What are the components of an existing floor?
Steel beams : of the IAO or NP type, these are not standardised-geometry beams. Their composition rules out any welding. Their stresses were in a range of 900 to 1,200 kg/cm².
Concrete : the concrete used is class C 25 or C 30, more than 5 cm thick. Structural lightweight concrete is preferred when the aim is to reduce the load on the strengthened slab while keeping its mechanical strength. Its use is particularly recommended in areas of high seismic risk.
Infill blocks : this is a levelling layer of concrete or plaster resting on prefabricated components. The fill was originally made of loose materials, but it is now made of clay or polystyrene. Bricks can also serve as formwork when the concrete is poured, if their condition allows it. Otherwise it is better to use a steel deck.
Welded mesh and connectors as described above.
Timber floors
A distinction must be made between existing timber floors and new ones to be laid.
Existing timber floors most often need to be strengthened and stiffened because, when they were designed, they were not intended to carry heavy loads. Examples of deformed floors are frequently found in old buildings. Excessive vibrations under the weight of footsteps are also very often noted. Rather than replacing the boards in this case, it is preferable to use composite concrete. This strengthens the floor while keeping it as it is, without any noticeable effect on the height of the structure.
To compete with concrete floors in terms of strength and stiffness, new timber floors must be built with deep-section beams. A mix of timber and concrete sections is a good alternative because it reduces the overall thickness.
In every case, the strength and stiffness of old timber floors can be increased by laying a thin concrete slab over the existing structure. This reinforced, connected concrete is then poured directly in place on the structure. The same operation can be applied to new floors, allowing beams with a much smaller section to be used.
This timber-concrete process is also used to build roofs, which can be either flat or pitched. When refurbishing an old timber floor, the benefit is twofold: the renovation significantly increases the floor's ability to carry a heavy load, and the connectors make it possible to reduce the section of the timber beam substantially while keeping its load-bearing capacity.
It is essential to place the connectors between the concrete slab and the timber beams. This is what allows the materials to work together. It creates an integral structure in which each material has a very specific function : as soon as a vertical load is applied, the concrete acts in compression and the timber acts in tension. This mixed timber-concrete structure is recommended over a simple timber structure for its high stiffness and strength.
On top of these properties, there are further significant advantages : vibrations are damped, and thermal insulation and thermal inertia are improved. Naturally, the first improvement is in load capacity, which is clearly increased. This composite assembly has the advantage of being shallower than other structures, and offers high stiffness and good fire resistance in the event of a fire.
Brick and concrete floors
Floors made of brick and concrete are those that were built in Italy at the beginning of the last century. Current regulations and standards for new construction make them poorly suited today. One of the main reasons lies in how they were built : the concrete used is insufficient or of poor quality, making it weak, or the steel reinforcement was not installed in sufficient quantity to guarantee the strength of the structure. Other cases of poor workmanship or faulty design have also been recorded.
In buildings where these brick and concrete floors were installed, strengthening work is needed. In their original state, they are weak when it comes to carrying a heavy load, and they do not meet the seismic standards, which require a reinforced concrete slab to distribute loads in the event of an earthquake.
A second concrete slab therefore has to be built and connected to the existing floor. This complementary solution is the cheapest for a very satisfactory result. The operation is all the more successful because the connection between the two parts of the floor makes it possible to create a section with perfect interaction.