Edge Distance and Anchor Spacing in Concrete: How c_a1 and s Govern Anchor Capacity

Two geometric parameters control concrete-anchor capacity more than anyone outside the calc realizes: the edge distance c_a1 and the anchor-to-anchor spacing s. ACI 318-19 Chapter 17 reduces breakout capacity (in tension and shear) when either is below a threshold, and the reduction is non-linear close to the edge. A ½-inch shift in baseplate position can swing the calc from passing to failing.

The geometry definitions

• c_a1: distance from anchor centerline to the nearest concrete edge, measured perpendicular to the shear direction.
• c_a2: distance to the second-nearest edge.
• s: center-to-center anchor spacing within a group.
• c_ac: critical edge distance — beyond this, no edge effect. Per ACI 318-19, c_ac = 1.5 h_ef for cast-in headed anchors and the value tabulated in the ESR for post-installed.

How edge distance drives concrete breakout in tension

ACI 318-19 §17.6.2.4 — the Ψ_ed,N factor:

• If c_a,min ≥ 1.5 h_ef → Ψ_ed,N = 1.0 (no reduction).
• If c_a,min < 1.5 h_ef → Ψ_ed,N = 0.7 + 0.3 · (c_a,min / (1.5 h_ef)).

At c_a,min = 0.5 h_ef, Ψ_ed,N = 0.8 — a 20% capacity hit on the basic breakout strength.

How edge distance drives concrete breakout in shear

Shear is far more sensitive. Per ACI 318-19 §17.7.2.1, the basic shear breakout capacity V_b uses c_a1^1.5directly. Halving c_a1 reduces V_b by a factor of 2^1.5 = 2.83. The Ψ_ed,V, Ψ_c,V, and Ψ_h,V further trim the capacity in tight or thin members.

Anchor spacing — the Ψ_cp,N and group effects

• The breakout area A_Nc for a group is the projected concrete area on the front face — adjacent anchors share the cone, reducing the per-anchor capacity.
• For a 4-anchor pattern at s = h_ef, the breakout strength is roughly 60% of 4 isolated anchors.
• Increase s to 2 h_ef and the group recovers to ~85% of 4 isolated anchors.
• Increase s to 3 h_ef and the group acts as 4 fully-independent anchors.

Worked example — ½″ post-installed expansion anchor, h_ef = 4 in.

• 4,000 psi cracked concrete, single anchor far from edges → φN_cb,seismic ≈ 2,300 lb.
• Move c_a,min to 4 in. (= h_ef): Ψ_ed,N = 0.7 + 0.3·(4/(1.5·4)) = 0.9. φN_cb,seismic ≈ 2,070 lb.
• Move c_a,min to 2 in. (= 0.5 h_ef): Ψ_ed,N = 0.8; basic A_Nc/A_Nco also drops from 1.0 toward ~0.7. φN_cb,seismic ≈ 1,290 lb — a 44% loss vs the ideal-edge case.

Worked example — same anchor, shear with c_a1 changes

• c_a1 = 6 in. → V_cb,seismic ≈ 1,800 lb per anchor.
• c_a1 = 4 in. → V_cb,seismic ≈ (4/6)^1.5·1,800 ≈ 980 lb.
• c_a1 = 2 in. → V_cb,seismic ≈ (2/6)^1.5·1,800 ≈ 350 lb.

Going from 6 in. to 2 in. of edge distance loses 80% of the shear capacity — exactly why curb-mounted equipment with anchors near the curb edge fails plan check so often.

Plan-check checklist

1. Tabulate c_a1, c_a2, and s on the anchor-pattern detail.
2. Compute Ψ_ed,N, Ψ_ed,V, Ψ_c,N, Ψ_c,V, and Ψ_cp,N per ACI 318-19.
3. Show A_Nc/A_Nco for the group.
4. Apply the 0.75 §17.10.6 reduction in seismic.
5. Apply Ω_0p per ASCE 7-22 §13.4.2 — see Ω₀ overstrength.

Common mistakes

• Measuring c_a1 from the baseplate edge instead of the anchor centerline.
• Ignoring the back edge when both edges are within 1.5 h_ef.
• Using uncracked breakout values when the slab is in flexural tension — see cracked vs uncracked.
• Spacing anchors at 2 in. on center because "they fit" — without checking the group A_Nc reduction.
• Forgetting that adhesive anchors have their own c_ac from the ESR (often larger than 1.5 h_ef).

How PANACHE ENGINEERING handles this

Our calculator computes every Ψ factor from first principles and flags any anchor near the c_ac threshold. See our anchor bolt design examples or request a stamped calculation.