Eng. Khalid Footing-Design (1)

7/31/2019 Eng. Khalid Footing-Design (1)

1/60

Footing Design


2/60

Types of Footing

Wall footings are used to

support structural walls that

carry loads for other floorsor to support nonstructural

walls.


3/60

Isolated or single footings

are used to support single

columns. This is one of themost economical types of

footings and is used when

columns are spaced at

relatively long distances.

Types of Footing


4/60

Combined footings usually

support two columns, or

three columns not in a row.Combined footings are used

when two columns are so

close that single footings

cannot be used or when one

column is located at or near

a property line.

Types of Footing


5/60

Cantilever or strap footings

consist of two single

footings connected with abeam or a strap and support

two single columns. This

type replaces a combined

footing and is more

economical.

Types of Footing


6/60

Continuous footings

support a row of three or

more columns. They havelimited width and continue

under all columns.

Types of Footing


7/60

Rafted or mat foundation

consists of one footing

usually placed under the

entire building area. Theyare used, when soil bearing

capacity is low, column

loads are heavy single

footings cannot be used,piles are not used and

differential settlement must

be reduced.

Types ofFooting


8/60

Pile caps are thick slabs

used to tie a group of piles

together to support andtransmit column loads to the

piles.

Types of Footing


9/60

Distribution of Soil Pressure

When the column loadPis applied

on the centroid of the footing, a

uniform pressure is assumed to

develop on the soil surface below the

footing area. However the actual

distribution of the soil is not

uniform, but depends on mayfactors especially the composition of

the soil and degree of flexibility of

the footing.


10/60

Distribution of Soil Pressure

Soil pressure distribution incohesionless soil.

Soil pressure distribution in cohesivesoil.


11/60

Eccentrically loaded footings


12/60


13/60


14/60

Eccentrically loaded footings Example

Isolated FootingD.L = 900 kN

L.L = 450 kN

Ms = 150kN.m

Mu=200kN.mqall=200kpa

M


15/60

Area required approximated

4

125.25.3

2

2

3

3

)(

2

)(

9.875.8

5.25.3

75.610200

10)450900(

200/20

3

mI

mA

mmuse

mq

PA

kPamtq

netall

sg

netall

kPaI

CM

A

P

kPaI

CM

A

P

P

Me

ss

ss

L

7.1249.8

150

75.8

1350

7.1839.8

150

75.8

1350

583.0111.01350

150

2

5.3

25.3

65.36

Check stress

183.7

124.7


16/60

Ultimate pressure under footing

kPaI

CM

A

P

kPaI

CM

A

P

mkNM

kNP

uu

uu

u

u

1669.8

200

75.8

1800

2459.8

200

75.8

1800

.200

1800)450(6.19002.1

2

5.3

25.3

245

245

166


17/60

Check Punching Shear

17985.25.3

5.24641000/37205303

2575.0

3

'

:equationsuitablecolumn thesquareFor3720)400530(4

4

)245166245166(kNV

kNdbf

V

isVcmb

U

o

c

C

C

o


18/60

Check Beam Shear

5.567

5.2*53.05.1*2

245223

facecolumnfromat

13.8281000/25005306

2575.0

CU

U

U

C

VV

kN

V

dV

kNV


19/60

directionlong/1675.131346100053000254.0

0.002542500*530*250.859.0

657.2*102-1-1

420

25*85.0

305d,2500b

.2.657)1(75.63)75.0(25.791)1()75.0(

75.63)5.2)(5.1)(211245(

25.791)5.2)(5.1(211

facecolumnfromdat

22

2

6

21

21

2

1

musecmmmA

mm

mkNPPM

kNP

kNP

M

S

U

U

Bending moment design

Long direction245

245

166

3.5m


20/60

directionshort1.1010601000530002.0

0.0023500*530*250.859.0

719.25*102-1-1

420

25*85.0

305d,3500b

.25.7195.05.312

166245

facecolumnfromdat

22

2

6

cmmmA

mm

mkNM

M

S

U

U

Bending moment design

Short direction

245

245

166

2.5m


21/60

0.832.42

12

1.42.5

3.5

Central band of short direction = 0.83 As = 0.83 (10.1)=8.6cm2

Central band ratio =


22/60

14/m7 142142

16/m7


23/60

Footing Design

Part II

Combined footing


24/60

Example 1

Design a combined footing As shown

kPamtq netall 200/202

)( 225 mmNfc

2420 mmNfy


25/60

Dimension calculation

The base dimension to get uniform distributed load

x2=6.2mx1=0.2m

800 kN 1200 kN

x

2000kN

A

800(0.2)+1200(6.2)=2000(x)

x = 3.8m

2x =7.6 m

800 kN 1200 kN

Try thickness

=80cm


26/60

kPaPa

A

Pq

mq

PA

kNPP

kPamtq

uu

netall

sg

su

netall

190101908.1*6.7

102600

8.1*6.71010200

102000

2600)2000(3.1)(3.1

,200/20

33

2

3

3

)(

2

)(

Area required


27/60

Check for punching Sheard = 730 mm

oK8.875190*765.0*13.1)3.1(800

60271000/22607301225

226073030275.0

12'2

3.20621000/22607303

2575.0

3

'

22601130)765(2

VkNV

kNdbf

b

dV

kNdbf

V

mmb

cU

o

csC

o

c

C

o

A

1.13m

0.765


28/60

oK4.1317190*13.1*13.1)3.1(1200

5.133221000/452073012

25

4520

73040275.0

12

'2

4.41241000/45207303

2575.0

3

'

4520)400730(4

VkNV

kNdbf

b

dV

kNdbf

V

mmb

cU

o

csC

o

c

C

o

B


29/60

Draw S.F.D & B.M.D

Stress under footing

= 190 *1.8 = 342 kN/m


30/60

34.762facecolumnfromat.

25.8211000/180073062575.0

CU

U

C

VV

kNdVMax

kNV

Check for beam shear

b = 1800mm, d = 730mm


31/60

Top/2095.28284710007300039.0

0.00391800*730*250.859.0

1366*102-1-1

420

25*85.0

307d,8001b

.1366

22

2

6

musecmmmA

mmmm

mkNMve

S

Bending moment Long direction

Bottom/1674.14144010008000018.0

0.00071800*730*250.859.0

246.7*102-1-1

420

25*85.0

307d,8001b

.7.246

22

min

min2

6

musecmmmA

mmmm

mkNMve

S


32/60

Bending moment Short direction

/147116.11017658000018.0

765*730*250.859.0

141.6*102-1-1

420

25*85.0

307d,765b

6.1412

4.08.1

2

765.0

)765.0*8.1(

1040

22

min

min2

6

2

musecmmmA

mmmm

M

S

Under Column A


33/60

/147116.11017658000018.0

1130*730*250.859.0

212.33*102-1-1

420

25*85.0

307d,1130b

33.2122

4.08.1

2

13.1

)13.1*8.1(

1560

22

min

min2

6

2

musecmmmA

mmmm

M

S

Under Column B

Shrinkage Reinforcement in short direction

/147116.11017658000018.0

22

min musecmmmAS


34/60


35/60

Footing Design

Part IIICombined footing, strip footing, & Mat foundation


36/60

Example 2

Design a combined footing As shown


)( 225 mmNfc

2420 mmNfy

Di i l l i


37/60



x1=0.2mx2=4.2

m

1200 kN 750 kN

x

1950kN

A

750(4.2)+1200(0.2)=1950 (x)

x = 1.75m

3

2

21

21 L

BB

BBx

Area required


38/60

mB

mB

B

BLBBBBx

BB

BB

BB

LBB

m

q

PA

kPamtq

netall

sg

netall

4

1

29.045.175.1

335.4

55

32

5

5.22

8.1035.4

2

8.102

8.10

10180

101950

,200/20

1

2

2

2

21

21

21

21

21

21

2

3

3

)(

2

)(

Area required

kPaPa

A

Pq uu 23510235

8.10

1019503.1 33


39/60

Check for punching Shearh= 750mm

d = 732 mm

oK6.1376235*733.0*065.1)3.1(1200

52221000/25906651225

259066530275.0

12'2

4.21601000/25906653

2575.0

3

'

25901065)732(2

VkNV

kNdbfbdV

kNdbf

V

mmb

cU

o

csC

o

c

C

o

A

B1=4m

B2=1m


40/60

B

oK5.896235*633.0*965.0)3.1(800

52731000/2231665

12

25

2231

66530275.0

12

'2

5.18541000/22316653

2575.0

3

'

2231965)633(2

VkNV

kNdbf

b

dV

kNdbf

V

mmb

cU

o

csC

o

c

C

o


41/60

Draw S.F.D & B.M.D


42/60

mkN

wlM

wave

.11748

65.3705

8

705)235940(235

22

max

32

Mmax

68270.0975

1092)70.0(1560

Empirical S.F.D & B.M.D

m

Convert trapezoidal load to rectangle

Clear distance between column

B in moment design = ave. width = 2.5m


43/60

668critical)mostthe(faceBcolumnfromat.

6961000/17006656

2575.0

17007.15.1)(21

0.150.815at

)(21

35.4965.0

CU

U

C

Lx

VV

kNdVMax

kNV

mmm

x

yb


d = 665mm

Y=1.5m

1m

4m

x

b


44/60

Top/16012019951000665003.0

0.0032600*665*250.859.0

1260*102-1-1

420

25*85.0

307d.1260

22

2

6

musecmmmA

mmmkNMve

S


Bottom/1495.13135010007500018.0 22min musecmmmAS

260060.25.1)(2135.425.2 mb

Top

Bottom


45/60

Bending moment Short direction

2001333325733665005.0

0.005733*665*250.859.0

583.57*102-1-1

420

25*85.0

665d

6.5832

4.075.3

2

733.0

)733.0*75.3(

1560

22

2

6

2

usecmmmA

mm

M

S

Under Column A

mmmb 35005.35.1)(21' 35.4 62.3

mmmmb 375075.32

45.3

mmmb 144044.15.1)(21'35.4633.0


46/60

1466.86.8546337500018.0

633*665*250.859.0

84.6*102-1-1

420

25*85.0

665d,633b

6.842

3.022.1

2

633.0

)633.0*22.1(

975

22

min

min2

6

2

usecmmmA

mmmm

M

S

Under Column B

Shrinkage Reinforcement in short direction

/1495.13135075010000018.0 22min musecmmmAS

mmmmb 122022.12

144.1


47/60

Reinforcement details


48/60

Example 3 (Strip footing)Design a combined footing As shown


)( 225 mmNfc

2420 mmNfy



49/60



Assume

L1=0.6 x1=5.2m

800 kN 1280 kN

x

3040kN

A

800(0.6)+1280(5.1)+960(10.6)=3040 (x)

x = 5.65m,2(x)=11.3m

L2=11.3-

(10.6)=0.7

960 kN

x2=10.7mL2


50/60

mmmq

PA

kPamtq

netall

s

g

netall

8.13.119.1610180

103040

,180/18

2

3

3

)(

2

)(

kPaPa

A

Pq uu 19510195

8.13.11

1030403.1 33

Check for punching Shear


51/60

p gh = 700 mm

d=630mm

oK1.1457195*03.1)3.1(1280

65841000/412063012

25

4120

63040

275.012

'

2

5.32441000/41206303

2575.0

3

'

4120))400630(4

2 VkNV

kNdb

f

b

d

V

kNdbf

V

mmb

cU

o

cs

C

o

c

C

o

B

Example

You can check other columns

Draw S.F.D & B.M.D Stress under footing


52/60

= 195 *1.8 = 351 kN/m



53/60

3.706)1009(7.0facecolumnfromat.

75.7081000/18006306

2575.0

CU

U

C

VV

kNdVMax

kNV


b = 1800mm, d = 630mm

L di i


54/60

Top/2296.33336210006300053.0

0.00531800*630*250.859.0

1365*102-1-1

420

25*85.0

307d,8001b

.1366

22

2

6

musecmmmA

mmmm

mkNMve

S


Bottom/1486.12126010007000018.0

1800*730*250.859.0

81*102

-1-1420

25

*85.0

307d,8001b

.7.246

22

min

min2

6

musecmmmA

mmmm

mkNMve

S

Design Short direction as example 1 (lecture 11)

R i f d il


55/60

Reinforcement details

Mat Foundation


56/60

Mat Foundation


57/60


58/60

Check for punching Shear


59/60

Modified load

General Example, Ref. 2

G l i f t d t il


60/60

General reinforcement details

Documents

Eng. Khalid Footing-Design (1)