Help I Need to Build a 20' Bridge (PS not a real bridge)


Postby tom66 » Sun Nov 11, 2018 3:56 pm

I need to build a footbridge (or boardwalk, as it has no railings) to span 18-20'. The 2x4 in the picture are 20' long, the shorter one (the one in the middle) is 16'. If I had a good way to build footings in between the two skimmers (square grey spots) I'd have only 14' distance to cover, but I don't. The soil is soft and I can't compact it. There are also pipes that I may have to access at some point.
A 20' bridge is best because it would have the length to reach further up into the curvature of the big pond, and that will also put the bridge right where I want it to be. The idea is to make it look like the two ponds are one, so there will be water on both sides of it and the tiny wall that separates the two body of water will be hidden underneath the bridge.

We want to build the bridge as thin as possible (in terms of lumber) for 2 reasons: 1) we need it not to look imposing, and 2) I'd like to be able to lift it and move it to the side in case I need to repair/service the 2 skimmers that will end up underneath.

I was thinking of making the bridge somewhere between 30 and 36" wide. Also the long row of cement block will be all filled up and will create a straight and level surface on which the bridge joists can sit, and if I build it well the joists will sit on the cement "wall' for most of its length (so the bridge is not really spanning 20'!).

So, I was thinking of building it with 2x6s 20' long!
As it's not easy to find lumber 20' long I have just a few options, one is to rip a 6x6 to make two 6x3s, and use these for the 2 joists. Then, I was thinking of placing several 2x6s perpendicular to the two joists, so that they would sit on the cement wall that runs in the middle. I was thinking of putting a third "joist" running the length, made of short pieces, functioning more like blocks, but necessary for the boards to sit on, in what would otherwise be a 25-30" span between the two joists. What do you guys think about this idea?

Also, what would be better, ripping a 6x6 or joining two 2x6 together? I know that strength-wise joining is better but I am also concerned about the quality of the lumber, while I can pick the 6x6 at my local lumber company, I'd have to special order the 2x6x20 with Lowes, so there is always a chance that they'd send boards with a bow, a twist, or a major defect. What do you guys think?

As for securing the two ends of the bridge to the ground the problem is that the soil is soft (there is 2' of fill dirt). So I am not sure how to do it. One way would be to dig 2-3' deep and build it up from there with either cement blocks or a pour. Another one I can think of is to stake a couple of 4x4s real deep. Or I could pour a small slab. On top of some kind of footing I was thinking of bolting a 30-36" long treated 4x6". On this I would somehow secure the bridge joist. And I'd do it in a way that I can just remove a couple of screws or nuts, lift it, and move the whole bridge a few feet to the side, if I ever need to.

For those of you that made it at the end of this post, thank you and I'd love to hear your ideas.
Tom
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Postby tom66 » Mon Nov 12, 2018 8:19 pm

a friend gave the idea of using two 20' aluminum ladder rungs and screwing the boards right on it , what do you guys think? the one in the picture is the one I use to go on my roof, and the pieces are 18' long, I looked on Craiglist and I can find a used 40' ladder for $150 which is close to what it'd cost me to buy joists, and cross beams ...
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Postby tom66 » Mon Nov 12, 2018 9:09 pm

what about the weight of all the boards? + people? they are rated at 200-250lb, but i think that is mostly about the strength of each step, do you think it'd hold that much weight used as a joist(s)?
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