Alternative analysis of pile cap

[ukengineer58]

Hi,

please refer to the attached sketch of a existing situation. The load down the column is being increased. This causes bending and shear failure. My thoughts are if I take the force in direct bearing through the cap to the pile and justify the top steel to hold it in place I can demonstate it will work. I will do this by working out the angle based on geometry, calculating the compressive and tension forces and checking. Please note that this is one cap connected to many others via tie beams. Any thoughts?

 

[ishvaaag]

Normally this situation uses what is called "viga centradora" (centering beam on the ground) as backspan that balances your cantilever. You can build the solution through excavation and dowels if the backspan is available for works. So more than just a tensile action atop that still leaves some problem to the pile, you build a device that provides moment that balances that of the cantilever. If the centering beam is significantly longer than the cantilever length, it only adds a small vertical force to the pile, what may be permissible. The centering beam is itself anchored at the other end to the foundation of other column; do that with dowels long enough as to ensure the there upwards force can be considered be only acting axially on that column.

 

[ukengineer58]

ishvaag, I am not at this stage concerned with the pile load I am concerned with the structural capacity of the cap. A backspan does not help in this regard. Plus there is no room to construct one in my case.

 

[hokie66]

This is a strut and tie application. Suggest you get up to speed on that method, as that is the way pile caps are typically designed these days. It is a truss analogy method, and you are thinking that way in your sketch. The key things which have to be learned are anchorage of the ties and confinement of the struts.

What ishvaaag was pointing out is that as drawn, your pile cap is unstable. Maybe there are rectifying members, but they are not shown.

 

[ukengineer58]

Homie I would have to disagree on that being the way pile caps are designed. Some yes some no. I did note that the cap is connected via tie beams to other caps. The system itself is not unstable. The cap has steel in top and bottom with vertical and horizontal steel which I am assuming is confining the concrete. The vertical side bars lapping down could I think be justified as vertical shear link legs.

 

[BAretired]

If the sketch is anywhere close to scale, I would be more concerned with bending in the pile than bending or shear in the pile cap but I agree they all must be checked.

BA

 

[hokie66]

ukengineer,
Your title was about an alternative way to design pile caps, and the way you proposed in your original post is essentially truss analogy. I simply pointed you in the way of the strut and tie method, which is a conservative lower bound approach commonly used in designing these non-flexural regions.

Please don't call me a Homie.