1. Foundation Engineering CE 483
6. Combined foundation

2. CONTENTS Introduction Design of combined foundation References
CE Foundation Engineering Combined foundations

3. Introduction Definition Types of combined foundations
Why do we use combined foundation?
Three Ws  three dimensions to find a space
CE Foundation Engineering Combined foundations

4. Definition Introduction
Spread footing
Spread footings such as square and rectangular footings are economical for support columns under normal condition.
Combined footing is a system of foundation that support more than one columns (usually 2 columns).
CE Foundation Engineering Combined foundations

5. CE 483 - Foundation Engineering - 6. Combined foundations

6. CE 483 - Foundation Engineering - 6. Combined foundations

7. Combined foundation may have several types.
CE Foundation Engineering Combined foundations

8. Types of combined foundations
Introduction
Types of combined foundations
Rectangular
Column
Column
Cross section
This is the simplest type.
Plan view
CE Foundation Engineering Combined foundations

9. Types of combined foundations
Introduction
Types of combined foundations
Rectangular
Trapezoidal
Column
Column
View plan of Trapezoidal combined Footing
CE Foundation Engineering Combined foundations

10. Types of combined foundations
Introduction
Types of combined foundations
Rectangular
Trapezoidal
Cantilever (Strap)
Column
Column
Strap
Cross section
Footing
Footing
Consist of two single column footings connected with a cantilever beam or a strap. 
This type is used to connect an eccentrically loaded column footing to an interior column so that a uniform soil pressure is generated beneath both footings.
Strap beam
Plan view
Strap is used instead of other types if distance between columns is large and/ or the allowable soil pressure is relatively large so that the additional footing area is not needed.

11. Types of combined foundations
Introduction
Types of combined foundations
Rectangular
Trapezoidal
Cantilever
Tee-shaped
Column
Column
Plan view
CE Foundation Engineering Combined foundations

12. Why do we use combined foundation?
Introduction
Why do we use combined foundation?
Combined footing is usually used when:
one column is located at or near the property boarder line.
two columns are so close that single footings cannot be used.
supporting two columns of unequal loads
Property boarder
CE Foundation Engineering Combined foundations

13. Design of combined foundation
Forces
Types of design
Rectangular combined foundation
Trapezoidal combined foundation
Three Ws  three dimensions to find a space
CE Foundation Engineering Combined foundations

14. Design of combined foundation
Forces
Flexural reinforcement
Tensile
Property boarder line

15. Design of combined foundation
Forces
Longitudinally, the footing acts as an upward loaded beam spanning between columns and cantilevering beyond.
Using statics, the shear force and bending moment diagrams in the longitudinal direction are drawn.
The footing is also subjected to transverse bending

16. Design of combined foundation
Types of design
The geotechnical design, will include:
Determining of allowable bearing capacity of the soil
Determine the horizontal geometries of the combined foundation such as width, length and total area A.
The structural design will include:
Determining the thickness of the foundation
Reinforcement of the concrete (for shear and bending)

17. Geometric Design of Rectangular combined foundation

18. Design of combined foundation
Design of Rectangular combined foundation
Let’s consider the rectangular combined foundation shown in the figure.
The allowable bearing capacity qall is estimated from the bearing capacity equation (see previous chapter).
The geotechnical design of this foundation will require to determine the design area A, and dimensions L, B (keeping uniformly distributed pressure).
The following steps are used:
area A

19. Design of combined foundation
Design of Rectangular combined foundation
Step #5.
The width B is then calculated from:
(5)

20. Geometric Design of Trapezoidal combined foundation

21. Design of combined foundation
Design of Trapezoidal combined foundation
Let’s consider the Trapezoidal combined foundation shown in the figure.
We will assume that the allowable bearing capacity qall is known for the soil.
The geotechnical design of this foundation will require to determine the design area A, and dimensions B1, B2 (keeping uniformly distributed pressure as much as possible).
The following steps are used:
is determined
from:
Note that A also equal to:
(6)

22. Design of combined foundation
Design of Trapezoidal combined foundation
Note:
Q1 and Q2 might be given as dead load DL and live load LL.
Some design code recommends to use a factor of safety (1.4 for DL and 1.7 for LL).
In this case the ultimate bearing capacity qult should be used.

23. Design of combined foundation
Design of Trapezoidal combined foundation
Class Example
Design a trapezoid-shaped footing, with 2 columns at ends of footing as shown below, where qall = 120 kPa and L = 7 m.

24. Class Example/ Solution
Note. Because the FS is applied on the loads we should use the ultimate bearing capacity qult

25. Class Example/ Solution (cont..)

26. CE 483 - Foundation Engineering - 6. Combined foundations
References
Braja M Das, 2011, Principles of Foundation Engineering, 7th ed, Chapter- 6.
Previous course materials and presentations at KSU.
Geotechnical on the web:
Andrew Bond and Andrew Harris, 2008, Decoding Eurocode 7, London.
The Institution of Structural Engineers library:
CE Foundation Engineering Combined foundations