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Structural engineers... I'm going to build a shooting hut, need some help with design

Discussion in 'Rallying Point and Range Discussions' started by CoRoMo, Jul 2, 2015.

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  1. CoRoMo

    CoRoMo Member

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    See the attached picture and note the ground depth indication I drew. Borrowed the photo from someone on Arfcom (will return it when done). :)

    I'm going to build a little building like this, but I'm not going to use wood throughout as this one pictured was made. I'm using a bunch of 6 gauge angle iron for most everything, but the floor will be wood framed and either planked or sheeted.

    The floor will not float as high off the ground as this pictured one does, I want mine to have only 6" or so between the ground and the bottom of the 2x6 and 2x4 floor joist frame. So the floor level will be approx 12" above ground.

    Ceiling height at the front will be 7' and 6' at the back, so a 12" total roof elevation/slope.

    Two rifle benches on the left and two pistol lanes on the right.

    I basically just don't want a big wind during any thunderstorm to take this thing down, so I'm going to dig the six post holes... how deep? 16"? 18"? 24"?

    I'll concrete them in, and post hole diggers dig what... a 6" diameter hole? I might shave them out to 8" or 10" diameter, but I don't know if that's really necessary if they are the right depth.

    So any pointers are more than welcome.


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  2. anothernewb

    anothernewb Member

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    what kind of soil? you're in Ca - so probably frost heave isn't going to be much of an issue.

    I've built a number of gazebos and pergolas in MN with a post depth in ground of 36" and none have yet to shift in the wind. Shingles have been ripped off - but not a roof. However, sandy or loose soil might affect that.
     
  3. CoRoMo

    CoRoMo Member

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    Thanks for the input!

    I'm in Colorado. The dirt isn't sandy. It's just as plain dirt as you might conceive of. I've dug several holes on this property and it's easy going with a shovel, but post hole diggers feel like your hitting concrete at 8"-10" down. It's quite dense down there and post hole diggers are what I'll be using since it's only six holes. I don't know if I can get much deeper than 20" or so without an auger.

    It'll be welded together, so the structure being nearly a solid frame, I'm hoping that I don't have to go as deep as you have.

    ps
    I don't like the name 'shooting hut' that much for this building. Been kickin' around '2nd Amendment gazebo' lately.
     
  4. MutinousDoug

    MutinousDoug Member

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    I got some recommendations off the Quickrete bags that suggest holes a minimum 25% of the finished height and 2x the diameter (or width) of the posts. So, for 8'-0" roof height, use 24" hole depth and for 4x4" posts use 8" dia holes minimum. 6" for your angle iron is probably plenty and a 6" dia hole uses much less concrete than an 8" one. If you fill your 20" holes with water and let it soak in, you should be able to dig another few inches.
    Note that a cone shaped hole where the top is wider than the bottom is more likely to frost heave than a straight sided, cylindrical plug.
     
  5. anothernewb

    anothernewb Member

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    Well, in my area, the depth is more to resist shearing forces from frost heave. too shallow, and the posts will move and simply tear the structure apart. sometimes better just to make a platform and anchor everything to that, and let the platform float on the fewest contact points you can get away with.

    if you're in the mountains - you might be hitting some serious boulders. Probably a better method there then to clear off a large portion of the base area, if it is solid rock, and just clean it off really good and bond it right to the concrete and set the posts in it.

    20" is a fair amount of dirt to move, but a 3'x3' area will give you a clear picture of it.

    if it is just compacted rubble, you might be better off drilling some anchors in. less than 2 feet isn't much to anchor with in a high wind. your alternative might be to just make a concrete pad heavy enough to resist the wind loads, and minimize the structures wind resistance.
     
    Last edited: Jul 2, 2015
  6. CoRoMo

    CoRoMo Member

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    Exactly what I was hoping for! Thanks guys.

    This project evolved to this from a simple car-port idea with a gravel bed underneath. I like the raised floor though. Not quite in the mountains, and this dirt hasn't produced any real amount of rock either, which surprised me being this close to those Rockies.

    Since it'll be an open-air gazebo, I do want to come up with some weatherproof storage integrated into this. Whether that be a Rigid box or a pickup tool box or some sort of closet/locker. Something.

    I'll probably put a half-wall on every side of this thing too. Maybe some lattice, I don't know.
     
  7. Slamfire

    Slamfire Member

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    Make sure you electrically ground the thing. Lightning happens and when it hits, it hits big. In a dry area you might need to use special grounding chemicals around any electrode.
     
  8. hso

    hso Moderator Staff Member

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    Bomb proof footings like the ones below should keep anything from blowing away.



    If you don't want to spend the money on a mushroom foundation remember that 1/3 of your post must be set below ground. That means you'll need about a 12' 4x4 set 4ft below ground to give an 8' post above ground. Since you're in CO you could get away with doing this, but in rainy areas I'd set the post on the galvanized fixture shown below and not pay for a 12' 4x4.

     
    Last edited: Jul 3, 2015
  9. horsemen61

    horsemen61 Member

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    tagg for later study
     
  10. Brin

    Brin Member

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    If the roof has wood rafters put hurricane brackets on them.
     
  11. X-Rap

    X-Rap Member

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    I've set hundreds of 4x4 posts in the CO dirt. 24" is adequate, I'd worry more about them snapping off at grade from wind.
    I don't think I'd use an elevated floor for a shooting platform simply because you will never get it rigid enough to not have movement when you are down on the bags. Anyone else on the floor sneezing will show movement at the bench and in the scope.
    We built ours on a concrete slab with 3 solid walls.
    If you used a slab you could use the Simpson Post Bases and drill and anchor after the concrete is cured and simplify the layout.
     
  12. Dave P

    Dave P Member

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    Shootin' Shack
     
  13. Evil-Twin

    Evil-Twin member

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    I would also suggest a concrete slab... a good building starts with a solid foundation... If you are going to invest in a 50 years + structure... you don't want to build it with 10 year specs. the shooti8ng benches and your personal stance with a pistol on a wooden floor is not the best idea... with a wooden floor any movement you make with the bench or even just standing your effect your shooting anchor. You don't want to feel movement under your feet or on the bench..
     
  14. wendellgj

    wendellgj Member

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    This has nothing to do with your foundation but will much improve your facility: place sound absorbing material in the interior under your tin roof and behind any tin surface. Beside anchoring; increase your comfort level with this simple measure using inexpensive materials to deaden the concussion when you torch-off!
     
  15. Schutzen

    Schutzen Member

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    shooting shack

    I too would vote for a concrete slab with the support posts cast in the floor. Even a sturdy wood floor will have some vibration in it if someone walks across it when you are shooting. Not a problem for close shots, but it can be a real issue for long range. Since you are spending the money for 2' angle for the roof, why not go all the way and use 4"-6" pipe or square tubes for posts? They would not wear out in your lifetime and probably not in your grandchildren's. On an open shed like the one posted, I would space the rafters no further apart than 16" and probably 12". I would attach my metal at each rib to each rafter. The up lift force of the wind on this type of roof is what destroys the building. A half wall or even better a full wall to the side of the prevailing wind helps to reduce the lift against the roof. If you do chose the 1/2 wall, do add the lattice above it. It too helps break the force of the wind.
     
  16. CoRoMo

    CoRoMo Member

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    Good info all around!!

    Not going to use 4x4 wood posts as some of you are posting, I've got a ton of heavy gauge (6 ga. - 1/4") angle iron that I paid zero $$ for. This whole structure will cost me not much more than time, welding rod, welder gasoline, and some odds and ends. I've been pack-ratting all kinds of metal and lumber that I've come across for free, for about a year now.

    So I didn't want to spring for a concrete truck; doubt any company would/could get one back in this spot anyhow. But I am now thinking that I might go back to a ground level over-hang only, simple design. I can get a trailer back to this spot and haul loads of gravel for the base material.

    The shack will face directly into the prevailing wind and there's no way around it. It's only 146 yards from the line to the dam that I shoot into. I had thought about the raised floor being an issue with movement, primarily from wind, but up until now I liked the design too much to reason my way out of it. So now I think I'll just go back to something like this...
    six-post-carport-3.jpg

    Gravel bed, I'll build a solid, heavy table, maybe lattice a couple sides to give me more shade and privacy. Still need a weather-proof storage, probably just a full-on shed setting by it, if I don't happen across a big free container of some type.
     
  17. oneounceload

    oneounceload member

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    You will want to use some rigid styrofoam insulation sheets to not only deaden sound echo but to also keep the heat from summer sun from turning it into an oven.
     
  18. CoRoMo

    CoRoMo Member

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    I hadn't been thinking of sound dampening or insulation factors, but by mere happenstance the roof material I came up with is some panels of sheet metal sandwiched around expanded foam.
     
  19. Evil-Twin

    Evil-Twin member

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    A couple of my gun club ranges... just to get ideas... you only need a smaller version
    Big bore rifle range.

    i-GXnFkFf-XL.jpg

    standing and prone windows..

    i-bzKj7qp-XL.jpg

    Air gun range

    i-dm9DsTX-XL.jpg

    i-kGhzrqK-L.jpg
     
  20. mac_hunter870

    mac_hunter870 Member

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    Tagging to follow
     
  21. jrdolall

    jrdolall Member

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    I will agree on just building a platform and then anchoring. I have done this with shooting houses (deer blinds) in many areas with a mind for possibly relocating at some point and they are all still viable. If they can anchor a mobile home or a "portable building" then you should be good with a shooting house. None of my buildings in Alabama have ever been affected by the wind and we are in tornado alley.
     
  22. Slamfire

    Slamfire Member

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    I heard the Alabama state bird is a mobile home. :neener: The devastation I saw caused by the April 2011 Alabama tornadoes https://en.wikipedia.org/wiki/April_25–28,_2011_tornado_outbreak was beyond words. :eek: I have seen big trees blown down and roofs torn off by winds less than tornado strength.

    I would think a big flat roof, as shown in post 16, is pretty much going to fly away in heavy winds. The basic problem is the surface area. There is a lot of surface area to a flat roof, if the winds get under the thing, the lifting force is psi times surface area. I have seen similar roofs just peel off. And of course, in something like a tornado, the debris carried by the wind will sweep anything before it. I have seen Lincoln towncars flipped hundreds of yards. Imagine concrete blocks traveling at 200 mph.

    I was told a primary safety issue with anchoring a mobile home is preventing wind from getting under the trailer. The anchoring system will support some side loads, but if enough wind gets under the trailer, I have been told the thing will go airborne.
     
  23. CoRoMo

    CoRoMo Member

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    Here's the latest design

    Thoughts?


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  24. Malamute

    Malamute Member

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    Hmm, concrete block walls, that shouldnt go anywhere. :)

    The hard layer you are encountering may be caliche https://en.wikipedia.org/wiki/Caliche

    I rebuilt a corral that required steel posts set in concrete. I had to use a digging bar to break through the caliche layer, its like low grade concrete and is brutally slow work at times. It can be done, but is not for the faint of heart.

    If you end up setting posts of some sort, just get a digging bar and expect a little extra time.

    If you(over) build it correctly from the beginning, it wont go anywhere. I've built some things in high wind areas (up to 135 mph @ 4000 ft el), and nothing has moved so far. Think it through, over build, thinking in terms of anchoring all the way down rather than just nailing or screwing it together, or relying on small fasteners in your steel. If you use rafter ties at the top, the same material should be anchored at its bottom. I used 1/8" to 3/16" x 2" steel strap on headers or logs going over posts, run a foot down each side and screwed/spiked into place, and 4' of similar steel set into the concrete bases with 8 or 10" to spike into the long post base. If you set posts directly into concrete, notch or otherwise give the concrete a way to grip the material.

    If you use steel for the roof material, use more fasteners. At least one in each flat valley (and two in every valley at the ends) and every 24" up the run if using common type 5v ribbed style metal. Stitch screws are good also. If going over plywood, be sure you arent hitting seams. Plywood is more rigid than OSB for sheathing. Use 5/8" for roof sheathing material if using sheet stock. Steel purlins works too. Read on roofing materials manufacturers site for purlin spacing for the max wind speed in your area. Going a bit more (closer spacing) wouldnt hurt either.

    Dont let the caliche deter you, once you get it figured out, its just a high nuisance level issue. You'll be pretty tough when youre done. Pace yourself and drink lots of water is all I can say.
     
    Last edited: Aug 28, 2015
  25. ColtPythonElite

    ColtPythonElite Member

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    No bigger than that is, I would just build it on skids....I have a 12 x 24 garage built on skids sitting on a crusher run pad. I have been keeping a full size truck in it for two years. It is still as level as the day I set it.

    If you are worried about wind, anchor it with mobile home anchors. I doubt if my building will blow away, but I anchored it because the building inspector required it.
     
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