Shear Friction Across an Interface (ACI 318-19 22.9)
The shear carried across a cold joint, a corbel interface, or a wall-to-footing plane.
Example
You enter
- Reinforcement crossing the plane Avf (in²) 2
- Steel yield fy (psi) 60000
- Interface area Ac (in²) 192
- Concrete strength f'c (psi) 4000
- Interface condition (mu) roughened
- Lightweight factor lambda 1
You get
- Friction coefficient mu 1.00
- Friction strength mu Avf fy 120.0 kip
- Interface cap 153.6 kip
- Nominal Vn 120.0 kip
- Design phi Vn 90.0 kip
Details, formula, and sources
Vn = mu Avf fy, with mu 1.4 monolithic / 1.0 roughened / 0.6 unroughened / 0.7 to steel, capped by the concrete interface. 2.0 in^2 of Grade 60 dowels across a roughened joint carry 120 kip (90 design); add more dowels past the 153.6 kip cap and nothing changes because the concrete itself limits the transfer. Perpendicular (no net tension) case. A design aid, not the engineer of record's stamped design.
mu = {monolithic:1.4, roughened:1.0, unroughened:0.6, steel:0.7} x lambda; Vn0 = mu Avf fy; cap = min(0.2 f'c, 480 + 0.08 f'c, 1600) Ac (normalweight mono/roughened); Vn = min(Vn0, cap); phi Vn = 0.75 Vn.
The ACI 318-19 22.9 shear-friction strength Vn = mu Avf fy, with the interface friction coefficients and the maximum-transfer caps, by name.
ACI 318 is readable free through the ACI online reading room at concrete.org; the 22.9 shear-friction provisions are in the published code.
Estimate. AHJ and licensed professional govern.
Field names used by the API: avf_in2, fy_psi, ac_in2, fc_psi, iface, lambda, mu_f, vn0_kip, cap_kip, vn_kip, phi_vn_kip
- Friction transfer Vn = mu Avf fy on the reinforcement crossing the planeACI 318-19 22.9.4.2
- Interface coefficient mu = 1.4/1.0/0.6/0.7 x lambda by monolithic / roughened / unroughened / steelACI 318-19 Table 22.9.4.2
- Maximum transfer capped by the concrete interface min(0.2 f'c, 480 + 0.08 f'c, 1600) AcACI 318-19 Table 22.9.4.4