Computerised interaction diagrams and moment capacity contours for composite steel–concrete cross-sections

A new incremental-iterative procedure based on the arc-length constraint equation is proposed in order to determine both interaction diagrams and moment capacity contours for composite steel–concrete cross-sections. This procedure adopts a tangent stiffness strategy for the solution of the nonlinear equilibrium equations thus resulting in a high rate of convergence. The proposed approach has been found to be very stable for all cases examined herein even when the section is close to the state of pure compression or tension or when there are multiple solutions, and it is not sensitive to the initial or starting values, to how the origin of the reference loading axes is chosen and to the strain softening exhibited by concrete in compression. Furthermore, the proposed method can be applied to provide directly the ultimate resistances of the cross-section, in the hypothesis that one or two components of the section forces are known, without the need of knowing in advance the whole interaction diagram or moment capacity contour. An object oriented computer program with full graphical interface was developed, aimed at obtaining the ultimate strength of composite cross-sections under combined biaxial bending and axial load. In order to illustrate the proposed method and its accuracy and efficiency, this program was used to study several representative examples, which have been studied previously by other researchers using independent fiber element solutions. The examples and the comparisons made prove the effectiveness and time saving of the proposed method of analysis.