RNeumann
Elite Member
Railings may be another answer- make two trusses that are also your railings.
2x6 joist span for simple foot bridge
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CobyRupert
Super Member
Or put 4 poles in ground at edge of bank, one at each corner. Run cables across stream to opposite pole. Suspend bridge off of cables.
Sort of like this:
Sort of like this:
quicksandfarmer
Elite Member
The thing to keep in mind is that the limiting factor in the span tables is the amount of sag, so you don't come close to the actual breaking loads. There are two reasons that sag is the limiting factor: first, it is disconcerting to walk on a deck that sags, it feels like it might give way at any time, kind of like standing on a branch that bends a lot. Second, when a wooden structure moves the fasteners tend to loosen. If you have a bridge that moves around a lot you're putting a lot of strain on the nails and bolts.
Others have suggested building it so that the handrails form a truss. That is what I would do if I were building a footbridge any more complicated than a flat deck. A couple of years ago I volunteered on a National Park Service project to install a footbridge. They used a kit from this company:
Pedestrian and Trail Bridges - ETTechtonics
While the pieces were made out of fiberglass, they were all dimensioned the same as 2x lumber. The pieces were noticeably less rigid than wood, the joists were so floppy it was hard to keep them in place while they were bolted. The bridge we built had an unsupported span of 65 feet and a width of 4 feet. It had five 2x6-sized joists. The joists had to be supported by scaffolding until the truss was assembled but once it was all connected together the scaffolding was removed and the frame was rock solid. I guess my point is that a material far less stiff than wood can be engineered into a very stiff structure.
I'll try to give an idea of the construction. The bridge was basically two trusses connected by bracing underneath. Each truss consisted of two 2x6 joists on the bottom and two 2x6 handrails on the top. A fifth joist ran down the middle to keep the decking from being too bouncy. Each pair of joists/handrails was spaced 1.5 inches apart, the thickness of 2x lumber. The joists and rails were 12 or 16 feet long and there would be a piece of 2x6 as a splicing block between them where they joined. Every five feet there was a vertical post made out of 2x2. It ended flush with the top of the hand rails and extended past the joists by six inches. There was a diagonal brace from the top of each post to the bottom of the next one, also made out of 2x2, and bolted to the joists and handrails where it went between them. Below and perpendicular to the joists was a pair of 2x6's, which were bolted on either side of the piece of post extending down. These pieces were 8 feet and extended two feet out on either side of the bridge. At the ends of these two pieces a diagonal piece of 2x2 was bolted between them and slanted back to meet the post. Between those two pieces, in the middle of the bridge, was a vertical piece of 2x2 11 inches long. The fifth joist was bolted onto that. The two trusses were tied together by diagonal 2x2's from the bottom of one post to the bottom of the next.
Here are some pictures that I hope make what I'm saying more clear:
Joists in place, you can see the splicing pieces and the cross braces, as well as the scaffolding:
Posts in place, along with diagonal bracing and the cross pieces. They did X-bracing but I think a single diagonal would be sufficient.
Center joist in, hand rails on, ready to take the scaffolding off. The NPS wanted pieces of 2x4 along the sides for safety and a piece of 2x6 along the top of the handrail, neither is necessary for strength.
If I were building a footbridge I would do something like this, but out of wood. I wouldn't go 65 feet with wood, but 15? Sure. I might upgrade some of the 2x2's to 2x4's, but mostly because the 2x2's you get at the store tend to be pretty lousy. It might be enough to make your own 2x2's by ripping graded lumber.
My recollection is that the cost of the kit for the 65-foot bridge was in the neighborhood of $30,000 or roughly $500 per foot. Which is really cheap for a 65-foot bridge and really expensive for a 14-foot one!
Others have suggested building it so that the handrails form a truss. That is what I would do if I were building a footbridge any more complicated than a flat deck. A couple of years ago I volunteered on a National Park Service project to install a footbridge. They used a kit from this company:
Pedestrian and Trail Bridges - ETTechtonics
While the pieces were made out of fiberglass, they were all dimensioned the same as 2x lumber. The pieces were noticeably less rigid than wood, the joists were so floppy it was hard to keep them in place while they were bolted. The bridge we built had an unsupported span of 65 feet and a width of 4 feet. It had five 2x6-sized joists. The joists had to be supported by scaffolding until the truss was assembled but once it was all connected together the scaffolding was removed and the frame was rock solid. I guess my point is that a material far less stiff than wood can be engineered into a very stiff structure.
I'll try to give an idea of the construction. The bridge was basically two trusses connected by bracing underneath. Each truss consisted of two 2x6 joists on the bottom and two 2x6 handrails on the top. A fifth joist ran down the middle to keep the decking from being too bouncy. Each pair of joists/handrails was spaced 1.5 inches apart, the thickness of 2x lumber. The joists and rails were 12 or 16 feet long and there would be a piece of 2x6 as a splicing block between them where they joined. Every five feet there was a vertical post made out of 2x2. It ended flush with the top of the hand rails and extended past the joists by six inches. There was a diagonal brace from the top of each post to the bottom of the next one, also made out of 2x2, and bolted to the joists and handrails where it went between them. Below and perpendicular to the joists was a pair of 2x6's, which were bolted on either side of the piece of post extending down. These pieces were 8 feet and extended two feet out on either side of the bridge. At the ends of these two pieces a diagonal piece of 2x2 was bolted between them and slanted back to meet the post. Between those two pieces, in the middle of the bridge, was a vertical piece of 2x2 11 inches long. The fifth joist was bolted onto that. The two trusses were tied together by diagonal 2x2's from the bottom of one post to the bottom of the next.
Here are some pictures that I hope make what I'm saying more clear:
Joists in place, you can see the splicing pieces and the cross braces, as well as the scaffolding:
Posts in place, along with diagonal bracing and the cross pieces. They did X-bracing but I think a single diagonal would be sufficient.
Center joist in, hand rails on, ready to take the scaffolding off. The NPS wanted pieces of 2x4 along the sides for safety and a piece of 2x6 along the top of the handrail, neither is necessary for strength.
If I were building a footbridge I would do something like this, but out of wood. I wouldn't go 65 feet with wood, but 15? Sure. I might upgrade some of the 2x2's to 2x4's, but mostly because the 2x2's you get at the store tend to be pretty lousy. It might be enough to make your own 2x2's by ripping graded lumber.
My recollection is that the cost of the kit for the 65-foot bridge was in the neighborhood of $30,000 or roughly $500 per foot. Which is really cheap for a 65-foot bridge and really expensive for a 14-foot one!
If you need to use 2x6 for height reasons, you can double and triple the 2x6's (construction glue and heavy nails in appropriate pattern should do it).
Assuming 16 o.c., I would think double 2x6 for the outside beams, and triple 2x6's for the inside beams will be plenty stiff for the loads you described. Even with four average people walking across the span at the same time, it's only about 10lbs/square foot of live load. You could also use 4x6 and 6x6 beams, and save yourself the struggle of squeezing a lot of glue and driving a lot of heavy nails. The span tables for deck building are necessarily conservative, because decks are supposed to attract a crowd, so 40 to 60/lbs of live load per square foot is considered a reasonable load for a deck to support.
Or, if it's not too difficult: Put a concrete footing down in whatever you're crossing, and add a support post, in the middle of the span.
P.S. I did a similar project. Spanned a creek with a 12' foot bridge using 2x10's, 16 o.c. The thing is so stiff, my neighbor actually laughed at me, calling it overbuilt. No detectable bounce with two or three people walking across at the same time.
Assuming 16 o.c., I would think double 2x6 for the outside beams, and triple 2x6's for the inside beams will be plenty stiff for the loads you described. Even with four average people walking across the span at the same time, it's only about 10lbs/square foot of live load. You could also use 4x6 and 6x6 beams, and save yourself the struggle of squeezing a lot of glue and driving a lot of heavy nails. The span tables for deck building are necessarily conservative, because decks are supposed to attract a crowd, so 40 to 60/lbs of live load per square foot is considered a reasonable load for a deck to support.
Or, if it's not too difficult: Put a concrete footing down in whatever you're crossing, and add a support post, in the middle of the span.
P.S. I did a similar project. Spanned a creek with a 12' foot bridge using 2x10's, 16 o.c. The thing is so stiff, my neighbor actually laughed at me, calling it overbuilt. No detectable bounce with two or three people walking across at the same time.
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aczlan
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I would lay down a membrane (like ice and water shield) on top of the main beams so that you don't have water sitting between the deck boards and the beam (especially if you make a laminated beam).
Aaron Z
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Yes, the weak spot in any outdoor wooden deck or bridge will always be the contact points between wood members. They collect moisture, and eventually that is where the wood rots. So, if you can cover the top of each Joist before you nail or screw the decking down, you have eliminated this problem.
RNeumann
Elite Member
This concept is theory- not fact. It's actually way more complex than that. It is good to be thinking about preventing rot (obviously) but their are several variables.
Larry Caldwell
Elite Member
It depends on the wood. Pine, cedar or redwood, no. Douglas fir or oak, yes. D. fir 2x8 12" o/c should be fine. Don't hold any parties. A bunch of people can weigh as much as a tractor. Every once in a while you see news stories of balconies collapsing because 20 people (4000 lbs) decide to stand on it. I did a footbridge a couple years ago and used 4x12 treated D. fir for the structural members, just in case a bunch of people decided they wanted to stand and watch the creek.
quicksandfarmer
Elite Member
Here is a sketch of a simple bridge that meets all of the original requirements:
It's based on the pre-fab bridge design I posted about earlier.
It's made from 2x6's (red),2x4's (green) and 6x6's (blue). Span is 14', width is 4', thickness of the deck is 2x6. The 2x6's are attached to the 6x6's with joist hangers, triple on the outside and single on the inside. Each joist has an unsupported span of 7' and there are five joists covering 4' which should be ample for 2x6's. If the spacing is too great for the decking to span an additional joist can easily be added.
You could take all of the 2x4's to 2x6 pretty simply if you wanted to. I'd put some sort of bridging between the joists, I didn't want to clutter the drawing.
Looks like a fun project, makes me wish I had a stream that needed crossing!
It's based on the pre-fab bridge design I posted about earlier.
It's made from 2x6's (red),2x4's (green) and 6x6's (blue). Span is 14', width is 4', thickness of the deck is 2x6. The 2x6's are attached to the 6x6's with joist hangers, triple on the outside and single on the inside. Each joist has an unsupported span of 7' and there are five joists covering 4' which should be ample for 2x6's. If the spacing is too great for the decking to span an additional joist can easily be added.
You could take all of the 2x4's to 2x6 pretty simply if you wanted to. I'd put some sort of bridging between the joists, I didn't want to clutter the drawing.
Looks like a fun project, makes me wish I had a stream that needed crossing!
EddieWalker
Epic Contributor
I would flip the truss on the railings if I was to do something like this. But I wouldn't want to rely on an exposed wood truss for support. Too much shrinkage and movement of the wood o er time for me to trust it.