Ground Mount Span

[callsign4223]

I'm attempting to setup an array as a back porch "cover". Basically, I'm looking at it as a ground mount 10ft in the air. The issue I'm running into is the porch is 25ft across. I was going to use the Iron Ridge ground mount system. It doesn't look like sched 40 pipe is strong enough to span this distance. I'm wondering is maybe steel tubing would be a better option and also wondering if steel tubing would work in the Iron Ridge system.

The one thing I REALLY don't want to do is have to put a support down in the middle of my porch. If that's the only solution this is likely a non-starter with the wife.

 

[Hedges]

Pipe isn't particularly strong against bending. My Unirac ULA mounts use 2" pipe horizontals, but only about 6 or 8' spans.
The load beam can carry per unit length drops to 1/4 as much when length is 2x.
The force it is subject to depends on wind speed, but I don't have figures for strength of pipe. You may be able to find them somewhere.
Round pipe easily buckles and collapses. For 25' you might need at least 1 additional upright, 3 total. They could be 7' apart, with the ends extending 2' beyond.
You may also need a diagonal brace to resist sideways motion, unless array attachment to house and and rigidity of array prevents that.

Pipe is strong under pure compression and tension, like the uprights and diagonals of my mounts. I-beams are optimized for bending loads. Trusses are strong.
You might be able to make trusses as horizontals, with 2" pipe top and bottom and smaller ones as zig-zag pieces. Or, get a pre-made truss like an antenna tower.

https://www.expressionevents.co.uk/2-5m-triangle-truss/

 

[OffGridInTheCity]

Ironridge has a PV mount online design tool that I used for my backyard ground mount. Built is using 2" x 10ft galvanized pipe as the framework and 10ft Unistrut across holding the panels. I went with nearly 9ft wide spans (even thought the design tool shows 6' 9" max) between upright and its held up well for over 2 years thru stormy/windy (80mph) weather.

Here's the entry link to the design tool - https://base.ironridge.com/gb/projects/new You have to create a login and click around - but you can put the details and it will show you max lengths etc. Its helped me get a handle on 'reality' as far as overall spans based on the size of pipe (2" or 3" etc)

Here's an example of the 2nd page of the design from my project:



Which lead to this....


And ultimately to this (20 panels)....

 

[Hedges]

No diagonals left to right? I have to deal with earthquake as well. Maybe wind can give some sideways push.
No diagonal from front top to bottom rear? Wind from the front will put a lot of torsion on the uprights, try to bend them at the bottom.
Maybe your location and fence/bushes as windbreaks make it no problem.
The reuse of occupied space, and clear unshaded exposure, look good.

OP's 25' total span on two supports vs. your 9' between supports will have about 2x the clear span, so could only take 1/4 the wind (or weight) loading. And if threaded joints rather than one piece, could be much weaker there.

My mounts grew from 10' lengths of conduit to 20'. When I rearrange them I plan to use 21' 2" pipe supported 3 places (either uprights or diagonals.)

I'm also considering an array as an awning extending out from eaves of house. There is a porch railing, from which I'll have multiple uprights. Location is north side of house, so awning will slope up not down.

 

[OffGridInTheCity]

No diagonals needed for mine - I'm not earthquake or hurricane/tornado (its a mountain valley). I worried about it - but once it was up.. its obvious that diagonals not needed. I used 20 x 60lb bags of concrete in 30"'ish wide x 40" deep hole on each vertical pipe so it wouldn't pull out of the ground due to wind up to 150mph? - but again, that was overkill as our wind never gets above 80mph and I don't have sandy (worst class) soil . Wanted to make sure it would pass city inspection without doing soil samples in case a neighbor complained.

The design tool offers a 'diagonal' checkbox as part of the planning which increases some of the dimensions.

 

[callsign4223]

I think I've decided to go with rectangular tubing instead. I found a calculator website that let me put in tubing dimensions and loading and get safety factors. Per this calculator, 2x4 tubing with a wall thickness of .120 will put me well over spec for a 25 foot span. Tubing Calculator

Even though I'm in an area with high wind speeds I plan on keeping the angle of the array pretty shallow. I know it's not ideal for PV efficiency, but in this case aesthetics are coming in to play. It's already going to be tilted in an angle opposite to the roof so I need to keep it reasonable. It's also mostly wind shaded by the roof and neighbors as it's in the back yard.

 

[OffGridInTheCity]

I understand the athletics - I went 15deg (instead of 25-40deg for better winter power) because of the neighbors... it would have raised the array high enough to 'be in their face' and the extra 20%? power would not have been worth the risk

A 25foot span seems pretty wide to me... if you happen to make it back to this (or other) thread and post results/pic I'd certainly be interested. Good luck on your build

 

[callsign4223]

Here is an aerial of the back porch. The red outline is my hot tub. The green is the door and back windows. The span of over the concrete is 25 feet. I could bring the hot tub about 3 feet over right next to the windows and put the post down onto the porch there, but that doesn't really make a huge difference in the calculations. I can't bring the other post any further in without obstructing the door.

 

[OffGridInTheCity]

This is 25feet. The 2" galvanized (sched 40 pipe) would not be strong enough for sure.
.

I'm in no way a structural expert, and don't mean to come off as negative/critical - it's just that 25ft is flagging the back of my mind. Even the main beams that support our deck/house are not anywhere near that long of a span. I'd recommend consulting someone... a structural engineer for example. Might only be $300-$500 dollars for a plan that would pass inspection (like for when you sell your house in the future).

Can you do diagonal bracing...? to reduce the length of that middle span?

 

[callsign4223]

Honestly, I hadn't thought of diagonal bracing as a mitigation technique. I'm not sure how much it would interfere, but it would be worth looking into.

I realize that 25 feet is really a rather absurd length for an open span. This project is in it's very beginning design phases and this is one of the first hurdles to come up. I'm also seriously considering trying to find a way to mount it to the edge of the house instead, but that comes with a whole other set of problems like the fact that I have water being channeled towards my roof and that section of roof is very shallow pitch.

 

[OffGridInTheCity]

I feel you're pain. I wish I could span 20ft !!!!

*Excuse the mess! its Cargo Trailer -> Camper construction mess.

 

[Q-Dog]

25 foot span? You may want to consider an Open Web Joist or maybe Steel Bar Joist.

 

[Hedges]

I think I've decided to go with rectangular tubing instead. I found a calculator website that let me put in tubing dimensions and loading and get safety factors. Per this calculator, 2x4 tubing with a wall thickness of .120 will put me well over spec for a 25 foot span. Tubing Calculator

The dimensions don't feel sufficient to me, although I'm not an ME.
The page you linked had 2000 lbs. for load. Was that your static weight of panels?
Find a wind load calculator, may be a much larger number.

I was envisioning a truss with at least two, 2" pipes about 12" apart.

 

[callsign4223]

The panels will be about 1300 pounds in total. Split between two supporting beams 2000lbs seemed sufficient.

 

[Hedges]

It should hold the weight. But for high winds I think it needs to be stronger.
I was looking for an on-line calculator but haven't found anything simple.

The following link has a table giving pressure vs. windspeed. I clicked on the highlighted words so it shows ft/min and psi.
60 mph is a mile a minute, 5280 ft/minute. That gave about 0.06 psi, 8.6 lbs per square foot.
At 90 mph, the force is shown as double.

That is static force. It can start to flap in the wind, if motion is undamped.
Remember the Tacoma Narrows Bridge!

 

[MurphyGuy]

I used 3 inch Sch40 pipes on 9 foot centers.

Couple years ago, we had a huge wind storm come through and I measured gusts to 75 mph standing next to the array.

My neighbor lost shingles on his roof, trees down everywhere, lawn chairs blown 30 yards across the lawn and into the pond, and it took them a week to get grid power working again.

The math works out to the wind exerting a bit over 6143 lbs of force on the array.. (Winds were coming from the northwest so had pretty much a perfect loading profile) it wasn't even vibrating.. With my hand on the array, I detected no movement, no vibrations, no stresses at all. In fact, judging from what I was seeing during that fun little experiment, I'd estimate we weren't even close to any failure point. Probably could have gone up over 100 to 125 before it was a concern.

And the more "stuff" you park under the array, the less the array is affected by the wind.

 

[OffGridInTheCity]

I used 3 inch Sch40 pipes on 9 foot centers.

Couple years ago, we had a huge wind storm come through and I measured gusts to 75 mph standing next to the array.

My neighbor lost shingles on his roof, trees down everywhere, lawn chairs blown 30 yards across the lawn and into the pond, and it took them a week to get grid power working again.

The math works out to the wind exerting a bit over 6143 lbs of force on the array.. (Winds were coming from the northwest so had pretty much a perfect loading profile) it wasn't even vibrating.. With my hand on the array, I detected no movement, no vibrations, no stresses at all. In fact, judging from what I was seeing during that fun little experiment, I'd estimate we weren't even close to any failure point. Probably could have gone up over 100 to 125 before it was a concern.

And the more "stuff" you park under the array, the less the array is affected by the wind.

Nice work! I used 2" pipe (pics a few posts above) but buried the concrete footers 6-12" under the ground because I read that freeze/thaw might crack the concrete over time. Did this come up in your design decisions? do you get freezing? - maybe what I read doesn't apply

 

[MurphyGuy]

Nice work! I used 2" pipe (pics a few posts above) but buried the concrete footers 6-12" under the ground because I read that freeze/thaw might crack the concrete over time. Did this come up in your design decisions? do you get freezing? - maybe what I read doesn't apply

I'm in Michigan where our frost line goes 42 inches down.

Our concrete pillars go 6 feet below grade and the 4 tallest at the bottom of the slope are reinforced with rebar cages. We used 5000 psi concrete and made sure it was air-free with vibrators.

 

[Hedges]

3" horizontals as well as verticals, that is strong. Mine are both 2"
The diagonals you have contribute greatly to stiffness, reduce the tendency to allow bouncing back and forth.
The left-right diagonals you've used mostly to support the short threaded-on extensions.
Optimum for diagonals would be forming triangles, from base of one upright to top of another. More important for an array with skinnier pipes (or like mine, bolted to concrete rather than set in it.)

6000 lbs over 36 panels, that's 160 lbs/panel or 4x their weight. So design should be at least 5x as strong as needed for dead weight. Force would be in a different direction, which could matter for OP's rectangular beam.

 

[MurphyGuy]

3" horizontals as well as verticals, that is strong. Mine are both 2"
The diagonals you have contribute greatly to stiffness, reduce the tendency to allow bouncing back and forth.
The left-right diagonals you've used mostly to support the short threaded-on extensions.
Optimum for diagonals would be forming triangles, from base of one upright to top of another. More important for an array with skinnier pipes (or like mine, bolted to concrete rather than set in it.)

6000 lbs over 36 panels, that's 160 lbs/panel or 4x their weight. So design should be at least 5x as strong as needed for dead weight. Force would be in a different direction, which could matter for OP's rectangular beam.

6000 lbs is the most I've seen so far with a 75 mph gust that lasted about 5 to 7 seconds..

Diagonals would be better, but would have seriously messed up my storage area under the array. I would have done it regardless if it was needed, but it isn't.

My brick house will blow away before the array does. The array is also somewhat shielded by the slope its on and lots of trees up wind.

What's really impressive is to stand at the far side where the array is tallest.. the wind is funneled to the end. Even with a 5 mph breeze, there's enough air current at that side to install a wind turbine and probably generate significant power from it.