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 component consisted of a beam-column subassembly with top-and seat-angle with double web-angle connecting the beam to the column. The analytical component is an inelastic finite element model with the connections modeled using a refined 2D continuum elements that is capable of capturing all relevant deformation and inelastic features of the connection.In addition to the hybrid simulation, nonlinear dynamic response-history analyses were conducted, on frames with three different connection capacities (70% Mpbeam, 50% Mpbeam, and 30% Mpbeam), using a collection of ground motion records scaled to the maximum considered earthquake (MCE). The analyses were aimed at investigating the effect of varying different design parameters on the seismic response and period elongation of the frames. The design parameters, in addition to connection strength, included yield strength of the angle material, coefficient of friction between faying surfaces, and the amount of slip allowed in the connection. The results of the hybrid simulation along with the analytical studies were used to evaluate more realistic fundamental code parameters needed for the seismic design of frames. The parameters included the equivalent damping ratio, ???eq, the inelastic period of the structure, Tinealstic, and a demand-based force reduction factor, Rdemand. The evaluated parameters can be used to better estimate the design base shear using a simplified design spectrum, allowing for safer and economical design of semi-rigid frames under seismic events.
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