Lecture 7 strap footing

INTERNATIONAL UNIVERSITY

FOR SCIENCE & TECHNOLOGY

�م وا�� ا�� ا��و�� 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 columnand obtain the soil reaction beneath the exterior footing.

Sum moments about the center of the exterior footingand obtain the soil reaction beneath the other footing.





� � = 0 �� + �� − � − � = 0(3)

To solve these three equations assume a value of eccentricity, e. Find and check equation (3).�, �


S

N1 N2

��

R1 R2e

��

� ��

� �

��

� �






�� !"(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 :




2- Evaluate factored net soil pressure under the footings.

� �� = 0 +� � − � − ,�� = 0

� +� = 0 ,� � − � − +� � − � − ,�� = 0

%+� = +�*�

%+� = +�*�

U1

��

Ru1

U2

��

Ru2



3- Draw shear and moment diagrams.( L- direction )


S.F.D

SU1 U2

%+ ∗ �� %+ ∗ ��

��


S

B.M.D

U1 U2

%+ ∗ �� %+ ∗ ��

�+-&. �+/

��





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.



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 .



a- Reinforcement in B- direction



For footing (1) : �� = �� − ��2

�+� = %+� ∗ ��

2For footing (2) : �� = �� − ��

2�+� = %+� ∗ ��

2Check the reinforcement and select bars.




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&34� �




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



Positive moment tension reinforcement shall be limited to a diameter such that ld computed for by equation :



c) Development of positive moment reinforcement at simple supports and at points of inflection.

78�'0+

+9& ≥ �;20Dr. Abdulmannan Orabi IUST



�'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


Design of Strap Footings.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

Propertyline
