Building substructure is the lower part of a structural system which is constructed beneath the ground level and is hidden from view.
Building substructure transfers the load of the superstructure to the ground beneath. It also supports and anchors the superstructure safely in the earth.
It should be designed with the proper technique because it assures the stability of superstructure too.
1.1. Functions of Building Substructure
a. It provides the stability to the structure as a whole.
b. It distributes the loads from superstructure to the soil beneath.
c. It anchors the superstructure safely into the earth.
1.2. Components of Building Substructure
Building Substructure has the following two components :
Foundation transfers the dead load, live load and all other loads coming from superstructure to the soil beneath. As stability of the building depends upon the strength of the foundation; it should be made more reliable and strong.
A. Functions of Foundation
a. To provide the stability to the entire structure.
b. To prevent the differential settlements.
c. To transfer the loads from superstructure to the soil beneath.
B. Types of Foundation
There are two types of foundations naming shallow foundation and deep foundation.
1. Shallow Foundation
It is the type of foundation which is placed near the earth’s surface that transfers the loads to the soil at shallow depth.
The depth of the shallow foundation is less than or equal to the width of the foundation. It is categorized into four types. They are:
a.Strip foundation(Strip Footings)
It is the foundation that supports the linear structure such as walls, columns, etc which are constructed above it.
The size and position of this foundation depend on the width of the linear structure, bearing capacity of the soil and extent of the load. This foundation is also known as pad foundation.
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This is the foundation that is made up of large concrete slab which can support the loads exerted by walls, columns constructed over it. It is also called the mat foundation.
As we know; when area increases the pressure decreases. Similarly, raft footing has a large surface area which reduces the stress on the soil. So, It is used when the soil has a low load-bearing capacity.
c. Isolated Footings
It is constructed beneath the column to distribute the concentrated loads safely to bed soil. It supports only for single columns. It is used when the load extent is low and the soil is homogeneous.
d. Combined Footings
This kind of footings is built for supporting two or more columns in a straight line when bearing capacity of the soil is least and the distance between adjacent isolated footings is very (or overlapping of foundations).
Example:-Strap footing (cantilever) is the independent footing that is formed of two columns connected by beams.
2. Deep Foundation
This kind of Foundation is placed at great depth from the surface level of the earth to transfer the load to the deep layers of soil beneath it.
Such type of foundation can mainly be made out of steel, timber, reinforced concrete or pre-stressed concrete.
For example; A good foundation which is made below the water level mostly for bridges & skyscrapers.
Types of Deep Foundation
A long cylindrical foundation which is analogous to a pole is said to be pile foundation.
Such kind of foundation can be made up of strong materials such as concrete, steel, timber, etc. so that it hardens the strata and transfers the heavy loads from the superstructure to below the ground level.
Pile foundation is considered as more compact, less compressible and stiffer.
When to use the pile foundation
a. When load extent is high & non-uniform.
b. When the groundwater table is high.
c. When the soil has a very low load-bearing capacity.
Most of the foundation depends on their instalment methods.
Its caissons are the watertight structures which are made up of steel, wood or reinforced concrete built above the ground level and the sunken into the ground.
There are drilled shafts, those are the caissons or bored piles. They support structure with large axial & lateral loads.
Plinth beam is generally constructed in areas which are more prone to earthquake. It is effective to use as it reduces the length of columns by reducing their slenderness.
Plinth beam is provided at plinth level and refers to a horizontal structural element that interconnects columns thereby preventing buckling of columns.
A. Functions of Plinth Beam
a. Prevents the differential settlements in foundation.
b. Transfers the load coming from superstructure to the foundation.
c. Check and prevents the ingress of moisture.
d. Acts as ties to interconnect columns.
|Read Also: Plinth Beam, Design and Construction of Plinth Beam, Function|