A side impact protective structure is provided for a vehicle floor pan. Right and left rear rails are attached, respectively, to right and left lateral sides of the floor pan. A kick-up cross member extends transversely between the right and left rocker assemblies. A right reinforcement is welded to the floor pan, the right rear rail, and the kick-up cross member. A left reinforcement is welded to the left rear rail, to the floor pan and the kick-up cross member. An assembly method includes the steps of selecting a thin mild steel floor pan between a thicker HSLA steel structural member and a thicker corner reinforcement and welding the components together with three thickness welds.