What Is Foundation?
The Foundation is the main supporting component of any building structure. It is the lowest part of the building which is in contact with soil and transfers all structural load to soil safely. Without a foundation, you may not see skyscrapers building in the world. There are different Types of Foundations used in construction depending upon the nature of the work.
In this article, we will learn about what are foundation and footing types used in construction with their image.
Types of Foundation In Building Construction
Following are type of foundaiton used in building construction,
1. Isolated Spread Footing
It is one of the simplest and worldwide popular foundations. An isolated footing is mostly used to support a single column. This type of foundation is suitable when columns are not closely spaced.
- Stepped footing
- Simple spread footing.
- Sloped footing.
This type of footings is provided with steps in foundation sides, which is also known as polestar. The step or polestar is starting from the bottom up to column size. Stepped footings are generally used when there is a heavy load coming from the upper superstructure.
Simple Spread Footing
In simple spread footing, only a foundation base is constructed and further followed by a column. This type of footings is suitable when there is no excessive heavy load coming from the upper part of the structure as in the case of stepped footings.
In this type of footing, there is a foundation base and further followed by a column. But the slope is given to connect the base to column size and which is also resembles the shape of trapezoidal. It is also known as trapezoidal slope footing.
Advantages of isolated footing
- In this type of footings shuttering material, the requirement is very less, which is making it economical compared to others.
- No skilled labor is required.
- It is time-saving because of simple shape.
- Where columns are not closely spaced.
- Loads on footings are less.
- The Safe bearing capacity of the soil is generally high at low depth.
2. Wall Footings
Wall footings are pad or spread and strip footings that are used to support structural or nonstructural walls to transmit and distribute the loads to the soil. All footing runs along the direction of the wall. The thickness and size of the footing are specified based on the type of soil at the site. The width of footing is generally kept between 2-3 times the widths of the wall.
In this foundation, the wall footing can be constructed from plain concrete, or reinforced concrete stone, brick. The wall footing is best suited for small buildings.
It can more effective for load-bearing structures and boundary wall construction.
3. Combined Footings
Combined footings are provided when the column is closely spaced. So that their footing overlaps with each other and when the soil bearing capacity of a soil is lower this type of footing are used. In such when columns are closely spaced and if we provide separate isolated footings would overlap, in such case, it is better to provide a combined footing than isolated footing.
Following are the situation when we can use combined footing,
- When center to center distance between columns is small and soil has a low bearing capacity. Individual column footing may overlap each other.
- In case when the column is located near to property line and sewer line, the column center of gravity will not coincide with footing. Then, it is necessary to provide combined this footing with that of the adjacent internal column.
- Dimensions of one side of footing are restricted to due to any reason, so that column footings may be combined.
- For Columns are closely spaced.
- Column located near to property line or sewer line.
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4. Cantilever or Strap Footings
When two or more footing is connected by a beam, it is known as a combined footing, and beam connecting footing is known as a strap.
This is an important foundation. When a square or rectangular footing located near to the property line and if it concentrically located under the column would extend into the adjoining property, which may not be permissible. For such a situation a trapezoidal combined footing may be an alternative.
Sometimes, when the distance between this column and the adjoining column is big, the combined trapezoidal footing will be quite narrow, with high bending moments. In such a case, strap footing may be provided.
The strap beam provided to connect two spread footing columns does not remain in contact with soil and thus does not transfer any pressure to the soil.
The main function of the strap beam is to transfer a load of the heavily loaded outer column to the inner one. While transferring this load strap footing has to experience shear force and bending moment. This should be taken into consideration while designing cantilever or strap footings.
The below figure shows the different positions in which this footing can be utilized and their choice depends upon the physical conditions of each specific case.
- The column is located near the property line and its dimension restricted
5. Raft or Mat Foundation
A Raft foundation also used for low-bearing capacity soil, as it distributes the weight of the building over the entire area of the building, and not over the smaller zone or at the individual point. Ultimately reduces the stress per area on the soil.
The stress concept is very simple for civil engineers. We know that stress is a ratio weight by area. For example, if a building has is 10m x 10 m weighs 100 tons and has a raft foundation, then the stress on the soil is weight/area = 100/100 = 1 ton per square meter.
In another case, if the same building has 4 individual footings, each of 1m x 1m, then the total area of the foundation would be 4 m2, and the stress on the soil would be 100/4, which is about 25 tons per square meter. So it increases load per unit area on the foundation.
It is recommended when compressed soil such as very soft clay, alluvial deposits, and compressible fill material where strip, pad or pile foundations would not provide a stable foundation without excessive excavation.
A foundation in which the depth of foundation is more than the width of the foundation is known as a deep foundation.
1. Pile Foundation
The pile foundation is a type of deep foundation is made of concrete, timber, or steel. It is like a small-diameter column that is driven into or cast in the ground. In simple words, the pile foundation has depth more than a shallow foundation. This foundation primly used in bridge construction.
This type of foundation is used when the soil below the foundation not having sufficient bearing capacity to carry the load of the building into deep soil up to hard strata. The major function of the pile foundation is to transmit loads to the lower level of the ground by the combination of friction pile and end-bearing pile at the pile point or base.
We use a pile foundation when:
1. The compressible or weak upper soil layer
2. Presence of horizontal forces3. Expansive soils in the foundation
4. Subjected to uplifting forces
5. Soil erosion
2. Pier or Caisson Foundation
A Caisson is one type of watertight retaining structure used in the construction of a concrete dam, as a pier of bridge construction in the river, or for the repair of ships. Caisson is a prefabricated hollow box or cylinder sunk into the water or ground to some desired depth and then filled with concrete thus forming a foundation.
Caisson foundation is majorly used for bridge construction & other structures that require foundation beneath rivers & other bodies of water. This is because caisson can be transported by floating to the construction site and sunk in water to use as a pier of foundation.
They are similar to pile foundations but are installed using a different method. Caisson foundation is used, when the soil of adequate bearing strength is found below surface layers of weak materials such as fill or peat.
It is one type of deep foundation, which is constructed above ground level, then sunk to the required level by excavating or dredging material from within the caisson.
Under Water Construction of bridge pier, Retaining wall or Waterlogged is construction.
Types of Footings
The classification of footings in the construction given below,
1. Shallow Foundation
When a depth of foundation is less than the width of the foundation then it is known as Shallow Foundation. Generally, a shallow foundation is placed no more than 6 ft depth from the lowest finished floor.
A shallow foundation is generally used when,
(1) The sufficient bearing capacity of soil available at shallow, and
(2) Foundation material or strata do not result in undue settlement.
The shallow foundation has a minimum depth of about 800 mm and the maximum depth, not more than 4 meters the ground. This type’s foundations are popular for low-rise building construction.
Footings are the important structural element that transfers the load of column, beam, slab, and retaining structures to the soil safely.
To transfer the above loads properly to the soil, footings must be designed to
- Prevent excessive settlement
- Minimize differential settlement, and
- Provide adequate safety against overturning and sliding.
Watch Video: Different Types of Footing With 3D Model