Concrete Anchor Tension-Shear Interaction (ACI 318-19 17.8)
The combined tension-and-shear check on an anchor.
Example
You enter
- Factored tension demand Nua (lb) 6000
- Factored shear demand Vua (lb) 3000
- Governing design tension capacity phiNn (lb) 9831
- Governing design shear capacity phiVn (lb) 5112
You get
- Tension ratio Nua/phiNn 0.610
- Shear ratio Vua/phiVn 0.587
- Combined sum 1.197 vs 1.2
- Limit 1.2
Details, formula, and sources
Full strength permitted when the companion demand is under 20%, else Nua/phiNn + Vua/phiVn <= 1.2, with each demand still required to clear its own capacity outright. The pinned example passes tension at 61% and shear at 59% yet clears combined at 1.197 of 1.2 - an anchor fine in each direction can be one bolt size from failing both. phiNn/phiVn = the least of the individual failure modes.
Vua/phiVn <= 0.2 -> full tension strength permitted; Nua/phiNn <= 0.2 -> full shear strength permitted; otherwise Nua/phiNn + Vua/phiVn <= 1.2. Each demand must also clear its own capacity (17.8.1, 17.8.2).
The ACI 318-19 Section 17.8 interaction of tensile and shear forces (trilinear rule, 17.8.3), by name.
ACI 318 is readable free through the ACI online reading room at concrete.org; the Chapter 17 anchoring provisions are in the published code.
Estimate. AHJ and licensed professional govern.
Field names used by the API: nua_lb, vua_lb, phi_nn_lb, phi_vn_lb, tension_ratio, shear_ratio, sum_ratio, limit
- Trilinear rule full companion strength permitted below a 0.2 demand ratio; 1.2 combined limit aboveACI 318-19 17.8.2, 17.8.3
- Individual caps Nua <= phiNn and Vua <= phiVn always hold on their ownACI 318-19 17.8.1
- Inputs the two governing design capacities come from the anchor-mode tiles; this tile does not recompute themACI 318-19 17.5