Keywords: moment-curvature analysis, composite steel-concrete sections, arclength method, biaxial bending..

This paper presents two new computer methods for moment-curvature analysis of arbitrary-shaped composite steel-concrete cross-sections that are subjected to biaxial bending and axial force. The complete moment-curvature diagrams are determined in two different ways such that axial force and bending moment ratio is kept constant: (a) for given path of loading in conjuction with arc-length control; and (b) for given value of the strain at the most compressed point. The main advantage of the second approach is that the solution is obtained by solving just two coupled nonlinear equations and the convergence stability is not sensitive to the initial or starting values of the iterative process and to the strain softening exhibited by the concrete in compression since the Jacobian of the resulting nonlinear system of equations is always positive definite. The procedures developed may be used to assess the main features of elasto-plastic behaviour: multiple yielding points, flexural and axial rigidity, moment-curvature relationship, curvature ductility. The methods developed show very good stability in the presence of the strain softening effect exhibited by the concrete in compression and tension and convergence stability is not affected by the shape of the stress-strain relationships of concrete, the type and amount of reinforcements or the residual stress distribution in encased steel elements. In order to illustrate the proposed method and its accuracy and efficiency, a computer program has been developed and used to study several representative examples, which have been studied previously by other researchers. The examples run and the comparisons made prove the effectiveness of the proposed method of analysis.

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