Section 9.6.3/Minimum Shear Reinforcement
ACI 318-19 Section 9.6.3 establishes when minimum shear reinforcement (stirrups) must be provided in beams.
Minimum shear reinforcement must be provided in all regions of flexural members where the factored shear force Vu exceeds 0.5 times phi times Vc (half the concrete shear capacity), except in slabs, footings, and certain shallow members. The minimum area of shear reinforcement is the greater of 0.75 times the square root of f'c times bw times s divided by fyt, or 50 times bw times s divided by fyt. Maximum stirrup spacing is d/2 or 24 inches, whichever is less.
Why this section exists
Concrete beams without stirrups can fail suddenly in shear with no warning. Even when the calculated shear demand is less than the concrete capacity, minimum stirrups are required to prevent a brittle failure if loads exceed expectations or if the concrete has localized weaknesses. The minimum stirrups hold diagonal cracks together, converting what would be a sudden collapse into a ductile failure that gives warning through visible cracking and deflection.
What plan reviewers look for
Plan reviewers check beam schedules and sections for stirrups throughout the span. They verify that minimum stirrups are provided wherever Vu exceeds 0.5 phi Vc. They check the stirrup size and spacing against the minimum area formula. They verify that the maximum spacing (d/2 or 24 inches) is not exceeded anywhere along the beam.
Common violations
Related ACI 318 requirements
Section 22.5.1 covers the shear strength calculation. Section 9.7.6covers reinforcement detailing at supports. Section 18.6 covers additional shear reinforcement requirements in seismic moment frames.Section 9.4.3 defines the critical section (d from face of support).