Recent research has identified that current code level seismic demands used for diaphragm design are considerably lower than demands in real structures during a seismic event. However, historical data has shown that steel deck diaphragms, common to steel framed buildings, perform exceptionally well during earthquake events. A new alternative diaphragm design procedure in ASCE 7-16 increases diaphragm seismic demand to better represent expected demands. The resulting elastic design forces from this method are reduced by a diaphragm design force reduction factor, Rs, to account for the ductility of the diaphragm system. Currently, there exist no provisions for Rs factors for steel deck diaphragms. This research was therefore initiated to understand inelastic steel deck diaphragm behavior and calculate Rs factors. A review of the literature showed that a large number of experimental programs have been performed to obtain the in-plane load-deformation behavior of steel deck diaphragms. To unify review of these diaphragm tests and their relevant results, a database of over 750 tested specimens was created. A subset of 108 specimens with post-peak, inelastic behavior was identified for the characterization of diaphragm behavior and ductility. A new recommended method for predicting shear strength and stiffness for steel deck diaphragms with structural concrete fill is proposed along with an appropriate resistance factor. Diaphragm system level ductility and overstrength are estimated based on subassemblage test results and Rs factors are then calculated based on these parameters. The effects of certain variables such as deck thickness and fastener spacing on diaphragm ductility are explored. / Master of Science / A building’s floor and roof systems active in the resistance of in-plane loads, known as diaphragms, can be a critical component of a structure. In the past, the majority of diaphragm systems have demonstrated exceptional performance during earthquake events. Diaphragms must be designed in compliance with building codes. ASCE 7 is a widely recognized document in structural engineering used to prescribe building loads and outline design methodologies. Recent research has identified that code level seismic demands in ASCE 7-10 used for diaphragm design are considerably lower than demands in real structures during a seismic event. However, historical data has shown that steel deck diaphragms, common to steel framed buildings, perform exceptionally well during earthquake events. A new alternative diaphragm design procedure in ASCE 7-16 increases diaphragm seismic demand to better represent expected demands. The resulting design forces from this method are reduced by a diaphragm design force reduction factor, R_s, to account for the diaphragm system’s ability to permanently deform without undergoing catastrophic failures – an engineering concept known as ductility. Currently, there exist no provisions for R_s factors for steel deck diaphragms. This research was therefore initiated to understand steel deck diaphragm behavior and calculate R_s factors. A review of the literature showed that a large number of experimental programs have been performed to obtain the in-plane load-deformation behavior of steel deck diaphragms. To unify review of these diaphragm tests and their relevant results, a database of over 750 tested specimens was created. A subset of 108 specimens with complete, inelastic behavior was identified for the characterization of diaphragm behavior and ductility. A new recommended method for predicting the in-plane strength and stiffness for steel deck diaphragms with overlaying structural concrete fill is proposed along with an appropriate resistance factor. Valuable information is extracted from previously tested steel deck diaphragms and R_s factors are then calculated based on these parameters. The effects of certain variables such as deck thickness and fastener spacing on diaphragm ductility are also explored.