EP3175057B1 - A pre-tensioned bearing structure - Google Patents

Description

TECHNICAL FIELD TO WHICH THE INVENTION RELATES

The present invention generally relates to the manufacture of structural elements, in particular beams, slabs or concrete moldings.

It relates more particularly to a prefabricated structural element comprising an elongated body, and at least a first tensioner which is fixed in the elongated body in such a way that it compresses and bends the elongated body in a first direction.

It also relates to a method of installing such a structural element in a structure.

The invention applies to any type of structure, for example buildings, bridges, dams, etc.

TECHNOLOGICAL BACKGROUND

It is common, during the construction of such a structure, to use concrete beams and slabs.

A well-known problem with concrete is that, although it resists compressive forces well, it cracks quickly when subjected to tensile forces. It is estimated that concrete resists compressive forces twenty times better than tensile forces.

It is then known to reinforce the concrete with metal reinforcements. We are talking about reinforced concrete.

However, although reinforced concrete has certain advantages, its use becomes counter-productive when the stresses exerted on the structure become significant, due to the increased weight of the reinforced concrete section.

The solution then considered is to use so-called prestressed concrete.

The idea is then to ensure that the concrete always works in compression and never (or only slightly) in tension. To do this, tensioners are placed in the concrete (generally steel bars or metal cables) on which traction is exerted so that at rest, the concrete is compressed.

In this way, when the concrete undergoes tensile forces, it decompresses but never works in tension, which prevents the appearance of cracks.

Currently, we know two processes for manufacturing beams in Prestressed concrete.

The first process, called pre-tensioning, consists of applying tension to the tensioners before the concrete has completely set. After the concrete has dried, the tensioners are released, thus putting the beam in compression by simple adhesion effect.

This process is simple to implement. Generally, when the beam is intended to be placed horizontally and to be loaded, the tensioners are off-centered relative to the neutral fiber of the beam in such a way that they allow the beam to be bent upwards (we speak of counter-deflection), opposite to the load that the beam will receive (once loaded, the beam will then find itself little deformed).

This process, however, has two drawbacks.

The first disadvantage is that when the beam is unmolded and the tensioners are released, the latter cause excessive forces on the beam which have the effect of bending the latter quickly, with the risk that these tensile or compressive forces generate cracks in the concrete. It is therefore advisable to wait until the concrete has dried well before releasing the tensioners and unmolding the beam.

Another disadvantage is that, when the beam is not yet loaded, its bending generates tensile stresses in the concrete (at the level of the convex face of the beam) as well as particularly strong localized compressive stresses (at the level of the concave face of the beam). The prestress generated by the first tensioners must therefore not exceed a threshold beyond which the beam bends too much and it would crack.

The second process, called post-tensioning, consists of placing tensioners through straight or curved sheaths incorporated into the concrete of the beams. After the concrete has set, the beam is placed in the structure (building, bridge, etc.) before the tensioners are tensioned. Once the beam is in place, it is gradually loaded (for example by placing slabs on top). It is during this loading stage that the tensioners will be gradually put in tension so as to compress and bend the beams as they are put under load.

This technique makes it possible to gradually compensate for the forces exerted on the beam when it is loaded. It then makes it possible to achieve the limits of concrete resistance to compression, so that the beam obtained by this technique can be more loaded than that obtained by pre-tension.

This technique, relatively complex, is however generally reserved for large works since it requires the use of bulky tensioning machines. It also turns out to be expensive not only because of the use of these machines, but also because the tensioners are threaded through special and expensive sheaths into which a special coating is injected which makes it possible to transmit the forces to the beam.

US 6,668,412 B1 discloses the features of the preamble of claim 1.

OBJECT OF THE INVENTION

In order to remedy the aforementioned drawbacks of the state of the art, the present invention proposes a new structural element which has the advantages of prestressing by pre-tension and which is adapted to support higher loads.

More particularly, according to the invention, we propose a structural element according to claim 1 and a method of constructing a structure according to claim 14. In particular, we propose a structural element as defined in the introduction, in which there is provided at least a second tensioner which is fixed at two distinct points to said elongated body such that it compresses and flexes the elongated body, and in which there is provided deactivation means for releasing the compression and flexing exerted on the body elongated by said second tensioner.

The first and second tensioners will then be put in tension during the molding of the body (beam, slab, etc.) of the structural element in the factory. The process for manufacturing the structural element will therefore be as easy to implement as the prestressing process by pre-tension.

The first and second tensioners will generally be located longitudinally in the body, along two opposite faces thereof. In this way, the tension exerted on the first and second tensioners will generate little or no deflection of the structural element (the counter-deflection exerted by the first tensioner will be compensated by the deflection exerted by the second tensioner).

We thus understand that this structural element can be demolded earlier since there will be no risk of cracking when the tensioners are released and demolded. Its manufacturing will therefore be faster, to the benefit of its production cost.

Thanks to the invention, the second tensioner(s) will make it possible to compress and flex the body temporarily. After the release of the second tensioners, the structural element will behave like a structural element obtained by a pre-tensioning process by pre-tension.

The major advantage of the invention will be that, as the body will not flex or only slightly, it will be possible to compress it further.

In fact, as long as the second tensioners are taut, the structural element will be little or not bent. We can then plan to release these second tensioners as the structural element is loaded, so that these successive releases of the second tensioners make it possible to compensate for the deformation of the structural element caused by the load.

Consequently, as the bending will be less significant, the risks of cracks appearing will be less, so that it will be possible to further tighten the first tensioners (and the second tensioners) until approaching the strength limits of the concrete at the compression.

It will also be possible to move the first tensioners as far as possible from the neutral fiber of the body so that after extracting the second tensioners from the body, the first tensioners exert a strong bending moment on the body which opposes the bending. generated by the body's own weight and by the loads resting on it.

Due to the greater prestressing, the structural element will therefore be able to support heavier loads.

To support a load of the same weight, it will otherwise be possible to reduce the section of the beam, which will generate savings in weight, size and costs.

• le corps allongé présentant deux abouts et chaque second tendeur présentant deux extrémités noyées dans les abouts du corps allongé;
• le second tendeur étant fixé audit corps allongé par des moyens de fixation, l'un au moins desdits moyens de fixation comporte une partie amovible qui forme ledit moyen de désactivation ;
• le second tendeur étant réalisé en deux parties qui sont situées dans le prolongement l'une de l'autre et qui sont raccordées l'une à l'autre par un moyen de fixation, ce moyen de fixation comporte une partie amovible qui forme ledit moyen de désactivation ;
• ladite partie amovible est fusible ;
• ladite partie amovible est démontable ;
• au moins une partie centrale du second tendeur est accessible depuis l'extérieur du corps allongé pour être découpée ou cassée ;
• le second tendeur étant fixé audit corps allongé par ses extrémités, les moyen de désactivation sont uniquement adaptés à relâcher la compression et le fléchissement exercés sur le corps allongé par la partie centrale dudit second tendeur ;
• chaque second tendeur est adapté à être entièrement ou partiellement extrait dudit corps allongé ;
• chaque second tendeur comporte un fil, un câble ou une barre métallique;
• au moins une partie centrale du second tendeur est enfilée librement dans une gaine fixée au corps allongé ;
• au moins une partie centrale du second tendeur est située à l'extérieur du corps allongé ;
• chaque premier tendeur comporte un fil, un câble ou une barre métallique, noyé dans le matériau du corps allongé ; et
• le corps est formé par une poutre en béton ou par une dalle en béton.

Other advantageous and non-limiting characteristics of the structural element according to the invention are as follows:

• the elongated body having two ends and each second tensioner having two ends embedded in the ends of the elongated body;
• the second tensioner being fixed to said elongated body by fixing means, at least one of said fixing means comprises a removable part which forms said deactivation means;
• the second tensioner being made in two parts which are located in the extension of one another and which are connected to each other by a fixing means, this fixing means comprises a removable part which forms said means deactivation;
• said removable part is fuseable;
• said removable part is removable;
• at least a central part of the second tensioner is accessible from outside the elongated body to be cut or broken;
• the second tensioner being fixed to said elongated body by its ends, the deactivation means are only adapted to release the compression and the bending exerted on the elongated body by the central part of said second tensioner;
• each second tensioner is adapted to be entirely or partially extracted from said elongated body;
• each second tensioner comprises a wire, a cable or a metal bar;
• at least a central part of the second tensioner is threaded freely into a sheath fixed to the elongated body;
• at least a central part of the second tensioner is located outside the elongated body;
• each first tensioner comprises a wire, a cable or a metal bar, embedded in the material of the elongated body; And
• the body is formed by a concrete beam or by a concrete slab.

1. a) d'installation d'une structure initiale,
2. b) de fixation d'un élément de structure tel que précité à ladite structure initiale,
3. c) d'installation d'une structure postérieure sur ledit élément de structure, de telle sorte que ledit élément de structure est mis en charge,
4. d) de désactivation de chaque second tendeur dudit élément de structure pour relâcher la compression et la flexion exercées par le second tendeur sur le corps allongé dudit élément de structure, en supprimant cette compression et cette flexion.

The invention also proposes a method of constructing a structure, comprising steps:

1. a) installation of an initial structure,
2. b) fixing a structural element as mentioned above to said initial structure,
3. c) installation of a posterior structure on said structural element, such that said structural element is loaded,
4. d) deactivating each second tensioner of said structural element to release the compression and flexion exerted by the second tensioner on the elongated body of said structural element, eliminating this compression and this flexion.

Preferably, steps c) and d) are carried out concomitantly.

• e) d'ajout d'une chape ou d'une table de compression ou d'une charge de poids permanente sur ladite structure postérieure, et
• f) de désactivation de chaque second tendeur de l'élément de structure de ladite structure postérieure pour relâcher la compression et la flexion exercées par le second tendeur sur le corps allongé dudit élément de structure.

Advantageously, said posterior structure comprising at least one structural element conforming to the invention, steps c) and d) are followed by the steps:

• e) adding a screed or a compression table or a load permanent weight on said posterior structure, and
• f) deactivating each second tensioner of the structural element of said posterior structure to release the compression and flexion exerted by the second tensioner on the elongated body of said structural element.

DETAILED DESCRIPTION OF AN EXAMPLE OF WORK

The description which follows with reference to the appended drawings, given as non-limiting examples, will make it clear what the invention consists of and how it can be carried out.

• les figures 1A et 1B sont des vues schématiques de côté et en coupe d'une poutre de section rectangulaire conforme à l'invention,
• la figure 1C est un diagramme des efforts et moments s'exerçant à vide sur le corps de la poutre représentée sur les figures 1A et 1B,
• la figure 1D est un diagramme des efforts et moments s'exerçant sous charge sur le corps de la poutre représentée sur les figures 1A et 1B,
• les figures 2A, 2B et 2C sont des vues de détail, en coupe, de trois modes de réalisation des moyens de désactivation d'un second tendeur de la poutre représentée sur les figures 1A et 1B,
• les figures 3A et 3B sont des vues schématiques de côté et en coupe d'une poutre de section en I conforme à l'invention,
• les figures 4A et 4B sont des vues schématiques de côté et en coupe d'une dalle conforme à l'invention, et
• les figures 5A et 5B sont des vues schématiques d'un ouvrage utilisant des poutres et des dalles conformes à l'invention.

On the attached drawings:

• THE Figures 1A and 1B are schematic side and sectional views of a beam of rectangular section according to the invention,
• there Figure 1C is a diagram of the forces and moments acting empty on the body of the beam represented on the Figures 1A and 1B ,
• there Figure 1D is a diagram of the forces and moments acting under load on the body of the beam represented on the Figures 1A and 1B ,
• THE Figures 2A, 2B and 2C are detailed views, in section, of three embodiments of the means for deactivating a second tensioner of the beam shown on the Figures 1A and 1B ,
• THE Figures 3A and 3B are schematic side and sectional views of an I-section beam according to the invention,
• THE Figures 4A and 4B are schematic side and sectional views of a slab according to the invention, and
• THE Figures 5A and 5B are schematic views of a structure using beams and slabs conforming to the invention.

Large structures (buildings, bridges) are generally made using prefabricated structural elements, which are assembled together on the site.

In the description, the terms “lower” and “upper” will be used in relation to this work once assembled, the lower part of an element designating the part of this element which is located on the ground side and the upper part designating the part of this element which is located on the opposite side.

On the figures 1A, 3A , 4A and 5A , four types of prefabricated structural elements 10, 20, 30, 40 are shown. On the Figures 1A and 3A , these are beams 10, 20. On the Figures 4A and 5A , these are slabs 30, 40.

The points common to these different structural elements 10, 20, 30, 40 will be described together in a first part of this presentation. These structural elements 10, 20, 30, 40 will then be described successively in detail in a second part of this presentation.

Each structural element 10, 20, 30, 40 firstly comprises an elongated body 11, 21, 31, 41. This body gives the structural element 10, 20, 30, 40 its general shape. It also incorporates reinforcements allowing it to be pre-stressed.

We will consider here the case where the body 11, 21, 31, 41 is made of concrete. Alternatively, another material could be used.

We will also consider the case where the body 11, 21, 31, 41 is designed to be placed horizontally in the structure, to be fixed there by its two ends, and to be loaded (that is to say to support heavy elements).

We understand that the structural element 10, 20, 30, 40 considered will then be subjected to two forces: its own weight and the weight of the load. Usually, we can subdivide this load into two components: a permanent component and a variable component.

Under the effect of these two efforts, the body 11, 21, 31, 41 will naturally tend to deform, bending downwards (the body is then said to have an “arrow”). This flexion will generate compressive stresses in the upper part of the body, and tensile stresses in the lower part of the body. The risk would then be that, due to these tensile or compressive stresses, the concrete would crack on the lower face or on the upper face of the body.

To prevent the appearance of such cracks, the structural elements 10, 20, 30, 40 are each equipped with at least a first tensioner 1 which is fixed in the body 11, 21, 31, 41 in such a way that the latter is compressed.

The compression obtained will then make it possible to compensate for the aforementioned tensile stresses, so as to avoid the appearance of cracks. The body 11, 21, 31, 41 is therefore prestressed by each first tensioner 1.

Generally, several first rectilinear tensioners 1 will be used. These first tensioners 1 could be formed by wires, by metal cables or by high-capacity steel bars. They will preferably be embedded in concrete, so as to gradually transmit the forces to the body of the structural element 10, 20, 30, 40. They are also eccentric with respect to the neutral fiber A1 of the body 11, 21, 31, 41 of the structural element 10, 20, 30, 40 considered .

As clearly shown by the figures 1B, 3B , 4B and 5 , these first tensioners 1 are preferentially distributed under the neutral fiber A1 of the body 11, 21, 31, 41 of the structural element 10, 20, 30, 40 considered.

As clearly shown by the Figures 1A and 1B (and it is the same for the other structural elements 20, 30, 40), the first tensioners 1 extend in length parallel to the neutral fiber A1 of the body 11 of the beam 10, and they are regularly distributed around 'a lower middle fiber A2.

The total traction exerted on the first tensioners 1 therefore makes it possible to compress the body 11 of the beam 10 along the lower middle fiber A2.

As shown in the Figure 1C , these first tensioners 1 therefore exert a compressive force E1 on the body 11 of the beam 10. This compressive force E1 generates compressive stresses distributed homogeneously over the entire section of the body 11.

As shown in the Figure 1A , the lower average fiber A2 is here located under the neutral fiber A1, at a distance from it denoted difference D1.

As shown in the Figure 1C , due to this difference D1, the first tensioners 1 exert a bending moment M1 on the body 11 of the beam 10. This bending moment M1 makes it possible to bend the body 11 upwards, that is to say in a direction opposite to that in which it tends to deflect under the effect of its own weight (the beam is then said to have a “counter-deflection). This bending moment M1 generates, in the upper part of the body 11, tensile stresses. It also generates compressive stresses in the lower part of the body 11.

When the body 11, 21, 31, 41 of the structural element 10, 20, 30, 40 is in place in the structure and is loaded, this bending moment M1 makes it possible to compensate for the bending of the body under the effect of its own weight and under the effect of the load it carries.

On the other hand, before the installation of the structural element 10, 20, 30, 40 in the structure, the bending moment M1 is not compensated and tends to bend the body 11, 21, 31, 41 (especially when the structural element is stored vertically and its own weight no longer causes the body to bend towards the down).

In practice, as long as the deviation D1 and/or the tensile force exerted on the first tensioners 1 remains less than a threshold, the tensile stresses generated by the bending moment M1 are compensated by the compressive stresses generated by the compressive force E1: the stresses remain within admissible limits. There is therefore no risk of cracking.

The disadvantage is then that the bending moment M1 then remains limited due to this difference D1, so that the loads that the structural element 10, 20, 30, 40 can support are also limited.

The invention then proposes a method making it possible to increase this gap D1 and/or to increase the forces exerted on the first tensioners in order to be able to further load the structural element 10, 20, 30, 40, without risking damage. see cracks appear in the concrete.

Thus, according to a particularly advantageous characteristic of the invention, the structural element 10, 20, 30, 40 comprises at least one second tensioner 2 which is fixed at two distinct points to the body 11, 21, 31, 41 by two means fixing means 3 such that it compresses and bends the body 11, 21, 31, 41 downwards, and there is provided a deactivation means 3B to release the compression and bending exerted on the elongated body 11, 21, 31, 41 by this second tensioner 2.

In practice, each second tensioner 2 is located on the body 11, 21, 31, 41 off-centered relative to the neutral fiber A1 so that, when the structural element is not yet placed in the work and is not yet loaded, the compression it exerts on the structural element helps prevent the appearance of cracks.

Preferably, each second tensioner 2 is located above the neutral fiber A1 of the body 11, 21, 31, 41, so that it exerts on the body a bending moment which at least partially opposes the bending moment M1.

We will generally use several second straight tensioners 2. These second tensioners 2 may be formed by wires, by metal cables or even by metal bars, preferably made of suitable steel.

As clearly shown by the figures 1B, 3B , 4B and 5 , these second tensioners 2 are preferably distributed above the neutral fiber A1 of the body 11, 21, 31, 41 of the structural element 10, 20, 30, 40 considered.

As shown, for example, by Figures 1A and 1B (and it is the same for the other structural elements 20, 30, 40), these second tensioners 2 are here elongated parallel to the neutral fiber A1 of the body 11 of the beam 10 and are regularly distributed around an average fiber superior A3.

The total traction exerted on the second tensioners 2 therefore makes it possible to compress the body 11 of the beam 10 along the upper middle fiber A3.

As shown in the Figure 1C , these second tensioners 2 exert a compressive force E2 on the body 11 of the beam 10, which is added to the compressive force E1. This compressive force E2 generates compressive stresses distributed homogeneously over the entire section of the body 11.

As shown, for example, by Figures 1A and 1B , this upper average fiber A3 is located above the neutral fiber A1, at a distance from it denoted difference D2.

As shown in the Figure 1C , due to this gap D2, these second tensioners 2 exert a bending moment M2 on the body 11 of the beam 10, in the opposite direction to the bending moment M1. This bending moment M2 makes it possible to bend the body 11 downwards, so that it makes it possible to compensate at least in part for the bending of the body 11 under the effect of the bending moment M1.

Otherwise formulated, this bending moment M2 makes it possible to simulate a load on the beam 10 when it is not yet loaded. The means for deactivating each second tensioner 2 will then allow, when the beam 10 begins to be loaded, to release the second tensioner 2 so as to cancel the bending moment M2.

Thanks to the invention, it is then possible to maximize the gaps D1 and D2 and the traction forces exerted on the first and second tensioners 1, 2 while avoiding excessive bending of the body when it is not yet loaded, which prevents the appearance of cracks in the concrete and which avoids exceeding the admissible compression limits of the concrete.

This maximization of the gaps D1 and D2 and the traction forces therefore makes it possible to increase the values of the bending moments M1, M2. In this way, when the second tensioners 2 have been extracted from the body 11, 21, 31, 41 of the structural element (the latter having been progressively loaded), the bending moment M1 will allow the structural element 10, 20, 30, 40 to endure heavier loads than those it could have supported if the gap D1 or if the force exerted on the first tensioners 1 had been less.

The only obstacle to this process remains the limit of the compressive strength of the concrete, which must not be exceeded. This is the reason why we will focus more on maximizing the gaps D1 and D2 than on increasing the tensile forces exerted on the first and second tensioners 1, 2.

On the Figure 1C , the sum of the forces and moments exerted by the first and second tensioners 1, 2 is denoted C1.

As shown in the Figure 1D , when the structural element 10, 20, 30, 40 is loaded, this load and the own weight of the structural element exert a bending moment M3 which is added to the aforementioned sum C1.

By releasing the second tensioners 2, it is then possible to eliminate the compressive force E2 and the bending moment M2 which were exerted on the structural element, so as to at least partially compensate for the bending moment M3.

• aucune contrainte de traction n'apparaît dans l'élément de structure, et
• les contraintes de compression que subit l'élément de structure demeurent inférieures à la limite de compression du béton.

Then, the sum of the forces and moments exerted on the structural element, denoted C2, makes it possible to ensure that:

• no tensile stress appears in the structural element, and
• the compressive stresses experienced by the structural element remain lower than the compression limit of the concrete.

In a variant of the invention where the ends of the second tensioners are fixed in a non-removable manner to the body of the structural element, provision can be made to release only the central parts of these second tensioners. It is thus possible, for example, to cut the second tensioners at their centers, so that their two ends remain fixed to the body of the structural element. In this variant, the ends of the second tensioners will therefore allow, after their cutting, to continue to exert compressive forces and bending moments at the ends of the body of the structural element. These efforts and moments will then make it possible to compensate for the forces that the first tensioners exert on these ends.

We can now look in more detail at the particular case of the beam of rectangular section represented on the Figures 1A and 1B .

In this mode, the body 11 of the beam 10 has a substantially parallelepiped shape.

The first tensioners 1 are entirely embedded in the concrete of the body 11 of the beam 10, with the possible exception of their ends which can project from the body. They are therefore immovable relative to the body of beam 10.

The second tensioners 2 are mounted slidingly in the body 11 of the beam 10. As shown in Figure 2B , the second tensioners 2 are for this purpose threaded into sheaths 4 cast in the concrete, such that their ends open out at both ends of the body 11. These sheaths 4, here made of plastic, prevent the concrete from coming hang on the second tensioners 2.

As explained above, we could however plan to embed the ends of the second tensioners in the ends of the body of the structural element.

Alternatively, as shown in Figure 2A , we could not use a sheath, in which case it would be necessary to plan to move the second tensioners 2 (for example by making them pivot on themselves) during the drying of the concrete, so that the concrete does not adhere to the second tensioners 2. Again as a variant, a product could be applied to the second tensioners which prevents the adhesion of the concrete. We could also plan to place the second tensioners outside the concrete, as will for example be explained with reference to the Figure 4A .

Here, as shown in Figure 1B , the first tensioners 1 are distributed over three rows and five columns. The first tensioner 1 which is located in the center of this matrix therefore extends along the lower middle fiber A2.

The second tensioners 2 are here half as numerous as the first tensioners 1 and are distributed relative to each other in substantially the same way as the first tensioners. The second tensioner 2 which is located in the center of this matrix therefore extends along the upper middle fiber A3.

The neutral fiber A1, which passes through the centers of the cross sections of the body 11, therefore extends between the upper middle fibers A3 and lower A2, at equal distances from them.

As explained above, each second tensioner 2 is fixed at two distinct points to the body 11 of the beam 10, by two fixing means, and a deactivation means is provided to release each second tensioner 2.

In practice, each second tensioner 2 is fixed by its two ends 2A to the two ends of the body 11 of the beam 10. In this way, these ends 2A remain easily accessible from outside the body 11.

The deactivation means is provided to allow, on site, to release the tension of the second tensioner 2.

This means of deactivation can be presented in various ways.

It can thus be integrated into at least one of the fixing means 3 of the ends 2A of the second tensioners 2 to the body 11 of the beam 10.

As it appears on the Figure 2A , this fixing means 3 comprises a socket 3A which is pressed against the corresponding end of the body 11 of the beam 10 and which internally houses two keys 3B.

The socket 3A has a cylindrical outer face of revolution, and a frustoconical inner face whose top is turned towards the body 11.

The keys 2B overall have the shape of a cylinder cut in two lengthwise. They each have a frustoconical outer face with a shape corresponding to that of the inner face of the socket 3A, and a notched inner face.

These two keys 2B thus form a sort of jaw which, when they sink into the socket 3A towards the body 11, make it possible to rigidly block the end 2A of the second tensioner 2.

In this embodiment shown on the Figure 2A , the aforementioned deactivation means is then formed by the two keys 3B. These two keys 3B in fact project outside the socket 3A, so that they are adapted to be mechanically pulled outside the socket 3A so as to release the end 2A of the second tensioner 2.

In the embodiment shown on the Figure 2B , the deactivation means is fuse. In this case, this deactivation means is formed by a layer of metal or by a non-metallic paste which covers the end 2A of the second tensioner 2 (or the internal face of the socket 3A or the keys 3B), the temperature of which melting temperature is relatively low, and which has satisfactory mechanical characteristics. It could thus be a layer of zinc or tin, since the melting temperature of these materials is low enough (less than 450°C, preferably around 200 to 300°C) to allow its fusion on site, and that its rigidity at ambient temperature is sufficient to ensure good grip of the second tensioner 2.

Alternatively, the removable fixing means could be in the form of another form.

It could thus be in the form of a threaded sleeve fixed on the second tensioner, which would screw into a nut engaged in the end of the sheath 4. To release the compression exerted by the second tensioner on the body of the beam, it would then be enough to unscrew the threaded sleeve so that it escapes from the nut.

According to another variant, the fixing means used could be in the form of a glue or a meltable paste. It could also be in the form of an easily destructible sleeve (for example by cutting), which would be slipped onto the second tensioner and which would rest against the body.

Still as a variant, the end 2A of the second tensioner 2 could be coated with a layer of zinc or tin and be directly sealed in the concrete. Thus, by heating the metal layer, it will be possible to melt it so that the second tensioner 2 can escape from the concrete and release the compression and bending that it exerts on the body 11 of the beam 10.

In any case, the second fixing means, intended to block the other end of the second tensioner 2, can also be presented in various forms.

It will preferably be removable. In this way, the second tensioner 2 can be extracted from the body 11 of the beam 10 and can be reused on other beams, which will reduce costs.

It can thus be in the same form as the first removable fixing means. It can also be in the form of a sleeve which will be slipped on and fixed on the second tensioner and which will simply rest against the body.

Alternatively, this second fixing means can fix the second end of the second tensioner 2 in an immovable manner to the body 11 of the beam 10, in which case the second tensioner 2 cannot be extracted from the body 11. Thus, one could plan to drown this second end of the second tensioner 2 in the concrete of the body 11.

As shown in the Figure 2C , it could alternatively be provided that the deactivation means is not at one of the ends of the second tensioner 2, but at a distance from them. In this variant, we can block the ends 2A of the second tensioner 2 to the body 11 of the beam 10 in an immovable manner (or not).

In this variant, the second tensioner 2 is made in two parts 2C, 2D located in the extension of one another and which are connected together by a fixing means 5.

This fixing means 5 comprises a sleeve 5A inside which two pairs of keys 3B are housed. The 3B keys are identical to those shown on the Figure 2A . The sleeve 5A has an interior face which has two frustoconical parts facing in opposite directions.

Here, the contiguous ends of the two parts 2C, 2D of the tensioner are each coated with a coating of zinc or tin or another material which represents suitable characteristics, in which a resistance wire runs.

When the second tensioner 2 is put into traction (during the molding of the body 11 of the beam 10), the keys 3B move towards the two ends of the sleeve 5A, which allows them to close in the manner of two jaws on the contiguous ends of the two parts 2C, 2D of the second tensioner 2.

To deactivate this second tensioner 2, it will then be necessary to supply the resistance wire with electric current (from the outside of the body 11 of the beam 10), so that the coating melts and the contiguous ends of the two parts 2C, 2D of the second tensioner escape out of sleeve 5A.

We can now give a specific example of beam 10 usable on site.

The beam could thus have a length of 7 meters, a height L2 of 60 centimeters, and a width L3 of 40 centimeters.

The first tensioners 1 can be distributed in such a way that the lower middle fiber A2 extends 6.8 centimeters from the lower face of the body 11 of the beam 10.

The second tensioners 2 can be distributed in such a way that the upper average fiber A3 extends 5 centimeters from the upper face of the body 11 of the beam 10.

This beam 10 can be prestressed using the first tensioners 1 with a compression force E1 equal to 192 tonnes.

We can also temporarily prestress this beam 10 using the second tensioners 2 with a compression force E2 equal to 120 tonnes.

On the Figure 3A , we have shown a second embodiment of a beam 20 according to the invention.

In this mode, the body 21 of the beam 20 has an I-shaped cross section.

As shown in the Figure 3B , the body 21 of the beam 20 therefore has two parallel flanges 23 between which a vertical wall 22 extends.

The first and second tensioners 1, 2 then extend over the entire length of the beam 20, parallel to each other.

Here there are provided five first tensioners 1 embedded in the concrete of the lower sole 23 of the body 21 of the beam 20. These first tensioners 1 are here again regularly distributed over the width of the beam 20. Thus, when they compress the body 21 of beam 10, they do not deform it in torsion.

These first five tensioners 1 are located near the underside of the lower sole 23. Thus, when they are tensioned, they make it possible to bend the body 21 of the beam 20 such that the center of the beam moves to the top.

Three second tensioners 2 are also provided, which are regularly distributed over the width of the beam 20. The means for fixing the ends of these second tensioners 2 to the body 21 of the beam 20 are identical to those described with reference to the figures 2B .

On the Figures 4A and 4B , we have shown a first embodiment of a slab 30 according to the invention.

In this mode, the body 31 of the slab 30 has a generally parallelepiped shape. Its neutral fiber A1 therefore coincides with the central longitudinal axis of body 31.

Here, the ends of the slab 30 however have, projecting from the upper face of the body 31, flanges 32. These two flanges 32 run along the two ends of the body 31. They delimit between them a cavity 33 into which a screed.

The first and second tensioners 1, 2 extend over the entire length of the slab 30, parallel to each other.

Here there is provided a plurality of first tensioners 1 embedded in the concrete of the body 31 of the slab 30. These first tensioners 1 are here again regularly distributed over the width of the slab 30, under the neutral fiber A1. They are located near the lower face of body 31 of slab 30.

Second tensioners 2 are also provided, which are regularly distributed over the width of the slab 30, above the neutral fiber A1. These second tensioners 2 pass through the two edges 32 of the body 31 of the slab 30 such that their ends project from either side of the body 31 of the slab 30. A central part of each of the second tensioners 2, which extends between these two edges 32, is on the other hand located in the cavity 33, outside the body 31.

Here, the second tensioners 2 are sheathed over their entire length, which ensures their sliding across the edges 32 and which guarantees their protection from the outside. These sheaths are also useful when a concrete screed is poured into the cavity 33, since they prevent the concrete of the screed from adhering to the second tensioners 2.

As a variant, it could be envisaged that the second tensioners 2 are only sheathed over part of their length, that located in the cavity 33. The parts of the second tensioners which pass through the edges 32 will be coated with zinc or tin. and cast in concrete. To allow the forces exerted by these second tensioners to be released, it will then be sufficient to heat the zinc or tin coating.

As a variant, it could also be envisaged that the second tensioners 2 are not sheathed, provided that the height of the concrete screed to be poured into the cavity 33 is sufficiently low so that, once this screed has been cast, the second tensioners are located at above this screed.

The edges 32 make it possible to offset the second tensioners 2 to a large distance from the neutral fiber A1 of the body 31. In this way, the tensile force exerted on each of the second tensioners 2 can be less than that exerted on the first tensioners 1. We can thus use here second tensioners 2 of a diameter smaller than that of the first tensioners 1 or reduce the number of second tensioners 2 used.

The fixing means provided at the ends of these second tensioners 2 are here again identical to those described with reference to the figures 1A, 1B And 2B .

According to a variant of the slab, it could have been expected that the body (31) is devoid of rim (32), in which case the second tensioners would be located entirely through the body of the slab.

On the figure 5A , a second embodiment of slabs 40 according to the invention is shown. The 40 slabs represented on this figure 5A do not differ from the slab 30 shown on the Figures 4A and 4B than by the honeycombed nature of their body 41.

It is in fact known to use cellular slabs 40, that is to say slabs 40 whose body is hollowed out with longitudinal conduits called alveoli. These cells reduce the weight of the slab while preserving its thickness to ensure good rigidity.

However here, as shown by Figures 4A and 4B , we will prefer to use a slab 30 whose body 31 is solid (as opposed to honeycomb).

Indeed, for the same slab section, the full surface area of the slab section is larger here due to the absence of cells. Consequently, the slab is able to undergo greater compressive forces.

It is thus possible to exert more forces on the first and second tensioners 1, 2, so that the slab 30 is able to withstand greater bending moments.

We can now give a specific example of a slab 30, 40 usable on site.

The slab could thus have a length of 9 meters, a height L2 of 20 centimeters, and a width L3 of 1.2 meters.

The first tensioners 1 can be distributed in such a way that the lower average fiber A2 extends 4 centimeters from the lower face of the body 31 of the slab 30, 40.

The second tensioners 2 can be distributed in such a way that the upper average fiber A3 extends 4 centimeters above the upper face of the body 31 of the slab 30, 40.

Let us first consider the case where the body 41 of the slab 40 is honeycombed ( Figure 5 ).

This slab 40 can be pre-stressed using the first tensioners 1 with a compression force equal to 140 tonnes. We can also temporarily prestress this slab 40 using the second tensioners 2 with a compression force equal to 60 tonnes.

The result will be that this hollow core slab 40 will be able to support loads twice as heavy as a hollow core slab which would not have been equipped with second tensioners 2. It is in fact designed to support a prestress of 140 tonnes which is much greater than the prestress that could not have been applied in the absence of second tensioners (which would have been approximately 82 tonnes).

Let us now consider the case where the body 31 of the slab 30 is full ( Figures 4A and 4B ).

This slab 30 can be pre-stressed using the first tensioners 1 with a compression force equal to 192 tonnes. We can also temporarily prestress this slab 30 using the second tensioners 2 with a compression force equal to 96 tonnes.

The result will be that this solid slab 30 will be able to support loads three times heavier than a hollow core slab which would not have been equipped with second tensioners 2. We understand in fact that a solid slab can store more prestress than a hollow core slab.

We can now describe how the aforementioned beams 10 and slabs 40 are used, with reference to the Figures 5A and 5B .

Beams 10 and slabs 40 are prefabricated in the factory.

Their manufacturing process consists of placing the first and second tensioners 1, 2 in molds (the second tensioners here already being sheathed and equipped with their frustoconical sleeves 3), applying tension to these tensioners, pouring concrete into the molds , and wait for the concrete to completely set. After drying the concrete, the first and second tensioners 1, 2 are released, thus putting the bodies 11, 31 of the beams and slabs in eccentric longitudinal compression.

The beams 10 and slabs 40 are then taken out of their respective molds. Due to the forces exerted by the first and second tensioners 1, 2, the bodies 11, 31 do not tend to bend excessively. Consequently, the exit of the beams 10 and slabs 40 outside their molds does not cause sudden bending of the bodies, which avoids the appearance of cracks in the concrete.

On the site, we will consider here that an initial structure of the work has already been assembled. In this case, vertical supports 50 will have already been installed at a distance from each other.

The first operation will then consist of installing two beams 10 in parallel and at a distance from each other, each cantilevered between two supports 50 spaced apart from each other. For this, the ends of each beam 10 will be placed on edges 51 provided on these supports 50.

The second operation will consist of gradually loading the two beams 10 by installing slabs 40 on them, the ends of each slab 40 resting respectively on the two beams 10.

The progressive installation of the slabs on the beams 10 will have the effect of gradually bending the beams 10 downwards. To stem this bending phenomenon, workers can gradually release the second tensioners 2 of the beams 10 so that these tensioners stop exerting a downward bending moment on the bodies 11 of the beams 10. We can thus plan to release the second tensioners 2 as the beams 10 are loaded, so that the beams 10 always maintain the best shapes (substantially rectilinear).

Once all of the slabs 40 have been put in place, we can plan to cover them with a concrete screed (also called a “compression table”).

Prior to this operation, we can remove all or part of the second tensioners 2 present in the slabs 40. If we wish to extract part of these second tensioners 2 at the time of pouring the screed or after the concrete has set, we can then advantageously use boxes at the ends of the second tensioners 2, in order to maintain access to these ends.

The present invention is in no way limited to the embodiments described and represented, but those skilled in the art will be able to make any variation consistent with their spirit.

In particular, when the slabs used will be honeycombed, provision may be made to place the second tensioners in the honeycombs themselves, so as to facilitate their installation and extraction.

According to another variant not shown of the invention, one could use, for the construction of a bridge, a beam of the type of that shown on the Figures 3A and 3B , with a length of 40 meters, a section of 2.5 meters in height, a lower heel of 70 centimeters in width, an upper heel of 1.2 meters in width, and a core thickness of 24 centimeters. The release of second tensioners makes it possible to obtain an effect similar to a second phase of prestressed by post tension.

Claims (16)

1. A prefabricated structural element (10, 20, 30, 40) comprising:

   · an elongate body (11, 21, 31, 41); and

   · at least one first tensioner (1) that is fastened in the elongate body (11, 21, 31, 41) in such a manner that it is suitable to compress and make the elongate body (11, 21, 31, 41) bend in a first direction;

   • at least one second tensioner (2) that is fastened to said elongate body (11, 21, 31, 41) so that it is suitable to compress and make the elongate body (11, 21, 31, 41) bend in a direction that is opposite to said first direction;

   the structural element being characterized in that said at least one second tensioner (2) is fastened to said elongate body (11, 21, 31, 41) at two distinct points,

   in that deactivation means (3B) are provided for relaxing the compression and the bending exerted on the elongate body (11, 21, 31, 41) by said second tensioner (2) .
2. A structural element (10, 20, 30, 40) according to claim 1, wherein the second tensioner (2) is fastened to said elongate body (11, 21, 31, 41) by fastener means (3), and at least one of said fastener means (3) includes a removable portion (3B; 3C) that forms said deactivation means.
3. A structural element (10, 20, 30, 40) according to claim 1, wherein the second tensioner (2) is made of two portions (2C, 2D) that are situated in alignment with each other and that are connected together by fastener means (5), and the fastener means (5) include a removable portion (3B; 3C) that forms said deactivation means.
4. A structural element (10, 20, 30, 40) according to claim 2 or claim 3, wherein said removable portion (3C) is meltable.
5. A structural element (10, 20, 30, 40) according to claim 2 or claim 3, wherein said removable portion (3B) is detachable.
6. A structural element according to any preceding claim, wherein at least a central portion of the second tensioner is accessible from the outside of the elongate body in order to be cut or broken.
7. A structural element according to the preceding claim, wherein the second tensioner is fastened to said elongate body via its ends, and the deactivation means are adapted solely to relax the compression and the bending exerted on the elongate body by the central portion of said second tensioner.
8. A structural element (10, 20, 30, 40) according to any preceding claim, wherein each second tensioner (2) is adapted to be removed from said elongate body (11, 21, 31, 41) completely or in part.
9. A structural element (10, 20, 30, 40) according to any preceding claim, wherein each second tensioner (2) comprises a metal bar, cable, or wire.
10. A structural element (10, 20, 30, 40) according to any preceding claim, wherein at least a central portion of the second tensioner (2) is passed freely into a sheath (4) that is fastened to the elongate body (11, 21, 31, 41).
11. A structural element (10, 20, 30, 40) according to any preceding claim, wherein at least a central portion of the second tensioner (2) is situated outside the elongate body (11, 21, 31, 41).
12. A structural element (10, 20, 30, 40) according to any preceding claim, wherein each first tensioner (1) comprises a metal bar, cable, or wire that is embedded in the material of the elongate body (11, 21, 31, 41).
13. A structural element (10, 20, 30, 40) according to any preceding claim, wherein the body is formed by a concrete beam (11, 21) or by a concrete slab (31, 41).
14. A method of building a work, the method comprising steps of:

    a) installing an initial structure (50);

    b) fastening a structural element (20) in accordance with any preceding claim to said initial structure (50), each first tensioner (1) being, at this step, fastened in the elongate body (11, 21, 31, 41) in such a manner that it compresses and bends the elongate body (11, 21, 31, 41) in a first direction, and each second tensioner (2) of the structural element (20) being fastened to said elongate body (11, 21, 31, 41) at two distinct points so that it compresses and bends the elongate body (11, 21, 31, 41) in a direction that is opposite to said first direction;

    c) installing a subsequent structure (40) on said structural element (20), so that said structural element (20) is put under load; and

    d) deactivating each second tensioner (2) of said structural element (20) so as to relax the compression and the bending exerted by the second tensioner (2) on the elongate body (21) of said structural element (20), by removing this compression and this bending.
15. A method according to the preceding claim, wherein steps c) and d) are performed in concurrent manner.
16. A method according to any one of the two preceding claims, wherein said subsequent structure comprises at least one structural element (40) according to any one of claims 1 to 13, and steps c) and d) are followed by steps of:

    e) adding a screed or concrete topping or a permanent heavy load on said subsequent structure; and

    f) deactivating each second tensioner (2) of said structural element (40) of said subsequent structure (40) so as to relax the compression and the bending exerted by the second tensioner (2) on the elongate body (41) of said structural element (40).

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