Seismic design criteria to improve the performance of X-CBFS

Cross concentrically braced frames are commonly used in seismic design of steel buildings at any seismicity, owing to their large lateral stiffness, simplicity of design and limited constructional cost. The seismic design criteria provided by current EC8 aim at assuring ductile global plastic mechanism occur, namely restraining the plastic deformation into the diagonals, while the remaining structural members and connections should elastically behave. However, several studies, show that the design process compliant to Eurocode 8 is affected by several criticisms, it entails several efforts and often leads to massive, uneconomical systems solution exhibiting poor seismic performance. EN1998-1 recommends estimating the lateral resistance of X-CBFs by performing structural analysis on a tension-only (T-O) diagonal scheme in which solely the tension braces resist the lateral loadings, while the contribution given by the compression diagonals is neglected. As highlighted by the comments given by European designers coming from the survey promoted by CEN TC 250/SC8 for the systematic review of EC8, this type of model can induce misleading interpretation of the structural behavior and relevant modelling of X-CBFs, leading to uneconomical and oversized systems characterized by heavy structural solutions and poor plastic engagement. The research presented in this paper is addressed to critically discuss and revise the detailing rules codified within the current EN 1998-1 for X-CBF, to simplify the design process and to improve the seismic behavior. Whit this aim, an extent set of nonlinear dynamic analyses has been performed on a set ow low, medium, and high-rise 2D frames extracted from residential steel buildings to assess the effectiveness of currently codified requirements and to validate the beneficial effects of the proposed revision.