Keywords: steel, ductility, rotation capacity, classification criteria, seismic codes.

The design of dissipative structural systems is based on the capacity of members to withstand deformations in the elasto-plastic field without significant losses of resistance. Such structural systems dissipate the earthquake energy through inelastic extensions assuring the complete development of the collapse mechanism. For these reasons, it is necessary to ensure that dissipative zones have suitable ductility, while non dissipative ones have adequate overstrength to allow the inelastic strains of the dissipative parts. In the specific case of steel frame structures, modern codes for construction in seismic zones adopt a design criterion which considers, during destructive seismic events, the possibility of developing plastic hinges at the ends of the beams and at the base of the first floor columns. For frame structures the attainment of a globally ductile behaviour of the structure and of a locally ductile behaviour of the members is required. A measure of local ductility is represented by rotation capacity of members which can be compromised by buckling phenomena that can include both the section (local buckling) and the whole member (global buckling) in the absence of torsional restraints. For this reasons the Eurocode 3 subdivides transversal cross sections considering their geometrical characteristics, the steel type and the inside actions. Considering the limitations of the flange and the web slenderness parameters four classes of cross-sections are defined: plastic or ductile, compact, semi-compact and slender [1]. The new approach adopted by the Italian code (OPCM 3274) is instead based on the evaluation of the overstrength factor, which can be defined as the ratio between critical tension that brings local instability of the compressed flange or the flexural-torsional instability and the yield tension [2]. An empirical expression of this parameter is given taking into account not only the geometrical characteristics of the section but also the influence of moment gradient on the member behaviour. In this paper several comparisons between the classification criteria are discussed. The results obtained show that the most reliable approach appears to be the one proposed in OPCM 3274 [2], which is less conservative and allows the concurrence between theoretical and experimental classes in a greater number of cases.

1

European Commitee for Standardization (CEN), Eurocode 3 "Design of steel structures, part. 1 general rules for building", Brussels, 2005.

2

O.P.C.M. n. 3274, "First elements in the matter of general criteria for seismic classification of the national territory and of technical codes for structures in seismic zones", Official Gazette of the Italian Republic, Roma, 2003.

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