Study On Seismic Behavior Of Concrete Frame Columns Strengthened By External Prestressing And Vertical Unloading

In order to meet the urgent requirements of the newly updated seismic code and the reconstruction of existing old buildings,the application of reinforcement construction technology for actual buildings and the research on the reinforcement theory,simulation and test of frame column are constantly promoted.At present,the research on the reinforcement of columns or frame columns is basically focused on axial compression test,and the reinforcement method of frame columns is limited to improving the bearing capacity by applying circumferential constraints to the column body.In this paper,through the application of prestressing force at the end of beam and column,the traditional circumferential prestressed direction pointing to the center of column is changed to vertical conduction along the column height,so that the stress analysis of beam column after reinforcement is connected.Compared with the existing strengthening method in which the prestressing force only stays on the column,the new reinforcement method proposed in this paper is more in line with the positioning of frame column reinforcement.For the new strengthening method and device of frame column,the concrete reinforcement effect is studied from the aspects of equivalent stress,equivalent plastic strain,effective stiffness and relevant seismic performance index of the strengthened frame column.(1)The history of experimental and theoretical research on strengthening concrete columns at home and abroad is comprehensively introduced.It is found that the recent research progress mainly focuses on the exploration of new material reinforcement and the attempt of composite reinforcement.The idea of vertical unloading reinforcement is put forward,the installation method of new reinforcement method is given,and the difference in construction and design between new patent reinforcement method and traditional reinforcement method is compared.(2)The failure criterion and constitutive relation of the frame column are determined.The most suitable finite element model is selected.Several basic assumptions about the previous modeling are put forward,and the concrete reinforcement and component size of the frame column are determined.Finally,the finite element model of the frame column strengthened by external prestressing is established in the finite element software,which provides a good foundation for the study of the nonlinear behavior of the strengthened frame column under different load mechanisms.(3)In order to obtain the stress level and distribution of frame columns under static load,twelve concrete frame column specimen models under static loading are established.The influence of axial compression ratio,vertical unloading prestress and steel strip prestress on the mechanical performance of frame columns is studied under the condition of only considering the axial compression of columns.The structure shows that the reinforcement can improve the stress distribution of reinforcement and concrete of frame columns even in static state.In order to measure the degree of vertical unloading prestressed reinforcement,the definition of prestress division is proposed by analogy with the concept of axial compression ratio.(4)In order to explore the performance of strengthened frame columns under horizontal monotonic load,ten model specimens of frame columns were established.The bearing capacity performance of frame columns under horizontal monotonic loading,such as characteristic value load and ductility,was studied by analyzing three reinforcement related parameters including axial compression ratio change,vertical unloading prestress change and steel strip prestress.It is concluded that the new strengthening method is better than the traditional one under monotonic loading.(5)Ten frame column models are established to carry out horizontal low cycle cyclic loading.The characteristic load values and seismic performance indexes such as ductility,energy dissipation capacity,stiffness degradation and hysteretic curve of frame columns under horizontal low cycle cyclic loading are studied by analyzing the change of axial compression ratio,the change of vertical unloading prestress and the presence or absence of steel strip prestress.This paper summarizes the new discovery of frame column in low cycle repeated simulation,analyzes and discusses the advantages of the new reinforcement method,which paves the way for the follow-up exploration of the new strengthening effect of multi-storey and multi span frame columns.