A STUDY OF REINFORCED CONCRETE COMPRESSION MEMBERS UNDER BIAXIAL BENDING
Abstract
The main objective of this investigation is to develop and to apply the finite segment method to study the behavior and strength of slender reinforced concrete columns and composite columns under biaxial bending. A review of previous works on reinforced concrete columns subjected to biaxial loading is presented. For convenience, this review is divided into two categories; short columns, and long columns. Finite segment method uses the displacement field obtained from the solution of the governing differential equations of equilibrium, while the finite element method assumes a displacement field in its formulation. The finite segment method is further developed, refined, and applied here to predict the behavior and capacity of both reinforced and composite concrete columns. The method of analysis is developed in two steps: (1) The force deformation equilibrium equations on a reinforced concrete section are first formulated, from which the moment-curvature-thrust relationships are developed. Various factors influencing these relationships have been studied along with different loading patterns. Bresler's load contour interaction equation has been extended and verified for its applicability to composite sections. Bresler's equation for reinforced concrete sections has been previously established by various investigators. (2) The moment-curvature-thrust relationships combined with the governing differential equations of equilibrium for segments and the corresponding stiffness matrix that relates force and displacement components of a segment are developed. Direct stiffness method is then utilized to obtain the structural stiffness matrix. An interative procedure is used to overcome both geometric and material nonlinearities. The parametric study shows that the solutions based on the lower bound iterative procedure give better and stable results than those of the upper bound procedure. The computer model developed for the analysis of biaxially loaded columns has been verified by comparison of the results with the tests reported by a number of investigators. Excellent agreement is observed between the analytically obtained failure loads and deflections with those of experimental results for both reinforced concrete columns and composite columns. Comparisons between the analytical results and the predictions based on ACI building code are also made for both slender reinforced and composite columns under biaxial as well as uniaxial loading. For all the cases studied it is found that the ACI equations for the effective flexural stiffness of columns appear to be very conservative, especially when axial load is high. As a result, improved design equations similar to those of ACI equations (10.9) and (10.10) are proposed. Based on the extensive cases studied, it is concluded that equation (10.10) or its improved version proposed herein, should not be used in the case of high reinforcement ratio because in this case this equation can significantly underestimate the strength of reinforced concrete columns. Furthermore, equation (10.10) should not be used if the steel distribution is not of a conventional type as in the case of composite columns because it may produce unsafe results.
Degree
Ph.D.
Subject Area
Civil engineering
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