The primary objective of the dissertation work is to examine the capacity of precast concrete diaphragms. This work is part of a multi-university research effort to develop a new seismic design methodology for precast/prestressed concrete floor diaphragms. To accomplish this, two-dimensional finite element (FE) models of precast floor diaphragms are created, including new elements to match the response of reinforcing details under combined forces. Using these models, nonlinear static "pushover" analyses are performed by applying body forces in the plane of the floor.The analyses are composed of three major parts:(1) Parametric studies to determine the required diaphragm shear strength relative to design (flexural) strength, termed "shear reinforcement overstrength", to promote a ductile mechanism in precast diaphragms. The performance of precast diaphragms with different shear reinforcement overstrength is examined. Appropriate shear reinforcement overstrength design factors are proposed to produce certain performance targets, in terms of a number of key parameters related to diaphragm geometry and the properties of the diaphragm reinforcing details.(2) Parametric studies to examine the effects of "secondary" diaphragm elements (spandrels, internal beams) on precast diaphragm behavior. Though not directly counted in design to participate diaphragm action, the secondary elements and their connections to the main diaphragm may modify the strength, stiffness and deformation capacity of the diaphragm. Analytical studies are performed to examine their effect on the global characteristics and local demands of precast floor diaphragms. The parameters evaluated include the characteristics of the connection details, the seismic hazard level used in design, diaphragm geometry, and layouts of spandrels and internal beams.(3) Development of a rational method for calculating the service stiffness and yield strength of precast concrete diaphragms. The method involves input of diaphragm geometry and reinforcing details. The method is verified analytically through comparisons to a set of FE analyses for an idealized diaphragm representation (regular single span diaphragm idealized with simple end supports). The method verified for a single set of diaphragm reinforcement details is used to estimate the properties over a range of untopped and topped diaphragm systems. Consideration of spandrel and internal beams in the method is discussed.