Concrete footing size

tons001
tons001 Solar Expert Posts: 71 ✭✭
I am having to switch from roof mounted to ground mounted because of insurance reasons. While this is a bigger pain in the a** to install, it actually gives me more sun time before my house shades the panels. Plus moving the potential for lightning 40 feet away from my house vs. mounted to it ... I consider an advantage. (Yes, I am still doing lightning control/protection) I have one hole dug that is below our frost line (42") but my question is how wide the base should be? Currently I will be using two 2" schedule 40 galvanized pipes in two 10" wide x 42" deep holes. I bellied out the bottom of the hole to allow more concrete to settle at the bottom so the top is 10" and the bottom is probably 14-16" wide. I am using two holes and Ironridge pipe to XRS racking connectors to hold up five 22" x 48". I know every setup is different based on wind loads and frost lines but is 10" sufficient or should I go up to 12" footings?

Thanks!
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Comments

  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Concrete footing size

    You will have to have an structural engineer sign off on it. Or your insurance may be voided. You might get some hints from ground mount websites, as to what they use for their recommendations. But for a structure holding thousands of $ of panels, you
    want to do it right, and not miss the wind load calculation.
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  • tons001
    tons001 Solar Expert Posts: 71 ✭✭
    Re: Concrete footing size

    Thank you Mike.
  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    Re: Concrete footing size

    With my AHJ, I have to defer to a structural P.E. for this calculation, but according to my 90mph wind loading analysis, you should figure on an uplift pressure of about 10psf normal to the panels. My P.E.'s software comes in way higher than this, but then windload calcs are some of the fuzziest math you will ever find. Figure how much weight per leg this translates to and figure 150lb per cu ft of concrete. Don't figure on any help from the ground unless you want to prove to the AHJ your soil conditions. Just go with dead weight of the concrete and be done with it. Guess what? your solar system foundation will need as much concrete as a house foundation of the same size.
  • Organic Farmer
    Organic Farmer Solar Expert Posts: 128 ✭✭
    Re: Concrete footing size

    I used 4' sonotubes set 4' deep filled with concrete, and 6X8 posts in them. We did not get the brunt of Sandy, but we did get 50knot winds from Sandy dying. My sonotubes snapped off at about 2' deep.

    Contractors here are telling me to use 4' corkscrew anchors and guy wires.
  • tons001
    tons001 Solar Expert Posts: 71 ✭✭
    Re: Concrete footing size

    It snapped a 48" wide concrete footing? How big was the array?
  • Organic Farmer
    Organic Farmer Solar Expert Posts: 128 ✭✭
    Re: Concrete footing size
    tons001 wrote: »
    It snapped a 48" wide concrete footing? How big was the array?

    No.

    Code here requires that permanent structures must be set 4' below grade to get under frost-heave. House foundations or posts must all be set below grade to that spec.

    Sonotubes are a foot in diameter. If you build a pole-barn, using a post and beam design, the posts to support a garage roof or whatever. Those posts are typically set into sonotubes, that reach 4' below grade.

    My array is 20 panels, each is 3' by 6'. So 360 sq/ft of sail surface [3 X 6 X 20]. Supported by 11 posts.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Concrete footing size
    Sonotubes are a foot in diameter. If you build a pole-barn, using a post and beam design, the posts to support a garage roof or whatever. Those posts are typically set into sonotubes, that reach 4' below grade.

    I think they has snapped because there was a weak place right were the posts end. There's no serious bond between the bottom of the post and concrete, so thee were only thin walls of concrete around the post. Thin concrete is very easy to break.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Concrete footing size

    In general, wood does not have a large load rating for bending loads... I forgot, but one time I looked up a "generic" 4"x4" post and figured a 8-10' post supported at one end (in ground, as a "derrick", etc., would be rated for something like 100-180 lbs maximum.

    And concrete has very little resistance to tension--when attempting to "bend" a post set in concrete, one side of the concrete will be in compression and the opposite side will be in tension (concrete is very strong in compression). So, put tall posts in unreinforced/non-post tensioned concrete) and it is very easy to see the bending moment of the beam just snapping off both beam and concrete a few feet into the ground.

    Adding guy wires/cross bracing/shear walls (such as structural plywood nailed to vertical posts or even made into shed walls), changes the forces from bending to shear--Both wood and concrete will be better able to resist those forces.

    -Bill "not a structural engineer" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tons001
    tons001 Solar Expert Posts: 71 ✭✭
    Re: Concrete footing size

    Ah ... We have close to the same frost depth here. Code says frost is 36" ... I have my tubes at 42" to play it safe. I am using 2" schedule 40 galvanized pipe instead of PT wood. My array is also no where near as big as yours though. It is 40 sq. ft. and at its max, it is 80 sq. ft.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Concrete footing size

    there would not have been much 'meat' left at the (bottom) corners of those posts and that is where all the forces would be concentrated...

    add: the best way to put those metal posts in is to set them all the way to the bottom of the concrete
     
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  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: Concrete footing size
    Sonotubes are a foot in diameter. If you build a pole-barn, using a post and beam design, the posts to support a garage roof or whatever. Those posts are typically set into sonotubes, that reach 4' below grade.


    Sonotubes (and other brand equivalents) can be ordered in several different diameters. The tube we used for the very large footing for my 1800 watt pole mounted array was 36 inch diameter - 5 feet long and set on top and tied in with rebar to a 1 foot thick, 5 foot square footing !!. For my recent 2700 watt PV shed - I set 9 4X6 posts in 12" Sonotubes - 3- 4 feet deep.
  • Organic Farmer
    Organic Farmer Solar Expert Posts: 128 ✭✭
    Re: Concrete footing size
    NorthGuy wrote: »
    I think they has snapped because there was a weak place right were the posts end. There's no serious bond between the bottom of the post and concrete, so thee were only thin walls of concrete around the post. Thin concrete is very easy to break.

    Yes, I agree.

    :)
  • Organic Farmer
    Organic Farmer Solar Expert Posts: 128 ✭✭
    Re: Concrete footing size
    BB. wrote: »
    ... Adding guy wires/cross bracing/shear walls (such as structural plywood nailed to vertical posts or even made into shed walls), changes the forces from bending to shear--Both wood and concrete will be better able to resist those forces.

    I am on a small budget.

    When I opened the trench to set the posts, [70' long, 4 foot deep, 1' wide] It filled with ground water, to 3' deep.

    We have a fairly high water table. So I have been hesitant to pour lots of concrete.

    The sonotubes were filled with concrete and later lowered into place. [OMG those things are heavy]

    It appears that 4' auger-style 'earth anchor's will be the cheapest solution; using braided steel cable, turnbuckles and tension springs.
  • Organic Farmer
    Organic Farmer Solar Expert Posts: 128 ✭✭
    Re: Concrete footing size
    mtdoc wrote: »
    Sonotubes (and other brand equivalents) can be ordered in several different diameters. The tube we used for the very large footing for my 1800 watt pole mounted array was 36 inch diameter - 5 feet long and set on top and tied in with rebar to a 1 foot thick, 5 foot square footing !!. For my recent 2700 watt PV shed - I set 9 4X6 posts in 12" Sonotubes - 3- 4 feet deep.

    Thank you, I had no idea that they were available any bigger than 12" wide. :)
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Concrete footing size

    take a look at these Big Foot tubes http://www.bigfootsystems.com/?gclid=CJL2rOaU87cCFQXZQgod0iwAXQ
     
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  • Coach Dad
    Coach Dad Solar Expert Posts: 154 ✭✭
    Re: Concrete footing size

    Rule of thumb is:
    The post hole should be at least 3 times the diameter of the post.
    1/3 plus 6 inches of the total length of the post should be in the ground.

    So if your hole is 42 inches deep, your post can't stick out of the ground more that 6 feet.
    2 inch schedule 40 is very small... I wouldn't use anything less than 4 inch as a post..

    Also.... make sure you add some rebar into the hole before filling with concrete
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Concrete footing size

    If your soil is that wet (I know, I had really wet back yard until I dug a drain system about 3-4' and installed a pump), you will have to research the augers... If the soil can be "liquified" (vibration from wind on cables, etc.), you may need to reevaluate your choice (larger/deeper anchors, pouring concrete for weight/mass to keep structure stable, etc.).

    A structural engineer who knows the area is probably the safe way to go.

    Virtually anything that fails in "normal use" failed because of under-design (there was series of old biplanes that were "losing" their tail feathers--failing wire cross bracing). People wondered how everything worked OK for 40+ years, and then a series of failures (different planes, models, ages, etc.). Modern structural analysis showed simply the cross bracing was under designed originally (many of the planes were use in aerobatics--which can put tremendous strain on airframes).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: Concrete footing size
    westbranch wrote: »

    Wow - I bet those are pricey. Just the cardboard sono tubes or equivalent are not cheap - more than the concrete to fill them. The 36" diameter was expensive and we had to buy a 10' length and cut it.
  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: Concrete footing size
    Coach Dad wrote: »
    Rule of thumb is:
    The post hole should be at least 3 times the diameter of the post.
    1/3 plus 6 inches of the total length of the post should be in the ground.

    True for a single pole mount with large wind forces on an array concentrated onto a single pole. It's different if you are setting several posts in concrete as the foundation for a pole barn type structure (like my PV shed). For a multiple pole/post array structure I'm not sure how you would translate that rule of thumb.
    2 inch schedule 40 is very small... I wouldn't use anything less than 4 inch as a post..

    I agree - we used a 8 inch diameter schedule 40 steel pipe filled with concrete for my pole mount array. (ADD: meaning for a single post/pole configuration - not with multiple posts tied together)
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Concrete footing size

    cheaper and less 'clean' and a bit more fussy is to pour a larger level footing in the hole i/c rebar and then when partially set add a sono tube with rebar and complete pouring. requires a working space sized hole (use a backhoe) though. Those bigfoot's make things a bit quicker and 'one stop'.. if you have a Hoe it seems cheaper ...
     
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Concrete footing size
    Coach Dad wrote: »
    2 inch schedule 40 is very small... I wouldn't use anything less than 4 inch as a post..
    +1
    Although the 2" pipe might support the dead weight load just fine, it will not do much for bending forces (wind load) or for torque (twisting) forces.
    Strength and rigidity in those modes goes up as at least the square of the diameter.
    SMA SB 3000, old BP panels.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Concrete footing size
    mtdoc wrote: »
    we used a 8 inch diameter schedule 40 steel pipe filled with concrete for my pole mount array.

    I'm sorry to tell you this, but filling the pipe with concrete is a very bad idea. I thought about doing that for my pole mount array and checked with several engineers... all of them said not to do that.

    Putting concrete in a pole is great for a lally column which is meant to hold a great weight. However, your panels do not weigh much.... most of the force they apply to the pole is bending force and the pole needs to be able to flex. The result of putting concrete in your pipe is that it cannot flex and may snap off at the foundation after a few years. :cry:

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • tons001
    tons001 Solar Expert Posts: 71 ✭✭
    Re: Concrete footing size

    IronRidge doesn't even sell a ground mount for a 4" plus size pipe. The options are 2" or 3" pipe for a ground support array. Maybe I did not make myself clear that is NOT a single pole array but a multiple support low to the ground tiny (by comparison to some of yours) array. I appreciate all of the feedback but I must not have made myself clear when I asked my question. Sorry. I am certainly not using four 8" schedule 40 pipes in a ton of concrete each to mount five 22" x 48" solar panels a foot off the ground regardless of the angle. :blush:
  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: Concrete footing size
    vtmaps wrote: »
    I'm sorry to tell you this, but filling the pipe with concrete is a very bad idea. I thought about doing that for my pole mount array and checked with several engineers... all of them said not to do that.

    Putting concrete in a pole is great for a lally column which is meant to hold a great weight. However, your panels do not weigh much.... most of the force they apply to the pole is bending force and the pole needs to be able to flex. The result of putting concrete in your pipe is that it cannot flex and may snap off at the foundation after a few years. :cry:

    --vtMaps

    Actually I had heard and thought the same thing until I researched it and checked with a structural engineer. Was told that engineering studies confirmed the "preventing it from flexing, causing it to snap" myth - is well, a myth. I was told concrete does flex. There's a reason that they use concrete poles and pilings and concrete filled steel tubes in structural engineering - even in high earthquake prone areas

    Apparently the failure of a steel pipe with lateral forces is by the pipe buckling which does not happen as easily with concrete filling. Perhaps you decrease flexion some but you decrease the failure rate by preventing collapse of the cylinder.

    At least that is how it was explained to me. I had some engineering research papers that I was referred to - will see if I can locate them.

    ADD:

    A quick google search turned up THIS paper. I think I have some others at home - will check later.
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Concrete footing size
    mtdoc wrote: »
    ... lateral forces is by the pipe compressing which does not happen easily with concrete filling. Perhaps you decrease flexion some but you decrease the failure rate by preventing collapse of the cylinder.
    Just be sure to vibrate the concrete to avoid large air bubbles when you fill the pipe. A rigid pipe with an air pocket as a weak spot can increase the chances of collapse and will also concentrate the flexing and resulting long term stress in one place.
    SMA SB 3000, old BP panels.
  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: Concrete footing size
    inetdog wrote: »
    Just be sure to vibrate the concrete to avoid large air bubbles when you fill the pipe. A rigid pipe with an air pocket as a weak spot can increase the chances of collapse and will also concentrate the flexing and resulting long term stress in one place.

    Makes sense. To avoid air bubbles, after each small pour of concrete A 2x4 was used to plunge up and down and then the pipe was whacked repeatedly with wood blocks to vibrate it. Hopefully that did it....
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Concrete footing size
    tons001 wrote: »
    IronRidge doesn't even sell a ground mount for a 4" plus size pipe. The options are 2" or 3" pipe for a ground support array. Maybe I did not make myself clear that is NOT a single pole array but a multiple support low to the ground tiny (by comparison to some of yours) array. I appreciate all of the feedback but I must not have made myself clear when I asked my question. Sorry. I am certainly not using four 8" schedule 40 pipes in a ton of concrete each to mount five 22" x 48" solar panels a foot off the ground regardless of the angle. :blush:


    I agree. I think others are over thinking over engineering this. If it were me, I would use 6" (or maybe 8") sono tubes, three across the front, three across the back, well mixed, with a couple of #4 bars in it vertically, in a clean bored hole. I would also consider using 6" drain tile as a form instead of a sono tube, much cheaper, and you can strip it back to grade by slicing and unraveling it. The dead load on the fitting is nearly nil. (get below frost, and deep enough, say 3' min)

    The total up load, wind load is not to great vertically, I would be more worried horizontally. If you provide a bit of vent space between the panels to spill wind I would sleep just fine, unless I was likely to experince a lot of Hurrican force winds. My neighbor has three 220 keoceras with a simple unistrut frame in the ground, withq few large rocks on the lowest rung. It has survived 80 mph winds from all directions without issue.

    Tony
  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: Concrete footing size
    icarus wrote: »
    I agree. I think others are over thinking over engineering this.

    I agree as well. I tried to make the distinction in an earlier post but perhaps I wasn't clear. A 2 " pipe is way too small for a single pole mount but that's not what you're doing. Also the rule of thumb mentioned for a pole being burried 1/3+ of it's total length only applies to a single pole.
    The total up load, wind load is not to great vertically, I would be more worried horizontally. If you provide a bit of vent space between the panels to spill wind I would sleep just fine, unless I was likely to experince a lot of Hurrican force winds. My neighbor has three 220 keoceras with a simple unistrut frame in the ground, withq few large rocks on the lowest rung. It has survived 80 mph winds from all directions without issue.

    This depends a lot of the angle of mounting of the panels of course. The steeper the angle the more lateral (horizonatal] wind load. An array that is tilted very steeply makes a good sail. That's part of the reason I decided to to build a "PV shed" -with enclosed walls. We get very large wind gusts.
  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: Concrete footing size
    BB. wrote: »
    If your soil is that wet (I know, I had really wet back yard until I dug a drain system about 3-4' and installed a pump), you will have to research the augers... If the soil can be "liquified" (vibration from wind on cables, etc.), you may need to reevaluate your choice (larger/deeper anchors, pouring concrete for weight/mass to keep structure stable, etc.).

    Wet soil.. Yeah, I know about that:

    Before:

    Attachment not found.

    I learned that concrete cures very well underwater..:roll:

    After:

    Attachment not found.

    Yes - that shading alder in the foreground has since been cut down.
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Concrete footing size

    As a side note about concrete. Concrete doesn't "dry". It cures. It is a chemical reation that is a process of absorbing water. The more water, the slower the cure, the stronger the concrete. Every once in a while you will see a road ridge with freshly poured concrete with lawn sprinklers going day and night and wonder why. The fact is, the are deliberate curing the concrete slowly.

    Concrete cures just fine under water. In fact, you can simply pour dry mix into a wet hole (or into an underwater form) and it will cure just fine.

    Tony