Section 22.5.1/Shear Strength of Concrete Members
ACI 318 Section 22.5 covers nominal shear strength including concrete (Vc) and stirrup (Vs) contributions.
The nominal shear strength (Vn) of a concrete member is the sum of the concrete contribution (Vc) and the steel reinforcement contribution (Vs). For non-prestressed members, Vc is typically 2 times the square root of f'c times bw times d (simplified formula). When the factored shear force (Vu) exceeds the capacity of concrete alone (phi times Vc), transverse reinforcement (stirrups) must be provided. The maximum stirrup spacing is d/2 or 24 inches, whichever is less. When Vs exceeds 4 times the square root of f'c times bw times d, the maximum spacing reduces to d/4.
Why this section exists
Shear failures in concrete beams are sudden and catastrophic, unlike flexural failures which are gradual and give warning through deflection and cracking. Diagonal tension cracks propagate rapidly through unreinforced concrete. Stirrups cross these diagonal cracks and hold the beam together, converting a brittle shear failure into a ductile mechanism. The shear design provisions ensure that every concrete beam has adequate capacity to resist diagonal tension.
What plan reviewers look for
Plan reviewers check beam sections and schedules for stirrup sizes, spacing, and extents. They verify that minimum shear reinforcement is provided where required per Section 9.6.3. They check that stirrup spacing decreases near supports where shear is highest and that the maximum spacing limits are met. For deep beams, they check for additional requirements per Section 9.9.
Common violations
Related ACI 318 requirements
Section 9.6.3 covers minimum shear reinforcement. Section 9.4.3covers the critical section for shear (d from face of support). Section 22.5.5 covers the Vc equations. Section 22.5.10 covers the Vs equations for stirrup contribution.