Analysis and modelling of the seismic behaviour of high ... - Ingegneria

1.1 Introduction
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1 INTRODUCTION
In the seismic design of framed structures the dissipation of input seismic energy
plays a fundamental role. The basic parameter in this approach is ductility,
considered as the ability of a structure to undergo serious plastic deformations
without losing strength. In design practice it is generally accepted that steel is an
excellent material for this purpose due to performance in terms of ductility. But the
recent earthquakes of Mexico City (1985), Loma Prieta (1989), Northridge (1994)
and Kobe (1995) have seriously compromised this idyllic image of steel as a
perfect material for seismic areas. A series of factors contributes to the poor local
ductility of steel structures. With regard to resistance these factors are the
discrepancies between real and design yield stress, the value of through thickness
resistance of steel, the need for requirements on toughness of the base and weld
material and the effect of strain rate. With regard to the action effect, other factors
contributed to a bad ductility: the past underestimates of needed plastic rotations,
the existence of 3D stress states created in welded connections of high beams, the
consideration of wrong stress distributions in beam ends, a bad design of
connections and the influence of the composite character of beams.
Aiming at making up for this lack, the world of civil structural research is moving to
give designers constructional rules that allow to design high-ductility structures in
seismic areas. An important evidence of these research activities is that the
verification of structure ductility must be quantified at the same level as the
strength and stiffness. It must be recalled that, in that concept, specific elements of
the primary lateral force resisting system are chosen and suitably designed and
detailed for energy dissipation under severe imposed deformations. The critical
regions of these members, often termed plastic hinges or dissipative zones, are