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Shallow foundation

Shallow Foundation: Types and BIM Design Methods

Beams, plynths, foundation slabs: Find out more about types of shallow foundations and which to adopt according to soil conditions

Shallow foundations are structural elements designed to transfer the load of buildings directly to the underlying soil without significant lateral friction.

In this article, we will delve into the concept of shallow foundations and the various types used in civil engineering.

You’ll also learn how to leverage a structural analysis software to accurately design foundations in detail.

Different types of foundations

Different types of foundations

What are shallow foundations

Shallow foundations are resilient structures placed between the ground and a building’s superstructure, designed to transfer loads from self-weight and imposed actions directly to the underlying soil. This is mainly achieved through the pressure exerted by the foundation on the soil beneath the bearing ground, without significant lateral friction. Shallow foundations are used when the underlying soil is sufficiently robust to bear the load without significant settlement risks.

This foundation type differs from deep or indirect foundations, which transmit loads to the soil through pressure under the bearing ground and friction along the shaft. This distinction is crucial in foundation design, as it affects the choice of the most suitable type for a specific situation and is also necessary for a thorough understanding of the subject matter.

Types of shallow foundations

There are various types of shallow foundations, each suitable for specific soil conditions and structural requirements. Below is a list of different types of shallow foundations commonly used in the construction industry.

Isolated footing

Isolated footing also called Pad footing, mainly represented by footing pads, are a common choice for residential and industrial buildings, particularly suitable for soils with good mechanical characteristics and where significant differential settlements are not expected. A footing pad is a structural element typically made of reinforced concrete, designed to distribute the concentrated loads from the superstructure over a sufficiently wide area of soil to ensure safety and structural stability.

During the design phase, it’s important to position the footing pad below the layer of surface soil subject to reworking or seasonal volume variations, such as groundwater level fluctuations. The behavior of a footing pad can be likened to that of a cantilever beam subjected to soil reaction from bottom to top.

There are two main types of footing pads: stout and slender pads. Stout pads have a height-to-width ratio of the upper wing’s fillet from the pillar greater than 2, while slender pads have a ratio less than 2.

The structural design of stout pads considers the strut-and-tie system, while slender pads can be designed using approaches based on the conservation of flat sections by Bernoulli and Navier.

Footing pads can be connected by beams or stringers, and when there is no connecting element between them, they are referred to as “isolated pads”. Connections between pads can be made via stringers or beams, contributing to the stability and continuity of the foundation.

Foundation pads

Foundation pads

Continuous footing/ Strip footing

When soils have characteristics that could lead to differential settlements or when the loads to be supported are high, continuous foundations are used. These include inverted beams, grids, and slabs, which distribute the load over a larger contact area, thus reducing the pressure of the load on the soil and minimizing differential settlements. This type of foundation can be further divided into two groups:

  • Unreinforced concrete continuous foundations – if the soil offers good resistance and the applied loads are not too high, unreinforced concrete continuous foundations can be adopted. This method offers several advantages, including cost reduction and simplification of the construction process. The foundation base is gradually widened using saw cuts, set at an inclination of about 60°. This helps to reduce bending and shearing stresses, ensuring greater stability to the structure. When the unreinforced concrete continuous foundation is cast directly into the foundation excavation, a trapezoidal-shaped section is obtained, with the smaller base being the lower one. This design allows for uniform load distribution on the underlying soil. The increased amount of concrete required for this type of foundation is offset by savings on formwork needed for casting. To ensure better connection with the base of the structure, it is advisable to construct a reinforced concrete foundation beam. This element is inserted throughout the thickness of the wall at the floor slab bearing ground, improving overall resistance and stability of the foundation;
  • Reinforced concrete continuous foundations – reinforced concrete continuous foundations, represent a versatile and robust solution, particularly suitable for weak soils. This type of foundation exploits the properties of reinforced concrete to withstand bending stresses, allowing for the construction of less massive structures compared to unreinforced foundations. The distinctive feature of reinforced concrete continuous foundations is the presence of a continuous foundation beam. This design allows the foundations to effectively absorb shearing and bending stresses, ensuring greater structural stability. This type of foundation is particularly suitable for weak soils, as it requires less concrete than unreinforced foundations. Thanks to reinforced concrete’s ability to withstand bending loads, reinforced concrete continuous foundations can be designed with smaller dimensions, thus reducing the overall weight of the structure. However, it should be noted that constructing reinforced concrete continuous foundations requires more work in terms of steel reinforcement and formwork. This is an important aspect to consider during the design and construction phase, as it affects project costs and completion times.
Continuous foundations

Continuous foundations

Strap or Cantilever Footing

Inverted T-beams are an important structural element used to distribute loads from columns over a wider area, thus ensuring greater stability and strength of the structure. These types of beams are made of reinforced concrete and feature an inverted T-shape, with two lower slabs or shelves that widen the bearing surface on the ground.

The main application of inverted beams occurs in frame structures, especially when it is necessary to distribute loads over large areas of ground and when the soil’s resistance is not uniform. In such cases, inverted beams help ensure a balanced distribution of loads while reducing the risk of differential ground settlements.

The process of constructing inverted beams involves several successive stages:

  1. it begins with the laying of a first layer of rubble, which serves as a base for the inverted beam;
  2. next, the metal reinforcement is formed, providing the necessary structural strength to the beam;
  3. then, the slabs of the beam are poured, creating the bottom part of the structure that will rest on the ground;
  4. once the concrete has hardened, containment walls are installed, and the upper part of the beam, known as the core, is cast, completing the structure of the inverted beam.

This construction process ensures that inverted beams can effectively withstand static and dynamic loads from the superstructure, while ensuring a uniform distribution of loads on the underlying soil.

Crossed continuous beams

This type of structural element consists of a succession of crossed inverted beams aligned according to the rows of columns, creating a static scheme of continuous beams on multiple supports, which are oriented parallel to one side of the building’s plan. These beams are then connected by transverse stiffening stringers, which help increase overall rigidity.

The distinctive feature of crossed inverted beams is that the stiffening stringers themselves become inverted beams, further increasing the load distribution capacity and structural strength of the foundation. This transformation occurs when it is necessary to make the foundation stiffer, for example, in the presence of unstable soils or particularly high loads.

A significant advantage of using crossed inverted beams is the ability to create beams with a high height-to-base ratio, which reduces the amount of reinforcement required and the stresses to which the concrete is subjected. This not only reduces construction costs but also improves the overall structural performance of the foundation.

Raft footing

Raft foundations are an effective solution when the ground cannot support the structure’s loads uniformly or adequately. This type of foundation replaces traditional inverted beams with a single “raft” base of reinforced concrete, which extends over the entire bearing area of the structure. This base, called a raft, is designed with adequate thickness to distribute loads evenly on the underlying soil.

In addition to the base raft, main and secondary beams can be provided at strategic locations to further stiffen the structure. These additional beams help ensure better structural continuity and more efficient load distribution.

A distinctive aspect of raft foundations is the presence of an inverted ribbed plate, formed by the main and secondary beams protruding from the base raft. This configuration allows for better load distribution and overall structural strength.

Among the advantages of raft foundations is their ability to efficiently distribute loads on the ground surface, ensuring good structural continuity. This makes them particularly suitable for soils with uneven settlement characteristics.

However, a disadvantage of raft foundations is the greater amount of material required compared to other types of foundations. This may entail additional costs in terms of materials and labor; however, this disadvantage is offset by the benefits in terms of structural performance and long-term durability.

Foundation slab

Foundation slab

How to design shallow foundations with a BIM software

Below you can find various videos showing how to design and size shallow foundations with a structural analysis software: