modular plastic structural composites having a web section disposed along a horizontal axis and at least one flange section disposed along a horizontal axis parallel thereto and integrally molded to engage the top or bottom surface of the web section , wherein said composite is formed from a mixture of high density polyolefin and a thermoplastic - coated fiber material , poly - styrene , or a combination thereof . composites molded in the form of i - beams and bridges constructed therefrom are also disclosed .