Inadequate concrete footers for EG4 Brightmount

[hwy17]

I'm installing an EG4 Brightmount on concrete footers. The installation guide recommends 3' by 20" wide concrete footers. These mounts are 4 panels / 4 footers so 1:1 footer to panel.

I'm not going to be installing to that spec because I don't want to buy all that concrete and I think I can get away with less. Peak gust wind speed I have observed in 3 years of monitoring here is 35mph. These will be in an open field surrounded by fences so I can't imagine the array blowing into a house or something.

I am really inclined to go with 2 foot depth because then I can buy 4ft form tubes and cut them in half, but if I decide I have to do 3ft I will.

I'm thinking along the lines of:

8" x 2ft protruding 8" from the ground, 2x60lb bags concrete
10" x 2ft protruding 8", 3 bags
8" x 3ft protruding 12", 3 bags
10" x 3ft protruding 16", 4 bags

And would you add reinforcement? I thought of using threaded rods that go all the way down, but that's pricey. Probably easier to add 4x rebar and jiggle them in to get bubbles out as well. Then 4 galvanized bolts set into the top to mount the brightmount base to.

 

[Sam50]

My footers were rather large. I saved on a lot of bags of concrete mix by putting large rocks into my footers as I poured the concrete mix in. Of course you need to make sure you get the concrete mix between all rocks as you fill. I also park my panels flat during wind events to be on the safe side.

 

[OzSolar]

Depth is a function of your frostline. Frost line plus 12" is pretty good rule of thumb that I go by. Not sure where I got it though.

FWIW: I'm not sure I'd design a system around 35 mph wind speeds in any area. For reference the lowest wind speed allowed by code to be used in any place in the US is 85mph. I'd sure hate to have a solar array come flying through my windows in a wind event, or even worse my neighbors windows. You might try to tell me you're miles from anyone but that doesn't change that I'm trying to give reasonable advice.

Check out Duckbill anchors or similar products. They are impressive and in most cases easy to install. We use them often with great success and no failures. They will allow you to use almost no concrete.

 

[hwy17]

Filler is an interesting idea. I don't have many rocks but I have some poor quality too fine sand that I could maybe make my own concrete with. But then I'd need aggregate and that kind of inconvenience is how I usually end up back at the bags.

No frost line here. I'll check out the duck bill and similar.

I didn't mean that I would design for 35, just that since I've only seen 35 I feel comfortable not designing for 100+. 85 sounds like a good minimum.

 

[Daddy Tanuki]

rent or buy a vibrator... mix to standard and ad some large rocks in to take up the extra volume. if you are really worried take some chem bolts bolt them into the larger rocks and build your rebar off of that.

 

[HarryN]

Seems optimistic given that this area has had 90 mph winds in the last 25 years.

You can always just have a truck come in and pump the concrete over to the footer location instead of dealing with all of those bags. It is so much easier and cheaper.

Mixing bags is kind of a pain and it always seems like something happens to interrupt / mess things up while trying to do multiple batches to fill a form.

 

[Daddy Tanuki]

Seems optimistic given that this area has had 90 mph winds in the last 25 years.

You can always just have a truck come in and pump the concrete over to the footer location instead of dealing with all of those bags. It is so much easier and cheaper.

Mixing bags is kind of a pain and it always seems like something happens to interrupt / mess things up while trying to do multiple batches to fill a form.

Yep and the inconsistent mix will have at your backside come the first winter…

 

[Hedges]

I would prefer bolts or brackets hooked on the rebar, rather than just inserted separately.
I have a rebar cutter-bender so bent a length back & forth, and hung it from bracket on sonotube.

That was a different structure. My PV arrays are bolted to existing concrete and have additional footings simply 2" pipe driven a few feet into the ground.
I observed a gas pipe heavily corroded where it came out of the ground. After replacing it, I cut the old one in half, found it actually had thick sound material. But the outer layer puffed with rust. I think an auger hole drilled about 1' deep before driving post, then filled with concrete, would protect steel from the more active corrosion environment.

 

[Ampster]

I am not an Engineer but when fooling around on the IronRidge ground mount design site I noticed that more smaller posts required less concrete than fewer bigger posts. The forces to be reconciled are lateral and uplift. You can improve lateral strength by diagonal bracing. Weight and depth of concrete compensate for uplift.

 

[Hedges]

And a wider stance. "6-DOF", degree of freedom, translation in 3 directions and rotation in 3 axis. In the limit, a single pole mount requires a massive foundation and large pipe like 6" or larger. Four corners and diagonals, the frame can be 2", maybe smaller. (Spans loaded in the middle need to be thick enough to resist bending.)

 

[Ampster]

What I also noticed was the price difference in materials. Sometimes more smaller pieces were less expensive overall than fewer bigger pieces. It may depend on factors for wind in the area where structure is being built and local material prices.

 

[Hedges]

Production costs vs. volume.

At work, a custom frame of 40mm aluminum (standard) extrusions and machined angle brackets is being fabricated. Quote seems absurdly high to me. I think if they just designed the one or two unique brackets and got a quote for the 48 pieces or whatever needed, those would not come to the same $500 or $1000 per piece the total represents. We can easily chop extrusions with a band saw. One frame, vs. many identical brackets. A system, vs. simple parts.

 

[SilverbackMP]

Seems optimistic given that this area has had 90 mph winds in the last 25 years.

You can always just have a truck come in and pump the concrete over to the footer location instead of dealing with all of those bags. It is so much easier and cheaper.

Mixing bags is kind of a pain and it always seems like something happens to interrupt / mess things up while trying to do multiple batches to fill a form.

This. We/they used ten 85lb bags per hole for this. If I was doing it by myself, I would have called in a truck and and moved it a skid loader bucket at at a time (too far for a pump). They opted for the hard way. We had a decent sized skid loader on site (plus my tractor).

 

[HarryN]

Especially at Santa Cruz - the sun doesn't really come out until later in the day. The panel angles desired might end up being completely different than conventional. Or you could take advantage of that very late sunshine to generate power when everyone else's panels have stopped generating and power is the most expensive.

 

[Daddy Tanuki]

This. We/they used ten 85lb bags per hole for this. If I was doing it by myself, I would have called in a truck and and moved it a skid loader bucket at at a time (too far for a pump). They opted for the hard way. We had a decent sized skid loader on site (plus my tractor).

View attachment 151635

thats where the japanese cement mixer trucks would hve been handy, they have models form the us sized all the way down to itty bitty little two ton models that are AWD. can't expect a japanese worker to hand mix ha ha ha

 

[SilverbackMP]

thats where the japanese cement mixer trucks would hve been handy, they have models form the us sized all the way down to itty bitty little two ton models that are AWD. can't expect a japanese worker to hand mix ha ha ha

Or Korean.

Walking by this time now:



But my problem was that it was on a pond dam.



Weight was a consideration. And it rained a few times. Tracks or large flotation tires were better.

 

[SilverbackMP]

Or Korean.

Walking by this time now:

View attachment 151709

But my problem was that it was on a pond damn.

View attachment 151710

Weight was a consideration. And it rained a few times. Tracks or large flotation tires were better.

View attachment 151711

Christ almighty. Come on Xenforo! I’ll fix it later on a computer.

 

[Hedges]

thats where the japanese cement mixer trucks would hve been handy, they have models form the us sized all the way down to itty bitty little two ton models that are AWD. can't expect a japanese worker to hand mix ha ha ha

Georgia Buggy:

(Front loader as mentioned is another way.)

 

[Daddy Tanuki]

Georgia Buggy:

(Front loader as mentioned is another way.)

they have tracked versions like your bottom photo with a small mixer that holds about 1 yards i think its about twice as large as the powered mixer you see on construction sites, and it fills from a large mixer truck and trundles it out to the spot mixing the hole time. with two or three of them rented you can empty a large mixer pretty quickly. a place like @SilverbackMP has is exactly what they rent them for.

probably not much use in the states for them, but here you got roads so narrow that only a K car (mini car) can fit down them and if a pump truck cant reach then its either crane it in, or drive it in with the mini mixers depending upon what is required.

 

[Farm Boy]

EG4 Brightmount with out concrete.