In order to refine plate analyses, many high-order theories have been presented, in which the transverse shear deformation is included. Especially, we have to introduce high-order displacement fields for the analysis of in-plane deformation. However, governing equations of those high-order theories are usually quite complicated. In this paper, we establisha low-order theory for thick plate analyses, in which in-plane deformation can be evaluated. In order to do that, we pay attention to constitution of lateral loads and introduce a transverse normal stress into a two-dimensional constitutive law. It follows form the present numerical examples that the new theory presented here gives us excellent results for in-plane deformation and stress analyses of thickp lates.
