I have a creek I need to cross with a footbridge. While the creek is only about 3' wide, the main span of the bridge will have to be somewhere between 14-17' feet long, due to low, wet, and loose ground around it. I also have to deal with wetlands regulations, so I don't want this bridge to be something large that is part of a major project. It'll be easier to get approval (due to wetlands regulations) if I stick with just wood rather than pouring concrete footings or using other materials or making something like a suspension bridge. I'm hoping to keep this bridge to a 3' wide walkway with simple railings.
I've found this site on building arched footbridges (he calls them "rainbow bridges"). If I went that way, it would be much more complex than just a flat bridge and I'd have the issue of having to make concrete footings specific to the bridge plan to handle the forces at the end of the arches.
I've seen at least one video of someone making a flat (non-arch) bridge with a 12' span using paired 2×6 boards for each beam. I have found tutorials on span tables, but I'm not clear if information designed for a larger surface area such as a floor or roof applies to a walkway or bridge that's only 3' wide, where the load is spread out over only 2 or 3 beams. Also, since the load would be people walking on it, I'm not sure how that breaks down to the amount of weight per square foot.
The idea of using paired or laminated boards as beams sounds appealing, since it's much easier than building an arch.
Will span tables work just as well for what I'm doing as for floors or other surfaces? If not, where can I find information that will give me an idea of what I'd need to use (such as paired or laminated boards for supports) to give me solid support for a span as long as I need?
Best Answer
The Scout Pioneering website contains information on how to build several bridges. I am guessing that most of these are more complicated than what you had in mind, so if you want something simpler, you can attempt flat span bridge using 2x12's for a span between 14 and 18 feet. On the latter page the author goes on to say that "[i]f you are thinking of building any bridge with a span greater than 20 feet, you better get a structural engineer involved, plain and simple", which I am inclined to agree with.
Note that none of those links say anything about the actual load-bearing capacity of the bridges, but I would assume that most of them would work fine for one or a few people crossing at a time.
If you want to calculate maximum load, it is complicated, especially as the materials in the bridge are likely to deterioriate over time - especially if the environment is wet. Some simple calculators for point loads and distributed loads can be found online to give you an idea of what your bridge can take, but as they make clear on their website, they provide no guarantees. In other words, if you want someone to guarantee you that your bridge is safe, you'll most likely have to hire an engineer, as circumstances change from bridge to bridge.