The important guidelines given in IS 456 : 2000 for the design of isolated footings are as follows:
Specifications for Design of
footings as per IS 456 : 2000
The
important guidelines given in IS 456 : 2000 for the design of isolated footings
are as follows:
1 General
Footings shall be designed to sustain the applied
loads, moments and forces and the induced reactions and to ensure that any
settlement which may occur shall be as nearly uniform as possible, and the safe
bearing capacity of the soil is not exceeded (see IS 1904).
1.1In
sloped or stepped footings the effective cross-section in compression shall be
limited by the area above the neutral plane, and the angle of slope or depth
and location of steps shall be such that the design requirements are satisfied
at every section. Sloped and stepped footings that are designed as a unit shall
be constructed to assure action as a unit.
1.2Thickness
at the Edge of Footing
In
reinforced and plain concrete footings, the thickness at the edge shall be not less
than 150 mm for footings on soils, nor less than 300 mm above the tops of piles
for footings on piles.
1.3
In
the case of plain concrete pedestals, the angle between the plane passing
through the bottom edge of the pedestal and the corresponding junction edge of
the column with pedestal and the horizontal plane (see Fig. 20) shall be
governed by the expression:
2 Moments and Forces
2.1In the case of
footings on piles, computation for moments and shears may be based on the
assumption that the reaction from any pile is concentrated at the centre of the
pile.
2.2For the purpose of
computing stresses in footings which support a round or octagonal concrete
column or pedestal, the face of the column or pedestal shall be taken as the
side of a square inscribed within the perimeter of the round or octagonal
column or pedestal.
2.3Bending Moment
2.4The bending moment
at any section shall be determined by passing through the section a vertical
plane which extends completely across the footing, and computing the moment of
the forces acting over the entire area of the footing on one side of the said
plane.
2.5The greatest bending
moment to be used in the design of an isolated concrete footing which supports
a column, pedestal or wall, shall be the moment computed in the manner
prescribed in 34.2.3.1 at sections located as follows:
a) At
the face of the column, pedestal or wall, for footings supporting a concrete
column, pedestal or wall;
b)
Halfway between the centre-line and the
edge of the wall, for footings under masonry walls; and
c) Halfway
between the face of the column or pedestal and the edge of the gussetted base,
for footings under gussetted bases.
2.4
Shear and Bond
2.4.1 The shear strength of footings is governed by
the more severe of the following two conditions:
a) The
footing acting essentially as a wide beam, with a potential diagonal crack
extending in a plane across the entire width; the critical section for this
condition shall be assumed as a vertical section located from the face of the
column, pedestal or wall at a distance equal to the effective depth of footing
for footings on piles.
Two-way
action of the footing, with potential diagonal cracking along the surface of
truncated cone or pyramid around the concentrated load; in this case, the
footing shall be designed for shear in accordance with appropriate provisions
specified in 31.6.
2.4.2In computing the
external shear or any section through a footing supported on piles, the entire
reaction from any pile of diameter D_{p} whose centre is located D_{P}/2
or more outside the
section shall be assumed as producing shear on the
section; the reaction from any pile whose centre is located D_{P}/2 or
more inside the section shall be assumed as producing no shear on the section,
For
intermediate positions of the pile centre, the
portion of the pile reaction to be assumed as producing shear on the section
shall be based on straight line interpolation between full value at D_{P}/2
outside the section and zero value at D_{P}/2 inside the section.
2.4.3The critical section for checking the
development length in a footing shall be assumed at the same planes as those
described for bending moment in 34.2.3 and also at all other vertical planes
where abrupt changes of section occur. If reinforcement is curtailed, the
anchorage requirements shall be checked in accordance with 26.2.3.
3.Tensile Reinforcement
The total tensile reinforcement at any section shall
provide a moment of resistance at least equal to the bending moment on the
section calculated in accordance with 34.2.3.
3.1Total tensile reinforcement shall be distributed
across the corresponding resisting section as given below:
a)In
one-way reinforced footing, the-reinforcement extending in each direction shall
be distributed uniformly across the full width of the footing;
b) In
two-way reinforced square footing, the reinforcement extending in each
direction shall be distributed uniformly across the full width of the footing;
and
c) In
two-way reinforced rectangular footing, the reinforcement in the long direction
shall be distributed uniformly across the full width of the footing. For
reinforcement in the short direction, a central band equal to the width of the
footing shall be marked along the length of the footing and portion of the
reinforcement determined in accordance with the equation given below shall be
uniformly distributed across the central band:
Reinforcement in central
band width / Total reinforcement_in_short_direction
where ? is the ratio of the long side to the short
side of the footing. The remainder of the reinforcement shall be uniformly
distributed in the outer portions of the footing.
4 Transfer of Load at the Base of
Column
The compressive stress in concrete at the base of a
column or pedestal shdl be considered as being transferred by bearing to the
top of the supporting Redestal or footing. The bearing pressure on the loaded
area shall not exceed the permissible bearing stress in direct compression
multiplied by a value equal to
but
not greater than 2, where A_{1} = supporting area for bearing of
footing, which in sloped or stepped footing may be taken as the area of the
lower base of the largest frustum of a pyramid or cone contained wholly within
the footing and having for its upper base, the area actually loaded and having
side slope of one vertical to two horizontal; and A_{2} = loaded area
at the column base.
4.1Where the
permissible bearing stress on the concrete in the supporting or supported
member would be exceeded, reinforcement shall be provided for developing the
excess force, either by extending the longitudinal bars into the supporting
member, or by dowels (see 34.4.3).
4.2Where transfer of
force is accomplished by, reinforcement, the development length of the
reinforcement shall be sufficient to transfer the compression or tension to the
supporting member in accordance with 26.2.
4.3Extended
longitudinal reinforcement or dowels of at least 0.5 percent of the
cross-sectional area of the supported column or pedestal and a minimum of four
bars shall be provided. Where dowels are used, their diameter shall no exceed
the diameter of the column bars by more than 3 mm.
4.4Column bars of
diameters larger than 36 mm, in compression only can be dowelled at the
footings with bars of smaller size of the necessary area. The dowel shall
extend into the column, a distance equal to the development length of the
column bar and into the footing, a distance equal to the development length of
the dowel.
5 Nominal Reinforcement
5.1Minimum
reinforcement and spacing shall be as per the requirements of solid slab.
5.2The nominal
reinforcement for concrete sections of thickness greater than 1 m shall be 360
mm per metre length in each direction on each face. This provision does not
supersede the requirement of minimum tensile reinforcement based on the depth
of the section.