Keywords: fire, numerical, modelling, steel, structures, non-uniform temperature, beam axial restraint.

This paper presents a comparison of a specialized program dedicated to the analyses of structures subject to fire, CEFICOSS, and the commercially available program, ABAQUS. CEFICOSS (computer engineering of the fire design of composite and steel structures) is a special purpose finite element program developed in Liège, Belgium [1]. This program integrates thermal and structural analysis and is the predecessor to the SAFIR program.

The main objective is to validate the utilization of the ABAQUS program for steel structures submitted to elevated temperatures in order to use it in the ROBUSTFIRE project, a RFCS funded European project, to simulate composite open car parks subjected to fire.

This benchmark example is based on the paper by Franssen [2] published in 1995 concerning a natural fire test on a fully loaded, two dimensional, unprotected steel framework carried out in a purpose-built compartment in Cardington. The influence of the model definition, axial restraint to beam, thermal expansion and non-uniform temperature is discussed, and the differences between the results of ABAQUS and CEFICOSS as well as the experimental results are illustrated in the paper.

The numerical analysis is performed using the non-linear finite element package ABAQUS, v6.7 [3]. Nonlinearities arise from large-displacement effects and material nonlinearities. Temperature dependent material properties and nonlinear temperature gradient over the cross-section are also considered. Beam elements are used to model the steel frame, so the coupled thermal-stress analysis available in this program could not be used. A heat transfer analysis is first realised to obtain the temperatures in the beam and column cross-sections, using two-dimensional deformable elements. The heat flow from the environment to the section is assumed to be convective and radiative. These first thermal results are then used in the static general analysis, where temperature gradients in the beam elements cross-section are applied as predefined temperatures.

ABAQUS performed the heat transfer analysis well to obtain temperature distributions in structures and showed a good ability to simulate structural steel behaviour under fire conditions using beam elements. An average coefficient of variation of about 3% between ABAQUS and CEFICOSS was measured for the fire resistance.

1

J.M. Franssen, "Etude du comportement au feu des structures mixtes acier béton", PhD Thesis, University of Liège, 1987.

2

J.M. Franssen, G.M.E. Cooke, D.J. Latham, "Numerical Simulation of a Full Scale Fire Test on a Loaded Steel Framework", Journal of Constructional Steel Research, 35, 377-408, 1995. doi:10.1016/0143-974X(95)00010-S

3

ABAQUS Theory Manual & Users Manuals, Version 6.7, Hibbitt, Karlsson and Sorensen, Inc., USA, 2007.

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