Abstract: A lightweight, composite beam with a reacted core for reinforcing a vehicle door is provided. The reinforcing beam comprises a metal tube having a longitudinal cavity which is partially filled with either a reinforcing polymeric core which in one aspect includes a thermoset or thermoplastic resin-based material or a cementitious core which includes Portland cement and a filler. The reacted core is friction fit or mechanically held within the metal tube at the mid-span of the tube. The composite beam includes end pieces which are adapted to be mounted to a vehicle door panel in a door cavity by spot-welds or the like. The horizontally mounted beam provides substantially increased resistance to side impacts to the vehicle door in the event of a collision.