Piles for lateral loads

[visitor]

I have to design the piles for supporting the bridge which is crossing a small stretch of water. The piles are partly embedded in water and then in soil. There is a pile cap connecting these piles above water level
( 3 nos piles in one plane). The bridge girders are directly resting on the pile cap.

The reactions ( Vertical and horizontal) from the girders are transferred to the pilecap.

Hecne the piles are subjected to vertical loads and horizontal loads at pile cap level. The piles are connected by a pilecap in one direction only and in the other direction it will be a cantilever. Can we assume the pile head as “ fixed “ ?

If we assume this as “fixed head", how to design the pile ? Based on soil properties, the embedment depth required will be more than 4 time T. So , this will be a long pile.

The charts given in “Pile design and construction practice – by Tomlinson “ for fixed pile head are for lateral loads at ground level only. How to design the pile for the externally applied moment and horizontal loads applied at higher level ?

It is a general opinion that the long piles are governed by the structural capacity of the pile. Does it mean that the soil can take any amount of load ? Please give suggestions.

 

[Qshake]

I suggest that you have a copy of the USS Steel reference Design of Steel Highway Structures. This reference has long been the staple of the industry for this application. Especially since the example is readily available here.

The piles are considered pinned in the longitudinal direction and fixed in the transverse direction. Also, the steel pile bent is considered fixed at the point of fixity within the soil as determined by an appropriate analysis. The remaining analysis is purely structural.

 

[ishvaaag]

Respect the growth of allowable vertical load with depth, such seems be the general consensus embedded in the ordinary design for piles, since both items pile tip strength and friction/adhesion at the shaft grow (er, in general) with depth, so either their sum controls or the compressive strength.

 

[pileupm]

Dear visitor,

We have a same problem in laterally-loaded pile, but i have to design a single pile under lateral load condition by using p-y method.

Based on my previous study, the pile is assumed "fixed" if the moment applied in negative sign (Prakash, 1990) which is applied at the ground surface.

But, i also have a problem in this method. So far, i don't know how to calculate LATERAL CAPACITY OF PILE (Hu) using p-y method for LONG PILE only. Based on my study, the failure will occour for long pile if Mmax=Myeild. Is it true if i just taking the value of lateral resistance of soil (Pu-static, Pb-cyclic) as a lateral capacity of pile?

I hope anybody can help me...

pileupm

 

[visitor]

Thanks for your reply.

I have seen some people using the modulus of sub grade
reactions ( horizontal) of soil and model the pile and
soil media ( as springs at every 1m height).Then they analyse the structure and calculate the BM at various levels
of the pile. But this way , the structural properties can be checked.
But I do not know t\how to check the soil capacity.

I do not know how this type of problem is dealt by the
book by Bowels.

 

[ishvaaag]

You can use as you say a horizontal modulus of subgrade reaction. For some cases of scarce relevance I have used alf to 1 third of the vertical one. This is merely a guess, and of course by itself inaccurate since response should vary at least in the upper zone.

Then you need either modulus of subgrade reaction (vertical) to start or a good E data to create a matrix of soil in which to embed your pile for the lateral analyse.
Then you also have the surely more adequate procedures in the geotechnical books but may lack the generality required by your structure, but the current practice is to use almos separate modeling or practices for the design of the foundation, so not so great a problem, since is what most times done.

In Bowles you have both data for tentative modulus of Young for different soilas, as well as procedures for piles laterally loaded.

For buildings and normal footings, footing strips or even mats, a poor but still usable soild should deliver a final 4 (kgf/cm2)/cm modulus of subgrade reaction (vertically). Normal soils should have 8 (kgf/cm2)/cm and 12 (kgf/cm2)/cm the good ones. It is reported that well compacted soils deliver up to 22 (kgf/cm2)/cm.

I think CRSI also has one program for piles in lateral load, but I think to remember were thick rigid piles, what may not be the best for your case.

FDOT has a pile foundation for indication panels that cover lateral loads, and torsion from the wind load atop the column is also included (free).

 

[DRC1]

As an alternate to P-Y curves and lateral analysis, if the horizontal loads are smaller than the vertical, and all loads are not unusually large, another solution would be to use batter piles. A batter is usually battered between 1:12 to 3:12. If the horizontal reaction componet of the pile capacity is less than the applied horizontal load, You are all set. This is often easier (and more reliable) than computing lateral capacity of vertical piles.

 

[Polecat]

I am frequently called upon to design foundations that have very heavy lateral loads (as in cantilevered electrical transmission pole structures), and have found the program L-Pile4 to be very useful for doing so. It uses the p-y method with the modulus of subgrade reaction, and also has some excellent graphics in the output that show the soil reactions, moments, shears and deflections all the way down.

Knowing the lateral deflection is a must if one is to account for the P-Delta effect and secondary moments in the structure.

You can read about it at

http://www.ensoftinc.com.

http://www.spiraleng.com

If you wish to visit my website, we can discuss these or other issues further.