EP0288350A1 - Bridge consisting of a deck and its supporting means, especially a large-span cable-stayed bridge, and its construction process - Google Patents

Abstract

PCT No. PCT/FR88/00157 Sec. 371 Date Nov. 28, 1988 Sec. 102(e) Date Nov. 28, 1988 PCT Filed Mar. 25, 1988 PCT Pub. No. WO88/07604 PCT Pub. Date Oct. 6, 1988.Bridge comprising a bridge floor, said floor consisting of an upper boom (4) which may form a road section, a lower boom (5) which may be formed by a tubular succession of sections, and diagonal linking girders (6), joining the booms, their axes converging on that of the lower boom. Those elements subjected to high stress are preferably to be provided with individual pretensioning elements. The gussets (15) linking the diagonals with the upper boom carry advantageously the attachment elements of the supporting stay cables (2).

Description

The present invention relates to a new bridge structure consisting of an apron and means for supporting this apron, and in particular to a new cable-stayed bridge structure of large span, as well as to a method of constructing such a bridge.

In the current state of the art, the crossing of large spans uses either suspension bridges or cable-stayed bridges. Suspension bridges are economically justified for exceptional spans, but their flexibility poses problems for traffic, especially rail, and for aeroelastic stability. Cable-stayed bridges, for their part, do not exhibit the wind sensitivity of suspension bridges, particularly if the deck is constructed of concrete, a material which gives the structure sufficient weight and great rigidity. The weight, however, limits the spans, so that beyond the field of application of concrete cable-stayed bridges, decks with a mixed steel / concrete structure or entirely metallic decks have been used.

In the current state of the art, guyed decks with a mixed steel / concrete structure have always consisted of an upper concrete member forming a paving slab, carried by transverse and longitudinal stiffening beams intended for transferring the loads to the guy wires. while ensuring sufficient rigidity to the deck. Achievements of this type are recent and highlight the current limitations of known means, on the following points:
- the coexistence of the metal frame and the concrete with regard to the effects of shrinkage and slow deformations of the concrete,
- the appearance of temperature gradients created by the exposure to the sun of metallic surfaces having a low thermal inertia,
- the risk of overall buckling of the structure by instability of the lower chord of the longitudinal stiffness beams, when the stresses due to the loads, combined with the above effects, approach the elastic limit in compression of the metal,
- the very low resistance of this type of structure vis-à-vis accidental forces such as the impact of a truck against a shroud.

Several of these drawbacks can be remedied by increasing the height and the importance of the longitudinal stiffness beams, but this is to the detriment of the wind resistance and the economy.

Lattice structures can also be used, because they make it possible to economically obtain great bending and torsional rigidity, while ensuring maximum transparency with respect to the wind. In the current state of the art, such trellis structures generally combine steel and concrete, but despite considerable research in this field, no truly satisfactory solution has been found to transfer the forces between the members and the diagonals at the different nodes of the trellis. The long-term behavior of such solutions is unknown and cost prices remain high.

The object of the invention is to overcome all the drawbacks mentioned above, by proposing a new structure which is both light, rigid, and easy to produce, therefore economical.

The invention provides, to obtain this result, a bridge consisting of an apron, and means for supporting this apron, the apron comprising:
- an upper chord forming a circulation slab,
- a lower chord forming a continuous longitudinal element,
- connecting beams, called "diagonals" connecting the upper and lower members, directed obliquely both with respect to the vertical and with respect to the length of the bridge, and forming, with the members, a spatial trellis, this bridge having for the particularity that the axes of the diagonals converge on the longitudinal axis of the lower member or the median plane of the upper member.

By "spatial trellis" is meant a structure made up of elements which can be assimilated to parts of a plane or to straight line segments and linked together, this structure not being included in a plane. In the following, we will call "nodes" the junction points of plane parts and / or line segments.

Preferably, the parts of members which are subjected to significant tensile forces, and the diagonals which are subjected to significant tensile forces are prestressed by means which are specific to each of said members and to each diagonal or to two concurrent diagonals. .

According to preferred methods:
the means of prestressing the diagonals comprise prestressing reinforcements anchored at their two ends at the points of junction of these diagonals with the upper member and drawing a V whose middle is at the point of junction of said diagonals with the lower member,
the lower chord is formed from successive assembled sections and is provided with longitudinal prestressing reinforcements which each compress several assembled sections,
- The means for prestressing the upper member are constituted by prestressing frames connecting together the nodes of the space lattice formed by the junction points of the diagonals with said upper member.

The combination of these last three modalities leads to a particularly interesting structure, because all the highly tensed parts of the structure of the deck form a network of prestressed elements.

The deck that has just been described can be integrated into bridges of various designs.

For large or medium-span structures, a cable-stayed bridge is preferred, and in this case provision is made for the means for supporting the deck to be constituted by guy wires connecting support masts to nodes of the space trellis formed by the junction points. diagonals with the upper chord. In this hypothesis, for medium spans, it is possible to provide that the bridge comprises at least two continuous lower members, and an equal number of spatial trusses comprising diagonals whose axes converge on the axis of a lower member, said members being interconnected by a bracing, these spatial trusses each including a part of the upper chord, and advantageously, that this bridge comprises two lower chords and two spatial trusses and in that the means for supporting the deck consist of guy lines connecting the nodes of the spatial trusses which are located in the axial plane of the bridge to support masts.

According to a variant, also at medium range, the means for supporting the deck consist of guy wires connecting support masts to nodes of the spatial trellis formed by the points of junction of the diagonals with the lower member.

The invention can also be applied to bridges with much smaller spans, and not guyed. In such bridges where the deck is constructed in the manner in accordance with the invention indicated above, the means for supporting the deck consist of transverse supports on which the upper frame rests, and additional prestressing frames are provided which follow a polygonal path connecting two successive transverse supports passing through reference points located on the lower member, and, advantageously, said additional prestressing reinforcements are not in the axial plane of the bridge.

The means of joining the diagonals to the frames are a very important element for the implementation of the invention. According to a preferred method, to ensure the junction between the diagonals and the lower member, there are provided folded sheet gussets, comprising two wings which are each in a longitudinal plane containing the axis of diagonals which are fixed on it, the gusset being fixed on the lower chord so that the fold axis of the gusset wings coincides with the longitudinal axis of the lower chord. Advantageously, provision is also made for the lower member to be formed from successive sections assembled, and at least some of the gussets are fixed to the points of assembly of successive sections.

According to another preferred method, which advantageously combines with the previous one, to ensure the junction between the diagonals and the upper chord, gussets are provided comprising a lower wing situated in a longitudinal plane containing the axis of the diagonals which are fixed on him, and an upper wing which is fixed on the upper chord, so that the fold axis of the wings of the gusset is in the median plane of the upper chord.

In this case, according to interesting embodiments:
- the gusset carries anchor points for diagonal prestressing reinforcements and anchor points for prestressing reinforcements of the upper member,
- the connection between the gusset and the upper chord is a concrete-steel connection,
- The upper wing of the gusset has its median plane located in a longitudinal plane containing the axis of shrouds supporting the point, these shrouds being fixed on said gusset. It can also be provided that the upper wing is split into two parallel wings between which the shroud is fixed, the fold axis being in this case formed by the intersection of the median planes of the upper and lower wings.

In order to obtain greater lightness, in very long span bridges, provision may also be made for the upper chord to constitute a concrete slab reinforced by continuous metal sections and prestressing reinforcements arranged perpendicularly to these metal sections.

According to the invention, the upper chord of the structure forming a pavement slab or carrying railway traffic is made of reinforced or prestressed concrete; the lower chord can be either in reinforced or prestressed concrete, or in a mixed steel / concrete structure, or entirely metallic. It is advantageous to use a metal tube filled with concrete, the characteristics of which are described further on. In the simplest form of the invention, the lower and upper members are interconnected by a series of diagonals located in two oblique planes forming an isosceles triangle in the cross section.

In the case of a guyed structure, the two edges of the upper slab have the anchors of the suspension shrouds at regular intervals, at the point of intersection of the diagonals mentioned above.

The advantages of this arrangement lie in the alliance of the torsional and bending stiffness of the section with the minimum weight and wind resistance, allowing significant savings compared to the currently known production methods.

The invention further provides an original construction method, adapted to the bridge structure which has just been described.

This process includes the following steps:
- place, on an already assembled part of the deck, two longitudinal beams with a temporary border, on either side of the upper chord, in cantilever with a length at least equal to the longitudinal dimension of a mesh of the space trellis, each beam being held by two successive nodes of the space truss already assembled,
- bring a new spatial lattice mesh, this mesh comprising at least one node located on the lower member, two nodes located on the upper member, and the diagonals corresponding to these nodes,
- securing this new mesh and the part of the deck already assembled, supporting the new mesh using the temporary edge beams,
- repeat the operations by advancing the provisional border beams along the mesh that has just been fixed.

In the case of a cable-stayed bridge, preferably, when the new link is secured to the deck part already mounted, a suspension guy wire is also fixed on said new link.

Advantageously, whether the bridge is guyed or not, use is made of temporary edge beams provided with means such as studs to immobilize them in the correct position relative to the meshes of the space trellis already assembled.

• La figure 1 est une élévation longitudinale d'un pont haubané conformé à l'invention.
• La figure 2 est la vue en plan du même ouvrage.
• La figure 3 est une coupe transversale courante d'un tablier haubané montrant la membrure supérieure en béton formant dalle de chaussée, les diagonales dans les plans obliques et la membrure inférieure tubulaire.
• La figure 4 est une vue en plan de l'ossature du tablier.
• Les figures 5a et 5b sont des vues axonométri­ques partielles du tablier montrant les mêmes éléments composants que sur la figure 4, pour un pont haubané ou non haubané.
• Les figures 6 à 9 incluses montrent le détail d'un tronçon élémentaire de la membrure inférieure, sa cons itution, le noeud de raccordement avec les diagonales et d'assemblage avec les tronçons adjacents et enfin le détail du joint entre deux tronçons.
• Les figures 10 à 13 montrent le détail du noeud d'assemblage supérieur selon les trois vues (coupe transversale, élévation longitudinale et vue en plan) ainsi qu'une variante de réalisation de l'attache des haubans.
• Les figures 14 à 17 montrent les éléments structuraux complémentaires nécessaires à la réalisation de l'invention selon l'intensité des charges supportées par le tablier et les dimensions géométriques de celui-­ci.
• Les figures 18 et 19 montrent deux autres modes particuliers de réalisation de l'invention selon lesquels une suspension haubanée unique est disposée au centre de l'ouvrage.
• Les figures 20 et 21 sont des coupes, respecti­vement transversale et longitudinale, partielles, illustrant un mode alternatif de réalisation de la dalle supérieure selon lequel des profilés métalliques sont noyés dans la dalle en béton, de préférence dans le sens longitudinal de l'ouvrage pour coopérer avec le béton à la résistance vis-à-vis de l'effort axial dans le tablier, le solidarisation entre le béton de la dalle et les profilés métalliques étant assurée par une précontrainte orthogonale à la direction des profilés.
• Les figures 22 et 23 sont une vue longitudinale et une coupe transversale montrant l'application de l'invention à la réalisation de travées non haubanées, par exemple les travées d'accès situées de part et d'autre d'une travée centrale haubanée.
• Les figures 24 à 27 montrent les phases constructives successives du tablier conforme à l'invention et les moyens particuliers nécessaires à cette construc­tion.

The invention will now be described in more detail, with the aid of practical, nonlimiting examples, illustrated with the aid of the drawings, among which:

• Figure 1 is a longitudinal elevation of a cable-stayed bridge according to the invention.
• Figure 2 is the plan view of the same work.
• Figure 3 is a common cross-section of a cable-stayed deck showing the upper concrete member forming the paving slab, the diagonals in the oblique planes and the lower tubular member.
• Figure 4 is a plan view of the framework of the deck.
• Figures 5a and 5b are partial axonometric views of the deck showing the same component elements as in Figure 4, for a guyed or non-guyed bridge.
• Figures 6 to 9 inclusive show the detail of an elementary section of the lower member, its composition, the connection node with the diagonals and of assembly with the adjacent sections and finally the detail of the joint between two sections.
• Figures 10 to 13 show the detail of the upper assembly node according to the three views (cross section, longitudinal elevation and plan view) as well as an alternative embodiment of the attachment of the shrouds.
• FIGS. 14 to 17 show the additional structural elements necessary for carrying out the invention according to the intensity of the loads supported by the deck and the geometric dimensions of the latter.
• Figures 18 and 19 show two other particular embodiments of the invention according to which a single guyed suspension is arranged in the center of the structure.
• FIGS. 20 and 21 are partial sections, respectively transverse and longitudinal, illustrating an alternative embodiment of the upper slab according to which metal sections are embedded in the concrete slab, preferably in the longitudinal direction of the structure for cooperate with the concrete to the resistance vis-à-vis the axial force in the deck, the connection between the concrete of the slab and the metal profiles being provided by a prestress orthogonal to the direction of the profiles.
• Figures 22 and 23 are a longitudinal view and a cross section showing the application of the invention to the production of non-guyed spans, for example the access spans located on either side of a central guyed span.
• Figures 24 to 27 show the successive construction phases of the deck according to the invention and the particular means necessary for this construction.

In the embodiment of Figures 1 and 2, the bridge according to the invention comprises an apron 1 consisting of a series of triangular spatial elements suspended from guy lines 2, at regularly spaced points. These shrouds are fixed towards the top of the support masts 3. For the sake of clarity, the central span is shown with only eight elements suspended by three shrouds on either side of the key. In long-span bridges, the spacing of the stays is variable from 10 to 20 m and the number of stays in the central half-span can reach 20 to 25.

In its simplest form, the cross section of the deck 1 shown in Figure 3 is an isosceles triangle consisting of an upper slab (or chord) 4, a lower chord 5 and diagonals 6, without intermediate supports of the upper slab 4 between the two banks of the bridge. The plan view of FIG. 4 also shows that the planes of the diagonals are cut into all identical triangles, the vertices of which are alternately located on the edges of the upper slab 4 and on the lower central member 5.

The lower member 5, the details of which are given in FIG. 6, is broken down, for the production of the structure, into sections of equal length separated by joints allowing rapid assembly during construction.

The lower chord 5 is, in the example described, a metal tube 7 whether or not filled with concrete, depending on the location along the length of the bridge and the nature of the stresses that stress it. Depending on the requirements of the project and in particular the intensity and direction of the forces applied to this member in the structure, it may be necessary and advantageous to provide separately or simultaneously the following ordinary reinforcements or prestressing reinforcements:
- passive reinforcements embedded in concrete in areas where the compression force is important to reduce the stress in the materials,
- pretensioned reinforcement put in tension before pouring the concrete and anchored on the end flanges of each section of tube, intended to place the metal tube under permanent longitudinal compression,
- post-tensioned prestressing reinforcements, placed under sheath 8 inside the filling concrete and intended to put the whole of the metal tube and the filling concrete under permanent longitudinal compression,
- Post-tensioned prestressing reinforcements 9 placed outside the tube and stretched in the structure over several sections after assembly of these.

The assembly between successive sections of the lower member is made by flanges 10 placed opposite one another and joined longitudinally by high-strength bolts 11. The end flanges of each section further include a gusset 12 folded according to the plane of the oblique diagonals allowing assembly by welding of these with the lower main chord. The flanges finally include and as necessary the anchors of the external prestress of the lower chord. The axis of folding of the gusset 12 coincides with the axis of the tube 7.

At least some of the sections of the tube are filled with concrete. The tube filling concrete, if it exists, can be used before or after assembling the frame in the structure. In both cases, it is advantageous to compress the filling concrete inside its metallic envelope to combat the subsequent effects of shrinkage and improve the relative adhesion of the two materials. Contrary to works using a composite metal tube / concrete filling member where the variations in force and consequently the adhesion stresses occur continuously along the member, in the object of the invention, such variations in force appear only at the right of the connecting nodes with the diagonals, in an area where the arrangements used make it impossible for any relative sliding of the concrete and the tube. For this purpose, stiffeners or connectors 13 are provided in the vicinity of the flange.

From a constructive point of view, the concrete filling of the tube and its compression is easily done with the cooperation of one or two temporary plugs placed at the ends of the tube and fixed to the end flanges by a series of temporary bolts.

In the case where the concrete is used before assembly of the frame in the structure, an injection device 14 is provided in the joint between two successive sections, to ensure perfect transmission of the longitudinal forces in the filling concrete .

The upper knot on the banks of the structure ensures the transmission of forces from the oblique diagonals to the upper slab at the same time as the suspension on the shrouds. According to the embodiment of the invention shown in Figures 10, 11 and 12, which are given respectively a cross section, a longitudinal view and a horizontal view of the node, the essential element of the assembly is a gusset 15 in folded sheet, the upper part of which coincides with the suspension plane of the shrouds 2, and the lower part is located in the plane of the oblique diagonals 6. The shroud is fixed there by known means such as yokes 16 and axis 17 or, depending the variant shown in Figure 13, by a splitting of the gusset allowing the fixing of the lower anchor of the stay. The diagonals are easily linked to the gusset by welding along a slot made in the tube. To ensure the decomposition of the forces according to the laws of statics, the fold edge 18 of the gusset is located in the mean plane 19 of the floor slab. The gusset also carries the anchors 20, 21 of the frames 22, 23 of the diagonal 6 and of the upper slab 4. According to the invention, the transmission of all the forces is thus carried out along a direct path, eliminating any welding or any assembly working at tearing which always presents a potential risk. In all the lower and upper nodes of the structure, the continuous gussets provided ensure an interpenetration of the members and the diagonals to achieve the direct path of the forces mentioned above.

In the overall balance of the structure and in the vicinity of the central part of the main span, significant tensile forces appear, either under the effect of the diffusion of the concentrated forces of the shrouds, or under the application of overloads, in the following three elements of the trellis:
- the upper chord between the attachment points of the shrouds,
- the diagonals oriented in the extension of the shrouds,
- the lower chord between the fastening nodes of the above diagonals.

These elements are indicated by dashes in Figure 1.

To ensure the balance of the bridge under these tensile forces, it is planned according to the invention to implement the following three families of prestressing reinforcements:
- upper longitudinal reinforcements 23 anchored in the upper gusset attachment gussets,
- oblique V-frames 22 placed inside the tensioned diagonals; the reinforcements are deflected at the bottom in the flange of the lower member and anchored at the top in the same assembly gusset as above,
- Lower longitudinal reinforcements 9 outside the lower chord and previously described.

The proposed arrangements ensure direct transfer of all the loads and complete continuity of all the efforts of the shrouds, the two members and the diagonals.

When the transverse dimensions of the bridge require it, it may be necessary to incorporate elements of complementary structures shown in FIGS. 14 to 17:
- intermediate posts 24 make it possible to reduce the spans of the roadway, therefore its thickness, its weight and its reinforcements,
- transverse bridge parts 25 bringing together for example the two connection gussets on the banks to divide the floor slab into panels working in both longitudinal and transverse directions.

In certain works of medium span (200 to 400 m for example), it is possible to envisage using only a single suspension plane, all the stays being arranged in the plane of symmetry of the bridge. Figures 18 and 19 give two possible arrangements, both forming part of the invention.

In the arrangement of FIG. 18, the shrouds 2 pass through the upper chord 4 through guides 26 designed to dampen the vibrations of the system, and are anchored in the lower chord, at the location of the nodes of the spatial trellis formed by the junction of the diagonals with the lower chord.

In the arrangement of FIG. 19, there are two parallel lower members 5, and two spatial trusses each consisting of a lower member, of one half of the upper member 4 which is above this lower member, and of diagonals 6 which connect each lower member to the upper member half which corresponds to it. A bracing 27 connects the two lower members 5 and stiffens the assembly by ensuring the continuity of the external contour and the stability of the cross section, and the torsional rigidity of the deck.

Finally, in very large structures (600 to 900 m for example), it is vital to reduce the weight of the deck as much as possible. To this end, the floor slab itself is made up of a mixed structure comprising continuous metal sections and concrete placed between them, the materials being joined together by a prestress orthogonal to the direction of the sections. The confinement conferred on the concrete of the slab by the metal profiles allows the minimum thickness of the slab to be reduced to 0.10 m without the risk of puncturing under the concentrated loads of the vehicles. In Figures 20 and 21 we can see metallic profiles 28, arranged in the longitudinal direction, and prestressing frames 29 arranged transversely, these sections and frames being placed in the thickness of the roadway. It is clear that we can also arrange the profiles and frames differently.

Designed essentially for making long-span guyed bridges, the method according to the invention can be extended to making non-guyed spans. The case arises when a large span to be built above a breach or a navigation channel is framed by access viaducts which can advantageously be built according to the same procedures as the main structure.

Figures 22 and 23 describe the construction of a typical span in elevation and cross-section respectively.

The longitudinal bending strength is imparted in the members by prestressing reinforcements such as 23 in the upper member and 9 in the lower member, supplemented as necessary by external prestressing members 30 of polygonal outline and overlapping at right supports constituted by crosspieces 31 carried by piles 32 and adjacent to diagonal junctions 6 at the upper frame 4. These external prestressing reinforcements 30 connect points 33 located on the crosspieces 31 near their ends, this is that is to say near the nodes of the spatial lattice formed by the junction of the diagonals to the upper chord, passing through deflection points 34 which are other nodes of the spatial lattice, constituted by the junction of the diagonals to the lower chord .

The resistance to cutting forces is supplemented by a prestressing of the diagonals such as 22, according to an embodiment identical to that which has been described for a guyed structure.

From a constructive point of view, the devices of the invention allow a remarkably simple embodiment, shown schematically in FIGS. 24 to 27.

Assuming the deck built up to the configuration shown in Figure 24 the steps below allow the realization of the next phase;
- Longitudinal advancement of two temporary edge beams 35 located on the bank immediately below the upper assembly nodes. These beams have sufficient strength to support the self-weight of a new section of frame over the distance separating two successive guy wires. Each beam has a length slightly greater than twice the distance above. In the center, the beam is immobilized by a centering pin 36 and suspension bars in line with the last guyed strut (Figure 24). At the rear, the beam finds its support to the right of the preceding shroud. The new frame section is transported and placed by known means (upper chord, four diagonals and two upper gussets provisionally joined by a cross beam ensuring the spatial rigidity of the element) which rests in its final position on the end of the two provisional longitudinal beams. You can dock the lower frame and secure the flanges. The upper gussets are then immobilized on the beams by pins (allowing at the same time the adjustment of the profile along the structure) and suspension bars, then a new stay cable is put in place.
- It is now possible to pour the concrete of the upper slab, the weight of which is carried at the rear by the already built part of the deck and at the front by the new guy wire, the tension of which can be adjusted to achieve the longitudinal profile desired of the book.

Of course, the invention is not limited to the exemplary embodiments described above and it is possible to make modifications to them without departing from the scope of the invention.

Claims (22)

1. Bridge consisting of an apron (1) and means (2, 3; 31, 32) for supporting this apron, the apron comprising:
- an upper member (4) forming a circulation slab,
- a lower chord (5) forming a continuous longitudinal element,
- connecting beams, called "diagonals" (6) connecting the upper and lower members, directed obliquely both with respect to the vertical and with respect to the length of the bridge, and forming with the members, a spatial trellis,
characterized in that the axes of the diagonals converge on the longitudinal axis of the lower member or the median plane of the upper member. 2. Bridge according to claim 1, characterized in that the parts of the members (4,5) which are subjected to significant tensile forces, and the diagonals (6) which are subjected to significant tensile forces are prestressed by means (9, 23, 22) which are specific to each of said members and to each diagonal or to two concurrent diagonals. 3. Bridge according to claim 1 or 2, characterized in that the biasing means of the diagonals comprise prestressing reinforcements (22) anchored at their two ends at the points of junction of these diagonals with the upper member and drawing a V whose middle is at the junction of said diagonals with the lower chord. 4. Bridge according to one of claims 1 to 3, characterized in that the lower member (5) is formed of successive sections assembled and is provided with longitudinal prestressing frames (9) which put in compression, each, several sections assembled. 5. Bridge according to one of claims 1 to 4, characterized in that the means for prestressing the upper frame are constituted by prestressing reinforcements (23) connecting together the nodes of the space trellis formed by the points of junction of the diagonals (6) with said upper chord. 6. Bridge according to one of claims 1 to 5, characterized in that the means for supporting the deck consist of guy wires (2) connecting support masts to nodes of the spatial trellis formed by the junction points of the diagonals (6) with the upper chord (4). 7. Bridge according to claim 6, characterized in that it comprises at least two continuous lower members (5), and an equal number of spatial trusses comprising diagonals (6) whose axes converge on the axis of a member lower (5), said members being interconnected by a brace (27), these spatial lattices each including a part of the upper member. 8. Bridge according to claim 7, characterized in that it comprises two lower members (5) and two spatial lattices and in that the means for supporting the deck consist of guy wires connecting the nodes of the spatial lattices which are located in the axial plane of the bridge to support masts. 9. Bridge according to one of claims 1 to 5, characterized in that the means for supporting the deck consist of guy wires (2) connecting support masts to nodes of the spatial trellis formed by the junction points of the diagonals (6) with the lower chord (5) 10. Bridge according to one of claims 1 to 5, characterized in that the means for supporting the deck consist of transverse supports (31) on which the upper frame rests, and prestressing reinforcements (30) are provided. additional which follow a polygonal path connecting two successive transverse supports passing through reference points (34) located on the lower chord. 11. Bridge according to claim 10, characterized in that said additional prestressing frames (30) are not in the axial plane of the bridge. 12. Bridge according to one of claims 1 to 11, characterized in that, to ensure the junction between the diagonals and the lower frame, there are provided gussets (12) in folded sheet metal, comprising two wings which are each in a longitudinal plane containing the axis of diagonals (6) which are fixed on it, the gusset being fixed on the lower member so that the fold axis of the wings of the gusset coincides with the longitudinal axis of the lower member. 13. Bridge according to claim 12, characterized in that the lower frame (5) is formed of successive sections assembled, and at least some of the gussets (12) are fixed to the points of assembly of successive sections. 14. Bridge according to one of claims 1 to 13, characterized in that, to ensure the junction between the diagonals (6) and the upper frame (4), there are provided gussets (15) having a lower wing located in a longitudinal plane containing the axis of the diagonals which are fixed on it, and an upper wing which is fixed on the upper member, so that the fold axis of the wings of the gusset is in the median plane of the upper member (4). 15. Bridge according to claim 14, characterized in that the gusset (15) carries anchor points (20) for frames (22) for prestressing the diagonals (6) and anchor points (21) for reinforcements (23) for prestressing the upper chord (4). 16. Bridge according to claim 14 or 15, characterized in that the connection between the gusset (15) and the upper chord (4) is a concrete-steel connection. 17. Bridge according to one of claims 14 to 16, characterized in that the upper wing of the gusset (15) has its median plane located in a longitudinal plane containing the stay axis (2) supporting the bridge, these stay cables being attached to said gusset. 18. Bridge according to claim 17, characterized in that the upper wing is split into two parallel wings between which is fixed the shroud (2), the fold axis being in this case constituted by the intersection of the median planes of upper and lower wings. 19. Bridge according to one of claims 1 to 18, characterized in that the upper member constitutes a concrete slab reinforced by continuous metal sections (28) and prestressing reinforcements (29) arranged perpendicular to these metal sections. 20. A method of constructing a bridge according to one of claims 1 to 19, characterized in that it comprises the following steps:
- Place, on a part of the deck (1) already assembled, two longitudinal beams (35) of provisional edge, on either side of the upper chord, overhanging with a length at least equal to the longitudinal dimension of a mesh of the space lattice, each beam being held by two successive nodes of the space lattice already assembled,
- Bringing a new mesh of the space lattice, this mesh comprising at least one node located on the lower member (5), two nodes located on the upper member (4), and the diagonals (6) corresponding to these nodes.
- securing this new mesh and the part of the deck already assembled, supporting the new mesh using the temporary edge beams,
- repeat the operations by advancing the provisional border beams along the mesh that has just been fixed. 21. The method of claim 20, characterized in that, during the joining of the new mesh with the deck part already assembled, a suspension strut is also fixed on said new mesh. 22. The method of claim 20 or 21, characterized in that one uses provisional edge beams provided with means such as studs (36) to immobilize them in the correct position relative to the meshes of the space trellis already mounted.

Images