A direct theory of a one-dimensional structured continuum is introduce
d in order to study the postbuckling behavior of thin-walled beams. A
simply supported beam bent by end couples is analyzed showing that, in
the case of nonsymmetric cross sections, lateral buckling gives rise
to imperfection sensitivity. Then an axially loaded beam is studied ta
king also into account the interaction between torsional and flexural
buckling. The results obtained prove that in this case imperfection se
nsitivity, though slighter than in the previous case, arises also for
symmetric cross sections.