Bridges are essential to access remote training areas on military installations. Department of Defense DoD elements are responsible for maintaining these bridges, which are experiencing significant deterioration of current steel and wood construction. Polymer compo-site materials that are resistant to corrosion and rot have been demonstrated as beneficial replacements however, the structural capacity and design considerations for future applications is not known. Structural performance tests were conducted on commercially availa-ble, thermoplastic polymer composite I-beams at U.S. Army Corps of Engineers, Engineer Research and Development Center-Con-struction Engineering Research Laboratory ERDC-CERL.Finite element models of the I-beams were used to predict the behavior of the beams and estimate maximum load, displacement, and shear at failure. Then, tests were conducted on actual beams. The thermoplastic beams displayed sudden, brittle failure modes under ultimate loading conditions. The thermoplastic material displayed viscoelastic properties, greater stiffness at higher deflection rates, and load decay under constant and ramped deflections, implying time-dependent deflection under constant load. The beams retained residual deformations after each load cycle.Therefore, a large capacity-reduction factor is advisable for future strength designs. Follow-on developmental work is recommended that refines material property estimates to create higher efficiency during the design process, yet maintains conservatism.