Strut alternatives

[Bluedog225]

I’ll admit I haven’t been paying attention to the prices of steel lately. But I hear they’re through the roof.

35 bucks a 10 foot stick of unistrut seems crazy.

Normally I buy my stuff off craigslist, but everything seems to have dried up.

I need some horizontal stuff to mount my panels. I don’t really care whether it’s strut or solar mounting rail, or heck, I might even go with treated wood.

But, I wanted to check with you guys to see if you had some cheaper alternatives that were metal.

Here is what I have so far.

Many thanks.

 

[ricardocello]

35 bucks a 10 foot stick of unistrut seems crazy.

And that price has come down over the last year. It was like $42 when I built mine.
Definitely buy 10 or more for the discount.
Really discouraging: 2" Rigid conduit 10' still $83.

 

[42OhmsPA]

Check your local electrical supply stores before purchasing at HD, especially if you have a friend with an account at the supply store...

 

[Lt.Dan]

I have found that home depots website does NOT have the same price as in the store! I had to buy 26x 10's of the superstrut, and they were something like $30 on the website, and $14 in store. It was crazy and I was pretty upset.

I work at a metal shop and we order millions of pounds worth of steel a year, and even getting P100 unistrut from them was still more expensive than Home Depot, but I was able to get them in 20' lengths from my steel supplier.

Electrical supply places in my area were all the most expensive, but again, they were able to supply 20' sticks also.

 

[42OhmsPA]

I didn't price 20' sticks, but wish I did during the install.
The 10' sticks were ~$15 cheaper than HD from one of my local supply stores (using a friend's account and paying cash)

 

[Bluedog225]

I was looking around for unistrut alternatives. The strut I was considering is 1 5/8 inches 12 gauge at about $3 per foot.

This galvanized square tubing is about $2 per foot.

The spans on my solar array are 40”. Corresponding roughly to the panel width (depends on the particular panel).

I’ll need to get a stick in hand to know for sure but I think it will work. It will mean I need to use washers and bolts instead of fancy (and expensive) panel hold-downs. But I’m ok with that.

What do you guys think? Or just bite the bullet and get strut.

 

[ricardocello]

I am not a mechanical engineer, but I know how to use internet calculators and calculate beam loading.
Let's compare the relative strength of Superstrut/Unistut to the 14 gauge galvanized square tubing 1.5 x 1.5 from the above.

14 gauge means 0.083 wall thickness (pretty weeny).
Using the calculator here: https://www.engineersedge.com/calculators/section_square_case_4.htm
a=1.5, b=1.334 (because of 2x wall thickness)

It says that the area moment is 0.158 in^4, which means it is only 85% as strong as the unistrut listed below.
Since you are already comfortable building with Unitstrut, this should give you a feel for the difference in strength.

Superstrut/Unistrut (from data sheet)
1⅝” x 1⅝” gold galvanized channel
Slots are 1⅛” x 9/16”, spaced 2” on center
Area Moment Ixx = 0.185 in^4
Steel Young Modulus 29 Mpsi

I noticed you've used the back to back unistrut in your photo (very nice). I think the Area Moment would double.

You can use online beam loading calculators to do things like compute the maximum deflection, calculate max wind loading, etc.

 

[ricardocello]

It will mean I need to use washers and bolts instead of fancy (and expensive) panel hold-downs. But I’m ok with that.

I went that route as well, using 1/4" and 5/16" hardware to mount the panels to the superstrut. No clamps!

I did put special tape to isolate the aluminum frame from the galvanized painted superstrut, and nylon washers.
Had to use the holes on the panel that lined up with the strut holes. It all worked out, but there was lots of preparation needed.

 

[ScrotusGobbleBottom]

View attachment 202454

Is this the base design? Is it going to be just like that with a single u clamp on a 2 inch pole?

How is the wind not going to move that? Not trying to be cynical. I have a very similar setup but with 2 masts. I am just wondering what you think of doing it the way you have.

 

[DPC]

I remember when we used to build racks for electrical equipment from slotted steel angle instead of Strut.
I don't know if it offers any advantages today but its still out there and available at hardware stores too

https://unistrut.biz/content/Resources/General/Prime_Angle.pdf

 

[Bluedog225]

Thanks Ricardo!

I bought a copy of engineering for dummies so I could calculate things like area of moment and young modulus and all that engineering stuff. Really so I could calculate deflection on steel beams.

The introductory page of the book said you have to have a good background and two more engineering topics I know nothing about. . I still haven’t tracked that down.

I haven’t quite decided how to firm this thing up. The horizontal members should help. I was thinking if it all comes together, I will have somebody come in and weld the bolts to the pipe. All except for the two bolts that could let me tilt the whole array. And if it doesn’t work out, I’ll just weld those as well.

It’s sort of a work in progress on the theory that I can build anything from steel pipe and strut. We will see.

 

[Marobel]

Honestly, at this point, you’re gonna mount a couple of thousand dollars worth of hardware on top of the struts. If you amortize this over the amount of time that these are going to last, factoring in all of the costs, you’ve put in so far, it’s probably worthwhile just sticking with unistrut and biting a little bit of the bullet on this.

 

[ricardocello]

I haven’t quite decided how to firm this thing up. The horizontal members should help.

See this thread
Post in thread 'Mounting panels with unistrut'
https://diysolarforum.com/threads/mounting-panels-with-unistrut.10109/post-891077

Note that @Hedges convinced me I needed a 45 to give the two vertical poles some lateral support, especially important when panels are at a 45 tilt to resist the wind. I ran an analysis and determined that at 100+ mph winds the 2” vertical poles would snap.

Yours look like 3” from the photo, which will certainly help

 

[DPC]

100mph winds ?
seriously thats a thing?

how do you keep your shingles on or just walk for that matter...

how often does that happen?

 

[ricardocello]

100mph winds ?
seriously thats a thing?

how do you keep your shingles on or just walk for that matter...

how often does that happen?

We had a Derecho in 2012 that flatted signs and trees and powerlines everywhere.

The 108 mph wind requirement is code as far as I know. I try to design to code, sometimes I even succeed.
Coastal areas have even higher numbers.

 

[Bluedog225]

Thanks. I’ll check out that thread.

They are actually 4 inch scheduled 40 rigid conduit for the vertical pipes and 3 inch horizontal. Additionally, the panels back up on many acres of woods. I’m anticipating wind load will be relatively light. And I’m in the middle of acreage, so if it leans over, or breaks, it’ll be OK. Those 4 inch pipes are 10 feet long with 5 feet driven into the ground. Took a big machine. A moderate tornado zone. If that happens all bets are off.

Ps-I was in 150+ mph wind once. Hurricane Celia. The house started coming apart. Fun times.

Edits-tipos

 

[ricardocello]

They are actually 4 inch scheduled 40 rigid conduit for the vertical pipes and 3 inch horizontal.

Gut feel: You won’t have any problems!
My issue was using 2”.

 

[ricardocello]

Those 4 inch pipes are 10 feet long with 5 feet driven into the ground.

No, you won’t have any problems!

 

[sumpower]

100mph winds ?
seriously thats a thing?

how do you keep your shingles on or just walk for that matter...

how often does that happen?

The rating is similiar to that of a 100 year flood rating, the highest level of water recorded in the last 100 years during a flood.

In my area we see the 90 mph winds once every 100 years.
We have had several occurances of wind gusts in the last 8 years that where recorded at more than 80 mph, one gust made 86 mph.
So installs are required to meet the 90 mph wind loading as well as a 70 pound snow load in the area I live.

 

[sumpower]

If you use schedule 40 rigid pipe, its tensile strength is around 35,000psi. Don't use it for any installs because it bends easier in high winds vs a product called Gatorshield. Gatorshield has a 50,000 psi tensile rating. Never had an install fail with it. Has a better corrsion resistance than Sch 40 also.