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 />
Performance objectives (POs) typically include multiple goals for <strong>the</strong> performance<br />
<strong>of</strong> <strong>the</strong> constructed building: for example that it will be fully operational in <strong>the</strong> 43-<br />
year-event, that it <strong>of</strong>fers life safety in <strong>the</strong> 475-year-event, <strong>and</strong> it will not collapse in<br />
<strong>the</strong> 970-year-event. The selection <strong>of</strong> POs sets <strong>the</strong> acceptability criteria for <strong>the</strong><br />
design. Design criteria are rules <strong>and</strong> guidelines, which must be met to ensure that<br />
<strong>the</strong> usual three major objectives <strong>of</strong> design, i.e. performance <strong>of</strong> function, safety,<br />
economy, are satisfied. The performance levels are keyed to limiting values <strong>of</strong><br />
measurable structural response parameters, such as drift <strong>and</strong> ductility (monotonic<br />
<strong>and</strong> cumulative), structural damage indexes, storey drift indexes, <strong>and</strong> rate <strong>of</strong><br />
deformations such as floor velocity, acceleration <strong>and</strong> even <strong>the</strong> jerk (in case <strong>of</strong><br />
frequent minor earthquake ground motions). When <strong>the</strong> performance levels are<br />
selected, <strong>the</strong> associated limiting values become <strong>the</strong> acceptability criteria to be<br />
verified in later stages <strong>of</strong> <strong>the</strong> design. Note that once <strong>the</strong> limit value <strong>of</strong> <strong>the</strong> parameter<br />
has been selected for a particular earthquake hazard level, in order to completely<br />
define <strong>the</strong> design criteria it is still necessary to define <strong>the</strong> acceptable conditional<br />
probability <strong>of</strong> going beyond that limit state (failure probability).<br />
2.3.2 Performance Levels<br />
A building can be subjected to low, moderate, or severe earthquakes. It may cross<br />
<strong>the</strong>se events undamaged, it can undergo slight, moderate or heavy damage, it may<br />
be partially destroyed or it can collapse. These levels <strong>of</strong> damage depend on <strong>the</strong><br />
earthquake intensities. Low intensity earthquakes occur frequently, moderate<br />
earthquakes more rarely, while strong earthquakes may occur once or maximum<br />
twice during <strong>the</strong> life <strong>of</strong> <strong>the</strong> structure. It is also possible that no devastating<br />
earthquake will affect <strong>the</strong> structure during its life. In <strong>the</strong>se conditions, <strong>the</strong> checks,<br />
required to guarantee a good <strong>behaviour</strong> <strong>of</strong> a structure during a <strong>seismic</strong> attack,<br />
must be examined in <strong>the</strong> light <strong>of</strong> a multi-level design approach. The structure<br />
design procedure on <strong>the</strong> basis <strong>of</strong> multi-level criteria is not a new concept. Under<br />
gravity, live, snow, wind loads, <strong>the</strong> limit state design considers <strong>the</strong> service <strong>and</strong><br />
ultimate levels. In <strong>the</strong> case <strong>of</strong> <strong>seismic</strong> loading, <strong>the</strong> declared intent <strong>of</strong> building codes<br />
is to produce buildings capable <strong>of</strong> achieving <strong>the</strong> following performance objectives<br />
(Fajfar, 1998):<br />
16<br />
• to resist minor earthquakes without significant damage;<br />
• to resist moderate earthquakes with repairable damage;<br />
• to resist major earthquakes without collapse.<br />
However, as a rule, <strong>the</strong> majority <strong>of</strong> codes considers explicitly only one performance<br />
objective, defined as protection, in cases <strong>of</strong> rare major earthquakes, <strong>of</strong> occupants