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 heat ed unrestrained beams. A series of different sections and spans has be en studied for different loading patterns and load ratios. The predict ed 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 fai lure temperatures tha does the code. This is in contrast to the extrem ely accurate limiting temperatures which the Code specifies for restra ined beams. A similar finding is obtained when the results are compare d with limiting temperatures obtained from EC3 Part 1.2. It can be see n that failure temperatures generally depend on the ambient-temperatur e ratio of lateral-torsional buckling resistance moment to in-plane mo ment capacity,with the last slender cases failing at the lowest temper atures for any given load ratio.