SUMMARYNon-linear dynamic time-history analyses conducted as part of a performance-based seismic design approach often require that the ground motion records are scaled to a speciÿed level of seismic intensity. Recent research has demonstrated that certain ground motion scaling methods can introduce a large scatter in the estimated seismic demands. The resulting demand estimates may be biased, leading to designs with signiÿcant uncertainty and unknown margins of safety, unless a relatively large ensemble of ground motion records is used. This paper investigates the e ectiveness of seven ground motion scaling methods in reducing the scatter in estimated peak lateral displacement demands. Non-linear single-degree-of-freedom systems and non-linear multi-degree-of-freedom systems are considered with di erent site conditions (site soil proÿle and epicentral distance) and structural characteristics (yield strength, period, and hysteretic behavior). It is shown that scaling methods that work well for ground motions representative of sti soil and far-ÿeld conditions lose their e ectiveness for soft soil and nearÿeld conditions for a wide range of structural characteristics.
Building structures are typically designed using the assumption that the oor systems serve as a rigid diaphragm between the vertical elements of the lateral load-resisting system. However, long-oor span structures with perimeter lateral load-resisting systems possess diaphragms which behave quite exibly. The dynamic behaviour of such structures is dissimilar to the behavior expected of typical structures. This di erence can lead to unexpected force and drift patterns. If force levels are su ciently underestimated, inelastic diaphragm behaviour can occur, exacerbating the e ects of diaphragm exibility. Such response may lead to a non-ductile diaphragm failure or structural instability due to high drift demands in the gravity system.Analytical models were developed which capture the diaphragm exibility of structures with long-oor spans and perimeter lateral-systems. Modal examination and time-history analyses were performed to determine the e ect of diaphragm exibility and diaphragm inelastic behaviour on the dynamic behaviour of these structures.
This paper investigates design relationships to estimate the following four seismic demand indices for single-degree-of-freedom (SDOF) systems: (1) the peak displacement ductility demand, μ; (2) the cumulative plastic displacement ductility demand, μp; (3) the residual displacement ductility demand, μr; and (4) the number of yield events, ny. The main objectives of the study are (1) to develop relatively simple regression relationships that can be used to estimate mean values of these demand indices; and (2) to investigate the effects of structure yield strength, hysteretic behavior, fundamental period, site soil characteristics, seismic demand level, site seismicity, and epicentral distance on these relationships. It is shown that the correlation between μ and the other demand indices is usually relatively strong. In some cases, the cross-correlations between the demand indices are weak, indicating that these indices carry independent measures of seismic demand.
Building structures are typically designed using the assumption that the floor systems serve as a rigid diaphragm between the vertical elements of the lateral force-resisting system (lateral system). However, perimeter lateralsystem structures with long floor spans possess diaphragms that behave quite flexibly. Difficulty can exist in predicting diaphragm force demand in these structures. Thus, current design may provide insufficient strength to maintain elastic diaphragm response. Inelastic diaphragm response exacerbates the effects of diaphragm flexibility. Such response may lead to poor seismic performance, including nonductile diaphragm failure or structural instability due to high drift demands in the gravity system. An analytical study was performed to determine the effect of diaphragm flexibility and strength on the seismic performance of perimeter lateral-system structures with highly flexible diaphragms. Nonlinear transient analyses were performed using ground motions suites corresponding to multiple levels of hazard for high seismic zones. Design recommendations for flexible diaphragms are presented.
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