Static structures (e.g. – buildings) – Machine or implement
Reexamination Certificate
2000-09-21
2004-07-20
Friedman, Carl D. (Department: 3635)
Static structures (e.g., buildings)
Machine or implement
C052S749100, C052S600000, C052S260000, C052S223400, C052S431000, C052S435000
Reexamination Certificate
active
06763646
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Technical Field of the Invention
The present invention concerns a method of introducing shear forces into a concrete body, in particular, through the side face of a concrete slab, in which the force is absorbed by an axially extended shear force rod, e.g. a shear force mandrel or a sleeve around the shear force mandrel, is diverted from the shear force rod by a shear force strap connected to the shear force rod at at least one point close to the surface of the concrete and one point distant from the surface of the concrete and is transmitted to the concrete.
The invention also concerns an element for introducing shear forces into a concrete body, in particular, through the side face of a concrete slab, with an axial shear force rod, e.g. a shear force mandrel or a sleeve around the shear force mandrel, and with a shear force strap fixed to the shear force rod at at least one point close to the surface of the concrete and one point distant from the surface of the concrete, for transmitting shear force to the concrete, and it also concerns a concrete body with a shear force element according to the invention.
2. Brief Description of Art
The most diverse forms of shear force absorbing pins are known, in which the cross section of the pin is increased in the stressed zones in order to reduce the local compressive forces occurring at the transition between pin and concrete. However, the load capacity of the concrete slab is reduced by the proximity of these pins to the surface of the concrete on the compression side.
An “Investigation into shear pins of reinforcing steel embedded in concrete” published in 1983 in Vol. 346 of the “Deutscher Ausschuss für Stahlbau” examined ten different reinforcements by which the shear forces introduced into the side face of a concrete body could be distributed in order to increase the load capacity of a concrete slab in the zone of the shear forces introduced. It emerged from this that only two of the test arrangements examined were fully effective. These were two-shear, rearwardly anchored loops in direct centric contact with the shear pin (p. 144, right column, paragraph 2). These loops were made of round steel bars at least 10 mm thick.
Because unified solutions are often more economic than individual ones on construction sites, prefabricated elements have been developed which can be incorporated in the shuttering as ready-made products. In the specialized trade two elements for introducing shear forces are known in particular, and are described below with reference to
FIGS. 1
to
3
.
The shear force strap in
FIGS. 1 and 2
is shown cast into a concrete slab
10
and has a steel plate
13
placed in the side face of the concrete slab
11
and two straps
15
,
15
′ welded to this steel plate. Plate
13
has a central opening
17
in which a mandrel
19
is inserted. The two straps
15
,
15
′ are set parallel to each other and away from the concrete surface of plate
13
and, at a distance from the mandrel axis
21
, are bent in a direction parallel to the mandrel axis
21
. The end portions
23
of the reinforcing steel forming the straps
15
,
15
′ are bent through about 165 to 170 degrees at a relatively large distance from the side face of the slab
11
and return to meet the end of mandrel
19
about halfway back to the side face of the slab. The straps
15
,
15
′ are formed symmetrically to axis
21
, so that shear forces in two opposite directions can be transmitted to the concrete slab by the same strap. The four ends
23
of the straps are linked to each other and to the mandrel
19
.
A part of the force (arrow
25
) introduced into this strap via the shear force mandrel is transmitted as tensile force to the upper part of strap
15
. Because of the eccentric introduction of the force into the mandrel, said mandrel has a tendency to twist away, so that it transfers compression forces to the ends
23
of the upper part of straps
15
,
15
′. Bending forces act, in particular, on the horizontal part of the strap. In addition, a part of the force introduced is transmitted to strap
15
as compression force in the lower part of strap
15
. This compression force is transmitted to the concrete slab
10
in the vicinity of the underside
27
of the slab.
The strap
15
shown in
FIGS. 1 and 2
is a two-shear loop anchored rearwards with direct centric contact to the mandrel, and differs from the arrangements which have proven effective in the study mentioned only in that the rearwardly anchored reinforcing rods
15
,
15
′ are in centric contact with the mandrel by means of a plate
13
and in that the ends
17
are bent round and fixed to the mandrel.
The device
31
shown in
FIG. 3
acts in a similar way to the device in
FIGS. 1 and 2
. Tensile and compressive forces are transmitted by the shear force mandrel
32
via the rigid plate
33
on the side face of a concrete slab to the two arms
35
of the flat steel strap
37
formed symmetrically on either side of the mandrel
32
, which arms are welded to plate
33
. Bending forces which are difficult to calculate occur in the strap arms
35
, which extend into the concrete in a convergent direction from plate
33
and are fixed to the shear force mandrel
32
at a distance from plate
33
. Here, too, the compressive forces are transmitted to the concrete lying in the direction of the pressure close to the surface of the concrete.
A disadvantageous feature of these prefabricated elements is that compressive forces arise close to the surface of the concrete on the compression side. There is therefore a danger that fragments of concrete will split away. A further disadvantage is that the resulting flow of forces in the concrete remains unclear. The forces arising can hardly be calculated, since the calculations cannot be based on any simple model.
It is therefore the objective of the invention to create a method and a device for introducing shear forces into a concrete body whereby the introduction into the concrete of compressive forces close to the surfaces of the concrete and directed towards these surfaces is avoided. Furthermore, the device shall be capable of being formed symmetrically, so that incorrect insertion of the device on the construction site is made impossible. In addition, the forces are to be calculable according to a simple model.
BRIEF DESCRIPTION OF THE INVENTION
This objective is achieved according to the invention in that, in a method of the type mentioned at the outset, the strap is so formed around a concrete core forming part of the concrete body that substantially only tensile forces can arise in the strap. The strap is advantageously formed in an arc around the concrete core. Thereby the compressive forces in the concrete occur inside the arc section. As the arc section is arranged on the tension side, and if the shear force rod is arranged at approximately the center of the thickness of the slab, a larger part of the slab's thickness certainly lies on the compression side than on the tension side of the strap.
The arc, preferably of flat material, is advantageously formed symmetrically around the concrete core. The symmetry permits simpler calculation of the forces arising, as the components aligned perpendicularly to the axis of symmetry cancel each other out. The sum of the force vectors therefore forms a vector on the axis of symmetry. The loading on the concrete is especially consistent when a planar arc section is used, as only compressive forces arise in the concrete, similarly to those in a vault. A planar arc section has the advantage of smaller local compressive forces and a three-dimensional play of forces, increasing the local load capacity of the concrete.
Advantageously, a first strap or strap section connected to the shear force rod is formed around a concrete core close to the surface of the concrete, and a second strap or strap section connected to the shear force rod is formed around a concrete core distant from the surface of the concrete. In this way the moment a
Friedman Carl D.
Nguyen Chi Q
Simons William A.
Wiggin and Dana
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