The mechanical behavior of plastic construction, due to visco-elasticity, depends mainly on parameters such as loading in the regions of different degrees of non-linearity, duration of loading causing creep and relaxation, history of loading with hereditary effect, atmospheric temperature changing material properties etc. The plastic construction can be considered for the design of structural parts of the automobile only if the negative creep effect can be minimized with proper construction form. At the same time it must also possess high static and fatigue strength with low brittleness to withstand forces at crash situation, long time dynamic loading, and must allow large scale production at a cost level comparable to steel. Due to these fundamental requirements plastic materials find their application possibilities rather limited in the highly loaded structural design of automobiles and only in the form of composite materials with high strength to weight ratio the application feasibility of plastic becomes greater. One example of this application possibility, a sandwich construction for a body floor with GRP and rigid polyurethane foam as cover and core materials, has been analyzed for this publication regarding its theoretical static, dynamic and creep behavior. New visco-elastic functions describing material properties of GRP and rigid polyurethane foam in the quasi-linear regions under long time loading history at different temperatures have been developed and used in the finite element structural analysis of the sandwich floor.