A building substructure is the lower part of a building system constructed beneath the ground level and hidden from view.
✔ Building a substructure transfers the load of the superstructure to the ground beneath.
✔ It also supports and anchors the superstructure safely in the earth.
1. Functions of Building Substructure
a. It provides 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.
2. Components of Building Substructure
Building Substructure has the following two components :
a. Foundation
b. Plinth Beam
2.1. Foundation
The foundation transfers the dead load, live load, and all other loads from the superstructure to the soil beneath.
As the stability of the building depends upon the strength of the foundation; so it should be made more reliable and robust.
Note: SBC refers to the soil bearing capacity.
i. Functions of Foundation
a. To provide stability to the entire structure.
b. To prevent the differential settlements.
c. To transfer the loads from the superstructure to the soil beneath.
ii. Types of Foundation
There are two types of foundation naming shallow foundation and deep foundation.
a. Shallow Foundation
A shallow foundation is also called an open or wide foundation.
Those foundations that transfer the loads to subsoil near the ground floor of the building are called shallow foundations.
For this type of foundation, the depth of the foundation is less than or equal to the width of the foundation.
The features of a shallow foundation are as follows:
a. Depth is less or sometimes equal to its width.
b. It is placed immediately beneath the lowest part of the superstructures.
c. It is spread more horizontally than vertically.
d. It transfers the loads to subsoil at a shallow depth, close to the ground level.
Types of shallow foundation
There are four types of shallow foundations. They are:
1. Spread footing
2. Strip footing
3. Mat foundation
4. Grillage foundation
1. Spread Footing
This footing is also known as pad foundation.
In this type of foundation, the base is wider than the top to distribute the load from the superstructure over a large area.
This type of foundation is suitable for walls and masonry columns.
These foundations are constructed after opening the trenches to the required depth.
It is economical for a maximum depth of 3 m.
While constructing this type of footing, trenches are opened to the required depth, and the soil is rammed well. Then a plain concrete mix 1:4:8 is placed. Its thickness differs from 150 to 200 mm. The stone-masonry footing is constructed over this bed. It is built in courses, and each course is projected 50 to 75 mm from the top course, and the height of each course is 150 to 200 mm.
In the case of wall footing, the projections are provided only in one direction, while in the case of columns, they are provided in both directions.
The projection of bed concrete from the lowest course of foundation masonry is usually 150 mm.
Types of Spread Footing
There are three types of spread footing. They are:
a. Independent Footing / Isolated
b. Combined Footing
c. Continuous Footing
a. Isolated Column Footings / Isolated Footings
Separate footings are built for each column in isolated column footings.
For distributing the load of the columns safely and uniformly over the soil, the size of the footing is kept according to the area required.
Such footings are usually constructed over 100 or 150mm concrete beds.
Design engineers evaluate the required reinforcement and thickness of footing.
Footing thickness may be uniform or sometimes vary.
b. Combined Footings
In this type of footing, two or more columns are supported by a single base.
This type of footing is required when a column is exceptionally close to the property’s boundary; hence, providing footing beyond the column face is worthless.
This footing may be rectangular or sometimes trapezoidal.
The footing should be designed and constructed to safely transfer loads from both columns to the soil.
A strap beam is provided for connecting two columns.
c. Continuous Footings
In continuous footings, a footing is common for more than two columns in a row.
If the columns in a row are nearer or the SBC of soil is low, the continuous footing will be more applicable.
2. Strip Footing
Strip footing is the independent footing of two columns connected by a beam.
It is of four types.
They are:
a. Wall footing
b. Inverted arch footing
c. Eccentrically loaded footing
d. Offset and strap (cantilever) footing
a. Inverted Arch Footing
This type of foundation applies to areas where the SBC of the soil is incredibly poor, and the structure’s load is through walls. Thus, inverted arches are built between the walls.
End walls should withstand the outward horizontal thrust due to arch action. So, it should be sufficiently thick and strong.
The outer walls may be provided with buttress walls to strengthen them.
b. Eccentrically Loaded Footing
As far as practicable, the foundation should be so shaped and proportioned in such a way that the centre of gravity of the imposed loads coincides with the C.G of the supporting area of the base.
But the footing which is so shaped that the center of gravity does not coincide with the C.G of the supporting area of the base is known as eccentrically loaded footing.
3. Mat footing / Raft footing
Whenever the load on the column is extensive (multi-story column) or when the SBC of the soil is low, the foundations overlap each other.
In such a situation, it is beneficial to provide common footing to several columns, and this footing is called mat footing.
Load distribution is uniform in this footing.
It is also called raft footing.
The raft foundation in which the beams are built in both directions over the footing slab for connecting columns may be called a grid foundation.
The settlement is uniform in this type of footing; hence, unnecessary stresses are not developed.
4. Grillage Footing
Most high-rise buildings are constructed with steel columns encased in concrete. Such columns carry a heavy load and require special foundations for spreading the entire load to a larger soil area.
So grillage foundation is one such special foundation that is used where the load of the structure is excessive, the bearing capacity of the soil is poor, and a deep foundation is not possible.
It has one tier or more tiers of I-section steel beams.
The top tiers consist of fewer numbers but large steel sections, while the lower tier consists of larger numbers but smaller steel sections.
Through the baseplate, column load is transferred to the top tier.
The unpainted grillage beams are enclosed in concrete beyond the edge of steel sections with a minimum cover of 100 mm.
A minimum clear space of 75 mm must be kept between the flanges of adjacent grillage beams, ensuring proper concreting.
Pipe separators are used to maintain spacing.
Based on the material of the foundation, the grillage foundation is of two types.
a. Timber grillage:
It is mainly used for masonry wall foundations. It avoids differential settlement.
b. Steel grillage:
It is used to carry heavy loads from steel columns and distribute them into the soil with low bearing power.
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b. 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.
The depth of this foundation is more than the width.
Such foundations can mainly be made out of reinforced or pre-stressed concrete.
Types of Deep Foundation
1. Pile Foundation
A Pile foundation is a type of deep foundation comprising a long, slender column used to transfer the load from the superstructure to the lower-lying ground, providing overall support to the structure.
Fig: Pile Foundation
Generally, a pile foundation is a long cylinder usually made of steel or concrete and driven deep into the ground to act as a base or support to the structure built on it.
Pile foundations transfer the load through skin friction or end bearing mechanism.
Sometimes, pile foundations made up of timber are also used.
Pile foundations are mainly used for large structures when the upper soil layer is weak and does not have the sufficient bearing capacity to withstand the imposed load.
A 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.
2. Pier Foundation
There are drilled shafts; those are the caissons or bored piles. They support structures with large axial & lateral loads.
2.2. Plinth Beam
A plinth beam is generally constructed in areas more prone to earthquakes.
It reduces the length of columns by reducing their slenderness.
A plinth beam is provided at the plinth level and refers to a horizontal structural element that interconnects columns, thereby preventing buckling of columns.
Fig: Plinth Beam
i. 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.
ii. Advantages of Plinth Beam
a. To sustain the masonry load at ground floor level.
b. Plinth beams aim to withstand the dead weight of the masonry wall
iii. Disadvantages of Plinth Beam
a. Increases Cost.
b. Need more reinforcement and concrete.
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