<jats:p> In the steel plate end diaphragm, welding the infill plate to the bearing stiffeners impose local damage on the main girders and their bearing stiffeners. To address this deficiency, a semi-connected-endplate diaphragm is proposed with the idea of semi-supported steel plate shear walls. To identify slab-connected steel plates with the maximum capacity for reducing seismic demand in composite steel/concrete bridges, 36 slab-connected steel plates with different thicknesses, lengths, and material properties were studied. Protecting the main girders from damage and reducing seismic load transmitted from the superstructure to the substructure were two main criteria to identify appropriate diaphragms. These criteria were quantified via seismic parameters such as ductility ratio, force reduction factors, and overstrength factor. The results showed that thin steel plates with small lengths cannot protect the main girders and also thick plates with large lengths transmit the whole seismic load from the superstructure to the substructure. In the next section of this study, the energy dissipation capacity of the optimised diaphragms was studied through cyclic analysis. The result of the time history analysis indicated that an appropriate end diaphragm could considerably mitigate the substructure seismic demand. </jats:p>