Modeling Schemes and Performance Comparisons of Unbonded and Partially Bonded Tendon in Post-Tensioned Concrete Beam

The modeling method of unbonded effects is a challenging and hot topic for the structural performance analysis of unbonded and partially bonded post-tensioned concrete beams. The main concerns accounting for the unbonded effects are the longitudinal free-slip behaviors and the vertical deformation compatibility relationship between the unbonded tendon and concrete beam. Three modeling schemes, namely, the beam-truss element model, the slipping cable element model, and the slack spring model, are presented in this paper. These modeling schemes are, for the first time, systematically compared regarding applicability, convenience, and accuracy. Then, these modeling schemes are applied to experimental beams with different tendon layouts and bonding conditions, including external tendons, internal unbonded tendons, and partially bonded tendons. The beam-truss element model and the slipping cable element model are only applicable to the fully bonded and unbonded members, respectively. The slack spring model is recommended as the generally applicable model for analyzing post-tensioned concrete beams with different bonding conditions. Crucial suggestions are put forward as to the zero-length slack spring element, which have the potential to improve the prediction accuracy for tendon stress. In addition, parametrical analysis is conducted to determine the influence of unbonded length on flexural performance. With the increase of unbonded length, the flexural capacity of the beam will decrease, but the self-centering performance can be improved. Interestingly, the effects of unbonded length on the structural deformability are not monotonic, and the reasons for this are clarified.