Seismic behaviour assessment of eccentrically split-X braced frames

Eccentrically Braced Frames (EBFs) are lateral resisting systems with appropriate ductility and strength against earthquakes. An important type of such systems, recommended by Popov and also presented in American Institute of Steel Construction (AISC), is eccentrically split-X bracing. The axial force applied to the beam outside the link beam is reduced causing improvement in the behaviour of this type of bracing. In this research, for the first time, the ductility factor, overstrength factor and response modication factor of eccentrically split-X braces are investigated through nonlinear static and incremental dynamic analyses and fragility curves are presented for different ratios of link beam length to span length. For this purpose, three buildings, 2-, 6- and 10-storey structures with the ratios of link beam length to span length (e=L) of 0.05, 0.1, 0.15, and 0.2 are considered. The ductility factor of Rµ = 3:55, overstrength factor of Rs = 2:31 and response modication factor of RLRFD = 8:06 are calculated under 10 earthquake records. It is concluded that the most appropriate values of e=L ratio in the eccentrically split-X bracing are 0.1 for tall structures and 0.05 for small ones. According to the log-normal distribution, the fragility curves are also plotted considering Collapse Prevention (CP) and immediate occupancy (IO) performance levels.