oa Civil Engineer in South Africa - A consistent mixed approach to computer analysis of frames : technical paper



The flexibility or force method of analysis of structural frames is complicated by the choice of appropriate unknowns. A mixed approach is presented in which member-end moments and sway deflections are adopted as unknowns, thereby providing uniformity of approach for computer analysis. The corresponding rotation-compatibility and sway-equilibrium equations are described in an example of linear frame analysis. The theory is amplified to include the influence of axial force on member flexibility and P-A effects. Results of examples of elastic instability are compared-with established solutions. A simple transition to plastic analysis is made by replacing plastic end-moments by hinge rotations as unknowns. The method is extended to represent more general material non-linearity and combined elastic-plastic instability analysis. This formulation represents a consistent and relatively simple solution to linear and non-linear analysis of frames based on a mixed flexibility and sway-deflection method.


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