Slab on grade foundation with inverted T-Beam

The slab on grade foundation represents an effective and versatile structural solution, particularly appreciated and widespread in contexts where it is necessary to ensure a uniform distribution of loads on terrains with variable geotechnical characteristics. This article will explore in detail the technical aspects of designing and implementing a slab on grade foundation, providing useful information for professionals in the construction industry.

When to use a slab on grade foundation

The slab on grade foundation is a type of continuous direct foundation (often also called a reverse T-beam, due to the shape of its section), used when the supporting structure is an independent frame, as in framed structures in reinforced concrete or steel.

Generally, reverse beams have a linear development and are normally used, in the construction field, when a larger contact surface is needed compared to foundation footings.

The name reverse beam comes from its static behavior: it behaves indeed like an inverted beam loaded by the reaction of the ground acting from bottom to top. Subjected to the loads transmitted by the columns, the bending moments will have opposite signs compared to those of the floor beams. Therefore, its static behavior is opposite to that of the beams in elevation.

Seismic vulnerability, structural calculation software

To achieve a more uniform distribution of loads on the foundation ground and ensure greater stiffness of the foundation structure, the slab on grade foundations are connected to the columns in two orthogonal directions, forming closed meshes that reproduce the building’s skeleton frame.

Assuming that the ground reaction is uniformly distributed on the footings, the slabs are calculated as continuous beams. The steel reinforcements are arranged to absorb the tensile and shear stresses that develop due to the elastic bending of the beams, stresses that the concrete alone could not support.

When there are multiple aligned pillars, loaded and spaced differently, and you want to standardize the unit load transmitted from the foundation to the ground, it is advisable to design a variable-width slab foundation (raft foundation) (see also Slab Foundation: from Calculation to Implementation). This allows the resulting reactions of the ground on the support surfaces to have the same line of action as the resulting loads acting on them.

The slab on grade foundation is used in a variety of specific situations where the characteristics of the ground and the structural requirements make this solution particularly advantageous. Below are the main contexts in which this type of foundation is used:

• grounds with low or variable bearing capacity. When the ground has a low bearing capacity or significant variations in its mechanical properties, the slab on grade foundation is preferred because it allows for a more uniform distribution of loads, reducing the risk of uneven settlements;
• reinforced concrete or steel frame structures;
• grounds subject to differential movements. In grounds subject to differential movements, such as clayey soils that can swell or shrink depending on the water content, the slab on grade foundations offer greater stability and resistance to ground uplift;
• areas with high seismicity. In seismic zones, slab on grade foundations are advantageous because they improve the structure’s ability to withstand the horizontal forces generated by earthquakes. Their configuration helps reduce the risk of overturning and improves the dissipation of seismic energies;
• structures with high concentrated loads. For buildings with high concentrated loads, such as multi-story building columns or heavy industrial structures, slab on grade foundations help distribute these loads over a larger area, preventing excessive pressures on the ground;
• constructions on heterogeneous grounds. When building on grounds with heterogeneous geotechnical layers, slab on grade foundations help standardize the ground response to structural loads, reducing the risk of irregular deformations.

To evaluate the design criteria of reinforced concrete slabs, it is important to consider the following aspects:

• building material;
• cross-sectional area of the beam section;
• number and type of constraints;
• ground type;
• type of stresses to which it is subjected.

What are the types of foundations

Foundations can have different characteristics based on design choices and ground characteristics.

Let’s specifically see all types of foundations based on the ground of the intervention area:

grounds with good resistance

• shallow foundations (direct)
  • continuous
  • raft
  • slab on grade
  • discontinuous
  • isolated footings
• deep foundations (indirect)
  • piles
  • driven piles
    • drilled
    • cast in place
    • driven

grounds with poor resistance

• shallow foundations (direct)
  • raft
  • slab on grade
• deep foundations (indirect)
  • suspended piles
• hydraulic foundations.

Types of foundations

Slab on grade foundation or raft foundation

The slab on grade foundation is a monolithic reinforced concrete structure, with a variable thickness, typically between 30 and 40 cm, which is designed to transfer all the weight of the building and the loads acting on it to the underlying soil.

It is a quick, simple, and very effective construction solution that ensures a uniform distribution of loads even in the presence of soils with low bearing capacity. It is the best solution to prevent the occurrence of differential settlements and to create a floating raft on the underlying soil.

Reinforced concrete structure created with EdiLus

Compared to other solutions (plinths, inverted T-beams), it requires the use of a greater amount of material (steel and concrete). Nevertheless, the cost of the slab on grade foundation is generally equal to, or in some cases even lower than, other solutions, due to its simplicity of installation and the shorter time required for its preparation (excavation, formwork, etc.).

In contrast, inverted T-beams consist of a network of rectangular or T-shaped beams, with a base ranging from 40 to 80 cm and a height from 40 to 50 cm, arranged orthogonally to each other and along the lines of the walls in elevation.

More generally, the thickness depends on the shear or punching stresses. The width, instead, depends on the bearing capacity of the soil and the loads coming from the superstructure.

The foundation beam is built on top of a cleaning layer, which will protect it from the chemical aggressions of the soil; it is a layer of concrete, generally without metal reinforcement, with a low cement content, called lean concrete, positioned at the excavation level.

The spaces between the beams are filled with a void former, generally made with gravel, polystyrene blocks, etc., and finished with a slab that constitutes the first floor of the building in elevation. In both solutions, it is possible to add a ventilation chamber between the foundation and the first slab, especially in the presence of damp soils and to reduce the negative effects of Radon gas coming from the ground.

The inverted beam foundation was widely used until a few years ago. Currently, however, the option of the slab on grade foundation is preferred for its simplicity of construction and labor cost savings. Although still a valid construction system, inverted beams are therefore less used compared to slabs.

How to design a slab on grade foundation

To improve and simplify all operations related to the design of foundations (but also those related to the structure as a whole), it is possible to use a structural calculation software.

Designing slab on grade foundations with the support of software that leverages the integration between BIM tools and structural calculation technologies offers numerous advantages in terms of precision, efficiency, material optimization, and compliance with current regulations. To proceed with the calculation, it is necessary to first model the structure in 3D, integrating the slab on grade with the vertical and horizontal elements in a detailed manner. The software allows simulating real load conditions, including static, dynamic, and seismic loads, providing an in-depth view of the structural behavior under different conditions.

Steel connections

3D modeling facilitates the visualization of the distribution of stresses and deformations through beams and foundations, helping to identify potential critical points and optimizing the arrangement of reinforcements. In addition, the powerful finite analysis capabilities of the software allow safety checks and regulatory compliance to be carried out, ensuring that the project meets all technical and safety requirements. This integrated approach improves the accuracy and reliability of the design, even in relation to the risk of conflicts with other elements involved in the design (such as plant components, architectural elements, etc.), reducing errors and increasing the overall efficiency of the design process.