investigating the effect of earthquake duration on concrete structures by analyzing the frequency content of acceleration time history

The occurred earthquakes in recent years have proved that in some parts of the world, they possess more duration and cause further damage to structures. due to the fact that major cities of iran are located in the high seismic areas and also a significant part of existing buildings are concrete structures, it is necessary to study their vulnerability under long duration earthquakes. in this study, the performance of concrete structures affected by earthquakes with different durations (from low to high) has been investigated. in order to investigate the duration of the earthquake, a new index is defined, which is the number of accelerated cycles where their period is close to the structure predominant periods (with 10% difference) and their corresponding acceleration is also higher than the plastic acceleration threshold of the structure. for this purpose, three concrete structures with, 3, 6, and 9 stories were designed with optimal sections and the predominant period and the threshold acceleration for entering nonlinear behavior were calculated. subsequently, all three buildings were investigated in two-dimensional seismo-struct software and nonlinear analysis was carried out considering the stiffness deterioration and strength reduction (takeda model). each of the 3, 6 and 9 story structures was subjected to 119 earthquake records, durations, distances, magnitudes, and different soil types. seismic performance of structures such as relative maximum displacement and number of plastic joints were investigated. the results of the analysis show that the use of the index introduced in this study is much more efficient than other definitions of duration for damage to concrete structures. in other words, duration of an earthquake, as defined by the number of cycles whose period is near the predominant period of the structure and accelerated beyond the plastic acceleration threshold of the structure, is very consistent with the rate of damage to the concrete structure. in continuation, the effects of the height and plastic acceleration threshold on various models are discussed in detail.