Steel Buildings in Europe

Part 7: Fire Engineering 7 - 54 Different types of convergence procedure are usually employed, such as the pure Newton-Raphson procedure and the modified Newton-Raphson procedure. The pure Newton-Raphson procedure is recommended for structures made of beam elements, and the modified Newton-Raphson procedure is recommended for structures made of shell elements. Static analysis is normally sufficient for modelling the behaviour of a structure in fire. However, local failure or instability of a structural member (such as lateral buckling of purlin) does not lead to overall structural failure. Consequently, analysis should be performed by a succession of subsequent static and dynamic analyses to pass instabilities and to obtain the complete failure mechanism to predict the influence of a local failure on the global behaviour of the structure and to follow eventually progressive collapse. It has to be kept in mind that here the aim is not the precise modelling of dynamic effects. So, default values of the main parameters fixed in models to determinate acceleration and damping effects can be used. Existing boundary conditions should be rightly represented. It is common to design structure by assuming pinned support conditions at the column bases. However, as fully pinned bases of columns are never achieved in reality, it is also possible, when data are available, to introduce semi-rigid connections. Where only a part of the structure is modelled, some restrained conditions from unmodelled part of the structure should be taken into consideration in appropriate way. The choices of restrained conditions that have to be applied at the boundaries between the modelled substructure and the rest of the structure have to be chosen by the designer. For example, in case of symmetry boundary, restraints to translation across the symmetry boundary and rotational restraint about the two major axes on the plane of symmetry are introduced in modelling. Usually, beam-to-column joints are assumed to be fully rigid in the fire design of steel and steel-concrete composite frames. However, in the case of steel frames based on lattice beams, joints between members of lattice beams and connections between top and bottom chords of lattice beams and columns can be assumed pinned or fully rigid according to the type of truss. Two types of action need to be applied to heated structures. The first type is static loading. It must correspond to that for fire situation. The second type consists of the temperature increase (above ambient) of the structural members obtained, from previous thermal analysis. Boundary conditions at supports as well as applied gravity loads are assumed to remain unchanged throughout the fire exposure It is important to choose an appropriate structural modelling strategy. Simulation of the mechanical behaviour of single-storey building in fire conditions can be performed either by a 2D or a 3D analysis. In a 2D analysis, simulation are performed in the plane of each portal frame, assuming a three dimensional behaviour of the frame to take into account the lateral instability of the members (columns, beams). In such modelling, adequate restraint conditions should be introduced to stabilize the frame laterally. In reality, these out-of-plane restraints are provided by roof structure