Open stirrups are the primary choice for concrete beams primarily subjected to shear forces, especially where torsional effects are negligible. U-shaped stirrups, on the other hand, are strategically positioned on the tension side of beams, where shear cracks are most likely to develop.
However, when concrete beams are tasked with resisting significant torsion, the design paradigm shifts. Closed stirrups become the preferred reinforcement type. Their enclosed shape offers superior confinement to the concrete core, enhancing its torsional capacity and mitigating the risk of diagonal cracking under torsional stresses.
In essence, the choice between open, U-shaped, and closed stirrups is contingent upon the dominant loading conditions. While open and U-shaped stirrups excel in resisting shear forces, closed stirrups are essential for beams experiencing substantial torsional loads, ensuring structural integrity and preventing premature failure.