In Japan, the building regulations do not allow for the conventional steelwork temperatures to exceed 350°C because the conventional carbon steel reduces its 0.2% proof stress at 350°C to 2/3 of its specified values at ambient temperature. This requirement generally leads to massive amounts of fire protection which is very costly. Consequently, the fire-resistant (FR) steels for buildings have been developed and commercialised.

Compared to carbon steels, FR steels have higher yield strength at elevated temperatures. Japanese Industrial Standard specifies that the 0.2% proof stress of FR steels at 600°C must retain at least 2/3 of the value at ambient temperature, that is, a value of 217 N/mm² for Grade SM490A steel with a yield strength of 325 N/mm² and a tensile strength of 490 – 610 N/mm². In other aspects, at ambient temperature, FR steels have the performance and weldability similar to carbon steels. Basically, FR steels have similar chemical composition to carbon steels of same strength grade, but have additions of chromium, molybdenum and other alloying elements to improve the yield point at high temperatures (Sakumoto et al. 1992).

In Japan, the FR steels have been widely used in building construction including multi-storey open car parks, sport facilities, atriums, railway stations, external steel frames and many other structures (Keira 1998). The use of FR steels successfully reduces the amount of fire protection and the structural members may even be unprotected in cases where the steel temperature would not exceed 600°C. However, little is known outside Japan about the performance of FR steels in fire. In fact, none of the codes and standards of European countries provide the provisions of using FR steels in building construction.

This section presents the basic information of FR steels, mainly the mechanical properties at elevated temperatures, obtained from literature on experimental research.