The last decades Fiber Reinforced Polymer (FRP) structures start to find their own place in the field of civil engineering applications and quite lately in bridge engineering. Composites are considered highly versatile materials that offer great tailoring on their design. Synthetic fibers and resins are already quite popular especially in applications where low weight, construction speed and high performance are the objectives. However, environmental concerns over the disposability of non-recycle composites is driving the engineering world towards the use of natural composites. Certain natural fibers appear to be advantageous not only in terms of mechanical properties but also in terms of sustainability compared to synthetic ones. However, environmental performance of bio materials is difficult to assess and predict due to their large natural variability. Durability assessment through literature review and optimization of the superstructure of an FRP bridge, accounting for failure modes mostly deriving from high concentrated vertical loading (wheel loading), are the main objectives of this report. In order to deal with the large number of variables and constraints for the design according to existing structural standards, advanced optimization algorithms are employed. In addition, due to lack of analytical formulas for local buckling resistance prediction, Finite Element (FE) analysis is used since it offers a better insight into buckling failure of composite laminates. Finally, an optimization workflow is developed that accounts for all variables and constraints and minimizes the objective, which in this case is the weight of the superstructure. The obtained results form recommendations for the optimum design of a flax and glass FRP bridge, regarding the thicknesses and orthotropy of the cross section’s laminates with L/300 and L/500 maximum deflection requirement, providing a deck slenderness L/16 and a spacing of the web sw = hSP /3. The assessment and the optimization reveals that flax or BioMid FRP could be promising candidates in replacing synthetic fibers for a bridge application as they can compete in performance and weight the synthetic glass FRP for the case of single span bridge and a span range of 10 - 30m.