Structural Aspects of Pile Foundation Design: A Practical Example

Geotechnical engineers are expected to hand over soil investigation report to structural engineers, who will proceed to provide the longitudinal reinforcements needed for the piles and also design the pile cap. Pile cap design is an important aspect of pile foundation construction, and has been presented in this post. The soil investigation report handed to the structural engineer for this purpose should contain the embedment length of the piles, the sizes of the pile, the safe working load of each pile size, and other information that may be necessary for the structural engineer to do his design properly.


 

The first step usually involves obtaining the number of piles required to support a column load. This is usually done using the service loads of the column, and relating them to the safe working load of the piles from soil investigation report. In this post, we are going to show how the structural design of pile foundations and pile caps can be done based on practical design and site experience.

Design Example
The frame of a 5 storey building is shown in Figure 1, and it is expected to be supported on piles with an embedment length of 20 m. The allowable working loads of the bored pile (CFA) is shown in Table 1. fy = 460 Mpa, fcu = 30 Mpa

Table 1: Pile safe working load

 

Building frame modelled on Staad Pro
Fig 1: Frame of a 5-storey building
Fig 2: Column load layout

Design for Column A1
Service axial load on column = 647 kN
Size of column = 450 x 230 mm

Using 2 No of piles
Service load per pile = 647/2 = 323.5 kN
Let us adopt 600 mm diameter piles for uniformity and to have fewer pile boring points

Safe working load of φ600 mm piles = 493.48 kN > 323.5 kN Okay

Spacing of piles = 3φ = 3 x 600 = 1800 mm
Overhang of pile cap edge from the pile = 150 mm

Total length of pile cap = 1800 + 600 + 2(150) = 2700 mm
Width of pile cap = 600 + 150 + 150 = 900 mm
Thickness of pile cap = 2φ + 100 = 2(600) + 100 = 1300 mm

The layout of the pile cap is therefore given as shown in Figure 3.

Structural arrangement of column supported by two piles
Fig 3: Pile cap type 1

Let us quickly carry out the structural design of pile cap Type 1 according to BS 8110-1:1997. You can also read Design of Pile Cap According to Eurocode 2.

From Table 3.61 of Reynolds et al. (2008), the tensile force to be resisted within the pile cap is given by;

Ft = N/(12ld)[3l2 – a2]


Where;
N = Column axial load at ultimate limit state
l = length of the pile cap
d = Effective depth of the pile cap 
a = dimension of the column side parallel to the length of the pile cap

N = 885 kN + (1.4 x 1.3 x 2.7 x 0.9 x 24) self weight of pile cap at ultimate limit state = 991.142 kN
l = 1.8 m
d = 1300 – 100 = 1200 mm = 1.2 m
a = 0.45 m

Ft = [991.142/(12 x 1.8 x 1.2)] x [3 x 1.82 – 0.452] = 364 kN
Ast = Ft/0.95fy = (364 x 1000)/(0.95 x 460) = 833 mm2
Asmin = 0.13bh/100 = 1690 mm2
Provide 6T20 @ 175 c/c (Asprov = 1974 mm2)

Check for shear
Critical position for shear on vertical section across full width of pile-cap occurs at distance from face of column given by:
av = 0.5(l – c) – 0.3φ = 0.5(1800 – 450) – (0.3 x 600) = 495 mm

The shear force carried by the piles V = 991.142/2 = 495.571 kN

The shear stress ν = V/bd = (495.571 x 1000)/(900 x 1200) = 0.458 MPa
Concrete resistance shear stress vc = 0.632(100As/bd)1/3(400/d)1/4


vc = 0.632 x [(100 x 1974)/(900 x 1200)]1/3 x (400/1200)1/4 = 0.632 x 0.557 x 0.759 = 0.275 MPa
For grade 30 concrete, vc = 0.275 x (30/25)1/3 = 0.292 Mpa
vc(2d/av) = 0.292 x [(2 x 1200)/495] = 1.415 MPa > 0.458 MPa  This is okay

Shear stress at column perimeter
ν = V/ud = (885 x 1000)/[(2 x 225 + 2 x 450) x 1200] = 0.546 MPa
This is less than 0.8√fcu = 4.38 Mpa. Therefore, this is okay.

Anti-burst bars of T12 @ 200 spacing should be provided
Main bars should be returned at least 900 mm into the sides to satisfy anchorage length requirements. You can take anchorage length to be conservatively 50 x diameter of reinforcement = 50 x 20 = 1000 mm

Design of Pile Cap Type 2
Service axial load on column = 1077 kN
Ultimate axial load on column = 1476 kN
Size of column = 450 x 230 mm
The number of φ600 mm piles required = 1077/493.48 = 2.184

Using 3 No of φ600 piles
Service load per pile = 1077/3 = 359 kN

Safe working load of φ600 mm piles = 493.48 kN > 359 kN This is okay
Let us adopt a triangular pile cap arranged in such a way that the column load will be equally distributed to the piles. This arrangement can be found in Table 3.16 of Reynolds et al (2008), and it is shown in Figure 4.

Layout of column supported by three piles for equal load distribution
Fig 4: Dimensions of triangular pile cap for equal load distribution (Reynolds et al, 2008)

hp = φ = diameter of pile = 600 mm
Spacing of piles = 3φ = 3 x 600 = 1800 mm
Overhang of pile cap edge from the pile = 150 mm
(α + 1)φ + 300 = (3 + 1)600 + 300 = 2700 mm
φ + 250 =  600 + 250 = 850 mm
φ + 300 =  600 + 300 = 900 mm
(6α/7 + 1)φ + 300 = 2442.857 mm (say = 2445 mm)
(2α/7 + 0.5)φ + 150 = 964.285 mm (say = 965 mm)
Thickness of pile cap = 2φ + 100 = 2(600) + 100 = 1300 mm

The layout of the pile cap is shown in Figure 5.

Structural layout of column supported by three piles
Fig 5: Structural arrangement of 3 pile caps

Self weight of the pile cap (ULS) = 1.4 x Area x depth x 24 kN/m3 = 1.4 x 5.166 m2 x 1.3m x 24 kN/m3 = 225.61 kN
Total load on pile cap at ULS = 1476 kN + 225.61 kN = 1701.61 kN

Tensile force to be resisted by the reinforcement in the direction parallel to X-X;
Ft,x = N/(36ld)[4l2 + b2 – 3a2]
Ft,x = [1701.61/(36 x 1.8 x 1.2)] x [4 x 1.82 + 0.452 – 3 x 0.2252]  = 284 kN

Tensile force to be resisted by the reinforcement in the direction parallel to Y-Y;
Ft,y = N/(18ld)[2l2 – b2]
Ft,y = [1701.61/(18 x 1.8 x 1.2)] x [2 x 1.82 – 0.452]  = 275 kN

Let us the highest value for design in anticipation that we will provide the same reinforcement in both directions
Ast = Ft/0.95fy = (284 x 1000)/(0.95 x 460) = 649 mm2
Asmin = 0.13bh/100 = 1690 mm2
Provide T20 @ 175 c/c in both directions (Asprov = 1974 mm2)

Shear resistance
The shear force carried by the piles V = 1701.61/3 = 567.2 kN
The shear stress ν = V/bd = (567.2 x 1000)/(1000 x 1200) = 0.472 MPa
vc(2d/av) = 0.292 x [(2 x 1200)/495] = 1.415 MPa > 0.472 MPa  This is okay
Shear will obviously not be a problem.

Design of Pile Cap Type 3
Service axial load on column = 1825 kN
Ultimate axial load on column = 2545 kN
Size of column = 400 x 400 mm
The number of φ600 mm piles required = 1825/493.48 = 3.69

Use 4 No of φ600 piles
Service load per pile = 1825/4 = 456.25 kN

Safe working load of φ600 mm piles = 493.48 kN > 456.25 kN This is okay

Let us adopt a square pile cap arranged in such a way that the column load will be equally distributed to the piles. This arrangement can be found in Table 3.16 of Reynolds et al (2008), and it is shown in Figure 6.

Structural layout of column supported by four piles
Fig 6: Pile Cap Type 3

Self weight of the pile cap (ULS) = 1.4 x Area x depth x 24 kN/m3 = 1.4 x 7.29 m2 x 1.3m x 24 kN/m3 = 318.43 kN
Total load on pile cap at ULS = 2545 kN + 318.43 kN = 2863.43 kN

Tensile force to be resisted by the reinforcement in both directions;
Ft = N/(12ld)[3l2 – a2]
Ft = [2863.43/(24 x 1.8 x 1.2)] x [3 x 1.82 – 0.452]  = 525.71 kN

Ast = Ft/0.95fy = (525.71 x 1000)/(0.95 x 460) = 1203 mm2
Asmin = 0.13bh/100 = 1690 mm2
Provide T20 @ 175 c/c in both directions (Asprov = 1974 mm2)

Shear resistance
The shear force carried by the piles V = 2863.43/4 = 715.9 kN
The shear stress ν = V/bd = (715.9 x 1000)/(1000 x 1200) = 0.595 MPa
vc(2d/av) = 0.292 x [(2 x 1200)/495] = 1.415 MPa > 0.472 MPa  This is okay

The structural engineer is expected to provide the following drawings;

(1) Setting out drawing showing the piling points and layout with a known reference point
(2) General column/pile cap layout/arrangement
(3) Pile cap/ground beam/ground floor slab layout
(3) Columns, piles and pile cap reinforcement drawings (detailing)
(5) Ground beams and ground floor slab reinforcement detailing
(6) Construction procedure sketches and other site instruction booklet

Construction Considerations
(1) Ground beams are usually provided to chain the pile caps together, and to provide the needed support for the ground floor slab. There are construction scenarios where the ground floor slab is placed directly on the pile caps, but note that this concept is quite different from piled raft foundation. The ground beams are usually embedded into the pile caps, or may be allowed to sit directly on the pile caps depending on the site level. A typical construction drawing showing this interaction is given in Figure 7.

(2) Contractor should maintain a minimum concrete cover of 75 mm

(3) There may be need to be cast the pile cap in two stages to achieve the configuration shown in Figure 7.  The first casting will get to the bottom level of the ground beams (see Figure 8), then the ground beam reinforcements are laid (see Figure 9), before the final casting of the pile cap and ground beams to the required level (see Figure 10). Read about bonding of old and new concrete.

Pile cap and ground beam arrangement
Concreting of pile cap
Figure 8: Typical casting of pile cap to ground beam level.
Ground beam reinforcement
Fig 9: Typical ground beam reinforcement arrangement
Reinforced concrete beam and pile cap
Fig 10: Completed pile cap and ground beam

On getting to the stage shown in Figure 10, the bays are filled with sharp sand, and the ground floor slab is cast as appropriate.

Do you need help in design, consultancy, production of construction drawings, supervision, and project management, contact us today at Structville Integrated Services Limited. We are excellent at what we do, and we pride ourselves in professionalism and integrity. Send an e-mail now to info@structville.com copy structville@gmail.com or send a whatsapp message to +2347053638996.

References
[1] Reynolds C.E., Steedman J.C., Threlfall A.J. (2008): Reynolds’ Reinforced Concrete Designer’s Handbook 11th Edition. Taylor and Francis, New York

6 COMMENTS

  1. Dear Sir,
    I was hoping to get a copy of this report/ design guide. It’s very nice. I am doing a research report for dynamic foundation on piles for my MS Degree and looking for good resources.
    Thanks,
    Jeff Leach

  2. what is the allowable settlement limits for the piles. what about the effect group efficiency factor. How about modeling them on the FEM packages with Modulus of subgrade reaction & what will be value of modulus of subgrade reaction to be adopted in that case.

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