shallow foundations ( combined, strap, raft foundation)

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Page 1: Shallow Foundations ( Combined, Strap, Raft foundation)

SHALLOW FOUNDATIONS 2DESIGN OF COMBINED, STRAP & RAFT FOOTING

Page 2: Shallow Foundations ( Combined, Strap, Raft foundation)

INTRODUCTION

Page 3: Shallow Foundations ( Combined, Strap, Raft foundation)

Contents :

Combined Footing

Strap Footing

Raft Foundation

1

2

3

Page 4: Shallow Foundations ( Combined, Strap, Raft foundation)

1. Combined Footing

Page 5: Shallow Foundations ( Combined, Strap, Raft foundation)

Combined Footing

A combined Footing is a long footing supporting two or more columns in (typically two) one row.

A combined Footing is a rectangular or Trapezoidal shaped footing.

Page 6: Shallow Foundations ( Combined, Strap, Raft foundation)

Using of Combined Footing

Construction Practice may dictate using only one footing for two or more columns due to:

a) Closeness of Columns

b) Due to property line constraint, which may limit the size of footing at boundary.

Page 7: Shallow Foundations ( Combined, Strap, Raft foundation)
Page 8: Shallow Foundations ( Combined, Strap, Raft foundation)

The Design of Combined Footing requires that the centroid of the area be as close as possible to the resultant of the 2 column loads for uniform pressure and settlement.

Which Means :

Page 9: Shallow Foundations ( Combined, Strap, Raft foundation)

x

y

Eccentricity = ZERO

Page 10: Shallow Foundations ( Combined, Strap, Raft foundation)
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Page 12: Shallow Foundations ( Combined, Strap, Raft foundation)
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Page 18: Shallow Foundations ( Combined, Strap, Raft foundation)
Page 19: Shallow Foundations ( Combined, Strap, Raft foundation)

STEPS OF DESIGN

Page 20: Shallow Foundations ( Combined, Strap, Raft foundation)

Dim

ensi

onin

g fo

r P.C

Find Area“L” & “B”

Des

ign

of R

.C F

ootin

g (lo

ngitu

dina

l)

Find “Max Moment”Find “d”

Check Shear

Check Punching

Des

ign

of R

.C F

ootin

g (S

hort)

Find Moment @ Hidden beam 1

Find Moment @ hidden beam 2

Ensure That C1 > 2.3

RFT

RFT in Long Direction

RFT in Short Direction

Page 21: Shallow Foundations ( Combined, Strap, Raft foundation)

C1 C2

L

Page 22: Shallow Foundations ( Combined, Strap, Raft foundation)

RC

MaxS.F.D

B.M.D

Zero shear

Page 23: Shallow Foundations ( Combined, Strap, Raft foundation)

c1 c2

Page 24: Shallow Foundations ( Combined, Strap, Raft foundation)

c1 c2

H.B.1 H.B.2

Page 25: Shallow Foundations ( Combined, Strap, Raft foundation)
Page 26: Shallow Foundations ( Combined, Strap, Raft foundation)

c1 c2

Page 27: Shallow Foundations ( Combined, Strap, Raft foundation)

EXAMPLE

C1 (30*70) C2 (30*90)

4 m

Design a combined footing to support a working load of p1=160 ton & p2=220 ton. Bearing capacity: 12.5 ton/m2Thickness of plain concrete 15 cm

Page 28: Shallow Foundations ( Combined, Strap, Raft foundation)

Solution

Page 29: Shallow Foundations ( Combined, Strap, Raft foundation)

1- Dimensions of footing :R = p1+p2 = 160 + 220 = 380 ton R . X = p2 . S 380 x X = 220 x 4 X= 2.32 m 

Lpc / 2 = X + b1 / 2 + 0.5 + tpc  

Lpc / 2 = 2.32 + 0.7 / 2 + 0.5 + 0.3 = 3.47m   Lpc = 6.94 m LRc = LPc - 2 x tPC = 6.94 - 2 X 0.3 = 6.34m

APC = 380 / 12.5 = 30.4 LPc X BPc = 30.4 6.34 x BPc = 30.4BPc = 4.8mBRc = 4.8 - 2 x 0.3 = 4.2m

Page 30: Shallow Foundations ( Combined, Strap, Raft foundation)

2Design of R.c footing 

P1u = 160 x 1.5 = 240 ton  P2u = 220 x 1.5 = 330 ton Ru = 380 x 1.5 = 570 ton   Wu = 570 / 6.34 = 89.9 ton   qu = 570 / 6.34 x 4.2 = 21.4 t/㎡

Page 31: Shallow Foundations ( Combined, Strap, Raft foundation)

Design of Footing in Longitudinal Direction

Page 32: Shallow Foundations ( Combined, Strap, Raft foundation)

At point of zero shear :  

P1u = wu . X

240 = 89.9 . X X = 2.67m  Mmax = 89.9 x 7.1289 / 2 - 240 x 1.82 = 116.3  D =5 × √116.35×10^7 / 25 × 4200 = 526.3 ㎜ D = 530㎜

Page 33: Shallow Foundations ( Combined, Strap, Raft foundation)

2- check shear  

Qsu = 0.16√25/1.5 = 0.653 N/m㎡ 

Qsumax = 155.56 - 89.99 × 0.53/2 = 131.7365

 Qs = 131.73×10^4 / 530×4200 = 0.59 ≤ qsu 

Safe

Page 34: Shallow Foundations ( Combined, Strap, Raft foundation)

c1 c2

1.23 1.43

0.83

0.83

Page 35: Shallow Foundations ( Combined, Strap, Raft foundation)

For c1 (30x70) Qpcu1 = 0.316 ( 0.5 + 300 / 700) √25/1.5 = 1.197

Qpu1 = 240 - 21.4 x (1.23x0.83)=218.15 ton Qpu1 = 218.15 x 10^4 / 530x2 (1230+830) =0.99 ≤ qpcu1 Safe

For c2 (30*90)

Qpcu2 = 0.316 x (0.5 + 300/900) √25/1.5 = 1.07 Qpu2 = 220 -21.4 (1.43x0.83) = 194.6 Qpu2 = 194.6 x 10^4 / 530 (1430+830)x2 = 0.81 ≤ 1.07 safe

Page 36: Shallow Foundations ( Combined, Strap, Raft foundation)

Design of Footing in Short Direction

Page 37: Shallow Foundations ( Combined, Strap, Raft foundation)

c1 c2

1.73 m 1.96 m

Page 38: Shallow Foundations ( Combined, Strap, Raft foundation)

For hidden Beam1 Qu1 = 240 / 4.2 x 1.73 = 33.03t/㎡M1 = 33.03 * 1.95^2 /2 = 62.9 t.m

For hidden Beam 2 Qu2 = 330 / 4.2 x 1.96 = 40.1t/㎡ M2 = 40.1 x 1.95^2 / 2 = 76.2 t.m   d = 530 = c1 √76.2x10^7 / 25 x 1000  

C1 = 3.1≥ 2.3

Page 39: Shallow Foundations ( Combined, Strap, Raft foundation)

Reinforcement

Page 40: Shallow Foundations ( Combined, Strap, Raft foundation)

Asmin =1.5 X d = 1.5 x 530 = 795 mm2 Asmin = 4 1 16

Page 41: Shallow Foundations ( Combined, Strap, Raft foundation)

RFT in long direction

Astop = 116.35 x 107 / 360 x 0.826 x 530 = 72382mm2/4.2m =1757.7 mm2 7 ϕ18

Asbottom = 48.61 x 107 / 360x 0.826 x 530 = 3084mm2 /4.2 = 734.3 mm2 4ϕ16

Page 42: Shallow Foundations ( Combined, Strap, Raft foundation)

RFT in short direction

As1 = 62.79 X 107 / 360 X 0.826 X 530 = 3984 mm2 11ϕ22

As2 = 76.2 X 107 / 360 X 0.826 X 530 = 4835 mm2 10ϕ25

Page 43: Shallow Foundations ( Combined, Strap, Raft foundation)

7ϕ18

4ϕ1611ϕ22 10ϕ25

4ϕ16

Page 44: Shallow Foundations ( Combined, Strap, Raft foundation)

c1 c2

7ϕ18

4ϕ16

4ϕ16

11ϕ2

2

4ϕ16

4ϕ16

10ϕ2

5

4ϕ16

Page 45: Shallow Foundations ( Combined, Strap, Raft foundation)

2. Strap Footing

Page 46: Shallow Foundations ( Combined, Strap, Raft foundation)

Design a strap footing to support an exterior column (30*50cm) and an interior column (30*90cm). The un factored Loads C1 =685 KN, C2 =1270 kN . Assume the allowable Bearing capacity is 150 kN/m2, fcu=25 N/mm2 and Fy = 360 N/mm2 , P.C. thickness = 40 cm

EXAMPLE

Page 47: Shallow Foundations ( Combined, Strap, Raft foundation)

C2C1

0.5 0.9

4.9 m

Page 48: Shallow Foundations ( Combined, Strap, Raft foundation)

Calculation of reaction:

Assume e=0.1 to 0.2 (L)

e = 0.5 to 1m (take it 1m)

R1= = 860.6 KN

R2=685 + 1270 - 860.6 = 1094.4 KN

C2C10.5 0.9

e

Page 49: Shallow Foundations ( Combined, Strap, Raft foundation)

Area of plain concrete footing:A1 = = 5.73 Dimension ((1+0.25)*2) = 2.5 (2.5*2.3)

A2 = = 7.29

Dimension = (2.7*2.7) = 7.29

R.C dimensions:F1 = (2.1*1.5) m

F2 = (1.9*1.9) m

Page 50: Shallow Foundations ( Combined, Strap, Raft foundation)

C2C10.5 0.9

0.50.90.50.5 2.11.6

M = 602720

695

432

262

B.M.D

S.F.D

Page 51: Shallow Foundations ( Combined, Strap, Raft foundation)

Design of strap beam:qu1= = 614 kN/m’

qu2= = 864 kN/m’

Point of zero shear:614X-1027=0 , X=1.67m , Mmax=602kN.m

d = 5 * =1227m

t=130cm d=123cm

Page 52: Shallow Foundations ( Combined, Strap, Raft foundation)

Reinforcement:Astop = =1646 = 7/m’

Asbot = = 295

Asmin = 0.15% b*d = 0.15%*400*1230 =738

= 4

Page 53: Shallow Foundations ( Combined, Strap, Raft foundation)

CHECK SHEAR:

Qs = (720*0.55)/1.17 = 341.53kN

qs = = 0.694N/

qcu =0.24 = 0.98N/

qs use min stirrups 5

0.550.62

720

Page 54: Shallow Foundations ( Combined, Strap, Raft foundation)

Design of Footings

Page 55: Shallow Foundations ( Combined, Strap, Raft foundation)

Critical Sec

Footing 1: qu = = 409.5 kN/

L1 = (1.5-0.4)/2 = 0.55 m

Mu = 409.5*(/2) = 61.9kN.m

d = 5 = 249mm

t = 35cm d = 28cm

Page 56: Shallow Foundations ( Combined, Strap, Raft foundation)

Check shear:Qs = 409.5*(0.55-(0.28/2)) = 168kN

qs = = 0.6 MPa

qcu = 0.16 = 0.65 MPa

qs safe

As = =743.44 = 5

Page 57: Shallow Foundations ( Combined, Strap, Raft foundation)

Footing 2:qu==455kN/

L2==0.75m

Mu=455*=128kN.m

d=5 =358mm

t=45cm d=38cm

Critical Sec

Page 58: Shallow Foundations ( Combined, Strap, Raft foundation)

Check shear:Qs = 455*(0.75-) = 255KN

qs ==0.67N/

qcu =0.65N/ qs unsafe

0.65=d=392mm

t=50cm d=43cm

As ==1001=5

Asmin = 1.5d = 1.5*430 = 645

Page 59: Shallow Foundations ( Combined, Strap, Raft foundation)

3ϕ18 + 4ϕ18 = 7ϕ18

4ϕ16

5ϕ10

2ϕ12

2ϕ12

Page 60: Shallow Foundations ( Combined, Strap, Raft foundation)

0.4

m

2.3

m1.

5 m

2.1 m2.5 m 2.7 m

1.9 m

5ϕ12 5ϕ12

5ϕ14

5ϕ16

Page 61: Shallow Foundations ( Combined, Strap, Raft foundation)

3. Raft Foundation

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