"Shallow" Concrete Pier design with Lateral Loading

[ndalle03]

I am designing an 18" diameter concrete pier (augured)to support solar modules. Lateral loads are in the range of 1.5-2kips. Typically the concrete piers are embedded 4-7ft; is this considered a long pier or short pier? I am struggling with how to determine the lateral resistance of the soil, given the lateral bearing pressure (ksf/ft).

If a short Pier, I assume it would be triangular loading: Therefore if given lateral pressure of 0.2 (ksf/ft) for 18" dia pier, 6ft deep, the lateral resistance would be -->

(0.2 ksf/ft)*(1.5ft)(1ft)(6ft)= 1.8 kips (I would guess @ 4ft below grade: 2/3 of 6ft embedment)

If this is considered a "long" pier, I am aware that there is a "pivot" point at which the loading reverses. I'm not sure how to calculate the lateral resistance, or how to calculate the depth to the pivot point. I read an article that suggested .67 to .75 times the length of the pier to the "pivot point". Any references or suggestions would be greatly appreciated on an approach to designing a 4-7ft deep, 18in dia. concrete pier to resist lateral loads.

 

[GeoPaveTraffic]

You really need to perform this analysis using a lateral pile analysis such as LPile.

I'm not sure where you got the 0.2ksf/ft or what it signifies. Is this a passive pressure?

Soil is non-linear the amount of resistance changes with the amount of deflection and the amount of deflection is somewhat dependent on the stiffness of the shaft (pier). There are some simplified methods, but they depend on the type of soil.

I suggest you either get with your project geotechnical engineer or someelse in your firm to discuss the analysis.

Mike Lambert

 

[ndalle03]

Mike,

Thanks for the response. The 0.2 ksf/ft is a "Lateral Bearing Pressure" from IBC 2012 Table 1806.2. It's just a conservative value based off the soil type, so essentially it is the same as passive pressure. I wasn't sure whether a 4-7ft, 18in dia. concrete pier would be considered "Rigid". Therefore, I didn't know which approach to go about for calculating the soil resistance. I have access to a program called All Pile which is similar, which I do use from time to time. With that, I input values and I get and design.. I'm relatively new in the engineering field so I want to know what and how the programs are analyzing the structure. Im not really a soils guy, more so structural. I hoped to get a better understanding from this forum. Thanks again,

-Nick

 

[GeoPaveTraffic]

I don't know the basis of the lateral bearing pressure from the IBC, but I would be very careful until you figure out how the code expects that value to be used, 200 psf is quite a lot of soil resistance particularly at shallow depths.

Mike Lambert

 

[FixedEarth]

In general, you don't want to design from guesses, articles and online forum postings. Suggestions:

1- You need to get a deep foundation text - Azizi, Bowles, Coduto and others all cover the design of drilled piers in their respective books. Do you have access to a foundation design book?

2- Software is a great tool once you learn how to work it by hand using textbooks. How else would you verify your computer results?

3- You never mentioned bending moment, uplift & downward loads? Sometimes on solar projects (depends on pivoting method of the axis beam), there is either a one way or a two way moment.

You should have a knowledgable boss that will train you and review your work. If that person was laid off or is not available- have the company owner hire an outside experienced engineer to train you.

 

[Mccoy]

Typically the concrete piers are embedded 4-7ft; is this considered a long pier or short pier?

That depends upon soil type and soil and concrete properties and exact lenght; by some quick calcs I carried out with average values, your piles could be considered short or rigid in clays, long or flexible in sands. In clays there is more uncertainty.

But again, a few more details should be known including soil type, water table, Su if clay, exact lenght...

And this only for the first step of the analysis (determination of rigid or flexible behavior). I agree with the suggestions on using a good, detailed handbook on deep foundations.

 

[RFreund]

Below is a good reference for lateral resistance to shallow foundations.

https://www.dropbox.com/s/7svb9fscmt1q9z1/ASAE Pole Embedment EP486_1.pdf

EIT

http://www.HowToEngineer.com

 

[Splitrings]

ASABE EP486 is another good reference. It discusses where the point of inflection is and how to calculate it.