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Uses and Characteristics of Foundation Plinths

Foundation phlinths are discontinuous structural elements used to transfer the loads of the superstructure to the underlying ground. Here’s when to use them

When constructing buildings with a frame structure, the use of discontinuous foundations can prove to be a particularly appropriate choice. Among these, foundation plinths play a key role as they provide a solid base to withstand the loads transmitted by the overlying pillars or columns.

If you are interested in delving deeper into this topic, we invite you to continue reading the article to discover the characteristics, possible applications, and design criteria of these important structural elements.

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What are foundation plinths?

Foundation plinths are structural elements used to transfer the load of a structure to the underlying soil safely and stably. They are part of shallow foundations and are used in the construction of buildings with a reinforced concrete or steel frame structure, bridges, trusses, and other structures characterized by discontinuous load-bearing elements.

The simplest form of a foundation plinth consists of a parallelepiped, usually made of reinforced concrete, which serves to expand the support base on the ground of the overlying structural element (such as a pillar), in order to achieve a sufficiently wide surface to ensure better load distribution and ensure compliance with safety, stiffness, and strength requirements.

Depending on the specific project requirements, foundation plinths can also be made in more elaborate forms, such as a truncated pyramidal shape, and can be interconnected using reinforced concrete beams to improve the cohesion and overall stability of the structural system.

Forms of foundation footings

Forms of foundation plinths

When to use foundation footings?

When it comes to shallow and discontinuous foundations, plinths can be used only in certain circumstances:

  • the soil must have adequate strength and deformability characteristics: plinths can be used when the soil has a sufficient bearing capacity to support the load without risks of settlements or significant deformations. If the soil is stable and strong enough, footings are a suitable choice for the foundation, as they are cost-effective and relatively simple to construct;
  • the applied loads must not be excessive: footings are designed to distribute point loads. It is essential to ensure that these loads do not exceed the bearing capacity of the soil or the structural strength of the footing itself, in order to avoid damages or foundation collapses.

To prevent any potential for uneven settlements or horizontal relative movements, especially in seismic areas, it is necessary to connect the footings together with a network of reinforced concrete beams or tie beams. This configuration improves load distribution and enhances the overall system resistance, reducing the risk of structural damages.

In case of weak soil resistance or high loads, it may be necessary to consider other types of foundations, such as piles or micropiles to reach deeper bearing layers or continuous foundations like rafts and inverted beams to distribute actions over a wider surface, thus reducing the pressure on the soil.

Configurazioni di fondazioni su plinti

Configurations of foundations on plinths

Characteristics of foundation plinths

Foundation plinths are very simple structural elements that are built below each column of the superstructure in order to transfer the load to the ground in a punctual manner, keeping the stresses within the permissible limits for the settlement area.

The main characteristics of these elements are outlined below:

  • shape and dimensions: plinths are usually made of a reinforced concrete block with a parallelepiped or truncated pyramidal shape, with a square base centered with respect to the column. In cases where the load transmitted by the column is strongly eccentric or the column itself is significantly elongated, a rectangular base of dimensions BxL and thickness H is adopted. The footing section can be rectangular or trapezoidal, depending on the stresses and structural requirements. Its dimensions are related to various factors, such as the loads from the superstructure, the acting stresses, and the bearing capacity of the ground;
  • thickness: unlike the BxL dimensions of the base, which are influenced by the factors listed above, the thickness H of the footing is determined by the expected shear or punching stresses. Choosing an adequate thickness is essential to ensure the structural resistance of the element and prevent settlements or significant damage;
  • reinforcement: foundation footings may or may not be provided with metal reinforcement. Unreinforced footings have a truncated pyramidal shape with a high height and inclined side faces of about 60°. This inclination increases their rigidity and enables them to support loads without the need for reinforcement, simplifying their execution. Reinforced footings, on the other hand, are made using inverted T-shaped reinforced concrete beams, resting on a non-reinforced concrete subbase. This approach significantly reduces the required excavation and the overall weight of the footing. However, to compensate for the lack of rigidity of the structural element, a complex reinforcement that can absorb bending, tension, and shear forces needs to be implemented. This reinforcement consists of iron bars arranged at the bottom of the footing along the two directions and perimeter stirrups;
  • special configurations: in particular situations, such as when pillars need to be built near existing buildings or on property boundaries, so-called “crippled footings” are used. These footings have an asymmetric shape, similar to a traditional footing cut in half along the vertical axis. These elements are employed when standard footings cannot be used due to spatial limitations or specific ground conditions that do not allow centering the element below the pillar. Careful design and a comprehensive assessment of the ground conditions are crucial to ensure that crippled footings can support the acting loads without causing structural issues or damage to surrounding buildings.
Geometry of foundation footings

Geometry of foundation plinths

Types of Plinths

The structural response of a foundation footing can be equated to that of a shelf inserted at the column’s level and subject to the ground reaction acting from bottom to top. Based on this scheme, foundation footings can be classified into two main categories, depending on the rigid or flexible behavior of the base:

  • stout plinths: designed to provide a rigid structural response since the ratio between the height of the footing and the dimension of the wing from the column’s edge is greater than two. They are characterized by a significant height compared to the base and can take on a stepped or truncated pyramid shape with a slope of the side faces between 45° and 60°. The thickness of the concrete section is sufficient to withstand bending and shear stresses. The stout footing may, therefore, be devoid of metal reinforcement, except for a simple grid placed at the base;
  • slender plinths: provide a flexible structural response as the ratio between the element’s height and the wing dimension is less than two. They are subject to the vertical load transmitted by the column and the ground reaction acting on the footing base. Slender footings have a parallelepiped or truncated pyramid shape with side faces inclined less than 45°. Reinforcement is always present, and steel bars are predominantly positioned on the lower part of the base.

Prefabricated Foundation Footings

In the case of buildings with a prefabricated frame structure, the use of so-called “glass-shaped plinths” is increasingly widespread. These are prefabricated structural elements with a specific housing, called “glass,” which allows the pillar to be positioned efficiently and safely. Glass footings are divided into different types, as listed below:

  • glass plinths with a rectangular base plate, suitable for standard uses;
  • glass plinths with a square plan, ideal for seismic areas;
  • plinths with prefabricated glass and base plate made on-site, used in case of significant stresses or terrains with reduced bearing capacity.

Prefabricated footings offer several advantages, including ease of pillar installation and a higher level of quality control and certification of the product. This type of solution can be particularly advantageous for projects requiring speed of execution, precision, and structural reliability.

Designing Foundations on Plinths

The design of foundations on footings usually begins with conducting geotechnical investigations to obtain information on soil properties, such as bearing capacity, cohesion, friction angle, and the presence of water tables. Vertical and horizontal loads transmitted by the structure to the foundations are then determined.

A series of steps follow, including the dimensioning of the structural element, the verification of stresses, and the evaluation of the footing-soil interaction.

Regarding stout footings, it is assumed that the curved sections of the shelf do not remain flat and orthogonal. Therefore, the structural dimensioning is carried out considering the strut-and-tie system. This approach takes into account the deformations and stresses that occur in the structural element. For slender footings, where the principle of conservation of flat sections applies, a punching check can be performed to ensure that the footing can withstand stresses without collapsing or being damaged.

To simplify and make the design process of foundations on footings more reliable, you can rely on the help of a structural calculation software with which you can:

  • choose the type of footing to model (stout footings and/or truncated pyramid-shaped footings);
  • insert the foundation footing and perform its dimensioning by indicating the properties (material, soil type, geometry, etc.) in relation to the loads the structure will have to bear;
  • carry out the reinforcement dimensioning. The reinforcements of the footing are designed based on the stresses and forces that the footing will have to withstand, ensuring that the structure can resist vertical, horizontal, and bending loads. During the reinforcement dimensioning of the footing, various factors are considered, including the bearing capacity of the soil, the footing geometry, the applied loads, and the structural design regulations. The reinforcements are strategically positioned inside the footing to ensure a uniform distribution of forces and to prevent the structure from settling or collapsing;
Dimensioning armature foundation footings |EdiLus

Dimensioning armature foundation footings |EdiLus

  • perform the punching shear check. This process ensures that the foundation footing is able to withstand the applied loads without excessive settlements or structural damage.
Verification of punching shear plinth foundation | EdiLus

Verification of punching shear plinth foundation | EdiLus

Don’t miss the opportunity to try this tool for free for thirty days and start revolutionizing your way of designing, just like other industry professionals. Model your structures in BIM environment and perform advanced structural analyses, leveraging the power of the integrated FEM solver. Visualize the results of your calculations graphically with easy-to-understand diagrams and color mappings. Additionally, carry out all the checks required by technical regulations and automatically extract the documentation to support your projects, all within a single integrated solution. Experience firsthand these and other features of the proposed system to appreciate its benefits and make the foundation footing design process (or any other structural element) more efficient, intuitive, and reliable than ever.

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