Lecture 7 strap footing

INTERNATIONAL UNIVERSITY
FOR SCIENCE & TECHNOLOGY
‫وا‬ ‫م‬ ‫ا‬ ‫و‬ ‫ا‬ ‫ا‬
CIVIL ENGINEERING AND
ENVIRONMENTAL DEPARTMENT
303421: Foundation Engineering
Strap Footing
Dr. Abdulmannan Orabi
Lecture
7

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
2Dr. Abdulmannan Orabi IUST

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
an eccentricity to the interior column footing so that
a uniform soil pressure is generated beneath both
footings. The strap footing may be used instead of a
rectangular or trapezoidal combined footing if the
distance between columns is large and / or the
allowable soil pressure is relatively large so that
the additional footing area is not needed.
Cantilever or Strap Footings.

Interior
column
Exterior
column
N1
N2

This is a special type of footing used for two columns.
The two columns are provided by two separate footings
connected by a rigid beam called “strap beam”.
S
B1
B2
L1 L2
N1 N2
Strap beam Interior
footing
Exterior
footing
Property
line
Strap Beam

Three basic considerations for strap footing design
are these:
1. Strap must be rigid—perhaps Istrap/Ifooting > 2.
This rigidity is necessary to control rotation of the
exterior footing.
2. Footings should be proportioned for approximately
equal soil pressures and avoidance of large differences in
B to reduce differential settlement.
3. Strap should be out of contact with soil so that there are
no soil reactions

It is common to neglect the strap weight in the design.
The strap should be adequately attached to the both
the column and the footing by the use of dowels such
that the footing and the strap act as unit.
The footing is subjected to one-way bending.
The strap beam is reinforced with main reinforcement
at top between the columns and at bottom under the
interior footing.

Summary of strap footing design is shown in the
following steps.
1- Proportion footing dimensions.
Design of Strap Footings.
= 0 − − = 0 (1)
= 0 − − − − = 0 (2)
Sum moments about the center of the interior column
and obtain the soil reaction beneath the exterior footing.
Sum moments about the center of the exterior footing
and obtain the soil reaction beneath the other footing.

following steps.
= 0 + − − = 0 (3)
To solve these three equations assume a value of
eccentricity, e. Find and check equation (3).,

S
N1 N2

R1 R2e


following steps.
!" (1) = 2 +
2
!# =
∗ %& ('())
!" (2) * = * = ∗* =
%& ('())
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 :

following steps.
2- Evaluate factored net soil pressure under the footings.
= 0 + − − , = 0
+ = 0 , − − + − − , = 0
%+ =
+
*
%+ =
+
*
U1

Ru1
U2

Ru2

Summary of strap footing design is shown in the following steps.
3- Draw shear and moment diagrams.( L- direction )
S.F.D
SU1 U2
%+ ∗ %+ ∗


S
B.M.D
U1 U2
%+ ∗ %+ ∗
+-&. +/

Summary of strap footing design is shown in the following steps.

following steps.
Estimate effective depth, d, for footing (1) by 3-way
punching shear under column (1) and for footing (2)
by 4-way punching shear under column (2)
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.

following steps.
a- Reinforcement in L- direction
5- Design footing reinforcement as a spread footing for
both direction
Select the moments ( refer to moment diagram ) and
estimate the required reinforcement .

following steps.
a- Reinforcement in B- direction
both direction
For footing (1) : =
−
2
+ =
%+ ∗
2
For footing (2) : =
−
2
+ =
%+ ∗
2
Check the reinforcement and select bars.

following steps.
6- Design strap as beam but check if it is a deep beam
a) Depth of strap beam
The shear is constant in strap beam. Assume that the width
of the strap, b (with the smallest of column 1 and 2 )
# =
0+1)2&3
4

following steps.
6- Design strap as beam but check if it is a deep beam
b) Reinforcement of strap beam
Select the appropriate moment . Using moment, d,
and strength of materials and estimate the reinforcement
56
*6
*6 = the total reinforcement in strap beam

following steps.
Positive moment tension reinforcement shall be limited to a
diameter such that ld computed for by equation :
both direction
c) Development of positive moment reinforcement at
simple supports and at points of inflection.
78
'
0+
+ 9& ≥ ;

following steps.
'
0+
+ 9& ≥ ;
Where :
! is nominal moment strength assuming all reinforcement
at the section to be stressed to the specified yield 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 db).
An increase of 30% in the value of shall be permitted
when the ends of reinforcement are confined a comp. reaction.
!
05

Example
Design a strap footing to support two columns, shown 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 kN and a live load of 300 kN. Column B is also 300
mm × 500 mm in cross section and carries a dead load of
700 kN and a live load of 500 kN.
Use 78 = 400 = and7>? = 20 = %& '() = 140 @=
6.0 m
Property
line

Lecture 7 strap footing

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Lecture 7 strap footing