ABSTRACT
Current seismic design procedures implicitly permit the inelastic structural deformations under strong ground motions for economical reasons. The local ductility is used as a parameter for evaluating the available inelastic performances of structures. It is widely accepted that the material ductility cannot be used alone in evaluating the capacity of structure to dissipate seismic energy input. Due to this fact the code provisions take into account the ductility at the level of section, considering that the loss in ductility can be correctly predicted in this case. The Eurocode 3 classification in plastic, compact, semicompact and slender sections just on the basis of sectional properties and static behaviour, provides a great flexibility for the designer, allowing some improvement of structure ductility. Unfortunately, the use of this classification leads to many questions and uncertainties about the erosion of ductility at the level of member, mainly caused by the dynamic characteristics of seismic actions. Indeed, the actual deformation of a structure is influenced by the global structure response to very complex seismic actions. The multiple buckling modes and the interaction between these modes, the continuous changing in moment gradient during the earthquake action, the strain-rate produced by high velocity of actions, the important accumulation of plastic deformations due to hysteretic behaviour under strong ground motions cannot be ignored in design. If these factors are out of control during the structure design, the erosion in ductility can by very high and the structure damage very important (Fig. 6.1).
