Keywords: performance-based design, steel structures, thermo-plastic material, geometric non-linearity, non-linear analysis, thermal buckling, fire resistance.

The purpose of this paper is to highlight the fundamental aspects of structural investigations carried out on steel structures subject to fire action. Steel structures are extensively used in multi-storey buildings because they have a great number of advantages: high ductility, and elevated strength while being lightweight. It is possible to quantify the structural performance of these steel elements subject to fire action using non-linear analysis and modelling the fire action with parametric curves [1]. The aim of this paper is to provide a performance based procedure [2] to analyze steel structures under fire action. This is made referring especially to isolated elements, which better help one to understand the complexity of the problem.

First a validation analysis is conducted. The results obtained are compared with those presented in the reference [3] and with those reported in the British Code [4]. In this manner, it is possible to evaluate that the modelling of the problem is correct.

The second application considers an isolated element with lateral expansion restrained subject to fire action. By means of non-linear analysis, it is possible to value how the coupling of the material [1] and the geometric non-linearity contributes to the collapse of the elements until the elastic-plastic buckling load is achieved. To assess the elastic-plastic buckling load, when the section enters a plastic field, the Euler's equation, used to determinate the critical buckling load in elastic field, can be still used on the condition that, the Young's modulus, is replaced by the value of the tangent modulus. This equation is called in literature Shanley's equation [5]. The collapse will be reach for values of critical temperature which will increase with the decrease of the slenderness of the element.

1

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

2

Foliente G., "Developments in Performance-Based Building Codes and Standards", Forest Products Journal, Vol.50, No 7/8, 2000.

3

Liew J.Y.R., Tang L.K., Holmaas T., Choo Y.S., "Advanced analysis for the assessment of steel frames in fire", Journal of Constructional Steel Research, Vol.47, 1998. doi:10.1016/S0143-974X(98)80004-7

4

BS5950: Part 8. "Structural use of steelwork in building, Part 8: Code of practice for fire resistance design", London: British Standards Institution, London, 1990.

5

Corradi dell'Acqua L., (in Italian), "Instabilità delle strutture", Edizione CLUP, Italy, 1978.

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