Nonlinear Analysis Of R/C Sections Subjected To Varying Axial Forces

In a seismic event, the reinforced concrete (RC) frame exterior columns and corner columns are subjected to varying axial forces due to existence of the overturning moment. Besides, the vertical component of the ground motion also may make the axial force in the column changing. According to reconnaissance of structural damage in recent earthquakes, the varying axial force may be the substantial factor inducing unexpected failure modes of RC structures. In this way, it is imperative to investigate effect of varying axial force on behavior of RC columns, that is, the bearing capacity, ductility and so forth. Variation of axial force in columns in a 7-story RC frame was investigated using nonlinear time history analysis program, IDARC-2D. Then the varying axial force was treated as varying axial loading path on a specific critical section of the column, which is also subjected to moment and shear while the latter is usually neglected. A sectional analysis algorithm was then developed to analyze M- relationship of such a section with varying axial loading.The main points of this paper are as follows:(1) Experimental data on seismic behavior of R/C rectangular, hollow square and circular columns under varying axial load were collected and analyzed. From the failure mode of the specimen subjected to varying axial load and experimental measurement of force-displacement relationship, the differences of seismic performance between varying axial load and constant axial load are pointed out. Besides, testing results with similar performance are chosen to validate the nonlinear section analysis program written by the author.(2) To count for axial loading paths in nonlinear section analysis, the varying axial forces of exterior and interior framed columns were discussed by pushover analysis and time-history analysis for a 2-bay 7-story frame with the design intensity of 8, by using EDARC-2D.(3) A program to account for axial load-moment interaction is developed. Any combination of axial load and moment when section failure occurs can be analyzed if the size, details of the section and the material properties are given. As a useful tool, it can help researchers to get the information of column critical section when a sectional failure occurs.(4) An algorithm to analyze moment-curvature of RC sections with constant axialload was developed by curvature control. Effects of various uniaxial constitutive models for concrete and steel on simulating results are compared by a numerical example. Furthermore, the quantitative effects of axial compression ratio on the section bearing capacity and the curvature ductility are discussed.(5) Based on the assumption that the plane section remain plane and basic idea of nonlinear incremental-iterative mixed method in finite element analysis, the cross-section stiffness matrix is built by using virtual-displacement principles and the central difference method respectively. To solve the nonlinear simultaneous equations of the cross-section, the Newton-Raphson method and modified Newton-Raphson method with incremental formation are employed. On this basis, an algorithm to analyze the moment-curvature relationship of RC sections subjected to varying axial load is put forward using the program, NARCS.(6) Numerical simulations of several numbers of column specimens subjected to varying and constant axial load were carried out. The validity and reliability of the program NARCS are thus proved and the effects of varying axial load on cross-section moment -curvature relations are discussed as a result.