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

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2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES safety under a rare earthquake ground motion) to fully operational in a maximum credible (capable or considered) earthquake ground motion. A PO is a coupling of expected performance levels with levels of seismic ground motions. A performance level represents a distinct band in the spectrum of damage to the structural and non-structural components and contents, and also considers the consequences of the damage to the occupants and functions of the facility. Four discrete performance levels are identified in Figure 2.3, which gives a table that define them in terms of the various components of the building. The seismic hazard at a given site is represented as a set of earthquake ground motions and associated hazards with specified probabilities of occurrence. For instance, the term ‘rare earthquake’ refers to a set of potential earthquake ground motions that can produce a defined level of damage with a specific mean annual frequency (e.g. 475 years return period for standard buildings). The set of earthquake ground motions will vary not only for different seismic regions but also from site to site within a region because of variations in site conditions (topography and soil profile). Note that the return period, TR, and the exceeding probability in N years, pN , are two different ways of expressing the same concept and they are related by: 1 N p = , p = 1 − (1 − p ) ( 2.2 ) 1 N 1 TR where p1 is the annual probability of exceedance. The validity of these equations assumes that earthquake occurrences are independent events. This is not strictly correct, but the above relations are widely used to simplify the discussion of probability. Figure 2.3. Recommended minimum seismic performance design objectives for buildings after Bertero and Bertero (2002) 15
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): 16 • 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
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3. SEISMIC BEHAVIOUR OF BOLTED END
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5. SEISMIC BEHAVIOUR OF RC COLUMNS
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6. SUMMARY, CONCLUSIONS AND FUTURE
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Analysis and modelling of the seismic behaviour of high ... - Ingegneria

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