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Trellis Arch Design Calculation

Trellis Arch Design Calculation

Trellis Arch Design Calculation

I am building an arched trellis for my apple plants.

Materials:  1x4 cedar boards 8' long (2)to make the arch out           of
            1/2" plywood 4x8' as support behind cedar cut for arch

            2x4 cedar boards 8' long (2)as the supports spaced 8 feet apart.

Desired dimensions:
          2x4s two feet underground concrete poured blocks (leaving six feet above ground to work with)

          2x4s spaced 8' apart to provide the diameter of the arch. I want the apex of the arch to be 4', so the radius is 4' also.

Question:  What are the cuts (measurements) of the 1x4 8' cedar planks so that they will make an arch four feet high at the top and eight feet at the base? Lets say there are 12 or 18 pieces of cedar to the arch, whatever provides the best stability and gives the arch look. (1/2 Plywood will be glued or nailed to the backside (not visible)for support.   Is this enough support? Then the arch will be screwed into the top 8" of the 8' 2x4s.

My design assumption is that I need one piece that I can cut 12 or eighteen times, but I need to be sure about the angles and measurements otherwise it will not meet the 4' arch height requirement and the 8' diameter.

Any help?  Thanks

RE: Trellis Arch Design Calculation

It sounds like you need to lay out a perfect half-circle.  18 pieces of 10 degrees each,yes?

RE: Trellis Arch Design Calculation

If you cut the plywood to a "U" shape for this you will likely lose much of its' strength as well as exposing ply edges to weathering.
What if you create your 1x4 arch as described  and a second rotated by half of an arc segment?  Then fasten (glue?) the two layers together. Corners could then be trimmed with a jigsaw to give a more true arc. A little more work but perhaps a much stronger, more aestheticly appealing result?


RE: Trellis Arch Design Calculation


Your observation that the plywood will be weakened in a U shape is correct. But I'm not sure of your description to remedy this.  I think now that you mean I should cut a second arch of ceder but maybe make it only half of the first arch.  Then it should be attached at the top?  to cover the largest part of the curve?  If so, then this will be somewhat of a   {ceder 180 } (plywood) [ceder 90] sandwich.  Will this actually make it stronger?  Will screws be better than glue for support or will it not make any difference?  Are screws a threat to cause splitting?  Also would you glue/screw it first and then cut? Also what is the strongest type of 3/4 plywood?  There were ISO, blends, etc.... about three kinds. Any thoughts?

RE: Trellis Arch Design Calculation

If you are using 18 pieces to make your arch then each segment covers 10 degrees. The second arch also covers 180 degrees but is cut to rotate it so that the joints occur at the midpoint of each of the front arch segments. The segments are cut the same for both but the starter piece for the second portion will be half of the length of the "front" starter.
Glue will be stronger than screws but you will need to clamp the two layers together while the glue cures.  you can use nails to do this and either leave them in place or remove them after curing.
If I were doing this, I would make a template to cut the arc on each segment before assembling.
The only thing I would use plywood for would be to lay out an assembly jig to assure consistency and OSB would work for this purpose.

Hope I have not confused too much.


RE: Trellis Arch Design Calculation

Got it thanks.

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