US3522682A - Multiple tendon anchorage - Google Patents

Description

8- 4, 1970 J. w. HOWLETT ETAL I MULTIPLE TENDON ANCHORAGE 3 Sheets-Sheet 1 I ENTORS GEORGE H. OWLETT JAMES W. HOWLETT I I 1 5 I y P 10mm, M122, mam ATTORNEYS Filed March 8. 1968 FIG.

kw/w

g- 1970 J. w. HOWLETT ETAL 3,522,532

HULTIPLE TENDQN ANCHORAGE Filed March 8. 1968 3 Sheets-Sheet 2 FIG. 8 j

FIG. 7 53 54 58 56 54 5B 53 FIG. 6

INYENTORS GEORGE HVJHOWLETT BY JAMES. v .HowLETT ATTORNEY? g-' 1970 J. w. HOWLETT ETAL 3,522,682

MULTIPLE TENDON ANCHORAGE- Filed March 8. 1968 FIG. 9 IO FIG. I0 I 3 Sheets-Sheet 5 FIG. I I

INVENTORS GEORGE H. HOWLETT BY JAMES w. HOWLETT WFZTMY/ ATTORNEY United States Patent Office 3,522,682 Patented Aug. 4, 1970 3,522,682 MULTIPLE TENDON ANCHORAGE James W. Howlett, Richmond Annex, and George H.

Howlett, Oakland, Calif., assignors to Howlett Machine Works, a corporation of California Filed Mar. 8, 1968, Ser. No. 711,608 Int. Cl. E04c 3/10, 3/26 US. Cl. 52-223 Claims ABSTRACT OF THE DISCLOSURE A tendon anchorage for use in connection with pretensioning and post-tensioning concrete is disclosed where in the anchorage includes an axially convergent bore in which a plurality of wedges are mounted for simulta neous convergent movement and are formed to receive and clamp tendons between opposed faces thereof. The wedges are supported between the center-most or apex portion of the wedge and the peripheral bore engaging wall to avoid fatiguing and failure of the wedges under forces generated by clamping of the tendons while not substantially lessening the clamping forces. A variety of structures suitable for providing this support are disclosed.

The present invention relates to anchorage devices for tendons, bar, wire and cable, and more particularly relates to anchorage devices suitable for securing a multiplicity of such tendons.

Tendon anchorages have previously been employed which are capable of simultaneously anchoring a multiplicity of tendons usually in connection with pro-stressing concrete. One such device is shown in Howlett patent application Ser. No. 434,978, now abandoned. The Howlett device employs a multiplicity of wedges of sectorshaped transverse cross section mounted within afrustoconical bore. The side faces of these wedges are formed with recesses to receive and clamp the tendons which are to be anchored. When the tendons are disposed in the recesses and axial loading applied, very high circumferential clamping forces are generated and the tendons can be gripped or frictionally engaged with a force sufficient to allow axial loading of the tendons up to and beyond the rated ultimate strength thereof without pulling free of the clamping recess. This result, as will be understood, is highly desirable and provides a tendon anchorage having many advantages. As will also be readily understood, one of the primary purposes of a multiple tendon anchorage device such as above described is to provide an anchorage in which the number of tendons in any given cross sectional area is maximized. Moreover, it is always important that the tendon anchorage not fatigue or fail under the extremely high clamping forces and that the clamping forces not be unduly reduced to avoid such failure.

For certain applications, a device constructed in ac- 'cordance with the Howlett device functions quite satisfactorily. This is primarily because the individual segmental wedges may be formed of sufiicient thickness and high strength material to eliminate problems of fatigue. It is often, however, desirable and necessary to reduce the size of the individual wedges in order to be able to reduce the diameter in which a given number of tendons may be anchored. In attempting to do this, we have discovered that under the multiplication of clamping forces resulting from use of a multiple interacting wedge system, the circumferential clamping force on the tendons disposed between opposed faces of the wedges induces very substantial forces in the wedges between the tendon clamping recesses, which forces fatigue the wedges and eventually cause their failure. Fatiguing and failure of the wedges between the tendon receiving recesses results in fracturing and falling away of pieces of the wedge including the recess defining portion and, therefore, results in the immediate release of the tendons secured in these pockets. The release of a single tendon in turn causes the entire anchorage to momentarily lose its circumferential clamping force. Thus, the remaining tendons may either pull completely out of the anchorage or will be under a much smaller axial load by the time the wedges move forward and converge to take up the slack left by the wedge which failed.

Failure of multiple tendon gripping wedges due to fatigue may not occur until months after the anchorage has been loaded. In most post-tensioning and some pretensioning applications, this is highly undesirable since by that time the anchorage will be incorporated into a part of a structure and failure of the anchorage can have a disastrous effect. Thus, it is highly advantageous to eliminate the problem of potential fatiguing or failure between the tendon clamping recesses in opposite sides of the tapered wedges. Accordingly, an object of the present invention is to provide a multiple tendon anchorage and method which is less susceptible to fatigue or failure due to stress forces induced by clamping the tendons, but is capable of achieving very high clamping forces.

Another object of the present invention is to provide a multiple tendon anchorage and method which enables the anchoring of a greater number of tendons for any given cross sectional area.

Still another object of the present invention is to provide a tendon anchorage and method which may be used to anchor clusters of tendons by generating the requisite magnitude of clamping forces required without danger of failure.

Still a further object of the present invention is to provide a tendon anchorage which is economical to manufacture, easy to assembly and is essentially maintenance free.

The invention possesses other objects and features of advantage, some of which of the foregoing will be set forth in the following description of the preferred form of the invention which is illustrated in the drawings accompanying and forming part of this specification. It is to be understood, however, that variations in the showing made by said drawings and the description may be adopted within the scope of the invention as set forth in the claims.

Referring to said drawings:

FIG. 1 is an end view of a tendon anchorage constructed in accordance with the present invention and shown in conjunction with an anchorage bearing plate.

FIG. 2 is a cross sectional view of the tendon anchorage illustrated in FIG. 1 and is taken substantially on the plane of line 22 of FIG. 1.

FIG. 3 is an end view of a modified form of the anchorage of the present invention.

FIG. 4 is a cross sectional view of the anchorage device as shown in FIG. 3 and is taken substantially on the plane of line 4-4 in FIG. 3.

FIG. 5 is a schematic view of one of the sectorshaped wedges of the device illustrated in FIG. 3 with the forces induced upon clamping action illustrated by arrows.

FIG. 6 is an end view of a modified form of a tendon anchorage device constructed in accordance with the present invention.

FIG. 7 is a cross sectional view of another modified form of the tendon anchorage of the present invention and is taken substantially on the plane of line 7-7 in FIG. 8.

FIG. 8 is an end view of the anchorage shown in FIG. 7 wherein the wedges are mounted directly in the anchorage bearing plate.

FIG. 8a is an enlarged perspective view of one of the tapered wedges of the anchorage shown in FIGS. 7 and 8.

FIG. 9 is an end view of another modified form of the tendon anchorage of the present invention.

FIG. is a cross sectional view of the tendon anchorage as shown in FIG. 9 and is taken substantially on the plane of line 10-10 of FIG. 9.

FIG. 11 is an end view of another modified form of the tendon anchorage of the present invention.

The tendon anchorage of the present invention comprises, briefiy, an anchor member 11 having a wall 12 defining an axially convergently tapered bore 13 dimensioned at the smallest point 14 thereof to receive a plurality of tendons 16 to be tensioned; a plurality of wedges 17 each a frusto-conical segment fitting together to provide a generally frusto-conical assembly mounted in said bore and having peripheral sides 18 formed for simultaneous sliding axial engagement with wall 12 in the direction of convergence of the bore, each of the wedges having a side face 19 extending internally of peripheral sides 18, wedges 17 being mounted with faces 19 disposed in opposed relation with a plurality of faces being provided with longitudinally extending tendon receiving and clamping portions 21 and at least one of the wedges, wedge 17a, being formed with at least two portions 21 formed in side faces 19 in opposed planes of faces 19 to define section 22 therebetween; and means 23 formed and disposed to support section 22 intermediately of portions 21 so that section 22 cannot fracture across the portions and pull away from and release tendons 17, means 23 being formed and dimensioned to allow convergence of the wedges up to a clamping force between faces 19 sufficient to retain the tendons in the anchorage under an axial load of at least 85 percent of the rated ultimate strength of the tendons.

As illustrated in 'FIGS. 1 and 2 the anchorage of the present invention will always include a wedge 17a having portions 21 formed in opposed planes of face 19. This is important because a wedge with portions 21 formed in opposed planes will have forces induced therein between portions 21 in section 22 upon convergent clamping of tendons 16 which will tend to shear off a part of the wedge. Portions formed in parallal unopposed planes of the wedge will not have forces induced in them which are substantial enough to cause failure of the wedges between the portions.

As used throughout this specification and the claims, opposed planes of the side face 19 shall mean a plane parallel to face 19 at the center of where the clamping or gripping portion 21 is formed in face 19, and planes shall be opposed if a plane cannot be passed through said centers which are parallel, or if parallel, the planes are in parallal spaced relation with clamping portions 21 disposed to bear upon the tendons in opposite directions. For example, wedge 17a has two faces 19 extending from peripheral side 18, although faces 19 could be connected at apex 28 by an arcuate surface and be considered as a single face 19, and portions 21 are substantially semicircular longitudinally extending recesses. Planes passing through the center of where the recesses 21 were formed in faces 19 which were parallel to faces 19 would coincide with the faces and would intersect at the apex of the wedge. Thus, when the planes must intersect they are opposed, and the portions 21 are disposed in opposed planes of the side face 19. By contrast in the remaining wedges 17 portions 21 are formed in parallel but unopposed planes in surfaces 19. Considering segment-shaped wedge 17 it is clear that portions 21 are formed in the same plane of face 19, and in the remaining wedge 17 a recess is formed at the apex of the wedge, thus allowing the planes to be passed through that point at a multiplicity of angles including a plane which is parallel to and coincides with a plane through the other portion 21. These recesses are not formed in opposed planes. If faces 19 of wedge 17a were parallel with wedge 17a being substantially rectangular in transverse cross section, portions 21 would be formed in parallel planes of face 19, but recesses 21 would extend toward one another and would bear upon the tendons in opposite directions. Such recesses would be formed in opposed planes of the Wedge side faces.

In FIG. 1, tendons 16a and 16b bear upon wedge 170 through gripping recesses 21, and induce a substantial force in wedge 17a along what may be termed a weakened plane between recesses 21 in section 22 of wedge 170. If the apex 28 of wedge 1711 did not bear upon tendon 16c, the forces induced in section 22 would tend to cause failure of wedge 17a between tendons 16a. and 16b, which would result in complete failure of the tendon anchorage. Tendon 16c bearing upon apex 28 causes a substantial compressive force to be induced in section 22 between apex 28 and peripheral side 18 of the wedge generally perpendicularly to the above-mentioned weakened plane. Thus, even if the circumferential clamping forces are sufficient to fracture wedge 17a at section 22 between clamping portions 21, the two halves of the Wedge will not separate or pull away from tendons 16a and 1612. We have tested anchorages which are formed in accordance with FIGS. 1 and 2 and have had a fracture between portions 21 through section 22 (the fracture was the result of heat treating) and have found that the anchorage will not fail and will achieve and maintain, over an extending period of time, clamping forces which are substantially equal to the yielding strength of tendons 16.

The ability to prevent failure of the tendon anchorage in the event of an actual fragmentation at section 22 is important in that it allows the wedges to be formed of a relatively thin section which in turn reduces the material required in order to have a reliable tendon anchorage and the diameter in which a given number of tendons may be gripped. Thus, the advantages of a multiple wedge, multiple tendon anchorage wherein the clamping forces on the tendons are proportional to the number of tendons gripped and very high clamping forces can be achieved may be fully exploited.

The anchorage of FIGS. 1 and 2 is illustrative of several structural features of the present invention for avoiding the shearing wedges by the tendons under the very high clamping forces. First, and as indicated above, tendon 16c and recess 21 at apex 28 operates to support wedge 17a intermediately of recesses 21 therein and through face 19 in a manner which prevents fracturing and falling away of section 22 and release of the tendons. Merely supporting wedge 17a at apex 28 to prevent falling away should the wedge fracture is a feature of the present anchorage. In addition, tendon 16c exerts a compressive load or force which, in the configuration of FIGS. 1 and 2, is proportional to the tendon clamping forces. This substantial transverse compressive load has the effect of greatly reducing the tendency to fracture at section 22. Wedges made of heat treated C-1117 carborized steel may be as thin as one-quarter inch at section 22 without fracturing or failing under sustained axial loads in excess of 130,000 pounds for the three tendon anchorage of FIGS. 1 and 2.

An important feature of the present invention is that the Wedge supporting means 23 does not interfere unduly with the clamping forces induced in the anchorage. This is essential because an important advantage of multiple tendon anchorages is to be able to frictionally engage tendons with enough strength to allow axial loading of the tendons to to percent of their rated or guaranteed ultimate strength and even above the rated ultimate strength. The anchorage construction of FIGS. 1 and 2 is particularly well suited to this end and, as an additional feature, is self-adjusting as well as providing the important advantage of preventing wedge failure without reducing clamping forces. The clamping forces generated in wedge 17a are transferred to the remaining wedges through tendon 160, which provides the desired compressive load. The frusto-conical sides 18 of the remaining wedges and bore 13 in turn causes forces, transferred through tendon to the remaining wedges, to be transferred through tendons 16a and 16b back to wedge 17a. Thus, means 23 is an integral part of and reacts to work with and further induces or increases clamping foces in the anchorage upon convergence of the wedges. The result is that means 23 does not interfere or reduce the clamping forces generated, but guides the joint convergent movement of the wedges to maximize the tendon clamping force on all tendons is proportional to the presavailable, an analysis of the geometry of the configurations of the wedges in FIGS. 1 and 2 indicates that the clamping force on all tendon sis proportional to the pressure or force on the peripheral sides of the wedges, that tendon 16a and 16b experience a clamping force which is equal, and tendon 16c probably experiences a clamping force which is slightly greater than the forces induced on 16a and 16b. Whether or not this is the case in actual applications where friction and deformations enter into the analysis is unknown, but in actual tests all tendons are clamped up to and beyond the rated or minimum ultimate strength without any indication of preferential clamping.

In the anchorage illustrated in FIGS. 3, 4, and 5 centrally located tendon 39, if not of proper diameter, will cause a substantial reduction in the clamping forces; therefore, apex recesses 38 and the diameter of tendon 39 and the tendon material, if the tendon is to be yieldable, should be selected so that the clamping forces are not reduced by the wedge supporting structure below that required to retain the tendons in the recesses under an axial load of at least 85 percent of the rated ultimate strength of the tendon. The rated or minimum guaranteed ultimate strength of a tendon shall mean the ultimate strength of the prestressing steel in the tendon which the manufacturer will guarantee. For example, seven wire strand or cable is commercially available with a rated or minimum guaranteed ultimate strength of about 270,000 pounds per square inch, or in a one-half inch nominal strand diameter, 41,300 pounds minimum loading. Onehalf inch bar tendons are commercially available with a rated or minimum ultimate strength of about 160,000 pounds per square inch or a tendon load of 28,000 to 31,000 pounds (C-5160 steel which may be modified to add carbon). Since these manufacturing figures are minimums, tendons, which as used throughout the specification and claims shall include strands or cables, bar and wire, will be susceptible to loading past the rated ultimate strength if the anchorage can retain the tendons. The anchorage of the present invention achieves that end without danger of failure of the wedges. Moreover, the economics of pre-stressing require that the tendon anchorages be able to retain the tendons at loads of at least 85 percent of the rated ultimate strength. This goal is similarly achieved by the tendon anchorage of the present invention.

In order to achieve the multiplication of clamping forces which allows frictional gripping of the tendons, bore 13 and Wedges 17 are formed with mating surfaces for sliding axial engagement, and, bore 13 is axially convergently tapered. It is preferable to form said axially convergently tapered bore as a frusto-conical opening since such a bore configuration is relatively easy to fabricate and urges mating wedges placed therein toward the longitudinal axis of the bore from all directions around that axis. In addition and as described in connection with the self-adjusting feature of the structure of FIG. 1, a frusto-conical bore allows rotation of the wedges in the bore as required to achieve uniform clamping forces. Other axially convergently tapered bores are suitable, such as, bores which are axially convergently tapered over short distances. An example of the latter type of bore is the incline cam planes as disclosed in Howlett patent application Ser. No. 434,978, and the bearing member of FIGS. 1 and 2 could readily incorporate such an inclined cam structure without in any way interfering with the advantages of the present invention.

As illustrated in FIGS. 1 and 2, anchor member 11 is mounted on anchorage plate 24 which is formed as a splay plate with bores or openings 26 therein to locate or position tendon 16 in a channel or bore 30 formed in the concrete mass 27. Bearing plate 24 functions to spread the very high axial loading which is typically encountered in concrete pre-tensioning or post-tensioning, and it will be readily understood that, if desirable, anchor member 11 can be formed as an integral part of bearing plate 24. Such a construction is illustrated in FIGS. 7 and 11. Moreover, it is a feature of the present invention in order to increase flexibility of manufacture, transportation of parts and field assembly, that anchor bearing plate 24, formed with a frusto-conical bore 13, may be fabricated at a location near the job site and a wedge assembly, constructed in accordance with the present invention be manufactured and packaged for mounting in the bearing plate at a more remote site.

It should be further noted that the longitudinally extending recesses 21 in Wedges 17 :are formed to a depth so that faces 19 of the wedges do not contact one another before maximum clamping forces are achieved. In this way, it is assured that the maximum clamping will be achieved. By way of example, the wedges illustrated in FIGS. 1 and 2 may be formed by drilling three bores about 0.470 inch in diameter in a solid frusto-conical wedge blank. The wedges are then cut into the desired shapes which remove 0.093 inch of wedge material as best seen in FIG. 1. This allows sufficient clearance between faces 19 to grip a strand of a nominal diameter of 0.500 inch up to the rated ultimate strength. Although the Wedges may occasionally come into touching contact at the periphery, they normally tend to adjust to the position shown in FIG. 1, and faces 19 do not contact each other over a substantial length thereof between recesses 21 and sides 18. In the above example, the 0.470 recesses typically clamp the 0.500 tendons until the space between wedges is about 0.093, the amount of material removed.

Recesses 21 may also be alternatively provided with teeth, serrations or a roughened surface (not illustrated in the drawing) to aid the clamping engagement of tendons 16. Serrations have the disadvantage of tending to cause localized stress concentrations, stress risers, but this effect may be tolerable in some applications or minmal depending upon the extent to which protrusion penetrates the tendons.

The method of the present invention can also be clearly illustrated by reference to FIGS. 1 and 2. First, a plurality of tendons 16 are mounted between wedges 17 which are constructed in accordance with the above description.

The tendons are then tensioned by jacking the tendons rearwardly, setting the wedges and releasing the tendons which draws the tendons and wedges axially in bore 13 in the direction of convergence or the small diameter end 14 of the bore. The wedges having a construction which will induce substantial shearing or fracturing forces between recesses 21 are then supported intermediately of portions 21 on face 19 in a manner which does not interfere with convergent movement of the Wedges. In FIG. 1, this is achieved by inducing a compressive force transversely of or generally perpendicularly to the weakened plane passing through recesses 21 which is proportional to and does not diminish the clamping forces.

The anchorage and method of the present invention can be further understood by reference to FIGS. 3, 4, and 5 which illustrate a modified form of the invention. In this form of the invention in order to obtain even higher tendon anchoring densities and ease of manufacture, wedges 31 are formed to have a substantially sector-shaped transverse cross section, as best may be seen in FIG. 3. Mounting wedges 31 in a frusto-conical bore 32 enables their simultaneous convergent movement and circumferential transfer and resultant multiplication of clamping forces. Each wedge has radially extending faces 33 in which tendon clamping portions 34 are formed.

Examining FIG. 5, it will be seen that each of the wedges 31 is constructed so that upon clamping of tendons 36 forces will be induced between gripping portions 34 in a manner analogous to the forces induced in wedges 17a. As the wedge 31a radially converges and clamps upon the tendons 36, substantial forces will be acting on peripheral surface 37 of the wedge, tendon gripping recesses 34 and apex recess 38. Balancing these forces, it will be immediately seen that a substantial component of force will be induced between pockets 34, as indicated by arrows A and B therebetween. If recesses 38 did not bear upon means 39, which in this case is a centrally disposed tendon, there would be no forces transverse of the forces induced between the clamping recesses. Thus, the wedge would be unsupported radially interiorally or internally of the forces between portions 34, and as described hereinabove, this is precisely the reason for and the area in which failure of the wedges will occur. Thus, wedge 31a is formed in an apex tendon receiving recess 38 in face 33 intermediately of portions 34 formed in opposed planes in face 33. Tendon 39 is disposed in recess 38 to induce substantial compressive loading, indicated by the arrows on FIG. 5, between peripheral side 37 and apex recess 38. Since each of wedges 31 is formed with recesses in opposed planes of faces 33 which induces substantial forces between clamping portions 34, the wedges have all been formed with complementary recesses 38, which together dons which may be effectively anchored is increased if means 39 is comprised of a tendon. In some applications,

however, means 39 may merely be an insert member having an exterior surface formed and dimensioned to mate with clamping or bearing portions 38 formed in the apexes f the Wedges. The insert and recesses must be formed and dimensioned to allow convergent movement of the wedges up to a clamping force sufficient to retain the tendons under an axial load of at least 85 percent of the minimum guaranteed ultimate strength of the tendons.

Constructing the wedges, as described in connection with FIGS. 1 and 2, with recesses 34 and 38 formed from bores of a diameter of about 0.470 and slots between faces 33 which are about 0.093, four seven-wire strand of 0.500 nominal diameter may be gripped without a reduction in the clamping forces below 85 percent of the rated ultimate strength.

Insert member 39 may also be formed of a yieldable material or a yieldable structure ,(best illustrated in FIGS. 9 and which yields under anchorage loading to allow convergence of wedges 31. Thus, means 39 may consist of a yieldable plug of an alloy which will cold flow. The use of a yieldable central plug 39 may be advantageous in that a cylindrical plug has the effect of stopping further convergents of wedges 31 and thus creating an upper limit of the magnitude of the clamping forces between faces 33. The limitation of further convergent movement of the wedges can be desirable if the degree of convergence is properly selected. That is, forming plug 39 and recesses 38 of carefully selected dimensions will result in the wedges converging to a substantial interference fit with tendons 36 and then further convergence will be prevented. This stopping of further convergence after a substantial interference fit prevents the multiplication of tendon clamping forces to the point of shearing off the tendons under the clamping forces. As will be understood, a yieldable central plug can achieve this effect without having to meet the exacting dimension tolerances in manufacture.

Another form of the tendon anchorage of the present invention is illustrated in FIG. 6. Clamping of tendons 41 is achieved by wedges 42 which are formed substantially in the manner as shown in FIG. 1. Wedges 42 may have a frnsto-conical exterior peripheral surface 43 for mounting in a mating frusto-conical bore, not shown. As is the case with the tendon anchorage of FIG. 1, only one of the wedges, wedge 42a, is shaped and provided with tendon gripping portions 44 which will induce a shearing or fatiguing force therebetween. The remaining two wedges are of substantially sector-shaped and segment-shaped transverse cross section to afford certain advantages in the ease of manufacturing the anchorage. It should be further noted that sector-shaped wedges 17a and 42a have faces which extend beyond the longitudinal axis of the bore and thus are not truly sector-shaped in that the radii are longer than normal. As is the case in the anchorage of FIG. 1, wedge 42a is provided with means which consists of a tendon 41 disposed at the apex of the wedge so as to induce loading intermediately of the gripping portions. The wedge configuration of FIG. 1 has the advantage of having both of the remaining two wedges 17 be formed of a substantially sector-shaped transverse cross section rather than one sector-shaped and one segment-shaped wedge. The wedge assembly of FIG. 6 is formed to clamp pairs of wedges between wedge 42a and the remaining Wedges, and this wedge assembly also in duces a compressive force which is proportional to the overall clamping force and is self-adjusting so as not to interfere with or reduce the anchorage clamping forces between faces 46.

Still another modified form of the anchorage of the present invention is illustrated in FIGS. 7, 8 and 8a. Mounted directly in bearing plate 51, which functions as an anchor member and is formed with frusto-conical bore 52, are a plurality of substantially sector-shaped wedges 53 having radially extending faces 54. In this form of the invention, the wedge support intermediately of the clamping recesses is achieved by means of a tapered insert member 56 which is formed to be selectively secured against axial movement relative to anchor member 51 in a direction opposite to the direction of convergence of bore 52. As here illustrated, tapered insert 56 is provided with screw threads 57 which matingly thread into member 51 and lock nut 62. In order to allow passage of tendons 58 through the anchorage, member 51 is formed with openings 59 therein. Member 56 is disposed within small diameter end adjacent to the small diameter end of bore 52, and wedges 53 are formed at their apexes 61 with a mating frusto-conical bore converging in the direction of bore 52 and having a pitch which is not substantially greater than the pitch of bore 52. The internally and externally tapered wedges 53 are best illustrated in FIG. 8a.

In operation, the use of a tapered insert has some substantial advantages. Tendons 58 are gripped by a hydraulic jacking tool of the type described in Howlett application Ser. No. 434,978 and tensioned. Wedges 53 are given in initial set and will automatically clamp down upon the tendons when the jack is released. Insert member 56 may be selectively locked in place either in a position to merely support the wedges should one fail and tend to move away from the tendons or with a predetermined degree of interference with Wedges 53 to induce a compressive loading thereof. Setting of the insert may be effected either before or after the release of the tendons from the jacking tool. Unlike the cylindrical insert, if the taper is selected to be substantially equal to the bore in the anchor member, the apexes of the wedges will be supported and yet be free to move axially and thus converge on the tendons. If, in addition, the taper of member 56 and apexes 61 is slightly less than the pitch of bore 52, wedges 53 will in effect experience a binding between the bore 52 and member 56 which will cause an increase in the compressive loading intermediately of the clamping recesses. Such a variation in the pitches of the bores, allows for a veneer or micro adjustment of the increase of the compressive load. As described above, the variation of pitch and initial setting of member 56 must not retard clamping action of wedges 53 below percent of the tendon ultimate strength, or the advantages of a multiple tendon anchorage of this type will be greatly diminished. Locking of member 56 relative to member 51 can be achieved through conventional means such as nut 62; other conventional constructions can be used to secure member 56 relative to member 52.

FIGS. 9 and illustrate another form of the anchorage of the present invention. In this construction, a plurality of outer wedges 71 each formed with two tendon gripping portions 72 on each radial side thereof for gripping of tendons 73 are provided. This portion of the anchorage is analogous to the anchorage construction disclosed in Howlett application Ser. No. 434,978 and has the advantages described and set forth in that application. In addition, a centrally disposed plurality of wedges 74 have been provided. These wedges are formed essentially as shown in FIG. 1, however, their peripheral sides 76 are formed as a cylindrical surface instead of the frusto-conical surface described in connection with the device of FIG. 1. Wedges 71 are formed with frustoconical peripheral sides for mounting in a frusto-conical bore, not shown, and are formed with apexes 77 having a portion of cylindrical surface formed to mate with the peripheral sides 76 of wedges 74. Thus, substantial converg'ent forces are transmitted to the wedges 74 upon convergence of wedges 71. The cluster of wedges 74 acts as a means for inducing a compressive force intermediately of portions 72 in wedges 71 by loading in wedges 71 between the peripheral walls 78 and apexes 77 to prevent fatiguing of the wedges as above described. In addition, the centrally disposed wedges 74 are formed to bear on tendon 79 which in turn induces a compressive transverse loading which will prevent failure of wedge 74a. As best may be seen in FIG. 10, wedges 74 and apexes 77 are formed with mating threads 81. Threads 81 prevent relative axial movement of wedges 74 and 71 during assembly and insertion of the tendons. It should be further noted that wedges 74 act as a yieldable insert.

As has been above described, the simultaneous converging of wedges under the axial urging of a multiplicity of tendons results in the generation of very substantial clamping forces between the internally extending faces of the wedges. The generation of these clamping forces is, of course, highly desirable in that it allows frictional engagement of the tendons and their retention up to axial loads which approach and exceed the rated ultimate strength of the tendon. It has also been found to be highly desirable particularly when cables are to be anchored, to be able to grip a multiplicity of tendons disposed in a single tendon receiving recess. As will be understood, this has the effect of increasing the density of the tendons which are anchored and avoids the necessity of manufacturing and handling tendons of extremely large diameters, which is economically disadvantageous. It is very important, When a multiplicity of tendons is to be gripped in each recess, that an anchorage be employed which will not fail or fracture between the tendon gripping portions since the clamping forces induced must be quite high in order to clamp internally disposed strands.

FIG. 11 illustrates a tendon anchorage of the present invention having recesses which are formed and dimensioned to receive a plurality of tendons, and more particularly, a plurality of tendons in which at least one of the tendons is internally disposed so that it is not in direct contact with the recess. In this configuration, wedges 91 are formed with recesses 92 in which clusters of seven tendons 93 are disposed, each having a central tendon 94 which does not contact recess 92. It will be noted that only wedges 91a and 91b are formed with recesses in opposed planes of the wedge side faces which would induce forces between clamping recesses 92 tending to cause failure of the wedges. In a manner analogous to the previously described anchorages, the apexes 96 of wedges 91a and 91b are formed with a recess which mates with and bears upon a cluster of tendons 93 to induce a substantial compressive force, proportional to the anchorage clamping force, between the frusto-conical peripheral wall 97 and apex 96. This anchorage construction is also self-adjusting in that the compressive force induced is transferred throughout the anchorage without a reduction in the clamping forces. As illustrated in the anchorage in FIGS. 7 and 8, wedges 91 are mounted directly in anchor plate 98 which acts as an anchor member and bears directly on the concrete mass. The wedge configuration shown in FIG. 11 is also well suited to the clamping of single tendons in each recess.

By way of further example of suitable materials for the construction of the anchorages of the present invention, cylindrical anchor members as illustrated in FIGS. 1 to 4 and 6 may be formed of 8620 carborized steel which is heat treated to a Rockwell hardness of 35-40 on the C scale. Anchor member 11, for example, is about 2 to 3 inches in diameter with a minimum wall thickness at the large diameter end of bore 13 of about /2 inch and a length of about 1% to 2 /2 inches. Wedges may be formed of C-1117 carborized steel heat treated to a surface hardness of about 60 to 65 Rockwell C and a core hardnes of about 35 Rockwell C. The wedge length substantially equals the anchor member length, although they may be slightly longer. The bore taper may be between 5 and 15 when a single taper is employed and as high as 30 when spiral or circular cam planes are used. Wedge tapers of 7 to 10 have been found to be particularly satisfactory for constructions as illustrated in the drawings and a taper of 22 for spiral or circular cam planes as described in Howlett application Ser. No. 434,978. Bearing plates such as members 24 and 29 may be found of 0-1040 heat treated to about 25 to 35 Rockwell C hardness, and they may be 1 to 3 inches thick.

We claim:

1. A wedge assembly for insertion into a tendon anchorage having an anchor member formed with an axially tapered wall defining a convergentzly tapered bore dimensioned at the smallest end thereof to receive a plurality of tendons to be tensioned comprising:

a plurality of wedge segments in excess of two each of generally segmental frusto-conical zform fitting together to provide a convergently tapered assembly mounted in and fitting said bore with contiguous peripheral segment surfaces in mated sliding engagement with said wall,

said segments having opposed side faces extending internally of said peripheral surfaces and having longitudinally extending tendon clamping portions, engaged with said tendons,

one of said segments being formed with at least one of said portions on each of two side faces thereof defining a weakened plane through a section of said segment between said portions, and

means engaging said last named segment and applying a compressive loading to said section generally perpendicularly to said plane to support said section against fracture between said portions thereon and to prevent said section from falling away from and releasing said tendons, said means guiding convergent movement of said segments for applying a clamping force between said faces sufilicient to retain said tendons in said anchorage under an axial load of at least percent of the rated ultimate strength of said tendons.

2. A tendon anchorage comprising:

(a) an anchor member having a wall defining a frustoconical bore dimensioned at the small diameter thereof to receive a plurality of tendons to be tensioned;

(b) a plurality of wedges in excess of two mounted in said bore and having peripheral sides in sliding engagement with said wall the direction of convergence of said bore, each of said wedges having a side face extending internally of its peripheral side, said wedges being mounted with said side :faces disposed in opposed relation with a plurality of said opposed side faces being provided with longitudinally extending tendon receiving and clamping portions, at least one of said wedges being formed with at least two of said clamping portions disposed in a side face thereof in relative position therein so that a line drawn through the longitudinal central axes of tendons clamped by said portion passes through a section of the last named wedge; and

(c) means for inducing a compressive force in said last named wedge transverse to said line between said tendon axes, said means being comprised of a loading surface formed in said face of said last named wedge on the side of said line opposed to the side of said line on which said peripheral wall is positioned, and loading means formed and positioned in said anchorage for applying a loading force to said loading surface and for guiding said wedges in convergent movement for applying a clamping force between said faces for retaining said tendons in said anchorage under an axial load of at least 85 percent of the rated ultimate strength of said tendons.

3. A tendon anchorage as defined in claim 2, wherein said portions are formed as parallel longitudinally extending recesses in said faces of suflicient depth dimension to allows contact with a substantial periphery of a tendon to be anchored and dimensioned to prevent contact of opposed wedge faces over a substantial length thereof between said portions and said peripheral walls, and at least one of said recesses is formed and dimensioned to receive a plurality of tendons.

4. A tendon anchorage comprising:

(a) an anchor member having a wall defining a frustoconical bore dimensioned at the small diameter end thereof to receive a plurality of tendons to be tensioned',

(b) a plurality of Wedges in excess of two mounted in said bore and having peripheral sides in sliding engagement with said wall in the direction of convergence of said bore, each of said wedges having a side face extending internally of its peripheral side, said wedges being mounted with said faces disposed in opposed relation with a plurality of said opposed faces being provided with longitudinally extending tendon receiving clamping portions, at least one of said Wedges being of substantially sector-shaped transverse cross section with the arcuate portion of said sector forming the peripheral side of said wedge and an internal apex portion of said sector being formed by said side faces, said apex defining side .faces being formed with at least one of said portions in each of said side faces, said portions being positioned in said side faces so that the apex of said sector extends internally beyond said portion; and

(c) means engaging said apex of said sector-shaped wedge internally of said clamping portions for applying a compressive loading force between said apex and peripheral side of said sector-shaped wedge, said means guiding convergent movement of said wedges for applying a clamping force between said faces for retaining said tendons in said anchorage under an axial load of at least 85 percent of the rated ultimate strength of said tendons.

5. A tendon anchorage as defined in claim 4, wherein said means is comprised of a tendon receiving and clamping portion formed by the apexes of said wedges and a tendon disposed therein.

6. A tendon anchorage as defined in claim 4, wherein said means is comprised of a tapered insert member mounted centrally of said wedges and formed to be selectively secured against axial movement relative to said anchor member 1n a direction opposite to the direction of convergence of said bore in said anchor member with said insert member converging in the direction of convergence of said bore, and said wedge formed with substantially sector-shaped transverse cross section having an internal apex portion formed to define a second frustoconical bore converging in the direction of the first named bore, and said second bore substantially mating with said insert member for axial sliding engagement therewith and the pitch of said insert member and said second bore being not substantially greater than the pitch of said bore 1n said anchor member.

7. A tendon anchorage as defined in claim 4, wherein three wedges are provided and a first wedge is formed with faces extending beyond the longitudinal axis of said bore and clamping portions on each face of said first wedge defining said section and a clamping portion at the apex of said first wedge, said means inducing said compressive force between said portion in said apex and said peripheral wall in said first wedge, the remaining two wedges each having two clamping portions forming to mate with said portions formed in each side face of said first wedge and said portion in said apex of said first wedge.

8. A tendon anchorage as defined in claim 4, wherein said means is comprised of an insert member and said wedges are formed at the apexes to receive and bear upon said insert member.

9. A tendon anchorage as defined in claim 8, wherein said insert member is yieldable and is comprised of a plurality of second wedges having cylindrical peripheral walls and said apexes of the first wedges are formed with segments of a cylindrical recess therein of not substantially greater diameter than said cylindrical peripheral walls.

10. A tendon anchorage as defined in claim 4, wherein said wedges are of substantially sector-shaped transverse cross section, at least one pair of wedges are formed with portions defining said sections and each of said pair of wedges are mounted with the apexes thereof disposed to bear upon a tendon clamped by portions formed in said faces of the other of said pair of wedges, and at least two additional wedges are provided each of which are formed with clamping portions and are disposed in said anchorage so that said portions mate with a portion formed in the side face and a portion formed at the apex of one of said first named wedges.

References Cited UNITED STATES PATENTS 820,037 5/1906 Canda 287-114 1,606,789 11/1926 Hooley 24-126 2,245,316 6/1941 Amsler 52-223 2,328,033 8/1943 Schorer 52-230 2,341,922 2/1944 King et al. 24-122.6 2,856,662 10/1-958 Clark et al. 24-1226 3,123,879 3/1964 Boduroff et al. 24-1226 FOREIGN PATENTS 265,844 4/ 1964 Australia.

75,670 4/ 1953 Denmark.

FRANK L. ABBOTT, Primary Examiner J. L. RIDGILL, ]R., Assistant Examiner US. Cl. X.R. 24-1226, 126

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