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

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