The in-plane flexibility of untopped precast concrete double-tee diaphragms is often ignored in current design practice. The performance of parking structures during earthquakes and subsequent research studies have shown, however, that in-plane flexibility of diaphragms may contribute to the development of large displacements, a concern for stability. The objective of this paper is to provide designers with a practical approach to judge the linear elastic in-plane flexibility of discretely connected untopped double-tee diaphragms. Based on the results of detailed finite element model analyses of commonly used untopped diaphragms, a simplified rational approximation is used to establish equivalent beam models. Methods of defining linear elastic stiffness parameters of the equivalent beam are derived and can be directly used in manual calculations or computer analysis. Compared to a complex finite element analysis, the proposed equivalent beam model can predict the linear elastic in-plane diaphragm deformation under wind or seismic load with reasonable accuracy for design. A deflection calculation example is provided in Appendix B to illustrate the proposed approach.
