CE 310 – PRC II- LECTURE 10

FOOTINGS

MILITARY COLLEGE OF ENGINEERING

RISALPUR
 Outline

Ø Introduction to Footings
Ø Types of Footing
Ø Behaviour of Footings
Ø Bending Moment in Footings
Ø Shear in Footings
Ø Provision of ACI Code 318-14
Ø Design of Footings and Examples
Ø Wall Footing
Ø Spread Footing
Ø Rectangle Footing
 Introduction to Footings

General
ØThe foundation or sub-structure is that part of a
structure which is usually placed below ground level
and which transmits the load to the underlying soil.
ØAll soils compress when loaded and cause the
supported structure to settle. Two essential
requirements in the design of a foundation are:
• Differential settlement of the various parts of a structure
should be eliminated as far as possible.
• Total settlement of the structure should be limited to
tolerable limits.
Introduction to Footings

Leaning Tower of Pisa, Italy

 Introduction to Footings

ØTo limit the settlement, it is necessary to: -

• Transmit the load to a soil of sufficient
strength.
• Spread the load over a sufficiently large area
to minimize the bearing pressure.
ØIf suitable soil is not found, it is necessary to use
deep foundation such as piles or caissons. If
satisfactory soil is found underneath, it is merely
necessary to spread the load of the footing and such
a footing is called “Spread footing”.
 Types of Footing

Different types of footing are: -

Wall Footings
Ø Spread Footings Isolated Column Footings
Ø Strap footing Combined Footings

Ø Pile foundation
Ø Raft foundation
Ø Grid foundation
 Types of Footing

Types of Column Footings.

ØSingle Slab. In the simplest form, they consist of a single
slab. Single column footings are usually square in plan.
Rectangular footings are used if space restrictions dictate this
choice or the supported columns are of strongly elongated
cross section.
ØStepped Footing. In stepped footing, a pedestal or cap is
interposed between column and footing slab. Pedestal
provides for a favourable transfer of load and is required to
provide necessary development length of dowels. All parts
must be cast monolithically
 Types of Footing
Ø Slopped Footings. They require less concrete than stepped or
single slab but require additional labour.
 Types of Footing

Wall footing Isolated column footing

 Types of Footing

Combined Footing Strap Footing

 Types of Footing

Raft Foundation

Pile Foundation
Types of Footing

Grid foundation
Types of Footing
Types of Footing
Types of Footing
Behaviour of Footings
 Behaviour of Footings

Notations

a = Width of wall
c = Width of column
B = Width of footing
L = Length of column footing
DL = Dead load
LL= Live load
qa = Allowable bearing pressure / capacity
qe =qn= Effective / net bearing pressure / capacity
qun = Ultimate net / design soil pressure ( Factored )
Behaviour of Footings
 Behaviour of Footings

Basic Definitions :
1) Ultimate Bearing Capacity (qu) :
The ultimate bearing capacity is the maximum gross pressure
at the base of the foundation just before the soil failure.
2) Net ultimate Bearing Capacity (qnu) :
It is the net increase in pressure at the base of foundation that
cause shear failure of the soil.
Thus, qnu = qu – γDf (overburden pressure)
3) Gross Allowable Bearing Capacity (qs):
It is the gross pressure which the soil can carry safely without
shear failure.
qs =qu /FOS
 Behaviour of Footings

4) Net Allowable/safe Bearing Pressure (qna ):

It is the net bearing pressure which can be used for design of
foundation.
qna = qnu/FOS = (qu – γDf)/FOS
It is also known as Allowable Soil Pressure (ASP). FOS - Factor
of safety usually taken as 2.00 -3.00
Behaviour of Footings
Behaviour of Footings
Behaviour of Footings
 Behaviour of Footings
ØArea of footing provided is larger of all possible options
discussed above. Loads are un factored and values are taken
at the base of footing.

ØFor eccentrically loaded footing

q max / min = P/A ± My/I

if eccentricity falls outside the kern, one value of q is negative

Behaviour of Footings
Behaviour of Footings
Behaviour of Footings
Behaviour of Footings
Behaviour of Footings
Behaviour of Footings
Bending Moments in Footings
Bending Moments in Footings
Bending Moments in Footings
Bending Moments in Footings
 Shear in Column Footings

ØThe thickness of footings is mostly governed by SHEAR.

Since column footings are subjected to two way action i.e.
bend in both major direction, their performance in shear is
much like that of flat plate in the vicinity of column.
Ø It is NOT economical and practical to use shear
reinforcement in footing and shear is carried only by the
concrete.
ØTwo different types of shear strengths are distinguished in
footings: Two way shear or punching shear and one way shear
or beam shear.
 Shear in Column Footings

One-way and Two-way Shear

 Shear in Column Footings
One-Way Shear
Ø Shear failure can also occur, as in beam or one way slab, at
a section located at a distance ‘d’ from the face of the
column.
Ø Shear strength provided by concrete for members subjected
to flexure and shear is given by
Vc = 2 λ√fc’ b.d
where ‘b’ is the width of slab
subjected to shear, can be taken
as 12 inch if unit width is considered
λ = 1.0 for normal density concrete.
Shear in Column Footings
One-Way Shear
 Shear in Column Footings
Two-Way Shear
Ø The average shear stress in concrete that fails in this manner
can be taken as that acting on a vertical plane through the
footing, around the column, on a perimeter, at a dist ‘d/2’ from
the face of the column.
Shear in Column Footings
Two-Way Shear
Shear in Column Footings
Two-Way Shear
Shear in Column Footings
Two-Way Shear
Shear in Column Footings
Two-Way Shear
Shear in Column Footings
Shear in Column Footings
Two-Way Shear
 Shear in Column Footings
Two-Way Shear

Ø A column supported by footing slab tends to punch through

it because of the shear stresses that act in the footing slab
around the perimeter of the column.
Ø At the same time, the concentrated compressive stresses
from the column spread out into the footing so that the
concrete adjacent to the column is in vertical or slightly
inclined compression, in addition to shear.
Ø In consequence, if failure occurs, the fracture takes the form
of the truncated pyramid (as shown in fig) with sides sloping
outwards at an angle approaching 45º.
Shear in Column Footings
Two-Way Shear
Provisions of ACI Code 318-14
(Chap. 13 )
Provisions of ACI Code 318-14
Provisions of ACI Code 318-14
Provisions of ACI Code 318-14
 Provisions of ACI Code 318-14

Df/B ≤ 1
Shallow foundation
Df ≤ 3 m.
 Design of Footings

Load, Bearing Pressure and Footing Size

Ø Allowable soil pressure is determined from the principles
of Soil Mechanics. FOS 2.5 to 3.0
Ø Net soil pressure = qe = qn = qa- wt of footing - wt of soil
on top of footing
Ø For concentrically loaded footing
Area of footing required = (D+L)/qn
Ø Most codes permit 33% increase in allowable soil
pressure when effect of wind or earthquake is included
Area of footing required =( D+L+E)/1.33 qn
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Design of Footings
Design of Footings
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