Steel Buildings in Europe

Part 7: Fire Engineering 7 - 51 voids (such as hollow steel section) should be considered in the thermal analysis. In principle, where the effects of a fire remain localised to a part of the structure, temperature distributions along structural members can be strongly non-uniform. So a precise calculation of temperatures should be determined by a full 3D thermal analysis. However, due to the prohibitive computing time of such analysis, it is often considered an acceptable simplification to perform a succession of 2D thermal analyses through the cross-sections of the structural members. Calculations are then performed at relevant location along the length of each structural member and the t emperature gradients are obtained, assuming linear variation between adjacent temperature profiles. This approach gives usually a reasonable approximation to the actual temperature profile through members and allows significant reduction of the modelling and numerical effort. In 2D thermal analysis, cross-sections of members are commonly discretised by means of triangular or quadrilateral plane elements with thermal conduction capability. All sections encountered in civil engineering can thus be modelled. Each plane element describing the cross-section can have its own temperature-dependent material such as steel, concrete or insulation materials. Boundary conditions can be either prescribed temperatures or prescribed impinging heat flux to simulate heat transfer by convection and radiation from fire to the exposed faces of structural members. Effects of non-uniform thermal exposure may be introduced in modelling with appropriate boundary conditions. Effects of mechanical deformations (such as buckling of steel element, cracking and crushing of concrete, etc.) on the temperature rise of structural members is neglected, which is the standard practice. Consequently geometry of structural members does not vary during the analysis As for simple models, the use of advanced models require knowledge of the geometry of structural members, thermal properties of the materials (thermal conductivity, specific heat, density, moisture...) and heat transfer coefficients at the member’s boundaries (emissivity, coefficient of heat transfer by convection). Usually for fire design, temperature-dependent thermal material properties of concrete and steel are taken from EN 1992-1-2 and EN 1993-1-2 and heat transfer coefficients are those given in EN 1991-1-2 respectively. 6.3 Structural models Advanced numerical models for the mechanical response should be based on the acknowledged principles and assumptions of the theory of structural mechanics. They are usually finite element models. They can simulate a partial or a whole structure in static or dynamic modes, providing information on displacements, stress and strain states in structural members and the collapse time of whole building if collapse occurs within the period of the fire. The changes of mechanical properties with temperature, as well as non-linear geometrical and non-linear material properties, can be taken into account in the structural fire behaviour. The transient heating regime of structures during fire