We present a set of new and improved criteria for balanced design of steel X-braced moment resisting frame (X-BMRF) dual systems. In a balanced design, premature failure modes are completely suppressed, overall system ductility is maximized in a Pareto-optimal sense, and a hierarchy of multiple secondary yielding mechanisms at selected performance levels is assured. The conventional X-BMRF design provisions fail to achieve these objectives. The proposed criteria conform to a non-dimensional formulation, integrating all system constituents to boost ductility, enhance energy dissipation, and mitigate failure modes through multiple stages of tensile yielding and compressive buckling of either continuous or discontinuous X-bracing diagonals-factors not addressed in previous balanced design strategies. The utility of these criteria is demonstrated through extensive parametric studies using highly refined, validated, and verified threedimensional finite element (FE) models in different X-BMRF model matrices. Thus, system ductilities, energy dissipation ratios/distributions, and elastic/inelastic load-shares of story shears are systematically obtained and used as metrics to measure/assess the benefits of the proposed criteria. The effects of corner gusset plate types/sizes, center-splice connection details, and demand-to-capacity ratios of X-bracing diagonals (D/C) for the specified versus the expected yield stress behavior are scrutinized. Nonlinear post-buckling monotonic and cyclic responses are incorporated in FE analyses to gain insights into the sequences of multiple secondary yield mechanisms and probable failure modes. Key findings recommend D/C approximate to 0.85 for linearclearance and D/C approximate to 0.75 for tapered/elliptical-clearance gusset plates, with optimal applications considering frame geometry (B/H) and mid-connection details. These criteria significantly strengthen the performance of both standalone and composite steel X-BMRF systems.
