Concrete Pier Design Calcs 1

[Samsclub]

I need a go-by example for designing a concrete pier in cohesionless soil. for the uplift and downward load carrying capacity we have to multiply the surface area with the skin friction, correct? I also have moment induced on the pier from the light pole on top. how do I check my pier for it?
Any calcs done by you or any reference book/article will be helpful. please pass it on. Thanks in advance.

 

[BridgeSmith]

Yes, for vertical loads, it would be the skin friction value multiplied by the surface area of the sides. For the moment from the pole, it's more complicated, and will very likely control the sizing of the pier (drilled shaft?). For a pole foundation, axial is usually not an issue. I'm sure there are some conservative 'rule of thumb' estimations readily available, but for an actual design, I've only seen it done using P-y analysis software such as Allpile (COM624P) or Lpile. Could maybe treat it as a soldier pile.

The best approach likely depends on how many pole foundations you need. If it's one or two, I'm sure there's guys here that can and will point you to some simple, conservative methods. If you have 20 or so to do, I'd look into getting a program like one of those I mentioned above.

 

[Teguci]

I've always started with CRSI and the Geotech. Give the Geotech the minimum and maximum axial loading along with the expected moment and ask them to give you the soil interaction diagrams (moment/shear/deflection). If the Geotech gives you a blank look that will be a good indication to get a second opinion on the soils. If they don't want to do the analysis that's fine, but make sure to involve them in deciding soil spring values for your model. You can use L-pile or SAP or any kind of beam analysis software that can handle spring reactions. Once you're OK with the deflection/rotation at the top - then use the moment and shear to design the drilled pier.

 

[bhiggins]

Here's a spreadsheet by Alex Tomanovich. This spreadsheet uses accepted but older empirical methods for designing a pier with moment and assumes a short and relatively rigid pier. You will need the angle of internal friction (phi) to do the calculation.

https://www.cesdb.com/polefdn.html

If you are looking for more accurate results then you need to do a P-y analysis as discussed.

As for gravity loads, for a short pier skin friction cannot be considered. The gravity loads will be resisted by end bearing. Skin friction is typically neglected for the first 5 feet of pier length.