In this paper, a confined infill panel previously experimented under out-of-plane loading is modeled and analyzed by the dynamic explicit method (DEM). This method is shown to have a good estimation of the experimental results, and is thus adopted to investigate the out-of-plane behavior of an infill model. Based on the analytical results, the stress distribution of an infill panel prior to and after cracking, and the effect of an arching mechanism on post-cracking behavior, is studied. A semi-empirical relationship is accordingly developed for predicting the transverse resistance of masonry infills due to one-way arching action, which accounts for the affects of the boundary frame stiffness, the masonry modulus of elasticity, and the infill slenderness ratio on the transverse strength. The theory also considers two distinct failure modes: boundary crushing and transverse instability. The relationship is shown to have good precision in predicting a large number of experimental results reported in the literature.