Torsional Strength of RC Beams Considering Tension Stiffening Effect

Stress-strain relationship for a steel bar embedded in concrete differs somewhat to that of a bare steel bar because the surrounding concrete is bonded to the bar. This behavior is known as tension stiffening. This paper presents the results of an analytical and experimental study on the performance of reinforced concrete beams subjected to pure torsion. In particular, the effect of the tension stiffening was discussed and included in the analytical study. Nine RC beams having different torsional reinforcements were tested. Although the torsional strength of RC beams according to the existing design codes (ACI 318-05, EC2, and JSCE-02) depends on neither the average yield stress of steel bars nor the tension stiffening effect, the test results indicated that the steel stress of the beams at peak load increased as the total percentage of reinforcement decreased due to the tension stiffening effect. A new equation including tension stiffening effect was proposed to predict the torsional moment capacities of RC beams. Comparisons between tested and calculated torsional moments of the seventy-one beams showed reasonable agreement.