Prediction of the collapse mode of axially crushed profiles

If thin-walled steel profiles are axially crushed, two basic collapse modes can be observed. In the first case the profile crushes by progressively developing local folds. This regular folding is favourable as it promises a high level of energy absorption. In the second case the profile buckles in a global bending mode. Based on results of crash tests and validated numerical analyses, this paper develops an analytical method to predict the relevant collapse mode as a function of the crushing distance. The proposed analytical method successively compares the global buckling load, which changes as the crushing distance increases, with the constant mean crushing force during regular folding. A change of collapse mode is predicted if the global buckling load falls below the mean crushing force. With the available analytical method, design criteria are formulated such as the maximum initial length of a profile allowed to ensure a regular folding mode.