Nonlinear analysis of thin-walled members under biaxial bending

A numerical method for the nonlinear and instability analysis of thin-walled members (NISAT) subjected to biaxial bending has been developed which includes the warping of degree freedom at each mode. The analysis includes the effects of geometric nonlinearity, out-of-plane bending and the consequent torsion, secondary effects of load applied away from the shear centre, large displacements and rotations in the three-dimensional space and initial imperfections. The effect of large displacements is included in the nonlinear analysis following the updated Lagrangian method where the nodal coordinates are updated before every load increment. The analysis does not consider distortion of the cross-section. The nonlinear biaxial bending behaviour of thin-walled Z- and sigma sections subjected to transverse load is studied analytically using the NISAT program and the results are compared with experimental values. The failure load, midspan displacements and the normal stresses at critical points of the midspan section are also compared. © 1994.