The four types of Shallow Foundation

Shallow foundation is applied in cases where we can find good load-bearing soil at a rather low depth. The foundation depth must meet the safety requirements of the breakdown. That is, after the application of load, the complete structure settlement will be within acceptable limits.

We can use the following four types of shallow foundations: Spread Footing, Combined Footing, Raft Foundation and Ring Foundation.

1. Spread Foundation: You need to spread the load from the column or wall to a larger area, you should use Spread Foundation. The width of the footing area is much wider than the wall or column.

The spread footing that is used to support a wall is called a wall footing or continuous footing. The top of the footing may be stepped or tapered, increasing the width gradually from wall or column to the base. They are of the following types:

a) Strip Footing: This primitive type of footing has been conventionally used in most constructions historically, before more modern inventions. They are mostly made of stones, masonry or concrete. The strip footing that is constructed of stone blocks generally has a stepped top. In modern days, however, the use of strip footing has become next to obsolete. Only in some light loading residential construction is strip footing still used.

b) Isolated Footing: When you provide footings under columns separately in a framed structure, it is called Isolated footing, pad footing or column footing.

In most cases, square footings are used under columns. However, space restrictions may force you to use rectangular footings. In case of circular columns, circular footings may be used, though it is not common. It may be used in special circumstances where construction work is difficult, or the load has to be dispersed very equally.

2. Combined Footing: When two columns are too close to make separate footings for each, then their footings are combined. These may be of the following three types:

a) Rectangular footing: These are the most common types of combined footing. It’s basically two square footings constructed together. This is used when each column is bearing the same load and of the same size.

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Published By
Rajib Dey
www.constructioncost.co
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Shallow Foundation and its types

The shallow foundation is categorized as follow :-

1. Spread Footing
2. Combined Footing
3. Raft Foundation
4. Annular Slab or Ring Foundation.

Type 1. Spread Footing: Foundation that transmits the load from a wall or column to a deeper width is called spread foundation or footing. The spread footing intended to the walls of a load bearing structure is called wall footing, continuous footing, or strip footing.

i. Strip Footings: Strip footing is made of stone masonry or concrete and it contains a stepped cross section. Now-a-days, the concrete is gaining popularity in construction industries and the load bearing structures & strip footings become outdated to some extent exclusive of small lightly loaded residential buildings. It is also called continuous footing or wall footing.

ii. Isolated Footing: Spread footing arranged to the columns of a framed structure is known as isolated footing, column footing, or pad foundation.

It is arranged underneath the column to disperse the loads securely to the bed soil. This type of footing is provided to support single-columns and when the columns are placed comparably at long span. It is very cost-effective.

Type 2. Combined Footing: Combined footing is effective when footings of two adjoining columns are provided too narrowly or overlap. It is categorized as follow :-

i. Rectangular Footing: Combined footing is usually rectangular in shape to resist equivalent column loads.

ii. Trapezoidal Footing: For irregular column loads, trapezoidal footing should be utilized to make sure that the centre of gravity (CG) of the column loads overlaps with the CG of the foundation in plan.

iii. Strap Footing: For footings located adjacent to the property lines, a strap footing should be applied to make sure that the edge of the footing adjacent to property line is not expanded into the nearby site. Under the situation, the footing adjacent to the property line (exterior footing) is attached with the footing inside the site (interior footing) via a strap beam.

The strap beam transmits the load of the exterior column footing merely to the interior footing during structural action.

Type 3. Raft Foundation: Raft foundations also known as raft footings or mat foundations, are provided with reinforced concrete slabs having consistent thickness (normally 150 mm to 300 mm) which are extended for a wide area, often the entire footprint of a building. They disperse the load enforced through a number of columns or walls over the area of foundation.

Raft foundation is mainly arranged when the entire area of all footings surpasses 50% of the loaded area. It is also suitable for heavy structures situated over highly compressible and weak soils expanding to large depth.

Type 4. Annular Slab or Ring Foundation: A ring foundation is sometimes arranged for a bigger water tank with its columns attached through a ring beam and supported across an annular slab.

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Published By
Rajib Dey
www.constructioncost.co
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