Pile Cap Design for Asymmetric Pile Group 1

[AshiqRumi]

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 ???

 

[hokie66]

A plan would help, but without a rectifying beam, you won't be able to have equal loading on all piles.

 

[AshiqRumi]

what do you mean by "rectifying beam" ??
would you please explain in more details ?
shall i inter-connect the piles with beam ???

 

[hokie66]

No, the beam would be to another structural element, probably the next pile cap in the direction of the eccentricity. Sometimes called a strap beam.

 

[SlideRuleEra]

Please show the plan that hokie66 requested, include the design location of all 4 piles and the actual location of all 4 piles.

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?

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[KootK]

Most designers will design a pile group and cap conservatively and for 3"-6" of pile misplacement to begin with. As a result, a 12" misplacement probably isn't that big of a deal unless the piles are spaced very tightly together.

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.

 

[Teguci]

Method for dealing with as-built pile locations.
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.

 

[AshiqRumi]

hokie66: thanks for your advise for a strap beam. I'm taking that as an potential option for balancing the moment due to eccentric loading. can you add anything more ?


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 ?

 

[hokie66]

No, you can't do what you are suggesting just by redesigning the pile cap. Without something like a strap beam, the piles will be take unequal loading. Perhaps they are capable of higher loads. How do you know the actual pile capacities?

 

[AshiqRumi]

hokie66 you are right. it will be difficult to find out the actual pile capacity on a unsymmetrical situation. it can be even more than the consideration.
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.....