A NUMERICAL MODEL FOR POST-TENSIONED PRECAST BEAMS WITH UNBONDED TENDONS

The modern seismic design of building is concerned with two distinct performance levels: protecting human lives in the case of large earthquakes, and provide structural systems with enhanced functionality by controlling structural damage. A 
\npromising resisting structural typology, to limit both generalized cracking as well as residual deformations on structural members, is adoption of unbonded post-tensioned precast concrete beams. 
\nIn this paper, a numerical model is presented and compared to experimental results. The model was developed to predict the shear-rotation relationship characteristic of unbonded post-tensioned precast concrete beam-columns connections with due attention to capturing the prestress loss due to cyclic loading. 
\nThe model adopts a recently proposed fiber model, developed within the diffuse plasticity approach. Great care has been devoted to selection of material laws for concrete, focusing on the capability to reproduce the gap-opening gap-closing mechanism at the beam-column interface, which is mainly responsible for of the prestress variation and contributes to possible yielding of the prestressing bars at a prescribed levels of column rotation. 
\nThe accuracy of the proposed models is illustrated by comparing the predicted behavior in terms of basic mechanical properties, such as shape of the hysteresis, sti ff ness degradation, failure mode and damage control, with results of recent experiments carried out on cyclically loaded precast beams, post-tensioned using unbonded tendons.