This paper examined the effect of outrigger braces with improved arrangements on the seismic performance of diagrid steel structures to enhance the performance of a diagrid structural system. To this aim, a tall steel structure was selected and equipped with outrigger braces by placing truss braces in three different positions and arrangements, including horizontal, V-shaped, and \( {\Lambda }\) -shaped (IV-shaped). The V- and \( {\Lambda }\) -shaped arrangements constituted an improved outrigger-braced system. Perform3D was utilized for modeling and nonlinear incremental dynamic analysis (NIDA). Finally, the fragility curves for the models were derived through IDA analysis and a log-normal probability distribution. The numerical analysis demonstrated that improved outrigger braces would significantly enhance the seismic performance of high-rise buildings with a diagrid system. The location of the truss belt on the 25th and 50th floors dramatically improved the structure’s seismic performance and collapse margin ratio (CMR). Thus, the CMR value of the outrigger-braced models on the 25th and 50th floors was about 2.33 times the base model. Besides, the V-shaped truss belt in the outrigger system exhibited the best seismic performance. The CMR value peaked in this arrangement. The results of the pushover analysis showed that equipping diagrid tall buildings with outrigger braces had little effect on structural flexibility and only increased the energy absorption of the system.
