Structural steel has been used more and more to replace concrete structure simply because of its lightweight and speedy construction.
Aspect of life safety and property conservation most building regulations require certain elements of structure to have fire resistance. Whether or not an element requires fire resistance depend upon decisive parameters as size, use of building and the function of the element. When exposed to fire all commodities used structural materials lose some of their strength.
Steel in full stressed in accordance with BS 449 or BS 5950 Part 1 will lose its design margin of safety at temperatures around 550°C regardless of the grade of steel.
Steel has a high thermal expansion which can lead to thermal movement in other parts of the structure, which in turn may cause loss of integrity and stability. Unprotected structural steel cannot normally achieve practical periods of fire resistance by virtue of the relatively low temperature of 500oC – 550oC (for mild steel) at which it loses 50% of its strength. Failure will occur if the safety factor with respect to ultimate load is 2 (a normal figure in engineering practice). To satisfy the criterion of loadbearing capacity therefore, a loaded steel section generally needs to be protected to ensure that this temperature is not attained during the fire resistance period required – due consideration being given to the way in which continuity, restraint, etc may affect this value.
The performance of structural steel depends on the mass of the steel and the surface area exposed to heating. In the past, steel section sizes adopted for fire tests were identical and thus deemed to satisfy tables referred to sections of a minimum mass. In order to provide design information for the protection of a range of steel section sizes, the research in conjunction with industry developed a program of fire resistance tests designed to provide a computational method of assessment for the thickness of protection needed. Methods of affording protection to steel beams and columns by passive encasement systems and using intumescent coatings have been published by the ASFPCM (Association of Structural Fire Protection Contractors and Manufacturers, now the Association of Specialist Fire Protection Contractors and Manufacturers).