# How do I build arched wood beams for bridge



## abrahamx (Feb 11, 2016)

As a sheet metal worker I can build but my wood working skills are not well tested. I am wanting to build a bridge in my back yard. I built one already using I beams and it was flat. Not I want to build an arched one that will be visible from the road. They want thousands for arched glulam beams. I was thinking of doubling up 2x's splicing and gluing them together in a rough arch shape then using a pattern to cut the arch. Is this doable? any help on how to go about this? Is there a better way? I will also need hand rails. The bridge has to be roughly 35' long and 4' wide. Its going over a 10-15' ravine with a small creek at the bottom. Something similar to the one pictured but no rope and probably not that much of an arch. Any help/advice on making the beams or any other help on the bridge itself is much appreciated. Really just need to know how to make the curved beams.


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## MrRon (Jul 9, 2009)

2x's won't work. The wood has to be perfectly flat in order to get a secure glue bond. If you get wood that is perfectly flat, you will need a whole lot of clamps to guarantee a solid glue joint. The glulam is the right way to go. One possible way would be to find a couple of trees, around 16" in diameter that have a natural curve; saw them to around 6" wide by 12" high. Trees like that can be found in areas that have a lot of wind activity and the trees develop a natural curve. It also depends on the species of tree. However you do it, it's going to cost a lot in terms of money and time. Another thought would be to use plywood cut into arch segments and laminated together into alternate layers and using natural wood for the outside layers. That's where the many clamps will be needed; you could spend hundreds of dollars for the clamps alone. For a glue, I would go with West system epoxy or resorcinol glue. Water tightness is mandatory.


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## bigblockyeti (Sep 9, 2013)

A form (a very large one) would be the best way to bend pieces to laminate together. 2"x construction lumber would be difficult to bend to much of a radius given the rigidity created by the thickness. The surface would need to be flatter & smoother for a good glue bond (you'd need a good water resistant glue), planing could help with both of these problems. Getting shorter pieces joined together to make the required length is best done with a deep groove finger joint. Another problem would be moisture exposure to typical 2"x non treated lumber would ensure it wouldn't last long. An exotic, moisture resistant species would yield better results, but probably be cost prohibitive unless you can find a great deal on something.


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## bbasiaga (Dec 8, 2012)

Have you thought about getting a metal beam arched the way you want it? Could then veneer it with some wood of your choice by epoxying on the top and bottom, then routing flush with the sides. Next, glue some wider boards on to the sides and route flush with the tops and bottoms. Not sure how that compares cost wise to gluelam pices, but you'd have the strength of metal, and be able to use thinner wood in any weather resistant species you wanted, as long as you could find boards wide enough.

-Brian


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## HermitStudio (Jun 25, 2015)

Circular work in carpentry and joinery - click the title link at the very top for a pdf
http://gen.lib.rus.ec/book/index.php?md5=3827837fa942534d4ee8c2de40e7f610

I recently read an appendix to a woodworking book that went into this in some detail. Unfortunately, I do not recall which, but I will try to find it for you.

Also:




View on YouTube


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## AandCstyle (Mar 21, 2012)

Abraham, I have never done this, but will offer this suggestion anyhow. 

Plane 2" x 8" dry treated planks so they are flat on the sides. Cut the desired arch on the tops and bottoms. Cut them to length and laminate three together for each side of the bridge so the ends are offset by half the length. Glue and clamp them together with 1/2" bolts every 1-2 feet.

For example: For each side of a bridge that is 35' long, assuming a 2' rise. the length of the arc will be 35.3'. Therefore, use 2 bys that are 12 feet long. The three outside and inside 2 bys will be about 11.75' long and the center ones will be about 5.875', 11.75', 11.75' and 5.875' long. Think of laying up bricks in a running bond. Is this understandable?

These laminated beams will be heavy so you will need a crane or a tractor with a bucket to get them across the ravine. Once they are in place and well secured (rebar into concrete pads?), laying the decking will be a trivial effort.

Also, if you do this be sure to wear a good respirator when cutting the treated stock and please post pix of the finished project.


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## abrahamx (Feb 11, 2016)

> 2x s won t work. The wood has to be perfectly flat in order to get a secure glue bond. If you get wood that is perfectly flat, you will need a whole lot of clamps to guarantee a solid glue joint. The glulam is the right way to go. One possible way would be to find a couple of trees, around 16" in diameter that have a natural curve; saw them to around 6" wide by 12" high. Trees like that can be found in areas that have a lot of wind activity and the trees develop a natural curve. It also depends on the species of tree. However you do it, it s going to cost a lot in terms of money and time. Another thought would be to use plywood cut into arch segments and laminated together into alternate layers and using natural wood for the outside layers. That s where the many clamps will be needed; you could spend hundreds of dollars for the clamps alone. For a glue, I would go with West system epoxy or resorcinol glue. Water tightness is mandatory.
> 
> - MrRon


Thats kind of what I meant with the 2×12's gluing two together with alternating seams to form a 4×12, cutting the ends to a small angle to make up a rough arch(hard to explain, would be an arch but not smooth radius). Then marking out a smooth radius on the wood and route or saw the pattern out to produce a 4×10 or 4×8 smooth radius arch. That way no bending is required. Is that advisable?


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## abrahamx (Feb 11, 2016)

glulam place wants 7k for 4 beams and decking.


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## abrahamx (Feb 11, 2016)

> Abraham, I have never done this, but will offer this suggestion anyhow.
> 
> Plane 2" x 8" dry treated planks so they are flat on the sides. Cut the desired arch on the tops and bottoms. Cut them to length and laminate three together for each side of the bridge so the ends are offset by half the length. Glue and clamp them together with 1/2" bolts every 1-2 feet.
> 
> ...


Exactally what I was trying to explain in my original post. The gluelam place said to exersize caution using this technique because of cutting against the grain or something. That why I was trying to see what you guys thought. That way no bending. thanks for putting it in better terms for me. Except I was talking about doing the glue up first then cutting one big arch


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## abrahamx (Feb 11, 2016)

I cant roll 35' but could roll 10' sectioins and weld them or cut out a arch out of metal and weld flanges on making my own I beam or c channel. But thats all at my shop and working with wood at home would be easier. I can buy c channel and have it rolled for about $400 a piece. Wanted cheaper. I still need a hand rail. I wanted to make a toerail and a middle rail also. Similar to the pic. Dont look like its going to happen tough. May just end up a flat bridge. Hopefully I'll work it out. I dread decking metal though. On my flat bridge I bolted 2x's to the top of the I beams and screwed my decking into that. I could get self tapping screws I figure that will cut through the metal if I go that route.


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## splatman (Jul 27, 2014)

Or predrill holes in the I beams and screw on the deck planks from underneath. That way, no visible screwheads.


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## abrahamx (Feb 11, 2016)

Just watched that video. thats what I was thinking of doing. Just was wanting to make sure it was an acceptable method. Now my question is what type of wood should I use? Can I just go with a standard pressure treated or something else. I went with oak for decking for my other bridge but I got the wood cheap and that is no longer an option. Any suggestions. Any idea on price in MI.


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## MrRon (Jul 9, 2009)

It's not necessary to drill holes in the flange for securing the deck. All you need are square flange washers that grip the edge of the flange. They have a hole where you can use a bolt and nut to fasten down the wood decking. Laminating is not practical. You will have to insure the mating surfaces are truly in contact for the full length of the lamination in order to get a complete bond. That will require a ton of C clamps. Companies that make laminated beams, have specialized machines to clamp the laminations under hydraulic clamping power.


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## gord09 (Jun 1, 2008)

why not build up a laminated beam with 1×4 x 8' ? The board will flex enough and will hold the flex while you glue and screw each board. You'll probably need an engineer to design the beam. Just a thought.

Gordon


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## abrahamx (Feb 11, 2016)

Cuz it sounds like a major pain in the butt. Im just going to roll some c channel. I cant do 40 feet but I can do 5 or 10' sections and weld them. About 600 for some c channel plus decking vs 7k for glue lam beam and decking. Wood would cost alot also plus alot more work that I am not familiar with. I can roll those up and weld them in probably a half a day. lts just kind of a big job for a side job at my shop so I may just go with a flat bridge seeing as how much of a pain in the ass it seems to be for an arch. Who'd a thunk it.


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## CharleyL (Mar 10, 2009)

With steel you could make your own curved I beams by cutting flat plates into an arch and welding them together with more flat plates welded top and bottom. I would not make it in sections, instead, I would overlap the joints so that no mating piece had a weld joint in the same position as an adjoining piece. For that length the flat top and bottom strips wouldn't take much to bend along the curved vertical rib as you weld it. The heat of welding would even help the curve. Steel is quite cheap right now, but you would need a good welder and someone really good at doing the welding. It's a lot of work, but it will last longer than any wood bridge design that you could come up with. For the look of wood it would be relatively easy to cover the finished steel arch with wood. You would only need to cover one side of the outer two beams with the wood. You could go with C channel for the decking or rough sawn white oak planking and it would likely last longer than you.A crane would be necessary for installation of the beams. Then you could plank it with your choice of material. Treated 2 X lumber is too soft and will wear away very quickly under traffic loads. As long as white oak can dry occasionally and not stay soaking wet it will last a very long time.

Charley


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