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The cross sections of reinforced concrete column with axial load and biaxial bending often have three unknowns : (1) maximum concrete compressive stress ,(2) the inclination and (3) depth of neutral axis. Theoretically,they can be obtained with three simultaneous nonlinear equations of (F=0 ,(Mx=0, and (My=88饗 ecause it is difficult and complex to derive these three equations and the poor of numerical methods and computing tools, many reseachers usually made some simplifying assumptions in attempt to overme these difficulties, but the results can not reflect the actual behavior of biaxially loaded column. For example, the inclination of neutral axis was proposed, material stress-strain relationship was assumed to be linear, equivalent compressive stress block of concrete was estimated as ACI Code, or adopting equivalent springs to represent cross section of columns.Therefore, the aim of this paper is to directly, really, and completely derive three simultaneous nonlinear equations, and use computer program carry out fastly and exactly. At the same time, this method can be adopted to study about the behavior of biaxially loaded column later. This analysis is based on the assumption that plane section remains plan before and after bending, and utilizes the modified Kent and Park stress-strain curve for concrete and a complete idealized stress-strain curve for steel to integrate directly resultant force and moment of biaxially loaded reinforced concrete column section. Based on those, three simultaneous nonlinear equations is derived and solved by modified the Newton-Raphson method to obtain maximum concrete compressive stress in the cross sectio the inclination and depth of neutral axis. Besides, this paper modifies the equation of effective flexural rigidity presented by ACI Code, and use it to find the load-deformation curves of biaxially or uniaxially loaded columns, including the failure and unloading step of member. In order to verify the directly analytical method and stiffness equation of column presented by this paper, this research analyzes 40 experimental specimens of ultimate loads and 11 experimental specimens of P-△ curve for columns. By comparing test and theoretical values, it is evident that results show an excellent agreement. Besides, this paper investigates load- contour diagrams and neutral axis angle of biaxially loaded RC columns, and presents an equation to determine neutral axis angle at compressive ilure. Keyword : RC column , biaxially bending, column stiffness, failure surface, load-contour curve, the neutral axis angle.
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