Help I need to build a 20' bridge

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Old 11-10-18, 06:50 PM
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Help I need to build a 20' bridge

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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Old 11-11-18, 01:44 AM
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2x6 will not provide sufficient stiffness, need something taller.

Here is my 24' walking bridge, it's made of (4) 2x12's.

https://www.doityourself.com/forum/a...1&d=1541929417

https://www.doityourself.com/forum/a...1&d=1541929429
 
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Old 11-11-18, 03:42 AM
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If you want something to span that distance and still be light enough to pick up I would buy a aluminum walk board. They are used in construction as scaffolding. I have a 24' long x 14" wide one on my pond and one person can pick it up. They come in wider widths but they get a bit heavier and more expensive.
 
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Old 11-11-18, 05:02 AM
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It would be better if you could have 2 rows of concrete blocks that are spread apart slightly... cross brace the concrete every 4' or so with a heavy angle iron set upside down into a mortar joint. 2x4s could work... but 2x6s might lay flatter. If you shop at a real lumber yard, not lowes, they might stock 20'ers. But I doubt that you would be able to budge the thing once all the decking is on. You might just want to plan on building it in 7 or 8' sections that are easier to move. The cement pads on the ends would be a good idea.
 
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Old 11-11-18, 06:26 AM
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it's not a real 20' span

I probably didn't explain well that it's a [size=3]FAKE[/size] 20' span.
The bridge will be supported by the one row of cement blocks the entire length. It's just for appearance, so as to make the two ponds look one. The cantilevered parts are so that the wall (covered by black liners on both sides) won't show. The problem is that it will be just "sitting" on the wall, ie it will not be bolted to it, as I'd like to be able to move it. I was thinking of bolting it only at the two ends.
 
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Old 11-11-18, 08:41 AM
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I was thinking the same as PD, use an aluminum bridge that can be easily moved.


PS... Marq, I like what you did. Looks good.
 
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Old 11-11-18, 11:50 AM
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Originally Posted by Norm201 View Post
I was thinking the same as PD, use an aluminum bridge that can be easily moved.


PS... Marq, I like what you did. Looks good.
I like them too but the ones I have been able to see are all quite expensive ...I am still looking but for the length I think they are all quite pricey
 
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Old 11-11-18, 11:55 AM
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Tom,

Yes they are. But consider what you get for the price. Longevity (nearly lifetime) use. Non- corrosive and almost indestructible. No maintenance. If moving it is critical then it's worth every cent. Suppose you get hurt trying move the homemade unit. Is it worth it? If you need to extend it I'm sure it's easier to work out something with the aluminum unit as opposed to t he wood unit. If aesthetics is important then a permanent bridge is what you want.
 
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Old 11-11-18, 02:10 PM
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Yes, aluminum walk boards the size you need are about $500+. But then you have the requirement that you must be able to pick up the bridge. Go to the home center and pick up 4 or 8 of the longest 2x6 they have. Then in your other arm grab a half dozen 12' long 5/4 deck boards. You may want to consider just building your "bridge" fixed in place with some lift off access covers to get to the skimmers.
 
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Old 11-12-18, 12:14 PM
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new idea

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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Old 11-12-18, 12:21 PM
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I looked on Craiglist and I can find a used 4' ladder for $150

I assume that's a typo, what size did you mean?
There are 4 group types of ladders, Type III, type II, type I and type IA. Type III is only rated for 200lbs, type I is rated at 250lbs. The planks you add over the ladder will have some weight along with using a ladder in a manner it wasn't designed for. Not saying it won't work but I would expect it to bow in the middle.
 
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Old 11-12-18, 12:43 PM
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I see goldfish. Is the other pond section for koi?

Since you are supporting the ladder along it's length it will probably work if you provide enough support for it. Have you figured out how much wood you'll be putting on top? That will add considerable weight.
 
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Old 11-12-18, 01:07 PM
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it's a typo, I meant 40 feet
 
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Old 11-12-18, 01:12 PM
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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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Old 11-12-18, 01:15 PM
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I don't know, most ladders tend to flex when extended and used how they are intended. With lumber, if it's laid flat it isn't as stout as when it's laid on end, I suspect [but don't know] the same is true for a ladder. The good thing is in your scenario a failure doesn't result in a fall or at least not much of one.
 
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Old 11-12-18, 03:56 PM
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I mentioned the weight because you say you want to be able to pick it up. The aluminum ladder is only a portion of the weight and I suspect that when you deck it solid the wood will weigh more than the ladder. You may end up with something too heavy for you to pick up.
 
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Old 11-13-18, 03:07 AM
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.... and a 40' ladder just by itself isn't light! They are a bear for one man to move much less get off the truck and set up.
 
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Old 11-13-18, 04:44 AM
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You may want to consider just building your "bridge" fixed in place with some lift off access covers to get to the skimmers.
This suggestion by PD seems like your best bet.
 
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Old 11-13-18, 06:45 AM
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Originally Posted by tom66
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.
Have you considered a causeway made of "upcycled" pallets?

There are lots of plans and sketches available online...
 
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