GB2157339A - Cable anchorages - Google Patents

Abstract

A tensioned cable of generally parallel elongate tensioned elements 22 is anchored in a concrete member 21 by an anchorage 20 having an anchorage body 28 with a passage through which said cable extends, and an anchoring plate 31 on said body having a plurality of bores 32 receiving said strands individually with anchoring wedges 33 anchoring the strands to said anchoring plate. To prevent or reduce damage to the strands through contact with each other and the anchorage body, especially under cyclic loading, there is a ring 36 axially spaced from said anchoring plate and surrounding the cable so as to contact the radially outermost strands, the strands splaying out so as to diverge from one another as they extend from said ring to said anchoring plate, and a deflector 39 between said ring and said anchoring plate, at least some of said elongate elements contacting the deflector so as to be radially located thereby. The ring and defector may be of plastics. <IMAGE>

Description

SPECIFICATION Cable anchorages This invention relates to cable anchorages and in particular to an anchorage for a'tensioned cable in the form of a plurality of generally parallel elongate tensioned elements, said anchorage serving to anchor the cable in a structural member. The invention is especially applicable to a cable stay in a cable-stayed structure where there is a particular need for an anchorage which can withstand cyclical loading, but is in principle applicable to other anchorages of other cables, e.g. prestressing cables. The invention also extends to a concrete structure having a tensioned cable anchored to it by an anchorage.

The tensioned cable with which the invention is concerned consists of a plurality of elongate tensioned cable elements, each element being for example a wire or a strand (a strand is a multi-wire element with usually 7 or 1 9 wires, one wire acting as a core with the others helically wound around it). For simplicity the term "strand" is generally used in this description, but this includes all other cable elements. The anchorage has an anchoring plate with bores to receive the cable elements, anchoring devices to anchor the elements to the plate and an anchorage body having a passage in which the elements diverge from each other to reach the bores of the anchoring plate.

British patent application no. 84.26506 (to be published on 30th May 1 985 under no.

GB-A-2,14 & 351) describes the use of a ring-shaped deflector element inside the anchorage to prevent or reduce damage caused by rubbing of the strands against each other and against the side of the anchorage, under cyclic loading. This is also discussed in GB-A-2095302.

The invention aims to provide further improvement to prevent or reduce such damage due to rubbing.

There is now proposed a deflector element arrangement having a first deflector element in the form of a ring so as to obstruct contact between the anchorage body and the strands at the point of first divergence of the strands.

A second deflector is located in the anchorage body between the first deflector element and the anchoring plate, and spaced from both of them, and serves to locate at least some of the strands radially. Preferably the second deflector is about midway between the first deflector element (ring) and the anchoring plate. The second deflector may be supported on leges from the anchor plate, or mounted on the wall of the anchorage body. At least some of the strands pass through the second deflector element, but the radially innermost may not be deflected by it and the radially outermost may not contact it.

With this arrangement of the deflector elements the first element is a ring passing around all the strands while the second element may be a disc or plate which, by locating the strands as they pass through it, determines the positions of the strands as they pass through the ring. In this way the deflector ring (first element) prevents the strands from rubbing against the anchorage under cyclic loading, while the deflector plate (second element) prevents them from rubbing against each other. It is not necessary for all the strands to pass through the second deflector in order to achieve this result. The outermost strands may pass outside the second deflector, running direct from the ring to the anchor plate. With a normal trumpet-shaped anchorage these outermost strands diverge within it, pressing outwardly against the deflector ring and away from the other strands.All the other strands, by contrast, should not press outwardly at the ring, to avoid contact with the outermost ones. All these other strands pass through the second deflector, which radially locates them so that they splay out from each other within the anchorage body without twisting of the whole cable and in a controlled manner. The outermost strands may alternatively also pass through the second deflector.

Two embodiments of the invention, given by way of non-limitative example, will now be described with reference to the accompanying drawings in which: Figure 1 shows schematically an anchorage for a cable stay in longitudinal axial section, and Figure 2 shows a second anchorage of a cable stay again in longitudinal axial section.

The anchorage of Fig. 1 has a body 1 which rests by means of a nut 5 screwthreadedly engaging its periphery on a bearing ring 7 embedded in a concrete member 8 and extends part way into a tunnel 9 through which runs the cable, composed of a plurality of strands 11 (only two are shown, for clarity).

Abutting the end of the anchorage body 1 is an anchoring plate 1 3 in which the strands are individually gripped by split conical wedges 1 3a in bores.

At the narrow end of the anchorage 1 a deflector ring 1 23 passes around all the strands 11 and fits against a shoulder of the anchorage body 1. The ring 1 23 is of a material softer than the strands, preferably a plastics material, so that the strands are not damaged by contact with it. Within the anchorage 1 there is a deflector disc or plate 1 25 which is supported on the anchor plate 13 of the anchorage by legs 127.

If the deflector plate 1 25 were absent, the strands 11 would bend at the ring 1 23 so as to deviate from running parallel in the cable and diverge within the anchorage body 1 as they extend to the anchoring plate 1 3. The strands would therefore press outwardly at the point of bending, the outermost strands 11 a pressing against the ring 1 23 and the other strands each pressing against the next strands outwards. Under cyclic loading the strands would rub excessively against one another and some could be damaged.

The effect of the deflector plate 1 25 is to divert the paths of most of the strands 11 so as to avoid this potentially damaging contact between them at the ring 1 23. The outermost strands 11 a do not pass through the plate in this embodiment, but pass beside it with their paths not diverted. The strands 11 a continue to press against the ring 1 23 therefore, but are not damaged thereby as the ring is relatively soft.

The remaining strands 11 pass through the plate 125, which diverts their paths, extending the parallel portion so that the strands turn and splay out at the plate 1 25. Thus at the ring 123 the strands 11 do not press on each other.

The holes through the plate 125, through which the strands 11 pass, can be of various shapes, for example there may be one hole per strand or there may be substantially ringshaped holes with a number of the strands passing through each hole. However, the arrangement should be such that as the strands 11 bend at the plate 1 25 they do not press against each other but only against the material of the plate. Like the ring 123, the plate 1 25 is made of a softer material than the strands, preferably a plastics material, and so it does not damage the strands if they rub against it under cyclic loading.

In an alternative embodiment, the outermost strands 11 a also pass through the plate 1 25. In this case the outermost wires 11 a may run parallel between the two deflector elements 123, 1 25 or they may splay out.

Fig. 2 shows, like Fig. 1, an anchorage 20 of the invention mounted in a concrete structure, in this case a vertical member 21 of a cable-stayed structure. The anchorage 20 anchors in the member 21 one end of a cable stay in the form of a bundle of strands 22, not all of which are shown for the sake of clarity. The stay extends within an inner sheath 23 in an outer tube 24 which forms the inner wall of the oblique aperture 25 through the concrete member 21 in which the anchorage is located.

A large nut 27 bears against a load-bearing plate 26 welded to the outer tube 24 at one end of the aperture 25 and engaging an oblique face 21a of the member 21. The nut 27 screw-threadedly engages an anchorage body or trumpet 28. The latter has a generally tapering inner wall 29 and is secured at one end 30 to the sheath 23. To its other end is secured an anchor plate 31 having a plurality of bores 32 suitably shaped to receive the strands 22 individually and to receive conventional split conical gripping wedges 33 which grip the strands so as to transfer their stress to the plate 31 and hence via the anchorage body 28, the nut 27 and the plate 26 to the concrete member 21. A thin-walled sleeve 34 secured to the rear face of the anchor plate 31 and closed by an end cap 35 protects the ends of the strands projecting through the plate 31.The strands are tensioned by pulling them through the anchor plate 23 using a suitable jack. The plate 31 has a central aperture 31a.

Within the sheath 23 the tensioned strands 22 lie parallel to each other, in contact or close together. Within the anchor body 28 they diverge to become spread out from each other in order to enter the individual bores 32 in the plate 31. At the narrow end of the body 28, a ring 36 of softer material than the strands, preferably a plastics material, is located against a shoulder 37 on the inner face of the body 28 and held in place by a grub screw 38. The ends of the inner face of the ring 36 are chamfered. The ring 36 surrounds the bundle of strands and contacts the outer ones so as to prevent any strands contacting the anchoring body 28 at its narrow end.

About half-way along the anchoring body 28 between the ring 36 and the anchoring plate 31 is a further locating member for the strands, in the form of a deflector plate or disc 39 also of softer material than the strands, preferably a plastics material, and seated against a shoulder 40 on the inner face of the body 28 and held in place by a grub screw 41. The locating disc 39 has a plurality of bores 42, which may be of slightly chamfered form at their ends, through which the strands pass. The strands are thus located by the disc 39 so that they diverge from each other in a controlled manner and reach the bores 32 of the anchoring plate 31 without excessive bending and without risk of contact with the anchoring body 28.

All of the strands may be located in individual bores 42 of the disc 39. Alternatively, as shown, the radially innermost strands, which are not required to bend within the anchoring body 28, pass through a central larger aperture 43 in the disc 39.

The arrangement shown in Fig. 2, like that of Fig. 1 allows accurate fensioning of the cable and also if required adjustment of individual strands. Particularly in the case of an anchorage subjected to cyclic loading, risk of damage to the strands by rubbing against each other or against the anchoring body is minimized.

The present invention may be used in conjunction with the developments proposed in GB-A-2,148,351 mentioned above.

Claims (9)

1. An anchorage for a tensioned cable in the form of a plurality of generally parallel elongate tensioned elements, said anchorage serving to anchor the cable in a structural member and having a) an anchorage body mounted on said structural member and having a passage through which said cable extends generally axially, b) an anchoring plate mounted at one end of said passage and having a plurality of bores receiving said elongate elements individually, c) anchoring devices anchoring said elongate elements to said anchoring plate, d) a ring located in said passage axially spaced from said anchoring plate and surrounding said cable so as to contact the radially outermost ones of said elongate elements, the elongate elements splaying out so as to diverge from one another as they extend from said ring to said anchoring plate, e) a deflector located in said passage between said ring and said anchoring plate and axially spaced from both, at least some of said elongate elements contacting the deflector so as to be radially located thereby.

2. An anchorage according to claim 1 wherein said radially outermost ones of said elongate elements do not contact the deflector.

3. An anchorage according to one of claims 1 and 2 wherein said deflector is a disc having a plurality of apertures through which at least some of said elongate elements pass.

4. An anchorage according to one of claims 1 and 2 in which said deflector is mounted on said anchoring plate by at least one support member extending from the anchoring plate.

5. An anchorage according to claim 1 wherein said deflector is mounted on the wall of said anchorage body bounding said passage.

6. An anchorage according to any one of claims 1 to 5 wherein said ring and said deflector are of plastics material.

7. An anchorage for a tensioned cable substantially as herein described with reference to and as shown in Fig. 1 of the accompanying drawings.

8. An anchorage for a tensioned cable substantially as herein described with reference to and as shown in Fig. 2 of the accompanying drawings.

9. A concrete structural member having a tensioned cable anchored in it by means of an anchorage according to any one of claims 1 to 8.