building substructure

Building Substructure | Components of Building Substructure | Functions of Building Substructure


In this article, we will discuss building substructures.


  1. Introduction  

Building substructure is the lower part of a structural system that 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 the 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 the 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 :

1. Foundation

2. Plinth Beam


  1.2.1. Foundation  

Foundation transfers the dead load, live load, and all other loads coming from the 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.


1. 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.


Read Also: Shallow Foundation, Types of Shallow Foundation


b. Raft Foundation 

This is the foundation that is made up of a 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 the 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 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 the bearing capacity of the soil is least and the distance between adjacent isolated footings is very less( 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 reinforced concrete or pre-stressed concrete.

For example; The deep foundation is made below the water level and is mostly used in bridges & skyscrapers.


Types of Deep Foundation


a. Pile Foundation 

A long cylindrical foundation that is analogous to a pole is said to be a pile foundation.

Such kind of foundation can be made up of strong materials such as concrete so that it hardens the strata and transfers the heavy loads from the superstructure to below the ground level.

Pile foundation is considered 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 installment methods.

Its caissons are watertight structures that are made up of steel, wood, or reinforced concrete and sunken into the ground.

There are drilled shafts, those are the caissons or bored piles. They support structures with large axial & lateral loads.


2. Functions of Foundation

a. To provide the stability to the entire structure.

b. To prevent the differential settlements.

c. To transfer the loads from the superstructure to the soil beneath.


3. Advantages of Foundation

1. Less Risk of Damage from Flooding or Leaking of Gasses.

2. Protection from Pests.

3. Cost Savings.

4. Transmits load of the super-structure to the ground.


4. Disadvantages of Foundation

1. Chance of settlement

2. Require more reinforcement and Concrete.

3. Increase Cost.



 1.2.2. Plinth Beam  

A plinth beam is generally constructed in areas that are more prone to earthquakes.

It is effective to use as it reduces the length of columns by reducing their slenderness.

A plinth beam is provided at plinth level and refers to a horizontal structural element that interconnects columns thereby preventing buckling of columns.


1. Functions of Plinth Beam 

a. Prevents the differential settlements in foundation.

b. Transfers the load coming from the superstructure to the foundation.

c. Check and prevents the ingress of moisture.

d. Acts as ties to interconnect columns.


2. Advantages of Plinth Beam

a. To sustain the masonry load at ground floor level.
b. The aim of plinth beams is to withstand the dead weight of the masonry wall


3. Disadvantages of Plinth Beam

a. Increases Cost.

b. Need more reinforcement and concrete.


Read Also: Plinth Beam, Design and Construction of Plinth Beam, Function