Eq. 13.3-1

Fp = 0.4 × 1.50 × 1.5 × 1,200 × (1.20 / 1.5) × (1.0 / 1.5) = 576 lb.

Step 5 — Apply the bounds

  • Fp,min = 0.3 × SDS × Ip × Wp = 0.3 × 1.5 × 1.5 × 1,200 = 810 lb ✱ governs
  • Fp,max = 1.6 × SDS × Ip × Wp = 1.6 × 1.5 × 1.5 × 1,200 = 4,320 lb

Because the calculated 576 lb is below Fp,min, the design force is Fp = 810 lb. (See the bounds article: Fp,min and Fp,max in ASCE 7-22.)

Step 6 — Vertical seismic component Fv

Fv = ±0.2 · SDS · Wp = ±0.2 × 1.5 × 1,200 = ±360 lb.

Step 7 — Free-body diagram and anchor forces

Apply Fp = 810 lb horizontally at the center of mass (30" up). Combine with gravity (Wp = 1,200 lb) and ±Fv. With four anchors in a 36" × 24" rectangle, the worst-case anchor carries:

  • Tension from overturning about the long edge: T = (Fp × hcg − (W − Fv) × d/2) / (n × d) = (810 × 30 − (1,200 − 360) × 12) / (2 × 24) = (24,300 − 10,080) / 48 ≈ 296 lb / anchor.
  • Shear: V = Fp / 4 = 203 lb / anchor.

Step 8 — Apply Ω0 for concrete-governed limit states

Per ASCE 7-22 §13.4.2, anchorage in concrete or masonry whose strength is governed by the concrete (breakout, pullout, side-face blowout) must use the over-strength force:

  • TΩ = Ω0p × T = 2.0 × 296 = 592 lb
  • VΩ = 2.0 × 203 = 406 lb

Steel-strength checks of the anchor itself use the un-amplified T and V. (More on this distinction in the Ω0 article.)

Step 9 — ACI 318-19 §17.8 tension–shear interaction

For each governing limit state (steel, breakout, pullout), check:

Nua/φNn+Vua/φVn≤1.2
ACI 318-19, Eq. 17.8.3

For a 5/8" Hilti KB-TZ2 in 4,000 psi cracked concrete with 4" embedment and ≥ 6" edge distance (Table from ESR-4266), φNn,steel ≈ 7,500 lb and φNn,breakout ≈ 4,200 lb. Demand ratio at steel: 296/7,500 + 203/3,200 = 0.10 → comfortable.

Step 10 — Document for plan check

  1. State the governing equation, code edition, and SFRS.
  2. Show every variable on the cover page (SDS, Ip, Hf, Rμ, CAR, Rpo, Ω0p).
  3. Include the Fp,min/Fp,max check explicitly.
  4. Show the free-body diagram and the orthogonal-direction check.
  5. Tabulate every limit state: steel tension/shear, concrete breakout, pullout, side-face blowout, pryout, and the §17.8 interaction.
  6. Reference the anchor's ICC-ES ESR for the assumed embedment, edge distance, and cracked-concrete factors.

Skip the arithmetic — use our calculator

Our Seismic Anchor Calculator performs every step above and outputs a stamp-ready PDF. For more context, see Equipment Anchorage Design and Anchor Bolt Design Examples.

Have a real project? Send us your equipment data sheet and we'll return a PE/SE-stamped calculation package.