Analytical method for assessment of seismic shear capacity demand for untopped precast double-tee diaphragms joined by mechanical connectors

Untopped precast double-tee diaphragms are assumed as rigid diaphragms in current design practice. This assumption, however, is lack of experimental and analytical evidence. The objective of this research is to provide experimental evidence of behaviors of connectors and to develop analytical method for identifying the role of diaphragm flexibility in earthquake response of parking structures and assessing diaphragm shear capacity demand under earthquake load. This thesis presents an experimental research of the behavior of mechanical connectors and an analytical investigation of performance of untopped double-tee diaphragm under earthquake loads. The commonly used connectors were tested to failure under different test load conditions and their behavior and resistance mechanism was identified. The methods for predicting the elastic limit strength and elastic stiffness of the connectors, as well as the ultimate strength of the connectors, were also proposed and correlated with test results. With the connector test results, a finite element model of untopped diaphragm was established and evaluated. A simplified method and rational formula for equivalent monolithic diaphragm model and equivalent beam model, which is adequate to reflect the flexibility of untopped double-tee diaphragm, was proposed for use in design. By conducting elastic static and inelastic dynamic analysis of prototype structures, the study examined (1) the elastic in-plane behavior of the diaphragms under the transverse lateral loads; (2) the influence of this behavior on the dynamic response of typical prototype parking structures. These results showed the rigid diaphragm assumption was inappropriate for typical prototype parking structures. The diaphragm shear demand at different floors is almost same and higher than that for rigid diaphragm assumption and the critical story drift occurs at first story between second floor diaphragms and ground. The deformation of double-tee diaphragm needs to be properly considered in design and to obtain diaphragm shear and ensure that the gravity-load system can undergo the deformations without collapse. The issues regarding the inelastic behavior of double-tee diaphragms under seismic loads are beyond the scope of this study.