WO1999067549A1

WO1999067549A1 - Anchoring jaw and strand anchoring device

Info

Publication number: WO1999067549A1
Application number: PCT/FR1999/001440
Authority: WO
Inventors: Pierre Jartoux
Original assignee: Freyssinet International (Stup)
Priority date: 1998-06-19
Filing date: 1999-06-16
Publication date: 1999-12-29
Also published as: AU4150699A; FR2780126B1; FR2780126A1

Abstract

The invention concerns a jaw comprising several wedges (6, 7) arranged in angular sectors about a generally cylindrical duct designed to receive the strand such that the wedges are urged to be engaged on the strand between a first zone located towards a jaw front side and a second zone located towards the jaw rear side. Each wedge has at least one portion whereof the thickness, radially measured relative to the duct, gradually increases from the jaw front towards the rear thereof. The wedges are so shaped that the jaw has substantially a rotational symmetry about the duct axis in the first zone and does not have rotational symmetry in the second zone, thereby preventing a wire layer of the strand from forming a shape retaining tube by the clamping effect.

Description

ANCHORING Jaw AND ANCHORING DEVICE OF A STRAND

The present invention relates to the techniques used to anchor cables working under tension. It finds a particular, but not exclusive, application in the field of construction of engineering structures.

Commonly, the strands constituting such cables are individually anchored by means of anchoring jaws which cooperate with anchoring bodies bearing on the structure.

In most cases, the jaw has a generally frustoconical shape, as illustrated in FIG. 1, and is divided by angular sectors into two, three or four keys 1. A cylindrical channel formed between the keys 1 receives the strand 2. The jaw, placed around the strand, is engaged in a complementary frustoconical orifice 3 formed in the anchoring body 4. When the cable is tensioned, the jaw is pushed into the frustoconical orifice in the direction of its narrowest end ( hereinafter called the front end). When the strand is released on the widest side of the jaw (rear side), the latter is pressed strongly against the strand due to the frustoconical shape of its contact surface with the anchoring body 4. This ensures the blocking of the strand, the tension of which decreases progressively from the front end towards the rear end of the jaw.

The angles of the truncated cones of the jaw and of the anchoring body, which are symmetrical in revolution about the direction of the strand, are chosen so that the clamping forces on the strand to be anchored are distributed regularly over its circumference ( Figure 2), and grow from the front side to the rear side of the jaw (Figure 3). The clamping forces are represented by the arrows F in FIGS. 2 and 3, and their distribution is illustrated by the dashed curves.

This type of anchoring is generally satisfactory for simple wires, or usual strands 2 formed of seven wires, namely a central wire and six peripheral wires twisted around the central wire (see FIG. 2). In the design of the seven-wire strand, a clearance J is left between the peripheral wires, the diameter of which is a few% less than that of the central wire. Thanks to this clearance J, it is avoided that the symmetrical tightening has a tendency to form, from the six peripheral wires, a non-deformable tube preventing the central wire from being tightened ("casing" effect).

In a certain number of applications, in particular in prestressing structures when it is desired to have elementary strands tensioned at more than 300 kN, it is necessary to use "multilayer" strands comprising more than seven wires. Multilayer strands often have many more wires (19, 25, 31, 37, 43, ...) and many more layers (3, 4, 5, 6, ...) than seven-wire strands.

Cross-sectional distributions of wires in common multilayer strands are shown in Figures 4 to 6. Figure 4 shows a "Seale" strand with 19 parallel wires (1 + 9 + 9), Figure 5 an "arrington" strand at 19 wires (1 + 6 + 12) and crossed layers, and Figure 6 a "Seale" strand with 25 parallel wires (1 + 6 + 6 + 12). The cable manufacturer endeavors to choose the assemblies of wires so as to maintain _clearances J _lf J ₂ , ... between the wires of each layer (Figure 4). But this becomes difficult when there are a lot of wires and layers, because the generally used drawn wires have a diameter which varies slightly from one wire to another and along the same wire, due to the wear of the die.

The difficulty in ensuring the clearance required between the wires of the multilayer strands makes them sensitive to the aforementioned casing effect at the level of their anchors. This results in the risk of poorly distributing the clamping forces, and therefore the resumption of the tension of the strand, between its various constituent wires. An object of the present invention is to combat this casing effect, particularly encountered with multilayer strands'.

The invention thus provides a jaw for anchoring a strand, comprising several keys arranged in angular sectors around a generally cylindrical channel intended to receive the strand so that the keys exert a clamping on the strand between a first zone situated towards a front side of the jaw and a second zone situated towards a rear side of the jaw. Each key has at least one portion whose thickness, measured radially with respect to the channel, gradually increases from the front side to the rear side of the jaw. The keys have shapes such that the jaw has substantially symmetry of revolution around the axis of the channel in the first zone and does not have symmetry of revolution in the second zone.

Engaged in an appropriate orifice of the anchoring body, which is normally symmetrical in revolution (typically a frustoconical orifice), this jaw exerts on the strand a clamping force which, in the second zone located towards its rear end, is not not evenly distributed around its circumference due to its non-symmetrical shape. The strand therefore undergoes in this second zone pinching stresses which prevent the casing effect.

On the other hand, in the first zone adjacent to the front end of the jaw, the clamping force is distributed uniformly over its circumference thanks to the symmetry of revolution. This arrangement is very advantageous because it avoids subjecting the strands of the strand to strong transverse pinching stresses in the first zone of the anchoring, where the longitudinal tension stresses are already maximum. The proposed jaw therefore combines a gradual recovery of the tension along its length with a pinch towards its rear end that is the least stressed in tension in order to avoid the casing effect. It is particularly suitable for multi-layer strands of more than seven wires.

It will be appreciated that the expression "symmetry of revolution", used to describe a jaw with multiple keys in a given zone along its axis, does not take account of the intervals naturally present between the angular sectors corresponding to the keys.

In a preferred embodiment of the jaw according to the invention, said portion of increasing thickness of each key is delimited internally by a generally shaped surface of cylindrical sector adjacent to the channel, and externally by a generally shaped surface of frustoconical sector, the shapes of frustoconical sector relating to the different keys of the jaw having the same dimensions in the first zone of the jaw, but having different half-angles at the top from one key to another.

This makes it possible to simply make the jaw by assembling keys of different shapes. The jaw can in particular be composed of two types of keys differentiated by the half-angle at the top of the shape of frustoconical sector of their external surfaces, and by an identification mark which are provided with the keys of at least one of the two types near their rear ends. The mark makes it easy to distinguish the keys during assembly, since the differences in angles are in practice quite small.

Preferably, the keys of the jaw are held together by an elastic ring engaged in a complementary groove formed coaxially with the channel on the external side of the keys near the rear end of the jaw, the ring having a cross section of substantially larger dimension. parallel to the channel only in the radial direction relative to the channel. This ring ensures simultaneous engagement of the keys in the orifice of the anchoring body when the tensioning of the strand.

It is observed that the structure of the anchoring device, which must clamp the jaw symmetrically towards its front side and asymmetrically towards its rear side (in the sense of the symmetry of revolution), could rest on the shape of the orifice of the body. anchoring, or on the constituent materials of the keys or of the anchoring body, although these solutions, which produce an equivalent technical effect, seem a priori to pose more manufacturing difficulties.

Another aspect of the present invention thus relates to a device for anchoring a strand, comprising at least one jaw and an anchoring body, the jaw comprising several keys arranged in angular sectors around a generally shaped channel. cylindrical intended to receive the strand so that the keys exert a clamping on the strand between a first zone situated towards a front side of the device and a second zone situated towards a rear side of the device. Each key has at least one portion whose thickness, measured radially with respect to the channel, gradually increases from the front side to the rear side of the jaw. The anchoring body has an orifice, at least a portion of which has a progressively increasing section from the front side to the rear side of the jaw, in order to receive and tighten radially the jaw placed around the strand. The jaw and the region of the anchor body adjacent to said orifice have substantially symmetrical constitutions of revolution around the axis of the channel in the first zone, while the constitution of the jaw and / or of said region of the anchor body does not has no symmetry of revolution in the second zone.

The differences in constitution between the first and second zones can result from differences in shape or characteristics of the materials used. For example, the Young's modulus of the material of the jaws and / or of the anchoring body can vary along the strand. As indicated, the convenience of manufacture would prefer to play essentially on the shape of the strand. In the preferred anchoring device, the jaw is therefore as described above, and the orifice of the anchoring body is symmetrical in revolution from the first to the second zone.

Other features and advantages of the present invention will appear in the following description of nonlimiting exemplary embodiments, with reference to the appended drawings, in which:

- Figure 1 is a diagram in axial section of means conventionally used to anchor a strand;

- Figure 2 is a diagram of the means of Figure 1, seen along the plane II-II, illustrating the distribution of the clamping forces on the circumference of the jaw, and Figure 3 is a diagram of the same means seen in half-section axial and illustrating the distribution of the clamping forces over the length of the jaw;

- Figures 4 to 6 are diagrams showing the transverse distributions of the son in three known types of multilayer strands;

- Figure 7 is a diagram in axial section of an anchoring device according to the invention, the two half-section planes VII-VII being indicated in Figures 8 and 9;

- Figures 8 and 9 are end views of the jaw and the strand shown in Figure 7, respectively along the planes VIII-VIII and IX-IX shown in Figure 7; - Figure 10 is a diagram similar to Figure 7 showing the distribution of the clamping forces over the length of the jaw;

- Figures 11 and 12 are views similar to Figures 8 and 9, respectively, and showing the distribution of the clamping forces on the circumference of the jaw;

- Figure 13 is a more detailed view of the jaws of Figures 7 and 10, according to the same section plane; - Figure 14 is a view from the rear end of an anchoring jaw according to the invention, illustrating two alternative embodiments of this jaw; and

- Figure 15 is a half-view in axial section of another alternative embodiment.

The multilayer strand 5, anchored by the device of FIGS. 7 to 9, is composed of more than seven wires. It can be part of a prestressing cable of a construction work. The strand 5 is anchored, for example at its two ends, by means of anchoring body 4 bearing on the structure of the structure. Such a strand can also find other applications, for example for producing suspension bridge suspensions which may require one or more anchors. An anchoring jaw is placed around the strand 5, and engaged with the latter in a frustoconical orifice of revolution 3 formed in the anchoring body.

In the example shown, suitable for a strand 5 with a diameter of the order of 20 to 35 mm, the jaw is composed of four keys each corresponding to an angular sector of approximately 90 ° around the strand. Two diametrically opposite keys 6 are of a first type, while the other two diametrically opposite keys 7 are of a second type. Each key 6,7 presents inwardly a surface in the general shape of a cylindrical sector of diameter slightly smaller than that of the strand 5. When they are assembled to form the jaw, the internal surfaces of the keys define a cylindrical channel in which place the strand.

Towards the outside, the key 6 or 7 is delimited by a surface in the general shape of a frustoconical sector, so that its thickness gradually increases from the front to the rear of the jaw. Near its front end (Figure 9), the jaw has a symmetry of revolution around the axis of the channel, the keys of the two types 6,7 presenting the same interior radius d / 2 and exterior radius 2/2. Thus, in the area closest to the front end of the jaw, the clamping forces F exerted by the keys are distributed uniformly over the circumference of the jaw as shown in FIG. 12.

In contrast, rearward end of the jaw (8), the pins 6 of the first type have an outer radius _φ χ / 2 greater than φ _2/2 wedges of the second type 7. Thus, when engaged around the strand in the frustoconical orifice 3 of the anchoring body, the external surfaces of the keys 6,7 are placed against the frustoconical surface of revolution of the orifice 3, so that the keys of the first type 6 compress more strongly the strand 5 as the keys of the second type 7, as in FIGS. 7 and 8 show it by exaggerating the distortion.

The deformation of the strand 5 which results therefrom avoids the casing effect of this strand which one would risk observing if the clamping was everywhere with symmetry of revolution. The structure of the jaw thus prevents the initiation of sliding of the layers of wires with respect to each other.

FIG. 11 shows that the tightening ₁ exerted by the keys 6 on their two sectors by 90 ° is greater than the tightening force F ₂ exerted by the keys 7 on the other two sectors by 90 °.

The distribution of the clamping forces over the length of the jaw is shown in FIG. 10. It can be seen that the clamping force gradually increases from the front end (where it has the same value F for the two types of keys 6,7 ) towards the rear end of the jaw (where the force F _χ exerted by the thickest keys 6 is greater than the force F ₂ exerted by the least thick keys 7).

Having the same force value F near the front end of the jaw avoids strongly pinching the strand 5 in this most sensitive area, where it has still the full value of its tension, which can be higher than 300 kN.

The jaw, shown diagrammatically in FIGS. 7 to 12, is shown in more detail in FIG. 13. As is usual, the interior surfaces of the keys 6,7 may have grooves 8 transverse to the axis of the channel receiving the strand, in order to grip it well. It will be noted that it could also be provided to have inserts on the inner surface of the keys, inserts formed from a material different from that of the keys as described in French patent 2,708,017. At the front end of the jaw, the interior surfaces keys 6,7 are preferably slightly flared towards the outside, at an angle γ with respect to the axis of the cylindrical channel, which improves the progressiveness of the clamping at the front of the jaw.

The half-angles at the apex α ₁ , α ₂ of the external surfaces of the keys 6,7 in the form of a frustoconical sector are such that α ₁ > α ₂ . This simple difference in angle ensures that, starting from the symmetrical shape of revolution of the jaw at its front end, its rear end will have the required asymmetry.

As an example, we can take α ₁ = 5 ° 30 'and α ₂ = 5 °. This dimensioning is suitable, for example, for a jaw of active length 75 mm used to anchor a multilayer strand with a diameter of 25 mm, the cylindrical channel formed between the keys having a diameter d = 24 mm. The outside diameter of the jaw at its front end being for example φ = 28 mm, its outside diameter at the rear end is then φ ₁ = 42.44 mm for the keys 6 of the first type and φ ₂ = 41.12 mm for keys 7 of the second type.

This numerical example shows that the difference of dimensions between the two types of keys is relatively small (φ _1/2-φ _2/2 = 0, 68 mm). For better differentiate the keys 6,7 from the two types during their assembly, the keys 7 are provided with one of the two types with an identification mark 9 such as a notch near its rear end. We could also provide the two types of keys with different marks.

FIG. 13 also shows an elastic ring 10 which holds the assembled keys together. This ring 10 is made from a curved elastic ring. Its cross section, visible in Figure 13 is in the form of a flattened rectangle, its dimensions being significantly larger parallel to the channel receiving the strand than radially with respect to this channel. Thanks to this shape, the ring 10 can be elastically separated to be engaged in the complementary groove 11 formed coaxially with the channel on the outside of the keys 6,7 in the vicinity of their rear end, and it offers a shear strength when the keys 6,7 are pressed into the frustoconical orifice 3 of the anchoring body. This shear resistance prevents the thinner keys 7 from entering the orifice 3 faster than the other keys 6, which would tend to attenuate the difference in tightness near the rear end and to move it towards the front end. .

The manufacture of the keys 6,7 can call upon lathe machining techniques with subsequent cutting into sectors (milling saw or band saw), foundry with lost wax or "lost foam", or even sintering.

Figures 14 and 15 show alternative embodiments of anchoring jaws according to the invention, well suited for strands of relatively large diameter, for example from 35 to 100 mm.

The right part of FIG. 14 shows the case of a jaw with six keys 12-14, and the left part the case of a strand with eight keys 22-25. When the number of keys increases, one can easily modulate the shape distortion of the strand layers by playing on the shapes or the arrangements of the different types of keys. In the preceding examples, we have considered anchoring jaws whose general shape corresponds to a truncated cone. It will be noted, however, that other forms of keys of gradually increasing thickness from front to rear could be used, the orifice of the anchor body having correspondingly a growing section from front to rear . The external surface of the keys could for example be concave, and the wall of the orifice 3 convex, seen in section along a radial plane.

On the other hand, there could be more than one portion of progressively increasing diameter per key. FIG. 15 shows such an embodiment, in which each key 16 comprises three successive frustoconical sections 17,18,19 from its front end towards its rear end, with recesses between these sections. This arrangement makes it possible to reduce the longitudinal stiffness of the jaw, as well as its radial size.

The progressiveness of the tightening can be improved by taking respectively for the frusto-conical sections 17,18,19 half angles at the top α, α ', α "decreasing from the front end towards the rear end of the key (α>' > "). To achieve the radial compression distortions of the jaw so as to avoid the casing effect, one can differentiate each of the angles α, α ', "from one key to another. One can also keep the same angle α for the sections 17 closest to the front end of the jaw, in order to obtain the symmetry of revolution of the clamping over the length of this section 17 if this length is not too great.

Claims

1. Jaw for anchoring a strand, comprising several keys (6,7; 12-14; 22-25; 16) arranged in angular sectors around a generally cylindrical channel intended to receive the strand (5) so that the keys exert a clamping on the strand between a first zone situated towards a front side of the jaw and a second zone situated towards a rear side of the jaw, in which each key has at least one portion whose thickness, measured radially relative to the channel, gradually increases from the front side to the rear side of the jaw, characterized in that the keys (6,7; 12-14; 22-25; 16) have shapes such that the jaw has substantially a symmetry of revolution around the axis of the channel in the first zone and does not exhibit symmetry of revolution in the second zone.

2. Anchoring jaw according to claim 1, wherein said portion of increasing thickness of each key (6,7) is delimited internally by a generally shaped surface of cylindrical sector adjacent to the channel, and externally by a shaped surface general of frustoconical sector, and in which the forms of frustoconical sector relating to the different keys of the jaw have the same dimensions (d, φ) in the first zone of the jaw, but present different half-angles at the top (a _lr CL ₂ ) from one key to another.

3. Anchoring jaw according to claim 2, composed of two types of keys (6,7) differentiated by the half-angle at the top (α ₁ , α ₂ ) of the shape of frustoconical sector of their outer surfaces, and by an identification mark (9) with which the keys (7) of at least one of the two types are provided near their rear ends.

4. Anchoring jaw according to any one of claims 1 to 3, in which the keys (6,7; 12-14; 22-25; 16) are held together by an elastic ring (10) engaged in a groove complementary (11) formed coaxially with the channel on the outside of the keys near the rear end of the jaw, the ring having a cross section of dimension substantially larger parallel to the channel than in the radial direction relative to the channel.

5. Anchoring jaw according to any one of claims 1 to 4, for anchoring a strand (5) formed by the assembly of more than seven wires.

6. Device for anchoring a strand, comprising at least one jaw and an anchoring body (4), the jaw comprising several keys (6,7; 12-14; 22-25; 16) arranged by angular sectors around a generally cylindrical channel intended to receive the strand (5) so that the keys exert a tightening on the strand between a first zone situated towards a front side of the device and a second zone situated towards a rear side of the device , in which each key has at least one portion, the thickness of which, measured radially with respect to the channel, increases progressively from the front side to the rear side of the jaw, and in which the anchoring body (4) has a hole (3 ) at least a portion of which has a progressively increasing section from the front side towards the rear side of the jaw in order to receive and tighten radially the jaw placed around the strand, characterized in that the jaw and the region of the anchoring body adjacent to said orifi this (3) have substantially symmetrical constitutions of revolution around the axis of the channel in the first zone, while the constitution of the jaw and / or of said region of the anchoring body does not exhibit symmetry of revolution in the second zoned.

7. Device according to claim 6, in which the Young's modulus of the material of the keys (6,7; 12-14;

22-25; 16) of the jaw and / or of the anchoring body (4) varies along the channel receiving the strand (5).

8. Device according to claim 6, wherein the jaw conforms to any one of claims 1 to 5, and the orifice (3) of the anchoring body (4) is symmetrical of revolution from the first to the second zoned.

9. Device according to any one of claims 6 to 8, in which the strand is a multilayer strand (5) subjected to a tension greater than 300 kN beyond the front end of the device.