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The widespread and unexpected damage to welded connections during recent earthquakes led to the investigation of alternatives for the construction of steel frames in seismic areas. Bolted semi-rigid connections have been recognized as an attractive alternative to welded connections. However, existing knowledge on the behavior of the connection is either from testing of beam- to-column subassemblies under idealized load and boundary conditions, or from analytical studies. In addition, the system-level experimental behavior of semi-rigidly connected frames using real earthquake motions to conclusively verify the full potential of semi-rigidity (implying also partial-strength) in earthquake resistance application is lacking. To this end, an advanced hybrid simulation approach for the seismic assessment of steel frames with semi-rigid connections was proposed and successfully completed. Furthermore, nonlinear dynamic response-history analyses of semi-rigid frames with varying design parameters were conducted to evaluate the system performance under seismic events. The results of the hybrid simulation and the parametric studies are used to quantify various fundamental code parameters needed for the seismic design of structures. The hybrid simulation included the most reliable, realistic, and computationally efficient experimental and analytical modules, which were developed and successfully integrated in a closed-loop system-level simulation. Three hybrid simulations were conducted on three different partial-strength semi-rigid frames with connection capacities that are a percentage of the plastic moment capacity of the beam (70% Mpbeam, 50% Mpbeam, and 30% Mpbeam). The simulations utilized the large-scale Multi-Axial Full-Scale Sub-Structured Testing and Simulation (MUST- SIM) facility at the University of Illinois and included a full-scale physical specimen for the experimental module and a 2D finite element model for the analytical module. The experimental

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