Pile cap design structural guide

Structural loads, structural analysis and structural design are simply explained with the worked
example for easiness of understanding. Element designs with notes and discussions have added to get
comprehensive knowledge. Also, construction materials, shoring system design, water retaining
structures, crack width calculations, etc. have discussed in addition to other aspects.
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FOUNDATION DESIGN
Pile Cap Design
BY PRASAD ·
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Pile caps are constructed to hold the pile and superstructure together at
the ground level or below the ground level while transferring loads of the
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superstructure to the foundation.
Generally, pile caps are constructed to connect one pile, two piles, three
piles, four piles or a group of files. Dimensions of the pile caps are decided
based on the loads and connecting arrangement of the superstructure and
the pile foundation.
Pile caps can be designed using the truss analogy or using the bending
theories.
Mostly, up to the four numbers of piles are connected by a pile cap to
support the concentrated load from the superstructure, truss theory is
used to design the pile caps.
Equations derived considering the strut and tie model are available to
calculate the area of reinforcement.
The following figure indicates the typical equations that can be used to
calculate the area of tension reinforcements. It was copied from the book
Reinforced concrete design to BS 8110.
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» Pile Cap Design
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» Pile foundations - Design, Construction
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When there are more piles, the design could be done using finite element
so ware which makes the analysis easier.
In general, the gap between the piles is kept at 2.5 times the diameter of
the piles. It is done to avoid the interaction of one pile on others.
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In addition to the calculation of the tension reinforcements, di erent shear
checks like punching and vertical line shear shall be done. When there are
more than two piles, the selection of the critical shear perimeter shall be
done very carefully. The guideline given in BS 5400 can be used to select
the shear perimeter.
Worked Example: Design of Pile
Cap
Consider the design of a pile cap supporting two pile and a single column
on the pile cap.
Data
Pile Diameter 600mm
Design Load 3000 kN
Cover to the reinforcement 50mm
Grade of concrete 30
Characteristic strength of steel as 500 N/mm2
Size of the column on the pile cap 500x500mm
Calculate the dimensions of the pile cap
consider 150mm o set from the pile and space between piles as 2.5 times
pile diameter.
Width = 500 + 150 + 150 = 800mm

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Length = 2.5 x 600 + 250 + 250+ 150 + 150 = 2150mm
Consider depth of 1000mm and main bar diameter as 20mm
E ective depth, d = 1000-50-20/2 = 940 mm > 750mm; (2.5×600/2) Ok.
Consider truss theory
Tension Force, T = Nl / 2d
T = 3000 x 0.75 / (2 x 0.94 ) = 1197 kN
As = T / 0.87fy
As = 1197 x 1000 / (0.87 x 500 ) = 2752 mm2
Provide 7 T25 (As provided = 3430 mm2
)
Check for Punching Shear
V = 3000 x 103
/ ( 4 x 500 x 940) = 1.596 N/mm2
Vall = 0.8 (fcu)0.5
= 0.8 x (25)0.5
= 4 N/mm2
< 5 N/mm2
Hence, punchig shear is Ok
Check for Vertical Line Shear

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The critical section of the pile shall be considered as 20% of the diameter
of the pile inside of the face of the pile. The following figures extracted
from the code (BS 8110) provide a clear location to be considered for shear
design.
When this section is considered as a critical section and the spacing
between the piles is less than or equals to three times the pile diameter,
enhancement (2d/av) for Vc can be considered. Here, av is the distance from
the face of the column to the critical section.

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In this example, we considered only two piles as shown in the following
image.
Vc and be calculated from Table 3.8 of BS 8110 based on the 100As/bd
value.
100As / bd = 100 x 3430 / ( 1000 x 940 ) = 0.365
Vc = 0.446 N/mm2

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Enhanced shear capacity ; (2d/av)Vc
(2d/av)Vc = ( 2 x 940 / 320 ) x 0.446 = 2.62 N/mm2
Desing shear stress = 1500 x 103
/ (1000 x 940) = 1.596 N/mm2
Hence Ok.
Calculate Area of Distribution Steel
Consider the minimum reinforcement requirement.
100As / Ac = 0.13
As = 0.13 x 1000 x 1000 / 100 = 1300 mm2
/ m
Provide T16@150 mm c/c (As Provided = 1340 mm2
Area of Horizontal Binders
Provide 25% of the design reinforcements
As binder = 0.25 x 2752 = 688 mm2
Provide T12@150 (As provided = 754 mm2
)
Similarly, other types of pile caps can also be designed following the same
procedure. However, when the number of piles is increasing in a pile cap,
the manual calculation becomes more complicated. Computer packages

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