The main role of double-tee diaphragms is to support gravity loads. In addition, they are important elements to transfer lateral loads such as wind and earthquake to vertical lateral load resisting system. Therefore diaphragms are the vertical part of lateral load resisting system. Most frequently used methods for analysis and design of this system which are described in PCI design handbook are based on assumption of rigid diaphragm behavior without consideration of behavior of connections. Due to assumption of rigid diaphragm DOF decreases and analysis becomes simpler. Regard to this assumption all of the elements will do the same lateral displacement. The greater in-plane diaphragm deformation would lead to the greater error in analysis results. In this study dynamic behavior of precast building which those roof and floor are covered by precast concrete double-tee is investigated. At experimental stage, three kinds of mechanical connectors and topping concrete connector were tested in full scale. Based on the results of this stage, a method for modeling of precast building is established by using of SAP 2000 software. In current research, the influence of plan shape, length to width ratio, number of stories and type of connection between double-tee panels on diaphragm rigidity is studied. Beside this, the influence of in-plane elastic deformation on the behavior of this type of building under seismic loading is investigated for determination of the error of in-plane rigidity assumption
