The development in welding and precision gas cutting techniques for heavy plating led to the introduction of steel box girders for bridges after WWII. Thus, new structural elements/ diaphragms were deployed into bridge construction. This book provides an experimental and analytical investigation of the behaviour of rectangular support box girder diaphragms up to collapse. It gives full details of the tested stiffened diaphragm models, initial geometric imperfections and testing procedures. A 3-D F.E. model using (ABAQUS) was used for the analytical investigation. The F.E. model is capable of reproducing the experimental results satisfactorily. Based on that, a wide parametric study was carried out to investigate the effects of several parameters on the collapse load. The book provides details of the features of the collapse of different arrangements of stiffeners, the effect of the B/D ratio, the D/td ratio, size of the stiffeners, imperfections and positions of the bearings, and guidelines for the design of rectangular diaphragms. It is vital for researchers and bridge engineers involved in the design of steel box Girder diaphragms and plated structures.