Study On Seismic Performance Of Post-Tensioned Self-Centering Concrete Frames Considering Long-Term Prestress Losses

Post-tensioned self-centering(PTSC)concrete frames belong to a class of prefabricated buildings.In these frames,the precast beams and columns are compressed together by the unbonded PT tendons,and the tensile restraints at their interfaces are released.During the connection rotation,the precast members separate from each other with the gaps forming and propagating at the interface.The nonlinear responses of connections are accommodated by the gap opening behavior instead of the formation of plastic hinges.Seismic energy dissipations are provided by various damping devices appended to the connections.The hysteretic responses are concentrated on these changeable nonstructural elements,with limited damage in structural members.After unloading,the gaps are closed by the clamping forces provide by PT tendons,which is referred to as "self-centering".As a form of resilient seismic-resistant structural system,the PTSC concrete frames have been intensively investigated during recent 30 years.Although plenty of research efforts have been focused on the seismic performance of PTSC concrete frames,little attention is paid on their life-cycle behavior.As the constantly loaded precast members are subjected to the shrinkage and creep of concrete,the prestress losses inevitably occur in the PTSC concrete frames.In these structures,the prestressing forces act as the main providers of connection strength.Hence,the estimation of prestress loss and the evaluation of long-term structural seismic performance are necessary complements of this field.On the other hand,the numerical simulations and finite element analyses are efficient approaches for the seismic studies.Because of the gap opening behavior of PTSC connections,redundant elements are required for the structural modeling.Simple yet accurate models are necessary for the improvement of life-cycle analysis efficiency.Based on this background,the analytical models of PTSC concrete connections and the time-dependent connection models are developed in this study.Furthermore,the long-term seismic performance of PTSC concrete frames is assessed using the proposed models.The major contents are listed as follows:(1)An analytical procedure for the mechanical behavior of PTSC concrete connections is proposed.The moment-rotation relation at the beam end is derived according to the section analysis,and the force-displacement response of connection is obtained by incorporating all deformation components.The problem with respect to the violation of strain compatibility is settled based on the parametric study with the aid of the ABAQUS software.The relation between the maximum concrete compressive strain and the gap opening angle is identified according to the multi variant regression analysis.The accuracy of proposed analytical model is verified by comparisons with the finite element analysis results and the experimental data.(2)The long-term mechanical properties of PTSC concrete connections are studied.The prestress losses in PTSC frames due to the shrinkage and creep of concrete are estimated first.The influences of several structural parameters on the loss extent are investigated.Subsequently,the time-dependent analytical model for PTSC concrete connections is proposed with the incorporation of time-variant factors.Finally,the long-term connection performance is analyzed with the consideration of prestress loss and the evolution of concrete properties.The results show that,the span number and initial prestress level considerably affect the prestress loss,while the reinforcement ratio of mild steel has negligible impact on the loss extent.The prestress loss would decrease the moment resistance of connections at the early stage of loading.Nevertheless,the connection behavior under large rotation is mainly affected by the concrete strength.Thus,the continuing hydration of cement could offset the disadvantage of prestress loss,leading to the increment of connection stiffness in the late loading stage.(3)The numerical model of a five-story PTSC concrete frame is developed with the rotational springs incorporating the formerly developed connection models.The results of both static and dynamic analyses are compared with those achieved using the model built by conventional fiber-based beam elements.The comparison results validate the superior capacity of proposed modeling method in the simulation of structural seismic responses.The bivariant seismic fragility with the simultaneous consideration of maximum and residual deformations is selected as the seismic performance indicator of PTSC concrete frames,and the methodology is improved.The uncertainties with respect to both material properties and long-term effects are considered in the definition of bivariant limit states,with the probabilistic structural seismic capacity incorporated in the fragility analyses.(4)Utilizing the proposed time-dependent connection model and the validated structural modeling method,the lifetime seismic performance of PTSC concrete frames with various structural properties are assessed via bivariant seismic fragility analyses.The PTSC characters such as the energy dissipation factor and beam end restraint condition,as well as the long-term effects are considered.The results show that,the restraint condition could decrease the fragilities with respect to serious damage under MCE for more than 30%,and such fragilities are reduced 10%-40%by additional damping.However,comprehensively considering the maximum and residual deformations,the lifetime seismic performance of PTSC concrete frames does not monotonically increase with the appended energy dissipation,which could be optimized by strengthened connection restraint and appropriate damping ratio.Moreover,under MCE,the long-term effects would cause less than 10%changes of seismic fragilities for the frames that are not collapsed.However,the collapse probabilities of non-damped structures considerably increase with time.