Something of that span would probably have to be engineered.What sort of load would it carry?- what material are you planning on using to construct it?I built a small footpath bridge over a winter creek-I just built a form and poured concrete over reinforcing rod.20' is quite a span if you are using stone or concrete, and something like that would require engineering, I'm sure, simply because of the weight alone.I think in the old days, they cut stone to fit.