<div class="htmlview paragraph">The regular deformation patters of axially compressed box section beams is analysed.</div> <div class="htmlview paragraph">Assuming a simple collaps mechanism, the theory is then extended to cover the influence of strain rate sensitive material.</div> <div class="htmlview paragraph">The dynamic strength of a beam-column is significantly higher than the static strength. Correlation of the present theoretical results with the available experimental data is discussed.</div>