Lecture 7 strap footing

Foundation Engineering

1. INTERNATIONAL UNIVERSITY
FOR SCIENCE & TECHNOLOGY
INTEREAOLR UVSEYFYRVC&IHR VCNRUGR VCF7I2R
CIVIL ENGINEERING AND
ENVIRONMENTAL DEPARTMENT
303421: Foundation Engineering
Strap Footing
Dr. Abdulmannan Orabi
Lecture
7

2. References
ACI 318M-14
Building Code Requirements for Structural
Concrete ( ACI 318M -14) and Commentary, American
Concrete Institute, ISBN 978-0-87031-283-0.
Bowles , J.,E.,(1996)
“Foundation Analysis and Design” -5th
ed. McGraw-Hill, ISBN 0-07-912247-7.
Das, B., M. (2012),
“ Principles of Foundation Engineering ”
Eighth Edition, CENGAGE Learning,
ISBN-13: 978-1-305-08155-0.
Syrian Arab Code for Construction 2012
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3. A strap footing is used to connect an eccentrically
loaded column footing to an interior column. The
strap is used to transmit the moment caused from
aneccentricity to the interior column footing so th at
auniform soil pressure isgenerated beneath both
footings. The strap footing may beused instead of a
rectangular or trapezoidal combined footing if the
distance between columns is large and / orthe
allowable soil pressure is relatively large so that
theadditional footing area is not needed.
Cantilever or Strap Footings.
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4. Cantilever or Strap Footings.
Interior
column
Exterior
column
N1
N2
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5. Cantilever or Strap Footings.
This is a special type of footing used for two colu mns.
The two columns are provided by two separate footin gs
connected by a rigid beam called “strap beam”.
S
B1
B2
L1L2
N1N2
Strap beam
Interior
footing
Exterior
footing
Property
line
Strap Beam
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6. Three basic considerations for strap footing design
are these:
1. Strap must be rigid—perhaps I strap/Ifooting> 2.
This rigidity is necessary to control rotation of t he
exterior footing.
2. Footings should be proportioned for approximatel y
equal soil pressures and avoidance of large differe nces in
B to reduce differential settlement.
3. Strap should be out of contact with soil so that there are
no soil reactions
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7. Cantilever or Strap Footings.
It is common to neglect the strap weight in the des ign.
The strap should be adequately attached to the both
the column and the footing by the use of dowels suc h
that the footing and the strap act as unit.
The footing is subjected to one-way bending.
The strap beam is reinforced with main reinforcemen t
at top between the columns and at bottom under the
interior footing.
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Dr. Abdulmannan Orabi IUST

8. Summary of strap footing design is shown in the
following steps.
1-Proportion footing dimensions.
Design of Strap Footings.
I
N
TERAO= 0

S Y F Y C
AS L AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA&HG
I

= 0C
O
S YF AYU
O
S Y F Y C
AF L AAAAAAAAAAAAAAAAAA&7G
Sum moments about the center of the interior column
and obtain the soil reaction beneath the exterior f ooting.
Sum moments about the center of the exterior footin g
and obtain the soil reaction beneath the other foot ing.
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Dr. Abdulmannan Orabi IUST

9. Summary of strap footing design is shown in the
following steps.
1-Proportion footing dimensions.
Design of Strap Footings.
I
2 L C
D C
OY U
AY U
O = 0 (3)
To solve these three equations assume a value of
eccentricity, e. Find and check equation (3).
,
O
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10. S
N1N2
.
.
OA
R1R2
e
b

d
OA
SulbmAaFbnA
a
OA
a

b
OA
d

Design of Strap Footings.
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11. Summary of strap footing design is shown in the
following steps.
1-Proportion footing dimensions.
Design of Strap Footings.
2iiu345A&H
).
= 2
F D
b

2
AAAAAAAAb46AAAAAAAAAAAAAAa
=


.
∗ %
1RR&0()G
2iiu345A&7G
*
OLAa
O
AAAAAAAAAilAAAAAAAA*
OL .
OA8a
O
*
O =

O
%
1RR&0()G
If B is too large or too small compared to L can be
repeated until satisfactory dimensions are obtained . B1
should not be greater than 1.5 L1
Find the required area for each footing :
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12. Summary of strap footing design is shown in the
following steps.
Design of Strap Footings.
2-Evaluate factored net soil pressure under the foo tings.
I
N
TERAO= 0
+
S Y F Y ,
AS L
I

+= 0,
O
S Y F AY U
+O
S Y F Y,
AF L
%
+O =

+O
*
O
%
+ =

+
*

U1
.

Ru1
U2 .
OA Ru2
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13. Summary of strap footing design is shown in the followi ng steps.
3-Draw shear and moment diagrams.( L -direction )
Design of Strap Footings.
S.F.D
S
U1U2
%
+8a
%
+8a
O
.
.
OA
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14. S
B.M.D
U1U2
%
+8a
%
+8a
O
N
+-&. N
+/
.
.
OA
Summary of strap footing design is shown in the followi ng steps.
Design of Strap Footings.
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15. Summary of strap footing design is shown in the
following steps.
Estimate effective depth, d, for footing (1) by 3-w ay
punching shear under column (1) and for footing (2 )
by 4-way punching shear under column (2)
Design of Strap Footings.
4-Find depth of concrete.
Design footing depth, d, for the worst case of two-
way action and wide-beam shear, obtain wide-beam
shear from shear force diagram.
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16. Summary of strap footing design is shown in the
following steps.
a-Reinforcement in L-direction
Design of Strap Footings.
5-Design footing reinforcement as a spread footing for
both direction
Select the moments ( refer to moment diagram ) and
estimate the required reinforcement .
16
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17. Summary of strap footing design is shown in the
following steps.
a-Reinforcement in B-direction
Design of Strap Footings.
5-Design footing reinforcement as a spread footing for
both direction
For footing (1) :

=
a
Y d

2
N
+ =
%
+8 S
O
2
For footing (2) :

O =
a
OYd
O
2
N
+O =
%
+O8 S
O
2
Check the reinforcement and select bars.
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18. Summary of strap footing design is shown in the
following steps.
Design of Strap Footings.
6-Design strap as beam but check if it is a deep be am
a) Depth of strap beam
The shear is constant in strap beam. Assume that th e width
of the strap, b (with the smallest of column 1 and 2 )
# =
0
+1)2&3
4
TAdA
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19. Summary of strap footing design is shown in the
following steps.
Design of Strap Footings.
6-Design strap as beam but check if it is a deep be am
b) Reinforcement of strap beam Select the appropriate moment . Using moment, d,
and strength of materials and estimate the reinforc ement
N
56ulbmA
*
6
*
6
= the total reinforcement in strap beam
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20. Summary of strap footing design is shown in the
following steps.
Positive moment tension reinforcement shall be limi ted to a
diameter such that ldcomputed for by equation :
Design of Strap Footings.
5-Design footing reinforcement as a spread footing for
both direction
c) Development of positive moment reinforcement at
simple supports and at points of inflection.
7
8
N
'
0
+
+ 9
&:.
;
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Dr. Abdulmannan Orabi IUST

21. Summary of strap footing design is shown in the
following steps.
Design of Strap Footings.
N
'
0
+
+ 9
&:.
;
Where : N
!
is nominal moment strength assuming all reinforceme nt
at the section to be stressed to the specified yiel d strength.
0
+
is factored shear force at the section.
.
&
is embedded length of bar past point of zero moment but
not exceed the greater of (d) or ( 12 d b).
An increase of 30% in the value of shall be pe rmitted
when the ends of reinforcement are confined a comp. reaction.
N
!
0
5
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22. Design of Strap Footings.
Example
Design a strap footing to support two columns, show n in
Figure below, spaced at a distance of 6.0 m center- to-center.
Column A is 300 mm
×
400 mm and carries a dead load of
500 kNand a live load of 300 kN.Column B is also 300
mm
×
500 mm in cross section and carries a dead load of
700kNand a live load of 500 kN.
Use 7
8L < AN=band 7>
?
L 7 AN=b
%
1RR
'()L H< AA@=b
6.0 m
Property
line
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Dr. Abdulmannan Orabi IUST