Bifurcation experiments on locally buckled Z-section columns

The paper describes a series of tests on thin-walled plain Z-sections compressed between fixed ends. Two tests each were performed on seven lengths covering the range from short stub columns to long columns. Most columns failed by interaction of local and overall flexural buckling. The test results are compared with theoretical solutions for the bifurcation load of a locally buckled point-symmetric column. The theoretical buckling loads require calculation of the instantaneous stiffnesses against minor and major axis flexure. These stiffnesses are obtained using a geometric and material nonlinear finite strip analysis. The theoretical results predict that while overall bifurcation of a non-locally buckled column will occur by pure flexure about the minor axis, the locally buckled column will bifurcate in a flexural mode involving coupling between minor and major axis flexure. This result is verified using a geometric nonlinear finite element analysis.