Examination of Ductility and Seismic Diaphragm Design Force-Reduction Factors for Steel Deck and Composite Diaphragms

This paper analyzes the ductility and overstrength of steel deck diaphragms and composite concrete on steel deck diaphragms and computes associated diaphragm design force-reduction factors, $R_s$. First, a database of past tests on steel deck and composite diaphragms was created, and more than 750 specimens were compiled into the database. A subset of 95 cantilever diaphragm specimens with postpeak inelastic behavior was identified for which specimen ductility and overstrength were calculated. The effects of variables such as fastener type, fastener spacing, and deck thickness on diaphragm ductility are explored. A method is then developed for converting cantilever diaphragm specimen ductility to diaphragm system-level ductility. Finally, diaphragm design force-reduction factors, $R_s$, are calculated consistent with an accepted methodology to be $R_s=2.09$, $R_s=1.13$, and $R_s=2.36$ for bare deck diaphragms with mechanical fasteners, bare deck diaphragms with welds to the supports, and concrete on steel deck diaphragms, respectively, for cyclically loaded specimens and assuming a long period structure.