The local buckling of RHS members at elevated temperatures

In Eurocode 3, Part 1.2 (ENV 1993-1-2 1993) simple calculation models are given by which the load-bearing capacity can be determined for different structures at elevated temperatures. These simple models are restricted to steel sections where the first order theory in global plastic analysis may be used. Eurocode 3, Part 1.2 gives no simple calculation method for a class 4 cross-section but does mention that the steel temperature of a class 4 cross-section should be below 350 °C, otherwise a general calculation method should be used. If the temperature is limited to 350 °C the insulation thickness increases, making structures of class 4 cross-sections uneconomical. The aim of this study is to find a simple calculation method for calculating the fire resistance of certain structures that may fail by local buckling. A series of fire tests was carried out for cold-formed rectangular hollow sections (RHS 200 x 200 x 5 and RHS 150 x 100 x 3) of structural steel S355. Both concentric and eccentric compression tests were performed. The fire tests were transient-state tests (heating rate 10 *C per minute under constant load) on unprotected specimens. A proposition for a simple calculation method is submitted. According to the proposition the effective width at elevated temperatures may be determined with the same formulae as at normal temperature, but the yield strength and the modulus of elasticity must be reduced. The yield strength of steel is determined corresponding the 0.2% proof strain at elevated temperatures. The stress-strain relationship and the modulus of elasticity at elevated temperatures are determined according to Eurocode 3, Part 1.2. The effects of actions during fire exposure may be deduced from the properties determined at normal temperature, using the reduction factor. With the typical value of the reduction factor, the end temperature according to load-bearing capacity calculated with a proof strain of 0.2% is clearly above 350 °C for rectangular hollow sections RHS 200 x 200 x 5 and RHS 150 x 100 x 3. The fire tests were analysed with the finite element method (FEM). Both geometrical and material non-linearity were included and the used stress-strain relationship at elevated temperatures was as determined in Eurocode 3, Part 1.2.