AN EQUIVALENT STRESS METHOD FOR CONSIDERING LOCAL BUCKLING IN BEAM FINITE ELEMENTS IN THE FIRE SITUATION

The use of slender steel sections has increased in recent years because they provide excellent strength to weight ratio. Yet, a major issue with slender sections is local buckling in compression zones. Several researchers have proposed design methods at elevated temperatures based on the effective width approach to calculate the capacity of the plates that compose these steel members, but this approach is not easily compatible with the implementation and use in Bernoulli beam elements. Another approach is the development of a stress based model, i.e. an “effective” constitutive law of steel. This approach was proposed previously by Liege University researchers for slender steel members exposed to high temperatures, and implemented within the framework of fiber type Bernoulli beam elements; however it was giving overly conservative results. This paper presents an improved temperature-dependent constitutive model for steel that accounts for local instabilities using the stress based method. The improved model is derived from refined plate analysis methodology and implemented in the SAFIR finite element analysis software. Validation shows good agreement against experimental and shell element analysis results.