First International Workshop « Structures in Fire » – Copenhagen – June 2000 . Lateral Torsional Buckling of Steel I-Beams in Case of Fire – Numerical Modelling Paulo M. M. Vila Real * ; Jean-Marc Franssen ** * Departamento de Engenharia Civil, Universidade de Aveiro – Campus Santiago - 3810 Aveiro, Portugal ( email: pvreal@civil.ua.pt , tel: 351-34-370049 ) ** Institut de Mécanique et Génie Civil - Service “Ponts et Charpentes”, Université de Liége – Chemin des Chevreuils, 1 - B 4000 Liège 1, Belgium ( email: JM.franssen@ulg.ac.be , tel: 32-4-3669251 ) ABSTRACT A geometrically and materially non-linear finite element program, i. e., a general model, has been used to determine the lateral-torsional resistance of steel I-beams under fire conditions, according to the hypotheses of Eurocode 3, Part 1-2. Two yield strengths, one cross section, one type of load and four durations of exposure to the ISO834 standard fire have been considered. The numerical results have been compared to the simple models presented in Eurocode 3, Part 1-2. These simple models lead to a safety level that depends on the slenderness of the beam, being unsafe for intermediate non-dimensional slenderness when compared with the general model. A new proposal has been made for a simple model that ensures a conservative result when compared to the general model. 1. INTRODUCTION Although the problem of lateral buckling of steel beams at room temperature is well known [12], the same problem at elevated temperature is not. Among the work done in this field there is the paper from Bailey et al. [16] who uses a three-dimensional computer model to investigate the ultimate behaviour of uniformly heated unrestrained beams. In this paper, a simple model for fire resistance of lateral-torsional buckling of steel I-beams is presented. It is based on the numerical results of the SAFIR program, a geometrically and materially non- linear code specially established for the analysis of structures submitted to the fire [3]. The capability of this code to model the lateral buckling of beams has been demonstrated [11] at room temperature by comparisons with the formulas of Eurocode 3, Part 1-1 [1]. 71