Torsional-Balanced Steel in Concrete Beams

Presently design of rectangular concrete beams under torsion is limited to underreinforced sections only. To ascertain the underreinforced character of a concrete beam it is necessary to know the balanced or maximum percentages of longitudinal and stirrup steel which can yield at failure of the beam. A semitheoretical approach based on space truss analogy has been presented to evaluate the torsional balanced percentages of reinforcements. For equal yield stresses, the balanced volume ratio between longitudinal and stirrup steel equals 1.3. Total percentage of torsional-balanced steel depends upon the concrete strength, yield stresses of reinforcements, the ratios of larger to shorter dimension of stirrups, and core area to total cross-sectional area of the beam. Beams failing under torsion can be classified according to yielding of both, either or none of the form of longitudinal and stirrup steel. Unlike flexure, a torsional-balanced failure is not accompanied by crushing of concrete.