shearwall pilecap

[v2]

How can you design straight and battered auger cast piles supporting a large shearwall pilecap when the effect of gravity and lateral wind loading combined must be taken into consideration?
Thank you in advance for sharing your experience.

 

[v2]

can anyone help on this one?!

thank you.

 

[jhall]

You may want to consider contacting the geotechical engineer which provided the soils report, if one was provided. If you give them the axial and lateral forces, they should be able to provide you with the number and length of piles required to resist those forces. They will at least, as a minimum, be able to give the end bearing capacity, soil friction capacity and lateral resistence of the soil based off of the soil properties. They should even be able to recommend the batter angle of the piles.

 

[v2]

Usually my geotech engineer provides the following:
1. Straight pile working capacity in compression and tension.
2. Straight pile working lateral capacity.
3. Batter angle (sometimes).
All that is associated with a pile diam. and pile centerline-to-centerline minimum spacing.

Assuming all the above is given. My task is size the pilecap with the required number of piles. I would also have to determine the location and number of batter piles within the same pilecap supporting a shearwall.

On the other hand the type of applied loads is as follows:
1. Gravity load from dead and live load
2. Overturning moment from wind load
3. Horizontal shear from wind load

The outermost piles are battered most of the times. The tips of all piles are assumed to be fixed and rigidly connected to the bottom of the pilecap.

Now as you can see those batter piles are loaded in compression or tension and shear simultaneously for different load conditions, and design combinations.
The problem becomes very complicated and at least time consuming or near impossible for hand calculations.

What procedure do you recommend in solving this problem effectively?
Is there special software that you suggest, for this type of problem?





the big question

 

[v2]

I still need help on this one.

thank you for your input.

 

[Qshake]

Sounds like you want to know how to design pile foundations.
You don't need specific software for this, refer to Peck, Hanson and Thorburn's classic reference Foundation Engineering. This will show you how to develop a pile layout and design of a row of piles (battered or not)for a pile cap with vertical loads, shear loads and moments.

You could, if so inclined, write a spread sheet, as many of us have done in the past, to allow for analyzing different pile layouts.

I have also duplicated Bowles matrix analysis of a pile group in a spreadsheet as well. however, this requires soil information that may not be readily available. A plus is that when no specific soil data is available the solution of the Bowles matrix method reduces to the P/A plus or minus My/I and indicated in PHT.

 

[v2]

Qwake,
Thank you for your thoughts.
but P/A + or - My/I is not valid for large mats. the assumption of infinitly rigid out of plane pilecap is not realistic and not economical.
There must be a software package that can handle this more efficiently.

Thank you for your input.

 

[v2]

ANY NEW IDEAS?

 

[v2]

can somebody please express his thoughts on this subject. and recommend a good easy to use and comprehensive software package?

thank you.

 

One thought:

I have used the finite element package FLOOR to analyze and design pile caps. By modeling the cap as a mesh of finite elements, and the piles as Winkler springs, a reasonable solution can be obtained without using the assumption of a completely rigid cap. If the size of your cap is not enormous compared to the size of your shearwall core, the rigid cap assumption is not terrible, since the wall group itself will lend considerable stiffness to the cap.

Hope this helps.

 

[ishvaaag]

Although Qshake has already expressed his contrary opinion elsewhere, I without doubt will rely in FEM packages to model foundation items that are not inmediately amenable to well established designs. The reason for that is that with some software packages yo can model the thing in minutes, and get good appraisal on what happens; also the flexibility of the mat or pilecap is immediately discounted, since the bending plates inmediately accounts for it.

When ground is involved I have modeled the thing as both Winkler springs and as plates of soil laid vertical under our foundation, or around our piles or so, all over a unyielding rock layer at depths bigger or at least in the magnitude of the building height.

All the stiffness bussiness in this case is passed and gauged through modulus of young etc. By the way that to model the things in ways that the settlements be credible one has to use stiffer modulus than those usually promoted for soils...the same thing that happens with well compacted highway top layers: are more stiffer than usually assumed.

Of course all this is incorrect, but through bracketing of the stiffnesses and changing your input member or plates you can get readily an envelope of design items compatible with what expected for settlements etc, that you can evaluate separately.

This same thing but well done is what one should expect of soil-structure interaction programs. I have not one such available, so I proceed this way for irregular shapes, investigation, and design with bracketing or safe assumptions such the dismissal of soil stiffness. For springs horizontally I use to take 1/2 or 1/3 of the vertical stiffness.

This is not to say that I don't use ready-made solutions to element-type designs; I have bought and made lots of programs for such thing and I do. But I am not hesitant to proceed this way, and I am not alone; for example for soils with heave or significant settlement it is accepted, yesterday I read the "complete" dismissal of any "ground" contribution to support the vertical loads, the total bearing on piles (which by the way is the usual way in which we design pile-caps).

ALso note that if in a high risk seismic zone battered piles are not recommendable.