Effects of Successive Earthquakes on Seismic Response of Reinforced Concrete Buildings in El Salvador

Abstract eng:
The effects of successive earthquakes in the response of RC moment-resisting frame systems for low ductility (intermediate moment frames) and high ductility (special moment frames) levels commonly designed in El Salvador is assessed in the present document. The outcome of the research may have great impact on the Response Modification Factor (R), Displacement Amplification Factor (Cd) and Allowable Drift (Δa), currently defined on the current national building code for seismic design in El Salvador, which still considers a single event as basis for the design of new structures. Three strong ground motion records from El Salvador are used, one near-fault source and two far-field source; with such input 15 sequences of one, two and three successive earthquakes are generated. Two sets of building are studied: a low ductility (1, 2, 3 and 4 stories) group and a high ductility (3, 6, 9 and 12 stories) group, 1353 analysis have been performed in total. The methodology comprised both nonlinear static analysis (static pushover) and incremental nonlinear dynamic analysis (dynamic pushover) for each model and earthquake sequence. Performance criteria at elements (local failure) and global failures are evaluated in all models. No damage evaluation in elements and torsion effects are considered. The results indicate that the Response Modification Factor “R” decreases as the number of records in the sequences of events increases, highlighting the importance of considering more than one event in its definition; additionally, for short periods (<0.43 sec.) the Response Modification Factor “R” is closely related to the elastic period of the buildings. The Displacement Amplification Factor “Cd” does not show significant variation by increasing the number of records in the sequence; however, it is observed this factor is linked to the elastic period of the building, which is opposed to the recommendations established on the current national building code of El Salvador. In addition, it has been found that low ductility buildings are being designed for greater forces than required, while high ductility buildings are being designed for lower forces than the ones recommended on the current national building code. Finally, results suggest the Displacement Amplification Factor found on the current Code is not conservative for buildings with periods longer than 1.0 sec whilst the Allowable Drift is considered conservative in all cases.

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