A multi-university research project is being performed to develop a comprehensive seismic design methodology for precast concrete floor diaphragms in the United States. The etfort, funded j ointly by the Prestressed/Precast Concrete Institute, the National Science Foundation, and the Charles Pankow Foundation, involves an integrated analytical/experimental research approach and strong industry oversight. Full-scale experiments of isolated diaphragm details were used to bui Id analytical models of diaphragms for use in nonlinear statie ("pushover") analyses and nonlinear transient dynarnic analyses. New deta ils have been developed and are being tested through hybrid (adaptive) experiments of precast panels integrated with computer dynarnic analyses. Diaphragm force arnplification factors, capacity design rules, and diaphragm detail classifications are being developed for the emerging seismic design methodology. A half-scale shake table test of a three-story diaphragm sensitive structure (one floor of untopped double tee, topped double tee and topped hollow core) will be performed to demonstrate the design methodology.