Foundation Loads - Concrete

FOUNDATION LOADS

Pile foundations are design to provide an allowable bearing capacity adequate enough to

resist the applied loads from the superstructure. Design of a pile foundation involves

solving the complex problem of transferring loads from the structure through the piles to

the underlying soil. It involves the analysis of a structure-pile system, the analysis

of a soil-pile system, and the interaction of the two systems, which is highly nonlinear.

These loads are generally transmitted to the foundation by means of the columns of the

structure. The first phase of the design of a pile foundation involves determining the

magnitude of the applied building loads which will be manifested in the form of axial loads

in the columns interacting directly with the foundation.

To determine the magnitude of the foundation loads a structural analysis was carried out

on the frame of the structure using the applied loads. This was done manually.

LOADING ANALYSIS

For design purposes, both dead and imposed loads were considered. The characteristic

dead load (Gk) used accounted for the self weight of the structural members complete with

finishes, fixtures and partitions. Characteristic imposed loads (Qk) were derived in

accordance with the recommendations as provided by BS 6399: Part 1: 1996 Code of

Practice for dead and imposed loads.

The maximum design ultimate load applied for all structural members were computed

using the following formula:

DUL = 1.4 Gk + 1.6 Qk a

a BS 8110: Part 1: 1996 British Standard, Structural use of Concrete, Part 1. Code of Practice for design and construction, British Standard Institution, 1996.

The following is a breakdown of the design loads utilized:

2.1.1 Floor loads

Breakdown of floor loads (internal slabs)

Element Description Magnitude of characteristic loadskN/m2

Dead Loads (Gk)

150 mm thk. R.C slab

50 mm screen

Services and ceiling

Partitions (walls)

Total DL

Imposed Loads (Qk)

Shopping Complex b

3.75

1.25

1.00

1.50

7.50

4.00

Design Ultimate Load =1.4 Gk + 1.6 Qk a

= 1.4(7.5) + 1.6(4.0) = 16.9 kN/m2

This will be taken as the applied uniformly distributed load on the internal floors slabs.

Since all floors carry similar uses this will apply for all other floors.

b Value taken from Table 1.0 BS 6339: Part 1: 1996 Code of Practice for dead and imposed loads. Pg 3.

2.1.2 Beam Loads

Breakdown of beam loads (External)

Element Description Magnitude of characteristic loadskN/m

Dead Loads (Gk)

Self Wt.

Hollow block + rendering

Total DL

Imposed Loads (Qk)

Any fixtures c

2.50

1.00

3.50

0.50


= 1.4(3.5) + 1.6(0.5) = 5.70 kN/m

Breakdown of beam loads (Internal)

Element Description Magnitude of characteristic loadskN/m

Dead Loads (Gk)

Self Wt.

Imposed Loads (Qk)

2.50

0.0


= 1.4(2.5) + 1.6(0.0) = 3.50 kN/m

2.1.1 Column Loads

Breakdown of column loads (both internal and external)

Element Description Magnitude of characteristic loadskN

Dead Loads (Gk)

Self Wt.

Imposed Loads (Qk)

7.80

0.0


= 1.4(7.8) + 1.6(0.0) = 10.92 kN

Since all floors carry similar uses this will apply for all other floors.

2.2 STRUCTURAL ANALYSIS

Manual structural analysis was used to calculated the

Reactions exerted on concrete columns

Column Reaction at Base (kN)A1B1C1D1A2B2C2D2A3B3C3D3A4B4C4D4A5B5C5D5A6B6C6D6

1366.402,556.242,556.24341.602562.564816.004816.004816.002562.564816.004816.004816.002562.564816.004816.004816.002562.564816.004816.004816.00341.602,556.242,556.24341.60

Figure 3 graphically illustrates the variation of the foundation loads for the proposed foundation layout.

3.0 DESIGN OF PILE FOUNDATION

The pile foundation design was done using established pile capacity formulae which take into account the two major contributing elements for pile capacity, mainly:

1. Skin resistance2. End bearing capacity

The final arrangements for the pile are those that provided allowable pile capacities in excess of the applied column loads which is transmitted to the foundation, with a factor of safety greater than 2 (SF˃2). The capacity of a single timber pile passing through the soil profile was determined first, to see whether one pile could have provided the required capacity. For cases where this was not possible, pile groups were used.

The analysis is summarized in the following design sheets.

3.1 Single Pile Capacity

ReferenceDesign Computations

OutputProject: Pile Foundation Design for Shopping Complex (R.C Building)Figure 4 shows the arrangement for a single pile.

Pile capacity is computed as follows:

Qu = Qp + Qs

Where Qu- Ultimate pile capacity

Qp- Load carrying capacity

Qs- Frictional resistance

The allowable pile capacity Qall is obtained by applying a suitable

factor of safety (SF) on the contributing parts as follows:

Qall = Qu/ SF

3.1.1 Skin Friction

Pile skin friction resistance is given by:

f = αcu

where α– empirical adhesion factor d

cu- undrained shear strength of soil

Qs = ∑ fp∆L = ∑αcup∆L


OutputProject: Pile Foundation Design for Shopping Complex (R.C Building)

Resistance will be considered for individual strata:

Strata 1 (0+00 to -1+80)

Soil properties: γ =15.6 kN/m3, c =20 kN/m2, Ø =0

Pile size = 300 mm (square)

Qs = ∑αcup∆L = 1 x 20 x 1.2 x 1.8 = 43.2 kN

Where p = perimeter of the pile

Qs =38.53kN

Reference Design Computations OutputProject: Pile Foundation Design for Shopping Complex (R.C

Building)

Strata 2 (-1+80 to -4+50)

Soil properties: γ =16.7 kN/m3, c’ =30 kN/m2, Ø =0

Qs = ∑αcup∆L = 1 x 30 x 1.2 x 2.7 = 97.2 kN

Strata 3 (-4+50 to -7+90)

Soil properties: γ’sat =18.6 kN/m3, c’ =40 kN/m2, Ø =100

Qs = αcuAs + KAs q tan Ø = 1 x 30 x 1.05 x 2.7 = 85.05 kN

Where K- coefficient of earth pressure = 1 –sin Ø = 0.83

Ø -Pile, soil frictional angle = 100

q- average vertical stress, computed as follows:

Qs =85.05kN


Building)

Overburden stress @ -1 +80

q= (15.6x 1.8) =28.08 kN/m2


q= (16.7- 9.81)x 2.7=18.6 kN/m2


q= (18.6- 9.81)x 3.4=29.89 kN/m2

qave.= 28.08 + 18.6 + 29.89 = 25.52 kN/m2

3

Ap- pile perimeter = 1.2 m

α= 0.95 as obtained from fig 8.19

Qs= (0.95x 40x 1.2x 3.4) + 0.83 (1.2x 3.4x 25.52x tan 100)

= 170.3 kN

Strata 4 (-7+90 to -16+00)

Soil properties: γ’sat =19.5 kN/m3, c’ =60 kN/m2, Ø =0

Qs = ∑αcup∆L = 1 x 30 x 1.2 x 2.7 = 97.2 kN

qave.=25.52 kN/m2

Qs =141.3kN




Qs = ∑αcup∆L = 0.8 x 60 x 1.2 x 8.1 = 466.56 kN

Strata 5 (-16+00 to -19+00)

Soil properties: γ’sat=20.0 kN/m3, c’ =140 kN/m2, Ø =0


Qs = ∑αcup∆L = 0.37 x 140 x 1.2 x 3 = 186.48 kN

3.1.2 End Bearing

The tip capacity of the pile is given by:

Qp = Nc*cuAp = 9cu Ap

Where:

Nc* = bearing capacity factor = 9

C = undrained shear strength of strata

Ap = cross section area of pile tip

Tip area Ap = π (0.252)/4 = 0.049m2

Qp =9x 140x 0.09 = 113.4 kN

Qs =350.7kN

Qs =128.7kN


Building)

Single pile capacity

Qu =43.2+ 170.3+ 97.2+ 466.56+ 186.48+ 113.4= 1,077.14kN

Allowable single pile capacity, Qall = 1,077.14/2 = 538.57kN

Since allowable pile capacity for a single pile is less than the

smallest foundation load, pile group will have to be employed in all

cases

Qu

=811.58kN

Qall

=405.79kN

3.2 Pile Groups



Pile group capacity will be determined using the Converse Labarre

group efficiency in conjunction with the single pile capacity.

As regards the pile group performance a factor of safety ranging

from 2 to 4 is required.

3.2.1 Pile Group for Columns B2, B3, B4, B5, C2, C3, C4, and C5.

No. of piles

Foundation Load = 4816.0 kN

Single pile capacity = 1077.14 kN an allowable pile group in excess

of 4816.0 kN is required.

Assuming a safety factor = 3.4

Required number of piles = (4816x 3.4) /1077.14 = 15.2, use 16

The pile arrangement will be as indicated in the drawing below.

qave.=25.52 kN/m2

Qs =141.3kN


Building)

Center to center pile spacing =900mm > 2D d

Extension of pile cap beyond the face of exterior pile e = 300 mm

Calculation of pile group efficiency by Converse Labarre

equation. f

η = 1- [(n1-1)n2 +(n2-1)n1] Ø

90 n1n2

Where: n1 – no. of columns in pile group plan = 4

n2 – no. of rows in pile group plan = 4

Ø=tan-1 (D/d)

Ø= tan-1 (300/900) = 18.430

η =1- [(4-1)4 + (4-1)4] 18.43 = 0.69 or 69%

90x 4x 4

Reference Design Computations Output

Project: Pile Foundation Design for Shopping Complex (R.C Building)

Group Capacity: Qg =NpQpη

Where:

Np – no. of pile in group

Qg =16x 1077.14x 0.69 = 11,892kN.

Check: Safety Factor should be between 2 to 3

S.F = 11892/4816 = 2.5

Requirement is satisfied.

Figure 5 below shows the arrangement of piles for the 16 pile

group, along with details of the pile cap.

Qg=11,892 kN



3.2.2 Pile Group for Columns A1, A6, D1, and D6.

No. of piles





Required number of piles = (1,366.4x 3.0) /1077.14 = 3.8, use 4







equation. f

η = 1- [(n1-1)n2 +(n2-1)n1] Ø

90 n1n2



Ø=tan-1 (D/d)

Ø= tan-1 (300/900) = 18.40

η =1- [(2-1)2 + (2-1)2] 18.4 = 0.79 or 79%

90x 2x 2




Where:


Qg =4x 1077.14x 0.79 = 3403.8kN.


S.F = 3403.8/1366 = 2.5




Qg=3403.8 kN



3.2.3 Pile Group for Columns B1, B6, C1, and C6.

No. of piles












equation. f

η = 1- [(n1-1)n2 +(n2-1)n1] Ø

90 n1n2



Ø=tan-1 (D/d)

Ø= tan-1 (300/900) = 18.40

η =1- [(4-1)3 + (3-1)4] 18.4 = 0.71 or 71%

90x 3x 4




Where:


Qg =7x 1077.14x 0.71 = 5353.38kN.


S.F = 5353.38/2556.24 = 2.09




Qg=5353.38 kN



3.2.4 Pile Group for Columns A2, A3, A4, A5, D2, D3, D4 and

D6

No. of piles







Qg=10,221.9kN






equation. f

η = 1- [(n1-1)n2 +(n2-1)n1] Ø

90 n1n2



Ø=tan-1 (D/d)

Ø= tan-1 (300/900) = 18.40

η =1- [(4-1)2 + (2-1)4] 18.4 = 0.74 or 74%

90x 2x 4




Where:


Qg =8x 1077.14x 0.74 = 6415kN.


S.F = 6415/2556.24 = 2.5




Qg=6415 kN

3.3 DESIGN SUMMARY SHEET

Pile Group Group Specifications No. of Groups

Requires

No. of piles required for this Group

4 Pile GroupColumns A1, A6, D1, and D6

1. Pile cap:1850mm x 2750mm x400mm R.C

2. Pile spacing: 900mm

3. Extension of pile cap beyond outside face of exterior piles = 300mm

4 16

8 Piles GroupColumns A2, A3, A4, A5, D2, D3, D4 and D6




8 64

16 Pile GroupColumns B2, B3, B4, B5, C2, C3, C4, and C5.

1. Pile cap: 4550mm x 3175mm x400mm R.C



8 128

7 Piles GroupColumns B1, B6, C1, and C6




4 28

Total No. Piles 236

Foundation Loads - Concrete

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