11'6" PT Plywood Arches

[Avadon]

Can I get everyone's take on this idea before I invest the time and money implementing it. It's an exterior archway

I want to create a 11' 6" wide arch (I'm thinking either lancet or rounded like a tunnel) that will span between two 6'x6' PT posts set 3' feet into concrete. They are really in there! I'll make the arch template in cardboard. The plywood will be home depot PT 23/32" plywood doubled up (overlapping laminated). I intend to cut something like 16"x16" squares and glue them in overlapping fashion with PL premium 8x and screw them together. Then when I have the rough shape of the arch I'll go back and cut the arches out with a scroll saw. I understand they'll be heavy, perhaps even so heavy that I'll need to lift them into the air with bobcat. They will be hung with joist hangers on both sides.

I'll stop them from racking using PT 2x4's between the arches. I'm guessing the arches will be roughly like 2x6 or 2x8 size but clearly much, much stronger. Once I have the cross 2x4's in place to stop it from racking I intend to install hog wire panels (metal lattice) to wrap around the underside of the arch and grow vines over it. The arches will be painted as will the hog wire with restoleum restore 10X paint. It's very, very thick paint. Also I live in the desert, not a lot of rain here.

Outright concerns:

Are the span of the arch and whether or not they'll sag in the weight. Do I have to maintain a certain element or greater of pitch to prevent sag?

Will it be too heavy and try to push my 6"x6" posts outward?

Guy at home depot said do not use lumber because it won't be anywhere near as strong as my laminated PT Plywood idea.


Will my idea pass?

 

[Kaizen]

This might require some sketches. What is the 11-6 measurement? Ground to top? A lancet is thin enough to use all 2x imo. From ground to top. Will there be just one of these? Any arch will exert outward force esp with vines. Any freestanding arch will have something countering the outward push.
Why not do it for real? You could lay out the form and cut hollow block like you would real stone. Put a footing and build up. Put rebar and concrete inside. Now that would be awesome. Some stone veneer to top it off


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[bczygan]

Photos of where this will go.

What is the purpose of this?

Drawing of what you are trying to describe...

Bill

 

[theoldwizard1]

Is the glue in the plywood truly waterproof not just water resistant ?

Is the laminating glue truly waterproof ?

Not sure what the radius is, but you might need to go down to 1/2" or even 3/8" plywood to make the curve without having it break.

 

[Avadon]

Another idea I had which may be less cool but still work is put in 3 sets of posts in the ground all parallel to each other. So there will be one strip of 11 posts about 4 feet away from another parallel set of 11 posts. And yet again about 8' feet away will be another set of 11 posts again all parallel to the other two sets. So you'll have 3 fences or lines of parallel posts and there will be a small archway about 4' apart between two of the parallel fences the third fence line will be connected by an 8' arch. It's a lot more work to build but it does decrease the span of the arch down to an 8' section. Is this worth while? or unncessary?

 

[Avadon]

This might require some sketches. What is the 11-6 measurement? Ground to top? A lancet is thin enough to use all 2x imo. From ground to top. Will there be just one of these? Any arch will exert outward force esp with vines. Any freestanding arch will have something countering the outward push.
Why not do it for real? You could lay out the form and cut hollow block like you would real stone. Put a footing and build up. Put rebar and concrete inside. Now that would be awesome. Some stone veneer to top it off


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The 11'6" is the width, Between the posts. So basically think of two parallel fence lines that are connected with 2x6's length wise but instead of privacy fencing on them they have hogwire streatched across them. So it will be basically like a wire tunnel if you will.

Probably something a little like this..
http://myzone.homesoftherich.netdna-cdn.com/wp-content/uploads/2012/03/2a-White-Garden-Through-Pear-Tunnel.jpg

or perhaps even like this

http://www.slate.com/content/dam/slate/blogs/atlas_obscura/2013/08/13/alnwick_poison_gardens_where_plants_can_kill/garden-tunnel.jpg.CROP.article920-large.jpg

 

[Avadon]

Is the glue in the plywood truly waterproof not just water resistant ?

Is the laminating glue truly waterproof ?

Not sure what the radius is, but you might need to go down to 1/2" or even 3/8" plywood to make the curve without having it break.

That's something i'll have to look into. The thing will be covered in a very, very thick paint.. almost like bed liner so I don't think that rot will be too much of an issue, especially in this dry climate. Mostly sag from the weight would be my main concern. Why would going down in plywood thickness be better?

 

[wssix99]

How high will this arch be?

That's a pretty long arch. Could you make one with some steel sandwiched in between the two layers of plywood? Bolt/screw all three and that will take up some of the pressure on the 6X6's.

 

[Avadon]

How high will this arch be?

That's a pretty long arch. Could you make one with some steel sandwiched in between the two layers of plywood? Bolt/screw all three and that will take up some of the pressure on the 6X6's.

I guess I could put like 16 or 18guage steel sandwiched in between. Do you really think that's warranted? It would be a lot more work clearly. The arch is about 7' in height so it's not terribly tall. I guess a taller arch is probably stronger than a more shallow arch, right? The span also worries me a bit.. I may have no choice but to bring the span of the arch to 8' feet.

 

[Kaizen]

If you are just after the green tunnel why not just use rebar? Not sure what thickness would bend. Then put your fence wire tied to the rebar


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[Avadon]

If you are just after the green tunnel why not just use rebar? Not sure what thickness would bend. Then put your fence wire tied to the rebar


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I did think about that. But doesn't bending rebar require torch work or at a bending machine to roll that correct contour. I don't think can be accomplished with just trying to bend it by hand right?

 

[Bearston]

Easy enough to create a template to bend the rebar around (sheet of ply and some blocks. Screw blox to ply in the shape that you need). Rebar is available in various thicknesses, 3/4" should be plenty. It is a soft metal made mostly from recycled cars and stoves, etc. that bends easily (remember to keep full length and only cut after bending). The only time that you will run into bending issues is when you're attempting to bend short pieces. For this I use my BFH (big....hammer) or a piece of pipe to extend the rebar. Rebar is certainly the quickest/cheapest method to create the tunnel. That's a really cool idea,I think I'll try that at home, too!

 

[wssix99]

I did think about that. But doesn't bending rebar require torch work or at a bending machine to roll that correct contour. I don't think can be accomplished with just trying to bend it by hand right?

You can bend it by hand but it won't be a perfect curve. If you order it from a commercial rebar/concrete supply, they will have robotic machines that can spit it out in whatever perfect shape you want. They do this for a very reasonable fee and can just deliver the arches to you on the back of a flatbed.

 

[Kaizen]

I did think about that. But doesn't bending rebar require torch work or at a bending machine to roll that correct contour. I don't think can be accomplished with just trying to bend it by hand right?

I was thinking if you make one side nonmovable and pull the other to where you want it the rebar will make the curve without much bending. You could use several small diameters like this wound together for a really strong support. Get twenty footers. That might get you a approx 7 foot tunnel. Then some running on the outside wired to each one. I would not use plywood. It will delaminate. The edges of any plywood are where it will start.


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[manwithtools]

Have a steel fabricator roll steel tubing into the shape you need. It can be pretty thin wall - maybe 1-1/2"" square with .060 wall. It would cost a little bit to do, might be not much more than the method you proposed with the Pt plywood and a whole lot less work on your end. Painted it should last a long time, for sure longer than the wood.

 

[kbs2244]

I like the re-bar idea.
Either bend it your self in a jig or pre-bent and delivered.
Before you plant your vines coat it good with rust-prevention stuff.
Road contractors have the best.

 

[matt_i]

They do make pressure treated plywood. That is what I would build it out of.

A continuous curve is going to have to be rolled on a 3-roller system, analogous to a slip-roll used in sheetmetal fabrication. I would not use rebar as I think it would be more difficult to roll on a smooth roller, the knurls on the surface are going to wreak havoc with the smooth rolls trying to maintain a center distance. 1018 Cold rolled round would be my choice if attempting to fabricate a metal skeleton.