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

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.

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.

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.

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.

tagg for later study

If the roof has wood rafters put hurricane brackets on them.

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.

Shootin' Shack

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..

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!

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.

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.

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



standing and prone windows..



Air gun range

Tagging to follow

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.

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.

Click to expand...

I heard the Alabama state bird is a mobile home. 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. 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.

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.

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.