Note: Descriptions are shown in the official language in which they were submitted.
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The present invention is directed to a method of
producing an annular steel anchor grip made up of a number of
wedge sections for anchoring a tension member or element in a
prestressed concrete structure. The anchor grip is formed
of wedge sections separated from one another along axially
extending radially directed planes. 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 frusto-conical outside surface is cut into the
individual wedge sections after the teeth are cut in the in-
side 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 anchor
grips 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
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 anchor grip is greater than
between the anchor grip 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 tension element.
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lZ767~ 21182-264
For the sake of simplicity, the shaping of the inside surface
is formed as a thread cut into an axial borehole in a frusto-
conically shaped wedge blank before it is cut into the in-
dividual wedge sections, Great Britain patent 869 923.
Known wedge anchor grips 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 anchor grips. Such failures
are caused substantially by high transverse compression in the
region of the introduction of force and by notches, particular-
ly 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 present invention provides, in the most economi-
cal manner possible, for the improved fatigue strength of a
wedge anchor of the aforesaid type.
In particular, the present invention provides
method for producing wedge sections for use in an axially ex-
tending steel anchor grip for anchoring tension elements in a
prestressed concrete structure, said wedge sections being
formed from an axially extending steel wedge blank which has
a generally cylindrical inside surface and a frusto-conical
outside surface; said method comprising:
i) cutting teeth into the inside surface of the wedge
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21182-264
blank in the form of a thread, said teeth encircling the axis
of the wedge blank and being spaced apart in the axial
direction thereof;
ii) cutting the steel blank along planes extending
radially and axially relative to the axis of the wedge blank to
form a plurality of axially extending wedge sections;
iii) subjecting the individual wedge section to a
tumbling operation along with loose abrasive bodies with bent
or curved surfaces; said bent or curved surfaces having a
radius less than the radius of the inside surface of the wedge
blank.
The invention also provides an axially extending
steel anchor grip for anchoring tension elements in a prestres-
sed concrete structure formed from a plurality of steel wedge
sections adapted to be arranged in a conical borehole of an
anchor member located in said prestressed concrete structure;
said wedge sections being separated from each other along
planes extending radially and axially relative to the axis of
the anchor grip, and said wedge sections having an axially
extending inside surface and outside surface, said inside sur-
face being formed with teeth extending circumferentially
relative to the axis of the anchor grip and spaced apart in the
axial direction of the anchor grip; each said tooth having a
pair of flanks converging to a radially inwardly directed tip,
and said flanks, only at said tips being rounded off and with-
out otherwise deforming said flanks, so that said teeth only
press into the surface to the tension element and do not cut
into the surface, said anchor grip having an outside surface
having a generally frusto-conical shape and an inside surface
having a generally cylindrical shape.
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21182-264
The invention is based on the method, known per se,
of removing imperfections from the surface of cast parts or the
like, such as removing burrs, fins and similar imperfections
by tumbling these parts in a drum with loose abrasive bodies
usually formed of ceramic material. The wedge sections move
along with the loose abrasive members during the rotation of
the drum so that they rub against one another with the surfaces
of the case parts being smoothed by the grinding action of the
abrasive members. This type of surface processing is character-
ized as tumbling or slide grinding.
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 indivi-
dual wedge sections are not destroyed, rather only the tips of
the teeth are rounded off. This tumbling action is preferably
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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 local 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.
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In the accompanying drawings which illustrate an
exemplary embodiment of the invention,
Figure 1 is an axially extending view, partly in
section, of an anchor grip for a strand or tension element
using an annular wedge made up of a number of wedge sections;
Figure 2 is a perspective view of the anchor grip
shown in Figure l;
Figure 3 is an end view of the larger end of the
anchor grip shown in Figure 2;
Figure 4 is a sectional view of the anchor grip
taken along the line IV-IV in Figure 3; and
Figure 5 is an enlarged view of the detail V in
Figure 4.
In the anchor grip shown in axially extending
section in Figure 1, a steel wire strand 1 is secured in a
conical ~orehole 3 in an anchor member 4 by means of an
anchor grip 2 made up of a number of wedge sections. The
anchor member 4 is supported in a known manner with its con-
tact surface 5 bearing against an abutment plate, not shown,
for transmitting the tensile force.
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21182-264
In Figure 2 the anchor grip 2 is shown in perspec-
tive. The anchor grip 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 combina-
tion, 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 Figure 3.
While the inside surface of the anchor grip generally is
cylindrical in shape its outside surface is frusto-conically
shaped with the smaller diameter end shown at the right in
Figure 1 and with the larger diameter or thicker end 7 shown
at the left. Each wedge section may have a groove 8 so that
in combination an annular groove is formed around the outside
surface of the wedge. A spring ring 9 may be inserted into
the annular groove 8 to hold the wedge sections together for
easier assembly.
As shown, particularly in Figure 4, each individual
wedge section 2a, 2b and 2c has a profiled or shaped surface
11 defining the inside surface 10 of the wedge. The shaped
surface 11 is formed as a thread with teeth 12 extending cir-
cumferentially around the inside surface of the axial bore
through the frusto-conically 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
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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 12. The tumbling of the wedge sections and the abrasive
bodies need only be carried out until the sharp edges of the
thread tee-th 12 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 12 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.
