Cast-In Anchor Shear Concrete Breakout (ACI 318-19 17.7.2)
Concrete breakout of a cast-in anchor loaded in SHEAR toward an edge.
Example
You enter
- Anchor diameter da (in) 0.75
- Effective embedment hef (in) 6
- Concrete strength f'c (psi) 4000
- Edge distance c_a1, toward the shear (in) 6
- Concrete condition at service cracked
- Lightweight factor lambda_a (1.0 normal weight) 1
You get
- Vb (lb) 8365.64
- Le (in) 6
- Area ratio 1
- Psi edv 1
- Psi hv 1
- Nominal breakout Vcb 8366 lb (8.4 kip)
- Design capacity phiVcb 5856 lb (5.9 kip)
Details, formula, and sources
Vb = lesser of 7(le/da)^0.2 sqrt(da) and 9, x lambda sqrt(f'c) c_a1^1.5; corner, thin-member, and cracking factors; phiVcb = 0.70 Vcb. Capacity scales with the EDGE DISTANCE^1.5, not embedment - a 3/4-in anchor 6 in from the edge checks 5.9 kip design, and a corner with a 4-in second edge cuts it to 2.9 kip. Routinely governs base plates, ledgers, and equipment anchors near slab edges.
Vb = lesser of 7 (le/da)^0.2 sqrt(da) lambda_a sqrt(f'c) c_a1^1.5 and 9 lambda_a sqrt(f'c) c_a1^1.5; le = min(hef, 8 da); AVco = 4.5 c_a1^2; Vcb = (AVc/AVco) psi_edV psi_cV psi_hV Vb; phiVcb = 0.70 Vcb (Condition B).
The ACI 318-19 Section 17.7.2 concrete breakout strength in shear (17.7.2.2.1 basic strength, 17.7.2.4.1 edge factor, 17.7.2.5.1 cracking factor, 17.7.2.6.1 thin-member factor, Table 17.5.3 phi), 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: anchor_dia_in, embedment_in, fc_psi, edge_distance_in, cracking, lambda, vb_lb, le_in, area_ratio, psi_edV, psi_hV, vcb_lb, phi_vcb_lb
- Basic-strength pair the lesser of the 7 (le/da)^0.2 sqrt(da) stiffness form and the 9-cap governs; the 9-form carries no sqrt(da); le = min(hef, 8 da)ACI 318-19 17.7.2.2.1
- Projected area AVco = 4.5 c_a1^2 (a full half-pyramid needs 1.5 c_a1 each way along the edge and 1.5 c_a1 of depth); a corner or thin member truncates AVcACI 318-19 17.7.2.1.3
- psi_cV cracking factor 1.0 cracked without supplementary reinforcement, 1.4 uncracked; the 1.2/1.4 edge-reinforcement credits are not offeredACI 318-19 17.7.2.5.1
- Scope single anchor, shear toward one edge; steel shear (0.6 Ase futa) and pryout (kcp Ncp) are separate checksACI 318-19 17.7.1, 17.7.3