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1. Re: Pile cap design
Re: Pile cap design
Source: http://newsgroups.derkeiler.com/Archive/Sci/sci.engr.civil/2008−03/msg00016.html
• From: "Chuck News" <cidinsmore@xxxxxxxxxxx>
• Date: Wed, 12 Mar 2008 01:33:57 −0700
<shalini.singh97@xxxxxxxxx> wrote in message
news:8db98a3e−1000−4371−9d55−a8cc5565d8c6@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Mar 11, 4:00 pm, shalini.sing...@xxxxxxxxx wrote:
On Mar 11, 12:59 pm, "Chuck News" <cidinsm...@xxxxxxxxxxx> wrote:
> <shalini.sing...@xxxxxxxxx> wrote in message
>news:b9bfee73−1bc0−4ec1−bea1−e56712b22be3@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> > Hi,
> > In regard to design of an RC pile cap, I have read the following in a
> > reference:
> > IF;
> > s = spacing between the piles
> > Nc = column axial laod
> > Mx = column moment about xx−axis
> > My = column moment about yy−axis
> > Pmax = equivalent maximum axial laod on pile group
> > Pmin = equivalent minimum axial load on pile group
> > mx = pile cap design moment about xx−axis
> > my = pile cap design moment about yy−axis
> > vx = design shear due to bending about xx−axis
> > vy = design shear due to bending about yy axis
> > Then,
> > 1. For 2 pile cap arrangement
> > Pmax = Nc + 2*My /S
> > Pmin = Nc − 2*My/S
Re: Pile cap design 1

2. Re: Pile cap design
> > N = P/2
> > my = PS/4
> > mx = 0
> > vx = 0
> > vy = P/2
> > Similarly for a 3 pile cap arrangement,4,5,6,7,8,9 pile cap
> > arrangement [that is 3,4,5,6,7,8,9 piles] formulae were given in the
> > reference.
> > I want to know the basis for these formulae−−−
> > How these formuale are given and based on what principle/concept?
> > Please please help−−−−
> It is very easy. It is a typical method of calculating forces on a > footing.
> You are basically calculating the neutral axis of the piles. When this > is
> done you are calculating the moment of inertia of the pile arrangement > then
> you can calculate the compression forces on the piles and the tension > forces
> on the piles.
> Basically P/N + Mx/I (compression) and P/N−Mx/I (tension) once the force
> is determined you do it for the 'y' access if the column is not on the
> center of axis of the pile arrangement. N is the number of piles Mx of > the
> column moment acting on the piles and being distributed for each pile. > 'x'
> is the distance from the neutral axis which is actually 'Mx'/I the > section
> modulus for the piles at that line.
> Once those are known you can design the pile cap using the forces that > are
> acting on the cap. This includes soil/water, concrete weight, pile. Then
> calculating the required reinforcing. This is done for each direction > and
> combined as needed to code requirements.
> Hope this is enough for you to understand the formulae above.
> CID...− Hide quoted text −
> − Show quoted text −
Thanks a million−−i still have some questions though you explained
very well−−
You said:
"You are basically calculating the neutral axis of the piles. When
this is
done you are calculating the moment of inertia of the pile
arrangement then
you can calculate the compression forces on the piles and the tension
forces
on the piles." Further my questions:
1. See, I have a pile cap which allows the load from the column to be
equally distributed to a series of piles.
Re: Pile cap design 2

3. Re: Pile cap design
Now, when you say "we are calculating the neutral axis of group of
piles".Do you mean the centroidal axis?Then if I have 9 piles arranged
in 3 rows and pile cap dimensions being 3100mm x 3100mm [plan
dimensions] and spacing between the piles being 1200mm,then what will
be the centroid/neutral axis of the pile group?
2. If the diameter of each pile is 400mm then what will be the moment
of inertia of the pile group?
3. Do you need the height of the pile in any calculation of these
formulae in my earlier post?
4. How will be the properties [centroid/moment of inertia] which
pertain to the pile group have influence on pile cap design?− Hide quoted text −
− Show quoted text −
Also,
for a 3 pile cap ararngement the formulae I have is:
Case 1:
Pmax = Nc + sqrt(3) *Mx / S + 3*My/S + Namax
Case2:
Pmax = Nc + 2* sqrt(3) *Mx / S + Namax
Pmax being the greater of the above 2 cases
Again:
Pmin = Nc − sqrt(3) *Mx / S − 3*My/S + Namax
Case2:
Pmin= Nc − 2* sqrt(3) *Mx / S + Namax
"Pmin" being the lesser of the two cases
N =P/3
mx = PS/3*sqrt(3)
my = PS/6
vx = P/3
vy = P/3
By the time you get through doing on the first set of description that I gave to you
you should know what to do with the formulae now. M=moment at the column base, N is the
Re: Pile cap design 3

4. Re: Pile cap design
compression force in the column the other portions are related to what I told you.
S is the section modulus which is the moment of inertia divided by the 'x' distance which is 1200mm.
Keep in mind if you don't have moment at the column base only compression all of this is not necessary.
Without moment all the piles will be the compressor force 'P' or 'N' divided by the number of piles or
P/N
CID...
.
Re: Pile cap design 4