Unistrut maximum span distance

RazerMackham

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Sep 3, 2021
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I am in the beginning stages of mounting my panels (20x REC N-Peak 310w), and I figured it would be a good idea to hop on here and ask some experts if I’m on the right track.
My wife and I recently moved into an off grid home, previous owners used a diesel generator for power, but I’m working on putting in solar to get away from having to run a generator all the time.
I stumbled across a big stack of 3” galvanized pipe, and it seems like the perfect material to use to support the solar array. We are located in Alaska, 62*N, and I’ve worked out that I’m going to mount my array at 77 degrees to minimize use of the generator in the winter.
My plan is to have one long row of panels mounted to galvanized unistrut, bolted onto poles that are set in the ground at a slight angle. Right now I’m looking at doing a single row of 10 poles, spaced 80” apart, so each pole would basically be supporting two panels in portrait. My question is, is that too big of a span for the unistrut? It seemed pretty solid when I made that span on the ground and walked on it. I have enough poles to space them every 40”, but it would be nice to save them for a second array that is more optimized for the summer months. I appreciate any input/guidance/criticism71900820-B595-4334-813C-94555CB4E100.jpeg
 

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stienman

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Jan 6, 2021
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327
Look at the loading tables on the unistrut site:


An 80" span has a maximum uniform load rating of 480LB.

The maximum takes into account wind loading and snow loading as well. While snow will mostly fall off, ice won't, so you'll need to figure out the maximum thickness of ice that could accumulate, then the weight of it across the two panels. Then, on top of that, add the wind loading using a calculator, or P = 0.00256 * V^2, which will give you pounds per square foot if the velocity, V, is miles per hour.

You'll enter the maximum velocity of wind for your area, the surface area of the two panels, and the angle of the panels. It'll produce newtons, which you can convert to pounds.

The weight of the panels + the maximum snow/ice load + the wind loading should be under 480LB for an 80" unistrut span.

Given the panels probably weigh about 25% of that, a half inch layer of ice will probably weigh more, leaving perhaps 240LB for wind loading.

A 60MPH wind gust is going to apply 9lb per square foot. Over a 30 square foot area (15 square feet per panel) it's 270LB. This will be reduced since the angle results in a smaller surface area (cosine, yada, yada, yada) but you're close enough to vertical that it won't be so very different.

Of course, you'll be using at least two struts per span, which means you should have a maximum load of 960LB, and that should give plenty of margin for the possible 500+lb load they would experience during a windy day after a little ice accumulation.

If you get stronger wind gusts, or thicker ice then you might want to add a third unistrut, or reduce the span length.
 

stienman

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Be aware that I'm not a mechanical or materials engineer, this is not professional engineering advice, and you should do the calculations since my assumed numbers will vary from your actual conditions.
 

Alphacarina

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Feb 22, 2021
Messages
214
I think you're probably OK for the space between the vertical pipes, but the one panel on the Unistrut outside that first and last pipe should probably have some additional support. A strong wind is very probably going to be moving that single panel around, flexing the Unistrut where you have it bolted to the 3" pipe. I predict you'll eventually break the Unitsrut at that flex point

I would redesign the rack so there is a pipe at each end of the rack and no panels outside those end pipes

Don
 

Mcgivor

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Jul 24, 2021
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The thing that stands out to me as being potential weakness the lack of triangulation , a simple ladder design without cross bracing would transfer load into the panel frames making them a structural member, which they are not designed to be. There also needs to be a bracing pole at around 45° at the rear from close to the top of the poles, with concrete footings, this will relieve stress at the vertical pole fulcrum point.

The space frame method results in an extremely ridged structure which prevents oscillating in winds, the longer the structure the more prone it becomes. This leads to another point, being in Alaska there may be frost heave which can create movement, perhaps it would be better to divide the array into smaller sections to reduce the possibility of flex over 80 feet.

Just a few observations and thoughts.
 

RazerMackham

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Sep 3, 2021
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Hey I just wanted to say thanks to everybody who responded to this. I did some math and adjusted the layout slightly after going over the unistrut engineering page. Recently got the array fully up and running, right now I’ve got a row of 11 poles, each is 4’ in the ground, filled in with concrete. It feels secure, but I will be adding a rear support for every pole, at about a 45 degree angle for extra support and to prevent oscillations.
 

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Maast

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Looks good! I would also add rear 45 degree bracing. The bending moment at the base of your poles in a wind gust will be a lot more than you think and pipe bends easier than you think.
 

Lynoise

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Mar 27, 2020
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Bringing this back to the top.
I am new here and am building a very similar ground mount. How is this holding up?
How did you mount the panels to the unistrut? is there a standard unistrut fitting that can be used? I was thinking of using a fender washer with a bolt through down to the unistrut between each set of panels.
Thanks for any info.
 

timselectric

If I can do it, you can do it.
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Feb 5, 2022
Messages
4,923
Bringing this back to the top.
I am new here and am building a very similar ground mount. How is this holding up?
How did you mount the panels to the unistrut? is there a standard unistrut fitting that can be used? I was thinking of using a fender washer with a bolt through down to the unistrut between each set of panels.
Thanks for any info.
Unistrut Clamping Nut 20 Threads 1/4 " https://www.amazon.com/dp/B0081HJ1FO/ref=cm_sw_r_cp_apan_i_KE1AECN88G8BA9RQJS5C
 
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