This paper proposes an extension of the Improved Forced Based Design procedure to 3D steel structures. The Improved Forced Based Design (IFBD) procedure consists of a more rational sequence of the design checks proposed in Eurocode 8 and involves a more realistic selection of the behaviour factor instead of selecting an empirical value based on the ductility class and lateral resisting system adopted. The design procedure was tested on a group of four 3D steel structures, composed by moment-resisting frames with three storeys height and the same plan configuration in all storeys. The plan configuration was defined in order to target lateral restrained or unrestrained systems as well as plan regular or irregular structures. The same group of structures was also designed according to the force-based process prescribed in Eurocode 8. The member sizes obtained through the two approaches were compared and the seismic performance was assessed through nonlinear static and time-history analyses. The limit states referred to structural and non-structural damage, considering the two levels design approach, which are the serviceability and the ultimate limit states, were examined. The results obtained reveal that the IFBD leads to more economical structures that still comply with the performance requirements prescribed in Eurocode 8.
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