Footings and Foundations Explained: Types, Functions, and IS 456 Guidelines

IS 456 recommendations are confined to the design of footing that supports isolated columns or walls and rests directly on the soil. (IS 1080 for spread foundation IS 2911 for pile foundation and IS 2950 for raft foundation).

Foundations are not easily accessible for periodic inspection and maintenance hence, care should be taken for the durability aspect and the clause of minimum cement content is vital.

Footings and Foundations Explained: Types, Functions, and IS 456 Guidelines — IS 456 Guidelines for Footings and Foundations

Deflection control can be neglected in the case of footing but control of crack width may have to be considered especially for very severe and extremely severe environments (Table 3 of IS456), As per IS1080 all soils have a minimum depth of 50cm is necessary.

D_{mni}=\frac{q_{0}}{\gamma}\left(\frac{1-sin\phi}{1+sin\phi}\right)^2

Design criteria will be flexural, shear, and development length with uniform settlement and uniform pressure for soil.

In the case of columns or walls subjected to eccentric loading conditions, it may be necessary to locate the geometric center of the footing deliberately away from the axis of the supported elements, so that the soil pressure and the expected settlement will be uniform.

Minimum thickness at the edge (Clause 34.1.2) Based on the dispersion of internal pressure in the concrete, minimum thickness shall not be less than 150mm for the footing resting on the soil and 300mm for the footing resting on piles

The minimum Cover to Reinforcement is 50mm. (clause 26.4.2.2)

Footing

The footing is the initial structural element of the building that safely transfers the weight of the upper story to the ground. The fundamental principle of foundation design is to distribute the load so that the strength—or bearing capacity—that the ground can withstand is less than the size per area of the force transferred from the top component of the building. The footing serves this purpose by acting as a framework between the ground and the column or wall directly above it. Its primary goal in terms of basic design is to minimize overall settlement and avoid the emergence of immobile settlement.

Type Of Footing

There are four different kinds of footing that builders use in home construction:

Individual footings:

These are the most common, and they’re also the type that appears under smaller structures, like decks and pergola posts. They are made up of a single, earth-buried concrete pier.

Combined footings:

These are two individual footings placed adjacent to one another under weighty pieces of a structure.

A combined footing is a footing that provides support for two or more columns. It is employed in situations where two or more columns are near to one another or where there is an overlap between two or more column footings. The plan shape of a combined footing might be rectangular or trapezoidal. When one column’s weight is higher than the other, a trapezoidal footing is supplied.
Use of Combined Footings: When columns face the plot border or are closely spaced apart, combined footings are utilized to transfer loads to the ground.

Raft footings:

Without any additional footings beneath, these enormous slabs sustain the entire structure above by sitting right on top of the ground. You’d see this in a floating slab foundation, one of the most common shed foundation types.

Strip footings:

They function similarly to subterranean walls in that they continuously and evenly support walls above ground.

Foundation

First and foremost, a structure or building consists of two main portions, the superstructure, and the substructure. Since the substructure transfers the superstructure’s weight, it is commonly referred to as the foundation. The superstructure’s weight is transferred to the foundation or substructure via columns or walls. The purpose of the foundation is to distribute the load to the soil with the right amount of pressure.

The base of every construction is its lowest point. The transmission of the higher components’ load to the soil is its primary purpose. It is a necessary component of any building. There are usually two types of foundations (shallow foundations and deep foundations).

A shallow foundation transfers a load of up to 1.5 meters and deep foundations transfer loads below the ground surface with more than 1.5 meters in depth. Shallow foundations are generally designed for low-rise buildings or buildings with a larger horizontal spread than their vertical height. They are also necessary for skyscrapers that are situated on unstable soil; a deeper foundation is only necessary if there are explicit plans to expand this building vertically in the future.

Pile, Well, and Pier Foundations

Deep foundations are comprised of pier, well, and pile foundation systems.

A pile foundation is utilized when the topsoil is somewhat poor. This is the most popular kind of deep foundation for construction. It is employed when a series of piles shifts the weight onto the ground.

A Well foundation is usually used when constructing a bridge where the soil is soft and/or moist. You’ll typically find them in water.

A pier foundation big cylindrical column makes up a pier foundation. The two types of pier foundations include masonry or concrete pier and drilled caisson. Large loads are to be transferred in order to firm the strata below.

A pier is dug out and cast in place.

Types of foundations

Slab foundation:

Slabs can either sit directly on the soil or shallow or deep footings. The building is right on top of the concrete slab, and there is no underhome area.

Crawl space foundation:

This is made up of a concrete slab that is often 4 feet below the surface and peripheral walls made of concrete block. It frequently has footings underneath for stability.

Basement foundation:

A basement is comparable to a crawl space but lies far deeper in the earth, often on footings. The inside of a basement is normally 7 to 10 feet in height, with concrete block walls or poured concrete walls surrounding the perimeter.

Pier and beam foundation:

It consists of concrete piers (footings) set into the earth and timber beams that rest on them and support the structure above.

Home Construction

A home is rarely built entirely on footings unless it is a pier and beam foundation (in which case the footings act as the foundation) or a floating slab foundation (which is unusual in residential construction). This is because building a home entails the construction of a large, heavy structure, so there is usually a foundation system that involves footings.

Outbuilding Construction

Smaller outbuildings, such as detached garages and sheds, can occasionally have footings without a full foundation system and weigh significantly less than dwellings.

You might see footings as a standalone foundation for sheds, but floating slab foundations are more popular options for larger sheds and detached garages. Floating slabs are regarded as footings since they carry the load of the structure to the soil, but they’re also considered foundations since they give even and level support to the structure above.

Other Minor Construction

Finally, you’ll almost always see footings and not complete foundation systems for things like pergolas, arbors, and fence posts. Individual footings are usually used in these lightweight buildings to distribute the load deep enough to prevent soil expansion due to freezing and moisture from runoff.

Whatever the use, a local foundation business can assist you in selecting the kind of foundation that will work best for you and be compatible with the local climate and weather.