engleski

Fire Resistance of Steel Structures

Fire resistance of load-bearing structures, particularly steel ones, is currently being investigated worldwide. For determining of safety of steel structure exposed to fire, as random accidental action, it is possible to use only probabilistic methods, together with adequately laboratory researches. The procedure for determining the safety of the load-bearing structures consists of four steps. The first step includes the determination of heat action in case of fire. By applying the model of zones, based on an approximate formulation, for different fire loads, obtained by varying the surface of ventilation openings, it was possible to obtain the temperature curves as a function of time which were used as the extreme temperature influence upon the load-bearing steel structure. The next step includes the determination of heat action upon the structure. Heat transfer through the basic material was modelled by using a transient, non-linear model according to the obtained values of extreme temperature curves. The paper also includes the analysis of the state of extreme temperatures in the case of steel structure protection by incombustible mineral plates. Heating the structure brings about two effects: the elongation of the structure and the degradation of the mechanical properties of the material. This step is at the same time a third step in determining the safety of the load-bearing structures. Mechanical characteristics of the raw material are get by tension test on high temperatures, while the element behaviour on high temperatures is determined as a part of experiments in special laboratory researches made on the real constructions. The safety of steel structures expressed by the safety index  was also analyzed for the extreme limit state and the serviceability limit state both for the combinations of permanent action and accidental fire action upon the structure, which represents the fourth phase of the process. For determining of safety index  it is necessary to know basis variables of action and basis variables of resistance. Basis variables of fire action upon the construction are determined with above mentioned models, while basis variables of resistance are determined by experimental methods. On the basis of values for basis variables of resistance and fire action, values of safety index    will be determined, as well as degree of safety of the steel structures exposed to the fire. These investigations of the actual temperature-time parameter curves, for large compartments and different fire loads have been carried out in Croatia within the National Application Documents in the field of accidental fire action.

fire; fire resistance; steel structures

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