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I am thinking of building a simple shaded Arch for agriculture. I have an idea of building it with 30 ft sections of rebar that I will bend into a rough art shape. I would then weld rebar perpendicularly to hold each Arch rigidly to the one next to it. I could then put some fabric/membrane to cover and shade the rebar. Overall, it should be very low cost and simple to build. I was thinking of putting rebar every 12 in, but I have no way of knowing if I should increase the frequency or not.

  1. For example, How do I calculate the weight that a structure like this can support? Could I hang a fan from the ceiling to push air through the arch. let's say the arch is 12ft high and ~17 ft wide (a bit less than 30 ft edge length). is there a formula for determining the amount of deflection?

    a. what thickness of rebar should be used?

    b. how frequently should rebar arches be built? ie spacing between arches.

  2. If I were to have a tree fall on it, would animals or items inside be safe?

  3. Is it okay to have a structure that's fairly flexible? I imagine it would move around quite a bit, but as long as it's anchored into the foundation....is that okay?

  4. is this just a bad idea?

Totally an experimental thing here so all ideas and criticisms are welcome! here's a quick pic of what i am thinking (minus the lateral pieces of rebar to prevent the arches from just falling over :P :

arch

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  • $\begingroup$ Compute the force of wind with a pressure change and tie it down well. A tree, comes in many sizes. $\endgroup$ Commented Aug 24, 2022 at 4:12
  • $\begingroup$ will the ridges on the rebar makes holes in the membrane easily over time due to wind moving the membrane? $\endgroup$
    – Solar Mike
    Commented Aug 24, 2022 at 7:00
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    $\begingroup$ No, It's way too heavy, expensive, and flimsy. You can buy kits made of cheap tubing for this. They would be 1/4 the weight and 1/2 the cost, and come with all the connectors, door, and ventilation options. The shade fabric can provide most of the lateral support, you only need a few longerons to support the structure enough to get the fabric on it. farmplasticsupply.com/shade-cloth/shade-cloth-black-40 $\endgroup$
    – Phil Sweet
    Commented Aug 24, 2022 at 9:07
  • $\begingroup$ "If I were to have a tree fall on it, would animals or items inside be safe?" Even a properly constructed warehouse might not be safe. Maybe you should deal with the trees if you are expecting them to fall over. $\endgroup$
    – DKNguyen
    Commented Aug 24, 2022 at 14:16

2 Answers 2

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Although rebar is stronger than ordinary carbon steel bar and tube ( depending on grade), a tube shape has better bending strength for a given weight. So it will be stronger if made of tube of the same weight. Some commercial green houses are made this way but I don't know the specific tubing. Depending where you are; Used oil production tubing is cheap and strong. I have seen may things like fences made with it. A commercial green house would likely have your questions worked out.

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  • $\begingroup$ this is good stuff - I'll do some reading. Thanks! $\endgroup$
    – Joe B
    Commented Aug 25, 2022 at 3:35
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It's a bad idea in general. You mostly need bending strength, a rebar, being a solid cylinder, has very low moment of inertia relative to it's weight.

Further, welding rebar makes it brittle, so you'll most likely will have shattering at connections, perhaps even due to residual stress due to heating while welding.

As for the strength of these structures, FEM is probably your best bet, the axial + bending interaction precludes a simple "back of the envelope" solution.

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