Study of Column Buckling Lengths of Braced and Unbraced Frames in Fire Situations

The problem of fire resistance of steel structures is still a very actual topic, because there is a need to suitably evaluate the fire resistance and safety of structures exposed to fire hazard. Rules given in the parts of structural Eurocodes devoted to fire design do not cover all relevant design issues. In the case of the buckling resistance of steel columns exposed to fire the Eurocode gives rules only for braced frames in which each storey comprises a separate fire compartment, where it is possible to reduce the buckling length to 0.5L (intermediate storey) or 0.7L (top storey) thanks to the fact, that the hot column is „fixed“ to the cold ones in the storeys above and under. For the case of unbraced frames there are no special rules, the columns should be verified with the procedures as for normal temperature design. Several authors dedicated their efforts to the problem of modelling the real behaviour of entire frame buildings in fire situation. The aim of this paper is to show the behaviour of braced and unbraced frame structure by simple analysis with a FEM programme and to compare the results with some other procedures from the literature.