EP0935034B1 - Method of manufacturing of an anchoring, anchoring piece and tensioning element for this purpose - Google Patents

Description

The present invention relates to the field of anchors in civil engineering, including blind anchors that are accessible only on one side.

For some anchors, an anchor head of a tension element prestressed or not, it is not possible to access the anchor from the rear. This case is encountered particularly in the case of a buried anchor, access to the anchor being only possible from the ground surface, or in the case where a sealing or corrosion protection must be specially treated so that the rear side anchor must be closed. This constraint prevents the use of a traditional anchor plate where the attachment of the tension element to the plate, for example by means of anchoring cones, requires the development of new types of anchoring.

Patent EP-0,351,582 shows an anchorage accessible from only one side; the disadvantage of the device described here being that each tension element, respectively the tube in which they are introduced, is maintained solely by longitudinal adhesion, which greatly limits the tensile force that can withstand such an anchoring and leads to a length anchoring very important to obtain a sufficient adhesion surface. Likewise, US Pat. No. 4,043,133 provides for a cladding tube for tension elements maintained solely by longitudinal adhesion in the surrounding ground. The tension members protrude from the lower end of the tube and are all attached to an anchor plate, without the manner in which the latter plate is introduced into the cavity and the manner in which the tensioning elements are attached to said plate. are described. In the case where this embodiment is feasible, the transmission of the anchoring force of the end of the tension elements in the surrounding ground through the injected cladding tube is achieved solely by longitudinal adhesion, without benefiting from the corner effect as described in the invention below.

Document DE-A-44 37 104, in the name of the applicant, relates to an anchoring device which, prefabricated in the factory or on the site of use (column 3, lines 17 to 21), is intended to be trapped in a massive anchor to allow the realization of an anchorage in such a massive.

• un pièce axiale consistant en au moins une tige de traction formée par assemblage de fils métalliques et présentant deux extrémités dont,
  • une première extrémité qui demeure accessible après la construction dudit ancrage,
  • une seconde extrémité qui, constituée par le déploiement radial et courbure desdits fils, devient inaccessible après la construction de l'ancrage, c'est-à-dire après incorporation du dispositif préfabriqué d'ancrage dans un massif d'ancrage, et
• une gaine de protection de la pièce centrale contre la corrosion qui, s'étendant entre la première extrémité et la seconde extrémité de l'élément de tension, est destinée à être remplie d'un coulis de ciment après emprisonnement du dispositif préfabriqué d'ancrage dans un massif d'ancrage,
• une masse de matériau qui est formée,
  • autour de la seconde extrémité de l'élément de tension,
  • avant incorporation du dispositif d'ancrage dans un massif d'ancrage, et
  • de manière à constituer une partie épaulée permettant l'ancrage de la pièce centrale dans un tel massif.

The prefabricated anchoring device of DE-A-44 37 104 uses:

• an axial part consisting of at least one tensile rod formed by assembling metal wires and having two ends,
  • a first end that remains accessible after the construction of said anchor,
  • a second end which, constituted by the radial deployment and curvature of said wires, becomes inaccessible after the construction of the anchor, that is to say after incorporation of the prefabricated anchoring device into an anchor, and
• a protective sheath of the central piece against corrosion which, extending between the first end and the second end of the tension member, is intended to be filled with a cement slurry after imprisonment of the prefabricated anchoring device in a massive anchor,
• a mass of material that is formed,
  • around the second end of the tension element,
  • before incorporation of the anchoring device into an anchor, and
  • so as to form a shoulder portion for anchoring the central piece in such a solid mass.

• à ce munir d'une tige de traction comportant une seconde extrémité radialement expansée ou à se munir d'un groupe de tiges de traction comportant chacune une telle seconde extrémité radialement expansée, puis
• à engager une gaine sur la tige ou sur le groupe de tiges, et
• à former la partie épaulée autour de la seconde extrémité ou autour des premières extrémités.

This anchoring device is prepared before being used on a specific site and the preparation consists of:

• to provide a pull rod having a second radially expanded end or to be provided with a group of pull rods each having such a radially expanded second end, and then
• to engage a sheath on the rod or on the group of rods, and
• forming the shoulder portion around the second end or around the first ends.

The push rods used in the case of the device of DE-A-44 37 104 can not be used for the construction of an anchor according to the invention.

Indeed, since the second end of these rods is constituted by radial deployment and the curvature of the metal wires, it does not make it possible to guarantee the achievement of the result sought by the invention, namely the sliding of a second end onto another second end already installed in a cavity in the anchor.

A first object of the invention is to propose a method for producing an anchorage accessible from a single side, not meeting the mentioned drawbacks of known anchors, that is to say making it possible to carry out an anchoring in which the elements tension are maintained so that the tensile force on each of them at the anchor is taken up by adhesion, this adhesion being substantially favored by the confinement induced by the general shape of the anchor, and by longitudinal mechanical locking of the ends of the tensioning elements, due to the particular shape of said ends and their arrangement in a cavity of substantially tapered shape.

A second object of the invention is to propose a device that makes it possible to perform such an anchoring.

These objects are obtained by a method of making an anchor as described in claim 1 and by a device according to claim 9.

• la figure 1 est une vue en coupe d'une forme d'exécution préférentielle d'une pièce d'ancrage selon l'invention,
• les figures 2A à 2L représentent chacune une étape particulière d'un procédé de réalisation d'un ancrage selon l'invention, et
• les figures 3A, 3B, 3C représentent trois formes d'exécution particulières d'un élément de tension selon l'invention, la figure 3D montrant un exemple obtenu sur une tige de traction constituée de brins assemblés.

The invention is described in more detail below, this description being to be considered with reference to the appended drawing comprising the figures in which:

• FIG. 1 is a sectional view of a preferred embodiment of an anchoring piece according to the invention,
• FIGS. 2A to 2L each represent a particular step of a method of producing an anchor according to the invention, and
• FIGS. 3A, 3B, 3C show three particular embodiments of a tension element according to the invention, FIG. 3D showing an example obtained on a traction rod consisting of assembled strands.

For the implementation of the method of the invention, it is first necessary to obtain an anchoring cavity of determined shape. The shape of this anchoring cavity is essentially oblong, tapered, with a first end open on the accessible side of the anchor and a second closed end on the non-accessible side of the anchor. In addition, the cross section of the first end of the anchor must be smaller than another cross section of the cavity, this section corresponds to that of the second end or to an intermediate section of the cavity.

Several means or devices make it possible to obtain such a cavity. A first means consists in using an anchoring piece, comprising a prefabricated internal cavity having the desired shape of the anchoring cavity. A preferred embodiment of such an anchoring piece is shown in FIG. 1. The anchoring piece 1 consists essentially of a wall 10, preferably thin, limiting an internal cavity 11. A first end of the anchor 1, the upper end of the part in the figure comprises an opening 12, and means 13 for securing a tubular protective sheath of the tension elements whose utility will be described later. The other end of the anchoring piece 1 is closed by a bottom wall 14. The external shape of the anchoring piece 1, or of the internal cavity 11 is essentially tapered, for example in a truncated cone or pyramid , with the weakest straight section close to the opening 12 and the strongest straight section close to the bottom wall 14. An inlet 15 is arranged near the bottom wall 14, a tube of injection 16 being fixed or attachable to said inlet port. Similarly, a discharge port 17 is provided near the opening 12, a discharge tube 18 being fixed or attachable to said discharge port. The usefulness of these orifices and tubes will be described below. Preferably the tapered outer surface, respectively truncated cone or pyramid, of the anchor 1 comprises one or more anchoring rings 19 disposed on the periphery of said surface, the purpose of which is to improve the transmission and the distribution of the anchoring force in the surrounding structure. The form embodiment shown in the figure comprises two such rings 19. The anchor 1 may be of synthetic material, metal or concrete, its dimensions depending essentially on the importance of the anchor considered.

Figure 2A shows the first step of the method of manufacturing an anchor according to the invention using such an anchor. While the surrounding concrete structure is not yet made, an anchor 1 is placed at the exact place where the anchoring must be made, the opening 12 being directed towards the future tension elements. The anchor piece 1 is held in place by a temporary scaffold or preferably by irons 20 of the reinforcement of the concrete. Preferably, but without this being essential to the invention is arranged around the anchoring piece 1 or more circular irons 21 forming one or frets, to improve the cohesion of concrete in this place.

In FIG. 2B, it can be seen that the concrete structure 2 to support the anchorage has been concretely concreted around the anchoring piece 1. The anchoring piece 1 is thus entirely surrounded and maintained in the concrete structure 2, with the exception of its first end provided with the opening 12 which is flush with the upper surface of the concrete structure 2 or which, as shown here, protrudes slightly above said upper surface of the concrete structure 2, and that ends of the injection tubes 16 and evacuation 18 which remain accessible outside the concrete structure 2.

It can therefore be seen that, at this second stage of the process, a cavity 11, of definite shape, has been obtained inside a concrete structure 2. As described up to now, this cavity 11 has been obtained using a part anchor 1 provided with a prefabricated cavity. The same cavity 11 in a concrete structure 2 can also be obtained in other ways, for example by making it on site. For example, there may be a removable formwork, wood or other material, having an outer shape in accordance with the desired shape of the cavity 11, placed at the desired location and around which the concrete structure 2 is then cast. As soon as the concrete hardens, the formwork is dismounted by acting from the opening 12 and is extracted from the cavity 11 by the same opening. In a fairly similar manner, one can have a flexible and inflatable piece having, after inflation, the desired shape of the cavity 11 and which is placed at the desired location. After concreting of the concrete structure 2, the inflatable part is deflated, leaving a cavity 11 of desired shape in the concrete structure 2. According to another way, the cavity 11 can be obtained by drilling a cavity 11 of the desired shape in an existing structure 2. The latter method of drilling is rather reserved for cases of an anchorage directly in the ground or for the installation of a new anchorage on an existing structure 2. The cavity 11, obtained in any of the ways described has two important dimensions, a passage area of the opening 12 called S12 and a maximum cross-sectional area called S11 (see Figure 1).

During the third step of the process, visible in FIG. 2C, the structural element to be prestressed 3 is laid or concreted, in known manner, over the concrete structure 2, said structural element 3 preferably comprising a conduit or a cladding tube 30, one end of which faces the opening 12 to be fixed on the fastening means 13 adjacent to said opening. The cross-section of the cladding tube 30 or duct provided in the structural element 3 for the tensioning elements corresponds essentially to the section of the opening 12 of the cavity 11. The cladding tube 30 or the corresponding duct arranged in the structural element 3 comprises at least one injection orifice 31, connected to an injection tube 32, at least one of said orifices 31 being preferably disposed near the end of the tube 30 close to the opening 12 , as well as at least one discharge orifice connected to a discharge tube, at least one of said discharge orifices being disposed near the other end (not visible in the figure) of the tube 30, respectively of the structural element 3.

The fourth step, shown in Figure 2D is to introduce the voltage elements.

For this, reference may be made to FIGS. 3A, 3B, 3C and 3D which show, by way of nonlimiting examples, four embodiments of such a tension element 4. The tension element 4 essentially consists of a pull rod 40 and an end portion 41. The end portion 41 on the pull rod 40 is designed such that said end portion 41 has a larger cross-sectional area S41 than the cross sectional area S40 of the pull rod 40, this for the reasons that will be explained below. The other end of the pull rod 40 has no end portion of this type and is formed for a normal anchor, known in the art.

The pull rod 40 may be of any known type, consisting of either a unitary strand or a plurality of strands helically assembled to form a pulling cable. The unitary strand or strands assembled to form the pull rod 40 may be steel, preferably a high tensile steel or synthetic material, for example based on carbon fiber or Kevlar.

The end portion 41 may consist of an end piece 41, made of metal or of synthetic material, which is firmly fixed to the end of the pull rod 40. The choice of the material of the end piece 41 thus that its mode of attachment to the pull rod 40 depends essentially on the material as well as the way in which the pull rod 40 is constituted. The end piece 41 essentially comprises a central body 42, bounded by an upper portion 43 and a lower portion 44. The central body may have a straight cylindrical shape, with a circular cross-section as in FIG. 3A or polygonal, or else a tapered shape in a truncated cone or pyramid, with a circular or polygonal straight section, as in FIG. Figure 3B. In the case of a tapered shape the portion of smaller section is that adjacent to the upper portion 43. The two portions 43 and 44 are curved or formed of inclined planes, so as to facilitate the sliding of an end portion being installed on another end portion already installed, as will be seen below.

According to another embodiment, the end portion 41 may be formed by deformation or machining directly on the end of the pull rod 40. FIGS. 3C and 3D show examples of end portions of this type. In FIG. 3C, the traction rod 40 consists of a unitary strand and the end portion 41 is obtained by deformation, for example by forging, stamping or stamping, of the end of the pull rod 40. FIG. 3D shows an example of an end portion 41 obtained on a traction rod 40 consisting of assembled strands. In this example, the end of each strand has been moved away from its normal position, a ring or ligation may be provided just before this separation to prevent undocking of the rest of the towing cable. The spaced apart ends of the strands may be held in position by a complementary holding piece 45, for example a circular disk welded or fixed in any way under the spaced strands or may be left free. According to an embodiment not shown, the part for holding the spaced strands may consist of an element having the shape of two conical portions contiguous by their base, a first conical portion being introduced between the strands to remove them, while the second conical portion has the same use as the lower portion 44 described above. Thus, according to any embodiment of the end portion 41, it may also have a circular or polygonal shape and include the upper and lower portions 43 and 44 as previously described.

The described examples of end pieces 41 or deformed end portion portions 41 are not limiting both in their form and their means of obtaining; any means for increasing the cross sectional area of the end portion of the pull rod 40 can be envisaged. In the following description we will talk about end piece 41, it being understood that it may also be an end portion as described above.

Returning to FIG. 2D, it can be seen that a first tension element 4 has been pushed into the guide tube 30 and then into the cavity 11, until its end piece 41 comes into contact with the lower surface. of said cavity. A second voltage element 4 is being installed in the same way.

FIG. 2E shows the utility of the curved or inclined shape that can be provided on the upper portion 43 and the lower portion 44 of the end piece 41. When a tension member 4 is being installed, it is strong. it is possible for its end piece 41 to abut against another end piece of an already installed tension element. Due to the curved or inclined shape of said portions, the second end piece does not jam against the first but is spaced apart and slides against it to find its final position next to it.

FIG. 2F shows that after a certain number of tension elements have been installed, a new end piece to be installed may not find its place at the bottom of the cavity 11; in this case, it is sufficient for the tension element in question to fulfill its role entirely later, that the end piece is pushed as low as possible into the cavity until it abuts against a or several pieces already installed or against the side wall of the cavity.

• pour permettre l'introduction du dernier élément de tension 4, respectivement pour permettre le passage de la dernière pièce d'extrémité 41 dans le tube de guidage 30, respectivement dans l'ouverture 12, on a: $$\left[\left(\mathrm{N}\mathrm{-}\mathrm{1}\right)\mathrm{\times }\mathrm{S}\mathrm{40}\right]\mathrm{+}\mathrm{S}\mathrm{41}\mathrm{<}\mathrm{S}\mathrm{12.}$$
  
  S12 correspondant à la surface de la section droite de l'ouverture 12 (fig 1).
• pour permettre une bonne disposition des pièces d'extrémité 41 sur le fond de la cavité 11, on a: $$\left(\mathrm{N}\mathrm{\times }\mathrm{S}\mathrm{41}\right)\mathrm{<}\mathrm{S}\mathrm{11.}$$
  
  S11 correspondant à la surface de la section droite de plus forte surface de la cavité 11 (fig 1).

For the implementation of the anchoring of the stay or the prestressed member, it is necessary to introduce a number "N" of tension elements 4 into the cavity 11. Knowing that the cross section of each pull rod 40 has a surface S40 and that the maximum cross-sectional area of the end piece 41 is S41 (see FIGS. 3A, 3B, 3C and 3D) the following relationships must be made:

• to allow the introduction of the last tension element 4, respectively to allow the passage of the last end piece 41 in the guide tube 30, respectively in the opening 12, we have: $$\left[\left(\mathrm{NOT}\mathrm{-}\mathrm{1}\right)\mathrm{\times }\mathrm{S}\mathrm{40}\right]\mathrm{+}\mathrm{S}\mathrm{41}\mathrm{<}\mathrm{S}\mathrm{12.}$$
  
  S12 corresponding to the surface of the cross section of the opening 12 (FIG. 1).
• to allow a good disposition of the end pieces 41 on the bottom of the cavity 11, we have: $$\left(\mathrm{NOT}\mathrm{\times }\mathrm{S}\mathrm{41}\right)\mathrm{<}\mathrm{S}\mathrm{11.}$$
  
  S11 corresponding to the surface of the cross section with the greatest area of the cavity 11 (FIG. 1).

When all the elements of tension 4 have been pushed through the conduit or the tube 30 so that all their end pieces 41 have been housed in the cavity 11 as indicated above, we can proceed to the next step of the process as seen in Figure 2G. During this step, a liquid sealing material 50 is introduced through the injection tube 16; this material enters the cavity 11 through the injection orifice 15 and fills the empty spaces between the end pieces 41 and the ends of the traction rods 40 in the cavity 11, until at least partially filling the cavity 11 During this operation, the discharge orifice 17 and the discharge tube 18 serves to evacuate the air contained in the cavity 11 during its filling and to control the filling level of the cavity 11. Preferably, the cavity 11 is filled until the liquid mass introduced reaches the level of the discharge orifice 17. The material contained in the cavity 11 then hardens to form a rigid block of high mechanical strength 5 in which the end pieces 41 and the ends of the pull rods 40 are recessed.

In the next step, shown in FIG. 2H, the tensioning elements 4 are each subjected to traction until reaching the prescribed prestressing tension value. This traction is effected in a conventional manner, by acting on the other end of each tension element 4, respectively of each pull rod 40, the tensioning elements being pretensioned simultaneously or sequentially. As can be seen in the figure, the tapered, frustoconical or pyramidal shape of the cavity 11, respectively of the cured mass in which the end pieces 41 and the ends of the rods 40 of the tensioning elements 4 are embedded, allows a effective corner anchoring in the concrete structure 2. Unlike the known prior art devices mentioned above, this wedge shape prevents any axial displacement of the cured mass 5 and causes transmission of the anchoring forces in the surrounding structure 2 by axial compression and not by simple adhesion. The length of this anchorage is therefore favorably reduced.

An additional anchoring security is ensured by the particular arrangement of the end pieces 41 inside the cavity 11. Considering that the end pieces 41 are arranged in a bundle in the cavity 11, the surface of the section straight line generated by the bundle of the assembled end pieces 41 is greater than the surface of the opening 12 of the cavity 11. The bundle of end pieces 41 is thus blocked in the cavity 11.

• pour permettre un blocage des éléments de tension 4 dans la cavité 11 en empêchant la sortie des pièces d'extrémités bloquées entre elles par l'ouverture 12, on doit avoir: $${\left(\mathrm{N}\mathrm{\times }\mathrm{S}\mathrm{41}\right)}^{\mathrm{*}}\mathrm{>}\mathrm{S}\mathrm{12.}$$
  

By repeating the expressions mentioned above, we have the following relation:

• to allow a locking of the tensioning elements 4 in the cavity 11 by preventing the exit of the end pieces locked together by the opening 12, one must have: $${\left(\mathrm{NOT}\mathrm{\times }\mathrm{S}\mathrm{41}\right)}^{\mathrm{*}}\mathrm{>}\mathrm{S}\mathrm{12.}$$
  

In the above expression, (N x S41) * generally represents the area generated by the bundle of the N-pieces of assembled ends, each having a cross-sectional area S41. In order to take into account that possibly one or two end pieces 41 may not have found their place, as indicated with reference to FIG. 2H, the individual sections S41 and the passage section S12 must be dimensioned for a blocking end pieces 41 when the tensile force is exerted simultaneously on all the tensioning elements 4.

It should be noted that the above-described step of pretensioning of the tensioning elements 4 can be carried out differently than described, in particular in the case of a simple unassembled guying.

In a final step of the process, seen in FIG. 2L, the empty space inside the cladding tube 30, respectively inside the duct arranged in the structural element 3 can be filled with a another sealing material 60 through the injection tube (s) 32 and the injection port (s) 31 in order to preserve the tightness of the claimed system and to prevent corrosion of the pretensioners. This last step is also optional, dependent if such protection is required or necessary.

It can therefore be seen that a very effective anchorage is thus obtained, the longitudinal tensile force of each tension element 4 being taken up mainly by its part or its end portion 41 and carried on the high-strength hardened sealing block. mechanical 5. An effective transmission of this effort is possible through the firm attachment of the end piece 41 on the pull rod 40; this fixing can be performed in the factory, its mechanical strength is very high. This force is then transferred by the oblique walls of the cavity 11 to the surrounding structure 2. By placing one or more anchoring rings 19 on the anchoring piece 1, it is still possible to improve the anchoring effect. mentioned in the surrounding structure 2. As mentioned, frets 21 may be provided to further improve the cohesion of the surrounding structure 2 around the cavity 11. In addition to the said longitudinal resistance, each pull rod end 40 being maintained in the sealing block 5, maintaining radial compression of each rod 40 is obtained in addition.

This type of anchorage is particularly well suited for prestressing anchoring of a prestressed structural element 3. It can also lend itself to anchoring of non-prestressed tension elements, such as for example stay cables of a mast or of a pylon, the stays can then not be protected by a protective tube 30. Similarly it is not essential that the cavity 11 is arranged in a surrounding concrete structure, a borehole or in rock allowing to obtain a required cavity can also be expected. The description was made for a cavity whose longitudinal axis is essentially vertical, with its opening 12 upwards. Other geometrical arrangements are also possible, the dimensions of the cavity 11 being adapted to obtain a sufficient filling of the cavity 11 by the sealing liquid 50.

Claims (18)

1. Method of constructing an anchorage having more than one tension element (4), said anchorage being accessible from one side only, this method comprising in particular the following steps:

   - making a cavity (11) in a surrounding structure (2), said cavity having a substantially oblong shape and having two ends, the area of the cross section (S12) of the end disposed on the accessible side of the anchorage being less than the area of the cross section (S11) of another portion of the cavity, the cavity comprising an opening (12) on the accessible side of the anchorage,

   - successively inserting, through the opening (12) of one end, each of the tension elements (4), each of said tension elements being made up of a traction rod (40) having a first cross-sectional area (S40) and of an end portion (41) having a second cross-sectional area (S41) larger than said first cross-sectional area (S40), and

   - filling the cavity (11) with an embedding material (50),

   this process being characterised in that,

   - the insertion of each tension element (4) consists in pushing said tension element (4) into a guide tube (30), then into the cavity (11), until its end piece (41) comes in contact with the bottom surface of the cavity (11) which presents a surface (S11) corresponding to the largest cross-sectional area of the cavity (11),

   - the insertion of a tension element (4) is repeated until a number of tension elements (4) have been installed in the same way, and

   • a new end piece (41) to be installed is not able to find its place at the bottom of the cavity (11), and that

   • for the tension element in question to play its full part later on, the end piece (41) is pushed down as far as possible in the cavity (11) until it comes up against one or more pieces (41) already installed or against the sidewall of the cavity (11),

   - the construction of the anchorage thus involving the introduction of a certain number "N" of tension elements (4) into the cavity (11), this introduction being preceded by the choice of tension elements (4) and of a cavity (11) verifying the following relations: $$\left(\mathrm{N}\mathrm{\times }\mathrm{S}\mathrm{41}\right)\mathrm{<}\mathrm{S}\mathrm{11}\mathrm{,}$$

   

   to allow a good arrangement of the end pieces (41) on the bottom of the cavity (11), and $$\left[\left(\mathrm{N}\mathrm{-}\mathrm{1}\right)\mathrm{\times }\mathrm{S}\mathrm{40}\right]\mathrm{+}\mathrm{S}\mathrm{41}\mathrm{<}\mathrm{S}\mathrm{12}$$

   

   to allow the insertion of the last tension element (4), respectively to allow the passage of the last end piece (41) into the guide tube (30), respectively into the opening (12) of cross section (S12) of the cavity (11), and $${\left(\mathrm{N}\mathrm{\times }\mathrm{S}\mathrm{41}\right)}^{\mathrm{*}}\mathrm{>}\mathrm{S}\mathrm{12}$$

   

   to allow a blockage of the tension elements (4) in the cavity (11) by preventing the exit of the mutually blocked end pieces (41) through the opening (12), (N x S41)* representing in a general way the surface generated by the casing of the bundle of the N assembled end pieces (41), each having a cross-sectional area (S41).
2. Method according to claim 1, characterised in that the cavity is made by installing a substantially oblong, tapering anchor member (1) having two ends, the area of the cross section of a first end being less than the area of the cross section of another portion of the anchor member, said anchor member being made up substantially of a wall (10) bounding a cavity (11) of a shape substantially similar to that of said member and provided with an opening (12) having a first cross-sectional area (S12) at said first end of the anchor member and comprising a bottom wall (14) at the second end, another cross section of said cavity having another area (S11) greater than the first area (S12), said anchor member (1) then being embedded or concreted in the surrounding structure (2), leaving said opening (12) free.
3. Method according to claim 1, characterised in that the construction of the cavity (11) is achieved by putting in a template capable of being dismantled and having a substantially oblong and tapering outside shape, the surrounding structure (2) then being concreted about said template, the template then being dismantled by one of its ends, leaving a cavity (11) of substantially oblong and tapering shape in the concreted surrounding structure (2), as well as an opening (12) toward said cavity, the area of the cross section of a portion (S12) of a portion of said cavity close to the opening (12) being less than the area (S11) of the cross section of another portion of said cavity.
4. Method according to claim 1, characterised in that the construction of the cavity (11) is achieved by putting in an inflatable flexible part which, once inflated, has a substantially oblong and tapering shape, the surrounding structure (2) then being concreted about said inflated part, the part then being deflated, leaving a cavity (11) of substantially oblong and tapering shape in the concreted surrounding structure (2), as well as an opening (12) toward said cavity, the area (S12) of the cross section of a portion of said cavity close to the opening (12) being less than the area (S11) of the cross section of another portion of said cavity.
5. Method according to claim 1, characterised in that the construction of the cavity (11) is achieved by boring a cavity of substantially oblong and tapering shape in the surrounding structure (2), said cavity having an opening (12), the area (S12) of the cross section of a portion of said cavity close to the opening (12) being less than the area (S11) of the cross section of another portion of said cavity.
6. Method according to one of the preceding claims, characterised in that it comprises, after the step of making the cavity (11), a step of putting in or concreting a structural element (3) to be prestressed, said structural element (3) comprising a longitudinal conduit (30) for the passage of the tension elements (4), one end of said longitudinal conduit (30) communicating (13) with the opening (12) provided in the cavity (11) of the anchorage.
7. Method as claimed in one of the preceding claims, comprising, after the step of filling the cavity (11) with an embedding material (50), a step of tightening each of the tension elements (4).
8. Method as claimed in claim 7, comprising, after the step of tightening the tension elements (4), a step of filling the longitudinal conduit (30) of the prestressed structural element (3) with a sealant (60).
9. Device for constructing an anchorage having more than one tension element, said anchorage being accessible from one side only, this device implementing

   • a member of substantially oblong shape, tapering and having two ends, the area of the cross section of a first end being less than the area of the cross section of another portion of the member, said anchor member being made up substantially of a wall (10) bounding a cavity (11) of a shape substantially similar to that of said member and being provided with an opening (12) having a first cross-sectional area (S12) at said first end of the anchor member and comprising a bottom wall (14) at the second end, another cross section of said cavity having another area (S11) greater than the first area (S12),

   • a predetermined number (N) of tension elements (4), each having an end intended to be inserted into an anchor cavity (11) of an anchorage accessible from one side only, said tension element being made up of a traction rod (40) provided at its end intended to be inserted in said cavity with an end portion (41), the cross-sectional area (S41) of which is greater than the area (S40) of the cross section of said traction rod (40),

   this device being characterised in that

   - the portion of end (41) of each tension element (4) comprises a central body (42) bounded by an upper portion (43) and a lower portion (43) <sic. (44)>, these two portions being domed or formed of inclined planes in a manner to facilitate the sliding of an end portion during installation on another end portion already installed,

   - the tension elements (4) and the cavity (11) verifying the following relations:

   • the area of the cross section (S12) of the opening (12) of the cavity (11) of the anchor member (1) is greater than the area made up of the sum of the areas of the cross-sections (S40) of (N-1) traction rods (40) added to the maximum area (S41) of the cross section (S41) of an end portion or of an end piece (41) of the tension elements (4),

   • the area of the maximum cross-section (S11) of the cavity (11) is greater than the sum of the maximum areas (S41) of the cross-sections of the end portions or of the end pieces (41) inserted into said cavity,

   • the area of the cross-section (S12) of the opening (12) of the cavity (11) is less than the area of the cross-section formed by the assembly of the areas of the cross-sections (S41) of the end portions or of the end pieces (41) inserted into said cavity.
10. Device for constructing an anchorage according to claim 9, characterised in that the anchor member has a substantially frustoconical shape.
11. Device for constructing an anchorage according to claim 9, characterised in that the anchor member has a substantially frustopyramidal shape.
12. Device for constructing an anchorage according to one of the claims 9 to 11, characterised in that the anchor member is equipped with an injection orifice (15) disposed close to its bottom wall (14), said injection orifice being connected or being able to be connected to an injection tube (16) of a liquid or semi-liquid substance (50).
13. Device for constructing an anchorage according to claim 12, characterised in that the anchor member is equipped with an outlet (17) disposed close to its opening (12), said outlet being connected or able to be connected to an exhaust tube (18).
14. Device for constructing an anchorage according to one of the claims 9 to 13, characterised in that the opening (12) is equipped with means of fastening (13) a guide tube (30) intended to contain tension elements (4) .
15. Device for constructing an anchorage according to one of the claims 9 to 14, characterised in that the anchorage is equipped with at least one peripheral widening (19) on its outer lateral surface.
16. Device for constructing an anchorage member according to one of the claims 9 to 15, characterised in that it is made at least partially of metal.
17. Device for constructing an anchorage member according to one of the claims 9 to 15, characterised in that it is made at least partially of synthetic material.
18. Device for constructing an anchorage member according to one of the claims 9 to 15, characterised in that the anchorage member is made at least partially of concrete.

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