Raft foundation

The extensive foundation (which is usually called raft foundation), is a unified foundation that extends throughout the entire area of the columns. As a rule, raft foundation is used as a building’s foundation when the soil has a low bearing capacity.

The behaviour of the raft foundation resembles the behaviour of a strip foundation grid. The stresses applied on the soil are larger in the columns area and smaller in the intermediate areas. The presence of beams acting as stiffeners helps in a more even distribution of the soil pressures between the columns’ areas and the intermediate areas of the raft foundation.

In a ribbed raft foundation apart from the unified foundation slab there are also beams which behave as stiffeners. The beams add stiffness to the foundation and they also level the soil stresses.

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The formwork’s assembling and the reinforcement implementation of a raft foundation stiffened by beams are two relatively strenuous procedures. A ribbed foundation can be stiffened either by beams or by walls. In the latter case, the reinforcement of the foundation slab does is independent of the wall’s reinforcement.

As shown at the figure, the ribbed raft foundation reinforcement can be separated into three categories:

(a) slabs’ reinforcement (in yellow color)

(b) slabs’ free edges reinforcement (in blue color)

(c) beams’ reinforcement (in green color)

The column rebars are in grey color.

In the solid raft foundation, there is only one unified foundation slab. The unified solid raft foundation is the simplest form of foundation and its formwork assembling as well as its reinforcement is usually very easy.

The foundation slabs are reinforced with two wire meshes, one placed at the lower fibers and one at the upper fibers. Since the most intense stresses appear along the columns’ axis, their surrounding areas are usually reinforced with stronger or double grates. The slabs’ free edges are reinforced with common hairpin bars or with a wire mesh shaped like a hairpin.

The reinforcement of a solid raft foundation can be separated into three categories, as shown at the following figure:

(a) slabs’ reinforcement

(b) slabs’ free edges reinforcement

(c) punching shear reinforcement (when necessary) in the area surrounding certain columns (in red color)

The column rebars are in grey color.

When the columns are subjected to large loads and the foundation slab’s thickness is analogically small, it is obligatory to use punching shear reinforcement. That reinforcement can be provided by stirrup cages, as it is in this example, by bundles of properly bent rebars or by special industrial elements.

In a raft foundation with hidden beams, the foundation slab is unified and has no additional stiffeners. This means that geometrically, it is as simple as the previous case. Its formwork assembling does not require a lot of effort as opposed to its reinforcement implementation.

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In a raft foundation with hidden beams, the foundation slab is unified and has no additional stiffeners. This means that geometrically, it is as simple as the previous case. Its formwork assembling does not require a lot of effort as opposed to its reinforcement implementation.

The stirrups placed inside the hidden beams may be two-legged or four-legged (as they are in this example). On other occasions, stirrups with more than four legs can be used.

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A mixed raft foundation includes solid parts, parts stiffened by beams and parts stiffened by walls.

The formwork assembling and the reinforcement implementation of a mixed raft foundation involve a lot of difficulties however, sometimes, it is an unavoidable solution e.g. in basements where it is obligatory to have shear walls.