Two parallel main steel beams extend from front to rear of a trailer which beams terminate at their upper web edges with aligned aluminum-extruded H-shaped beams having a depending leg which is bolted to a steel beam web. A plurality of aluminum-extruded upper floor sections including a floor member and depending beams are interlocked and welded to one another at their edges to form a floor assembly. Side rails extend from front to rear of the trailer and have an upper leg interlocked with and welded to the upper floor assembly. A lower side rail leg is glued to a bottom aluminum sheet skin member. A rigid polyurethane foam layer is within each of the volumes formed by the bottom skin layers, the side rails, the H-beams and the upper floor assembly to form a composite platform structure which resists bending and shear loads. Braces further support the steel beams and platform assembly.