Piles and Pile Caps 1

[Ahmed A. Alamin]

Hello fellow engineers, i have a question regarding the structural design of piles and pile cap. I know that piles are structurally designed as columns but do they follow the same classification " Slender/Short" in other words what is the effect of the surrounding soil? and how it could be modelled in softwares such as Etabs & Safe ? . How piles are connected to pile cap will the connection transfer moment to the pile?.

Finally, it's my understanding that pile caps are most likely to be like deep beams that is it should be designed using truss analogy not the conventional bending theory, is this right? and if so do programs such as csi safe take this into account?

 

[phamENG]

The soil will act as a series of springs acting perpendicular to the column, typically varying in stiffness from very stiff a the pile tip to nearly zero stiffness at the surface (or 12" down, or at the frost line - depends on the situation). There are programs that can analyze this pretty accurately, but I've never played with them. The geotechnical firms typically had them and would provide me the information I needed if asked.

That said, I've had to do a few analyses without the benefit of the geotech's help (smaller firm without the software, no geotech hired, etc.). The way I'm most comfortable with finds the "Depth of Fixity" of the pile. This is based on the horizontal subgrade reaction, which is in turn based on the lateral stiffness of the surrounding soil (like the springs I mentioned above). It's not perfect, but it's pretty well documented in the literature and it has worked so far. Despite "fixity" I generally assume that point as a pinned support unless I have a whole lot of pile below it, and even then I'll lower it down several feet before switching it to a fixed condition. That's partially due to the soil conditions in my area - mostly soft clays and silty sands for the first 50 to 90 feet below the surface.

Yes, pile caps should be designed using strut-and-tie methods (truss analogy). I can't speak to specific programs. It should be discussed in the program's documentation. If you can't find it, the company's customer service should be able to help you.

 

[Lomarandil]

To answer a few of your questions:

Any reasonable soil (e.g. not marsh or marine clays) will have enough support to brace piles against buckling for their embedded length. Piles are only designed for buckling under axial loading if they extend above grade by some height.

If you have lateral loads on your piles, then you would go into the "soil spring"/"P-Y"/Lpile analysis to determine depths of fixity, bending and shear below grade, etc.

Piles are most commonly connected to the pile cap by embedment, and considered a pinned connection. Developing a moment connection is possible, but generally extremely difficult and cost prohibitive.



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just call me Lo.

 

[Ahmed A. Alamin]

I can"t thank you enough this is very helpful

 

[canwesteng]

Your Geotech should have given you a soil stiffness, varying with pile diameter, that you can use in a simple FEA model to determine pile moments and shears.

 

[BridgeSmith]

Piles are most commonly connected to the pile cap by embedment, and considered a pinned connection. Developing a moment connection is possible, but generally extremely difficult and cost prohibitive.

I agree with your other statements, but I have to disagree with you here. Full scale testing of pile caps (I believe it was by TXDOT) showed that 12" embedment of an HP12x53 into a cap with confining reinforcing provided sufficient restraint to develop the full plastic moment capacity of the pile. While it requires some thought and some extra reinforcement to provide confinement, I wouldn't characterize it as difficult or expensive.

Generally, the pile cap itself is embedded in soil, which is typically adequate to resist the small lateral shear forces using passive soil resistance on the side of the cap.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[phamENG]

BridgeSmith - while possible, it usually isn't relied upon in building construction.

1) The analysis is easier if you ignore fixity (and is generally conservative).
2) Accounting for the fixity could tempt an engineer to ignore the usual lateral restraint rules (3+ piles, 2 piles and grade beam in one direction, or 1 pile and grade beams in 2 directions). I could see that not passing the standard of care smell test if something went wrong.
3) No pile driving exercise in building construction is ever terribly accurate. I don't think I've ever gotten a pile survey back with every single pile in spec. Being able to fall back on the fixity provided can be a nice buffer to not have to change the design of the foundation.

 

[r13]

Fixed head piles are usually required in cases the lateral displacement is excessive, or the structure is sensitive to lateral displacement.

 

[BridgeSmith]

Another way building construction apparently differs from bridge construction, I suppose, phamENG. Those all look like good reasons to not count on fixity.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Boiler106]

Yes, pile caps should be designed using strut-and-tie methods (truss analogy). I can't speak to specific programs. It should be discussed in the program's documentation. If you can't find it, the company's customer service should be able to help you.

I wanted to point out that CRSI's Design Guide for Pile Caps (available in pdf format) does not recommend using the strut and tie method as it 'can be unconservative' in certain conditions and CRSI does not illustrate the method in the guide because of their research. Something to look into.

 

[phamENG]

Thanks, Boiler106 - I was unaware of that. Not sure what other method you could use, unless they've fit some design equations to research results. I'll look into it.

 

[r13]

I personally stayed with the old ways - design the pile cap as one way beam, or two way plate, depending on the pile layout. Strut and tie method should be good for simple pile cap without continuity over several spans.