Design of
Pile Caps
10.1
Introduction
A pile cap is defined as a concrete block cast on the head of a group of
piles, to transmit the load from the structure to the
group of piles. Generally, pile cap transfers the load
form the structures to a pile group, then the load further
transfers to firm soil.
External pressures on a pile are likely to be greatest near the ground
surface. Ground stability increases with depth and
pressure. The top of the pile therefore, is more
vulnerable to movement and stress than the base of the
pile. Pile caps are thus incorporated in order to tie the
pile heads together so that individual pile movement and
settlement is greatly reduced. Thus stability of the pile
group is greatly increased.
The functions of a pile cap are:
1.
To distribute a
single load equally over the pile group and thus over a
greater area of bearing potential,
2.
To
laterally stabilise individual piles thus increasing
overall stability of the group. And
3.
To
provide the necessary combined resistance to stresses set
up by the superstructure and/or ground movement.
Pile caps
are thick slabs used to tie
a group of piles together to support
and
transmit column loads to the piles.

10.2
Pile Cap Arrangement
§
Spacing of the
piles in the pile group
The following should be considered when determining the spacing of the
piles:
1.
Overall cost of
the foundation
2.
Nature
of the ground
3.
Pile
behaviour in the group
4.
Resulting
possible heave or compaction of ground causing damage to
adjacent structures
5.
Cost
of pile cap
6.
Size
and effective length of ground beam
7.
Type
and size of pile
§
Piles should be placed in a suitable arrangement so
that the spacing between piles ranges from (23) D
(pile diameter) in case of isolated pile caps and
(26) D in case of rafts supported on piles.
§
The C.G. of piles should be placed as far as
possible in the C.G. of loads transmitted from
the structure to the group of piles.
§
In the case of presence of neighbors, piles should
be away from the property line by a distance not less than
D or as the pile installation method requires.
§
The projection of the pile cap should be 1015 cm.
Initial Layout:
The simplest pile layout is one without
batter piles. Such a layout should be used if the
magnitude of lateral forces is small. Since all piles do
not carry an equal portion of the load, axial pile
capacity can be reduced to 70 percent of the computed
value to provide a good starting point to determine an
initial layout. In this case, the designer begins by
dividing the largest vertical load on the structure by the
reduced pile capacity to obtain the approximate number of
pile. If there are large applied lateral forces, then
batter piles are usually required. Piles with flat batters
2.5 (V) to 1 (H), provide greater resistance to lateral
loads and the less resistance to vertical loads. Piles
with steep batters 5 (V) to 1 (H) provide greater vertical
resistance and less lateral resistance.
Final Layout:
After the preliminary layout was
developed remaining load cases should be investigated and
the layout revised to provide an efficient layout. The
goal should be to produce a pile layout in which most
piles are loaded as near to capacity as practical for the
critical loading cases with tips located at the same
elevation for the various pile groups within a given
monolith. Adjustments to the initial layout by the
addition deletion, or relocation of piles within the
layout grid system may be required. Generally, revisions
to the pile batters will not be required because they were
optimized during the initial pile layout. The designer is
cautioned that the founding of piles at various elevations
or in different strata may result in monolith instability
and differential settlement.
Typical Arrangement of Piles
•
Requirements for
Pile Caps
Same as spread footings with the following additions:
1.
Design must
satisfy the punching shear in the vicinity of the
individual piles or shafts
2.
The effective
depth d must be at least 30 cm. This implies a
minimum thickness T of 40 cm.
3.
The bearing
force between the individual piles or shafts and the caps
must not exceed the capacity of either element.
•
Pile Cap
Reinforcement
The amount of pile cap
reinforcement is governed by:
1.
The loading on the pile cap,
2.
The spacing of the piles, and
3.
The depth of the pile cap.
10.3 Load Distribution
To
a great extent the design and calculation (load analysis)
of pile foundations is carried out using computer
software. For some special cases, calculations can be
carried out using the following methods
For
a simple understanding of the method, let us assume that
the following conditions are satisfied:
1.
The pile is
rigid
2.
The pile is
pinned at the top and at the bottom
3.
Each pile
receives the load only vertically (i.e. axially applied );
4.
The force P
acting on the pile is proportional to the displacement U
due to compression.
حيث M_{x} , M_{y} العزوم
حول
المحورين x ,y
x
, y المسافة
بين
المحور y
والمحور x
الى أى
خازوق فى
المجموعة
åx^{2}
, å
y^{2} عزوم
القصور
للمجموعة
محسوبة
كما فى
المعادلة
التالية :
I_{x} = I_{o} + A . X^{2
}
بإهمال
I_{o} لصغر
قيمته،
وحذف الحد A (حيث (A) مساحة
مقطع
الخازوق )
من
المعادلة
، نجد أن
حمولة
الخازوق
الناتجة
عن العزوم
المطبقة
على
القيمة هى
المبينة
فى
المعادلة :
Eccentricity of load
( Single )
Eccentricity of load
( Double)
Graphical
Method
Installation error:
Until now we have been calculating
theoretical force distribution on piles. However during
installation of piles slight changes in position do occur
and piles may miss their designed locations.
So the designer must compare theoretical
and the actual load distribution as a result of
misalignment after pile installation.
Deviation of the piles
Most piling specifications permit a deviation in pile position of not
exceeding 75 mm in any direction from the intended
position. Additional deviations of 1:75 from the vertical
piles and 1:25 from the designed rake for raking piles are
also permitted.
Thus, the pile cap should be large enough to accommodate those piles
which have deviated from the intended position. The pile
cap should extend for a distance of 100 to 150 mm outside
the outer face of the piles in the group.
Location and Alignment
Tolerance:
The pile head at cutoff elevation shall be within 50 mm of plan locations
for bent caps supported by piles, and shall be within
150mm of plan locations for all piles capped below final
grade. The as – driven centroid of load of any pile
group at cutoff elevation shall be within 5% of the plan
location of the designed centroid of load.
No pile shall be nearer than 100mm from any edge of the cap. Any increase
in size of cap to meet this edge distance requirement
shall be at the Contractor’s expense.
Piles shall be installed so that the axial alignment of the top 3m of the
pile is within 2% of the specified alignment. For piles
that cannot be inspected internally after installation, an
alignment check shall be made before installing the last
1.5m of pile, or after installation is completed provided
the exposed portion of the pile is not less than 1.5m in
length. The Engineer may require that driving be stopped
in order to check the pile alignment. Aligned section on a
misaligned section shall not be permitted.
If the location and/ or alignment tolerances specified in the preceding
paragraphs are exceeded, the extent of overloading shall
be evaluated by the Engineer. If in the judgment of the
Engineer, corrective measures are necessary, suitable
measures shall be designed and constructed by the
Contractor. The Contractor shall bear all costs, including
delays, associated with the corrective action.
10.4 Design of Pile Cap
•
If the pile group is analyzed with a flexible base,
then the forces required to design the base are obtained
directly from the structure model.
•
If the
pile group is analyzed with a rigid base, then a separate
analysis is needed to determine the stresses in the pile
cap.
•
An
appropriate finite element model (frame, plate and plane
stress or plane strain) should be used and should include
all external loads (water, concrete, soil, etc. ) and pile
reactions.
•
There are many methods for designing pile caps from
which we could mention the following:
1
Circulage Method
2
Beam Method
3
FEM methods
10.4.1 Circulage Method
•
Circulage method can only be used when the column is
loaded with an axial force and piles are arranged on the
circumference of a circle. Piles are not allowed to carry
horizontal forces in this case.
•
As it is shown in the following figure, the force
T’ for which the reinforcement is calculated is
calculated using the shown force diagram.
Force
Transmission in Circulage Method
Strutandtie model
The strutandtie model should be considered for the
design of deep footings and pile caps or other situations
in which the distance between the centres of applied load
and the supporting reactions is less than about twice the
member thickness.
Struts
and ties in a pile cap
The main reinforcement (A_{s})
can then be calculated from the following relation:
10.4.2 Beam Method
•
The Beam Method is the most widely used method as it
suitable for any type of loading and any shape of the pile
cap.
Design Procedure:
A Required Data:
Pile Data: 1 Pile
diameter and length,
2
Pile allowable bearing capacity
Column Data: 1 Column load (N + M + H),
2 Column dimensions
B Design Steps:
1 Determine required number of piles:
Notes:
§
In case of (N) only multiply by 1.1
§
In case of (M+N) multiply by 1.2
§
Number of piles used is rounded to the upper integer
2 Pile Cap Arrangement and Plane
Dimension:
§
Piles should be placed in a suitable arrangement so
that the spacing between piles ranges from (23) D
in case of isolated pile caps and (26) D in case
of rafts supported on piles, where D is the pile
diameter.
§
The C.G. of piles should be placed as far as
possible in the C.G. of
loads.
§
In the case of presence of neighbors, piles should
be away from the property line by a distance not less than
D or as the pile installation method requires.
§
The projection of the pile cap should be about 1015
cm.
3 Pile Cap Preliminary Depth:
The depth of the pile cap could be
preliminary estimated assuming an allowable punching
stress of 10 kg/cm^{2} on the column face.
4 Check Forces in Piles:

5 Check for punching shear:
6 Check for shear:
7 Design for moment:
The critical section for moment is taken
at the column face.
8 Check for Bond:
•
The reinforcement used in resisting flexural moment
should be checked for bond stress acting on it.
•
Shear at the same section of the bending moment is
calculated.
9
Details of reinforcement:
10.4.3 FEM Method
Grid used for FLAC 3D analysis of pile groups
(After Poulos, 2001)
10.5 Grade Beams
•
Deep foundations are
sometimes connected with grade beams.
•
Grade beams are required for
all deep foundations subject to seismic loads. For seismic
design, they must resist a horizontal load equal to 10% of
the column vertical load.
•
Grade beams must be designed
without the support of the underlying soil.
•
In the British Standard Code
of Practice BS 8004, a ground beam is defined as a beam in
a substructure transmitting load(s) to a pile, pad or
other foundation. The ground beam connects the two pile
caps.
•
Ground beams should not be
confused with capping beams. Capping beams perform the
same function as pile caps. However, the function of a
ground beam is to connect adjacent pile caps to ensure
stability of the foundation and to ensure stability
against lateral forces.
•
Ground beams are designed to
connect a group of pile caps in a continuous manner.
•
The top and bottom
reinforcement of a ground beam are usually made equal to
overcome lateral forces or settlement of one pile cap
relative to the adjacent one.
•
Ground beams may also
require shear reinforcement in the form of binders.
•
The depth of the ground beam
is usually more than 1/15 of the span. The width of the
beam depends on design requirements.
Ground beams can also be
designed to transmit loads from walls to pile caps.
