Section 22.6.1/Two-Way Shear (Punching Shear)
ACI 318-19 Section 22.6 covers punching shear at slab-column connections including the d/2 critical section and three capacity equations.
Two-way shear (punching shear) must be checked at a critical section located at a distance of d/2 from the face of the column, concentrated load, or reaction area. The shear stress on the critical perimeter must not exceed the smallest of three values: 4 x sqrt(f'c), (2 + 4/beta) x sqrt(f'c), or (2 + alpha_s x d/bo) x sqrt(f'c), where beta is the column aspect ratio, alpha_s depends on the column location (interior, edge, corner), d is the effective slab depth, and bo is the critical perimeter length. For slab-column connections transferring unbalanced moment, a portion of the moment is transferred by eccentricity of shear per Section 8.4.4.2.
Why this section exists
Punching shear failure is a sudden, brittle failure mode where the column punches through the slab. Unlike flexural failures that show warning signs (cracking, deflection), punching shear failures occur with little warning and can trigger progressive collapse if the falling slab pulls adjacent connections. This failure mode was responsible for several notable building collapses. The three capacity equations address different failure mechanisms: the basic concrete shear capacity, the effect of rectangular (non-square) columns, and the effect of the critical section perimeter relative to slab thickness.
What plan reviewers look for
Plan reviewers check the punching shear calculation at every slab- column connection. They verify the critical perimeter is at d/2 from the column face. They check all three capacity equations and verify the smallest governs. They check for unbalanced moment transfer at edge and corner columns. For connections that fail the punching shear check, they verify that shear reinforcement (stud rails or stirrups) is provided per Section 22.6.8.