Pile Cap Design for Asymmetric Pile Group
Pile Cap Design for Asymmetric Pile Group
(OP)
Hello Everyone,
Last day i went to visit a site and found 2 piles of a 4 piles group has been shifted 1 ft away from drawing location.
Now, how can i solve the problem to ensure all the piles will be active under column loading ???
if i need to design the pile cap according to as cast location, can anyone suggest for a good analysis method or software for better solution ???






RE: Pile Cap Design for Asymmetric Pile Group
RE: Pile Cap Design for Asymmetric Pile Group
would you please explain in more details ?
shall i inter-connect the piles with beam ???
RE: Pile Cap Design for Asymmetric Pile Group
RE: Pile Cap Design for Asymmetric Pile Group
Depending on what the plan shows, would driving 1 or 2 additional piles (with appropriate pile spacing) to create a larger 5 or 6-pile group be an option?
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RE: Pile Cap Design for Asymmetric Pile Group
Is the pile group resisting only applied compression load? No shear, moment, or tension? I'll assume so for now.
There isn't much out there for pile cap software and what does exist usually deals only with regular pile layouts. You'll likely need to do some hand calculations. if you decide that a reanalysis is justified. CRSI publishes an excellent pile cap design guide which would be my go to tool for such an exercise.
In a potential reanalysis, you'll want to consider:
1) the revised load on the piles which shouldn't be a problem.
2) the revised flexure in the cap which shouldn't be a problem.
3) one and two way shear. The angle of the compressive struts connecting the piles to the column will have changed.
4) development of flexural reinforcement beyond the column face. In a tight, four column group, this can be an issue.
5) you may now violate geotechnical requirements for minimum pile spacing.
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 Cap Design for Asymmetric Pile Group
1. Design the pile cap for a selected eccentricity in both directions. In the past I have added an Mxx and Myy of P x 3" eccentricity with reasonable success. If allowed and desired, consider an additional pile overstress in the eccentric design (I believe IBC has a 10% allowance for as-built conditions). Warning - this may require additional piles to be added to the design and I suggest this be discussed with the owner prior to implementing.
2. During installation, have the piles surveyed on a regular schedule. Don't wait until you go to the field to find the piles out of location (hint - they are always out of location). In rare cases, you will need to tell the installer to place another pile. If he hasn't demobilized, you get better options. If you are partnered with the installer, you can even run the numbers while he is installing the piles so that you can provide corrected locations for the last piles in each pile cap.
3. Once you have a pile group, run the numbers for final pile cap eccentricity (P/A +- M y / I). If its less than what was designed for, great. Otherwise, tighten the numbers using the actual pile allowables. If a pile is overstressed then consider an additional pile or use a grade beam tied to another pile cap to balance out the eccentricity - I use the term "pump handle" or strap beam, but whatever you call it, you use it to transfer some shear and bending from one adjacent cap into another.
4. Not done yet. Some of those piles will be outbound from the edge of the pile cap by more than what was designed. You need to detail a bump out for those piles in the pile cap so that the hooked reinforcement is developed to the outside edge of each outbound pile and lapped in with the main reinforcement. Further, if the design eccentricity was exceeded, then you need to check the main reinforcement. Finally, have a quick look at 2-way shear for any piles that encroach on the adjacent pile spacing.
As for software, Excel has worked for me. If the design assumptions were reasonable, this becomes more of a data entry issue than design.
RE: Pile Cap Design for Asymmetric Pile Group
SlideRuleEra: that building wasn't designed by me so i don't have the soft copy of that drawing yet. but as soon i will get them i will post to this thread for more discussion and group brainstorming.... :)
KootK: i know that most of the as cast pile location is not as per drawing but still it needs some verification. i will check the flexural reinforcement as suggested.
Teguci: the project is a 6-storied building of one of my colleague. i just payed a random site visit and found these. the contractor has demobilized all the equipment already. so, further pile installation will be difficult now. transferring shear and bending by connecting beam to the adjacent pile cap is now available option. but still i will make some calculations
Now, can anybody give me any idea about constructing a irregular pile cap to adjust the C.G of the reactions of pile group into the column C.G ? is it possible ?
RE: Pile Cap Design for Asymmetric Pile Group
RE: Pile Cap Design for Asymmetric Pile Group
so, it is better that that we try to stabilize the eccentric loading of column by the strap beam.
thanks for your advice
if anyone find something more then please do share.....