Rigid or Flexible Drilled Pier?

[Rhoadies]

I have thoroughly researched via Google and the forum searches here and I did not find anything of use to me on this topic.

With that in mind, here I go.

I have a building being supported on piers and gradebeams. The piers are 18" and 24" diameters respectively and are 25' long socketed a minimum of 3' into limestone. I do not have access to LPile (I am working on that) but I need to determine the lateral capacity of my piles to determine if the above sizes are adequate.

Per the geotech, I was given this information to determine lateral capacity.

depth k(pci)
0-8' ---
8'-13' 90
13'-20' 60
20'-22' 2000
22'-30' 4000 (socketed only 3' into this material)

Not having LPile I used RAM Advanse to set up my piers and grade beams as it would be in the field. I then broke the pier lengths into 1' sections with spring constants consistent with the values above modified to an 18" section (k*.85*18"*12").

By doing this, I was able to show that my lateral deflection was under 1/2".

Is this an acceptable analysis procedure? I know that the soil stiffness isn't linear, with that in mind, am I totally screwed without using LPile?

Oh, to tie in the subject line, if this is a rigid pile, how do you determine the lateral capacity for that? My geotech textbook from school was not helpful by any means. It said that long piers are L/D>35 whereas mine is 16. But, it quickly brushed by the short pier lateral capacity directly to the long piers.

Again thanks for reading, I appreciate any help you could lend to this subject. Have a great weekend!

 

[BigH]

Why not try elastic theory as per Poulos and Davis? Oh what did we do before the PC?

 

[BridgeEI]

Try COM624P. It is a free program from FHWA. Here is the link

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

The program is virtually the same as LPILE but it is not as user friendly, ie no graphical display. It is the old dos based way of doing things.

 

[jhoulette]

I understand why we need to design the piers for lateral resistance, however I wonder how much lateral load would ever really reach the piers when grade beams are used.
For most pier/grade beam foundations (a lot used in Texas and Colorado two states I have done a lot of design work in) you will always have the exterior grade beams braced by the soil and in many situations the interior grade beams are braced as well.

I might be getting away from TexEIT's question, but is there any useful information about this grade beam interaction with the backfill or native material if trenches were used?

For the record, I hate drilled pier and grade beam foundations. I design them where needed or required thanks to Ft. Hood. But they are expensive and I've seen a lot of them fail when poorly designed by other engineers. Typically, failure is identified by improper drainage above or below the surface and when piers may become lightly loaded and no additional embedment is specified.

Jim Houlette PE
Web:

http://www.evstudio.us

Online Magazine:

http://www.evstudio.info

 

[Rhoadies]

Thanks you guys.

BigH, your comment about Poulos and Davis helped a lot. It led me to another thread that discussed a very similar topic and helped me to refine my analysis.

BridgeEI, I had tried to use COM624P; but without a manual I found it unusable. I tried to guess, but ultimately, without being sure of anything, I didn't trust any answers that I was able to get out of it.

Jhoulette, I don't follow how you could use grade beams as lateral members when you have the beams spanning between piers. The soil is bracing only the sides of the grade beams, do you assume that they take lateral load through the out of plane bending against the soil?

I still have one issue though. Using the length of 25' and pier diameters of 18" and 24", is my member of adequate flexibility to allow this kind of analysis? I have gone ahead and treated these piers as if they are flexible, but how do you handle a rigid pier analysis if it is rigid?

 

[BridgeEI]

Text is linked here

http://www.fhwa.dot.gov/engineering/geotech/library_listing.cfm?sort=FHWA_PubNo

if you are still interested. The publication is FHWA-SA-91-048 (Users manual for laterally loaded...). Your best bet, if not familiar w/ the program, is to use one of the examples they have set up and modify it.

 

[mudman54]

you can change the flexibility of the pile in COM624 to see what effect it has on the deflections; typically, the modulus is divided by 2 to get the "cracked" modulus.

 

[jhoulette]

Don't get me wrong, I understand that the lateral forces will transfer to the piers and that's how I design them for large projects. However, in the residential world (a scary world ;o) it is assumed that no all lateral forces are transferred through the foundation walls to the soil, no lateral load is designed for the piers. In residential the lateral forces are much smaller so I don't have much of an issue.

I'm just wondering if anyone has read anything about how the grade beams may distribute some of the lateral forces into the soil and not 100% to the piers. A floor diaphragm would need to be used and the geometry of the foundation grade beams (spacing, height) would play a big part in the rigidity to transfer the loads.

Sorry, again I should start a new thread for this.

Jim Houlette PE
Web:

http://www.evstudio.us

Online Magazine:

http://www.evstudio.info

 

[jhoulette]

oops - should say "it is assumed that all lateral forces"