Pile to Cap Interface/Design
Pile to Cap Interface/Design
(OP)
This is related to a previous question I posted here (http://www.eng-tips.com/viewthread.cfm?qid=391181).
Same pile assembly, steel casing w/grout and a large center bar - typically #18 75ksi. I've still yet to determine how the force is transfered into the casing but I'm setting that aside for now (seems like it relies on a local overstress that isn't accounted for).
The center bar carries a lot of load. If this is bar is developed by extension into the cap how does this affect the stm? Seems like this really complicates things - is the strut only a portion of the load, i.e. the load minus the component in the bar? Seems silly.
The only sort of logical solution I see is a bearing cap plate on the casing with the thread bar poking through. However this detail doesn't address load into the center bar unless it's got a nut on the underside, which implies that the plate is held high not tight to the casing.
The pile assembly gives huge capacities however they seem unworkable based on the stm strength at the node. As a cct node I'm capped at capacity .80 x phixf'c, so it seems like the only proper way is to come up with some kind of funky cap system that is large enough to make the node work and also transfer force to the various components.
I would like to say this doesn't work or do the funky detail but this pile with no special capping provisions is as common as white bread here and it's swimming upstream to say it doesn't work. These are high capacity piles though (600k to 3000k working load) so would like to know how (if) it works.
Same pile assembly, steel casing w/grout and a large center bar - typically #18 75ksi. I've still yet to determine how the force is transfered into the casing but I'm setting that aside for now (seems like it relies on a local overstress that isn't accounted for).
The center bar carries a lot of load. If this is bar is developed by extension into the cap how does this affect the stm? Seems like this really complicates things - is the strut only a portion of the load, i.e. the load minus the component in the bar? Seems silly.
The only sort of logical solution I see is a bearing cap plate on the casing with the thread bar poking through. However this detail doesn't address load into the center bar unless it's got a nut on the underside, which implies that the plate is held high not tight to the casing.
The pile assembly gives huge capacities however they seem unworkable based on the stm strength at the node. As a cct node I'm capped at capacity .80 x phixf'c, so it seems like the only proper way is to come up with some kind of funky cap system that is large enough to make the node work and also transfer force to the various components.
I would like to say this doesn't work or do the funky detail but this pile with no special capping provisions is as common as white bread here and it's swimming upstream to say it doesn't work. These are high capacity piles though (600k to 3000k working load) so would like to know how (if) it works.






RE: Pile to Cap Interface/Design
RE: Pile to Cap Interface/Design
1) Adjust the height of your nodes such that the rebar can compression develop between where it enters the nodes and where it exits the struts. This means that you need to lift your tie bars.
2) Use four struts instead of two with the lower going to the top of the pile and the upper going to the compression dowels. This will lead to a complex node at the top of the pile and the need for a second tie placed higher than the conventional one. It also means that you've got a tie crossing your struts. That's allowed but I generally try to avoid it.
3) You use two shear resisting mechanisms concurrently: your struts and punching shear for the portion of the compression dowel force that you're not able develop across your struts. Obviously, this method is pretty far off reservation and there are inherent difficulties in trying to properly account for the complexities associated with the two mechanisms working in tandem.
If it can be made to work reasonably, I'd prefer option #1. It's simple and the easiest to defend from a code standpoint. It may be quite inefficient with respect to the ties. I suspect that #3 is what is really taking place for common detailing out in the wild.
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 Interface/Design
I'm struggling with this concept, there seems to be an incompatibility with high capacity piles and strut and tie - or it's revealing a big deficiency in how I typically see these detailed.
Unless I'm missing something you will always be limited by the nodal zone strength at the pile face. Lets say this is a 10" round pile of any type. This means you can get a max capacity at this node of Area x phi x 0.85 x Beta x f'c = PI()/4 x 10in^2 x 0.75 x 0.85 x 0.80 x 5ksi = 200 kips. 200 kips for a 10" pile is low, I have geotech designs for 600 kips (300T) 10" round. Seems like a high capacity pile requires other mechanisms to work, i.e. development into the pile which makes a simple stm not realistic.
RE: Pile to Cap Interface/Design
I agree with the nature/scope of the problem. We've discussed both of the following in other threads but they apply here in spades:
1) With these connections, I often worry that there's a gap in who is taking responsibility. What I see in practice is geotechs providing suggested details for foundations and designers taking those recommendations as gospel without much additional checking. The truth is that the design of these kinds of connections is not within the scope/capability of most geotechnical engineers. Nor should it be.
2) Very similar conversations have been had here regarding column compression dowels. There's a cadre of structural engineers out there who seem to think that the only thing that matters for compression dowels is the compression development length. So what if the bars involved in the compression transfer are in different time zones. I see this as an upside down version of that to a degree.
I really hope that we get some feedback from some other folks on this. It's an interesting topic and potentially represents a pretty serious load path shortcoming.
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 Interface/Design
RE: Pile to Cap Interface/Design
3.1.2 Limiting Stresses in Bearing Zones
Current Specifications (5.6.3.5): The limiting stresses in the current specifications do not
account for the beneficial effects of confinement.
Proposed Change: Introduce a factor to account for confinement for cases where “the
supporting surface is wider on all sides than the loaded area”. For simplicity and
consistency the “modification factor, m”, used in Article 5.7.5 – “Bearing” could be used
in Article 5.6.3.5 to modify the permissible bearing stresses where appropriate.
Effect of Proposed Change: This would reduce conservatism and maintain consistency
with the specifications in Section 5.7.5.
http://onlinepubs.trb.org/onlinepubs/nchrp/docs/NC...
RE: Pile to Cap Interface/Design
Section 5.3.3 of this provides some unofficial support as well: Link
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 Interface/Design
The problem is that I'm not trying to go rogue but explain away already rogue behavior to avoid reinventing the wheel and getting a lot of pushback.
RE: Pile to Cap Interface/Design
http://www.pci.org/uploadedFiles/Siteroot/Publicat...
RE: Pile to Cap Interface/Design
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 Interface/Design
RE: Pile to Cap Interface/Design
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 Interface/Design
ftp://ftp.dfi.org/SuperPile2015Presentations/02-Ma...
RE: Pile to Cap Interface/Design
1) Damn thread's closed already and;
2) The iPad only business makes it difficult for me to clip snippets.
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 Interface/Design
Found a paper on micropiles testing without bearing caps - Link