Anchor for post-tensioning prestressed concrete

[57] ABSTRACT

This invention is an anchor for post-tensioning prestressed concrete including a case having a central bore through which a tendon passes, the internal wall of the case defining the central bore being tapered in a manner to form a compound angle with respect to the longitudinal axis of the case. A chuck comprising chuck segments is positioned within, and movable longitudinally with respect to, the case, the inner faces of the chuck segments being provided with a series of teeth engageable with the outer surface of the tendon which passes through the case and chuck. The compound angles of the case offers relief to the forward portion of the chuck segments in order to better distribute the forces exerted by the chuck segments on the tendon over the length thereof. Variations in the compound angles and/or the chuck segments permit use of the anchor for either locking engagement with a tendon or non-locking engagement therewith when anchor releasing is desired.

Claims, 10 Drawing Figures

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PATENTED NOV 2 8 1972 3.703,748

SHEET 1 OF 2

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THEORETICAL POINT OF INTERSECTION PIQ γ -- B &gt;C INVENTOR

----------- WILLIAM E KELLY

TRANSITIONAL CURVE AT ACTUAL JUNCTION OF B+C

ATTORNEYS

PATENTED NOV 2 81972 3,703,748

SHEET 2 OF 2

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60

FIG. 10

BY

ATTORNEYS

3,703

ANCHOR FOR POST-TENSIONING PRESTRESSED CONCRETE

BACKGROUND AND OBJECTS

Application Ser. No. 743,406 relates to an anchor for gripping and holding tendons or the like, wherein a locking effect on the tendon is obtained. It has been found, however, that in some uses of the anchor, it is desirable to provide a non-locking anchor in those instances where release and/or retensioning of the tendon is required.

It is therefore an object of this invention to provide a tendon-gripping anchor or the like for ppst-tensioning concrete, the anchor being constructed for use either as a locking or non-locking anchor with minor modifi- ¹⁵cation.

Another object is to provide a tendon-gripping anchor or the like for post-tensioning prestressed concrete, the anchor including means for securely ₂θ holding a tendon without exerting severing pressure on the strand at a point or points, but distributing the gripping force gradually over the length of the tendon in contact with the holding chuck.

Another object is to provide a tendon-gripping ₂g anchor of the character described wherein the anchor case is provided with an axial bore through which the tendon passes, the inner wall of the case defining the axial bore being tapered at compound angles to the longitudinal axis of the bore through a substantial portion 30 of the case’s length, the one angle of the taper comprising a working angle for engagement of the chuck segments with the wall of the case to apply pressure to the strands ofthe tendon, the other angle of the taper being decreased at the small end of the bore to provide a re- 35 lief angle to better distribute the forces exerted by the chuck segments on the tendon over the length thereof.

A further object is to provide a device as described having means to align and support the tendon in true axial relationship to the center line of the anchor so 40 that chuck segments will engage the tendon in a uniform location and manner.

A still further object is to provide an anchor which includes a universal casing, wherein the anchor is adapted for use as a locking or non-locking unit and for 45 use with different sized tendons by means of an adapter fitted in one end of the casing and the substitution of different sized chuck segments.

Additional objects will be obvious from a consideration of the following description of the presently 50 preferred forms of the present invention, taken in conjunction with the appended drawings.

DESCRIPTION OF FIGURES OF DRAWING

FIG- 1 is a longitudinal sectional view of the anchor ⁵⁵of the present invention, illustrating its use as a ‘dead end’anchor;

FIG. 2 is a longitudinal sectional view of the anchor of the present invention, illustrating its use as a ‘live end’ anchor, and showing the chuck end in in-operative ⁰position;

FIG. 3 is a view similar to FIG. 2, illustrating the chuck ip operative position;

FIG. 4 is a sectional view taken along the line 4—4 of ₆₅FIG. 2, looking in the direction of arrows; \

FIG. 5 is a sectional view taken along the line 5—5 of FIG. 3, looking in the direction of the arrows;

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FIG. 6 is a perspective view of an end cap cover for a ‘dead end’ anchor, forming a part of the present invention;

FIG. 7 is a diagrammatic view of the transitional 5 curve formed by the junction of the two angles of the tapered bore sections, forming a part of the present invention;

FIG. 8 is a longitudinal sectional view of a modified form of the anchor of the present invention;

¹⁰ FIG. 9 is a sectional view taken along the line 9—9 of

FIG. 8, looking in the direction of the arrows, and

FIG. 10 is an end elevational view of the form of invention illustrated in FIG. 8 taken from the opposite end of the modified form of anchor.

DESCRIPTION OF FORM OF INVENTION ILLUSTRATED IN FIGS. 1 TO 7

In FIG. 1, there is illustrated an anchor constructed in accordance with one form of the present invention for use as a ‘dead end’ anchor, and including a case 10 in which is positioned a chuck 12. The case and chuck are shown in an arrangement such as set out in my prior U.S. Pat. No. 3,399,434, including reaction plates 14 and 16 which are fixed to the outer periphery of case 10, reaction plate 14 being secured by nails 18 or other suitable means, to a form 20. Chuck 12 is held within case 10 by means of a helical spring 22 one terminal of which lies within a cup 24, and the opposite end thereof abutting closure cap 26 made of a semi-flexible translucent material, and having a flanged portion 28 for engagement over an annular flange 30 at one end of case 10 for securing the cap in place.

As shown to advantage in FIG. 6, the central portion of cap 26 is internally thickened to provide a circular boss 32, around which the final turn of helical spring 22 is wound, as illustrated in FIG. 1.

A tendon 34 extends through the anchor and a thimble-like finder 36 is placed over the extremity of the tendon to facilitate insertion into the anchor and to prevent the chuck from engaging the tendon until the tendon is completely inserted in the anchor. That portion of tendon 34 which extends beyond the anchor is greased and covered by a loose fitting jacket 38, a sleeve or masking tape 40 being placed over the terminal of the jacket and the adjacent terminal of the case 10 to prevent entry of concrete or other foreign matter into the anchor.

In FIGS. 2 to 5, there is illustrated the anchor of the present invention as modified for use as a ‘live end’ anchor. In this connection, no helical spring or end closure cap is employed.

Referring now to FIGS. 1 to 5 it will be seen that case 10 comprises an elongated cylindrical body 42, which is non-uniformly tapered from one end to the other, and having an axial bore 44, extending through body 42, the cross-sectional area of which bore is decreased form one end of the body to the other. The small end of the body is provided with a flanged extension or bearing support 46.

As indicated in FIGS. 1 and 2, that portion of body 42 adjacent flange 30 extends for a distance A forming a small angle a with a line parallel to the longitudinal axis of case 10. The angularity of body 42 is then increased through a distance designated B providing a first working bare section and forms an angle b with «005

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3,703,748 respect to a line drawn parallel to the longitudinal axis of case 10. The angularity of the taper of body 42 is altered again for a distance C providing a second working bare section and forms an angle c with respect to a line drawn parallel to the longitudinal axis of the case. In actual manufacture of the anchor of the present invention, the angularity of that section of the case extending through a distance C is not sharp with respect to B, but the junction of the two angles is a transitional curve (See FIG. 7). The angularity of body 42 is once again altered for a distance D and issues into an inwardly directed flange portion extending longitudinally for a distance E, the flange portion being angularly disposed to the body, following which it connects with bearing support 46.

In the drawings, there is illustrated an anchor adapted for use with a tendon of ½ inch diameter and the size and taper of the case and chuck are designed to obtain expected optimum results with that size tendon, using a low angle, for locking taper, and a larger angle of taper when anchor releasing is desired. Properly sized cases and chucks, with necessary variations, may be employed with different sized tendons. A locking taper anchor has been developed with a case and a chuck comprising jaw segments having approximately a 4° angularity and a non-locking taper anchor is effected by means of a case and a chuck wherein the jaw segments have approximately a 7° angularity. Optimum results are obtained when the anchor is constructed having the following relationships.

EXAMPLE 1 1/2 INCH TENDON CASE

<td> Locking Taper - 4° Chuck A-l” a=l°</td><td> Non-locking Taper - 7° Chuck :A=1 a=l°</td>

<td> B- 2-3/8- b = 4°30' 2-7/16</td><td> :B = 2 3/8b = 6°3O'</td>

<td> C-11/16- c = 3°15'</td><td> :C = 5/8 c = 4°30'</td>

<td> 5/8 n = 1/8 d = 0°</td><td> :D=l/8” d = 0°</td>

<td> E = 3/16</td><td> :E = 3/8</td>

EXAMPLE 2 3/8 INCH TENDON CASE

<td> Locking Taper - 4° Chuck</td><td colspan="2"> Non-Locking Taper - 7° Chuck</td>

<td> A = l a=l°</td><td> A= 1</td><td> a= 1°</td>

<td> B = 2 b = 4°</td><td> B = 2”</td><td> b = 6°30'</td>

<td> C=1 1/2 c = 3°3O</td><td> C = 3/4</td><td> c — 4°30'</td>

<td> D—1/8 d = 0°</td><td> D= 1/8</td><td> d = 0°</td>

<td> E = 3/16</td><td> E = 3/8</td><td></td>

Case 10 is preferably of die-formed material, in which the contours above-described can be accurately imparted thereto. As indicated by the above examples, the case may be of various sizes to accommodate tendons of different diameters.

Chuck 12 includes a plurality of like, elongated, tapering chuck segments or jaws 48 which are transversely arcuate in cross section to nearly complement the contour of the inner wall of case 10 (see FIG. 4). Longitudinally, the tapered face is straight. However, upon exertion of pressure thereon by a tendon, the segments bend transversely and longitudinally to conform to the contour of the case, and tendon.

The inner face of each segment 48 is provided with a series of individual, tendon-engaging teeth 50 extending through a substantial portion of the length of each segment. Segments 48 are held together at the large end by a suspension ring assembly 52 which also spaces the segments from each other around the tendon, to effect uniform gripping engagement therewith.

The jaw segments are preferably of a forged, ductile, surface-hardened steel, with the teeth having a brinnel 5 hardness of approximately 58 and approximately 0.015 deep for the anchor size shown.

It will be noted from a comparison of FIGS. 4 and 5 that the radius of curvature of the chuck segments is less than that of the inner wall of case 10 defining the ¹⁰ axial bore thereof. Upon exertion of tension on the chuck, the segments are deformed into contiguous engagement with the inner wall of the case.

In accordance with the present invention, either ] locking or non-locking type anchors may be employed as ‘dead end’ and ‘live end’ anchors, or the two types may be used together.

OPERATION

2o In the use of the present invention, tendon 34 is inserted into the ‘dead end’ anchor 10 until finder 36, which is fitted over the tendon terminal, engages boss 32 of end cap closure 26. This is readily determined by visual inspection through the translucent cover to 25 determine when the tendon and finder are properly positioned. At this time, chuck 12 lies adjacent that portion of case 10 extending through distance B and a portion of distance C. It will be noted that the smaller end of the chuck is spaced from the wall of case 10 and ³⁰ that teeth 50 are in engagement with, but not gripping, tendon 34.

Upon exertion of tension on tendon 34, chuck 12 moves longitudinally through case 10 in the direction of the case taper to wedge the outer walls of the seg³⁵ ments against the inner wall of the case and forcing teeth 50 into gripping engagement with the tendon.

By virtue of the angularity of chuck segment 48, and the taper of distance B of the case through angle b, a ₄₀ wedging effect takes place. As the chuck segment continues to be drawn through the case, the angularity of the case decreases as indicated at c. This allows relief of the leading portion of the chuck segments and, due to the ductile nature of the chuck segments, the latter are 45 deformed in a direction away from the tendon.

Accordingly, instead of the forward teeth of the chuck segment exerting severing pressure on the tendon, this relief acts to distribute the force exerted by the teeth proportionately over the full holding portion 50 of each of the chuck segment’s length. It, in effect, makes a practical, uniform force triangle for holding the tendon. In addition, the angles of the case’s and chuck segment’s taper bring about the pressure molding of the tendon into the chuck and the deformation of 55 the chuck during the molding process to conform to the walls of the case and tendon. As indicated in the above examples, with a chuck having segments of 4°, when the working taper angle b of the case is approximately 4°, a locking together of the chuck, the tendon and the ⁶⁰ case is effected to the point of creating substantially a cold pressure weld between the three major parts which prevents any unwanted release of the anchorage. As a result of this welding effect, it has been found that ₆₅ a reverse pull of up to 80 percent of the force required to lock the parts together is necessary to release them again, with the tendon cut free. This cold forming of the chuck against the case and tendon is made possible

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3,703,748 /' by the tremendous transverse pressure developed by the angle of the chuck and the case, not exceeding 5°, plus the Compounded reduction of the case taper.

‘ With a case and a chuck having segments of 7°, hav- ing a working taper angle preferably between 6° and 7°, a non-locking relationship between the chuck, the tenγ don and case is effected, permitting release and/or retensioning of the tendon.

Tendon 34 extends from the ‘dead end’ anchor to the ‘live end’ anchor illustrated in FIGS. 2 to 5, the tendon passing through chuck 12 which is positioned in case 10. Continued exertion of force on the tendon towards the small end of the case effects gripping engagement of teeth 50 of jaw segments 48 with the tendon for holding the tendon under predetermined tension between the ‘dead end’ enchor and the ‘live end’ enchor.

By virtue of the relationship of the taper angle of the wedge segments to the angle b and c comprising the compound taper in the case and forming the working angle and the relief angle, the ‘live end’ anchor holds the tendon in position, in either a locking or nonlocking relationship, depending on the type of anchor employed.

Therefore, if a non-locking anchor is used and it is desired to release and/or retension tendon 34, a force is exerted on tendon 34 in the direction of the large end of case 10, thereby moving chuck 12 from the position shown in FIG. 3 to the position Of FIG. 2, at which point the grip of chuck 12 on tendon 34 is released.

MODIFIED FORM OF THE INVENTION ILLUSTRATED IN FIGS. 8-10

In FIGS. 8 to 10, there is illustrated a modified form of the anchor of the present invention including a case 54 having integrally formed therewith spaced reaction plates 56 and 58 which extend in peripheral relationship to the case. As shown to advantage in FIG. 9, reaction plate 56 is of rectangular shape and is provided with a series of openings 60 at the corners thereof through which nails or other suitable securing means pass for securing the anchor to a form.

A chuck 62 is held within case 54 by means of a helical spring 64, one terminal of which lies within a cup 66 and the opposite end thereof engages a closure cap 68 made of a semi-flexible translucent material, which is similar to that illustrated in FIGS. 1 and 7. Closure cap 68 includes a flanged portion 70 for engagement over an annular flange 72 at the large end of the case 54. The central portion of cap 68 is internally thickened to provide a circular boss 74, around which the final turn 4 of helical spring 64 is wound, when the anchor is to be used as a ‘dead end’ anchor.

A tendon 76 extends through the anchor and a finder 78 is placed over the extremity of the tendon to facilitate insertion into the anchor. Thimble 78 engages boss 74 of cap 68 to limit the movement of the tendon into the anchor. That portion of tendon 76 which extends beyond the anchor is greased and covered by a loose fitting jacket 80, a sleeve or masking tape 82 being placed over the terminal of the jacket and the adjacent terminal of the case to prevent admission of concrete or other foreign matter into the anchor.

Case 54 includes an elongated cylindrical body 84 which is non-uniformly tapered from one end to the other, and having an axial bore 86, the cross section of which bore is decreased from one end of the body to the other. Body 84 may be of any desired thickness as required for strength. The small end of the body is pro⁵ vided with an inwardly directed flange portion 88 which serves as a bearing support for tendon 76.

The portion of body 84 adjacent flange 72 extends for a distance A', forming a small angle a' with a line parallel to the longitudinal axis of case 84. The angularity of body 84 is then increased through a distance designated B' and forms an angle b' with respect to a line drawn parallel to the longitudinal axis of case 54. The angularity of the taper of body 42 is altered again 15 for a distance C and forms an angle c' with respect to a line drawn parallel to the longitudinal axis of the case. The junction of angles b' and c' is not sharp but a transitional curve. The terminal portion of body 84 extending through a distance D'-E' is approximately 20 parallel to the longitudinal axis of the case.

The modified form of anchor is particularly adapted as a non-locking type anchor and has been developed with a chuck comprising jaw segments having approximately a 7° angularity. Optimum results are obtained ²⁵ when the case and chuck are constructed having the following dimensions.

EXAMPLE 3 an Non-Locking - 7° Chuck

A'= 1 a'= 1°

B'= 113/16 b' = 6°30

C' = 3/4 c' = 4°

D'-E'= 3/8” d' = 0°

Chuck 62 includes a plurality of like, elongated tapering chuck segments or jaws 90 which are transversely arcuate in cross section to nearly complement the contour of the inner wall of the case, as set out above in connection with the form of invention illustrated in ⁴⁰ FIGS. 1 to 7. The inner face of each segment 90 is provided with a series of teeth 92 extending through a substantial portion of the length of each segment. Segments 90 are held together at the large end by a suspension ring assembly 94 which also spaces segments from ³ each other, to effect uniform gripping engagement therewith.

It is a salient feature of this form of the present invention that it readily adapts for use in holding either % 5Q inch, 7/16 inch, ½ inch, or 0.06 inch tendons. This is effected first of all by selectively employing chuck segments having bores for accommodating either % inch, 7/16 inch, % inch or 0.06 inch tendons. Secondly, a cylindrical adapter 94 may be inserted into the small 55 end of the case 54, when the anchor is to be used for different sized tendons, the adapter fitting between the tendon and flange portion 88 or case 54. Adapter 94 further includes an annular terminal portion 96 for abutting engagement with the terminal face of case 54 ⁶⁰ to limit the movement of cylindrical adapter 94 into the case.

The operation of the modified form of anchor is the same as that set out above in connection with the form ₆₅ of invention illustrated in FIGS. 1 to 7, the anchor being used as illustrated in FIG. 8 for use as a ‘dead end’ anchor, end closure cap 68 and spring 64 being removed for use as a ‘live end’ anchor.

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While the modified form of anchor has been shown and described as a non-locking type anchor, the case taper angles and wedge segments taper can be modified for use as a locking type anchor in the manner described above in connection with the form of invention illustrated in FIGS. 1 to 7.

With the anchor of the present invention, a locking effect is obtained when the working angle taper of the case does not exceed 5°. When a non-locking anchor is desired, one that may be easily retensioned and/or reset, it is necessary to increase the taper angles involved to a maximum of 10°, with a wedge segment tapered to match, to reduce the locking effect resulting when the lower taper angles are used. This still results in a substantially cold pressure weld between the three major parts which prevents accidental release of the anchorage.

While there has been herein shown and described the presently preferred form of the present invention, it is to be understood that such has been done for purposes of illustration only, and that various changes may be made therein within the scope of the claims hereto appended.