A composite floor structure and method of making the same are disclosed. The composite floor structure may include a platform and a plurality of transverse beams. The composite floor structure may also include at least one longitudinal beam and a plurality of insert beams to accommodate the longitudinal beam. The composite floor structure may also include an underlayment between the plurality of transverse beams and the at least one longitudinal beam. Some or all of these components may be integrally molded together to form a fiber-reinforced polymer structure. The composite floor structure may be used for cargo vehicles and other applications.