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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2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES<br />
3. Survivability-ultimate limit state (ULS) for earthquakes which may rarely<br />
occur represents <strong>the</strong> strongest possible ground shaking. For <strong>the</strong>se<br />
earthquakes, both structural <strong>and</strong> non-structural damage are expected, but<br />
<strong>the</strong> safety <strong>of</strong> inhabitants has to be guaranteed. In many cases damage is<br />
so substantial that structures are not repaired <strong>and</strong> demolition is <strong>the</strong><br />
recommended solution.<br />
Although it is recognized that <strong>the</strong> ideal methodology would use four or three levels<br />
for design, today current European Code methodologies <strong>and</strong> <strong>seismic</strong> design<br />
philosophy can be based on just two levels (Eurocode 8, 2002):<br />
1. Serviceability limit state, for which structures are designed to remain<br />
elastic, or with minor plastic deformations <strong>and</strong> <strong>the</strong> non-structural elements<br />
remain undamaged or have minor damage;<br />
2. Ultimate limit state, for which structures exploit <strong>the</strong>ir capability to deform<br />
beyond <strong>the</strong> elastic range, <strong>the</strong> non-structural elements being partially or<br />
totally damaged.<br />
In <strong>the</strong> Eurocode 8, <strong>the</strong> accelerations corresponding to <strong>the</strong> serviceability limit state<br />
are given as a fraction <strong>of</strong> <strong>the</strong> corresponding ones for <strong>the</strong> ultimate limit state.<br />
Generally, this methodology cannot assure controlled damage, also because <strong>the</strong><br />
determination <strong>of</strong> this relationship is not clearly assigned in <strong>the</strong> code.<br />
When <strong>the</strong> number <strong>of</strong> performance levels is discussed, one must recognize that <strong>the</strong><br />
use <strong>of</strong> four levels is <strong>the</strong> most rational proposal <strong>and</strong> that two levels represent <strong>the</strong><br />
minimum acceptable option. Since it is questionable to ask design engineers to<br />
perform too much verification, it seems more rational to introduce no more than<br />
three levels <strong>of</strong> verifications: serviceability, damageability <strong>and</strong> ultimate (survivability)<br />
levels. To achieve <strong>the</strong>se levels <strong>of</strong> verifications, <strong>the</strong> <strong>seismic</strong> design problem is laid<br />
out through required-available formulation. Currently, <strong>the</strong> required-available pairs<br />
<strong>of</strong> three mechanical characteristics considered in <strong>seismic</strong> design are:<br />
Required rigidity < Available rigidity ( 2.3 )<br />
Required strength < Available strength ( 2.4 )<br />
Required ductility < Available ductility ( 2.5 )<br />
The rigidity verification is a problem <strong>of</strong> <strong>the</strong> serviceability level, <strong>the</strong> strength<br />
verification is related to <strong>the</strong> damageability level <strong>and</strong> ductility verification is a<br />
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