Basic Engineering. Any substance that can take tensile and compressive stresses can be shaped into something strong. An I beam (blame Digg's text for not having an I that looks like and I beam) is very little material for maximum loading capability.Hell, you can make a bridge out of stale bread and peanut butter and it could probably hold a tank, if its done right.The actual *Important* issue here is how cheap can these bridges be made and how easy are they to erect/move. It being made of recycled plastic is irrelevant. The bottom line is price and function. Chances are this is a dirt cheap and light weight bridge so they plan to use it instead of wood/steel temporary bridges.