Capacity of existing pile foundation.

[avnei]

I am trying to investigate whether the exisiting pile cap with 13 pile beneath them is capable of taking an increased load or not. I am considering modelling the pile cap in Staad Pro as solid elements. how do I define the supports?. Existing piles were designed to take compression only and no tension. So, I am thinking of defining the support points where pile exists as PINNED. Is this the right approach?. This gives me the reaction on the pile which I will check with the axial load capacity of the pile. Should I rest the entire pile cap only on the piles or do I consider the soil beneath it. In that case will I have to define spring supports?.
Is there a way to find the modulus of subgrade of soil based on bearing capacity of soil?. Comments appreciated.

 

[VAD]

This topic requires discussion with a geotechnical engineer to ensure that parameters that will go into your model is realistic or else you will end up with GIGO. As well, one should try first to address the problem from a simple approach. This requires an understandinhg of how the pile cap was designed - why 13 piles were some required based on geometry of the cap. What are the soil conditions and what factors were taken in consideration in the design- end bearing, end bearing and shaft resistance. What factor of safety was used etc.

A good example of where the geotechnical and structural engineer should work together. If you are wearing both hats then you should be able to figure out some of the requirements.

 

[DRC1]

Agree with what VAD said. You don't need FE to solve a pile cap w/13 piles See Lui & Evert or similar geotech text Make cap stiff enough to evenly distribute loads. Treat piles as 2 force members. Pile load = P/A +/- (My/Sum(Ay^2)
or something like that. I don't have a reference handy. Piles are assumed to carry all loads. Do not have the subgrade participate. Over time the subgrade will settle and be unable to supprt the cap.

 

[Qshake]

The problem I see with this is that most engineered facilities are not over designed to begin with. So if designed properly, the pile and pile cap were designed for a specific load case and taken to nearly the allowable of the pile, bearing strata, or soil/pile capacity.

I think it would be very difficult to show that an existing pile and cap has capacity to take additional load without some upgrade.

As for determining the capacity I agree with the rigid pile cap idea noted above but also encourage you to determine the shear and moment capacity of the pile cap. Also you should develop a interaction diagram for the capacity as you would perhaps for a column but based on different controlling cases.

I don't believe that a FEA is the answer to this problem. Please recall that FE is a numerical solution based on assumptions of the constructed environment. without good assumptions you cannont expect to achieve applicable or practical results.

Regards,
Qshake

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