US3333819A

US3333819A - Wire tensioning jack having alignment means

Info

Publication number: US3333819A
Application number: US507890A
Authority: US
Inventors: Brandestini Antonio
Original assignee: Individual
Current assignee: Individual
Priority date: 1964-11-17
Filing date: 1965-11-15
Publication date: 1967-08-01
Anticipated expiration: 1984-08-01
Also published as: GB1123233A

Description

Aug. 1, 1967 A. BRANDESTINI WIRE TENSIONING JACK HAVING ALIGNMENT MEANS 5 Sheets-Sheet 1 Filed Nov. 15, 1965 1967 A. BRANDESTINI 3,333,819

WIRE TENSIONING JACK HAVING ALIGNMENT MEANS Filed Nov. 15, 1965 3 Sheets$heet z:

x 8 274 1a L 1 r. M 1 1 Q zm/ 2 5 p 33 j I v I 31 M l T 5: E a}; 529 1 j 1967 A. BRANDESTINI 3,333,819

WIRE TENSIONING JACK HAVING ALIGNMENT MEANS Filed Nov. 15, 1965 3 Sheets-Sheet Fig.3

United States Patent 3,333,819 WIRE TENSIONING JACK HAVING ALIGNMENT MEANS Antonio Brandestini, 60 Alte Landstrasse, Kusnacht, Zurich, Switzerland Filed Nov. 15, 1965, Ser. No. 507,890 Claims priority, application Great Britain, Nov. 17, 1964,

17 Claims. (Cl. 254-29) This invention relates to hydraulic jacks, and in particular to hydraulic jacks adapted to prest-ress tensioning wires located in ducts within the structure of concrete pressure vessels.

Trends in the development of pressurised concrete vessels, for use particularly in the field of nuclear power generation, require the prestressing of vessels of pseudospherical shape, whose outer surface has a multiple step formation. These vessels are of large size, with diameters of the order of 100 feet. A further application of such vessels is in the storage of gases, or of liquefied gases, under pressure.

The prestressing of such vessels is conveniently achieved by means of tensioned wires, a proportion of which are located within ducts extending between upper and lower faces of the pressure vessel.

Such ducts may lie in a curved path, and may be of a total length of the order of 100 ft. In order to satisfactorily prestress a set of wires located within such a duct, it may be necessary to apply simultaneous tensioning force at either end of the duct. In some cases satisfactory results may be achieved by attachment of the wires at one end of the duct prior to tensioning the wire bundle at the other end. According to which method is to be used, either two jacks or a single jack will be required.

In view of the large size of modern concrete pressure vessels and the high interior pressures required to be withstood, the total number of prestressing ducts is large and their layout is complex.

Any means by which the total tension achieved within a single duct can be maximised affords a reduction in the required number of ducts and thus reduces the construction cost of the vessel.

It is a main object of the present invention to provide a' prestressing jack permitting the application of large tensile forces, for example, of the order of magnitude of 800 tons, in tensioning wires and sets of such wires.

It is another main object of the invention to provide alignment means whereby a pullrod of such a jack may be aligned with an anchor head to which one end of the wires within a wire duct are attached.

These and further objects of the invention will be understood from the following description, with reference to the accompanying drawings, in which:

FIGURE 1 is a part ghosted section of a tensioning jack in location adjacent one end of a wire duct of a concrete pressure vessel.

FIGURE 2 shows in detail coupling means between the pull-rod of the jack-and an anchor head of a set of prestressing wires.

FIGURE 3 is an end view of the attachment face of the jack, showing retaining chocks placed in position following a prestressing operation.

In FIGURES l and 2, the jack is shown in engagement with an anchor head 1 forming part of a prestressing wire system, the anchor head receiving the ends of a number of stressing wires 2 located within a duct 3 in a concrete pressure vessel 4. The duct 3 may contain, for example, 163 wires each of 7 mm. diameter.

A wire button 5 (FIG. 2) is integrally formed at the ends of each wire so that the wire is retained in position within the anchor head when tension is applied to the wires by the jack;

The jack includes a body portion 6 within which a pull-rod 7 is arranged for forward or rearward sliding motion with respect to the concrete vessel 4. Annularly arranged about the pullrod 7 is a hydraulic piston 8 sliding within an annular cylinder. Hydraulic fluid may be fed under pressure through ducts 9, 10 to cylinder regions 11, 12 located to either side of the piston 8. Rea-rward movement of the pullrod is achieved by pressurisation of region 12, and forward movement of the pullrod by pressurisation of region 11. Fluid pressures up to approximately 11,000 p.s.i. may be used. The movement of the piston is communicated to the pullrod through the annular ram 13 and the thrust limiter assembly 14.

At the forward end of the jack body 6 is attached a trestle 15 into which a forward enlarged portion 16 of the pullrod projects. Bolts 15a serve to secure the trestle to the sack body. The trestle 15 is a partly open structure permitting access to the forward end face of the pullrod and to a faceplate 17 attached thereto by bolts 18. The forward extremity of the trestle has integrally cast lugs 19 by means of which the jack is attached, by keyed studs 20, to a bearing block assembly 21 cast into the pressure vessel 4 around the end of the duct 3. The bearing block assembly 21 comprises a containing coil 22 of high tensile wire, a bearing plate 23, -a duct end section 24, and four fishtail anchorages 25 set in high strength concrete 26 of compressive strength, for example, 5,000 1b./sq. in. This high strength concrete extends between the containing coil 22 and a duct wall 27. The concrete forming the main body of the pressure vessel 4 may have a compressive strength of, for example, 1,000 lb./sq. in.

In pretensioning operation of the jack, the bearing block 21 is required to take and distribute a compressive loadof up to 800 tons. The weight of the jack itself, which may be hung vertically beneath an under face of the pressure vessel 4, is 3 /2 tons.

Returning now to a consideration of the anchor head 1, this is seen in FIGURE 2 to comprise a multi-holed threaded core 27A receiving the tensioning wires from the duct, and a core sleeve 28, screw threaded on its inner and outer surfaces and screwed about the core 27. In view of the high lateral loads to which the screw threads of the anchor head are subjected, these threads are of buttress-section.

In order to effect concentric alignment of the anchor head 1 and the pullrod portion 16 of the jack prior to wire tensioning, a centering disk 29 as attached by bolts 30 to the anchor head 1 and its angular location on the anchor head is fixed by a pin 57 entering a location hole 57A in the sleeve 28 of the anchor head. The centering disk 29 is dished at its face adjacent the anchor head to permit accommodation of the wire buttons 5 within the recess so formed. A centering stud 31 projects centrally from the rear face of the disk 29, and the shape of the stud 31 matches with a recess 32 in the end of the pullrod portion 16. The recess 32 connects into a central bore 33 extending the length of the pullrod. At its forward part the recess has a ring 32a that engages the stud for positively centering the latter.

The forward enlarged portion 16 of the pullrod is screw threaded with buttress-section threads 36 and carries a coupling sleeve 34 having a corresponding interior screw thread in its rearward portion. A forward portion of the sleeve 34 is of greater internal bore and has an interior buttress screw thread 35 adapted to receive the screwed anchor head 1 prior to tensioning of the wires located therein. Before such engagement of the coupling sleeve is made, the sleeve is located in a rearward posi- 3 tion upon the pullrod enlarged portion 16, as shown in chain lines in FIGURE 1. The coupling sleeve has blind holes 37 drilled in its outer surface to receive a tommy bar for the purpose of screwing the sleeve 34 manually upon the pullrod portion 16.

The thrust limiter assembly 14 located towards the rear of the pullrod 7 comprises a counter nut 38 screwed upon the pullrod and an annular faceplate 39 bolted to the ram 13 and slidable upon the counter nut 38. A ring 40 is located against a step formed in the outer surface 'of the counter nut, and coil springs 41 in tension are located between the ring 40 and faceplate 39, being seated in recesses formed in these elements. The ends of the springs are positively held within these recesses by means of formed bushes 42 and holding bolts 43. The effect of the thrust limiter assembly 14 is to transmit rearward movement of the ram 13 directly to the pullrod 7 through the annular faceplate 39 bearing on the counter nut 38. In the case of forward movement of the ram, however, the thrust transmitted to the pullrod is limited by the strength of the springs 41, any extension of which results in separation of the faceplate 39 and counter nut 38.

In order to measure the tensile stress applied during tensioning of a wire bundle, a stress gauge 42A is located at the rearward end of the pullrod 7. The principle of operation of the stress gauge is to detect small variations in length of a portion of the pullrod when under tensile stress, and to magnify such variations and indicate them in terms of applied tensile stress in the strain gauge 42A.

To this end, two

collets

43A and 44 are held in a press fit within the central bore 33 of the pullrod, at a known separation from each other. The collet 43A is attached to a tube 45 passing through the bore 33 to the stress gauge 42A. The other collet 44 is attached to a rod 46 passing through the tube 45 likewise to the stress gauge. Differential movement of the tube 45 and rod 46 is detected, magnified and indicated by the stress gauge in terms of pullrod stress in tons.

The operation of the jack may be considered from an initial state in which the jack is attached by the studs 20 to the vessel 4, the unstressed tensioning wires 2 are in position in the duct 3, their end portions being located within the core of the anchor head 1 and a button having been formed at the end of each wire 2. The centering disk 29 has been bolted to the anchor head 1 with the pin 57 in its locating hole.

The anchor head 1 will lie close to the bearing plate 23 of the bearing block 21, but will not necessarily be concentric with the duct 3 since the wire bundle is loose in the duct.

The pullrod of the jack is, however, concentric with the duct and it is therefore necessary, before the coupling sleeve 34 of the jack can be engaged with the anchor head 1, to align these two latter components. It is further necessary to ensure continuity of the threads on the coupling sleeve and on the anchor head, so that when the sleeve is screwed outwardly from the pullrod towards the centered anchor head, the threads on these two elements will mate. In view of the size and weight of the components involved, centering and continuity means are provided in the jack to effect these alignments.

Thread continuity is ensured by adjustment of the angular position of the pullrod before coupling up to the anchor head. On the outer peripheries of the faceplate 17 and of the centering disk 29 are

indented marks

47, 48, so located that when the faceplate and centering disk are in contact one with the other and the peripheral marks are matched to each other in angular position, the screw threads of the coupling sleeve will mate with those on the anchor head when the coupling sleeve is screwed forward to meet it. To permit rotation of the pullrod to its correct angular position for alignment of the

marks

47, 48, blind holes 49 are provided in the rear exposed portion of the pullrod, for acceptance of a tommy bar.

Centering of the anchor head and coupling sleeve is effected by means of the centering stud 31. The pullrod is advanced towards the anchor head while the coupling sleeve 34 is in its retarded position on the pullrod, as shown in chain lines in FIGURE 2. The centering stud 31 of the centering disk 29 then enters the shaped recess 32, and when the faceplate 17 is in contact with the rear face of the centering disk 29, the stud 31 is positively located in the recess 32 to ensure concentricity of the pullrod and anchor head. The strength of the springs 41 in the thrust limiter assembly 14 is arranged to ensure precise abutment of the faceplate and centering disk while limiting the resulting draw force on the attachment studs 20 to an acceptable value.

Having now satisfied the three conditions necessary for engagement of the-coupling sleeve with the anchor head, viz., angular alignment of the pullrod and anchor head, face to face contact of the faceplate 17 and centering disk 29, and engagement of the stud 31 in the shaped recess in the pullrod, the coupling sleeve 34 can be screwed forward by means of a tommy bar in the blind holes 37 until the sleeve is in its fully advanced position, shown in full lines in FIGURE 1, and in screwed engagement with the anchor head 1. The jack is now ready for the tensioning operation, which in a typical example will be to tension a bundle of 163 wires within a duct to an overall prestress of 660 tons. In this operation the anchor head will be drawn outwards from the duct entrance to the position shown in FIGURE 1. While in this position two chocks are placed between the anchor head 1 and bearing plate 23 of the bearing block 21 before releasing the pull of the jack. FIGURE 3 shows the

chocks

50, 51 in position. Each chock is of substantially semiannular shape, with a metal strip 52 bolted thereto to form two

location arms

53, 54. When in position, the

location arms

53, 54 rest against end portions 55 of the jack trestle 15, which portions include the lugs 19. Each chock further comprises a handle 56 welded to the strip 52.

After release of the jack pressure, the load due to the prestressed wires 2 is taken by the

chocks

50, 51 and the coupling sleeve 34 is screwed backwards out of engagement with the anchor head. The jack is then independently supported by lifting tackle or otherwise prior to its disengagement from the attachment studs 20 and removal of the jack from the concrete vessel. The studs 20 may then be unscrewed from the fishtail anchorages 25.

Depending on the particular prestressing application, the jack may be used singly to prestress a wire bundle from one end of a duct, or by a pair of jacks one at either end of the duct to permit a more even application of stress throughout the duct.

The invention is not limited to jacks intended for use in connection with the particular wire anchorage system described in the example.

I claim:

1. A hydraulic jack for stressing tensioning wires of prestressed constructions, including a housing, a rigid member slidably mounted within said housing defining an axis and first and second ends along said axis, said first and second ends each including a face; hydraulic means mounted within said housing for sliding said member along said axis in a first direction to advance said first end and in an opposite direction to withdraw said first end, which latter movement stresses the tensioning wires; a hollow coupling means, defining first and second ends and interiorly threaded at either end, for engaging, at said first end of the coupling means, the threads of the first end of said member and for engaging, at said second end of the coupling means, an anchor head for securing the tensioning wires; centering means for aligning said rigid member with an anchor head; and thread continuity means whereby, when said member and an anchor head are in alignment, the threads at said second end of the coupling means may be engaged with screw threads of an anchor head when the coupling means is partly unscrewed off the threads of the first end of said member,

said centering means including a central recess in the face of said first end of said rigid member extending along said axis, and a centering plate for rigid attachment to an anchor head and having a centering stud that is to be received by said central recess.

2. The jack of claim 1, wherein said recess diverges in the direction of said front end.

3. The jack of claim 1, wherein said recess includes means for engaging said stud and positively centering the latter.

4. A hydraulic jack for stressing tensioning wires of prestressed constructions, including a housing, a rigid member slidably mounted within said housing defining an axis and first and second ends along said axis, said first and second ends each including a face; hydraulic means mounted within said housing for sliding said member along said axis in a first direction to advance said first end and in an opposite direction to Withdraw said first end, which latter movement stresse the tensioning wires; a hollow coupling means, defining first and second ends and interiorly threaded at either end, for engaging, at said first end of the coupling means, the threads of the first end of said member and for engaging, at said second end of the coupling means, an anchor head for securing the tensioning Wires; centering means for aligning said rigid member with an anchor head; and thread continuity means whereby, when said member and an anchor head are in alignment, the threads at said second end of the coupling means may be engaged With screw threads of an anchor head when the coupling means is partly unscrewed off the threads of the first end of said member, means for measuring the tensile stress applied to the tensioning wires by said rigid member, said rigid member having an opening therein extending parallel to said axis and terminating at the face of the second end of said rigid member; a stress gauge in communication with the terminus of said opening at the face; a first rod within said opening secured at its one end to said rigid member at a first position and at its other end operatively connected to said stress gauge; a second rod within said opening secured at its one end to said rigid member at a second position intermediate said first position and the face of the second end of said rigid member and at its other end operatively connected to said stress gauge, whereby the difference in elongation of the two rods is detected by said stress gauge and indicated thereon as stress applied by said rigid member.

5. The jack of claim 4 wherein said second rod is hollow and said first rod is located within said second rod for the length of their common distance. i

6. A hydraulic jack for stressing tensioning wires of prestressed constructions, including a housing, a rigid member slidably mounted within said housing defining an axis and first and second ends along said axis, said first and second ends each' including aface; hydraulic means mounted within said housing for sliding said member along said axis in a first direction to advance said first end and in an opposite direction to withdraw said first end, which latter movement stresses the tensioning wires; a hollow coupling means, defining first and second ends and interiorily threaded at either end, for engaging, at said first end of the coupling means, the threads of the first end of said member and for engaging, at said second end of the coupling means, an anchor head for securing the tensioning wires; centering means for aligning said rigid member with an anchor head; and thread continuity means whereby, when said member and an anchor head are in alignment, the threads at said second end of the coupling means may be engaged with screw threads of an anchor head when the coupling means is partly unscrewed off the threads of the first end of said member, means for limiting the thrust transmitted by said hydraulic means to said rigid member during forward movement thereof.

7. The jack of claim 6, including first means mounted on said rigid member and spaced from the front end thereof and defining a surface facing the front end of said rigid member; second means normally bearing on said surface; resilient means connecting said second means with said first means; and third means rigidly connected to said second means and moved by said hydraulic means.

8. The jack of claim 7, wherein the rear end of said rigid member is threaded, and said first means is a counter nut threaded on said rear end; fourth means located between said second means and the face of the rear end of said rigid member, said fourth means supported by said counter nut to prevent movement thereof towards the front end of said rigid member; and said resilient means comprise a plurality of tension springs connected between said second and fourth members.

9. A hydraulic jack for stressing tensioning wires of prestressed constructions, including a housing; a rigid member slidably mounted within said housing defining an axis and first and second ends along said axis, said first and second ends each including a face; hydraulic means mounted Within said housing for sliding said member along said axis in a first direction to advance said first end and in an opposite direction to withdraw said first end, which latter movement stresses the tensioning wires; a hollow coupling means, defining first and second ends and interiorily threaded at either end, for engaging, at said first end of the coupling means, the threads of the first end of said member and for engaging, at said second end of the coupling means, an anchor head for securing the tensioning wires; centering means for aligning said rigid member with an anchor head; and thread continuity means whereby, when said member and an anchor head are in alignment, the threads at said second end of the coupling means may be engaged with screw threads of an anchor head when the coupling means is partly unscrewed olf the threads of the first end of said member, said hydraulic means including first and second hydraulic chambers for forward and rearward movement, respectively, of said rigid member; a piston separating said first and second chambers and mounted for movement therein; a ram in rigid connection with the piston for moving said rigid member, means for limiting the thrust transmitted by said ram to said rigid member during forward movement thereof.

10. The jack of claim 9, including a first means mounted on said rigid member and spaced from the front end thereof and defining a surface facing the front end of said rigid member; a second means normally bearing on said surface and rigidly connected to said ram; resilient means connecting said second means with said first means.

11. The jack of claim 10, wherein the rear end of said rigid member is threaded, and said first means is a counter nut threaded on said rear end; fourth means located between said second means and the face of the rear end of said rigid member, said fourth means supported by said counter nut to prevent movement thereof towards the front end of said rigid member; and said resilient means comprise a plurality of tension springs connected between said second and fourth members.

12. A hydraulic jack for stressing tensioning wires of prestressed constructions, including a housing; a rigid member slidably mounted within said housing defining an axis and first and second ends along said axis, said first and second ends each including a face; hydraulic means mounted within said housing for sliding said member along said axis in a first direction to advance said first end and in an opposite direction to withdraw said first end, which latter movement stresses the tensioning wires; a hollow coupling means, defining first and second ends and interiorly threaded at either end, for engaging, at said first end of the coupling means, the threads of the first end of said member and for engaging, at said second end of the coupling means, an anchor head for securing the tensioning wires; centering means for aligning said rigid member with an anchor head; and thread continuity means whereby, when said member and an anchor head are in alignment, the threads at said second end of the coupling means may be engaged with screw threads of an anchor head when the coupling means is partly unscrewed off the threads of the first end of said member, an anchor head for holding the tensioning Wires and having threads for engaging those of said second end of said coupling means, such anchor head including a location hole; said thread continuity means comprising an angular alignment means associated with the first end of said rigid member and with said centering means; means securing said centering means to such anchoring head; and means for permitting rotation of said rigid member for aligning said angular alignment means.

13. A hydraulic jack for stressing tensioning wires of prestressed constructions, including a housing; a rigid member slidably mounted within said housing defining an axis and first and second ends along said axis, said first and second ends each including a face; hydraulic means mounted Within said housing for sliding said member along said axis in a first direction to advance said first end and in an opposite direction to withdraw said first end, which latter movement stresses the tensioning wires; a hollow coupling means, defining first and second ends and interiorly threaded at either end, for engaging, at said first end of the coupling means, the threads of the first end of said member and for engaging, at said second end of the coupling means, an anchor head for securing the tensioning wires; centering means for aligning said rigid member with an anchor head; and thread continuity means whereby, when said member and an anchor head are in alignment, the threads at said second end of the coupling means may be engaged with screw threads of an anchor head when the coupling means is partly unscrewed olf the threads of the first end of said member, a hollow frame, having first and second ends, for receiving said first end of the rigid member and said coupling means; means for securing said first end of said frame to said construction; and means securing the second end of said frame to said housing.

14. A hydraulic jack for stressing tensioning wires of prestressed constructions, including a housing; a rigid member slidably mounted within said housing defining an axis and first and second ends along said axis, said first and second ends each including a face; hydraulic means mounted within said housing for sliding said member along said axis in a first direction to advance said first end and in an opposite direction to withdraw said first end, which latter movement stresses the tensioning Wires; a hollow coupling means, defining first and second ends and interiorly threaded at either end, for engaging, at said first end of the coupling means, the threads of the first end of said member and for engaging, at said second end of the coupling means, an anchor head for securing the tensioning wires; centering means for aligning said rigid member with an anchor head; and thread continuity means whereby, when said member and an anchor head are in alignment, the threads at said second end of the coupling means may be engaged with screw threads of an anchor head when the coupling means is partly unscrewed off the threads of the first end of said member, said hydraulic means including first and second hydraulic chambers for forward and rearward movement, respectively, of said rigid member; a piston separating said first and second chambers and mounted for movement therein; a ram in rigid connection with the piston for moving said rigid member, means for limiting the thrust transmitted by said ram to said rigid member during forward movement thereof, a first means mounted on said rigid member and spaced from the front end thereof and defining a surface facing the front end of said rigid member; a second means normall hearing on said surface and rigidly connected to said ram; resilient means connecting said second means with said first means, said centering means including a central recess in the face of said first end extending along said axis, and a centering plate for rigid attachment to an anchor head and having a centering stud that is to be received and engaged by said central recess.

15. The jack of claim 14, wherein there is provided an anchor head for holding the tensioning Wires and having threads for engaging those of said second end of said coupling means, such anchor head includes a location hole; said thread continuity means comprising an angular alignment means associated with the first end of said rigid member and with said centering means; means securing said centering means to said anchoring head; and means for permitting rotation of said rigid member for aligning said angular alignment means.

16. The jack of claim 14, including a hollow frame, having first and second ends, for receiving said first end of the rigid member and said coupling means; means for securing said first end of said frame to said construction; and means securing the second end of said frame to said housing.

17. The jack of claim 14 wherein there is provided an anchor head for holding the tensioning wires and having threads for engaging those of said second end of said coupling means, a hollow frame, having first and second ends, for receiving said first end of the rigid member and said coupling means; means for securing said first end of said frame to said construction; and means securing the second end of said frame to said housing.

References Cited Martter 25429 X WILLIAM FELDMAN, Primary Examiner..

MILTON S. MEI-IR, Examiner.

Claims

1. A HYDRAULIC JACK FOR STRESSING TENSIONING WIRES OF PRESTRESSED CONSTRUCTIONS, INCLUDING A HOUSING, A RIGID MEMBER SLIDABLY MOUNTED WITHIN SAID HOUSING DEFINING AN AXIS AND FIRST AND SECOND ENDS ALONG SAID AXIS, SAID FIRST AND SECOND ENDS EACH INCLUDING A FACE; HYDRAULIC MEANS MOUNTED WITHIN SAID HOUSING FOR SLIDING SAID MEMBER ALONG SAID AXIS IN A FIRST DIRECTION TO ADVANCE SAID FIRST END AND IN AN OPPOSITE DIRECTION TOWITHDRAW SAID FIRST END, WHICH LATTER MOVEMENT STRESSES THE TENSIONING WIRES; A HOLLOW COUPLING MEANS, DEFINING FIRST AND SECOND ENDS AND INTERIORLY THREADED AT EITHER END, FOR ENGAGING, AT SAID FIRST END OF THE COUPLING MEANS, THE THREADS OF THE FIRST END OF SAID MEMBER AND FOR ENGAGING, AT SAID SECOND END OF THE COUPLING MEANS, AN ANCHOR HEAD FOR SECURING THE TENSIONING WIRES; CENTERING MEANS FOR ALIGNING SAID RIGID MEMBER WITH AN ANCHOR HEAD; AND THREAD CONTINUITY MEANS WHEREBY, WHEN SAID MEMBER AND AN ANCHOR HEAD ARE IN ALIGNMENT, THE THREADS AT SAID SECOND END OF THE COUPLING MEANS MAY BE ENGAGED WITH SCREW THREADS OF AN ANCHOR HEAD WHEN THE COUPLING MEANS IS PARTLY UNSCREWED OFF THE THREADS OF THE FIRST END OF SAID MEMBER, SAID CENTERING MEANS INCLUDING A CENTRAL RECESS IN THE FACE OF SAID FIRST END OF SAID RIGID MEMBER EXTENDING ALONG SAID AXIS, AND A CENTERING PLATE FOR RIGID ATTACHMENT TO AN ANCHOR HEAD AND HAVING A CENTERING STUD THAT IS TO BE RECEIVED BY SAID CENTRAL RECESS.