Unequal pile lengths

Are you looking for the capacity of different length friction pile? As you have identified yourself as a contractor, I don't see why you would need to know the load to the pile group (assuming you've been given the required design bearing to achieve) and the load seen by each individual pile is not really of any interest. A matrix analysis of the pile group would take into account the pile length, but would unless you've got soil parameters, the matrix method would ignore those parameters and you would be back to the old P over A plus or minus M over I.

If you're looking for the capacity of friction pile based on driven length, which would be variable. That value is a function of the soil properties and would vary from site to site. The geotech on the job should be able to help you out and they have already determined this information in order to find the terminal depth for the friction pile.

I hope this helps, if not, please elaborate on what you need. [sig][/sig]

Fairly common for piles to be different lengths, depending upon soil conditions. If you want an initial capacity estimate of the piles, you can use WEAP, a public domain wave equation analysis program. If you want something more accurate, you can use a Pile Driving Analyzer (a specialty service provided by some geotechnical consultants) during placement to get capacity, or you can load test individual or pile groups with a static load test. [sig][/sig]

Perhaps, I was not very clear. I really want to know how to modify the P/A +M/Z technique to calculate the forces in a pile group whose pile lengths differ.

Thanx [sig][/sig]

As I stated in my previous response, you will have to run a matrix analysis which includes the soil parameters along with the appropriate member stiffness terms for the pile. This can be accomplished via a spreadsheet. I have completed such a spreadsheet application based on the information in Bowles' Foundation Engineering and Design. I am not aware of any commercial programs availiable for this. Perhaps you should check with the geotech forum. Remember if you don't have the necessary soil parameters, the analysis is nothing more than a lot of work for nothing more than P/a(+/-)M/z. [sig][/sig]

CHARECTERISTIC OF THE UNDERNEATH SOIL VARIES VERY SIGNAFICANTLY. I BELIEVE IF THE GROUP OF PILES AND THE PILECAPS ON WHICH THESE PILES ARE RESTING AND THE INTERACTION OF THE PILECAPS AND THE SOIL OVER WHICH THE PILE CAPS REST ARE CAPABLE TO TAKE THE TOTAL SUPERIMPOSED LOAD THEN NOTHING TO WORRY. HOWEVER IT MAY HAPPEN THAT IN THE SAME GROUP SOME PILS ARE SHORTEN DEPENDING ON SKIN FRICTION AND BEARING ON WHICH THE PILE IS RESTING. IT IS MORE IMPORTANT TO SEE (BEFORE DESIGN) THE SOIL STRATIFICATION. AND ALSO THE PILES AND ALONG THE PILE CAPS AND INTERACTION WITH THE SOIL IS MUCH MORE RIGD AND LOAD CARRYING CAPACITY GETS INCREASED. [sig][/sig]

Any program package capable of analysis soil-pile-structure interaction
for offshore platforms can analyse a set of piles with individually
different sizes, lengths, for any combination of loads (Axial, lateral,
torsional, all simultaneously). Some of the packages are:
1. SACS of Engineering Dynamics Inc., Kenner, La
2. StruCAD-3D of Zentech. Inc, Houston, Tx.
3. Splice
4. Splinter
The last two are used in Europe. One is part of SESAM package of DNV.

This is a typical problem in Offshore structural design.

Incidentally, Qshake's statement of P/A + My/I would be valid only if you
have a rigid pile cap. Pilecap flexibility can significantly affect the load
distribution in a pile group. This effect is probably more important
than length effect! Also for piles on slope, lateral load carrying capacity
(shear distribution in piles) may also be important since the fixity lengths
wpuld be different.

I agree that the effect of the rigid pile cap is more important than the length and am trying to point out the difference in loads due to the length are nothing without the proper soil information. And that only comes from a geotechnical person familar with the project. Thus I can't figure out why the length is of interest.

Pile lengths are not a concern provided the unknowns are at minimum. As it was mentioned earlier, the fixity on caps is equally important. However, a proper settlement analysis is necessary if you go for varying pile lengths. A pdelta analysis is essential too. The question therfore, I feel is not specific as it may attract solutions that were not asked for.

The question arose when the building control officer requested that the difference in pile lengths be considered to determine the differenct axial, horizontal and bending effects on rock socketed large diameter bore piles.

This was actually for a spriral bridge built on 30 degree fill slope overlying the rock. I did have some mathematical equations which some whizz had given me on an earlier job unfortunately I never took time to learn their derivation and i did not have the confidence to use them.

Since the strucutre was a bridge it had umteen load conditions and in the end we just used the computer to analyse the actual pile lengths.

I just wanted to see if the problem had been encountered by others and if there was a quick method to overcome the analysis.

Incidently the bridge is built and looks great!

NB