GB2162449A

GB2162449A - Producing an annular wedge for anchoring a tension element in prestressed concrete

Info

Publication number: GB2162449A
Application number: GB08518109A
Authority: GB
Inventors: Otmar Langwadt; Wolfgang Westerweller
Original assignee: Dyckerhoff and Widmann AG
Current assignee: Walter Bau AG
Priority date: 1984-07-28
Filing date: 1985-07-18
Publication date: 1986-02-05
Also published as: GB8518109D0; DE3427901C2; JPS6138826A; GB2162449B; CA1276777C; IT8567694A0; NO163873B; CH669630A5; ATA222085A; DE3427901A1; IT1182546B; ES8703564A1; ES545582A0; NO852858L; AT396167B; JPH0513770B2; NO163873C

Abstract

An annular steel wedge (2) divided into individual wedge sections (2a, 2b, and 2c Fig. 2, not shown) is used to anchor a tension element (1) in a prestressed concrete structure (4). Prior to forming the individual wedge sections from an axially extending annular wedge blank, the inside surface of the blank is threaded forming circumferentially extending teeth (11) so that the wedge can grip the tension element and provide the requisite friction coefficient in the anchorage. To prevent the sharp teeth formed in the threading step from cutting into the surface of the tension element, the teeth are ground down in a tumbling operation in which the wedge sections (2a, 2b and 2c) are mixed together with ceramic grinding bodies in a tumbling drum. The grinding bodies have bends in at least a portion of their surfaces and the bends have a smaller radius than the radius of the threaded inside surface of the wedge. <IMAGE>

Description

SPECIFICATION Method of producing an annular steel wedge formed of a plurality of parts and used for anchoring a tension element in prestressed concrete The present invention is directed to a method of producing an annular steel wedge made up of a number of parts for anchoring a tension member or element in a prestressed concrete structure. The annular wedge is formed of wedge sections separated from one another by axially extending radially directed joints.

The inside surface of the wedge sections which contact the tension element are formed with a plurality of circumferentially extending teeth. A wedge blank with a frustoconical outside surface is cut into the individual wedge sections after the teeth are cut in the inside surface of the blank forming an axially extending threaded bore.

Numerous prestressing methods with tension elements of steel bars, steel wires or steel wire strands, are used in prestressed concrete construction utilizing the principle of wedge anchors. For individual tension elements, annular wedges are used formed of at least two and at most three wedge sections which laterally enclose the tension element to be anchored. In anchoring a prestressed tension element of this type, the wedge sections are drawn into a conically shaped borehole in an anchoring member in the direction of the tension of the tension element axis by a tensile force applied to the tension element.

As a result, clamping forces directed at right angles to the tension element axis are developed in the wedge sections and these clamping forces prevent the movement of the tension element. To achieve this effect, the friction coefficient between the tension element and the wedge is greater than between the wedge and the conical borehole. Accordingly, the inside surface of the wedge sections are shaped so that the wedge can securely grip the surface of the inside surface of the tension element. For the sake of simplicity, the shaping of the inside surface is formed as a thread cut into axial borehole in a frusto-conically shaped wedge blank before it is cut into the individual wedge sections.

Known wedge anchors for high-strength smooth wires and strands are sensitive with respect to fatigue strength. In practice, it has been found that the wires and strands always break in the front region of the anchor, that is, at the smaller diameter end of the wedges.

Such failures are caused substantially by high transverse compression in the region of the introduction of force by the notches, particularly formed at the commencement of the anchoring operation and caused by the sharp edges of the thread teeth. Since the diameter of the individual wedge sections is somewhat smaller than the outside diameter of the tension element, the sharp edges on the thread teeth at the axially extending edges of the wedge sections, that is, where the thread teeth end along the elongated edges of the wedge sections, cut into the surface of the tension element.

The invention provide method of producing an annular steel wheel made up of a number of parts for anchoring tension elements in a prestressed concrete structure with the annular wedge being formed from a wedge blank into axially extending wedge sections separated from one another by radially and axially extending joints, the wedge blank having an annular axially extending inside surface and outside surface and the inside surface being formed with teeth encircling the axis of the wedge blank and spaced apart in the axial direction of the wedge, the outside surface of the wedge having a frusto-conical shape, the method including the steps of cutting the teeth in the inside surface of the annular wedge blank in the form of a thread, dividing the wedge blank into axially extending wedge sections, after cutting the teeth and subjecting the individual wedge sections to a tumbling operation in combination with loose abrasive bodies with bent or curved surfaces in the abrasive bodies having a radius less than the radius of the inside of the wedge blank. Thus imperfections in the surface of the wedge sections such as burrs, fins and similar imperfections are removed by tumbling these parts in a drum with loose abrasive bodies. The wedge sections are fumbled along with the loose abrasive members during the rotation of the drum so that they rub against one another with the surface of the cast parts being smooth by the grinding action of the abrasive members.

The invention is based on the surprising discovery that when the wedge sections, formed by cutting the wedge blank after a thread is cut in its inside surface, are tumbled along with abrasive bodies in a drum, the thread teeth in the individual wedge sections are not destroyed, rather only the tips of the teeth are rounded off. This tumbling action is preferably performed after hardening or case hardening the wedge sections so that the thread teeth are not ground excessively, however, the grinding operation can also be carried out before hardening under certain circumstances in order to save time.

The abrasive bodies are preferably formed of ceramic material, such as glass powder, alumina, or the like with the possible addition of corundums, metallic oxides or the like.

There is the advantage according to the invention that the thread teeth in the wedge sections no longer cut into the tension element in a manner that damages the surface of the element, rather the teeth only press into the tension element surface. Accordingly, the surface of the tension element is not cut or notched, instead it is only deflected or depressed so that a iocal increase in strength occurs which is approximately comparable to the cold coiling of a thread. With wedges processed in this manner, an increase in the fatigue strength and the static tensile strength of the anchor of up to 100% of the actual strength of the tension element is attained in the region of the free length of the element without any additional steps.

The following is a description of some specific embodiments of the invention, reference being made to the accompanying drawings in which: Figure 1 is an axially extending view, partly in section, of a wedge anchor for a strand or tension element using an annular wedge made up of a number of parts; Figure 2 is a perspective view of the annular wedge shown in Fig. 1; Figure 3 is an end view of the larger end of the wedge shown in Fig. 2; Figure 4 is a sectional view of the annular wedge taken along the line IV-IV in Fig. 3; and Figure 5 is an enlarged view of the detail V in Fig. 4.

In the wedge anchor shown in axially extending section in Fig. 1, a steel wire strand 1 is secured in a conical borehole 3 in an anchor member 4 by means of an annular steel wedge 2 made up of a number of parts.

The anchor member 4 is supported in a known manner with its contact surfaces 5 bearing against an abutment plate, not shown, for transmitting the tensile force.

In Fig. 2 the annular wedge 2 is shown in perspective. The wedge 2 is made up of three axially extending wedge sections 2a, 2b and 2c each of which extends for approximately 120 around the axis of the wedge. In combination, the three wedge sections 2a, 2b and 2c form a closed annular cross-section separated by narrow wedge joints 6, each extending in the axial direction of the wedge, note Fig. 3. While the inside surface of the wedge is cylindrically shaped its outside surface is frusto-conically shaped with the smaller diameter end shown at the right in Fig. 1 and with the larger diameter or thicker end 7 shown at the left. Each wedge section has a groove 8 so that in combination an annular groove formed around the outside surface of the wedge. A spring ring 9 is inserted into the annular groove 8 to hold the wedge sections together for easier assembly.

As shown particularly in Fig. 4, each individual wedge section 2a, 2b and 2c has profiled or shaped surface 11 defining the inside surface 10 of the wedge. The shaped surface 11 is formed as a thread with teeth 1 2 extending circumferentially around the inside surface of the axial bore through the frustoconically shaped wedge blank before it is cut into the individual wedge sections 2a, 2b and 2c.

In accordance with the invention, the wedge sections 2a, 2b and 2c are preferably placed in a grinding or tumbling drum after they are case hardened. Loose abrasive members formed of a ceramic material, such as glass powder, alumina or the like, are mixed with the wedge sections within the drum and the drum is rotated so that a continuous circular motion is developed with the abrasive bodies moving relative to the wedge sections.

Basically, the abrasive bodies can have any desired shape. It is important that the surfaces of the abrasive bodies, at least in a partial region of their surfaces, are provided with bent or curved surfaces having a radius less than the radius of the threaded opening through the annular wedge so that the abrasive bodies can provide a grinding effect on the thread teeth 1 2. The tumbling of the wedge sections and the abrasive bodies need only be carried out until the sharp edges of the thread teeth 1 2 formed when the thread is cut, are rounded off so that they no longer cut into the surface of the tension element, note Fig. 5. The rounding off action also takes place where the thread teeth 1 2 end along the wedge joints 6 so that at this location the thread teeth do not cut into the surface of the tension element 1 and cause it damage.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

Claims

1. Method of producing an annular steel wedge made up of a number of parts for anchoring tension elements in a prestressed concrete structure with the annular wedge being formed from a wedge blank into axially wedge sections separated from one another by radially and axially extending joints, the wedge blank having an annular axially extending inside surfaces and outside surfaces and the inside surface being formed with teeth encircling the axis of the wedge blank and spaced apart in the axial direction of the wedge, the outside surface of the wedge having a frustro-conical shape, the method including the steps of cutting the teeth in the inside surface of the annular wedge blank in the form of a thread, dividing the wedge blank into axially extending wedge sections after cutting the teeth and subjecting the individual wedge sections to a tumbling operation in combination with loose abrasive bodies with bent or curved surfaces in the abrasive bodies having a radius less than the radius of the inside surface of the wedge blank.

2. Method, as set forth in claim 1, includ ing the step of hardening the wedge sections and tumbling the wedge sections with the abrasive bodies subsequent to the hardening step.

3. Method, as set forth in claim 1 or claim 2, wherein the loose abrasive bodies are formed of a ceramic material, such as glass powder, alumina or the like.

4. Method, as set forth in any of claims 1 to 3 least one of corundum, metal oxides and the like to the abrasive bodies

5. A method of manufacturing annular steel wedges substantially as described with reference to and as illustrated in the accom panying drawings.

6. An annular wedge manufactured in ac cordance with the method of any of claim 1 to 5