Due to the recent increase in domestic seismic activity and the proliferation of PC structure buildings, there is a pressing need for a fundamental study to develop and revise the design criteria for Half-PC slabs. In this study, we propose criteria for determining the rigid diaphragm based on the aspect ratio of Half-PC slabs and investigate the structural effects based on the presence of chord element installation. This study concluded that Half-PC slabs with an aspect ratio of 3.0 or lower can be designed as rigid diaphragms. When chord elements are installed, it is possible to design Half-PC slabs with an aspect ratio of 4.0 or lower as rigid diaphragms. In addition, the increase in the aspect ratio of the Half-PC slab leads to a decrease in the in-plane stiffness of the structure, confirming that the reduction effect of the maximum displacement in force direction (max ) due to the increase in wall stiffness is predominantly influenced by flexibility.