TORSIONAL ANALYSIS OF FLANGED CONCRETE ELEMENTS WITH TENSION SOFTENING

An efficient algorithm for the prediction of the torsional behaviour of concrete elements has been developed. The proposed approach is initially based on Saint Venant's elastic theory and uses a finite difference scheme resulting from a second order finite element shape function. It is applied to structural elements with arbitrary cross-section shapes, since it utilizes numerical mapping. It has no requirements of extended computing power because it does not involve the use of any stiffness matrix calculation. The present scheme preserves the practicality of the finite element method in treating irregular boundaries, while it makes use of the simple analysis of the finite difference method. Furthermore, the proposed method employs a stress-strain law for the modelling of concrete behaviour under tension, which includes the tension softening phenomenon. In this way, it models the real behaviour of the element after the development of microcracks at high torque and, particularly, near to the failure loading. Special failure criteria for the concrete are employed for cases of combined loading. Comparisons between the predicted results and the experimental ones for a series of concrete elements are presented. The examined elements are beams with rectangular, L and T shaped cross-sections, which are tested under torsion and under combined torsion, flexure and shear. These comparisons proved that the proposed algorithm properly describes the torsional behaviour of concrete elements, even under combined loading. Furthermore, it is proved that, taking into account the influence of the concrete tension softening, the predicted ultimate torque moment is in good agreement with the experimental one.