This paper deals with the design of steel plate elements of rectangular industrial ducts. Because of the practical aspect ratios and the stiffener arrangements, such a plate element is often treated as a long plate (one way slab), fixed supported at the edges and subjected to transverse pressure. The design pressure and the deflection limit determine the plate thickness and the stiffener spacing. Current design practice considers the geometrical nonlinearity, however, uses the strength criterion of first yield. This investigation postulates that consideration of partial yielding of the plate may result in an economical design option, while meeting the serviceability criterion. The study is based on nonlinear finite element analyses of long plates subjected to increasing transverse pressure. Dimensionless parameters that characterize the behavior of such plates were identified and varied in a parametric study. Based on these analyses, design equations have been established for plate thickness and for stiffener spacing corresponding to three different scenarios namely; 0%, 16.5%, and 33% of through thickness partial yielding of the plate. Results show that allowing for 16.5% yielding results in 50% increase in stiffener spacing, at the expense of about 40% increase in deflections. Partially yielding plates can satisfy the serviceability limits states and can lead to economical stiffened plate systems for large rectangular industrial ducts.