Effect of temperature profile

A joint test programme by Corus (now Tata Steel) and the Building Research Establishment has shown that the temperature profile through the cross-section of a steel structural member has a marked effect on its performance in fire.

The basic high temperature strength curve, Figure 31, has been generated by testing a series of small samples of steel in the laboratory, where the whole of each test sample is at a uniform temperature and is axially loaded.

When these conditions are repeated in full scale member tests, eg unprotected axially loaded columns, then failure does indeed occur at 550°C. But if a member is not uniformly heated then, when the hotter part of the section reaches its limiting temperature, it will yield plastically and transfer load to cooler regions of the section, which will still act elastically. As the temperature rises further, more load is transferred from the hot region by plastic yielding until eventually the load in the cool regions becomes so high that they too become plastic and the member fails.

The most common situation in which temperature gradients have a significant effect on the fire resistance of structural steel is where beams support concrete slabs. The effect of the slab is both to protect the upper surface of the top flange of the beam from the fire and to act as a heat sink. This induces temperature differences of up to 200°C between the upper and lower flanges in standard fire tests. Test data shows that the limiting (lower flange) temperature of fully loaded beams carrying concrete slabs is about 620°C. This compares with 550°C for beams exposed on all four sides.