MotherEarthNews.com provides a tutorial on how to build a simple log bridge online. BridgeBuilders.com also has instructions on building a timber bridge, ranging from simple to difficult for more robust projects. A Google search for the keywords "build a simple bridge" reveals more resources.
PBS.org provides a comprehensive resource on building different kinds of bridges, including the arch, cable-stayed, beam bridge and suspension. The most important aspect of bridge-building is to conduct a survey of the site, to determine which type of bridge best suits the situation. Building a simple model is the next step to ascertain how well the physics stand up.