UNIT - II

Materials and Construction

• Sub Structure

• Settlement

• Types of Load

• Super Structure

- Basavaraj S Tavade

FOUNDATION or SUBSTRUCTURE

• Every building consists

of two basic components :

1. The Super-structure.

2. The Sub-structure.

FOUNDATION or SUBSTRUCTURE

The substructure or foundation is the part of structure

which is usually below the ground level and in direct

contact with the soil, through which the load of

superstructure is transmitted to the soil.

or

It is the lowest artificially prepared part of a structure

below the ground level which provides base for

superstructure and transmits all loads from

components parts of the building to the soil on which

building rests.

Functions of Foundation:

1. Reduction of load intensity.

Foundations distribute the load of the superstructure to a

larger area so the total intensity of load doesn't exceed the

Safe bearing capacity of soil.

2. Even distribution of load.

Foundations distribute the non uniform load of the super

structure evenly to the subsoil.

3. Provision of level surface.

Foundations provide a levelled and hard surface over which a

super-structure can be built.

Functions of Foundation:

4. Lateral stability.

It anchors the super-structure to the ground thus imparting

stability to the building.

5. Safety against undermining.

It provides safety against undermining or scouring due to

burrowing animals & flood water.

6. Protection against soil movements.

Special measures prevent or minimise the distress (cracks) in

superstructure, due to expansion or contraction of sub-soil.

Types of Foundation:

Shallow Foundation:

– when the depth of foundation is less than or equal

to its width, it is defined as Shallow foundation.

Deep Foundation:

-- when the depth of foundation is more than its

width, it is defined as Shallow foundation.

Types of Foundation:

Foundation

Shallow Foundation

Wall Footing/ Strip Footing

Isolated Footing

Combined Footing

Cantilever / Strap Beam Footing

Mat / Raft Footing

Deep Foundation

End Bearing pile

Friction pile

Wall Footing / Strip Footing

• It is the footing provided throughout the length of the

wall in the load bearing walls, then it is called wall

footing.

• Simple footings are used for light structure.

• the projection of Simple footings base is usually 15 to

20cm.

• Stepped footings are similar to simple footing except

that they have more than one projection or steps.

• Stepped footings are used to transfer heavy load.

Isolated / Spread footing

• When footing is provided to support an individual

column, it is called “isolated footing”.

• They may be square, rectangular, circular or sloped

in shape depending on the distribution of load

required.

• Isolated sloped column footing is the most

commonly used footing.

Combined Footing :

• A footing which Supports more than one column.

Situations where Combine Footing is provided

1. It is used when the two column are so close to each

other that their individual footings would overlap.

2. A combined footing is also provided when the

property line is so close to one column that a spread

footing would be eccentrically loaded when kept

entirely within the property line. By combining it

with that of an interior column, the load is evenly

distributed.

Combined Footing :

• A combine footing may be rectangular or trapezoidal

in plan.

• Rectangular Combined footing is used if sufficient

space is available beyond each column.

• If the load on the both column is same.

• And if the distance between the column is less

• Trapezoidal combined footing is provided when the

load on one of the column is different, because more

footing area is required for supporting the column

carrying heavy load and also if the distance between

the column is large.

Here in the above figure we can clearly see that, two columns footings are overlapping. So instead of providing individual isolated / spread footing , they have provided the combined

footing

Here in the above figure we can clearly see that, two columns footings are overlapping. So instead of providing individual isolated / spread footing , they have provided the combined

footing

Strap Beam / Cantilever Footing :

• A strap (or cantilever) footing consists of two

isolated footings connected with a structural strap or

a lever.

• A strap footing is more economical than a combined

footing when the allowable soil pressure is relatively

high and the distance between the columns is large.

Mat / Raft Footing :

• In case when the soil conditions are poor and a pile

foundation is not economical than the load of the

columns is transferred to a mesh or a mat supporting

all of the columns.

• A mat or raft is a thick reinforced concrete slab

which supports all the load bearing wall or column

loads of a structure & covers the entire area of the

building.

• Mat foundations are useful in reducing the

differential settlements on non-homogeneous soils

or where there is large variation in the loads on the

individual columns.

COLUMN

Deep Foundation / Pile Foundation:

• A pile is basically a long cylinder of a strong

material such as concrete that is pushed into the

ground to act as a steady support for structures built

on top of it.

Pile foundations are used in the following situations:

1. When a building has very heavy concentrated loads,

such as in a high rise structure, bridge, or water tank.

2. When there is a layer of weak soil at the surface.

There are two types of pile foundations.

1. End bearing pile :

• In end bearing piles, the bottom end of the pile rests

on a layer of especially strong soil or rock.

• The load of the building is transferred through the

pile onto the strong layer.

• End bearing piles are useful when hard strata is

available within reasonable depth in loose soil

having low bearing capacity.

2. Friction Piles :

• The principle of this type of piles is, “The pile

transfers the load of the building to the soil across

the full height of the pile by friction”

• These piles are used where hard strata is available at

a very great depth.

• Friction files are useful in cohesive soil with low

bearing capacity

Settlement:

• The downward movement of the base of footing is

known as settlement

The rate of settlement depends on

1. Magnitude of bearing capacity and load acting.

2. Time elapsed since loading

The settlement is classified into two categories

1. Uniform Settlement:

If the vertical movement of all the parts structure

is equal in magnitude, it is known as uniform

settlement

Structures on rigid foundation undergo uniform

settlement.

Reasons for uniform settlement

1. Lowering of the ground water table.

2. Swelling and shrinkage of expansive soil caused by

seasoned variations.

2. Differential Settlement:

If one part the structure settles more than the

other part, then the difference between two

settlements is known as differential settlement.

It induces heavy stresses in the structure leading

to cracks in superstructure.

The ratio of the differential settlement between

two footings to the distance between them is known

as angular distortion.

2. Differential Settlement:

Causes of differential settlement:

1. Unequal intensity of loading on the foundation by

superstructure.

2. Unequal swelling and shrinkage of soil.

3. Overstressing of soil by changes in adjoining

structure.

4. Erosion of soil.

5. Presence of deep roots of trees below one

foundation.

SUPER STRUCTURE :

Dead Load:

“It is defined as the force whose magnitude, position, and

direction remains constant”.

The weight of all the permanent parts of a building such

as walls, columns, beams, floors, roofs, stairs, doors etc is

considered as dead load.

The unit weight of common materials used in building

construction is given by IS 875-1 1987

Building Material Unit Weight (KN/m3)

Stones 20 to 28

Steel 78

Plain cement concrete 22 to 24

Reinforced cement concrete 25

Structural Timber 7

Sand and Loose gravel 16 to 20

Tiles 18

Commonly Used Building Materials and their Unit

weight

SUPER STRUCTURE :

Live Load:

The load which consist of moving or variable loads due to

occupants, weight of furniture, home appliances, equipments

etc.

Live load should be considered as per the

recommendation of IS 875-2 1987.

Type of FloorMinimum Live Load

(KN/m2)

Residential building, hospital

work2.0

Office rooms, small work

places2.5

Bank and reading rooms 3.0

Shops, class rooms, assembly

halls4.0

Warehouses, workshops,

factories, dance halls5.0

The minimum live load to be considered as per IS 875-

2 1987 is as below

Number of Floors Reduction in Live Load

One floor No reduction

Two floor 10%

Three floor 20%

Four floor 30%

Five floor 40%

Six floor and all other

subsequent floor50%

The live load for upper floor is reduced by applying

reduction factor as below

SUPER STRUCTURE :

Wind load:

When wind is obstructed by the structure, it exerts a

pressure on the structure known as wind pressure.

as per IS 875-3 1987 basic wind pressure may be obtained

by

P = KV2

where P = wind pressure in kg/m2

V = Wind velocity in kmph

K = Coefficient which depends on wind velocity,

air temperature, size of the building.

SUPER STRUCTURE :

Types of construction:

1. Load bearing structures :

• In this type of construction, load of the structure is

transferred to the walls as, roof and floors are directly

supported on walls.

• walls in turn transfer the load to the underlying soil

through wall footing.

• this type of construction is suitable and economical up to

two storey's.

• The load transfer mechanism is shown in figure.

SUPER STRUCTURE :

Types of construction:

2. Frame structures :

• In this type of construction, load is transferred through a

frame of slab, beam, column and footing to the underlying

soil.

• Slab and beams are the flexural members of the structure.

• Columns and footings are the compression members of

the frame.

• Walls do not carry any load.

• The load transfer mechanism is shown in figure.

SUPER STRUCTURE :

Types of construction:

3. Composite structures :

• This is a combination of load bearing and frame structure

• The composite structure is shown in figure.

• Here the outer walls can be of load bearing type where as

column and beam structure can be provided internally.