However, the influence of seismic sequence on performance of structures has not been addressed in majority of existing seismic codes. Structures in earthquake-prone areas are not subjected to a single seismic event, but also to a seismic sequence consisting of mainshock and several aftershocks. The low-damage performance of composite frames is mainly emphasized as the number of storeys increases, while both the IDR and DI tend to fall within lower performance levels than those of the corresponding all steel frames for the same seismic intensity.
Compared to all steel framed structures, the composite frames considered here exhibit a better seismic performance with beams and columns exhibiting a lower DI. Design examples reveal that the DDC design method successfully estimates the targeted IDR for the desired seismic performance level as well as controls the DI in critical beam-to-column joints in order to avoid a soft-storey failure mechanism or partial loss of structure. This response databank is created by performing extensive parametric incremental dynamic analyses of many composite framed structures of the kind considered here under many seismic motions and different soil types. The necessary empirical expressions of the design method are derived by means of statistical and sensitivity analysis of a large response databank consists of IDR and DI that cover all the way from elastic behavior to final global dynamic instability. A reduced number of design iterations is achieved while the computationally demanding non-linear time-history analysis can be avoided. Through empirical expressions this method can estimate the inter-storey drift ratio (IDR) of a designed structure and evaluate the damage index (DI) of critical members for a given seismic intensity. The proposed seismic design method controls displacement and damage in a direct way for all seismic performance levels including the one near collapse. The result of the study indicates that with the increase in the number of aftershocks, significant deterioration of strength and stiffness takes place, resulting in the residual demand parameters exceeding the permissible limit.Ī displacement/damage controlled (DDC) seismic design method for composite (steel/concrete) frames, consisting of circular concrete-filled steel tube (CFT) columns and composite beams (steel beams connected with concrete floor slabs) is developed in this study. Several seismic demand parameters, namely, transient and residual top displacements, maximum inter-storey drift, residual storey drift, base shear and a number of plastic hinges, are used to evaluate the seismic performance of the frame for each earthquake shock. The present study investigates the adequacy of the current design practice to cater to the aftershock events using 11-storey RC frame subjected to a sequence of main- and aftershocks synthetically generated from the design response spectrum as specified in the IS code. No consideration is paid to the aftershock event, which is commonly associated with any major shock.
The seismic performance of the designed structure is evaluated using non-linear time history analysis for some specified ground motion or synthetically generated ground motion from the response spectrum. Two-level earthquakes are used along with the specified design response spectrum for arriving at the seismic design forces. The current practice of the seismic design of buildings relies on a single event of earthquake having a specified intensity measure.
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