building substructure

Building Substructure

Building substructure is the lower part of a structural system which is constructed beneath the ground level so, hidden from view. It transfers loads of the structural system i.e building, bridge, etc. to the ground which has the load-bearing capacity. It supports and anchors the superstructure safely. It should be designed with the proper technique because it assures the stability of superstructure too. Example:- the multistory structure has basement parking which is a kind of substructure. 

Components of Building Substructure

Building Substructure has the following two components :

1.Foundation

2.Plinth Beam

 

1.Foundation

Foundation transfers dead loads, live loads and all other loads which are exerted on it to the soil beneath. As stability of building depends upon the strength of the foundation, it should be made more reliable and strong. 

 

Types of Foundation

A.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. It is categorized into four types. They are explained below:-


a.Strip foundation(Strip Footings)

It is the foundation that supports the linear structure such as walls, columns, etc which are constructed above. 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.


b.Raft Foundation 

This is the foundation that is made up of large concrete slab that 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 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 where soil bearing capacity is least that cause overlap between adjacent isolated footings.

Example:-Strap footings (cantilever) is the independent footings of two columns connected by beams.

 

B.Deep Foundation

This kind of Foundation is placed at great depth from surface level 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.

 

Types of Deep Foundation

 

a.Pile 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.

 

b.Pier Foundation

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.

 

2.Plinth beam

This is a reinforced concrete beam which is in framed or structured form constructed between the wall and the foundation. When the foundation suffers from the settlement, it is provided in such a way to prevent the extension or the propagation of cracks from its foundation into the wall above.

Plinth beam is generally constructed in such areas which are prone to earthquake. It is effective to be used as it reduces the length of columns by reducing their slenderness. Here’s something that needs to be noted, the strength of this plinth beam shouldn’t be smaller than 20 MPa and the minimum dimension of the plinth beam is about 20 cm depth having with the matching width of the foundation. To prevent the steel base from aggressive elements, the formwork for it has to be compacted sufficiently.

 

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