Floor strengthening
The company Ouverture Murs Porteurs Trémies (OMPT) puts its building expertise at your service
Load-bearing wall openings, floor strengthening, reinforcement with steel structures, floor openings and glass partitions : these are OMPT's specialities. Major remodelling work like this must be carried out by approved professionals, after a feasibility study by the structural engineering office (BET), in accordance with the legal provisions on planning. Here is an explanation of our working methods.
HOW DO YOU REINFORCE A FLOOR ?
Several reasons can make it necessary to reinforce a floor. The need may come from storing loads that are too heavy, from wear of the reinforcement due to rust or fire, or from correcting the consequences of design calculation errors, poorly determined concrete cover during construction, or poor-quality concrete. Cutting floor openings next to load-bearing walls also makes it essential to reinforce the reinforced concrete floor with steel, a guarantee of strength and durability.
Different floor strengthening techniques can be used depending on the layout of the premises and the nature of the materials. Twin-beam clamping (moisage) is one : it consists of bringing together timber beams or concrete pieces to reinforce the floor and tie it back if there is a floor opening. Another method is to embed compact sections by thickening. A last process, known as the « Hermite » method, uses a connected beam as reinforcement together with bonded steel plates, like the technique used to strengthen a bridge. Although this technique offers excellent strength, it is not ideal in the event of fire, because the fire resistance of the adhesive is not optimal.
Reinforcing a timber or concrete floor is not a good solution when the surface is too badly deteriorated. In that case it is better to strengthen the whole structure beam by beam. Starting from what exists, the steel parts of the reinforced concrete must be treated to remove any trace of corrosion and given an anti-rust treatment. Steel beams are then added to reinforce the existing structure.
This combination of concrete and steel offers a double advantage, both economic and structural. The connectors joining the steel structure and the concrete slab reduce the installation cost and ensure a mechanical bond between them for greater strength and less deformation.
HOW ARE STEEL FLOORS MADE ?
With concrete : essentially structural concrete of at least class C 25 or C 30, more than 5 cm thick above the steel deck, taking care never to go right through the slab. A lighter concrete can be used instead. Remember to include a welded mesh or reinforcement.
With a nail gun : the gun drives SPIT P 560 standard nails. These are highly resistant nails sold with the connector, and they fix the steel deck to the beam. The nail gun guarantees a perfect fixing. This fastening system is quick and safe. Be careful with plaster materials, whose fragility may be tested by the vibrations of the nail as it is driven in.
With welded mesh : this is a reinforcement for reinforced concrete, made of crossed, welded bars, cut to the same size as the slab. The ST 20 reference is generally used, placed at mid-height of the connectors, with no need to tie it to them.
With corrugated sheet : the steel deck or decks, with a total thickness of less than 2 mm, are firmly attached to the beam, which allows the connector to be fixed. The recommended deck type is Cofraplus 60, with a rib height of 55 to 60 mm. It must act as one with the beam so that the connectors can work.
With connectors : two types of connector are possible : first, the CTF lugged connector, made of galvanised steel, cold-pressed onto an anchor plate and fixed with two nails ; it is mainly used for floors that need no particular reinforcement. Second, the tuning-fork connector, made of 3 mm thick galvanised sheet, U-shaped, with two wings whose angle gives a strong anchorage in the concrete. 4 highly resistant steel nails fix the base to the steel beam and guarantee excellent mechanical performance.
With steel sections : steel beams of type S 235, S 275 or S 355. They are either hot-dip galvanised or painted. The nails are driven into the steel or into the flange of the section. Make sure the thickness is greater than 8 mm.
HOW DO YOU FIX A FLOOR ?
Propping the floor
Once the concrete is poured, it takes 28 days to cure. It is during this period that the floor must be propped, so that it can carry loads without any risk of sagging. If the floor is not easily accessible, hangers must be provided to suspend it.
Connecting the slab to the wall
The slab must be tied to the load-bearing walls on every side of the floor. This makes it stiffer and gives it resistance to earthquakes. Depending on the type of wall, the work is done differently.
Strengthening an existing steel floor
Existing floors are often made of steel beams of the IAO type resting on the wall part of the foundations, with hollow or solid brick elements between them. When designed, the floor surface was levelled with a fill layer, serving as a base for the floor covering. Seismic standards introduced since then require this type of existing floor to be strengthened.
Fixing the connectors
It is not advisable to weld the steel elements of the steel joists, because of their chemical composition, but also because dust, rust or mortar may be stuck to them. It is therefore recommended to fix the connectors cold, so that the nails go straight into the metal. This is also easier to carry out.
Positioning the connectors
About 4 per m² are needed, close together on the wall side and more widely spaced in the middle of the beam. Of course, take care not to place them in the splice zones.
Adding insulation
Installing insulation as a structural element is useful to increase the section of the beam made of steel and concrete without adding any incidental load to the floor. These are panels made of rigid materials offering various benefits : from strength to dual thermal and acoustic insulation.
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 installed.
Existing timber floors most often need to be strengthened and stiffened, because when they were designed they were not meant to carry heavy loads. Deformed floors are in fact frequently found in old buildings. Excessive vibration under footsteps is also very common. Rather than replacing the boards in this situation, it is preferable to use composite concrete. This reinforces 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.
What are the steps to follow ?
The first step before any work is to assess the condition of the structure : is it well preserved, and have the beams kept their mechanical properties ? These findings can be made with the help of a surveyor, who tests the strength of the concrete and identifies the quantity and quality of the reinforcement in the existing joists.
If it is difficult to check beforehand whether the roughness of the surface to be joined provides enough resistance, a connector should be fixed. Indeed, if there is slip between the components of the structure, the load-bearing capacity is reduced and the risk of deformation increases.
One way to reduce the natural tendency to slip is to pour the concrete in two clearly separate stages. This alone is not enough, however, to let the whole act together fully. Hence the importance of positioning a connector, which makes it possible to check the level of cohesion of the materials.
There are 4 surface classifications in terms of roughness. Very smooth, when cast against smooth formwork ; smooth, the most common, for a concrete screed with a vibrated surface ; rough, obtained artificially by mechanical processes ; and indented, which is prepared and then cast using profiled elements made for the purpose. For exposed brick or crumbly smoothing, the contribution should be considered nil from a safety point of view.
In addition, the strength and deformation properties of the connection affect how the composite element will behave. What matters most is the ductility of this connection, because it provides the best load-bearing and bending capacity of the structural assembly. It also influences the even distribution of all the connectors along the whole length of the structure. This shows how important it is to know how a connection behaves in order to design and correctly assess the structural reliability of the floor before any reinforcing slab is added.
The connection is most often made with lugged connectors, with anchors to the various elements depending on the materials ; this is to make installation easier.
Concrete and fibre slabs
New techniques have made it possible to create thin slabs that offer high strength. They are made of fibre-reinforced concrete. This material is now very often used to strengthen concrete and brick floors. It is known as FRC, the English acronym for Fiber Reinforced Concrete. A cement-based mix reinforced with various fibres, this material guarantees high resistance to compression as well as to tension. It is also ductile and withstands shear better than conventional concrete.
As these new materials are not yet classified, their fields of application are not precisely known. They are in any case already used to meet seismic standards and to stiffen thin floors with a limited load.
The effectiveness of such a rigid plane is optimal provided there is a degree of constraint on the existing structure at the double junction of beam and slab and of slab and wall. FRC designers frequently advise carrying out laborious preparation of the surface to be strengthened in order to reinforce brick and concrete slabs. They recommend, for example, dulling the substrate by mechanical abrasion, then cleaning it and starting the strengthening with a primer applied with a roller. Where the surface is not smooth in places, a metal connection is essential so that nothing is left in doubt.
What are the components ?
Floors without a screed : many floors have no upper screed. These are so-called hollow floors, for which a composite concrete slab must be built so that loads can be distributed and the structure meets the requirements of the seismic standards.
Floors with a screed : some floors have screeds that are relatively thin or lightly reinforced. In this kind of configuration, they must be reinforced to stiffen the whole plane and increase the load-bearing capacity.
Floors with low load capacity : for floors characterised by low load capacities, any possible increase in loads must be allowed for. A composite slab is then installed to increase the distance between the internal forces and improve the section's resistance to bending. The more the depth of the section is increased, the more the strength has to be increased.
The dimensions of the existing joists
Take the time to examine the dimensions of the joists already in place. Their width must be sufficient for the connector to be covered laterally by an adequate volume of concrete. This precaution must be taken over the whole embedment depth of the connector. The minimum width of the joist must therefore be 10 cm, or its maximum thickness 2 cm, if the floor has no existing slab. The same thickness requirement applies if the floor has a slab, but its width must be at least 8 cm. If the existing floors have smaller joists, no connector can be installed at all. The type of precast joists makes it easy to tell which case applies. If the floor is made of Bausta elements, there is no problem fixing a connector ; for Varese or Sap floors, which are small in size, it is quite different.
The nature of the existing joists
For existing concrete joists, the compressive strength must be tested. It must give values greater than or equal to Rck 20 MPa. The connector's resistance, its shear and the bending checks all depend on this. If the concrete has carbonated and the steel has then oxidised, it is impossible to position connectors.
The reinforcement of the existing joists
The strength of the lower steel bars of the existing floors to be reinforced must be tested. To do so, their characteristics in terms of diameter, steel type and number should be recorded. Tests carried out in a specialist laboratory make it possible to check the steel's ability to resist up to failure.
Which connectors should you use?
Several connectors are available, and the right one depends on the floor configuration and how the floor will be used. For example, a floor that has to carry heavy loads calls for a CTEM connector. It has a plate at its base that anchors onto the existing slab, giving the best possible mechanical performance.
For lighter applications, the VCEM connector may be suitable. It has a single screw with a threaded section that bites well into the concrete.
Finally, the CTC MINI connector is used to bond several very thin layers of high-performance concrete, in order to create composite slabs with a thickness of at least 20 mm.
Which type of concrete should you use?
Most often, structural concrete of at least class C 25/C 30 is used, with a thickness greater than 4 cm. Above all, take care never to exceed the level of the composite slab, and make sure the floor is properly dampened before the concrete is poured.
Lightweight structural concrete is recommended to reduce the load on the reinforced slab while keeping its optimal mechanical strength. This is the material used in new buildings, as it meets the standards required in any area with significant seismic risk.
The overall thickness of the reinforced slab must be greater than 1/25 of its length. This is a basic rule. A simple calculation is needed to determine the number of connectors to fix. They should be placed close together near the walls and spaced further apart towards the middle of the beam.
The floors must be properly propped before the concrete is poured. If the spaces below cannot be accessed, hangers should be used to suspend the floor.
Fibre-reinforced concrete, for its part, is used to keep the thickness of the work within a range of 20 to 30 mm and to reduce loads.
If there are distribution slabs that are not tied to the walls, or if there is no slab at all, they must either be tied to the walls or bars must be fixed with resin.
Do not forget to add welded mesh at mid-height of the slab. This mesh does not necessarily have to be tied to the slab with connectors.
Finally, it is important to provide for an insulating panel with the right properties of rigidity, strengthening, strength, and thermal and acoustic insulation.
The problem of brick failure
Some floors that are sensitive to significant deflection can bend to the point where the lower plate and the hollow floor block sag. In this case, the floor should be strengthened accordingly, then connectors fixed to the new slab to prevent this brick failure from happening again.
If the floor cannot be strengthened, for example because it is undersized, steel beams must be inserted inside it. Various solutions are possible, from outright demolition to repair using carbon fibre. The steel beams are then placed in place of rows of hollow floor. The beam is then connected to a composite slab with a CTF-type connector and finally properly reinforced with welded mesh.
We are here to help. If you have any questions, do not hesitate to contact us.