The Lateral-torsional Buckling of Unrestrained Steel Beams in Fire

A three-dimensional computer model which is capable of predicting the structural behaviour at elevated temperatures of skeletal steel frames has been used to investigate the ultimate behaviour of uniformly heated unrestrained beams. A series of different sections and spans has been studied for different loading patterns and load ratios. The predicted results indicate failure by lateral-torsional buckling in all cases. A comparison has been made with the limiting temperatures which are given in BS5950 Part 8, and in most cases the model predicts lower failure temperatures tha does the code. This is in contrast to the extremely accurate limiting temperatures which the Code specifies for restrained beams. A similar finding is obtained when the results are compared with limiting temperatures obtained from EC3 Part 1.2. It can be seen that failure temperatures generally depend on the ambient-temperature ratio of lateral-torsional buckling resistance moment to in-plane moment capacity, with the last slender cases failing at the lowest temperatures for any given load ratio.