Steelwork fire resistance

Fire resistance is usually expressed in terms of compliance with a test regime outlined in BS476 Part 20 and 21.  It is a measure of the time taken before an element of construction exceeds specified limits for load carrying capacity, insulation and integrity. These limits are clearly defined in the standard. The characteristics of the time-temperature relationship for the test fire from BS476 are shown in Figure 30 shown below.

All materials become weaker when they get hot. The strength of steel at high temperature has been defined in great detail and it is known that at a temperature of 550ºC structural steel will retain 60% of its room temperature strength, Figure 31 shown below.  This is important because, before the introduction of limit state design concepts, when permissible stress was used as a basis for design, the maximum stress allowed in a member was about 60% of its room temperature strength. This led to the commonly held assumption that 550ºC was the highest or critical temperature that a steel structure would withstand before collapse.

Recent international research has shown, however, that the limiting (failure) temperature of a structural steel member is not fixed at 550ºC but varies according to two factors, the temperature profile and the load.