Keywords: steel structures, sensibility analysis, thermo-plastic material, geometrical non-linearity, non-linear analysis, fire scenario, reticular covering.

The mechanic effect of fire action on the steel material has been already presented [1]. From this reference, the use of analysis with thermo-plastic material and geometric non-linearity and the modelling of fire action by the use of parametric curves permits the correct evaluation of the real behaviour of steel structures subject to the effects of fire [2].

The first application of this paper is conducted to evaluate what happens when, in an isolated element, the lateral expansion, due to the increase in temperature, is not restrained. Analyzing a simply supported beam, the so-called bowing effect is the main feature that is considered. This particular deformative mechanism can be taken only with an appropriate formulation of the analysis that implements geometric non-linearity. In fact, the coupling between material and geometry non-linearity, permits a progressive approach of extreme nodes of the beam, when the temperature increases and the resistance and the stiffness decreases. This effect is very important for the assessment of the real behaviour of steel elements subject to fire action [3].

Once these two basic aspects are understood, the next step is that to consider a real structure with reticular covering. Some considerations are done about the structural collapse when the construction considered is subject to localized fire. The main considerations are with regard to the static scheme of this structure. Where a structure is very redundant, as in the reticular covering, there are many alternative load paths, and large deformations that can develop without a loss of its capacity to carry the imposed loads, and the failure must be defined in a different way. This phenomenon creates sufficient reserve capacity to allow most such structures to survive fires with little structural damage [4]. Considering structural collapse, it is possible to state that the collapse of a single bar of a reticular covering is an important aspect but that does not compromise the global behaviour of the whole structure. Therefore, the local collapse of some cover of the bars must be distinguished from the collapse or loss of resistance of the vertical elements. The collapse must be evaluated as a function of the global behaviour of the whole structure, assigning particular importance to the elements of most resistance such as the columns, on which the numerical evaluation is focused.

1

Crosti C., Bontempi F., "Performance assessment of steel structure subject to fire action", in Topping B.H.V., Papadrakakis M., Editors, "Proceedings of the Ninth International Conference on Computational Structures Technology", Civil-Comp Press, UK, paper 213, 2008. doi:10.4203/ccp.88.213

2

Eurocode 3, Design of steel structures, Part 1.2: Structural fire design. Commission of the European Communities, Brussels, 1993.

3

Usmani A.S., Rotter J.M., "Fundamental principles of structural behaviour under thermal effects", First International Workshop on Structures in Fire, Copenhagen, Denmark, June 2000.

4

Bontempi F., (in Italian), "Robustezza strutturale", Proceedings of CRASC'06. Messina, Italy, 2006.

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