FR2862076A1 - LONG-RANGE BEAM FOR CONSTITUTING A CARRIER COMPONENT OF A MODULAR SUPERSTRUCTURE - Google Patents

Abstract

The invention relates to a long-span beam intended to constitute a carrier element of a modular superstructure, said beam consisting of a main body made of wood with glulam structure, and an internal reinforcing reinforcement made of composite of According to the invention, the internal reinforcing armature (13) is formed of high and low armature parts (13.1, 13.2) each having a plurality of superposed elements (13.11, 13.12; 13.21, 13.22). ) whose length decreases identically from the two ends of the beam as one moves away from the respectively upper or lower free edge of the beam according to a predetermined law of variation, and whose section is such that that the total section of the elements is at least equal to a minimum value calculated according to the dimensions of the beam and the loads applied to said beam.

Description

FIELD OF THE INVENTION

  The present invention relates to a long-span beam intended to constitute a carrier element of a modular superstructure.

  BACKGROUND OF THE INVENTION Long-span beams with a laminated timber structure are already known, consisting of a superposition of a large number of wooden boards which are glued to one another. The ranges sought for such beams traditionally range from 25 to 40 meters. The mechanical strength calculations then lead to total beam heights ranging from 2.20 meters to 3.15 meters, respectively. This means that the ratio between the range and the height varies for such traditional beams between 1/11 and 1/13. These traditional beams of glulam thus have necessarily high heights, and therefore high masses.

  To reduce the height of such beams with comparable range, as well as the weight thereof, it has been proposed beam structures laminated with an internal reinforcement reinforcement.

  The document WO-A-00/17465 thus describes a long-span beam formed of a main body made of wood with glulam structure, and an internal reinforcing reinforcement consisting of elongate carbon fiber composite elements. high strength which extend in the longitudinal direction of the beam, each of said elongate elements being housed and glued in a reservation associated laterally non-emerging. Such an approach is interesting because the presence of an internal reinforcing reinforcement made of a high-strength carbon fiber composite makes it possible to reduce the height of a beam substantially for a comparable range compared to traditional glulam beams, as well as the weight of said beam, using a frame twice that of traditional beams.

  Other documents may also be cited as such to complement the technological background. It will thus be possible to refer to the documents EPA-0 177 350, FR-A-2 764 622, WO-A-96 13378, EP-A-1 260 648, and WO-A-01 96108.

  Although the approach illustrated in the above documents is interesting on the principle, it is clear that, for very long spans, using carbon fiber composite frame elements that are relatively massive. As a result, the total weight of the laminated beam with its internal reinforcing reinforcement is still important.

  OBJECT OF THE INVENTION The object of the present invention is to design a longer-span beam that is more efficient than the glued-laminated beams with the aforementioned internal reinforcing reinforcement, in particular making it possible to optimize the dimensioning and the weight of the reinforcement elements. and this without affecting the mechanical strength of the beam.

DESCRIPTION OF THE INVENTION

  This problem is solved according to the invention thanks to a beam consisting of a main body made of laminated wood structure, and an internal reinforcement frame consisting of elongate elements made of high-strength carbon fiber composite. which extend in the longitudinal direction of the beam, each of said elongated elements being housed and bonded in a non-emergent laterally associated reservation, said beam being remarkable in that the reinforcing inner reinforcement is formed of a reinforcement part high and a low armature portion each having a plurality of superimposed elements whose length decreases identically from the two ends of said beam as one moves away from the free edge respectively higher or lower of the beam according to a predetermined law of variation, and whose section is such that the total section of the elements of the said high or low plurality of elements is at least equal to a minimum value calculated according to the dimensions of the beam and the loads applied to said beam.

  Thus, rather than choosing elongate elements all having the same length to form the internal reinforcing reinforcement, the invention suggests to have different lengths for these reinforcing elements, without these length decreases do not affect substantially the strength of the beam. The optimization of the section of the reinforcing elements also makes it possible to use, if necessary, elements sufficiently thin to tolerate bending for easy packaging.

  Preferably, the variation law determining the lengths of the reinforcing elements substantially corresponds to the curve of the bending moments of said beam.

  Advantageously, the high reinforcement portion and the low reinforcement portion are arranged symmetrically with respect to the neutral fiber of said beam.

  According to a particular embodiment, the high reinforcement portion and the low reinforcement portion each comprise two or three levels of superimposed reinforcement elements. It can then be provided that each level of high or low reinforcement comprises two identical reinforcement elements juxtaposed.

  The elongated reinforcing elements may also be flat flat blades, or alternatively solid bars of round section or other.

  It can be provided that the flat blades juxtaposed each pair are arranged side by side in a common reservation, each common reservation being defined by a non-emerging engravure practiced in a wood blade and by the face opposite the directly adjacent wood blade .

  It may alternatively be provided that the juxtaposed reinforcement bars of each pair are arranged in adjacent adjacent but non-communicating associated reservations, each of said reservations being defined by two non-emerging complementary grooves formed in two adjacent wooden strips. In particular, it may be advantageous to provide, against each free end reinforcement bars of greater length, an abutment plate embedded in an associated reservation communicating with the individual reservation receiving the reinforcing bar concerned.

  It is also advantageous to provide that each common or individual reservation also has at least one longitudinal groove opening into said reservation, in order to avoid delamination of the wood of some blades, or cracking of the carbon of some reinforcing elements.

  Advantageously, the reinforcing elements constituting the high and low armature parts are all embedded in the bonding material used, the latter being preferably the same as the bonding material solidarisant the wooden strips between them. In particular, the bonding material is a resorcinol adhesive, and preferably a resorcinoleate-formaldehyde glue.

  Preferably, finally, the beam of the invention comprises wood strips and curved reinforcing elements, so as to have a counterfleche which is maximum mid-length of said beam. The presence of a counter-arrow makes it possible to better control the efforts of the bending moments, and also to encourage the water flows in the roof thanks to the slope thus formed which guides the rainwater towards the ends of the long-span beams.

  Other features and advantages of the invention will appear more clearly in the light of the description which follows and the accompanying drawings, illustrating a particular embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

  Reference will be made to the figures in which: FIG. 1 illustrates in perspective a part of a modular superstructure using long-span beams in accordance with the invention; FIG. 2 is a perspective view illustrating, on a larger scale, the head zone of one of the vertical columns of the preceding superstructure, with two long-span beam ends having an internal reinforcing armature arranged according to FIG. invention; FIG. 3 is an elevational view of a beam according to the invention, the high and low armature parts of which each comprise two levels of superposed elements, here visible by transparency, the lengths of which are determined from FIG. a variation law according to a curve also illustrated, which is in this case the curve of the bending moments of said beam; - Figure 4 is an elevational view, on a larger scale, of the end of the beam of Figure 3, and Figure 5 is the section along V-V; Figure 6 is an elevational view illustrating a variant of the beam shown in Figure 3, wherein the upper and lower frame portions each comprise three levels of superimposed elements; FIG. 7 is an elevational view of the end of the beam of FIG. 6, and FIG. 8 is the section along VIII-VIII; - Figures 9 and 10 are detail views, respectively in the assembled position and exploded, to better distinguish the housing of juxtaposed elongate elements constituting a portion of the inner frame high or low, arranged between two adjacent wood strips ; FIG. 11 is an elevational view illustrating an end of a beam made according to another variant of the invention, in which the upper and lower reinforcement parts comprise elongated elements made in the form of composite solid bars. high-strength carbon fiber; - Figure 12 is a section along XII-XII of Figure 11 to better distinguish the upper internal armature constituted by two pairs of carbon bars superimposed; FIG. 13 is an exploded view in perspective making it possible to better understand the arrangement of the reservations associated with the longer carbon bars, with associated end stop plates; FIG. 14 is a view in elevation making it possible to distinguish a bearing flange fixed laterally on each side of the long beam, in the upper part of the latter, said flanges being intended to serve as support for metal girders; bracing a long-span horizontal beam in a modular-type superstructure; and FIG. 15 is a sectional view, on a larger scale, for illustrating the support zone of beams associated with one and the same beam, the assembly being coated with an insulating roofing element ensuring the watertightness of the superstructure.

  DETAILED DESCRIPTION OF THE INVENTION

  In Figure 1, there is a modular superstructure S, which is here arranged to form the supporting structure of a building of parallelepiped shape of large dimensions, such as a warehouse or a large area store.

  The superstructure S comprises a plurality of vertical support posts 1 embedded in a ground P via concrete foundations, which posts are interconnected by horizontal beams of long reach. In the present case, poles 1 are illustrated which are made of concrete, but which could alternatively be metal poles, for example with an H-shaped structure. Vertical poles 1 of considerable height, for example about ten, have also been illustrated. meters, in order to make a superstructure carrying a roof or an upper floor of a building, but this is only an example of application. In particular, it can be provided that the posts 1 of the central row are of slightly greater height than the other vertical posts, to form a slope facilitating the flow of water to the roof edges.

  The vertical support poles 1 are interconnected by long-span horizontal beams denoted 2, 10. The beams 10, which extend parallel to one another, are supported at their ends on hoofs of 11, as is best seen in Figure 2, and they have an internal frame (not visible in Figure 1) arranged according to the invention. Long-span horizontal beams 2, the direction of which is orthogonal to the direction of the beams 10, are here of traditional type, that is to say that they are of lamellar-bonded type, but without internal reinforcement. , simply by being embedded on load-bearing shoes fixed at the top of the posts (hooves 6 better visible in FIG. 2).

  For one of the modules superstructure S, there is illustrated here metal beams 5 of conventional type, often called beams "joist canam" by specialists, these metal beams used instead of failures bracing two horizontal beams long-range 10. Thus, a module or mesh of the superstructure S is constituted by four vertical support poles 1, two long-span horizontal beams 2 of traditional type, and two long-span horizontal beams 10 having, in accordance with the invention, a internal reinforcing reinforcement having optimized dimensioning. As an indication, the arrangement of the invention makes it possible to produce modular superstructures of very large dimensions, a unitary unit being able to be 24 meters wide by 36 meters long, the height being given by the height of the vertical post 1 which will be for example from 10 meters up to 15 meters, especially for poles of a central row. The bracing is in this case provided on the one hand by embedding the vertical concrete columns 1, and on the other hand by means of flat beams 5.1 interposed between two beams "joist canam" 5 (such beams are illustrated on Figure 1) and two 5.2 cross cables stretched by turnbuckles 5.3.

  Alternatively, we can ensure the bracing, for each module, by a failure made in the form of a traditional beam laminated glue identical to the beams 2, arranged at mid-length between two groups of beams "joist canam" (four or five beams for each group in general depending on the case), or for slightly smaller mesh widths (15 meters for example), it will be possible exclusively to provide traditional glulam beams identical to the beams 2 (for example eleven beams between two beams 2), without metal faults of the type "joist canam" or others. It was found that the glue-laminated faults of 15 meters reach a weight very close to that of metal beams of the type "joist canam" of 24 meters range. These two variants, not shown here, can substantially reduce the cost of the modular superstructure made.

  Long-span horizontal beams 2 and 10 are laminated, but are not made in accordance with the same arrangement. Indeed, the transverse beams 2 are of the traditional type, being constituted by a unit block glued-laminated rectangular section, with a height of 1 to 1.2 meters, and a width of the order of 0.2 meter . The ends of the beams 2 are embedded on metal shoes 6 which are best seen in Figure 2. These metal shoes are of conventional type, and are screwed to the top of the posts 1 concerned.

  On the contrary, the horizontal beams of long reach 10, which are supported on metal shoes 11, also of traditional type, screwed to the top of the posts 1, have a very special internal reinforcement reinforcement which will be described in more detail below. after referring to the other figures.

  Each beam 10 thus consists of a wooden main body 12 with a glulam structure, and an internal reinforcing armature 13 which is formed of a high armature portion 13.1 and a reinforcement portion 13.2 bass each having a plurality of superimposed elements.

  In Figure 2, there is illustrated a particular embodiment in which the upper and lower armature parts each comprise two levels of superimposed elements. Thus, the high reinforcement portion 13.1 comprises two superimposed elements 13.11, 13.12, and the lower reinforcement portion 13.2 also comprises two superimposed elements 13.21, 13.22. In this case, the elongated reinforcing elements are flat blades coated in high-strength carbon fiber composites.

  It is interesting to note that the flat slats illustrated in FIG. 2 are totally integrated into the wooden main body 12 of the beam 10, that is to say that each carbon blade is housed in an associated reservation which is non-emergent laterally, and also non-emergent at the ends noted 17 of the beam 10. It is also noted that the flat slats forming the high armature portion and flat slats forming the low armature portion are disposed substantially symmetrically. relative to the neutral fiber of the beam 10, in order to have a maximum balance for the bending behavior of said beam and thus avoid generating undesirable torsion moments.

  In the variant of FIG. 2, an embodiment has thus been illustrated in which the high and low armature parts each comprise two levels of superposed elements. More generally, however, more than two superimposed levels may be provided, in particular three levels, as illustrated in FIGS. 3 to 10, to which reference will now be made.

  According to an essential characteristic of the invention, the high reinforcement parts 13.1 and the low reinforcement parts 13.2 each comprise a plurality of superimposed elements whose length decreases identically from the two ends of the beam as the one moves away from the free edge respectively upper or lower of the beam according to a predetermined law of variation. In addition, the section of the reinforcing elements is such that the total section of the elements of said plurality of high or low elements is at least equal to a minimum value calculated according to the dimensions of the beam and the loads applied to said beam.

  The variation law determining the lengths of the reinforcing elements may be of several types, generally having a substantially parabolic shape. In particular, it will be possible to determine the lengths of the reinforcing elements from the curve MF of the bending moments of the beam, said curve being illustrated in FIG. 3 in line with the long-span beam 10, the part of which high and low armatures each comprise two levels of superimposed elements.

  The high reinforcement portion 13.1 firstly comprises a first reinforcing member 13.11 arranged just below the upper edge 19 of the beam. As is better seen in FIG. 4, the ends of this reinforcing element 13.11 do not reach the end edges 17 of the beam, that is to say that the associated reservations are axially non-emerging. .

  In practice, it will be possible to provide a distance of the order of ten centimeters below the free edges 17. Then, while moving away from the upper edge 19, there is a second reinforcement element 13.12 of shorter length . The length of this last element is given by the law of variation chosen, in this case the curve of the bending moments MF of the beam. For the reinforcing element 13.12, a length L2 is obtained which is of the order of 0.55 L1, where L1 is the length of the reinforcing element 13.11.

  For the lower armature portion 13.2, there is a symmetrical arrangement of the previous one with respect to the neutral fiber of the beam, with a reinforcing element 13.21 of the same length as the element 13.11, and an inner reinforcing element 13.22 the same length as the reinforcing element 13.12. The reinforcing element 13.21 is likewise slightly distant from the lower edge 18 of the beam. In addition, the section of the reinforcing elements 13.11, 13.12, 13.21, 13.22 is chosen such that the total section of these elements is at least equal to a minimum value calculated according to the dimensions of the beam and the loads applied to said beam.

  In practice, it will be attempted to arrange the high and low armature parts as close as possible to the upper and lower edges respectively of the beam. In the present case, it is found that the reinforcing element 13.11 is arranged just below the upper blade 12.1 noted the beam body, and that in the same way the reinforcing element 13.21 is disposed just above the last blade 12.n of said beam, the number n corresponding to the number of blades constituting the beam body 12.

  As this is better visible on the section of FIG. 5, each level of high reinforcement 13.1 or low 13.2 comprises two identical reinforcing elements juxtaposed.

  This allows to leave a free seal between the longitudinal edges facing the two juxtaposed blades, said seal allowing free expansion of the carbon, which is significantly different from that of wood.

  Thus, the arrangement of the internal reinforcing armature has a double symmetry, both with respect to the neutral fiber (which corresponds to the dashed line L at the representation of the curve of the bending moments MF), and with respect to a transverse median plane denoted Q. FIGS. 6 to 8, which respectively correspond to FIGS. 3 to 5 previously described, illustrate another variant in which the high reinforcement portion 13. 1 and the low reinforcement portion 13.2 each comprise three levels of superimposed elements. There are thus elements 13.11, 13.12, 13.13 for the high armature portion 13.1, and elements 13.21, 13.22, 13.23 for the lower armature portion 13.2. Each time, the lengths vary decreasingly as one moves away from the upper free edge 19 or lower 18 of the beam, in accordance with the variation law retained. For the reinforcement elements of smaller length 13.13, 13.23, the length is here terminated from the curve MF, and corresponds substantially to the relation L3 = 0.25 L1.

  Here again, each level of high reinforcement 13.1 or low 13.2 comprises two identical reinforcement elements juxtaposed, in this case two flat plates lying between which is defined a free expansion joint.

  As can be seen more clearly in FIGS. 9 and 10, each of the pair of juxtaposed flat blades is disposed in a common reservation 14. In these figures, the first two upper wooden blades 12.1, 12.2 of the beam body are distinguished. lower blade 12.2 has a central groove 14.2. This engravure 14.2 delimits with the lower face 14.1 of the blade 12.1 a common reservation 14 for the two flat blades lying side by side 13.11. One could have provided two half-engraves symmetrically arranged on each of the facing blades to accommodate the juxtaposed reinforcing elements, but the embodiment illustrated here makes it considerably easier to manufacture, and also reduces the precision requirements for machining.

  It should be noted that the lateral edges 14.5 delimiting the engravure 14.2 define large bonding surfaces with the facing lower face 14.1 of the wood blade 12.1, which makes it possible to reduce the delamination phenomena of the wood in a boom situation. maximum of the beam.

  FIG. 9 corresponds to the assembled position, in which the reinforcing elements are embedded in the bonding material, which is advantageously that used to join together the wooden boards, this bonding material being for example a resorcinol adhesive, and preferably a resorcinoleate-formaldehyde glue.

  Note also the presence of longitudinal grooves 14.3, 14.4 opening the reservation 14. These grooves help to avoid possible wood delamination of some blades, or carbon cracking of some reinforcing elements.

  As an indication, using thirty wooden blades 45 mm thick to form the main body of wood 12, it will be used to form the internal reinforcing armature 13 flat blades coated high strength carbon fiber composite whose thickness ranges from 10 to 20 millimeters depending on the length of the intended range. This means that each wood blade having a receiving engravure will be thinned locally 10 to 12 millimeters of its thickness at said engravures. The length of the carbon reinforcement strips will be approximately 29.76 meters for the L1 length which is the largest (ie about 30 meters minus a distance of about 12 centimeters from each end) , the length of the next element will be of the order of 14 meters, and when it will be provided, the length of the third constituent element even shorter will be of the order of 8 meters.

  With a beam having the aforementioned dimensioning, the calculation makes it possible to determine the minimum value of desired section, that is to say in this case about 8160 square millimeters, which gives, for an embodiment in which the parts d high and low frame each have two levels of superimposed elements, flat blades 120 millimeters wide and 34 millimeters thick.

  This last value shows that one can be satisfied with weakly dimensioned reinforcement elements, which opens up extremely interesting perspectives that one could not obtain previously with the classical techniques. In particular, in addition to the ease of manufacture of composite plates, the small thickness also makes it considerably easier to transport since the dishes can be packaged in the form of rolls. In addition, a considerable saving of carbon fiber composite plates is achieved in the implementation.

  FIGS. 11 to 13 illustrate another variant in which the internal reinforcing reinforcement made of a composite of high-strength carbon fibers consists of elongated elements made in the form of solid bars, of round section or the like. As can be seen more clearly in the section of FIG. 12, pairs of carbon bars juxtaposed are used both at the top (as in the lower part) of the beam 10.

  There is thus distinguished a high armature portion 13.1 which here consists of two pairs of bars 13'.11 of great length, followed by two pairs of bars 13'.12 of shorter length. The low armature portion 13.2 similarly comprises two pairs of bars 13. 21 of great length, and two pairs of bars of shorter length 13.22. The lengths of the bars are as previously determined from the variation law retained, for example in correspondence with the curve of the bending moments of the beam.

  Because of the importance of the carbon bar section, there may be significant forces exerted on the free ends of the carbon bars, which are exerted directly on the surrounding wood. To avoid any risk of tearing or delamination of this highly stressed area of the wooden body, it is advantageous to provide end plates for distribution of forces, such as the plates 15 illustrated here. These abutment plates will preferably also be made of carbon. As best seen in the exploded view of FIG. 13, the abutment plate 15 is embedded in an associated reservation 16 which communicates with the reservation 14 receiving the two reinforcing bars 13 '11 juxtaposed. The two adjacent wood strips concerned 12.1, 12.2 then each have two 14.21, 14.22 half-cylindrical moldings, intended to receive the reinforcement bars of high-strength carbon fiber composite. At the outlet of these semi-cylindrical moldings, there is then the transverse engravure 16 which communicates with the previous ones, corresponding to half a height of the abutment plate 15.

  As before, the round section bars are embedded in the bonding material used for the assembly of the wooden boards. The adhesive material will preferably be a resorcinol adhesive, and more particularly a resorcinoleate-formaldehyde adhesive. Such an adhesive achieves an extremely strong bond between the surface of the carbon fiber reinforcing elements and the adjacent wood fibers. The reinforcing armature 13 thus forms an integral part of the glulam beam body, and provides an optimal mechanical behavior for the armed beam thus produced.

  It will be noted in the case of the realization of a reinforcing reinforcement in the form of carbon bars, that we find a high reinforcement portion 13.1 and a low reinforcement portion 13.2 arranged symmetrically with respect to the neutral fiber of the beam.

  Thanks to the internal reinforcement carbon fiber composite described above, we obtain a surprising compatibility and high performance of deformation between wood and carbon fibers. In practice, the high strength carbon fiber composite members will be constituted by an association between carbon fibers and an associated resin obtained by polymerization in a mold. Under the action of the loads applied, it is found that the wood is deformed, and if we push the loads as and when, increasing the load so as to achieve the plasticization of the wood blades, those that are on the extreme fibers reach the limits of its resistance and plasticize, causing the glue which flows and transmits the load to the second blade, and so on until the resistance of the wooden blades is exhausted. We thus obtain a deformation diagram which is no longer triangular, but rectangular.

  Moreover, for spans of 25 to 40 meters, the use of an internal reinforcement according to the invention makes it possible to limit considerably the height of the beams, with a gain of 30 to 40% compared to traditional structures made of laminated wood. glued with no internal reinforcement.

  Furthermore, as can be seen in FIGS. 3 and 6, it is advantageous to provide for the beam 10 to have wooden boards 12.i and reinforcement elements 13.1j, 13.2j curved so as to present a counter-arrow noted c which is maximum mid-length of said beam. Thus, the lower edge 18 of the beam 10, or intrados, is curved so that the long-span beam 10 has a counter-arrow which is maximum mid-length of the beam, the parameter c being preferably given by the relation c = 0.013 x (1 / 2L), where L is the length of the beam 10. This corresponds to a counter-arrow of 1.3%.

  The presence of such a counter-arrow makes it possible to better control the efforts of the bending moments, and also to promote the flow of water on the roof thanks to the slope thus formed which guides the rainwater towards the ends of the long-span beams. .

  Another advantage of the invention lies in the participation of the long-span beam in the partitioning of a roof portion of the superstructure.

  Indeed, with reference to FIGS. 14 and 15, it can be seen that each long-span beam with internal reinforcement 10 is equipped laterally, in the upper part and parallel to the upper edge thereof, with two fixed support pads 20 21. These bearing flanges 20 thus extend longitudinally over almost the entire length of the beam 10. The upper edge of each of the flanges 20 is disposed a little lower than the upper edge of the beam. , which provides a support space for end elements 5.5 of the beams of the type "joist canam". It is then possible to arrange a roof element 25, with its upper sealing plate 26, its intermediate insulating mat 27, and its corrugated support pan 28, resting on the sets of beams 5, the fixing being ensured precisely on the soles. As is easily understood, the long-span beams 2 and 10 provide a natural partitioning of this roof portion, which naturally generates spaces forming smoke extraction cantons. The cantons being thus made allow to remain in accordance with the most stringent standards in terms of fire resistance, without the need to add components.

  Finally, the reinforced beam according to the invention remains extremely homogeneous thanks to the use of a maximum wood-on-wood bonding surface, and to the excellent work of the carbon-wood pair. This technique also allows better protection against carbon fire, and, by concealing it, keeps the beam all the wood look.

  The invention is not limited to the embodiments that have just been described, but on the contrary covers any variant using, with equivalent means, the essential characteristics mentioned above.

Claims (15)

  1. Long-span beam (10) intended to constitute a carrier element of a modular superstructure, said beam consisting of a wooden main body (12) with glulam structure, and an internal reinforcing reinforcement (13) constituted by elongated composite members of high strength carbon fibers which extend in the longitudinal direction of the beam, each of said elongated elements being housed and glued in a laterally associated non-emergent associated reservation, characterized in that the internal reinforcing armature (13) is formed of a high armature portion (13.1) and a low armature portion (13.2) each having a plurality of superposed elements (13.11, 13, 12, 13.13). ;  13.21, 13.22, 13.23) whose length decreases identically from both ends of said beam as one moves away from the respectively upper (19) or lower (18) free edge of the beam in accordance with a predetermined variation law, and whose section is such that the total section of the elements of said plurality of high or low elements is at least equal to a minimum value calculated according to the dimensions of the beam and the loads applied to said beam.   2. Beam (10) according to claim 1, characterized in that the variation law determining the lengths of the reinforcing elements (13.11, 13.12, 13.13, 13.21, 13.22, 13.23) substantially corresponds to the curve (MF) of the moments bending said beam.   Beam (10) according to claim 1 or claim 2, characterized in that the high reinforcement portion (13.1) and the low reinforcement portion (13.2) are arranged symmetrically with respect to the neutral fiber (L). of said beam.   4. Beam (10) according to one of claims 1 to 3, characterized in that the high armature portion (13.1) and the low armature portion (13.2) each comprise two or three levels of reinforcement elements bunk.   5. Beam (10) according to claim 4, characterized in that each level of high reinforcement (13.1) or low (13.2) comprises two identical reinforcement elements juxtaposed.   6. Beam (10) according to one of claims 1 to   5, characterized in that the elongated reinforcing elements (13.11, 13.12, 13.13, 13.21, 13.22, 13.23) are flat flat blades.   7. Beam (10) according to one of claims 1 to 5, characterized in that the elongated reinforcing elements (13'.11, 13'.12; 13'.21, 13'.22) are solid bars , of round section or other.   8. Beam (10) according to claims 5 and 6, characterized in that the juxtaposed flat blades (13.11) of each pair are arranged side by side in a common reservation (14).   9. Beam (10) according to claim 8, characterized in that each common reservation (14) is defined by a non-emerging engravure (14.2) made in a wood blade and the opposite face (14.1) of the blade of directly adjacent wood.   10. Beam (10) according to claims 5 and 7, characterized in that the juxtaposed reinforcing bars (13'.11) of each pair are arranged in adjacent reservations (14) adjacent but not communicating with each other, each of said reservations being defined by two complementary non-emergent moldings (14.21, 14.22) made in two adjacent strips of wood.   11. Beam (10) according to claim 10, characterized in that there is provided, against each free end reinforcement bars of greater length (13'.11, 13'.21), a thrust plate (15 ) embedded in an associated reservation (16) communicating with the individual reservation (14) receiving the reinforcement bar concerned.   12. Beam (10) according to claim 8 or claim 10, characterized in that each common or individual reservation (14) further has at least one longitudinal groove (14.3, 14.4) opening in said reservation, in order to avoid wood delamination of some blades, or carbon cracking of some reinforcing elements (13.11).   13. Beam (10) according to one of claims 1 to 12, characterized in that the reinforcing elements constituting the parts of high reinforcement (13. 1) and low (13.2) are all embedded in the bonding material used , it being preferably the same as the bonding material solidarisant the wooden blades between them.   14. Beam (10) according to claim 13, characterized in that the bonding material is a resorcinol adhesive, and preferably a resorcinol-ureaformol adhesive.   15. Beam (10) according to one of claims 1 to 14, characterized in that it comprises wood blades (12.i) and reinforcing elements (13.1j, 13.2j) curved so as to present a counter-arrow (c) which is maximum mid-length of said beam.