Study On Seismic Capacity Of Unboned Post-tensioned Hybrid Precast Concrete Frame

Prefabricated prestressed concrete frame structure is a kind of structure form of more energy-saving,environmental protection,compared to the traditional cast-in-site concrete structure.It is suitable for the industrialized production,conforming to the requirements of the ’outline of national medium and long-term science and technology development’.A new kind of unboned post-tensioned hybrid precast concrete frame(PTHP)is proposed.The PTHP connection is formed by the prefabricated beams,columns with corbel and stiffened angles through the high-strength friction bolts and the tendons.The tendons provide the self-centering capability,the stiffened angle provides the energy-dissipation capability,and the corbel supports precast beam and bears shear during construction.Because the damage concentrates in the steel angles and local beam end,the connection can achieve easy reparation,rapid recovery and implement industrial production after the earthquake.In order to further study the seismic perfonnance of the connection,low cycle loading tests,the finite element analysis and theoretical analysis are implemented for angle and joint respectively,furthermore,the design of the PTHP frame and the overall performance of static and dynamic elastic-plastic analysis of it have been carried out,according to the idea of the local connection to the whole structure.Based on the test results,the restoring force model of the common angle is modified.The calculation method is put forward for the yield and after yield bearing capacity of angle steel,according to geometric and material hardening.The finite element software ANSYS is used to establish the 3D modeling of steel angle.The force-displacement relationship of common angle and ribbed angle under monotonic loading is obtained.The correctness of the finite element analysis is proved by comparing with the experimental results.According to the analysis results,the relationship of the yield point and the initial stiffness value between the ribbed angle and no rib angle is obtained.The results show that the stiffener can effectively improve the mechanical properties of steel angle under cyclic loading.The seismic behavior of the PTHP joint is studied through the low cycle reciprocal load test.The bearing capacity,deformation capacity and failure mode of the joint are analyzed.The low cyclic loading tests of 3 PTHP joints with different average initial compression and angle specifications are carried out.Hysteretic hoops,skeleton curves,stiffness degradat:ion and energy dissipating capacity,ductility and so on are obtained.The tendons forces,deformation along the beams and strain of angles are analyzed.According to the energy dissipation coefficient,ductility coefficient and residual deformation rate of the joints,seismic capacity of the joint is evaluated.The modeling method of the joint based on fiber model is studied by using the finite element software OpenSees.According to the force-displacement relationship of the stiffened angles,the angle element is established.The equivalent static analysis of the PTHP joint under low cyclic loading is carried out.The results got from simulations agree well with the test results and are correct and reasonable.Several main factors that affect the PTHP joint behaviors,such as the initial concrete stress of the connection section,the effective stress of the post-tensioned tendon,and the strength of the concrete are analyzed and some design recommendations are given.Based on the theoretical analysis,the restoring force model of the unboned post-tensioned hybrid joint is proposed.Based on the theory of elastic beam and the principle of rotation of the rigid body,the composition of the beam end displacement at different stress stages is analyzed in detail.According to the stress equilibrium of the beam end section,rotation characteristics of the joint,the restoring force model of the angle under repeated tension and compression loading and the force-displacement relationship of the rib angle,the computing method for the load value of feature points on the skeleton curve are deduced.The deformation increment of prestressed tendon is deduced according to angle of the joint,and the resultant force is obtained.The theoretical calculation method is based on reasonable mechanical theory.The skeleton curve of the joint can be better predicted,which is the basis of the design of the joint.Based on the displacement based seismic design method,an improved design method is proposed for the PTHP frame structure.The design process is introduced,including connection construction of the joint,flexural design of interface of beam and column and mid-span of the beam,design method of precast beams and columns and corbels,the core zone of the joint,tendon and angle,and the corresponding calculation formulas are given.On this basis,as an example,a three span two storey PTHP frame structure of a typical multilayer workshop is designed.This method is simple and easy to understand.The design principle of ’strong column weak beam’ and ’strong shear weak bending’ can be effectively realized.The pushover analysis and dynamic elastoplastic analysis are carried out for the PTHP frame structure which is designed above by using the Opensees software.The seismic performance of the overall framework is evaluated.First,pushover analysis of the PTHP frame is carried out;the base shear-displacement of the vertices(V-u)curve,maximum drift,and the development and distribution of plastic hinges at the end of the supporting points are obtained.Finally,the article 6 seismic wave is selected to apply to the overall framework for dynamic elastoplastic analysis;the displacement,base shear and distribution of plastic hinges are analyzed,compared with the results of static elastoplastic analysis.The research results show that the comprehensive seismic performance of the unboned post-tensioned hybrid precast concrete frame(PTHP)is excellent and worthy of application.