Appropriate Overstrength of Shear Reinforcement in Precast Concrete Diaphragms

Current precast concrete diaphragm design may not necessarily protect the diaphragm from a nonductile shear failure in an overload situation. As evidence exists that diaphragms can attract large inertial loads during strong ground motion, emerging design methodologies are aiming to develop the diaphragm flexural strength. However, the performance target desirable or achievable in design may depend on a number of other factors. An analytical study examines the performance of precast diaphragms with different shear strength relative to design (flexural) strength, termed here “shear reinforcement overstrength.” The objective of the study is to determine the required shear reinforcement overstrength in the precast diaphragm to produce certain performance targets. Appropriate shear reinforcement overstrength design factors are proposed in terms of a number of key parameters related to diaphragm geometry and the properties of the diaphragm reinforcing details.