Abstract: Considering the effect in the middle plane, in this paper the bending problem of rectangular plates stiffened by ribs of open section that have little torsional resistance with simply supported edges along the direction of stiffeners is researched. All analytical formulas of both displacements and internal forces are derived. By using the results a calculating exam ple to treat engineering problem is discussed. The solution with more simple form of the paper is better thart that of Huber's Iheory.