In this paper, the seismic behavior of steel dual frames with large-scaled concentrically bracings have been evaluated at special ductility level, and then the ductility reduction factor and the over-strength reduction factor for this type of structural system have been calculated. To do so, multi-story buildings with large scaled X- bracing form were considered. Then, the push-over static analysis, non-linear incremental dynamic analysis and linear dynamic analysis were performed to determine the effect of building height on the behavior factor. This study shows that the seismic behavior, the ductility reduction factor and the over-strength reduction factor decrease when the number stories of building increase. But the decrease rate in ductility reduction factor is more than the decrease rate in over-strength reduction factor. Finally, the behavior factor for each building has been calculated separately and, mean values, 5.0 and 7.5 have been recommended for limit-states design and permissible (or Allowable) stress approaches, respectively.
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