Stability Check of Steel Frames Exposed to Fire P. V. REAL, C. COUTO, N. LOPES and J. P. RODRIGUES ABSTRACT This paper aims at evaluating the validity of using in fire situation the same procedures used at normal temperature for checking the stability of unbraced steel frames. A comparison is made between simple and advanced calculation models. Based on a parametric study a procedure is proposed for evaluating the buckling lengths of columns in unbraced frames in case of fire. INTRODUCTION At normal temperature, where it is necessary to consider the influence of the deformed geometry of the structure (2 nd order effects) to verify the stability of columns belonging to a structural framed system, when frame imperfections are considered but member imperfections are not taken into account [1,2], two procedures can be adopted: i) perform a 2 nd order analysis including the effects of lateral displacements and check of the member instability with non-sway buckling lengths; and ii) perform a 1 st order analysis and check of the member instability with sway buckling lengths. For the first procedure, it should be noted that non- sway effective lengths can be used because no sway will occur in addition to that which causes the second-order effects calculated by a P- second-order analysis. For sake of simplicity, the column length of a member may be taken as its system length, which is safe and suggested by the Eurocode 3 [1] for normal temperature design. In fire situation, Eurocode 3 [3] states that, using simple calculation methods, a global analysis of the frame should be done as for normal temperature and the buckling length of a column for the fire design should generally be determined as for normal temperature design. However, in the case of a braced frame in which each storey comprises a separate fire compartment with sufficient fire resistance, the buckling length, l fi , of Paulo Vila Real, Carlos Couto, Nuno Lopes, LABEST—Department of Civil Engineering, Univ of Aveiro, Portugal. João Paulo Rodrigues, ISISE—Department of Civil Engineering, Univ of Coimbra, Portugal. 27