US 3659321 A
The units of a prestressing cable made of multiple parallel tendon units are individually clamped between two wedges of a crown of wedges clamping all the units being itself located in a frusto-conical hole of a bearing part of which the apex angle is greater than 15 DEG .
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Description (OCR text may contain errors)
United States Patent Laurent [451 May 2,1972
MULTIPLE WEDGE ANCHORAGE DEVICE FOR PRESTRESSING TENDONS inventor: Michel Laurent, Bagnolet, France Assignee: Societe Technique Pour L'Utilisation De La Precontrainte, Boulogne, l-lauts de Seine, France Filed: June 12, 1970 Appl. No.: 45,643
Foreign Application Priority Data June 24, 1969 France ..6921 121 US. Cl ..24/122.6, 24/126, 52/230 Int.Cl... ..F16gl1/04,Fl6g 11/10 Field of Search ..24/l22.6, 126 L, 136 L, 126 C;
Primary Examiner--Bernard A. Gelak Attorney-Watson, Cole, Grindle & Watson [5 7] ABSTRACT The units of a prestressing cable made of multiple parallel tendon units are individually clamped between two wedges of a crown of wedges clamping all the units being itself located in a frusto-conical hole of a bearing part of which the apex angle is greater than 15.
3 Claims, 4 Drawing Figures z /ii'- Z- V i a 97 I Q S/ I PATENTEDMAY 21972 3,659,321
' SHEET 2 0F 2 a WWW MULTIPLE WEDGE ANCHORAGE DEVICE FOR PRESTRESSING TENDONS The present invention concerns an anchorage with multiple wedges placed in a female anchorage cone for anchoring tendons or cables made of several parallel wires or strands, which in the following description will be called tendon units.
An anchorage for tendons of this type is known in the prestressing technique in which each tendon unit is located between two grooves cut in wedges and parallel to the tendon unit, all of which wedges form a crown having a frusto-conical surface placed in a frusto-conical hole of a female bearing block.
The tension of the tendon pulls the wedges into said hole and the lateral clamping force produced by the reaction of the bearing block on each wedge is transformed in a circumferential force, pressing all the tendon units together along a cylinder, due to the space left between adjoining wedges, and the force is thus transmitted in acirclefrom wedge to wedge through all the tendom units.
Usually, and by analogy with other types of anchorage using wedges, the angle of the frusto-conical hole in the bearing block is small, i.e. of about 6 to 10. As a matter of fact, when the tendon units are not clamped individually between the wedges, but are arranged around a single central frusto-conical wedge, it is necessary to keep the angle of the wedge small in order to have a good clamping action and furthermore to prevent ejection of the wedge under the action of the component of the lateral force directed in the sense opposite to the thin end of the wedge, this kind of ejection is called orange pip effect.
However, if the angle of the set of wedges clamping the individual tendon units one by one is reduced to as little as 6 or 10, the lateral clamping force becomes excessive as the tensioning force of the tendon increases.
To reduce the lateral stress, it was proposed to make the wedges longer, but this only makes the anchorage, more expensive and more difficult to handle. In lieu of lengthening the wedgesit was necessary to limit the number and the section of tendon units.
This excessive lateral pressure on the tendon units is due to the fact that when there are many tendon units, the radial angle of each wedge is small. Since the lateral pressure then results from a double wedge effect, the effects due to the angle of the frusto-conical cavity of the bearing block and of the radial angle of the wedges, are additive. Moreover, if the average lateral clamping force F on a tendon unit is expressed mathematically, the following formula may be set forth:
In this formula n is the number of tendon units, Tis the tension of one tendon unit, is the friction angle between the set of wedges and the frusto-conical cavity and 0 the angle of the outer face of the wedges relative to the axis of the cable.
It can be seen that, for small angles, the clamping force is inversely proportional to the product of the radial half-angle of the wedged (11'/ n) by the half-angle of the frusto-conical cavity (0) which are both small.
The invention is based on the finding that in an anchorage, in which tendon units are clamped one by one between the wedges, there is no danger of orange pip effect. The tendon units are in contact with the wedges along a surface parallel to their own axes and these tendon units thus only tend to pull the wedges towards the narrow end of the frusto-conical hole as soon as friction contact is established between tendon units and wedges.
Under these conditions, following the main feature of the invention, the bearing part and the wedges being made of rigid solid metal angle of the external surface of the wedges with the tendon is selected over and above 15. This value is quite superior to the usual values and can be, for example, of for a tendon made of seven units. Thus the lateral clamping force is considerably decreased and can be reduced to acceptable values. Moreover, by varying this angle it becomes possible to match the clamping force with the number of tendon units, according to theabove formula. It is obvious that the larger the number of tendon units, i.e. the smaller the radial angle of the wedges, the larger may be the angle of the frusto-conical cavity.
This limitation of lateral pressure is particularly important in the case of strands to which the invention mainly applies whereas the previous anchorages of this type were principally concerned with plain wires. In fact, strands are more easily damaged by lateral pressure which is applied on points of contact with the individual wires composing the strands, rather than along entire generatrices of the wires as in case of plain wires.
In order that the clamping force be progressive and increasing when approaching the free end of the tendon, the angle of the outer surface of the wedges can be advantageously chosen 20 to 2 larger than the angle of the hole of the female bearing block. In this way the clamping is stronger towards the free end of the cable.
The nature of this anchorage is such that the wedges can be made solid and therefore practically indeformable which gives to the difference of angle its full efiiciency for obtaining progressive clamping. This difference of angle is already known, but had not always the desired efficiency because the wedges were too deformable partly because of their shape, partly because of the constituent-material (mortar).
Preferably, the wedges have a length 50 percent greater than the height of the bearing block. Owing to the rigidity of the bearing block and of the wedges a suitable distribution of the clamping force on the tendon units can be assured in spite of the localized pressure of the bearing block on the wedges.
The-important industrial advantage of the'invention is the possibility of considerably increasing the strength of tendons used with this type of anchorage and thus meeting the needs of the users of prestressed concrete.
Two embodiments of anchorages in accordance with the invention are shown in the attached drawings.
FIG. 1 is a plan view of a first example of anchorage.
FIG. 2 is a side elevation and a partial sectional view taken along the line IIII of FIG. 1.
FIG. 3 is a plan view of a second example of anchorage.
FIG; 4 is a side elevation and a partial sectional view taken along the line IVIV of FIG. 3.
The anchorage shown on FIGS. 1 and 2 comprises a female bearing block 2 having a frusto-conical cavity 3 bearing on the concrete 1 of a structure to be prestressed. The tendon is made in this case of seven units 4 which may be strands, for instance, each made out of seven wires.
This cable is placed in a duct formed by means of a sheath 5 cast into the concrete. Each tendon unit is clamped between two edges 6 in shape of a sector, of which the in frusto-conical periphery bears on the frusto-conical cavity 3. These wedges form a crown in the center of which remains a free space which can be advantageously cylindrical. Each radial surface of each wedge is provided with a groove 9, a little less deep than a half cylinder of a diameter equal to the diameter of the tendon units, so that each tendon unit is clamped between two opposite grooves.
In order to achieve a better clamping, grooves can be provided with serrations in the form of saw teeth machined perpendicularly to the direction of the tendon units. The grooves may alternatively be coated, for example by metallization, with hard grains in order to increase the friction factor between the grooves and the tendon units.
In an arrangement of this kind, the tendon units, due to their friction in the grooves, tend to pull the wedges deeper into the cavity 3. The radial reaction of the wall of this cavity on each of these wedges is transformed, by vault effect, in a circular pressure which is transmitted from one wedge to the other through the tendon units, because each of them is clamped between two wedges and the wedges are spaced by narrow intervals l0 and thus are not directly in contact.
According to a first feature of the invention, the angle of the generatrices of the frusto-conical cavity with the axis of this cavity is greater than Moreover, the apex angle of the frusto-cones formed by the set of wedges, which is the angle of the generatrices of the surface 7 of each wedge with the axis of the tendon, is greater than the angle 0 by to 2, preferably by about 30. 5
Thus, the clamping of the crown of wedges by the bearing block 2 is maximum at the outer side of this bearing part and progressively decreases towards the side of the part bearing on the concrete. Thus in a known manner, the clamping of the tendon units is greater there, where their stress has been progressively reduced by the friction, the clamping itself, being progressive.
According to another feature of the invention, the length L of the wedges, measured axially, is greater than the height H of the bearing block by about 50 percent. Moreover, the wedges have, at the side opposite to their portion inside the frustoconical cavity 3, another frusto-conical surface 11 in opposite direction to the surface 7.
Owing to this arrangement, the reaction force exerted by the surface 3 of the cavity on the wedges 6 near the edge 12 of the intersection of the surfaces 7 and 11 is evenly distributed along the length L of the wedge. As a matter of fact, assuming that the wedges are very tightly inserted in the block opening, pressure reaction of the hole opening against the wedge bundle is substantially uniform and each longitudinal section of the anchorage device has a resultant reaction, substantially applied midway of the block height and perpendicularly to said hole wall. By reason of the large angle of the block opening and of its relatively large diameter with respect to the small height of the block, said resultant reaction passes through the plane of the outer face of the block substantially at the same place as the tendon units and at the level'of the edge 12.
Hence, the frusto-conical wedge portion ll,'being equal to approximately one-half the height of the wedge portion inserted in the frusto-conical hole, pressure against the tendon units is, as is well known, distributed according to the triangular law, i.e., pressure is substantially zero at the inner tip of the wedge, maximum at the outer end thereof and linearly distributed along the grooves 9 between these two extreme values. I
Hence, at the clamping start, i.e., when the wedge bundle with the intermediate tendon units begins to enter the block opening, the difference between the wedges and block opening wall angles provides a clamping of the units by the wedges which is greater towards the outer ends of the wedges than towards the inner ends or tips thereof. Now, when the wedges are very tightly engaged within the hole and when, owing to the yielding of the different parts of the anchorage device, said small angle difference is cancelled, reaction pressure between the wedges and the block opening wall becomes uniform, while distribution of the clamping pressure on the tendon units remains favorable. By the reduction of the depth of the hearing block and the tapering of wedges outside the block, the volume of .the anchorage is considerably reduced, which represents saving, of material and a better use of it. Nevertheless, it is possible to build such anchorage for tendons of a very highprestressing force.
As a matter of fact, the angle of the frusto-conical cavity may be determined and chosen for obtaining, on each tendon unit, a clamping force adjusted for being not detrimental for said unit whereas, with the conventional hole reduced angle, local clamping force on the tendon may be so high that the anchorage device should be lengthened for distributing said pressure and reducing the same.
In the embodiment shown, the length of the wedges is almost double of the depth of the bearing block 2 and the angle 6 for seven tendon units can be taken at about 20.
In order to avoid crossing of tendon units 4 and in order to keep them effectively parallel at the entry into the intervals between wedges 6, an ogive 13 is located between these tendon units in order to separate them before placing the wedges 6. This 0 've has a central duct 14 prolonging the axial space 8 provide between the wedges which allows, after anchoring,
the protective grouting of the sheath 5 of the tendon.
The tendon units 4 being parallel, tensioning can be carried out in a known manner, by means of a jack provided with an axial hole for the passage of all the said tendon units and with a surface bearing on the external surface 17 of the anchorage block 2.
The embodiment shown on FIGSQ3 and 4 is the same as shown on FIGS. 1 and 2, except for the following differences:
The number of tendon units is twelve, therefore the radial angle of each wedge 6 is l5, whereas the angle 0 is of about Moreover, the grooves 9 provided in the wedges 6 are askew relative to the axis of the tendon, which allows a tensioning of this tendon with a jack without a central hole,-but provided with means of securing the tendon units at its periphery.
As before, wedges 6 are longer than the height of the, bearing block 2 and are essentially protruding from the external surface of this block.
The divergence of the units of the tendon at their entry in the anchorage device may require a widening of the sheath 5 which may be carried out by means of a connection piece 15. For preventing friction of this cable againstthe sheath, a ring 16 is placed in this connection piece in order to progressively deviate the parallel tendon units until they reach the slope of the grooves 9 where they are located.
The ring 16 must obviously be strong enough to keep the wires or strands in position when stressed, It will be noted that the friction of the tendon units on the internal surface of the ring 16 reduces the force necessary to anchor these units.
As already mentioned, the anchorages which were, described are particularly suitable for large strength tendons intended to supply big prestressing forces for concrete structures of any kind, and in particular for civil enginerring works.
, What I claim is:
1. An anchorage device for prestressing a plurality of similarly shaped units, comprising, a flat bearing block having a frusto-conical opening therein, the wall of said opening being greater than 15 with respect to the axis of said block; a plurality of similar sector-shaped wedges equal to the number of said units having, in gathered active condition, a first frustoconical outer surface engaging said wall of said opening, and a second frusto conical surface, said surfaces defining an apex, the slope of said second surface being greater than the slope of said first surface, the distance parallel to said block axis between said apex and the end of said first surface slope being greater than the distance parallel to said block axis between said apex and the end of said second surface slope, the slope of said first frusto-conical outer surface being slightly greater than the shape of said block opening, and a pair of adjoining ones of said wedges each having, in their opposing radial plane faces, a facing groove for accommodating one of said tendon units.
2. An anchorage device according to claim 1 wherein said distance between said apex and the end of said second surface slope is about half the said distance between said apex and the end of said first surface slope, and wherein, in active condition, a perpendicular to said first frusto-conical surface midway along the length thereof passes through the outer plane face of said block substantially in the same location as said one tendon unit.
3. An anchorage device according to claim 1 wherein said grooves are straight and, in operative condition, are parallel to the axis of said block.