Section 9.4.3/Factored Shear and Critical Section
ACI 318-19 Section 9.4.3 establishes the critical section at distance d from support face for shear design of beams.
For non-prestressed beams and one-way slabs, the maximum factored shear Vu for the design of shear reinforcement is permitted to be computed at a distance d (effective depth) from the face of the support, provided the support reaction introduces compression into the end region of the member, loads are applied to the top of the member, and no concentrated load occurs between the face of support and the critical section. If these conditions are not met (hanging loads, concentrated loads near supports, or tension reactions), the critical section is at the face of the support. For two-way shear (punching shear at columns), the critical section is at d/2 from the face of the column.
Why this section exists
The region between the face of the support and distance d acts as a compression strut that transfers shear directly to the support through a diagonal compression field. This region has higher shear capacity than the beam theory predicts. Designing for the shear at distance d (rather than at the face) avoids unnecessarily conservative stirrup designs. However, when the conditions are not met (for example, a hanger connection applying tension or a heavy point load near the support), the compression strut mechanism does not develop, and the full shear at the face must be used.
What plan reviewers look for
Plan reviewers check the shear design calculations to verify the critical section location. They check whether the conditions for using the d-from-face location are met: compression reaction, top loading, no concentrated loads in the end region. For transfer beams, hangers, and members with point loads near supports, they verify the critical section is taken at the face of the support.