Modelling of unbonded post-tensioned concrete beams critical in shear

An attempt is made to analyze unbonded post-tensioned beams using numerical modelling based on the finite element method. A bond stress-slip model is formulated for use with two-node bond link elements to represent friction losses in unbonded tendons arising from profile curvature and tendon wobble. The formulation is incorporated into the two dimensional nonlinear finite element program VecTor2, to model a series of shear-critical concrete beams post-tensioned using unbonded tendons. The results from the analyses have demonstrated an ability to accurately simulate load deflection response, load capacity and variation of tendon forces, along with a good agreement of the crack patterns and failure mode. It is shown that neglecting friction effects in these beams has a minor influence on the computed response, while ignoring the increase of tendon forces during loading has a significant impact. The shear strengths predicted by the North American code provisions are generally over-conservative.