Pile to Cap Force Transfer
Pile to Cap Force Transfer
(OP)
This is related to but different than a recent post here.
Pile type: steel pipe casing, center large diameter bar steel core, grout filled (to rock). For example 13-3/8 x 0.5" wall pipe with one #24 center bar, capacity ~400 tons (800 kips).
The design of the pile capacity comes from code allowable stresses. The casing is designed using 0.5Fy (max 32ksi). What is the force transfer mechanism between the casing and the pile cap? The grout makes sense in straight bearing, center core bar can be developed or provided with additional anchor plates, but the casing? There is no way that the contact bearing based on 0.5Fy works for concrete and the embedment isn't sufficient for any kind of friction. Have used these many times but never thought about it. Am I missing something?
Pile type: steel pipe casing, center large diameter bar steel core, grout filled (to rock). For example 13-3/8 x 0.5" wall pipe with one #24 center bar, capacity ~400 tons (800 kips).
The design of the pile capacity comes from code allowable stresses. The casing is designed using 0.5Fy (max 32ksi). What is the force transfer mechanism between the casing and the pile cap? The grout makes sense in straight bearing, center core bar can be developed or provided with additional anchor plates, but the casing? There is no way that the contact bearing based on 0.5Fy works for concrete and the embedment isn't sufficient for any kind of friction. Have used these many times but never thought about it. Am I missing something?






RE: Pile to Cap Force Transfer
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Pile to Cap Force Transfer
RE: Pile to Cap Force Transfer
RE: Pile to Cap Force Transfer
Yup. It's still the same load path though if you plan to take some of the compression in the casing.
Couldn't your centre #24 bar do this job? Perhaps I'm not understanding the detail.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Pile to Cap Force Transfer
The way I have always seen this implies that the center bar gets developed but I have never seen anything account for how the .5Fy x A_casing gets transferred. That force can be large and doesn't make sense (on paper anyway) in bearing just at the surface of the casing tip.
RE: Pile to Cap Force Transfer
These two statements seem incongruous. Could it not be that some portion of the load approaching 0.5FyA of the center bar might be getting transferred from the center bar over to the casing?
It would be useful to see what the top and bottom details look like.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Pile to Cap Force Transfer
The base detail is a socket to rock, varying in length depending on how much capacity you need. At the top it's usually a 6" embed of the casing into your cap the core bar either extends up to develop or gets a compression anchor plate.
RE: Pile to Cap Force Transfer
I guess they're not. As you surmised, I was thinking partial utilization of the steel in ULS.
No cap plate at the bottom of the pile, right?
As you and Hokie have both touched on, it sounds as though the casing needs to be non-participating at both the top and bottom of the pile with respect to bearing, unless there are cap plates involved. So if the casing is to participate meaningfully at all, there must be shear transfer between the casing and the idealized "plugs" at each end. Well confined concrete should be much stronger than the equation would indicate. Maybe some of the margin that makes it work resides there.
So I guess my final answer here is that I don't know a darn thing beyond what you started with.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Pile to Cap Force Transfer
I guess maybe there isn't a logic but just enough cushion in the allowable stresses that it works. I thought maybe there was a trick that i was missing.
RE: Pile to Cap Force Transfer
I think that you may have identified a bonafide mystery here. And if we can't identify the logic, that makes me a little queasy about applying the capacity equation. The tip detail that you uploaded suggests strongly, to me at least, that the force in the pile both leaves and enters the pile through the grout/rebar plug alone, just as you suggested initially. I don't understand why the casing should even enter into the capacity equation.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Pile to Cap Force Transfer
Some details that I have seen of the micro pile top-to-footing connection are as follows:
RE: Pile to Cap Force Transfer
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Pile to Cap Force Transfer
RE: Pile to Cap Force Transfer
This download Link by Dr. Donald BRUCE, et al, on micropile design maybe useful.
RE: Pile to Cap Force Transfer
@Bookowski: it appears that, in the absence of some kind of casing cap assembly, the plug capacity based on equation five of Ingenuity's attachment would require attention. I'm curious to see where you land with this.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Pile to Cap Force Transfer
Seems like it just doesn't make sense in terms of load path but isn't causing any problems. I will update this if I figure out that it does make sense and I'm missing something.
RE: Pile to Cap Force Transfer
0.40 x f'c x A_grout + 0.5 x Fy x A_Center_Bar
...would be as applicable to the uncased length as...
0.33 x f'c x A_grout + 0.5 x Fy x A_Center_Bar + 0.5 x Fy x A_casing.
...would be to the cased length.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Pile to Cap Force Transfer
If your assumptions are correct then at the uncased length you gain 7% on the grout strength but drop off the entire casing. That 7% would not come close to what you've lost so the casing would be essentially useless. On the pile I'm looking at the casing is 15.5 square inches of 50ksi steel, that's a lot of force.
RE: Pile to Cap Force Transfer
I worry that responsibility for the connection between piles and supported structure tends to fall into a sort of no man's land (it should be EOR responsibility IMO). I use micro piles and screw piles all the time. The geotechnical engineer or piling contractor usually provides some conceptual detailing recommendations which may or may not suit the supported structure.
Designers then often take these details as gospel and consider their work done once the recommendations have been incorporated. I can't tell you how many pile cap and grade beam details I've seen on IFC structural drawings where piles would punch right through the supported structure if loaded anywhere near capacity.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.