Evaluation of the ASCE 7-05 Standard for Dual Systems: Response History Analysis of a Tall Buckling-Restrained Braced Frame Dual System

In mid-to-high rise structures, dual systems (DS) enable a structural designer to satisfy the stringent drift limitations of current codes without compromising ductility. Currently, ASCE 7-05 permits a variety of structural systems to be used in combination as a dual system yet the design requirements are limited to the following: Moment frames must be capable of resisting 25% of the seismic forces while the moment frames and braced frames or shear walls must be capable of resisting the entire seismic forces in proportion to their relative rigidities. This paper assesses the significance of the 25% design requirement for the secondary moment frames (SMF) in dual systems with consideration of current structural engineering practice. Three 20-story buckling-restrained braced frame (BRBF) dual system structures have been designed with varying relative strengths between the braced and special moment frame systems. The SMF system has been designed for 15%, 25%, and 40% of seismic demands and the BRBF system design has been adjusted accordingly based on its relative stiffness with respect to the moment frame. These structures have been examined with nonlinear static and nonlinear dynamic procedures with guidance from ASCE 41-06. The drift, displacement and ductility demands, and the base shear distribution results of this study show similar responses of the three prototype structures. These results indicate a secondary moment frame designed to less than 25% of seismic demands may be adequate for consideration as a dual system regardless of the 25% rule.