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Slab Foundation: from Calculation to Implementation

Discover more about slab foundations, when to prefer them to plinths and inverted T-beams, how to calculate them, and what implementation phases to follow

The foundation is the structural element that allows the loads transfer from the construction to the ground. The floors load is first transmitted to the columns and, added to their weight, reaches the foundation which, in turn, distributes the entire weight of the building onto the ground.

There are different types of foundations to choose from and calculate based on the building loads and soil characteristics. In any case, it is always preferable to use a structural calculation software to avoid design errors that can also be very costly for the technician and for the property’s users.

In this article, we’ll delve into slab foundations, one of the most commonly used systems for small and large size constructions.

What are slab foundations?

Foundations can essentially be of two types:

  • shallow;
  • or deep.

Deep foundations intercept good soil at depth to reach a solid support. A typical example includes:

  • piles;
  • micropiles.

By definition, shallow foundations are the ones that lay on the ground or that are only slightly embedded in it, such as:

  • footings;
  • raft foundations;
  • slab foundations.

Slab foundations are a type of shallow foundation that extends over the entire building support surface. Generally made of reinforced concrete, they have a thickness ranging from 40 to 100 cm depending on the loads to be supported and the soil characteristics. They can be of various types: from the classic constant thickness plate, to the ribbed slab (a plate stiffened by a mesh of beams), up to the waffle slab, also known as a grid slab, is a two-way reinforced concrete slab with square or rectangular recesses, resembling a waffle or grid pattern.

Implementation of slab foundations

Implementation of slab foundations

Thanks to its rigidity, the slab minimizes the risk of settlements, but it is important to consider the presence of soft soil layers in depth, as they could compromise its stability.

Foundations, in fact, constitute the point of contact between the building and the ground, absorbing the loads that travel through walls and columns. The strength and compactness of foundations depend on the specific characteristics of the soil.

Slab foundations represent an excellent solution to counteract settlements related to low resistance soils, rising damp problems, etc.

The slab, in fact, compared to punctual type shallow foundations (such as footings) offers a very wide support surface, distributing the load more evenly on the ground and avoiding differential settlements, which are potentially more dangerous for the structure.

Thanks to its large surface area, the slab exerts very low and better distributed pressures, suitable even for weak and compressible soils. If correctly sized and reinforced, a slab can allow the structure to move monolithically, even in the presence of significant settlements.

Finite Element Method solver for structural calculation of slab foundations

Finite Element Method solver for structural calculation of slab foundations

Slab Foundations Calculation

To take advantage of all the benefits of a slab foundation, it is necessary to properly size the slab, calculate the reinforcements, choose the concrete and steel characteristics appropriately, request a geological survey of the foundation soil, and comply with regulatory requirements based on the seismicity level of the area under examination.
For the calculation of slab foundations, various methods can be used, very complex, one of which considers the slab as an isotropic plate on Winkler soil. The differential equation describing the phenomenon is not easily solvable, which is why simplified mathematical methods, such as the Finite Element Method (FEM), are used.

Currently, to carry out a calculation that responds to the real situation and to be truly sure to respect all the parameters defined by the regulations, it is possible to resort to the use of a structural calculation software. Design errors or construction defects, in fact, could cause damage even to structures under construction.

The use of software makes the calculation process simpler but above all avoids making mistakes.

Here’s a video showing you how to model a foundation slab with a specific software.

Implementation Phases of a Slab Foundation

The implementation of a foundation slab requires precision and attention to detail from the earliest stages. Even the layout of the slab is an operation that requires a lot of precision.
In this phase, the full-scale drawing of the slab outline is carried out on the blinding concrete, using templates and strings to ensure precision and accuracy. A laser level is used to mark the levels of the concrete to be poured. It is advisable to verify the measurements several times to ensure correct positioning of the building, also in relation to the plot and distances from roads and buildings. A measurement error at this stage could affect the entire construction of the building.

Reinforcements placement follows their layout. It is possible to reinforce the slab with welded wire meshes or with steel rebars appropriately calculated. The placement of rebars certainly requires more time and greater planning. Choices regarding the type of steel, the diameter of the rebars to be used, and the width of the mesh of the welded wire mesh are the result of evaluations and analyses made using a structural calculation software.

The installation starts by placing the lower concrete cover profiles, on which the lower reinforcement bars are then placed first in one direction and then in the other. The ties between the bars orthogonal to each other are made as the reinforcement placement progresses.

After completing the bottom reinforcement mesh, proceed by installing the perimeter forks and the lattice spacing-rebars inside the slab for accommodating the upper reinforcement.

Subsequently, the formwork installation is carried out, used to contain the concrete during pouring. The formwork is placed around the perimeter of the slab and can be made of wooden panels or prefabricated steel panels.

During this phase, the layout of the pipes of the systems serving the building is also carried out. In particular, hydraulic drains and main utility passages (for water supply, electricity, and telecommunication) are prepared.

Before pouring the concrete, the level marking is carried out.

In practice, with the help of a laser, the finished level of the slab is indicated (marked with nails fixed on the perimeter panels) which will serve as a guide during the concrete pour.

At this point, the concrete pouring can proceed using a concrete pump with a hydraulic arm, which allows easy access to all points of the foundation. The concrete is carefully vibrated with special equipment to eliminate air bubbles in the mix and completely wrap the reinforcements. The pour is completed with a slightly more fluid final layer to obtain a leveled and smooth surface with the help of specific floats. During this phase, concrete samples are also taken, which, properly labeled, will be indispensable for carrying out tests on the material.

After a few days from the pour, the formwork is removed, a process known as stripping or shuttering.