Download (a) Steel grillage foundation

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 The Foundation Can be classified
broadly into two types:
1. Shallow foundations
2. Deep foundations
 When the depth of foundation is equal to or less
than the width of foundation, then it is termed as
shallow foundation. It is also know as open
 A shallow foundation is placed immediately
below the lowest part of the superstructure.
 A footing is a foundation unit constructed in brick
work, stone masonry or concrete below the base
of the wall or column for the purpose of
distributing the structural-load over a wide area.
1. Spread footing
2. Combined footing
3. Strap footing
4. Mat or Raft foundation
1. Spread footing:
 The spread footing are those which spread the
superimposed Load of wall or column over a
larger area. The spread footing support either a
column or a wall.
 The spread footing may be of the following kinds:
i. Single footing for a column
ii. Stepped footing for a column
iii. Slopped footing (Trapezoidal) for a column
iv. Wall footing without step
v. Stepped footing for wall
vi. Grillage foundation
(i) Single footing for a column:
 A spread footing for a single column is either
known as the isolated or pad footing.
 In this case, the footing may consist of simple
concrete block projecting out from the column
face on all sides.
 The base dimensions of the concrete base should
not be less than twice the appropriate lateral
dimension of the column in that direction.
 The thickness of concrete block should atleast be
equal to side offset from the column face.
(ii) Stepped footing for a column:
 If the column load is more or if the safe bearing
pressure of the soil is less, the base area will be
 In such a case, it is necessary to provide masonry
offsets, to achieve larger spread, before the load is
transferred to the concrete base.
 The height and width of each should be so
proportioned that the rate of spread does not
exceed the permissible value for the masonry.
(iii) Slopped footing for a column:
 These are also known as Isolated or Individual column
footings, They have the projection in the concrete base.
 Due to the low bending strength, the footing
constructed with brick, stone or plain concrete required
considererable depth to be safe to carry heavy loads.
 The depth of plain concrete footing can be reduced
much, by providing reinforcement at its base to take up
tensile stresses.
 RCC column footing may be circular, rectangular or
square in plan.
Figure : Spread footing
(vi) Grillage foundations:
A grillage foundation is an isolated footing
generally provided, when heavy structural
loads from columns, piers or steel stanchions
are required to be transferred to a soil having
poor or low bearing capacity.
It can be broadly divided into two categories,
depending upon the material used:
a) Steel grillage foundation
b) Timber grillage foundation
(a) Steel grillage foundation:
 Fig. 3.4 shows steel grillage foundation for steel
 Steel grillage foundation consists of steel joints
or beams (Rolled Steel Joints – RSJ) which are
provided in single or double tiers.
 A Minimum cover of 10cm. is kept on the outer
sides of the external beams as well as above the
upper flanges of the top tier.
 The depth of concrete below the lower tier should
be atleast 15cm.
Figure : steel grillage foundation
(b) Timber grillage foundation:
 Times type of foundation is provided for heavily loaded
masonry walls or timber columns. This foundation is
specially useful in waterlogged areas, where the bearing
capacity of soil is very low and where. The loading on
the soil is limited to 50 to 60 kN/m2.
 The foundation uses timber planks and timber beams in
the place of steel joists. No concrete is embedded
between the timber joints. However, the bottom
concrete provided in steel grillage foundation is
replaced by timber platform constructed of timber
Fig(a) Timber Grillage foundation for
wooden post
Fig. 3.5(a) shows a typical timber grillage
foundation for a timber column. The
excavation for the base is levelled. The bottom
layer of timber planks of size 20 to 30cm.
Wide and 5.0 to 7.5 cm. thick is laid, side by
side; without any gap between them.
Over the top of this layer, a timber beam of
same section as that of the wooden post is
placed at right angles.
Fig. 3.5(b) Timber Grillage Foundation
For Masonry wall
Fig. shows the timber grillage
foundation for wall. The
foundation consists of bottom
layer of planks, over which the
wooden beams are placed at right
angles to the direction of the
3. Strap footing:
 A strap footing consists of two or more footing of
individual column by a beam, called a strap.
 The strap beam, connecting the spread footing of
the two columns, does not remain in contact with
soil and thus does not transfer any pressure to the
 The function of the strap beam is to transfer the
load of heavily loaded outer column to the inner
one. In doing so, the strap beam is subjected to
bending moment and shear-force and it should be
designed to withstand these.
Fig. 3.7 Strap Footing
Construction of building on black cotton or
expansive soils is very much dangerous due to
its volumetric changes with the change of
atmospheric conditions.
The differential settlement of the buildings,
caused by the movement of the ground due to
the alternate swelling and shrinkage, results in
formation of cracks thus formed are sometimes
15 to 20 cm. wide and 2.5 to 4.0 m. deep.
1.Precaution for Foundation in
Black Cotton soils:
- The foundation should be taken to such depths,
where the cracks cease to extend. The minimum
depth of foundation should be atleast 1.50m.
- The construction in black cotton soil should be
carried out during dry season.
- The external walls should be provided with plinth
protection at ground level, so that moisture does
not enter in foundation during monsoon.
2.Types of foundation in Black
Cotton soils:
i. Strip or pad foundation
ii. Pier foundation
iii.Under-reamed pile foundation
(i) Strip or pad foundation:
The strip foundation for walls and the pad
foundation for columns may be provided for
medium loads.
When the soil is soft and having poor bearing
capacity, a 30cm thick layer of ballast and
moorum should be provided and rammed and
then 30 cm. thick layer of coarse sand may be
placed. Fig 3.9
(ii) Pier foundation:
 For the walls carrying heavy loads, the pier
foundation with arches may be provided. The
piers are dug at regular interval and filled with
concrete, which are connected by concrete or
masonry arch and the wall may be constructed
over it.
 The arches are constructed with a gap above the
ground level, which may permit the movement of
soil during swelling and shrinkage operatings.
Causes of failure of foundation
• Unequal settlement of sub-soil.
• Unequal settlement of masonry.
• Horizontal movement of adjoining the
• Shrinkage due to with drawl of moisture from
soil below the foundation.
• Lateral pressure on walls.
• Action of atmosphere.
• Lateral movement of soil below foundation.
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Building Planning and Drawing
Building Planning and Drawing