EP0242238A1 - Steel and reinforced concrete structures, particularly for producing girders, in particular girders used as purlins or girders with a large span - Google Patents

Abstract

The structure consists essentially of concrete in the case of all the members in compression (B) and of steel in the case of all the members in tension (A). The assembly of steel sections is largely outside the concrete, and is anchored in the concrete giving it its lateral rigidity and its buckling strength. <IMAGE>

Description

The present invention relates to reinforced concrete and steel structures intended in particular to constitute beams and, in particular, beams for use of purlins.

• - le béton armé,
• - la charpente métallique

For various types of construction and, in particular, for the roofing structures of industrial buildings, two principles of construction clash:

• - the reinforced concrete,
• - the metal frame

To achieve economic structures, it is obviously necessary to use a minimum of material. This phenomenon is particularly sensitive for large spans because unnecessary material constitutes a parasitic overload.

To consume a minimum of material in the realization of a structure, the ideal would be to saturate in all points the capacities of resistance of the material. However, in the two main principles of construction cited above, we note:

Reinforced concrete structures.

The steels entering the reinforced concrete structures are calculated to resist mainly the tensile stresses since the concrete material is considered as cracked in the tensed parts during the failure calculations.

• - résister dans certaines zones aux contraintes de compression et de cisaillement,
• - dans d'autres zones, son rôle principal est l'enrobage des aciers.

Concrete material has several functions:

• - withstand compression and shear stresses in certain areas,
• - in other areas, its main role is the coating of steels.

In a conventional reinforced concrete beam, a significant amount of material is underused supporting insignificant stresses in the sense of the resistance of the materials.

• - coût de la matière,
• - surcharge parasite

The concrete material is heavy and the underused material leads to a double waste:

• - cost of the material,
• - parasitic overload

Metallic structures.-

In a metal frame, the tensioned parts are calculated by making full use of the steel's resistance capacities.

On the other hand, the compressed parts must have a certain inertia to resist the phenomenon of buckling and, under these conditions, the steel in the compressed zones is in a sense underused.

• - afin de présenter une certaine inertie pour résister au flambage,
• - afin de présenter des formes propices aux assemblages par boulonnage ou par soudage.

The metal profiles (angles, U-profiles, I-profiles) have a cut section:

• - in order to present a certain inertia to resist buckling,
• - in order to present shapes suitable for bolting or welding assemblies.

In the first case, drilling the holes weakens the sections and, in the second case, a certain length of weld bead is necessary.

The double problem of buckling and assembly therefore leads to complex shapes and overabundant sections for the profiles.

On the other hand, a metal frame, during the assembly of large preassembled elements, must have a certain overall rigidity, often difficult to obtain with very resistant profiles but having in their conventional forms a low inertia. This leads to complementary triangulations and only intended to avoid overall buckling.

The present invention relates to mixed structures having the characteristics of using reinforced concrete in all the compressed parts and steel in the form of profiles in all the tensioned parts.

Typically, a structure according to the invention is essentially a rigid structure. consisting of reinforced concrete and steel intended to withstand compression forces and tensioning forces in service, this structure being characterized in that the parts intended to withstand the compression forces are essentially constituted by a mass of concrete and in this that the parts intended to resist tensile stresses are essentially constituted by an assembly of steel profiles substantially devoid of intrinsic rigidity, this assembly being for the most part external to the concrete and being anchored in the concrete which provides it with its lateral rigidity and its resistance to buckling.

In a typical embodiment, the steel part is constituted by an assembly of profiles of simple shape, for example rectangular plates or, better still, concrete bars.

• - la structure mixte de l'invention est plus légère qu'une structure qui serait en béton armé,
• - la partie acier de la structure de l'invention peut être beaucoup plus économique qu'un treillis métallique car la rigidité de la structure sera essentiellement due à La partie béton et non à la partie acier. En outre, lorsqu'on utilise comme acier du rond à béton, ce qui n'est jamais le cas d'une charpente métallique, on obtient des caractéristiques de résistance mécanique supérieures à celles que l'on obtient avec les profilés généralement utilisés en charpente métallique. De plus, par rapport à une charpente métallique classique, les assemblages peuvent être très simplifiés grâce à l'invention puisqu'une partie des assemblages est réalisée par le béton et le reste des assemblages peut être réalisée par des soudures simples,
• - la partie béton peut être réalisée de façon simple par moulage sur le chantier si bien que la structure de l'invention convient tout à fait pour des petits chantiers où l'on désire cependant utiliser des structures de grandes longueurs. Actuellement, pour une telle application, on rencontre de sérieuses difficultés dues au fait que les structures utilisées sont généralement des structures précontraintes qui sont trop onéreuses à réaliser sur un chantier et qui doivent donc être réalisées en usine, ce qui pose le problème de leur transport et ce problème est particulièrement ardu lorsque les structures ont une grande longueur.

A structure according to the invention is significantly more economical than known structures, in particular for the following reasons:

• the mixed structure of the invention is lighter than a structure which would be made of reinforced concrete,
• - The steel part of the structure of the invention can be much more economical than a wire mesh because the rigidity of the structure will be essentially due to the concrete part and not to the steel part. In addition, when using concrete reinforcing steel as steel, which is never the case with a metal frame, mechanical strength characteristics are obtained which are higher than those obtained with the profiles generally used in metal framework. In addition, compared to a conventional metal frame, the assemblies can be very simplified thanks to the invention since part of the assemblies is made by concrete and the rest of the assemblies can be made by simple welds,
• - The concrete part can be produced in a simple way by molding on the site so that the structure of the invention is entirely suitable for small sites where it is however desired to use structures of great lengths. Currently, for such an application, there are serious difficulties due to the fact that the structures used are generally prestressed structures which are too expensive to produce on a site and which therefore have to be produced in the factory, which poses the problem of transporting them. and this problem is particularly difficult when the structures are very long.

Typical embodiments of structures in accordance with the invention will be described below, the most typical embodiment being a beam made of a concrete frame of relatively small cross section relative to its length and of a steel frame made of a hull which rises at its ends in the concrete frame and which is, on the other hand, connected from place to place to the concrete frame by couples arranged in transverse planes which are perpendicular to the concrete frame, the ends of these couples being embedded in the concrete frame.

The hull is preferably made up of a single profile but can also be made up of several profiles joined together end to end and / or by several metal profiles in parallel.

As a variant, the hull is constituted by a hollow tube which contains a prestressing cable, which in particular makes it possible to produce beams with a large span.

• - la figure 1 est une perspective d'une poutre réalisée selon la présente invention ;
• - la figure 2 est une demi-coupe longitudinale de la poutre ;
• - la figure 3 est une demi-coupe longitudinale d'une variante de réalisation de la poutre ;
• - la figure 4 est une coupe schématique de la poutre par un plan vertical transversal passant dans la région d'un noeud de la membrure en acier de la poutre ;
• - les figures 5 et 6 sont des vues agrandies de la figure 4 respectivement dans la région de la membrure en béton et dans la région de la membrure en acier ;
• - la figure 7 est un schéma de la poutre dans sa position de moulage de la membrure en béton ;
• - la figure 8 est une perspective de la poutre avant coulage de la membrure en béton ;
• - la figure 9 est une coupe par un plan vertical transversal du moule de fabrication de la membrure en béton avant et après mise en forme du ferraillage de la membrure ;
• - la figure 10 est une coupe, par un plan vertical longitudinal, des extrémités en regard de deux poutres sur un appui commun ;
• - la figure 11 est une perspective d'une extrémité d'une poutre ;
• - les figures 12, 14 et 16 représentent schématiquement, en vue longitudinale, des variantes d'exécution d'une poutre ;
• - la figure 13 est une coupe de la poutre de la figure 12 par un plan vertical transversal ;
• - la figure 15 est une coupe de la poutre de la figure 14 par un plan vertical transversal,
• - les figures 17 à 19 représentent des exemp-es de réalisation de jonctions dans la membrure en acier,
• - la figure 20 est un schéma en long d'une poutre selon une variante de l'invention, dans laquelle la carène formant la membrure en acier est constituée par un tube creux qui contient un câble de précontrainte,
• - la figure 21 est une coupe schématique par le plan transversal aa de la figure 20,
• - la figure 22 est un schéma en long d'une poutre plus complexe portée par deux poteaux,
• - la figure 23 est une coupe schématique de la poutre de la figure 22 par le plan transversal aa de la figure 22,
• - la figure 24 est une coupe schématique de la poutre de la figure 22 par le plan transversal bb de la figure 22,
• - la figure 25 est une coupe schématique agrandie des extrémités voisines de deux membrures en béton contigues,
• - la figure 26 est une perspective schématique de l'extrémité de l'une des membrures en béton de la figure 25.
• - la figure 27 est une coupe transversale d'un tube de carène, et
• - la figure 28 est un schéma en long d'une structure constituée de plusieurs travées avec continuité au droit des appuis.

In the figures:

• - Figure 1 is a perspective of a beam made according to the present invention;
• - Figure 2 is a longitudinal half-section of the beam;
• - Figure 3 is a longitudinal half-section of an alternative embodiment of the beam;
• - Figure 4 is a schematic section of the beam through a vertical transverse plane passing through the region of a node of the steel frame of the beam;
• - Figures 5 and 6 are enlarged views of Figure 4 respectively in the region of the concrete member and in the region of the steel member;
• - Figure 7 is a diagram of the beam in its molding position of the concrete member;
• - Figure 8 is a perspective of the beam before pouring the concrete member;
• - Figure 9 is a section through a transverse vertical plane of the mold for manufacturing the concrete member before and after shaping the reinforcement of the member;
• - Figure 10 is a section through a longitudinal vertical plane of the opposite ends of two beams on a common support;
• - Figure 11 is a perspective of one end of a beam;
• - Figures 12, 14 and 16 show schematically, in longitudinal view, alternative embodiments of a beam;
• - Figure 13 is a section of the beam of Figure 12 by a transverse vertical plane;
• FIG. 15 is a section of the beam in FIG. 14 through a transverse vertical plane,
• - Figures 17 to 19 show examples of making junctions in the steel frame,
• FIG. 20 is a longitudinal diagram of a beam according to a variant of the invention, in which the hull forming the steel frame is constituted by a hollow tube which contains a prestressing cable,
• FIG. 21 is a diagrammatic section through the transverse plane aa of FIG. 20,
• FIG. 22 is a longitudinal diagram of a more complex beam carried by two posts,
• FIG. 23 is a diagrammatic section of the beam in FIG. 22 by the transverse plane aa of FIG. 22,
• FIG. 24 is a diagrammatic section of the beam in FIG. 22 through the transverse plane bb in FIG. 22,
• FIG. 25 is an enlarged schematic section of the neighboring ends of two adjoining concrete members,
• - Figure 26 is a schematic perspective of the end of one of the concrete members of Figure 25.
• FIG. 27 is a cross section of a hull tube, and
• - Figure 28 is a long diagram of a structure consisting of several spans with continuity to the right of the supports.

The beams of the invention which are shown in Figures 1 to 19 consist of two superimposed members, one B of concrete and the other A of steel.

In the most common embodiments (Figures 1 to 3 and 14), the concrete member is systematically located above the steel member when the beam is in service.

However, it also falls within the scope of the invention to produce beams such that the concrete members can be locally below the steel member (as shown in Figures 12 and 16 by way of example) .

It will be understood that, in general, most of the concrete member remains above the steel member when the beam is in the service position.

The concrete frame B is generally constituted by an elongated rectilinear block of reinforced concrete and this block is preferably given a section which is easy to demold on a site, such as for example a straight section of trapezoidal shape (FIGS. 4, 5 and 7).

This frame is reinforced by a reinforcement F according to the rules of the art so that the frame mainly resists compressive forces. In addition, the member must have sufficient inertia to withstand buckling phenomena.

The steel frame A is constituted by a longitudinal section L which extends along the length of the concrete frame along a curve whose distance from the concrete frame is maximum in the middle of the beam and decreases progressively towards the ends of the beam, this longitudinal section penetrating into the concrete member in the region of the ends of this member and even coming out on the end edges of the concrete member, as seen in Figures 2, 3 and 10.

The steel frame A comprises, on the other hand, steel connectors C which connect from place to place the profile L to the concrete frame. Preferably, these sections L and these connectors C have a part C1 welded to the section L and a part C ₂ anchored in the concrete frame. In the examples shown, these connectors have a general V shape, the top of the V being flattened and welded along its length to the section L while the ends of the wings of the V are also flattened and embedded in the concrete of the member B. Furthermore, in the embodiments shown, the connectors C are arranged in pairs respectively to the right and to the left of the longitudinal vertical median plane P of the beam, as best seen in Figures 5 and 6 which are sections (at two scales different) of the concrete frame (Figure 5) and the steel frame (Figure 6) by the transverse vertical median plane of the beam in Figure 3 (plane IV-IV) of Figure 3. We see in this figure that the longitudinal section L of the steel frame is constituted by a round of relatively large concrete steel while the connecting connectors C, C ′ of the steel frame A to the concrete frame B are constituted by rounds concrete steel of smaller diameter and it can also be seen in FIG. 6 that the local connection between the concrete rounds of the - couples C and C 'and the concrete round of the hull L are constituted by welds S.

As an indication, a concrete bar whose diameter is in the range 20 to 60 mm is used for the L profile while a concrete bar whose diameter is in the range 10 to 30 mm is used for the connectors, these ranges being given on a preferred but nonlimiting basis.

The longitudinal profile L is preferably formed by a single profile but it is not excluded to constitute it by several assembled profiles. FIG. 18 shows, by way of example, an assembly of two sections L ₁ , L ₂ by means of an end-to-end assembly sleeve M in which the ends of the sections, previously shaped into truncated cones and threaded, are pressed in by screwing. It is not necessary to describe in detail this assembly means known per se. Profile L can, on the other hand, consist of several profiles arranged in parallel instead of being constituted by a single profile and FIG. 17 shows, by way of example, a mixed solution where profile L comprises a profile L ₃ followed by two parallel profiles L ₄ , L ₅ , these various profiles being assembled by a plate p which is freely traversed by the profiles whose through ends are then welded to the plate. This type of assembly makes it easy to adjust the length of the L profile on request.

The number and shape of the connectors C is also chosen as a function of the performance requirements of the beam, knowing that the steel frame A essentially only has the role of supporting the tensile forces. On this subject, it will be noted that in the examples shown in FIGS. 1 to 3, the lower longitudinal section L of the steel frame follows the curve of the moments and supports a substantially constant tensile force over its entire length under symmetrical loading, while the upper concrete member B, which provides longitudinal rigidity, withstands a substantially constant compressive force under symmetrical loading.

The manufacture of a beam according to the invention can easily be done on site using a mold of simple design, a prototype of which has been shown in FIG. 8. The molding position is upside down from the service position in the sense that during molding the steel frame A is above the mold Z, supported from place to place by suitable supports, as seen in FIG. ₈ . In the mold is arranged the reinforcement F which is produced according to the rules of the art. By way of example, there is shown in FIG. 9 the cross section of a reinforcement, the parts in broken lines representing the reinforcement before shaping. The ends of the connectors C are introduced into the mold as well as the ends of the longitudinal profile L which pass through the reinforcement to come out at the longitudinal ends of the mold. At each of the longitudinal ends of the concrete member B, is fixed a metal part S in the shape of an L (FIGS. 10, 11) which is intended to ensure the fixing of the concrete member on a support by bolting. The fixing part S is incorporated into the concrete frame during molding.

This fastening part S has a flat S ₁₀ embedded in the end edge of the concrete frame and this flat has an orifice T _lo for the passage of the end of the longitudinal section L of the steel frame, the end protruding then being welded to the flat, as best seen in Figures 10 and 11. On the other hand, the fixing part S has another flat S ₂₀ intended to rest on the support of the beam, and this another plate S ₂₀ has a light T ₂₀ for the passage of a stud which will serve to fix the part S to the support of the beam. When two beams are brought into alignment on the same support, S dishes ₂₀ and S _'20 S fasteners and S' are superimposed Preferably, as seen in Figure 10, to be crossed by the same pin; alternatively, they could be independent of each other and crossed by respective studs.

The support on which rests one end of the beam is any support in itself depending on the circumstances of use. It can be for example a wall, a post or a transverse beam. In the case where the beam according to the invention is intended to constitute a breakdown in a structural truss, the support will generally be a beam arranged as a crossbowman. In such an application, the concrete member provides lateral rigidity and works very slightly in bending under the loads of the material which constitutes the covering. The forces introduced by the local bending phenomena are second order compared to the main forces.

To make very long beams or for any other reason, it may be useful to give the concrete member B a special longitudinal profile other than straight and Figures 12, 14 and 16 illustrate exemplary embodiments where the concrete member has a long trapezoidal profile (Figures 12 and ¹ 4) or convex (Figure 16). In the case of Figure 12; continuity must be ensured between the upper steel members of the two beams which follow each other on the same support.

The fixing of the profile L to the fixing piece S can also be obtained by mechanical means (bolting, etc.), for example by threading or machining the end of the profile L.

FIGS. 22 to 28 relate to a variant of the invention, more particularly with a view to the production of long-span beams.

• - il n'existe pas, dans le commerce, de ronds à béton de très gros diamètre,
• - les ronds à béton disponibles et transportables sont d'une longueur limitée.

When you want to make beams with large spans, you run into two problems:

• - there are no commercially available concrete rods of very large diameter,
• - the concrete bars available and transportable are of limited length.

Under these conditions, if we want to make beams of great span, we are led to assemble several sections end to end. This assembly is expensive and presents difficulties.

The present invention can in particular allow the production of long-span beams, in particular for making frames for industrial or sports roofs.

This is achieved, according to the present invention, by constituting the hull by a hollow steel tube capable of receiving a prestressing cable which will later be tensioned. In particular, the tube consists of a single tube section or of several tube sections fixed end to end by welds or by fixing pieces, for example by bolted plates. .

More precisely, the structure shown in FIGS. 20 and 21 comprises a beam S consisting of a rectilinear concrete member B and a steel member A consisting on the one hand by a tube T of arcuate shape, the concavity of which faces up when the beam is in the service position and, secondly, by diagonals, e.g., concrete reinforcing bars arranged in pairs with a C _- end is welded to the tube and whose other end is embedded in the concrete of the concrete member B.

These beams can be made according to the technique described above, where the beams are made with the steel frame disposed above the concrete frame.

A multi-strand prestressing cable F is threaded into the tube and tensioned according to the calculated force, by any means known per se and so as to bring into the tube compressive stresses always below the elastic limit. These means are shown diagrammatically in FIG. 20 by cones W. Subsequently, the tube is injected with mortar or other curable medium to block the prestressing cable and protect it from rust.

The prestressing is calculated so that under normal use load of the beam, the tube remains slightly compressed.

If the load is increased, the prestressing cable first lengthens by a few centimeters, then the tube begins to work in tension: at break, all the steel is tensioned.

This process has great advantages because it allows the prefabrication described above under normal conditions.

The tubular member is possibly made up of several sections, connected by bolted plates, or welded end to end. The welding of large diameter tubes is a known and simple operation.

After assembly, we benefit from the efficiency of the prestressing cable.

The invention thus uses prestressing cables which are delivered in great length while - that the tubular profiles are limited due to the difficulty of transport.

It should be noted that this technique does not penalize the quantity of metal used for the following reason: if one loads such a structure until rupture, the steels of the tube and prestressing work at the maximum of their capacity taking into account their possibilities relative elongation.

However, in the failure calculation, the authorized load represents a percentage of the breaking load.

The relationship between these two loads - working load and breaking load - represents what we usually call the "safety factor" of the frame concerned.

In addition, under normal use load, the tube remains compressed and thus the beam is clear of the risk constituted by a bad weld in the assembly of the tubes end to end; noting however that the resistance capacity of the tube was tested by the tensile force generated by the self-weight before tensioning of the cable.

In this embodiment, the prestressing is carried out after the beam has been placed in its service position. It is possible to use the beam supports to tension the prestressing cable. In a particular variant, the cable is tensioned before the beam is placed in the service position.

A beam as described above is suitable for spans from 20 to 50 meters, for example.

For larger spans, for example 50 to 100 meters, it is preferable to use a more complex beam consisting in fact of several beams arranged end to end and crossed by a common prestressing cable.

Figures 22 to 27 schematically show an embodiment of such a beam.

. The structure shown in FIG. 22 comprises a central beam S ₁ and two end beams S ₂ , S ₃ . The central beam S ₁ is made up of a concrete frame B ₁ , of arcuate shape, the concavity of which turns downwards when the beam is in the service position, and a steel frame, formed on the one hand by a tube T ₁ , of arcuate shape, the concavity of which is turned upwards when the beam is in the service position and, on the other hand, by diagonals, arranged in pairs C ₁ , one end of which is welded to the tube and the other end of which is embedded in the concrete of the concrete member B.

The end beams 8 ₂ and S ₃ have a similar structure but the curvatures of the members are less accentuated and the relative positions of the members are reversed.

to set up lap _or t _r e on two supports or posts ^P , first place the two end beams S _{2 '} S ₃ which are each fixed to one of the posts P, so that the steel frame is above the concrete frame, then place between these two beams the central beam S ₁ after having turned it over (in relation to its manufacturing position) so that the steel frame is below of the concrete member and so that the longitudinal tube T ₁ of the central beam and the longitudinal tubes T ₂ , T ₃ of the end beams S ₂ , S ₃ are in continuity. These tubes are welded end to end, or connected by metal plates bolted together, while the concrete beams are secured by any suitable means, for example: welded or bolted metal plates.

FIGS. 25 and 26 illustrate an exemplary embodiment of the junction of the central beam S ₁ and an end beam, for example the beam S ₂ . The concrete members of the two beams are provided at their ends with plates K ₁ , K ₂ which are anchored in the members by rods (for example concrete rods) r _i , r ₂ welded to the plates and embedded in the concrete. To assemble the two frames, the plates are applied one against the other and temporarily fixed by bolts. The two plates are then welded to each other.

The structure at this stage is stable under its own weight. Under the dead weight of the beam, the tube is stretched.

A multi-strand prestressing cable F is then threaded into the three tubes and into the posts P, from F ₁ to F ₂ , and tensioned according to the calculated force, by any means known per se and so as to bring into the tube of the compressive stresses always lower than the elastic limit. Later, the tube is injected with mortar or other curable medium to block the prestressing cable and protect it from rust.

FIG. 28 relates to a very long-span structure produced using the technique explained in connection with FIG. 22 from span to span, the prestressing cables being installed by span, from F ₁ to F ₂₁ from F ₂ to F ₃ , from F ₃ to F ₄ , etc ...

It goes without saying that in a complex beam, the number of successive concrete members is generally two or three, but it can also be greater. It will also be understood that, depending on the case, the steel hull tube has its ends which are embedded in the concrete member or which pass through the concrete member.

The beams and structures of this variant of the invention (described with regard to FIGS. 20 to 28) are in particular intended to produce roof frames, the cover being located in the extension of the concrete frame, that is to say - say along the curved surface formed by straight and horizontal lines resting on the concrete members. It goes without saying, however, that the invention is not limited to this application: another important application of the invention is the production of bridges.

Claims (23)

1. Rigid structure essentially consisting of reinforced concrete and steel intended to withstand compression forces and tension forces in service, characterized in that the parts intended to support the compression forces are essentially constituted by a mass of concrete ( B) and in that the parts intended to resist tensile stresses are essentially constituted by an assembly (A) of steel profiles substantially devoid of intrinsic rigidity, this assembly being mainly external to the concrete and being anchored in the concrete which gives it lateral rigidity and buckling resistance. 2. Structure according to claim 1, characterized in that the steel assembly (A) of the structure consists essentially of concrete rods. 3. Beam made up of a concrete frame (B) of relatively small section compared to its length and a steel frame (A) made up of a hull (L) connected from place to place to the concrete frame by couples (C) arranged in transverse planes which are perpendicular or oblique with respect to the concrete member, the ends of these couples being embedded in the concrete member. 4. Beam according to claim 3, characterized in that the hull (L) and the couples (C) are constituted by concrete rods. 5. Beam according to claim 3 or 4, characterized in that the hull (L) rises at its ends in the concrete frame. 6. Beam according to one of claims 3 to 5, characterized in that the concrete member (B) has a trapezoidal cross section. 7. Beam according to one of claims 3 to 6, characterized in that the hull (L) is constituted by a longitudinal profile which extends along the length of the concrete member according to a curve whose distance from the member in concrete is maximum in the middle of the beam and gradually decreases towards the ends of the beam. 8. Beam according to claim 7, characterized in that the couples are connectors (C) which have a part (C ₁ ) welded to the hull (L) and a part (C ₂ ) anchored in the concrete member. 9. Beam according to claim 8, characterized in that the connectors (C) have a general V shape, the top of the V being flattened and welded along its length to the hull, while the ends of the wings of the V are also flattened and embedded in the concrete of the concrete member (B). 10. Beam according to claim 8 or 9, characterized in that the connectors (C) are arranged in pairs, respectively to the right and to the left of the longitudinal vertical median plane of the beam. 11. Beam according to one of claims 3 to 10, characterized in that the hull (L) follows the curve of the moments and supports a substantially constant tensile force both over its entire length under symmetrical loading, while the upper member in concrete (B), which provides longitudinal rigidity, supports a substantially constant compressive force under symmetrical loading. 12. Beam according to one of claims 3 to 11, characterized in that at a longitudinal end of the frame (B) of concrete is fixed a metal part (S) in the form of L which is intended to ensure the fixing of the concrete member, on a support, by bolting. 13. Beam according to claim 12, characterized in that said fixing part (S) has a flat (S ₁₀ ) embedded in the end edge of the concrete frame, that this flat has an orifice (T ₁₀ ) for the passage of the end of the hull (L), and that the protruding end of the hull is welded to the flat. 14. Beam according to claim 13, characterized in that the fixing part (S) has another dish (S ₂₀ ) intended to rest on the support of the beam, and this other dish (S ₂₀ ) has a lumen ( T ₂₀ ) for the passage of a stud which will serve to fix the part (S) to the support of the beam. 15. Application of a beam according to one of claims 3 to 14 as a failure in a structural truss. 16. Beam according to claim 3, characterized in that the steel hull consists of a hollow tube (T). which contains a prestressing cable (F). 17. Beam according to claim 16, characterized in that the tube (T) of the hull is injected with mortar or other curable medium to block the prestressing cable after prestressing. 18. Beam according to claim 16 or 17, characterized in that the concrete member (B) is substantially rectilinear. 19. Beam according to one of claims 16 to 18, characterized in that the tube (T) has its ends which pass through the concrete frame (B). 20. Beam according to one of claims 16 to 19, characterized in that the tube (T) consists of several sections of tube fixed end to end by welds or fixing parts. 21. Beam characterized in that it consists of a plurality of beams (S ₁ , S ₂ , S ₃ ) according to claim 16 or 17, whose concrete members are arranged in continuity and secured, and whose tubes hulls (T ₁ , T ₂ , T ₃ ) are arranged in continuity and secured, these tubes being traversed by a common prestressing cable. 22. Beam according to claim 21, characterized in that the concrete members (B ₁ , B ₂ , B ₃ ) of the beam are curved and / or straight. 23. Beam according to claim 21 or 22, characterized in that it comprises a central beam (S ₁ ) between two end beams (S ₂ , S ₃ ), the concrete frame (B ₁ ) of the central beam (S ₁ ) located above the steel frame, and the concrete frame (B ₂ , B ₃ ) of the end beams (S ₂ , S ₃ ) located below the steel frame these beams.

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