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Anchor Bolt Design Examples

Worked examples for seismic anchor bolt design per ACI 318-19 Chapter 17 — tension, shear, combined interaction, and the Ω0 amplification rule that catches engineers new to ASCE 7-22.

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 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:

Fp = 0.4 · 1.0 · 1.5 · 1,200 · (2.0/1.5) · (1.0/1.5) = 640 lb

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):

Tua = (640 · 30 – (1,200 – 240) · 18) / (2 · 36) = (19,200 – 17,280) / 72 = 27 lb/anchor
Vua = 640 / 4 = 160 lb/anchor

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 = 0.4 · 1.0 · 1.5 · 4,500 · (1.0/1.5) · (1.4/1.5) = 1,680 lb

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 = 0.4 · 1.0 · 1.0 · 250 · (1.5/1.5) · (1.0/1.5) = 67 lb

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.

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.

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