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