An upper coupler for a semi-trailer has a generally planar bottom plate. A kingpin attaches thereto to couple with a fifth wheel of a tractor. A reinforcement member extends across a width of the bottom plate to reduce bending of the bottom plate as it rests on the fifth wheel of the tractor. Along the reinforcement member's length, the reinforcement member has varying second moments of area taken in cross sections perpendicular to the reinforcement member's length.