A rocker assembly for a vehicle body structure includes a rocker rail having a boxed cross-section defining a rocker rail interior space, a rocker rail length, and a rocker rail outer surface along the length. The rocker rail defines at least one aperture connecting the interior space and the panel outer surface. The rocker assembly also includes an insert member configured to fit within the rocker rail interior space and extend along the panel length to reinforce the panel. The insert member includes at least one projection configured to match up with and extend at least partially through a respective at least one aperture. The projection(s) are configured to reinforce the rocker rail by opposing deformation of the boxed cross-section. The projection(s) oppose deformation via interference with the boxed cross-section at the respective aperture(s) when the rocker rail is subjected to an applied load perpendicular to the panel outer surface.