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6.1 PILE CAP FOR 3 PILE GROUP(3PC1)
These pile caps provided at three locations in north west of the platform columns. The staad reactions from the steel structure
received from IEC and the reactions are converted into forces on foundations with respect to design sheet access as given below.
Reactions have been attached in Appendix-A for reference.
A) Pile Cap Details & Load Calculations
Pile Cap Details
Size of pile cap
Length
Width
Lx =
Lx1 =
Lz =
1.850
0.750
2.000
Lz1 =
0.750
Depth of the pile cap
Hpc =
Height of soil below HPP
h1
1.50 m
100 - 99
1.000 m
Height
Hp =
Length
lx
1.300 m
0.75 m
Width
lz
=
=
0.75 m
0.80 m
Pedestal Details
Size of Column/Pedestal
Centre of the pedestal from to top edge of pile cap
400
1200
400
Design Sheet Axis
750
P1
P2
400
650
1850
C.L of Fdn
Z
P3
400
Staad Model Axis
400
750
C.L of Fdn
2000
(U/S of base plate)
EL
100.300
EL
100.000
H.P.P
750x750
EL
99.000
EL
97.500
1500
H =2.80
40 thk Grout
As the column centre line is located in line with the centre of the piles P1 & P2 and placed in between the piles. So,
considered the pile cap acting like a two pile cap with pile P1 & P2 tied with the another pile P3.
Based on the above assumption, super structure loads i.e., vertical load(Fy), horizontal loads in z-direction(Fz) and respective
moment(Mx) are transferred through piles P1 & P2 and remaining loads horizontal loads in x-direction(Fx) and respective
moment(Mz) are transferred through piles P1, P2 & P3.
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Pile Details
S.No
1
2
3
Pile No.
P1
P2
P3
z
m
0.400
1.600
1.000
3.000
REV.
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x
m
0.400
0.400
1.450
2.250
Total number of piles
Number of piles in line with column in Z-direction
C.G of the pile group xg
n=
n1 =
C.G of the pile group zg
3
2
(x )/n
0.750
=
=
(z )/n =
x1 - x3
1.050 m
z1 - z2
1.200 m
1.000
Lever arm between the piles about Z axis
Zzz
Section Modulus of Pile group about X axis
Zxx =
Self Weight of Foundation
Area of pile cap
Area of Pedestal
Area of soil above the pile cap
Weight of the pile cap
Weight of the Pedestal
Weight of the soil above the pile cap
Consider Surcharge Load =
10 kN/m2
Total weight of foundation excluding surcharge weight
Total weight of foundation including surcharge weight.
Apc =
=
=
Apd =
=
=
As =
=
=
W pc =
=
=
W pd =
=
=
Ws =
=
=
W sur =
Lz*Lx1+0.5*(Lz+Lz1)*(Lx-Lx1)
2*0.75+0.5*(2+0.75)*(1.85-0.75)
3.01 m2
lz * lx
0.75*0.75
0.56 m2
Apc -Apd
3.01-0.5625
2.45 m2
Apc*Hpc*c
(3.01*1.5*24)
108.45 kN
Apd*Hp*c
(0.56*1.3*24)
17.55 kN
As*h1*s
(2.45*1*18)
44.10 kN
10*As
=
=
10*2.45
24.50 kN
W pc+W s+W pd
=
=
108.45+44.1+17.55
170.1 kN
W pc+W s+W pd+W sur
=
=
108.45+44.1+17.55+24.5
194.6 kN
Eccentricity of the Pile Cap
C.G of the pile cap weight from top edge of pile cap
in X-direction
(Lz*Lx1*Lx1/2+0.5*(Lz+Lz1)*(Lx-Lx1)*(Lx1+1/3*((2*Lz1+Lz)/
(Lz+Lz1)))/ (Lz*Lx1*Lx1/2+0.5*(Lz+Lz1)*(Lx-Lx1))
(2 0.75 0.75/2+0.5 (2+0.75) (1.85
= 0.75)*(0.75+1/3*(2*0.75+2)/ (2+0.75)))/
(2*0.75+0.5*(2+0.75)*(1.85-0.75))
m
=
0.776
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C.G of the pile group and the pile cap are closer to each other. So, consider the 50% of the dead weight due to self weight of
the pile cap and soil weight transferring through pile P3 and remaining 50% equally shared by piles P1 and P2.
Pile-P1 Pile-P2 Pile-P3
Foundation dead weight on piles * -170.1*0.25
= Without Surcharge Load(Fd) 42.53*
42.53 85.05
With Surcharge Load(Fsd)
** -194.6*0.25
48.65** 48.65 97.3
Each support is checked for all the unfactored load combinations and only critical load cases are summarised.
a) Loads at the top of pedestal
STAAD
S.No
Load comb
NodeNo.
1
2
3
4
603
629
603
649
FX
kN
62.00
60.50
82.27
0.50
1300
1300
1200
1400
FY
kN
FZ
kN
-62.00
-60.60
-1.00
-82.00
1032.70
-866.00
723.00
-561.50
MX
kNm
MY
kNm
MZ
kNm
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Max.Vertical
Min.Vertical
Max.Hor
Min.Hor
b) Loads at the bottom of pile cap(Except Dead Weight of Foundation)
MXB = MX + FZ * H
FXB = FX
FYB = FY
MZB = MZ - FX * H
FZB = FZ
S.No
STAAD
NodeNo.
Load comb
FXB
FYB
FZB
MXB
MYB
MZB
kN
kN
kN
kNm
kNm
kNm
603
1300
62.00
1032.70
-62.00
-173.60
0.00
-173.60
629
1300
60.50
-866.00
-60.60
-169.68
0.00
-169.40
603
1200
723.00
0.00
-230.36
649
1400
-1.00
-82.00
-2.80
82.27
0.50
-229.60
0.00
-1.40
-561.50
B) Analysis of Pile Group
a) Check for compression capacity of the pile
Maximum compression load on the Pile
Node No. 603 , L.C
Max compressive load, P1
110
= FYB/n1 + Fsd - MXB/Zxx - MZB/(Zzz*n1)
= CHECK N.No. & L.C
= CHECK kN
P2
= FYB/n1 + Fsd + MXB/Zxx - MZB/(Zzz*n1)
= CHECK N.No. & L.C
= ECK N.No. & kN
P3
Fsd - MZB/(Zzz)
= CHECK N.No. & L.C
= CHECK kN
Pile Loads (kN)
P2
P3
Pmax
Pmax,allow
kN
kN
S.No
STAAD
NodeNo.
Load comb
603
1300
792.34
503
262.64
792.3
870.0
Safe!
2
3
4
629
603
649
1300
1200
1400
-162.29
522.18
-40.1
-445.09
517.52
-422.77
258.64
316.7
98.64
258.6
522.2
870.0
870.0
Safe!
Safe!
98.6
870.0
Safe!
P1
The Pile capacity increased by 25%, as the critical loads are combination of the wind loads
Check
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b) Check for tension capacity of the pile
Minimum load on the Pile
12
, L.C
Node No.
Min compressive load, P1
110
= FYB/n1 + Fd - MXB/Zxx - MZB/(Zzz*n1)
= CHECK N.No. & L.C
= ECK N.No. & kN
= FYB/n1 + Fd + MXB/Zxx - MZB/(Zzz*n1)
P2
= CHECK N.No. & L.C
= ECK N.No. & kN
P3
Fd + MZB/(Zzz)
= CHECK N.No. & L.C
= CHECK kN
S.No
1
2
3
STAAD
NodeNo.
Load comb
603
629
603
1300
1300
1200
Pmin
Pmin,allow
P1
P2
P3
kN
kN
786.22
-168.41
516.06
496.88
-451.21
511.4
-80.29
-76.29
-134.35
-80.3
-451.2
-450.0
-450.0
Safe!
Unsafe!
-134.4
-450.0
Safe!
Pile Loads (kN)
Check
Safe!
-428.9
-450.0
4
649
1400
-46.22
-428.89
83.72
The Pile capacity increased by 25%, as the critical loads are combination of the wind loads
The tension capacity of the piles at node 12 need to increase by increasing the length of the reinforcement in the pile.
c) Check for shear capacity of the pile
Resultant Horizontal Force
12
, L.C
Node No.
Min compressive load, P1
127
= Sqrt((Fx/n)^2 + (Fz /n1)^2)
= CHECK N.No. & L.C
= CHECK kN
P2
= Sqrt((Fx/n)^2 + (Fz /n1)^2)
= CHECK N.No. & L.C
= CHECK kN
P3
= Fx/n
= CHECK N.No. & L.C
= CHECK kN
STAAD
NodeNo.
Load comb
1
2
603
629
603
S.No
Pile Loads (kN)
Hmax
Hmax,allow
Check
1300
1300
P1
37.26
36.4
P2
37.26
36.4
P3
20.67
20.17
kN
37.3
kN
87.0
36.4
87.0
Safe!
1200
27.43
27.43
27.43
27.4
87.0
Safe!
The Pile capacity increased by 25%, as the critical loads are combination of the wind loads
Safe!
