Abstract: The base frame structure for containers or load carrying platforms has an outer rectangular frame structure with longitudinal side beams (13) and transverse beams extending between the side beams. Between the longitudinal side beams and transverse beams is mounted a dished sheet metal underpan, preferably welded to the side beams (13) and transverse beams. Sheet flooring material (4) such as sheet plywood or wooden planks are supported on the underpan and preferably fixed one to the other by adhesive bonding. A space (5) defined between the underpan and flooring material is filled with a rigid load distributing filling material (6) such as polyurethane foam having a high density, 60 Kg/m.sup.3 for example. The underpan is curved in at least two directions to distribute load forces applied to the flooring. The underpan is of a reduced thickness resulting in a lighter base frame structure.