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I am installing a 20' x 100' hoop house with 5' spacing between each of the hoops. The arch is about 10' tall. The hoops are galvanized chain link fence top rail. 3 rails inserted into one another makes one arch. The hoops are inserted into 2.5' vertical posts which have been pounded into the ground about 2'. From there it is a caterpillar style hoop house. There will be a three ridge poles. One down the center and one on each side. The end hoops will have double hoops with 1" x 8" fastened to them with wiggle wire channel fastened to that. There will be diagonal supports at both ends spanning two arches. There will be a single layer of plastic pulled over the building. From there I will use 550 cord (parachute cord) to lace over the plastic in crisscross fashion.

Is pounding the vertical posts into the ground good enough in a strong wind? Or should I also put some cement around the base? Any other ideas to keep this style of structure from being up-rooted in a storm?

Tester101
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Andrew Siemer
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  • Besides the structural improvements, you could also consider protecting your building from storms by planting trees/bushes around your house. – Earthliŋ May 10 '13 at 12:03

3 Answers3

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I'm not sure if you have already built this or not, but ideally you should build footings just like you would for a deck or a fence. How should I build the concrete footings for my deck? is a particularly good example.

Essentially, dig post holes down past the frost line or whatever is suggested in your area. The holes should be bigger (mushroom out) on the bottom. Set your posts and fill the hole with concrete. Now as long as you have a solid way to attach everything to the posts, it's probably about as strong as you can get it.

Community
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gregmac
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  • In this case I took 3' long galvanized posts and pounded them about 2.5' into the ground. Well into the clay layer beneath my sandylome top layer. I was following a PDF on hoop house construction. I am worried that the wind might uproot my building though! :) – Andrew Siemer May 09 '13 at 19:35
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There is a distinct possibility of this structure being uprooted in a wind storm. You have a structure of significant area and very little mass, not a good combination for wind stability. A significant factor is how strong the wind can be expected to be in your area. Unfortunately, adding more wind resistance to an existing structure is difficult to do economically. As mentioned, the ideal anchorage is allowing a deep concrete pier to be mushroomed out at the bottom.

On the other hand, on a larger scale, I know uplift resistance of driven piles by soil friction alone can be substantial, depending on soil types. So posts driven into clay can be quite beneficial. Just be sure the hoop pipe is through bolted into the driven pipe or otherwise connected. Adding concrete after the fact at the surface will be of marginal benefit because it is simply extra dead weight, there is little benefit from resistance of the surrounding soil, that only comes with depth. In addition, for concrete to be effective, you need to weld studs to the driven pipe, concrete bond is not enough. The problem is welding galvanized steel produces poisonous gas and must not be attempted without proper protective equipment.

Another possibility for enhancing uplift resistance may be employing ground augers intended for anchoring mobile homes. One problem with this scheme is the distance the anchors can be driven into the clay may be limited, and anchorage from sandy-loam is limited. Another is to be most effective, they should be driven at an angle parallel to the direction of pull. I'm unsure how well this can be accomplished with typical driving equipment adjacent to existing structures. The arc shape certainly offers more opportunity than a boxy mobile home. When properly installed, these anchor can offer substantial resistance.

bcworkz
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Standard practice uses pounded in posts. Commercial greenhouse catalogs often offer options for either base plates that are set in concrete, or pound in posts. The decision usually comes down to whether they have a concrete floor interior. Container based operations like concrete floors as it makes equipment easier to move, and everthing stays level. If you are growing in the ground, the pound in posts are cheap cheap and easy to set up. That said, most greenhouses are set up where there is at least some shelter from wind, if nothing else than to reduce the heating costs.

I have just finished taking apart a greenhouse and moved to to my site, which has some additional wind exposure. Pulling the posts took a front end loader.

Remember too that a wind is going to put a sideways as well as an upward force on the frame. The sideways component will increase the frictional force needed to uproot the post. (Don't believe me? Pull a fence post with a front end loader with the chain 30 degrees off vertical.)

Two easy ways to hedge your bet:

  1. Drill a couple of half inch holes in each post. Put bolts through them extending a couple inches out each side. (Or sucker rod.) Repaint with rust resistant paint. Now you are dragging more junk through the soil.

  2. Concrete for this application can be very low cement in content. Use a foot of washed gravel around the base of your post, mix up a slurry of 2 parts cement 1 part water, and as much gravel as it will wet (It will be about 8 parts gravel) Dump into hole. the surplus cement water, will coat some of the rock below.

More information on this in Kern's "The Owner Built Home" on 'no fines' concrete.

Finally, in your description, you omitted a base board on either side for anchoring your plastic cover.

Most accounts of greenhouse collapse in my area (Alberta) have been due to snow loads, rather than wind loading.

Note: I'm not an engineer. Your millage may vary. With my recycled green house I am going to pound the posts in with a sledge hammer.

Sherwood Botsford
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