Equipment anchorage design is the process of sizing the bolts, welds, frames, and embedded plates that keep a mechanical or electrical component attached to the structure during a design earthquake. Done correctly it is a defensible, code-compliant calculation package that an Authority Having Jurisdiction (AHJ), HCAI/OSHPD plan checker, or hospital DPOR will accept on first submittal. Done poorly it is the single most common reason a nonstructural submittal gets back-checked. This page lays out the workflow we use on every PE/SE-stamped seismic anchor calculation.
Step 1 — Establish the seismic demand
From the project structural drawings, gather:
- SDS — short-period design spectral response acceleration.
- Risk Category (I–IV) and the resulting Ip per ASCE 7-22 §13.1.3.
- SFRS (Seismic Force-Resisting System) — drives Rμ from Table 13.3-1.
- Building height h and the component elevation z — drive Hf.
- Approximate fundamental period Ta for the building (or use the §12.8.2 approximation).
Step 2 — Compute Fp and Fv
Per ASCE 7-22 Eq. 13.3-1 (see our full Chapter 13 guide):
Then bracket the result between Fp,min = 0.3 · SDS · Ip · Wp and Fp,max = 1.6 · SDS · Ip · Wp. The vertical component is Fv = ±0.2 · SDS · Wp.
Step 3 — Free-body diagram and load path
Place Fp at the component center of gravity (CG). Resolve into the anchor group:
- Direct shear: V = Fp / n (split by friction only if a positive bearing connection is present).
- Overturning tension: T = (Fp · hcg – Wp · d/2) / (nrow · d) where hcg is the CG height above the base, and d is the distance between tension-side and compression-side anchor rows.
- Vertical uplift: include ±Fv; the up-acting case usually governs unanchored or lightly loaded equipment.
Always check both orthogonal horizontal directions. For square anchor patterns you can also check 45° per ASCE 7-22 §12.5 (bidirectional) for items susceptible to corner uplift.
Step 4 — Select the anchor type
- Cast-in-place headed studs/bolts — best for new construction, full ACI 318 Ch. 17 capacity, no concrete cracking penalty.
- Post-installed expansion anchors — Hilti Kwik Bolt TZ2/HUS3, Simpson Strong-Bolt 2; use only ICC-ESR-listed seismic-qualified anchors per ACI 318 §17.10.
- Screw anchors — Hilti KH-EZ, Simpson Titen HD; fast install, smaller edge distances.
- Adhesive anchors — Hilti HIT-HY 200, Simpson SET-XP; required to be installed by certified installers in seismic regions.
- Welded studs / Nelson studs — for steel substrates, designed per AISC J7.
Cracked vs. uncracked concrete matters: in seismic regions assume cracked concrete unless you can demonstrate otherwise. Use the manufacturer's seismic ESR values, not the static values.
Step 5 — Check ACI 318 Chapter 17 limit states
For each anchor in the group, check tension (N), shear (V), and combined N+V interaction against:
- Steel strength (Nsa, Vsa) — controlled by the anchor itself.
- Concrete breakout (Ncbg, Vcbg) — controlled by edge distance, embedment, group geometry.
- Pullout (Npn) — adhesive or undercut anchors.
- Side-face blowout (Nsb) — deeply embedded anchors near a free edge.
- Concrete pryout (Vcp) — short, stocky anchors in shear.
Per ASCE 7-22 §13.4.2, when concrete breakout, side-face blowout, or pryout governs, the anchor design forces are amplified by Ω0 unless a ductile yield mechanism is provided in the attached part. The combined-load interaction (ACI 318 §17.8) is:
Walk through fully worked numbers on our anchor bolt design examples page.
Step 6 — Design the support frame
The anchorage check is only one of three required checks. The support — the skid, frame, stand, or bracket — must independently carry Fp and remain stable under drift. Common failures we see in plan check:
- Skid base channel undersized for the resulting prying moment at the anchor.
- Welds at the column-to-baseplate connection sized only for gravity, not for the Fp moment.
- Bracing missing in the cross direction — engineers check the long axis and forget the short axis.
Step 7 — Stamp the package and submit
A defensible submittal includes:
- Cover sheet with project info, code references, and PE/SE stamp.
- Component data sheet (weight, CG, dimensions, mounting).
- Seismic demand calculation (SDS, Ip, Hf, Rμ, Fp, Fv).
- Free-body diagram and anchor group geometry.
- Anchor capacity calculation per ACI 318 Ch. 17 (or AISC for welds).
- Support frame calculation (member, weld, bolt).
- Anchor cut sheet and current ICC-ESR.
- OSP/OPM reference if applicable for HCAI/OSHPD projects.
Special cases
Vibration-isolated equipment
Spring-mounted equipment requires a seismic restraint that engages only during the earthquake. Travel limits, snubber gaps, and impact loads all need to be considered. See our patented SSI-Series isolators for an integrated isolator + restraint solution.
Housekeeping pads
Pads less than 4 in. thick generally do not develop full anchor capacity and need to be poured monolithically with the slab or doweled in. Always check breakout from the pad edge separately from the slab.
Designated Seismic Systems (Ip = 1.5 active equipment)
Analytical anchorage design is necessary but not sufficient — the equipment itself must carry an OSP or be qualified by AC156 shake table testing.
Why retain a PE/SE for equipment anchorage
Equipment anchorage sits at the intersection of mechanical, structural, and code disciplines. The IBC and CBC require it to be designed by a licensed engineer; in California's high seismic zones and on HCAI/OSHPD projects, an SE stamp is typically required. PANACHE ENGINEERING's board-certified engineers deliver thousands of stamped equipment anchorage calculations every year — see our project list.
Frequently asked questions
- What code governs equipment anchorage design in California?
- Equipment anchorage in California is governed by the 2022 California Building Code (CBC) Chapter 16, which adopts ASCE 7-22 for seismic loads and ACI 318-19 Chapter 17 for anchorage to concrete. HCAI/OSHPD hospital projects additionally require compliance with CAN 2-7 and the OSP/OPM pre-approval program for designated active equipment.
- How do I calculate Fp for equipment anchorage under ASCE 7-22?
- ASCE 7-22 Eq. 13.3-1 gives Fp = 0.4 × S_DS × I_p × W_p × (H_f / R_μ) × (C_AR / R_po), bounded by Eq. 13.3-2 (max) and Eq. 13.3-3 (min). H_f accounts for the floor amplification with height, R_μ is the structural ductility reduction, and C_AR / R_po come from the new component-amplification and component-overstrength tables in Table 13.5-1 and 13.6-1.
- When is overstrength factor Ω₀ required for anchors?
- Per ACI 318-19 §17.10.5.3 and ASCE 7-22 §13.4.2, anchors used to resist seismic forces in concrete must be designed for Ω₀ × Fp (typically Ω₀ = 2.0) unless the anchor itself is qualified for the seismic force, the connection is ductile, or the anchorage is governed by a non-brittle limit state such as steel yielding.
- What is the difference between cast-in-place and post-installed anchors?
- Cast-in-place anchors (headed bolts, J-bolts, anchor rods) are placed before concrete is poured and develop the highest capacity through full embedment. Post-installed anchors (expansion, screw, and adhesive) are drilled into hardened concrete after the fact — they require an ICC-ESR for seismic use, must match cracked vs. uncracked conditions, and are sensitive to edge distance, spacing, and inspection requirements.
- What deliverables does a stamped equipment anchorage submittal include?
- A complete PE/SE stamped submittal includes the anchorage drawing with anchor pattern and embedment, a calculation package showing Fp derivation and ACI 318 Chapter 17 limit-state checks (steel, breakout, pullout, side-face blowout, pryout), the support frame design if applicable, the anchor cut sheet with current ICC-ESR, and the OSP/OPM reference number for HCAI projects.