Shear Friction Across an Interface (ACI 318-19 22.9)
The mechanism that carries shear across a cold joint, a corbel interface, or a wall-to-footing plane - not the beam-web diagonal-tension of rc-beam-shear: 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.
Formula and source
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.
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