Lightweight sectional doors for cargo bodies and the like are reinforced by a reinforcing member supported on an inner wall surface of at least alternate ones of the panels of a multipanel door. The reinforcing member comprises an elongated, preferably extruded member having spaced apart coplanar flanges and an arcuate web interposed the flanges. An integral rib is disposed at the midpoint of the arcuate web to provide additional stiffness for the reinforcing member. The reinforcing member may be secured to selected door panels by conventional mechanical fasteners.