In reinforced concrete beams, flexural strength (bending) is always accompanied by shear forces. Shear failures are characteristically brittle, sudden, and catastrophic, providing little to no warning compared to flexural failures. Therefore, design codes stipulate a lower strength reduction factor ($\phi = 0.75$) to guarantee that a flexural failure, if one occurs, will precede a shear failure.

Pure shear stress rarely causes concrete to fail. Instead, the combination of flexural tensile stress and shear stress creates principal tensile stresses that act diagonally across the member's cross-section. Because concrete is weak in tension, it cracks perpendicularly to these principal tensile stresses. This phenomenon is called diagonal tension, and it is the true cause of "shear" failure in concrete beams.

The nominal shear strength $V_n$ of a reinforced concrete section is the combined resistance provided by the concrete ($V_c$) and the shear reinforcement ($V_s$).

The fundamental LRFD requirement for shear design is:

Where $V_u$ is the factored shear force at the critical section, and $\phi = 0.75$.

The concrete shear capacity $V_c$ is derived from three primary mechanisms: the uncracked concrete compression zone, aggregate interlock along the diagonal crack surfaces, and dowel action of the longitudinal tension reinforcement.

The basic ACI 318 / NSCP 2015 equation for members subject to shear and flexure only is:

For a more detailed analysis that accounts for the longitudinal reinforcement ratio ($\rho_w$) and the shear-to-moment ratio ($V_u d / M_u$), a more rigorous equation can be used:

Where $\lambda$ is the lightweight concrete modification factor ($1.0$ for normal-weight concrete, $0.75$ for all-lightweight).

When $V_u > \phi V_c$, shear reinforcement (stirrups) is required to carry the excess shear. Stirrups act like the vertical tension members of a parallel-chord truss, resisting the diagonal tension.

For vertical stirrups crossing the $45^\circ$ diagonal crack:

Where:

The required shear reinforcement varies along the beam span as $V_u$ changes (e.g., maximum near supports, zero near midspan). The design is divided into specific zones:

In situations where a direct shear failure along a specific plane is possible (such as a construction joint, the interface between a beam and a slab, or a crack in a bracket), the concept of shear friction is applied.

Brackets and corbels are short, cantilevered members projecting from columns or walls to support heavy concentrated loads (like precast beams or crane girders). Due to their small span-to-depth ratio ($a/d < 1$), standard flexural theory does not apply.

To ensure every potential $45^\circ$ diagonal crack is intercepted by at least one vertical stirrup, maximum spacing limits are imposed.

Torsion occurs when a member is twisted about its longitudinal axis, such as spandrel beams supporting off-center slabs or curved continuous beams. Torsion causes diagonal tension stresses that spiral around the perimeter of the cross-section, leading to spiraling cracks.

Torsional effects can be safely neglected if the factored torque $T_u$ is below the threshold torsion ($T_{th}$). Below this value, torsion will not significantly reduce the shear or flexural strength of the beam. For nonprestressed members subject to axial load:

Where:

If $T_u > \phi T_{th}$, the beam must be designed for combined shear and torsion. Torsion is modeled using a space truss analogy, resisted by the concrete shell, closed stirrups, and longitudinal bars.