Help with a Drilled Pier Foundation for a Monopole 6

[calpolyholley]

Are there any suggestions for a design approach of a drilled pier foundation for a new monopole in Northern Califonia? The foundation will have a high overturning moment (Mo = 406 kp-ft) and very little axial load (P = 6.3 kp). The 2001 CBC Section 1806.8.2 provides a formula that dictates the foundation depth and diameter as a function of the allowable lateral soil pressure. What I am looking for is a design approach for the design of the concrete pier. Specifically, the allowable bending capacity as a function of reinforcing & concrete strength. Right now I have designed the pier as an unbraced cantilevered column with a large applied moment at the top; essentially ignored any benefit from the soil, but I would like to know if there is a better approach. I have consulted ACI 336.3R-93, but it more theprectical than practical. Thanks, Jason

 

[GeoPaveTraffic]

You need to have your geoetechnical engineer do a lateral pile analysis using a software package such as LPile. These packages model the soil as a series of non-linear springs taking into account the applied loads, soil/rock type, and the structural stiffness of the pile.

 

[JAE]

I'd second what GeoPaveTraffic states. I've used data from my geotech's analysis using LPILE in which they determine an appropriate pier diameter and depth and report to me the max. lateral deflection at the top of the pier as well as shear and moments along the length.

Alternatively, you can do your own (more approximate) analysis using a 2D analysis program (such as RISA 2D) and model the pier as a series of short (1 ft long) lengths extending down into the ground.

The surrounding soil is modeled using springs - we usually use about 50 to 75 pci for the soil subgrade modulus if we don't get that from the geotech. Take a stab at a pier diameter and depth and calculate your spring as (pier dia.) x (50 pci) x (12 inches vertical) = k (pounds/inch). Use that "k" value as your spring coefficient in your model at every 1 ft. segment.
We usually discount the top 3 to 5 feet of soil due to freezing and apply no springs there.

Apply your vertical and lateral forces to the top of the pier and you will see that there is usually (or should be) a point of rotation in the pier about 2/3 down its length. You then can cycle back through your design, adjusting pier diameter and length to get your top of pier lateral deflection down to a reasonable value (1/2" or less).

Your individual spring reactions can be converted to psf to check for lateral bearing pressure on the surrounding soil - we usually try to keep it fairly minimal - 1000 psf max but your geotech can assist in this as well.

Take your moments and axial forces in each segment and design as a round column per ACI. We use PCA Column for this but any method would do.

 

[JackTrades]

I'm presently working on a drilled pier foundation for a jib crane and JAE's post was very helpful. Thanks.

-Jack

 

[GeoPaveTraffic]

One thing to keep in mind when using JAE's method. He is modeling the soil with springs that have a constant reaction no mater the amount of movement. This is not how soils behave. They have a non-linear reaction to movement. Therefore, any analysis done with constrant spring constants will be off from the behavior of real soils. Typically, the spring constant goes down as the amount of movement increases. This leads to larger deflections than an analysis with constant spring values.

Just keep these limitations in mind.

 

[rochplayer]

FHWA offers a free lateral pile analysis program COM624P that can be downloaded from the following site:

http://www.fhwa.dot.gov/engineering/geotech/software.cfm

It is similar to LPILE in that it uses the p-y curve method for calculating the lateral response of piles. It is DOS based.

 

[JAE]

GeoPaveTraffic,

Yes, I'd rather use LPile but if you can't....

 

[rochplayer]

JAE, I totatly agree!

 

[jheidt2543]

You might find the following paper very usefull:

"Analysis and Design of Laterally Loaded Plies and Caissons in a Layered Soil System" by Tarun R. Naik and Alain H. Peyrot, pages 589-606, Methods of Structural Analysis, Vol.II, Edited by William E. Sauland Alain H. Peyrot, American Society of Civil Engineers, 1976.