Foundation Engineering
CE 483
6. Combined
foundation
CONTENTS
– Introduction
– Design of combined foundation
– References
CE 483 - Foundation Engineering - 6. Combined foundations
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Introduction
 Definition
 Types of combined foundations
 Why do we use combined foundation?
CE 483 - Foundation Engineering - 6. Combined foundations
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Introduction
 Definition
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).
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CE 483 - Foundation Engineering - 6. Combined foundations
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CE 483 - Foundation Engineering - 6. Combined foundations
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Combined foundation
may have several
types.
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Introduction
 Types of combined foundations
• Rectangular
•
•
•
Column
Column
Cross section
This is the simplest type.
Plan view
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Introduction
 Types of combined foundations
• Rectangular
• Trapezoidal
•
•
Column
Column
View plan of Trapezoidal combined Footing
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Introduction
 Types of combined foundations
• Rectangular
• Trapezoidal
• Cantilever (Strap)
•
– 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.
Column
Column
Cross section
Strap
Footing
Footing
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.
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Introduction
 Types of combined foundations
• Rectangular
• Trapezoidal
• Cantilever
• Tee-shaped
Column
Column
Plan view
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Introduction
 Why do we use combined foundation?
Combined footing is usually used when:
1.
2.
3.
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
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Design of combined foundation
 Forces
 Types of design
 Rectangular combined foundation
 Trapezoidal combined foundation
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Design of combined foundation
 Forces
Property boarder line
Flexural reinforcement
Tensile
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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
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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)
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Geometric Design of
Rectangular combined
foundation
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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
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Design of combined foundation
 Design of Rectangular combined foundation
Step #5.
The width B is then
calculated from:
(5)
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Geometric Design of
Trapezoidal combined
foundation
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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)
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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.
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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.
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Class Example/ Solution
Note. Because the FS is applied on
the loads we should use the
ultimate bearing capacity qult
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Class Example/ Solution (cont..)
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References
1. Braja M Das, 2011, Principles of Foundation Engineering, 7th ed,
Chapter- 6.
2. Previous course materials and presentations at KSU.
3. Geotechnical on the web:
http://environment.uwe.ac.uk/geocal/foundations/founbear.htm.
4. Andrew Bond and Andrew Harris, 2008, Decoding Eurocode 7, London.
5. The Institution of Structural Engineers library:
www.istructe.org/resources-centre/library
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