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 (2-3) D (pile diameter) in case of isolated pile caps and (2-6) 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 10-15 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 Mx , My العزوم حول المحورين x ,y

 x , y المسافة بين المحور y والمحور x الى أى خازوق فى المجموعة

 εx2 , ε y2 عزوم القصور للمجموعة محسوبة كما فى المعادلة التالية :

Ix = Io + A . X2

بإهمال  Io  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



Strut-and-tie model

The strut-and-tie 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 (As) 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:    


§        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 (2-3) D in case of isolated pile caps and (2-6) 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 10-15 cm.


3- Pile Cap Preliminary Depth:

The depth of the pile cap could be preliminary estimated assuming an allowable punching stress of 10 kg/cm2 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.