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BACKGROUND OF THE INVENTION 
     The present invention relates to the use of cables in certain construction techniques and more specifically to the methods used to attach load-transmitting elements to a structural cable. 
     An important, though not exclusive, application of the invention is that of suspension bridges. In this application, hangers transmitting the load of the deck have to be attached to a bearing cable of the bridge, formed of one or more bundles of strands. 
     This attachment is generally performed by means of collars, each being made up of several shells which are clamped around the cable using means such as bolts, each hanger being fixed to one of the shells of its collar. The clamping force applied to the collar enables the bearing cable to absorb, by friction, the tangential components of the forces transmitted by the hangers. 
     French patent 2 739 113 proposes a bearing cable made from a bundle of “coherent” strands. Each coherent strand comprises a strand strictly speaking, made up of a stranded assembly of seven or more metal wires, coated with an adherent elastomer material and surrounded by an individual sheath of plastic material, whereby the strand and the elastomer material fill the interior space of the individual sheath. 
     This strand structure limits the force needed for the clamping action, which improves the reliability of the system. 
     European patent application 0 789 110 discloses the insertion of filler elements in a cable made up of individually protected strands (for example coherent strands), the filler elements being placed at the level of the attachment collars so that they fill the interstices between the individual sheaths of the assembled strands, the cross-section of said interstices being triangular in shape with curved sides. 
     This arrangement ensures that the clamping forces of the collars are transmitted more evenly to the strands under tension. At the level of the collars, the bearing cable is contained in a tubular envelope made of plastic material and deformable elements are inserted between the envelope and the bundle of strands. This assembly is crushed instantly when the collars are clamped and is also subjected to a delayed crushing action due to the creep of the materials, generally plastic, from which the envelope, the deformable elements and/or the filler elements are made. 
     In the typical example of a collar having an internal diameter of 400 mm, the delayed settling of this assembly under the effect of creep is typically in the order of 1 mm. This value may cause the collar to loosen, which could have disastrous consequences. As a result, regular maintenance work has to be carried out to check the tightness of the clamped collars and re-clamp them as required. 
     An object of the present invention is to improve the clamping action of the attachment collars on a structural cable. 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention proposes a device for attaching a load-transmitting element to a cable, comprising a collar formed of at least two shells and means for clamping the collar around the cable, at least one of the shells having means providing a link with the load-transmitting element. The cable comprises an assembly of tensioned strands contained, at the level of the collar, in a deformable matrix filling the interstices between the strands, at least part of the matrix being in a plastic material that is susceptible to creep under the action of the clamping means. According to the invention, the clamping means comprise elongate clamping elements which transmit a clamping force to the shells of the collar and which are stressed so as to have a longitudinal elastic deformation that is substantially greater than the maximum settlement of the matrix due to creep. 
     The pre-stressed clamping elements provide a sort of reserve capacity for deformation, allowing any settlement of the assembly located underneath the collar due to the effect of creep to be absorbed. By making the elastic extension of these clamping elements much greater than the foreseeable settlement of the cable due to creep (typically at least five times greater), the clamping action can be guaranteed to remain at a controllable value and close to the initial value. 
     In preferred embodiments of the device according to the invention: 
     each tensioned strand of the cable consists of a strand coated with an adherent elastomer material and surrounded by an individual sheath of plastic material, whereby the strand and the elastomer material fill the interior space of the individual sheath (coherent strand); 
     the matrix comprises inserts having a triangular-shaped cross-section with curved sides, placed so as to fill, at the level of the collar, the interstices of matching shape located between the individual sheaths of the strands assembled in a bundle, and optionally elastomer elements arranged at the periphery of the bundled strands, and a tubular envelope made from a plastic material surrounding the elastomer elements, on which the shells of the collar bear; 
     the elongate clamping element is a metal rod, at least one end of which has a thread for receiving a nut used to apply stress to it, or else a tensioned cable, at least one end of which is clamped by means of anchoring jaws; 
     the elongate clamping element is housed in at least one bracing tube filled with a protective substance such as a petroleum wax, a grease or a resin. 
     A second aspect of the present invention relates to a suspension bridge, comprising at least one bearing cable anchored at its two ends and a deck suspended from the bearing cable by means of hangers, at least some of the hangers being attached to the bearing cable by means of attachment devices as defined above. 
     Advantageously, the attachment device has a portion which projects substantially above the bearing cable and which contains a stressed clamping element, the top end of said portion being fitted with means for fixing a hand rail to allow personnel to move along the bearing cable. 
     Another aspect of the present invention relates to a of attaching a load-transmitting element to a cable by means of a collar formed of at least two shells, the cable comprising an assembly of tensioned strands contained in a matrix at the level of the collar, at least part of the matrix being in a plastic material susceptible to creep under the clamping action exerted on the collar. In that method, the shells of the collar are clamped around the cable by means of elongate clamping elements which are stressed so as to have a longitudinal elastic deformation substantially greater than the maximum settlement of the matrix due to creep. 
     The method advantageously comprises a step of simultaneously applying the stresses to the elongate clamping elements by means of a hydraulic system, and a step of clamping the elongate clamping elements in the stressed position. This makes it possible to balance the forces exerted on all the clamping elements of a same collar and avoid subjecting the elements to individual clamping forces that would cause twisting. 
     In specific approaches to implementing the method: 
     the steps of simultaneously applying stresses and clamping in the stressed position are carried out after the collar has been fitted around the cable, as well as during subsequent monitoring and re-clamping operations. 
     the step of simultaneously applying stresses comprises a first phase in which the hydraulic system exerts an excessive stress followed by a second phase in which the applied stresses are reduced before proceeding with the clamping stage. 
     the steps during which stresses are applied simultaneously and the clamping action is applied in the stressed position are performed after the collar has been fitted around the cable, as well as subsequent checking and re-clamping operations; 
     the step during which stresses are applied simultaneously incorporates a first phase during which the hydraulic system exerts an excessive stress, followed by a second phase in which the stresses applied are reduced before proceeding with the clamping step. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general schematic view of a suspension bridge according to the invention; 
     FIG. 2 is a view in cross section of one of the coherent strands of a bearing cable of the suspension bridge illustrated in FIG. 1; 
     FIG. 3 is a view in cross section illustrating the layout of such strands inside the bearing cable at the level of a hanger attachment collar; 
     FIGS. 4 and 5 are views in cross section of two embodiments of attachment devices proposed by the invention; 
     FIG. 6 is a diagram illustrating one way of applying the initial stress to the clamping elements of such devices; and 
     FIG. 7 is a schematic illustration of a alternative embodiment of the means for clamping the collar. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The suspension bridge illustrated by way of example in FIG. 1 conventionally comprises a deck  1 , two pylons  2 , two parallel bearing cables  3 , only one of which is visible in the drawing, and a set of hangers  4  attached to the cables  3  by means of respective collars  10 , and which carry the deck  1 , transmitting its load to the bearing cables  3 . The bearing cables  3  are tensioned between two ground anchorings  5  at the two ends of the bridge and are supported by the pylons  2 . As illustrated, the bearing cables  3  may be discontinued at the level of the pylons  2 , in which case they will be anchored on these pylons. 
     Each bearing cable  3  is made by assembling individually-protected strands  6  of the type illustrated in FIG.  2 . 
     Each of these strands is made up of several twisted steel wires  7 , which may be optionally galvanised, seven being provided in this case, which are embedded in an elastomer  8  such as polybutadiene or the like. The elastomer  8  is in turn covered by an external sheath  9  made from a flexible plastic material which might be a polyolefin, in particular high-density polyethylene (HDPE) or alternatively a polyamide. The elastomer  8  adheres to the twisted wires  7  forming the strand per se by surface adhesion and by conforming to its shape. This elastomer  8  is also adhered to the individual HDPE sheath  9 . For more details on how this coherent strand is made, reference may be made to French patent 2 739 113. 
     The individual sheath  9  is integral with the steel wires  7  of the strand so that the forces applied to this sheath parallel with the axis of the strand  6  will be duly transmitted to the steel wires  7 . 
     FIG. 3 shows the structure of a bearing cable  3  made up of a bundle of thirty seven coherent strands  6 . Seen in cross-section, these strands  6  are arranged in a hexagonal lattice defining interstices between them in the shape of a triangle with curved sides. 
     At the level of the collars  10 , the bundle of strands is housed in a tubular envelope formed by joining two semi-cylindrical shells  12  made from a plastic material such as HDPE. Elastomer elements  13  are arranged between the hexagonal bundle of strands and the internal face of the envelope  12  so as to hold the bundle in position inside the envelope and transmit the clamping forces exerted by the attachment collars  10  to the bundle. 
     Inserts  14  made from a plastic material, for example HDPE, are placed in the triangular interstices with curved sides, defined between the individual sheaths  9  of the strands  6 , so as to fill these interstices at the level of the collars  10 . The inserts  14  ensure that the clamping forces are evenly distributed between the different coherent strands  6 , as explained in European patent application 0 789 110. 
     Accordingly, at the level of the collars  10 , the bundle of strands  6  is contained in a matrix made up of the tubular envelope  12 , the elastomer elements  13  and the inserts  14 . 
     This matrix is likely to settle due to creep under the effect of the clamping action exerted by the collars  10 . As mentioned above, a typical value for this settlement in the case of a cable with a diameter of 400 mm is 1 mm. If the collars are clamped in a conventional manner using steel bolts having a Young modulus of 20,000 kg/mm 2  having an active length of 150 mm with a tensile stress of 50 kg/mm 2 , the elastic extension of these bolts is 150×50/20,000=0.375 mm, which is not enough to cope with the 1 mm settlement which can be expected. The deferred creep of the matrix containing the cable at the level of the collars will therefore cause the clamping forces initially applied to disappear. 
     In order to avoid this disadvantage, a hanging device of the type illustrated in FIG. 4 may be used. The collar  10  is made up of two (or more) semi-cylindrical shells  10   a ,  10   b  which bear on the tubular envelope  12  of the cable. The collar has a linking member  15 , on its bottom shell  10   b  for example, for an articulated connection of the top end of the hanger  4 . 
     At the level of the median plane where they face one another at a small distance apart, the shells  10   a ,  10   b  have side extensions  16   a ,  16   b  in which orifices are provided for inserting clamping elements  17 . 
     The clamping elements  17  are elongate in shape. Four or more may be provided for each collar  10 . 
     Each clamping element  17  is inserted through two bracing tubes  18   a ,  18   b , one bearing against a side extension  16   a  of the top shell  10   a  and the other bearing against a side extension  16   b  of the bottom shell  10   b.    
     The bracing tubes  18   a ,  18   b  may be integral with the shells  10   a ,  10   b  or be separate components. Each shell  10   a ,  10   b  may be provided with ribs  19  in planes transverse to the direction of the cable, so as to support the bracing tubes  18   a ,  18   b.    
     In the embodiment illustrated in FIG. 4, the elongate clamping elements  17  consist of metal rods, the section of which may be in the order of one tenth of the internal section of the bracing tube and the ends  20  of which are threaded. A nut  21  is screwed onto each threaded end  20  and bears on the corresponding end of the bracing tube  18   a ,  18   b  opposite the side extension  16   a ,  16   b  of the shell. 
     These nuts  21  are tightened so as to pre-stress the metal rods  17 . This pre-stress is such that the rods  17  undergo a longitudinal elastic deformation that is greater than the maximum settlement due to creep of the matrix containing the strands at the level of the collar. This elastic deformation or extension is preferably more than five times greater than the maximum settlement of the matrix due to creep. 
     By selecting rods  17  made from a steel with a Young modulus in the order of 20,000 kg/mm 2  and capable of withstanding tensile stress of 120 kg/mm 2  (a common value in pre-stress applications), it will be possible to use rods  17  of a length of 1 m, which will give an elastic extension in the order of 1000×120/20,000=6 mm. Accordingly, the typical settlement of 1 mm which occurs due to the effect of creep will give rise to a loss of only 16% of the clamping force initially applied. 
     In the clamping device illustrated in FIG. 4, the general design of the clamping means is symmetrical on either side of the median plane of the collar. The bracing tubes  18   a ,  18   b  may therefore be of a length in the order of 50 cm. 
     In the embodiment illustrated in FIG. 5, a single bracing tube  18  is provided for each pre-stressed rod  17 . The bottom nut  21  bears directly on the side extension  16   b  of the bottom shell  10   b , whilst the bracing tube  18 , the length of which is 1 m, bears on the extension  16   a  of the top shell  10   a.    
     FIG. 5 also shows a platform  25  provided above the bearing cable  3  to enable personnel to move along the cable. In particular, this platform  25  may be fixed onto the top shells  10   a  of the collars. The fact that the bracing tubes  18  and pre-stressed rods  17  project approximately 1 m above the bearing cable  3  allows their top ends to be fitted with members such as rings  26  so that a and rail can be installed for personnel moving around on the platform. These members  26  are screwed onto the threaded ends  20  of the rods  17  projecting above the nuts  21 , for example. 
     FIG. 6 illustrates an advantageous way of applying the pre-stress to the clamping rods  17 . 
     Once the shells  10   a ,  10   b  have been fitted on the cable  3 , the bracing tubes  18  (or  18   a ,  18   b ) are set in place as well as the top and bottom nuts  21 , which are not tightened at this stage. 
     A plate  28  in which orifices  29  are provided at the level of the top ends  20  of the rods  17  is mounted above the assembly so that the ends  20  of the rods  17  project through the orifices  29 . A nut  30  is then tightened on each of the threaded ends  20  projecting above the plate  28 . A hydraulic system  31 , which might consist of one or more jacks, is arranged between the top face of the shell  10   a  and the bottom face of the plate  28 . A similar arrangement (plate  28 , nuts  30  and hydraulic system  31 ) may optionally be provided in a symmetrical arrangement underneath the bottom shell  10   b  of the collar. 
     In a first step, the hydraulic system  31  is energised so that a force F is applied which places tension on the rods  17 , the top end  20  of which is raised by the nut  30  biased by the plate  28 . When the nominal force F is applied or once the required deformation of the rods  17  is obtained, the upper nuts  21  are brought into contact with the top end of the bracing tubes  18 , clamping the rods  17  in the stressed position. The hydraulic system  31  can then be deactivated, the provisional nuts  30  and plate  28  dismantled and the hydraulic system  31  removed. 
     This method of clamping avoids applying any torsion to the rods  17  and provides a good balance of forces between the different rods  17  used to clamp the same collar. 
     In the step of energising the hydraulic system  31 , it is possible to start by applying an initial excessive clamping force, i.e. a force F greater than the nominal force, so as to induce creep of the matrix containing the bundle of strands more rapidly. This initial excessive clamping is suppressed after a period, which may be several hours. The nominal force is then applied before the rods are clamped by the nuts  21 . 
     The clamping method described above using the plate  28 , nuts  30  and hydraulic system  31  may be applied at the time when the collars  10  are mounted but may also be effected during subsequent monitoring and, optionally, re-clamping operations. However, these operations will not have to be carried out as frequently as they would have to be if the clamping elements  17  had not been pre-stressed as proposed by the invention. 
     FIG. 7 shows another embodiment of the invention, in which the elongate clamping elements are not threaded rods but cables  37  made up or one or more strands (one in the example illustrated). 
     The strand  37  is housed in the bracing tube  18  and its top end  38  is clamped by an anchoring jaw  39  of a generally frusto-conical shape comprising several wedges. The outer side of the anchoring jaw  39  is supported against a matching frusto-conical surface of an orifice  40  provided in a socket  41 . Underneath the portion in which the orifice  40  is provided, the socket  41  has a threaded extension  42  on which the clamping nut  21  engages. 
     As illustrated in FIG. 7, the socket  41  has a shoulder between its threaded portion  42  and its portion having the frusto-conical orifice  40 , engaged by the plate  28  used to apply tension to the clamping elements  37 . When this plate  28  is biased by the force F exerted by the hydraulic system  31 , the socket  41  is raised, which causes the anchoring jaw  39  to clamp firmly on the strand  37 . Once the requisite force is applied, the nut  21  is lowered along the threaded portion  42  so that it engages the top end of the bracing tube  18 . 
     The bottom end of the strand  37  may also be anchored by means of a jaw  39 , or simply clamped by means of a protuberance formed at the end of the strand by drawing or pressing. 
     As illustrated in FIG. 7, the bracing tube  18  containing the strand  37  is advantageously filled with a protective substance  44  which might be a petroleum wax, a grease or alternatively a resin. A protective substance  44  of this type may also be provided if the elongate clamping element is of a different type, for example a threaded rod  17 . 
     Although the invention is described here with reference to its preferred application to suspension bridges, it may clearly be applied to other structures requiring the use of cables subjected to transverse loads.

Summary:
The device comprises a collar consisting of two shells, designed to provide a link with the load-transmitting element. The cable has an assembly of tensioned strands contained, at the level of the collar, in a matrix filling the interstices between the strands. This matrix is totally or partially made from a plastic material susceptible to creep under the effect of the clamping action of the collar. The clamping action is applied by elongate elements transmitting a clamping force to the shells of the collar. These elongate elements are stressed so as to produce a longitudinal elastic deformation that is much greater than the maximum settlement of the matrix due to creep. This prevents any undesirable loosening of the collar.