Reasons for fire damage

All materials weaken with increasing temperature and steel is no exception. Strength loss for steel is generally accepted to begin at about 300ºC and increases rapidly after 400º.  By 550ºC steel retains about 60% of its room temperature yield strength. This is usually considered to be the failure temperature for structural steel.  However, in practice this is a very conservative assumption; low loads, the insulating effects of concrete slabs, the restraining effects of connections etc. mean that real failure temperatures can be as high as 750ºC or even higher for partially exposed members.