Influence of imperfections on the stability of beams with intermediate flexible supports

Abstract

For the prediction of the ultimate load of beams and columns, respectively susceptible to lateral torsional buckling and flexural buckling, numerical FEM-calculations including geometric imperfections and residual stresses (so-called “GMNIA analyses”), are becoming more commonly employed and recognized. In these analyses, the shape of the geometric imperfections is often based on the 1st buckling mode, seen as the worst case for these members. In this paper, the buckling behaviour of beams and columns with additional intermediate flexible supports is studied and illustrated, using additional spring elements with systematically varied stiffness. As expected, a decrease of the spring stiffness always leads to a reduced elastic critical buckling load. Nevertheless, when assuming geometric imperfections based on the 1st buckling mode, this may lead to a higher ultimate load than for the system with rigid intermediate supports, when identical imperfection amplitudes are used. These interesting results are presented and analyzed for two simple cases, in order to clearly show the main effects. These cases are: 1.) Columns under compression with additional flexible support at midspan; 2.) Beams under bending with additional flexible supports of the compressed flange only. For the latter case, a parametric study was performed, varying the number of intermediate supports and stiffness of the lateral supports.