As a brain needs a skull, a ship needs a hull, and a train needs railway tracks and stations, so do the buildings need their foundations. Before understanding the different types of foundations, let's learn what a foundation or footing is.
Foundations (also known as footing) are necessary to support everything built on land. These include the beautiful structures and buildings that we create on it. The term ‘footing’ is correctly used for steel construction to create a foothold on the earth to support the weight of a structure evenly and remain intact. Most of the foundations consist of two parts:
Superstructure - Foundation that lies above the ground level.
Substructure - Foundation that lies below the ground level.
The solid ground on which the foundation lies is termed as the foundation bed. The foundation transmits the load of the structure and it’s self-weight. The type of foundation required depends on the substructure of the building. Foundations are classified into two sub-categories on the basis of load transmission to the ground - shallow foundations and deep foundations.
Shallow Foundations
A shallow foundation is a type of building foundation that distributes loads from the building into the upper layers of the ground. They support imposed loads when surface soils are sufficiently strong and stiff. For sites with weaker soils, it may be more economical to improve the ground if you are using shallow foundations.
Wall Footing
This type of foundation continuously runs parallel to the direction of the wall. It helps to transmit and spread the weight of the load of the wall into the ground across an area of soil. The basic purpose of this foundation is to spread the weight of the load so that the soil can withstand the stress. Wall footing is suitable in cases where the loads transmitted are small and economical in dense sands and gravels. In this case, the width of the foundation is 2-3 times the width of the wall at ground level.
Column Footing
A column footing is suitable and economical for supporting columns in steel constructions. Column footing is in the form of a flat slab and may be constructed with plain or reinforced concrete. The foundation base of the column is large to support the column of the building. This type of foundation distributes the weight placed on the column through a more extensive area, thus preventing the columns from sinking into the ground.
Combined Footing
These foundations are made to support two or more steel columns in a row in steel construction. When the footing for a column extends beyond the property line, combined footing is preferred. It is also suitable for closely-spaced columns and when the soil if of low bearing capacity. Isolated footings for each column are generally more economical.
Strap Foundation
Strap foundation is a type of combined footing. When an edge footing cannot be extended beyond the property line, it is linked up with other interior footings using a strap beam or grade beam. The strap beam distributes the internal forces from one footing to another by one column to eccentrically loaded column footing. In order to distribute these forces to a near footing, a strap beam is required.
Raft or Mat Foundation
Raft or mat foundations are the type of foundation that spread across the entire area below the building. These help to support heavy structural loads from columns and walls. It prevents differential settlement of individual footings. Thus, the design of the mat foundation can bear the load of the elements of the structure of the building when the soil conditions are poor.
Deep Foundations
When the soil bearing capacity is very low, it may not be economical, or even possible to opt for a shallow foundation. In such cases, deep foundations are ideal solutions to transfer loads of the structure vertically to a stronger layer lying at a significant depth below the ground surface.
Pile Foundation
Pile is a slender member with a small cross-sectional area relative to its length. It is a type of deep foundation which is used to transfer heavy loads from the structure to hard rock layer deep below the ground level during the steel construction. They can transfer the load either by bearing or friction. This is also used to prevent uplift of the structure due to lateral loads during an earthquake and the wind forces.
Drilled Shafts or Caissons
A drilled shaft (also known as A caisson) is a type of deep foundation and functions similar to the pile foundation. It has a higher capacity cast-in-place concrete foundation poured in, formed by a drilled excavation. It can resist loads from the structure through shaft resistance, toe resistance, or combination of both of these. The steel construction involves drilling shafts or caissons using an auger. Drilled shafts can transfer larger column loads compared to pile foundations. But, this type of foundation is not suitable if deep deposits of soft clays and loose, water-bearing granular soil exists.
The foundation lends itself as a worthy investment when it comes to steel construction. No matter what type of foundation you decide to have for your structural building, you need to consult with a steel construction expert to use the steel at its best form in the steel construction and complete the construction flawlessly.
