Existing design methods for web stiffeners differ widely in concept and in the stiffener size which results. An investigation was conducted to provide a rational, usable and validated design method for transverse stiffeners. The proposed design method takes account of the axial compression in stiffeners caused by tension field action (J Constructional Steel Res 59 (2003) 1035) and external forces, the transverse forces on the stiffeners required to enforce effective division of the web plate into plate panels, and the tendency for the stiffened plate to buckle overall. In this paper, a knife edge model is used to investigate the transverse forces and bending moments exerted on the stiffener caused by the buckled panels (the ''panel bending effect''). A simple approach for predicting the stiffener moment due to the panel bending effect has been developed, incorporating the finite element results of nonlinear elastic buckling analyses of plates subjected to combinations of shear, compression and bending. Further papers will consider the overall buckling effect and the total moment in real stiffeners, and will describe the proposed design model.