Fire design of steel

Steel does not burn, but it loses strength and stiffness as it heats. Above about 400 °C its yield strength starts to fall; by around 550 to 600 °C it retains only about half to two-thirds of its normal strength, and its modulus has dropped further. Because steel conducts heat well and thin sections heat quickly, unprotected members in a building fire can reach these temperatures within minutes.

Fire resistance is the time a member keeps its load-bearing function in a standard fire test, expressed as 30, 60, 90 or 120 minutes, set by building regulations according to occupancy and height. In India, required periods are in the National Building Code, and IS 800 has a section on fire resistance. The design load in fire is lower than at ambient, because full imposed load and wind rarely coincide with a fire, so the member only needs to reach a critical temperature at which its reduced strength equals that lower load.

How fast a section heats is measured by its section factor: heated perimeter divided by cross-sectional area. A light section with a large perimeter heats fast; a heavy compact section heats slowly. Protection slows the heating: sprayed fire-resisting material, boards, intumescent coatings that swell into insulating char, or concrete encasement and filling. The thickness required depends on the section factor, the critical temperature and the required period, and comes from tested data for each product.

Performance-based fire engineering goes further, modelling realistic fires and the behaviour of the whole frame, including the tensile membrane action of composite floors. Connections, which can be stressed by restrained thermal expansion and later by cooling, need particular care.