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Old 06-18-2010, 09:46 AM   #1
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Fabricating curved wood beams for bridge


I am just looking for any advice/tips on putting together some wood beams for a bridge I am building. The bridge is roughly 30 ft long and will be 4ft wide. I am looking to have it arc up about 2 ft or so. I was thinking of using 3 sets of doubled up 2x12's. I would think that is enough but am open to suggestions. I need to know how to go about getting a pattern and what technique I should use. I am a sheet metal worker by trade so wood is not my thing but I dont want a flat bridge. Any and all advice welcome. thanks.

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Old 06-18-2010, 09:44 PM   #2
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Fabricating curved wood beams for bridge


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Originally Posted by abraham View Post
I am just looking for any advice/tips on putting together some wood beams for a bridge I am building. The bridge is roughly 30 ft long and will be 4ft wide. I am looking to have it arc up about 2 ft or so. I was thinking of using 3 sets of doubled up 2x12's. I would think that is enough but am open to suggestions. I need to know how to go about getting a pattern and what technique I should use. I am a sheet metal worker by trade so wood is not my thing but I dont want a flat bridge. Any and all advice welcome. thanks.
Are planning a bridge without a center support?

As a metal worker, you probably know how to weld. Using steel would be much simpler.
Get an I beam, cut it into sections, on a slight angle. Turn one section over and weld it together, to form an arc.

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Old 06-18-2010, 09:52 PM   #3
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Fabricating curved wood beams for bridge


You could build a clamping form and use 1 by material and glue to make a laminated beam.
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Old 06-19-2010, 11:03 AM   #4
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Fabricating curved wood beams for bridge


Your post does not indicate the dimensions of the beam you are trying to build, and it does not indicate how you sized the beam. The required size of the beam is controlled by the load you are planning to drive over the bridge, i.e. is it a pedestrian bridge, are you going to drive a tractor or something similar over it?

Assuming you know how big the beam needs to be, there are at least three ways to fabricate curved beams.

1. Glulam process: You take relatively thin strips of wood, bend them to shape on a jig (a brake in sheet metal speak), and glue them together. For information on how to do this, see the Forest Products website at the University of Wisconsin - Madison, they have a whole manual on the process.

2. You take larger dimension lumber and bend it using steam. For this, you need to build a steam box, you put the wood beams in the steam bath, and when they get soft you bend them to shape. They will regain some of their shape when they cool, so the process is more complex than simply bending to shape when they are hot and saturated. Google steam bending for information. You can also bend wood by saturating it with ammonia, but this is not a DIY process since ammonia is very caustic and poisonous.

3. You start with relatively large elements like long 2x4's, glue them up, and cut to shape using a jig saw. This is wasteful of material, but you don't need a brake.

I build wooden canoe and kayak paddles using the thin strip glue up technique. For exterior applications like your bridge, you need to use a qualified exterior class glue, with bonding strength that exceeds the strength of the wood you are working with.
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Old 06-20-2010, 05:28 AM   #5
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Fabricating curved wood beams for bridge


thanks. I did mention 30 ft long roughly and I mentioned using doubled up 2x12's like in your last suggestion. I will get a arch pattern made out of sheet metal from the shop and line up 2x12's over that. My last bridge I built I did use i beams and c channel with no center support. this one I would like arched though. It is a pedestrian bridge with only a lawn mower going over it. I think a 4x10 beam should work for support but am open to suggestions. I will probably have two sets of supports under the bridge roughly every 10 feet. thanks for the replies
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Old 06-20-2010, 10:37 AM   #6
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Fabricating curved wood beams for bridge


Sounds like an interesting thing to build and you have a good vision of what you want. Can't help you with the engineering but it would be great if you posted a picture or two after you get it completed.
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Old 06-21-2010, 07:22 AM   #7
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Fabricating curved wood beams for bridge


I made an arched bride for my sister/bro-in-law some years back, certainly not 30 ft. long, more like 16-18 ft. long to cross a wet-weather stream. I laid 2 x 10's flat on my shop floor, one under the other (or one on top of the other), then drew my arches for top and bottom using a long 2x4 and trammel points until it looked good. The I figured out where I needed to put long lag bolts coming up from the bottom to secure the 2 x 10's together, even counter-boring for them. I stayed out of the cut-out area. Once all was secured, I cut out the arc with a sawz-all and stood it on edge. So far, so good. The bridge was supported at each end on concrete piers and about 1/4 of the way from the ends with decorative cut 4x4's to which the rails were added. The walkway was plain 5/4 boards for decking. The darn thing turned out pretty good. It's been there since '94 so far. David
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Old 06-21-2010, 08:22 AM   #8
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Fabricating curved wood beams for bridge


thanks, sounds exactally like what I am doing. How thick/wide were your arches when finished? A full 10 inches or less? I was going to cut 10 inches out of 2x12's. My main concern was getting the pattern, but remembered I could have one cut out of sheet metal at the shop. I will lay out the boards along the pattern and cut the ends to the proper angle. Then trace the pattern on the wood. Glue and bolt/screw the boards together then cut. Any other tips/pointers you may have would be appreciated. How did you secure the arches to the footing? I was thinking of using a sill plate and rim joist just as in building a house floor, but am worried about soil rotting the wood.

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