Ground Mount Uplift Calculations

[Jalapeno1042]

I got partway started with a 24 volt ground mount system when I discovered (at my wife's urging) that I need a permit since I live in a township in AZ. Even though its on my property (~ 1 acre). Sparing you all a hilarious and ridiculous diatribe, here's where this is at. I have 6 panels ("modules") mounted in 2 "panels" sets, 4 in one and 2 larger ones in the other, all of which are mounted to my latitude of 32°. They are concreted in the ground to a depth of 12 inches by 14 gauge slotted angle iron at each corner. For the permit, among other things, I have to calculate wind uplift for each set of panels at a design wind speed of 90 miles per hour. (this is actually an old value since ASCE 7-05 has been replace by ASCE 7-10, which is 115 mph. but who's complaining?).

The formula for such calculation apparently is F = A (projection area of panels) x P (wind pressure) x Cd (drag coeff.) x Kz (exposure coeff) x Gh (gust response).

After several days of hunting, head shaking and rarely used language, I think I have figured it out for the smaller 4 panel array. But the answer I get seems awfully high: 395.6 lbs. Subtract the weight of the array and it becomes 332.2 lbs. So my anchor system of angle iron, concrete, and A307 1/2 inch bolts and nuts would have to hold down that force before being ripped out of the ground.

Do these numbers seem right to anyone? Does anyone else have to go through this? IS ANYBODY OUT THERE????

 

[OzSolar]

They are concreted in the ground to a depth of 12 inches

What's your frost depth there? If it's over 12" then you could have a bit of problem.

Do these numbers seem right to anyone?

They do. Is the permit going to require a PE to sign off?

 

[Jalapeno1042]

Hi Oz... thanks for the reply
1) HAHAHAHA..... good one. Tucson weather MAYBE gets down to 26-28 a couple of days a year. But then I didn't let you know that I'm in Tucson.
2) I don't know. I spoke to the person who is reviewing my permit application, and an inspector does have to come inspect, but otherwise permits are issued electronically. If I show calculations, a PE would know them. the concrete is one of the things they do inspect.

 

[Jalapeno1042]

What's your frost depth there? If it's over 12" then you could have a bit of problem.


They do. Is the permit going to require a PE to sign off?

I am also assuming that since the panels are mounted as a unit at 4 corners, that the 332.2 lbs they would have to hold on to would be divided by 4, so that each corner must only hold down to a force of 83.5 lbs...?

 

[OzSolar]

I am also assuming that since the panels are mounted as a unit at 4 corners, that the 332.2 lbs they would have to hold on to would be divided by 4, so that each corner must only hold down to a force of 83.5 lbs...?

I'd have to dig into the notes from the engineer on the last ground mount project we did but from memory the back legs will be responsible for somewhere around 2x's what the front legs are so your case it will be ~100 lbs for each of the back legs and ~50 for each of the front.

In many soils you can also add in a coefficient of friction from the surrounding undisturbed soil resisting the uplift. But in that sand of yours I suspect you can't.

Did you say how much concrete you have down there? I'm not a civil engineer, I just pretend to fight with them so they will go into overdrive to prove thier designs to me. I listen very closely and learn a lot.

I'd add some ~"36 duckbill anchors at 15 to 25 degrees angle in opposing directions on each corner. They would likely add 300lbs to each corner for less $100 and an hour of work.

 

[OffGridForGood]

I am also assuming that since the panels are mounted as a unit at 4 corners, that the 332.2 lbs they would have to hold on to would be divided by 4,

The people reviewing are not PE either, just show them the calculations you came up with, and the duck bills Oz suggested are capable of more than the 332 lbs each, provide a copy of the spec for the duck bills with your calculations, they will see you have pleanty of capacity and likely just sign off collect their fee and you never hear from them again.

 

[Jalapeno1042]

I just pretend to fight with them so they will go into overdrive to prove thier designs to me. I listen very closely and learn a lot.

Smart! Duck bills make good sense, as does the idea of the back legs taking more of the load.
Thank you, gentlemen!!!

 

[12VoltInstalls]

concreted in the ground to a depth of 12 inches by 14 gauge slotted angle iron at each corner

That’s insufficient - 14ga ‘might’ calculate out just fine, but on the grand scale of things I’d just use pretty healthy 1/4” wall 2x2 angle or square tubing. Those won’t flex and fail over time.

add some ~"36 duckbill anchors at 15 to 25 degrees angle in opposing directions on each corner. They would likely add 300lbs to each corner for less $100 and an hour of work.

good idea

I’d prefer to go deeper with concrete. For anchors like solar racks it isn’t the mass but the surface area (which your duckbills add)