What are Structural Foundations?
Structural foundations are the part of the building that transfers its full weight into the soil. They support the columns and walls that transmit the structure’s weight.
Importantly, structural foundations are an essential part of any building.
These foundations are typically made from reinforced concrete material that is specifically tailored to their needs.
The structural designer must distinguish between each type of foundation in order to determine which one is the best for the design. It is easy for a structural engineer to select the right foundation for the design. Albeit, it is still a difficult task for beginners.
This article will help you understand the different types of structural foundations and how to choose the right foundation for building design.
Structural Types of Foundations
There are two categories of structural foundation design for buildings in the world of Structural Engineering that you should be aware of:
1. Shallow Foundation
The shallow foundation, as its name implies, is a foundation with a lower depth. The depth of the shallow foundation is usually between 1 and 3 meters below the natural grade.
Because shallow foundations require less excavation, they are cheaper than deep foundations.
They also tend to be more cost-effective than deep foundations because they are typically located near the ground surface.
Raft slab and slab-on-grade are other kinds of shallow foundations employed in construction. They are more affordable than deep foundations when the loads aren’t too heavy and the structure can withstand differential movements. There are some risks with slab-on-grade since there is a chance for the ground below to shrink and swell which could cause movement in the slab.
Types of Shallow Structural Foundations
1.1 Isolated Footing
The most widely used foundation type is the isolated footing. It is often shaped in a rectangular, square, or circular shape that supports a single column
This is an ideal choice for light structures such as residential or medium-sized buildings with columns that are not very closely spaced.
Isolated spread footings are usually rectangular, square, or even a geometric frustum block or stepped concrete that supports the weight of one pillar or column. The width of the foundation will depend on the amount of weight to be carried and the bearing capacity that the soil can support.
Individual or isolated spread footing is the most common type of foundation used to construct buildings. It is built to support a single column. Moreover, it is also known as the pad foundation.
1.2 Combined Footing
The combined footings can be used to support multiple columns. This is useful when columns are too close together that making isolated bases to them creates overlapping configurations.
When designing a combined footing, it is important that the centroid for the footing coincides with the centroid for the combined loads.
This will ensure that the soil-bearing pressure beneath the footing is evenly distributed to avoid uneven soil settlements.
1.3 Strap Footing
The strap footing is also called neighbor footings or cantilever. It consists of two separate footings, concentric and eccentric, connected with a strap beam.
This type of foundation works best when a column is close to a property line. An eccentric footing is required due to space limitations.
A strap beam is used to attach the soil bearing pressure distribution to the adjacent closed column with a concentric footing.
The strap should be designed with a rectangular beam that spans between the columns.
The typical width of the strap beam is equal to the width plus 100mm of the largest column. Its depth is determined by the maximum bending moment.
The main reinforcement is located on the top of the strap beam, with links or stirrups near the support. To resist settlement stresses, bottom reinforcement may be required.
1.4 Strip Footing
A strip footing is a continuous base that supports a retaining wall. This can be used when multiple columns are arranged vertically or horizontally and are close to each other.
This is a cost-effective way to build if you have overlapping columns that overlap.
1.5 Raft, or Mat Foundation
A raft foundation or mat footing is a single-thick foundation that supports the entire structure’s weight.
This foundation is used when the column resists too heavy load or the soil bearing capacity is too low to support the foundations of shallow foundations.
Rafts can be used to avoid differential settlement of individual footings, and thus designed as the combined footing of all load-bearing components in the structure.
For buildings with a maximum height of 12-16 stories, the raft foundation is suitable. These structures can have basements provided the soil bearing pressure is strong enough to withstand the structural load.
Spread footings also are used in an enlarged form for high-rise structures. Other structures that support heavy loads include concrete caisson columns, piles as well as construction directly on exposed rock.
2. Deep Foundation
A deep foundation is required when the soil’s hard strata make it difficult to excavate. These foundations work best in areas that have the low or inadequate ground bearing capacity, high water tables, excessive soil settlements, and naturally soft soils.
Pile foundations are the most popular deep foundations type.
This type of foundation is typically utilized in coastal flood zones to raise structures above flood levels, expansive or weak soils to reach a firm stratum, or in steeply sloped areas. This structural foundation is also used to prevent the uplift of the building due to lateral loads such as earthquakes, wind forces, or backfill forces.
To reach the soil’s hard strata, pile caps are often combined with piles (either 2, 3, 4, 5 piled groups or multiple pile foundations).
Types of Deep Structural Foundations
2.1 Driven Pile
Driven piles were used to be driven into the ground with a vibro-hammer or other equivalent machines. concrete piles and steel (H beam) or tubular piles of steel with capacities between 3000kN – 800kN.
Although this type is economical and simple to install, it’s important to take into account the potential for pile head injuries and shaft bending when driving.
The pile is susceptible to vibration and noise. It is not recommended to use this pile on soils that have rocks or boulders.
The piles are long and slender which can transfer loads from a building by friction between the sides of the pile and surrounding soil and bearing between the base piling and strata of soil below. The piles can be constructed from wood, steel, or concrete. Steel piles typically consist of hot-rolled, I-shaped sections or screw piles that are helical. Wood piles are typically huge timber sections that have been pressure treated.
2.2 Mini or Micropiles
Mini piles or micro piles which are smaller bore piles with a diameter of 0.3m, can be used to support loads up to 1000kN in restricted spaces and difficult access.
They usually have a diameter of around 0.3m, and they require smaller rigs that are generally less than 2 meters high.
2.3 Continuous Flight Auger (CFA) Piles
As the pile construction goes at a quick pace, there is no need for support fluid or casings. Drilling, concreting, and filling the drilled soil must be done almost simultaneously.
Once the concrete had been poured, the prefabricated reinforcement cage was installed and lowered down to the concrete.
2.4 Bored Piles
Bored piles are the most popular type of deep foundation used in building construction. Based on the location of the pile holes, pre-drilling can be used.
Borehole stabilization requires a casing and bentonite system.
There are two kinds of pile foundations: end-bearing and friction piles. Both are made up of large solid columns that are bored deep into the ground.
What is the difference between foundation and footing?
Foundation is a component of a structure that transfers loads from the superstructure to the footing. Footing is the structure in contact with the ground. Footings are structural elements of the foundation combination. The combination is composed of structural elements called footings, as well as supporting soils.
What’s the difference between mat foundation and raft?
In terms of foundation, raft and mat are the same. This type of foundation is suitable when the soil has a very low load-bearing capacity or when a very narrow gap exists between successive columns. In such conditions, a mat foundation is helpful because it distributes loads uniformly across the entire building, as opposed to all other foundation types.
Does every foundation have a footing?
In general, most foundations have footings. However, footings are not necessary beneath a foundation when the soils are weak or lose under the structure’s weight. Footings perform well in good soil conditions but are not suitable for poor soils with low bearing capacity.