New engineers tend to size anchor bolts by the worst single force — either pure tension or pure shear. Experienced engineers size them by the combination. In a real seismic event, an anchor sees both at the same time, and ACI 318-19 §17.8 enforces a tension–shear interaction limit that often governs the design even when neither force, taken alone, looks problematic.
Where the forces come from
- Shear at the base of an anchored component is the lateral seismic force Fp divided across the anchor group: V = Fp/n.
- Tension arises from overturning of the component about its base: the horizontal Fp applied at the center of mass creates a moment that lifts the anchors on the leeward side.
- Vertical seismic ±Fv = ±0.2·SDS·Wp reduces the gravity stabilizing moment in the worst load combination.
The free-body diagram for a four-anchor base
For a rectangular footprint with side d in the direction of Fp, center of mass at height hcg, weight Wp, and a line of n anchors on the tension side:
If T < 0, gravity stabilizes the anchor and there is no net tension. Apply Fp in both orthogonal directions per ASCE 7-22 §12.5.3 — the governing tension is usually the diagonal case, not a single edge.
The ACI 318-19 §17.8 interaction equation
Apply this for each limit state pair: steel tension / steel shear, concrete breakout tension / concrete breakout shear, pullout / pryout, and so on. The lowest demand-to-capacity ratio (DCR) governs.
ACI also allows two simpler bounding rules:
- If Vua/φVn ≤ 0.2 — neglect shear; check tension alone at full φNn.
- If Nua/φNn ≤ 0.2 — neglect tension; check shear alone at full φVn.
Why the interaction matters even when each ratio looks small
Two anchors carrying 50% tension and 50% shear DCR are not at 0.5 — they are at 1.0 by §17.8. Combined loading is the most common reason a "comfortable" hand check fails plan review. Expect the interaction ratio, not either individual ratio, to drive the design.
Steel vs concrete-controlled anchors
- Steel-controlled anchors fail at the bolt itself. Steel limit states use the un-amplified T and V.
- Concrete-controlled anchors (breakout, pullout, side-face blowout) must use the over-strength load TΩ = Ω0p·T and VΩ = Ω0p·V per ASCE 7-22 §13.4.2 (and the parallel ACI 318-19 §17.10 requirement). See our Ω0 deep-dive.
Common mistakes
- Distributing tension to all anchors in the group instead of the leeward side only.
- Using gross Wp instead of (Wp − Fv) in the resisting moment for the worst-case combination.
- Forgetting to check the orthogonal direction (45° corner case).
- Applying Ω0p to steel-controlled limit states (over-conservative) or omitting it from concrete-controlled limit states (under-conservative).
- Ignoring the §17.8 interaction because each individual DCR is < 1.0.
- Using cracked-concrete capacities from the ESR but assuming uncracked behavior, or vice versa — see our cracked vs uncracked article.
Worked DCR example
For T = 1,200 lb, V = 1,500 lb, φNn,steel = 5,000 lb, φVn,steel = 3,000 lb:
- Tension DCR = 1,200/5,000 = 0.24
- Shear DCR = 1,500/3,000 = 0.50
- Interaction = 0.24 + 0.50 = 0.74 ≤ 1.2 ✓
Reduce edge distance and the breakout limit drops fast — the same anchor at 4" edge distance instead of 6" might fail interaction at the breakout limit even when the steel check is comfortable.
How to design anchors so this is never tight
- Increase the anchor spacing in the direction of Fp — bigger d means smaller T per anchor.
- Lower the center of mass with a stiff base frame — hcg drives overturning.
- Move from 4 anchors to 6 — but only if the added anchors are on the tension side; symmetric extras don't help.
- Use a deeper-embedment anchor (or cast-in-place) to push concrete breakout out of the limit state set — see post-installed vs cast-in-place.
Skip the algebra
Our Seismic Anchor Calculator automates the §17.8 interaction across every limit state and shows you which one governs. For worked examples, see Anchor Bolt Design Examples.