Solar PV Pergola Footing / Wind uplift question

[rossTessien]

Hello,

I'm designing a solar pergola and intend to use 12 inch diameter by 3 to 4 foot deep concrete poured footings with stirrups to hold posts for structure. We have 100mph wind loading requirements. I have an estimated wind force of 4000 pounds. If I just calculate a torque for wind force and then for structure, each footing (4) would need to resist 1000 pound uplift. Each footing will weigh about 450 to 600 depending on depth. Is there a value I can use for uplift resistance in clay soil for the footings............simple round, auger drilled into the dirt and poured?

The pergola will be 36x26 feet with 3 rows of 4 piers per row inset 3ft, ie, piers are 30x20 ft to outside corners of layout.......4 along 30 foot dimension.

Thanks,
rt

 

[Ron]

Provide a sketch. I suspect your overturning calculations might be off a bit. Also, for overturning you will need a safety factor of 1.5 at least.

 

[rossTessien]

Hello Ron, thanks for the reply and questions. I'm new to forum and clumsy with CAD. My decades of engineering have been machine design so I'm a bit out of my area working to design this solar PV pergola. That said, I was a carpenter long ago so hopefully can figure it out.

I tried to upload a picture of a similar pergola but couldn't figure it out.

Here's an image of a very similar system to what I'm designing:

http://www.primalenergysystems.com/img/pergola-project.jpg

We will tilt closer to 20 degrees to shed dirt better, and the array size is perhaps double....that array is 6 x 3 or 18 panels, ours will be 11x3 or 33 panels. So basically, just place a second entire array right next to that one and you'd have what I'm designing.

I will have a friend who is structural engineer stamp the project before permits. But I want to get things close before handing them off to him.

That array is 4 rows of 3 columns each. Mine is at present, 3 rows of 2 colums per side with 2 sides. So, that's in total, 3 rows with 4 columns in each row. And I will have Large diagonal braces (6x6 posts and 6x6 diagonal braces, whereas that picture has little bracing to speak of.

I think I'm going to be ok on bracing for winds. But what I really don't know about is the foundation / footings. They used bored and poured footings like I intend. I could change diameter if I needed more weight.

Does this help? I haven't figured out how to upload a drawing....and at any rate it won't be until next week that I have one. I was hoping to settle this issue first and then draw the solution.

rt

 

[allgoodnamestaken]

You're on the wrong subforum for this question, try Geotechnical Engineers -> Foundation Design.

I don't think you should rely on skin friction for uplift resistance on such a shallow foundation, but I am not a geotechnical engineer by profession. I remember in Foundation Design: Principles and Practices by Coduto, when discussion uplift resistance of friction piles, coduto notes that because the concrete "stretches" and the section gets thinner, he conservatively uses 0.75 factor on regular side skin friction due to this deformation. In compression this isn't an issue.

That being said there are soil properties involved for determining skin friction and it doesn't sound like you know what yours are. And having a structural engineer friend stamp a design he didn't make and that you don't know enough about to design properly isn't a good idea for either of you.