Structural Foundations: Definition, Types & Construction

Instructor: Hassan Alsaud

Earned my B.S. in Civil Engineering back in 2011. Have two years of experience in oil and gas fields and two year as a graduate research assistant. Earned my Master degree in Engineering from Tennessee State University in 2016.

In this lesson, we are going to learn about different types of structural foundations and what characteristics they each have. We will also learn about their construction.

Structural Foundations

Have you ever walked around a city, looked at the high rise buildings and thought to yourself, ''What keeps these building standing?'' The answer is similar to what keeps trees standing. Like roots of a tree, the roots of buildings are the foundations.

A structural foundation is the part of a building that fixes it into the soil. These structures provide support for the main structures that appear above the soil level, much like the roots of a tree support the stem.

One of its functions is to transfer loads from the structure to the ground. For example, slabs transfer their weight to girders, which in turn transfer that load as well as loads applied to them to the beams. Beams transfer that load and any additional loads applied to them to the columns, and finally, columns transfer that load to the foundations.

One of the duties of civil engineers is to consider the bearing capacity of the soil, and design these foundations to resist shear stress, overturning, and sliding.

Types of Foundations

There are many types of foundations, each with its own benefits and drawbacks. Let's go over some now.

Spread Footings

A spread footing foundation can either support one column or multiple columns (called 'joined footing'). This type of foundation is shallow, usually reaching no more than three meters deep.

Spread footings are used when there is a small load and the top soil layers are not weak. For example, flag poles or a single story buildings can be supported by a spread footing.

To construct these types of footings, the required depth is excavated, reinforcing steel bars are placed according to design, framework is placed according to design dimensions, and concrete is poured. After the concrete reaches its maximum compressive strength, the excavation is filled with soil, and the soil is compacted.

A spread footing foundation supporting a single column.
spread footing

Mat Footings

Mat foundations, also called raft foundations, have large areas with multiple supported columns. Usually all the columns in the building are supported on one foundation. Mat foundations are preferred when the soil has low bearing capacity.

All the loads are distributed to a large area, which reduces the stress exerted on the low-bearing capacity soil. Mat foundations are economic in situations where piles cannot be constructed and spread footings are not practical, such as the case with low-rise, multistory buildings, if the top layers of the soil are weak.

mat foundation

Pile Foundations

Pile foundations are long columns that reach into deep layers of the soil and act as fixed ends for the structures that they support. Pile foundations can be made of reinforced concrete, steel or wood.

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