3. FOOTINGS
What is Footing ?
• Footings are structural members used to support
columns and walls and to transmit and distribute
their loads to the soil.
4. Different types of Footings:
1. Spread or isolated
footing:
A spread footing is provided to
support an individual column.
It is circular, square or
rectangular of
uniform thickness.
5. 2. Continuous or
strip footing:
The footing beneath a wall is
known as a strip or
continuous footing. It is
provided for a raw of columns
which are so closely spaced
that spread footings are
overlap or nearly touch each
other.
6. 3. Combined
footing:
A Combined footing
supports two columns.
It is provided when two
columns are close
together, causing
overlap of adjacent
isolated footings and
where soil bearing
capacity is low, causing
overlap of adjacent
isolated footings.
7. 4. Strap or cantilever
footing
A strap footing consists of
two isolated footings
connected with a structural
strap or a lever. The strap
connect two footings such that
they behave as one unit.
8. The Design Procedure.
• 1.Determine the structural loads and member sizes at the
foundation level;
• 2.Collect all the geotechnical data; set the proposed footings on
the geotechnical profile;
• 3. Determine the depth and location of all foundation elements,
• 4.Determine the bearing capacity,
• 5.Determine possible total and differential settlements; check
effects at 2B depths;
• 6.Select the concrete strength (and possibly the mix),
9. The Design Procedure:
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7.Select the steel grade,
8.Determine the required footing dimensions,
9.Determine the footing thickness, T (or D in some textbooks),
10.Determine the size, number and spacing of the reinforcing
bars,
• 11.Design the connection between the superstructure and the
foundation, and
• 12.Check uplift and stability against sliding and overturning of
the structure-soil system.
10. Ground investigation for
footings
The recommended minimum depth of
investigation, za, for spread foundations
supporting high-rise structures and civil
engineering projects is the greater of:
za ≥ 3bF and za ≥ 6m, where bF is the
foundation’s breadth.
For raft foundations:
za≥ 1.5bB, where bB is the breadth of the
raft.
The depth za may be reduced to 2m if the
foundation is built on competent strata†
with ‘distinct’ (i.e. known) geology.
With ‘indistinct’ geology, at least one
borehole should go to at least 5m. If
bedrock is encountered, it becomes the
reference level for za.
Fig : Recommended depth of investigation
for spread foundations
11. Design situations and limit states
Examples of some ultimate limit states that footings must be designed to withstand
12. Design considerations for footings
Euro code 7 lists a number of things that must be considered when choosing
the depth of a spread foundation
13. Footings subject to vertical actions
• For a spread foundation subject to vertical actions, Euro code 7 requires
the design vertical action Vd acting on the foundation to be less than or
equal to the design bearing resistance Rd of the ground beneath it:
Vd ≤ R d
Vd should include the self-weight of the foundation and any backfill on it.
• This equation is merely a re-statement of the inequality:
• Ed ≤ Rd.
• Rather than work in terms of forces, engineers more commonly consider
pressures and stresses, so we will rewrite this equation as:
• qEd ≤ qRd
• where qEd is the design bearing pressure on the ground (an action effect),
and qRd is the corresponding design resistance.
14. Figure 1 shows a footing carrying characteristic vertical actions VGK
(permanent) and VQK (variable) imposed on it by the super-structure. The
characteristic self weights of the footing and of the backfill upon it are both
permanent actions (WGK). The following sub-sections explain how qEd and qRd
are obtained from VGK, VQK, WGK, and ground
Figure 1. Vertical actions on a spread
foundation
15. Effects of actions
The characteristic bearing pressure qEK shown in Figure 1 is given by:
Where Vrep is a representative vertical action; VGK, VQK, and WGK are as defined above; A' is the footing’s
effective area and ψi is the combination factor applicable to the ith variable action.
If we assume that only one variable action is applied to the footing, this equation simplifies
to:
since ψ = 1.0 for the leading variable action (i = 1).
The design bearing pressure qEd beneath the footing is then:
Where γG and γQ are partial factors on permanent and variable actions, respectively.
16. Eccentric loading and effective foundation area
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The ability of a spread foundation to
carry forces reduces dramatically when
those forces are applied eccentrically
from the centre of the foundation.
To prevent contact with the ground
being lost at the footing’s edges, it is
customary to keep the total action
within the foundation’s ‘middle-third’.
In other words, the eccentricity of the
action from the centre of the footing is
kept within the following limits:
where B and L are the footing’s breadth and
length, respectively; and eB and eL are
eccentricities in the direction of B and L (see
Figure 2)
Figure 137. Effective area of spread
foundation
17. Different types of load:
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Dead load
Live load
Earth pressure
Water pressure
Earthquake loads
Wind loads
Snow loads
