Every seismic anchor calculation eventually reduces to a few numbers from ACI 318-19 Chapter 17: tension capacity, shear capacity, and a combined-load interaction check. This page walks through three worked examples we use as internal teaching cases for new engineers — a roof-mounted condenser, a floor-mounted switchgear, and a wall-mounted electrical panel. All numbers are illustrative; for project work see our equipment anchorage workflow, start with the SDS, Ip & z/h calculator, and request a stamped calc.
Reference — ACI 318-19 limit states
- Steel — tension: φNsa = φ · Ase,N · futa
- Steel — shear: φVsa = φ · 0.6 · Ase,V · futa
- Concrete breakout — tension: φNcbg = φ · (ANc/ANco) · ψec,N · ψed,N · ψc,N · ψcp,N · Nb
- Concrete breakout — shear: φVcbg = φ · (AVc/AVco) · ψec,V · ψed,V · ψc,V · ψh,V · Vb
- Pullout: φNpn = φ · ψc,P · Np
- Pryout: Vcp = kcp · Ncb
- Combined interaction (§17.8): (Nua/φNn) + (Vua/φVn) ≤ 1.2
Seismic Ω0 rule (ASCE 7-22 §13.4.2): when concrete breakout, side-face blowout, or pryout governs, multiply the anchor design force by Ω0 (typically 2.0–3.0 depending on the SFRS) unless a ductile yield mechanism is provided in the attached part.
Example 1 — Roof-mounted condenser, post-installed anchors
Given: 1,200 lb HVAC condenser on a 4-bolt pattern, 24″ × 36″, anchored to a 6″-thick normal-weight concrete roof slab. Hilti Kwik Bolt TZ2 5/8″, hef = 3¼″. SDC D, SDS = 1.0g, Ip = 1.5, Rμ = 1.5, Hf = 2.0, CAR = 1.0, Rpo = 1.5, Ω0p = 2.0.
Compute Fp:
Check bounds: Fp,min = 0.3 · 1.0 · 1.5 · 1,200 = 540 lb. Fp,max = 1.6 · 1.0 · 1.5 · 1,200 = 2,880 lb. Use Fp = 640 lb. Fv = 0.2 · 1.0 · 1,200 = 240 lb.
Per-anchor demand (CG height 30″ above slab, 36″ between tension/comp rows, 4 anchors total = 2 in tension):
Capacity check (concrete breakout governs ⇒ apply Ω0 = 2.0):
Amplified Vua = 320 lb. From Hilti ESR-4266 seismic, 5/8″ KB-TZ2 with hef = 3¼″ in 4,000 psi cracked concrete: φVcbg ≈ 1,810 lb at the perimeter anchor (8″ edge). DCR = 320/1,810 = 0.18. ✓
Tension is trivial; combined check = 27/φNn + 320/1,810 ≪ 1.2. ✓
Lesson: even on a 1,200 lb condenser at the roof of a moderate building, the Fp,min floor often governs. Always evaluate both.
Example 2 — Floor-mounted switchgear, cast-in-place anchors
Given: 4,500 lb switchgear lineup, 96″ long × 36″ deep × 90″ tall. 8 cast-in-place 3/4″ A36 anchor bolts in a rectangular pattern, hef = 8″, edge distance 6″. SDC D, SDS = 1.0g, Ip = 1.5 (life-safety), Rμ = 1.5, Hf = 1.0 (ground floor), CAR = 1.4 (flexible cabinet), Rpo = 1.5, Ω0p = 2.0.
Fp,min = 2,025 lb governs. Use Fp = 2,025 lb. Fv = 900 lb.
Long-axis check (CG at 45″, base 96″, 4 tension-side anchors): Tua = (2,025·45 – (4,500–900)·48) / (4·96) = negligible (gravity dominates). Vua = 2,025/8 = 253 lb/anchor.
Short-axis check (base 36″ — typically governs): Tua = (2,025·45 – (4,500–900)·18) / (4·36) = (91,125 – 64,800)/144 = 183 lb/anchor.
Concrete breakout governs ⇒ Ω0: Tua,amp = 366 lb, Vua,amp = 506 lb.
For 3/4″ A36 cast-in headed bolt, hef = 8″, c = 6″, in 4,000 psi cracked concrete (per ACI 318 §17.6.2): φNcbg ≈ 8,200 lb (group of 2 tension-side anchors), φVcbg ≈ 6,400 lb (perimeter group). DCRs < 0.10. Comfortably ✓.
Lesson: switchgear is usually anchor-rich; the governing case is almost always the short-axis overturning. Don't forget to check both directions.
Example 3 — Wall-mounted electrical panel, screw anchors
Given: 250 lb panel, 24″ × 30″ × 6″ deep, mounted to an 8″ CMU wall with 4 Hilti KH-EZ ¼″ screw anchors, hef = 1¾″. SDC D, SDS = 1.0g, Ip = 1.0, Rμ = 1.5, Hf = 1.5, CAR = 1.0, Rpo = 1.5, Ω0p = 2.0.
Fp,min = 75 lb governs.
Per anchor (panel CG 3″ off wall, 4 anchors at corners spaced 18″ vertically): pullout from CG offset = 75 · 3 / (2 · 18) = 6 lb/anchor; in-plane shear from gravity weight + Fv = (250 + 50)/4 = 75 lb/anchor; out-of-plane shear from Fp = 75/4 = 19 lb/anchor.
Apply Ω0 for masonry breakout: amplified shear = 188 lb/anchor. Per Hilti ESR-3027 KH-EZ ¼″ in 8″ CMU grouted: φVn ≈ 540 lb. ✓.
Lesson: small wall-mounted equipment is governed by Fp,min, gravity shear, and minimum edge distances — not by Fp.
Anchorage design workflow — the six steps we follow
Every anchorage design we stamp follows the same sequence, whether the component is a 200 lb panel or a 40,000 lb chiller. The examples above are that workflow applied to three geometries.
- Demand — Fp from ASCE 7-22 Eq. 13.3-1, bounded by Fp,min and Fp,max, plus the vertical effect Ev = 0.2·SDS·Wp.
- Distribution — resolve Fp and Wp through the center of gravity into per-anchor tension T and shear V for the actual bolt pattern.
- Capacity — φNn and φVn for steel, breakout, pullout, side-face blowout, and pryout per ACI 318-19 §17.6–17.7.
- Seismic modifiers — the §17.10 0.75 factor on concrete-controlled limit states and Ω0p unless a §17.10.5 escape clause applies.
- Interaction — the combined tension–shear check of §17.8.
- Documentation — anchor model and ESR number, embedment hef, edge distance ca1, spacing, cracked-concrete assumption, and the governing limit state.
Need this run on your equipment? Request a stamped anchorage design package.
Common pitfalls in anchor bolt design
- Using uncracked concrete capacities in seismic regions — almost never appropriate.
- Ignoring Ω0 on concrete-controlled limit states.
- Forgetting to check perpendicular-to-edge shear separately from parallel-to-edge.
- Treating an L-bolt as if it had headed-bolt pullout capacity (it doesn't — use the hooked-bolt formula).
- Using anchor manufacturer software that defaults to ASCE 7-16 inputs.
- Missing the §17.10.6 seismic ductility requirement for tension-loaded anchors.
For more, see Common Anchorage Design Mistakes.
Related pages
- Seismic Anchor Calculations
- SDS, Ip & z/h Calculator
- ASCE 7-22 Chapter 13 Guide
- Equipment Anchorage Workflow
- Common Anchorage Mistakes
- Engineering Calculation Tools
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Read articleCracked vs Uncracked Concrete: Which Anchor Values to Use
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Read articleCast-In Anchor Bolts vs Post-Installed Anchors: How to Choose
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Read articleFrequently asked questions
- What is anchorage design?
- Anchorage design is the engineering of the connection between a component (equipment, baseplate, brace, or rack) and its concrete, masonry, or steel support. For concrete, it means proving every ACI 318-19 Chapter 17 limit state — steel tension and shear, concrete breakout, pullout, side-face blowout, and pryout — plus the ASCE 7-22 §13.4 seismic demand Fp and the Ω0p amplification on concrete-controlled failure modes.
- What are the steps in an anchor bolt design calculation?
- 1) Compute the seismic demand Fp per ASCE 7-22 Eq. 13.3-1 and check the Fp,min / Fp,max bounds. 2) Distribute Fp and the component weight into per-anchor tension T and shear V using the center of gravity and bolt pattern. 3) Compute φNn and φVn for every ACI 318-19 limit state. 4) Apply the §17.10 seismic 0.75 factor and Ω0p where required. 5) Run the tension–shear interaction check. 6) Document edge distance, spacing, embedment, and the ESR used.
- Which limit state usually governs anchorage design?
- Concrete breakout governs the large majority of equipment anchorage in cracked concrete, especially with shallow embedment or a nearby edge. Steel strength only governs when embedment is deep, edges are generous, or anchor reinforcement is detailed to suppress breakout — which is also the condition that lets you escape Ω0p under ACI 318-19 §17.10.5(b).
- How many anchor bolts does a piece of equipment need?
- The count follows from the demand, not from a rule of thumb: divide the amplified per-anchor tension and shear by the governing φNn and φVn. In practice, four anchors at the corners of the base is the minimum for most floor-mounted equipment because it gives an overturning couple in both directions; larger skids and generators commonly use six to twelve.
- Do anchorage design calculations need a PE or SE stamp?
- For California HCAI/OSHPD projects and most Risk Category III and IV buildings, yes — the anchorage calculation package must be stamped by a licensed California PE or SE and submitted with the equipment cut sheets, ESR, and anchor details. We issue stamped packages through our seismic anchor calculation request form.
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