Dr.: Youssef Gomaa Youssef

CE 402: Part A

Shallow Foundation Design

Lecture No. (5): Strap Footing

Fayoum University

Faculty of Engineering

Department of Civil Engineering

Strap Footing

Cantilever footing. A cantilever or strap footing normally

comprises two footings connected by a beam called a

strap. A strap footing is a special case of a combined

footing.

A strap footing is used to connect an eccentrically loaded

column footing close to the property line to an interior

column as shown in the Figure.

Dr. Youssef Gomaa Youssef 2

Dr. Youssef Gomaa Youssef 3

Strap Footing

be

ae

Be1 bb bi

ai

Bi1 Bi Be

Ae

Ae1

t

ae

t1

L

X X

Ai

Ai1

ai

Dr. Youssef Gomaa Youssef 4

Plain Concrete (P.C.) of Exterior Footing

1.30: e

e

a

PAssume A

q

*0.10e

e e

p LR P

S

Assumptions for Design :

1. Uniform stress distribution below

footings.

2. No stress below the strap beam.

3. The strap beam is weightless.

Get (Re)

Dim. of P.C. = A * B * t

Ae

t

ae

t1

L

X X

Ai

ai

Pe

* ee e

a

RArea A B

q Get (Be)

Re Ri

S

Pi

2 2

e ea AS L

Assume t=0.25 to 0.40

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Reinforced Concrete (R.C.) of Exterior Footing

1

* cu

Md C

b F

1e eA A X

1 covt d er

2

1( ) / 2

2

e b

e n

B bM P

1

1 1

1.50

*

en

e e

Rp

A B

1 2e eB B X

Ae1

ae

t1

L

Ai1

ai

Pe

Re1 Ri1

S1

Pi

1

1

*ee

P LR

S

11

2 2

e ea AS L

Be1 be

ae

bb bi

ai

Me

* *

ese

y

MA

f d j

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Interior Footing

*

i

cu

Md C

b F

1 2i iA A X

1 covt d er

2

1( ) / 2

2

i b

i n

B bM P

1

1 1

1.50

*

in

i i

Rp

A B

1 2i iB B X

Ae1

ae

t1

L

Ai1

ai

Pe

Re1 Ri1

S1

Pi

* *

isi

y

MA

f d j

1.15* i

i i

a

PArea A B

q

i iA B Area

Mi Be1 be

ae

bb bi

ai Bi1

Dr. Youssef Gomaa Youssef 7

Strap Beam

1.50 0.0e ew Z P

max

* cu

Md C

b F

1i i e ieR P P R

2

max 22 2

e ew Z aM P Z

Get

(Z) Ae1

ae

L

Ai1

ai

Pe

Re1 Ri1

S1

Pi

M1

Mmax

M2

1

1

ii

i

Rw

A

1

1

ee

e

Rw

A

Q2

Q1

Q3

Q4

Z

2

1

1

2

2

i i

i

A a

M w

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Strap Beam (Shear)

1 *e e eQ P w a

3 *s iQ Q w d Ae1

ae

Q1

L

Ai1

ai

Pe

Re1 Ri1

S1

Pi

Q2 Q3

Q4 Mmax

M1

M2

Z

2 1*e e eQ P w A

13 *

2

i ii i i

A aQ P w a

14 *

2

i ii

A aQ w

0.75 cusu

c

fq

*

ss su

b

Qq q

b d

Use Stirrups

Dr. Youssef Gomaa Youssef 9

Footing Reinforcement

max

* *top

y

MA

f d j

1

* *bot

y

MA

f d j

Ae1

ae

L

Ai1

ai

Mmax

M1

M2

2ϕ10

2ϕ10

2ϕ10

t1

Ase Asi

Dr. Youssef Gomaa Youssef 10

Design of Strap Footing

Example(1):

Make a complete design for a strap footing for the two columns

shown in figure (2). The allowable gross bearing capacity is 1.40

kg/cm2 and the foundation level is 2.0m below ground surface

ae = 0.40m. be= 0.25m Pe=50t

ai = 0.60m bi= 0.25m Pi=100t

qa = 1.40kg/cm2 = 14.0t/m2.

fcu = 250kg/cm2.

fy =3600kg/cm2

0.25

100t 0.40

0.25

0.60

3.50

50t

Dr. Youssef Gomaa Youssef 11

Plain Concrete (P.C.) of Exterior Footing

1.30: e

e

a

PAssume A

q

*0.10e

e e

p LR P

S Get (Re)

Dim. of P.C. = 2.10 * 2.45 * 0.30

Ae

t

ae

t1

L

X X

Ai

ai

Pe

271* 5.07

14

ee e

a

RA B m

q Be=2.45m

Re Ri

S

Pi

2 2

e ea AS L

Assume t=0.30

1.30*502.10

14eA m

0.40 2.103.5 2.65

2 2S m

50*3.500.10*50 71.0

2.65eR t

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(R.C.) of Exterior Footing

57.50*105.0* 27.32 30

100*250d cm

1 2.10 0.30 1.80e eA A X m

1 30 5 35t cm

2

(1.85 0.40) / 228.41 7.47 . / '

2eM m t m

21.50*63.0628.41 /

1.80*1.85np t m

1 2 2.45 0.60 1.85e eB B X m

Ae1

ae

t1

L

Ai1

ai

Pe

Re1 Ri1

S1

Pi

1

1

* 50*3.5063.06

2.775

ee

P LR t

S

11

0.40 1.853.50 2.775

2 2 2 2

e ea AS L

Be1 be

ae

bb bi

ai

Me

527.5*10

10.063600*30*0.69

seA cm

6 12/ 'm

Dr. Youssef Gomaa Youssef 13

Interior Footing

512.80*105 35.78 40

100*250d cm

1 2.90 0.60 2.30iA m

1 40 5 45t cm

2

(2.30 0.40) / 228.36 12.80 . / '

2iM m t m

1.50*10028.36

2.30*2.30np

1 2.90 0.60 2.30iB

Ae1

ae

t1

L

Ai1

ai

Pe

Re1 Ri1

S1

Pi

5212.80*10

10.303600*40*0.869

siA cm

1.15*100* 8.22

14i iA B

8.22 2.90i iA B m

Mi Be1 be

ae

bb bi

ai Bi1

6 12 / 'mm m

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Strap Footing Program

Dr. Youssef Gomaa Youssef 15

Strap Footing Excel Sheet