An early concept was to relate performance levels to the physical condition of the building as it was subjected to increasing lateral deformation (SEAOC, 1995). Figure 1.9 illustrates three performance levels introduced in FEMA 273 (1997) and continued in ASCE 41 (2013). The performance level Immediate Occupancycorresponds to a state in which some damage may have occurred, but after cosmetic repairs the structure can be occupied and functional. Collapse Preventionis a point in the response just prior to onset of collapse. Life Safetyis a term used to define a performance state with a “comfortable” margin below the collapse state. In ASCE 41, the margin is set at about three-quarters of the displacement corresponding to the collapse performance state, but in ASCE 7 and the building code, this margin is two-thirds. Figure 1.9 implies that performance states are a function of the deformations imposed on the structural and nonstructural systems. Performance of contents and other items that are not rigidly fixed to the structural system can instead be a function of floor acceleration or velocity.Building performance should be defined by the performance of the building system as a whole. It can be difficult, however, to quantify performance metrics for building systems. Therefore, as a practical matter, a common practice is to define system performance based on the performance of individual structural (or nonstructural) components that compose the building system. In effect, the building performance is defined as being equal to the worst performance of any of the components of the building. This approach, which is adopted in ASCE 41, tends to be a very conservative approach.FIGURE1.8 Performance objectives suggested by SEAOC (1995).OperationalImmediateoccupancyLifesafetyCollapsepreventionFrequent(43 years)Occasional(72 years)Rare(475 years)Very rare(970 years)Basic objectiveEarthquake design levelSystem performance levelxxxx++Unacceptable performanceSafety critical objectiveEssential/hazardous objective01_Moehle_Ch01.indd 148/20/14 11:44 AMFor use in CE 244 2014 only
7 3/8 x 9 1/4 Technical / Radar Signal Processing / Richards / 474-2 / Chapter 1S e i s m i c D e s i g n a n d P e r f o r m a n c e V e r i f i c a t i o n 15The preceding discussion emphasizes the current approach of gauging performance using structural engineering metrics, such as displacement, story drift, floor acceleration, inelastic deformation, and component forces, all compared with values that are considered acceptable. It is also feasible to translate these engineering metrics into damage states, and from there into consequences such as casualties, repair costs, and downtime. This approach is not commonly applied today, but the capabilities exist and are occasionally applied for special buildings.
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