Hot finished carbon steel begins to lose strength at temperatures above 300°C and reduces in strength at steady rate up to 800°C. The small residual strength then reduces more gradually until the melting temperature at around 1500°C. This behaviour is similar for hot rolled reinforcing steels. For cold worked steels including reinforcement, there is a more rapid decrease of strength after 300°C (Lawson & Newman 1990). In addition to the reduction of material strength and stiffness, steel displays a significant creep phenomena at temperatures over 450°C. The phenomena of creep results in an increase of deformation (strain) with time, even if the temperature and applied stress remain unchanged (Twilt 1988).

High temperature creep is dependent on the stress level and heating rate. The occurrence of creep indicates that the stress and the temperature history have to be taken into account in estimating the strength and deformation behaviour of steel structures in fire. Including creep explicitly within analytical models, is complex. For simple design methods, it is widely accepted that the effect of creep is implicitly considered in the stress-strain-temperature relationships.

The thermal properties of steel at elevated temperatures are found to be dependent on temperature and are less influenced by the stress level and heating rate. This simplified the consideration of the thermal properties of steel in design methods.

Thermal and mechanical properties of different types of steel at elevated temperatures are discussed. These include:

• Hot rolled carbon steel
• Stainless steel
• Fire resistant steel
• Light gauge steel

The performance of bolts and welds in fire will also be discussed.