Wooden Bridge

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Old 08-04-18, 08:18 AM
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Wooden Bridge

Not sure this is the correct forum for this, but it's probably the closest fit!

I have a small stream that I need to cross, into a wooded area on my property. It's very muddy, so I want to build a wooden bridge to span it.

My plan is to use four pressure treated (4) 4x4s @ sixteen (16) feet long each, and then top it with 2x8s @ a width of 8. My question is, on a 16 span like that, do you think I need something in the middle to keep it from bowing under weight, or will the decking and the four 4x4s provide enough stiffness to support the bridge? Obviously putting any type of support in the middle will be difficult given the mud in that little stream. Id appreciate your insight here!

It needs to support a John Deere Gator which weighs in empty @ 1,630 lbs.
 
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Old 08-04-18, 08:28 AM
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Do not use posts, they are not structurally strong for what you are proposing.

I have similar bridge, 22' long and built with (4) 2x12's.

Top boards are just 1x? deck boards.

At 16' you might be able to get away with 2x10's but at 22' the 2x12's are very solid.

Looking for a picture will post when found,

t needs to support a John Deere Gator which weighs in empty @ 1,630 lbs.
Sorry, I just saw this, no way would I ever consider putting something that heavy on any kind of simple wood foot bridge,

You are looking at a completely different and much more elaborate structure!

Further down stream I have my crossing for getting the tractor to the back acres, it's an earth structure with culvert pipes, it's was simple to build.
 
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Old 08-04-18, 08:29 AM
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I'd be using 4 X 6's standing on end. (so the 4"side is sitting toward the ground)
I'd also have the 4 X 6's sitting on 12 X 6 X 4" solid concrete blocks so there not making direct ground contact and to increase the surface area there resting on.
4 X 4's love to twist, curl, check and have less side load strength.
 
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Old 08-04-18, 08:33 AM
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Well, I suppose I should have added that I already bought the lumber as described in my original post...So I guess the better question now would be, given what I have to work with, is there a way anyone might suggest that would make it strong enough for the job? Should have asked here first, but, hindsight and all that....
 
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Old 08-04-18, 08:42 AM
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https://www.doityourself.com/forum/a...1&d=1533397187


Here is the foot bridge, whatever you do I would not drive anything that heavy over a 16' span with the construction you are suggesting, I would not even let my kids walk on something like that!

You need some serious structure, even some steel beams possibly!
 
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Old 08-04-18, 03:37 PM
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4x4, whether you've bought them already or not, are not good beams for spanning a distance. The biggest thing for spanning a creek is height. You need height in your beams. So a 2x10 will be stiffer and better able to span a distance than a 4x4. If you lay a 16' 4x4 horizontally and only support it at it's ends you'll seeing sagging just under it's own weight.

First, how wide is the area you need to span? Minimizing this distance is helpful because as you go up in span it takes a lot more beam to carry the load. So, a 12' span is easier to span than 14'.
 
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Old 08-04-18, 03:53 PM
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What about 6x6s laid side by side with a cross beam in the center and supports running to the embankment?
 
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Old 08-04-18, 04:54 PM
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Your last idea would work fine assuming you are using concrete footings because your span would only be 8'. Nothing wrong with marq's footbridge either.

keep in mind that as far as bending is concerned, a 4x4 is no stronger than two 2x4s. And a 6x6 no stronger than three 2x6s. They aren't beams. But a 2x12 CAN span 16'. That's why Marq's bridge works.
 
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Old 08-04-18, 05:53 PM
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2X's are not direct contact rated.
 
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Old 08-04-18, 06:48 PM
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Don't even know what to say to that?

example
 
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