The effects of structural continuity on the fire resistance of concrete filled columns in non-sway frames

Abstract

This paper presents the results of a series of theoretical studies on the effects of structural continuity on the fire resistance of composite columns of concrete filled steel tubes. In particular, these theoretical studies concern the changes in column effective lengths, axial loads and bending moments under fire conditions and the effects of these changes on column fire resistance. These studies are carried out to confirm (or otherwise) the recommendation on column effective length in Eurocode 4 Part 1.2 and to put forward additional suggestions on column axial loads and bending moments at the fire limit state. Finite element analysis is used to evaluate the non-uniform temperature distributions in composite cross-sections and to analyse the structural response at elevated temperatures. The results of these studies have validated the recommendation in Eurocode 4 Part 1.2 that the boundary conditions for continuous columns may be assumed to be built-in. They also suggest that at the fire limit state, additional axial loads in columns due to restrained thermal expansions are very small and that column bending moments become much lower than those at ambient temperature. For calculating its fire resistance, a column may be assumed to be under pure axial load equal to its ambient temperature value, the result of which may only slightly overestimate the exact column fire resistance.