Does the 0.5% minimum seismic reinforcement for deep foundation apply to solar panel pole footings?

[structure567]

The solar panel is governed by wind force and I was wondering if the minimum reinforcement also apply to foundations governed by wind force. The pole footing is 5'6" deep and 30" wide. Are there any codes that specify pole footing reinforcement?

 

[r13]

You need to calculate the required strength of the foundation. I suspect you will need more reinforcement than the code specified minimum, which is mainly serving for shrinkage and crack control.

 

[structure567]

You need to calculate the required strength of the foundation. I suspect you will need more reinforcement than the code specified minimum, which is mainly serving for shrinkage and crack control.

Thank you for our reply. What if the rebar needed for the required strength of the foundation is less than what it is needed for the specified minimum. Do we need to strictly follow the 0.005 minimum? I read in another forum that pole footings with small loads can be just plain concrete as long as the steel pole is embedded into the ground.

https://www.eng-tips.com/viewthread.cfm?qid=368597

 

[RPMG]

The 1/2% is for columns for creep, not seismic, and I would adhere to it because I generally consider it a pedestal, unless it checked out as Plain Concrete per ACI. The IBC contains some ASD Code designs in the soils and foundations chapter.

 

[r13]

Unless there is a clear reference (code/standard/research) that tells me "NO, you don't...", usually I'll go conservative on foundation, for it is something we couldn't see and detect when problem starts. Why trade safety of your equipment for a little saving here.

 

[azcats]

Is your column embedded the entire depth of the foundation? If so, would that column not be your structural element? And one could argue that the concrete isn't really acting as structural concrete but simply soil improvement to transfer fairly minor (WRT concrete strength) compressive loads.

Would this argument hold up in the face of a pesky plan checker? Not likely.

 

[structure567]

[s your column embedded the entire depth of the foundation? If so, would that column not be your structural element? /quote] Yes, I am planning on having the column embedded the entire depth. On top of that I plan on placing additional reinforcements to withstand the moment. Therefore, I'm wondering if I need to satisfy the specified minimum just for reinforcements.

 

[Lomarandil]

For deep foundation elements with axial compression, there often comes a point where the bending/shear loads have been sufficiently dissipated through lateral bearing on the surrounding soils that reinforcement can be curtailed, and plain concrete can deliver the remaining compression to the bearing strata. Check that it works with plain concrete provisions, and save your contractor the trouble of a long cage.

Based on your geometry (not that deep) and application (assuming mostly lateral load, either not much compression or possibly tension), I don't think you'll get there in most soils. Especially if like most solar jobs, you're trying to come up with a design that's applicable across a wide range of soils, I would probably include 0.5% reinforcement for the full length.

AZcats may be on to something counting the embedded column as your structural element/reinforcing if it can handle the below-grade moment. (Don't forget that this can be larger than the grade-line moment).



----
just call me Lo.