This gives you the basic parts to build a self-supporting bridge, as conceived by Leonardo da Vinci. For a simple bridge, 5 notchless and 10 notched beams are required (single stl files provided). Print more to make more elaborate designs (and please post images of the results!). You can play around with the dimensions and proportions in the supplied SCAD-file.
You can find a very nice animation of the assembly of the bridge on http://www.giocomania.org/pagine/15587/pagina.asp
(the dimensional parameter values in my SCAD file were taken from this site as well).