Seismic behaviour assessment of eccentrically split-X braced frames

Eccentrically braced frames (EBF) are lateral resisting systems with appropriate ductility and strength against earthquakes. An important kind of arranging such systems, recommended by Popov and also presented in AISC, is eccentrically split-X bracing. The axial force applied to the beam outside link beam is reduced causing the improvement of the behaviour of this type of bracing. In this research, for the first time, ductility factor, overstrength factor and response modification 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. Ductility factor of R𝛍=3.55, overstrength factor of Rs = 2.31 and response modification 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.