Designing reinforced concrete beams for shear involves a crucial aspect – determining the spacing between stirrups. Shear forces act perpendicular to the longitudinal axis of the beam. Typically, these forces are highest at beam supports and diminish towards the center of the span.
Reinforced concrete beams often experience cracks in flexure due to principal tension stresses. These stresses change from horizontal at the longitudinal reinforcement to 45 degrees at the neutral axis and vertical at the point of maximum compression stress.
Shear reinforcement, in the form of stirrups, plays a vital role in preventing and resisting shear cracks. Shear design considers flexural failure, diagonal tension failure, and shear compression failure as potential modes of failure.
Modes of Failure
Stirrups in shear reinforcement perform four essential functions:
The critical section for shear design depends on beam support conditions. ACI Code provisions guide shear strength calculations using the formula Vu = Vn, where Vn is the sum of concrete and stirrup contributions.
Shear strength (Vn) combines the strengths of concrete (Vc) and stirrups (Vs). For circular sections, ACI specifies area calculation using the product of diameter and effective depth.
The shear design involves various steps, including computing ultimate shear force, estimating concrete shear strength, and selecting appropriate stirrup spacing.
Consider a simply supported beam with a 400 mm width and 550 mm effective depth, subjected to a total factored load of 137 KN/m over a 6 m clear span. The beam is reinforced with tensile steel, and shear design using vertical U stirrups is illustrated.
In conclusion, designing reinforced concrete beams for shear involves a systematic approach considering various factors to ensure structural integrity and safety.
