5-Strap Footing

Fayoum University
Faculty of Engineering
Department of Civil Engineering
CE 402: Part A
Shallow Foundation Design
Lecture No. (5): Strap Footing
Dr.: Youssef Gomaa Youssef
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
Strap Footing
ae
ai
Be B b
e1 e
bb
Bi1 Bi
bi
L
ae
ai
t1
X
X
t
Ae1
Ae
Ai1
Ai
Dr. Youssef Gomaa Youssef
3
Plain Concrete (P.C.) of Exterior Footing
Assumptions for Design :
L
1.
ae
Uniform stress distribution below
footings.
2.
No stress below the strap beam.
3.
The strap beam is weightless.
ai
Pi
Pe
t1
t
1.30Pe
Assume : Ae 
qa
S
Re
Ae
ae Ae
S L 
2 2
p *L
Re  e
 0.10Pe
S
X
X
Area  Ae * B e 
Re
qa
Ri
Ai
Get (Be)
Assume t=0.25 to 0.40
Get (Re)
Dim. of P.C. = A * B * t
Dr. Youssef Gomaa Youssef
4
Reinforced Concrete (R.C.) of Exterior Footing
L
Ae 1  Ae  X
ae
ai
Pi
Pe
B e 1  B e  2X
S1  L 
ae Ae 1

2
2
t1
P *L
Re 1  e
S1
1.50R e 1
pn 
Ae 1 * B e 1
S1
Re1
Ae1
ae
(B e 1  bb ) / 2
2
M e  Pn
d C
Ri1
Ai1
Me
Be1 be
ai
bi
bb
2
M1
b * Fcu
t1  d  cov er
Dr. Youssef Gomaa Youssef
Ase 
Me
f y *d * j
5
Interior Footing
L
1.15Pi
Area  Ai * B i 
qa
ae
ai
Pi
Pe
Ai  B i  Area
Ai 1  Ai  2X
t1
B i 1  B i  2X
pn 
S1
Re1
Ae1
1.50R i 1
Ai 1 * B i 1
Ri1
Ai1
ae
(B i 1  bb ) / 2
2
M i  Pn
d C
Mi
b * Fcu
Be1 be
ai
Bi1
Mi
bi
bb
2
t1  d  cov er
Dr. Youssef Gomaa Youssef
Asi 
Mi
f y *d * j
6
Strap Beam
R i 1  Pi  Pe  R ie
wi 
L
ae
ai
Pi
Pe
Ri 1
Ai 1
Re 1
we 
Ae 1
w e Z  1.50Pe  0.0
M max 
weZ
a 

 P2  Z  e 
2
2

2
 Ai 1  ai 
 2 
M 1 w i
2
d C
Get
(Z)
S1
Re1
Ri1
Ae1
Ai1
Q3
Q2
Z
2
Q1
Q4
Mmax
M2
M max
b * Fcu
M1
Dr. Youssef Gomaa Youssef
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Strap Beam (Shear)
Q1  Pe w e * ae
L
ae
ai
Pi
Pe
Q 2  Pe w e * Ae 1
A a

Q 3  Pi w i *  i 1 i  ai 
 2

A a 
Q4  w i *  i 1 i 
 2 
S1
Re1
Ri1
Ae1
Q s  Q 3 w i * d
Ai1
Q3
Q2
Z
qs 
Qs
 q su
bb * d
q su  0.75
Q1
f cu
c
Q4
Mmax
M2
Use Stirrups
M1
Dr. Youssef Gomaa Youssef
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Footing Reinforcement
L
ae
ai
2ϕ10
2ϕ10
2ϕ10
t1
Ase
Ae1
Ai1
Asi
Mmax
Atop 
M max
f y *d * j
Abot 
M1
f y *d * j
M2
M1
Dr. Youssef Gomaa Youssef
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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
0.40
0.60
100t
50t
0.25
bi= 0.25m Pi=100t
qa = 1.40kg/cm2 = 14.0t/m2.
0.25
3.50
fcu = 250kg/cm2.
fy =3600kg/cm2
Dr. Youssef Gomaa Youssef
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Plain Concrete (P.C.) of Exterior Footing
Assume : Ae 
1.30Pe
qa
L
ae
ai
Pi
Pe
Ae 
1.30*50
 2.10m
14
S L
ae Ae

2 2
t1
X
X
t
0.40 2.10
S  3.5 

 2.65m
2
2
Re 
pe * L
 0.10Pe
S
Re 
S
Re
Ae
Get (Re) Ae * B e  Re  71  5.07m 2
qa
50*3.50
 0.10*50  71.0t
2.65
14
Ri
Ai
Be=2.45m
Assume t=0.30
Dim. of P.C. = 2.10 * 2.45 * 0.30
Dr. Youssef Gomaa Youssef
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(R.C.) of Exterior Footing
L
ae
Ae 1  Ae  X  2.10  0.30  1.80m
ai
Pi
Pe
Be 1  Be  2X  2.45  0.60  1.85m
S1  L 
ae Ae 1
0.40 1.85

 3.50 

 2.775
2
2
2
2
Re 1 
Pe * L 50*3.50

 63.06t
S1
2.775
pn 
1.50*63.06
 28.41t / m 2
1.80*1.85
S1
Re1
Ae1
Ri1
Ai1
ae
Me
(1.85  0.40) / 2
 28.41
t1
Be1 be
ai
bb
bi
2
Me
2
 7.47m .t / m '
7.50*105
d  5.0*
 27.32  30cm
100*250
t1  30  5  35cm
Dr. Youssef Gomaa Youssef
7.5*105
Ase 
 10.06cm 2
3600*30*0.69
612 / m '
12
Interior Footing
L
1.15*100
Ai * B i 
 8.22
14
ae
ai
Pi
Pe
Ai  B i  8.22  2.90m
Ai 1  2.90  0.60  2.30m
t1
B i 1  2.90  0.60  2.30
pn 
S1
Re1
Ae1
1.50*100
 28.36
2.30*2.30
(2.30  0.40) / 2
 28.36
ae
2
Mi
2
 12.80m .t / m '
12.80*105
d 5
 35.78  40cm
100*250
Ri1
Ai1
Be1 be
t1  40  5  45cm
Dr. Youssef Gomaa Youssef
ai
Bi1
bb
Mi
bi
12.80*105
Asi 
 10.30cm 2
3600*40*0.869
612mm / m '
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Strap Footing
Program
Dr. Youssef Gomaa Youssef
14
Strap Footing
Excel Sheet
Dr. Youssef Gomaa Youssef
15