Seismic response modification factors of eccentrically braced steel structures

Determination of the behaviour factor and displacement amplification factor is one of the most important issues in the seismic design of structures. Seismic codes use the behaviour factor to consider the inelastic behavior of structures. The inelastic behavior of structures is defined by ductility factor. This factor refers to the ratio of the maximum structural displacement to the displacement corresponding to the yield strength. So, the determination of the behaviour factor is directly dependent on the ductility factor so that a greater ductility factor leads to the greater behaviour factor. The behaviour factor is related to other factors such as the overstrength, degrees of redundancy and damping. The calculation of the real lateral displacement of structures including the elastic and inelastic parts is essential for design purposes. Seismic codes use displacement amplification factor for the determination of the maximum real lateral displacement of structures  The main objective of this study is to evaluate the effects of the height of the structure and the length of the link beam on the seismic response modification factors including the behaviour factor and displacement amplification factor in eccentrically braced frames (EBFs). Since the seismic behavior of eccentrically braced frames depends on the link beam length, several eccentrically braced building structures with different lengths of the link beam and various numbers of stories including 2, 5, 10 and 15 story buildings were considered. The ASCE 7-10 code was used for seismic loading and the AISC 360-10 and AISC 341-10 codes were used for the design of structural models. The nonlinear static and nonlinear time history analyses were used to obtain the behaviour factor and displacement amplification factor. The values obtained for the behaviour factor and displacement amplification factor were compared with those prescribed by the ASCE 7-10 code. The seismic codes recommend constant values for the behaviour factor and displacement amplification factor for eccentrically braced systems. The results of this study show that the behaviour factor and displacement amplification factor for eccentrically braced structures significantly depend on the height of the structure and the length of the link beam. So, considering constant values for the behaviour factor and displacement amplification factor for eccentrically braced frames in seismic codes is not reasonable. Then, the effects of the height of the structure and the length of the link beam should be considered in these factors. Finally, with regard to the values obtained in this study, some equations were proposed to calculate the mentioned factors based on the length of the link beam and the fundamental period of structures. The results indicate that the behaviour factor and the displacement amplification factor decrease with the increase in the height of the structure and also by the increase in the length of the link beam.