Modeling of unbonded post-tensioned concrete beams critical in shear

A stress-slip constitutive model is formulated for use with two-node bond-link elements to represent friction effects occurring in unbonded tendons of post-tensioned concrete beams. The friction effects considered are those arising from profile curvature and tendon wobble; thus, changes in tendon force during stressing, setting, and subsequent member loading are inherently modeled. The formulation is incorporated into a nonlinear finite element algorithm, where it is found to simulate friction effects well tinder all stages of stressing and loading. A series of shear-critical beams is modeled and analyzed; the load capacity cracking patterns, and deformation response of all beams and the accompanying variations in tendon forces, are accurately simulated. It is shown that neglecting friction effects in unbonded post-tensioned beams has a minor influence air the computed response, whereas neglecting increases in tendon forces during loading has a major influence. Analyses based oil code specifications tend to be overly conservative with respect to shear capacity and are marked by wide scatter.