GB2106947A

GB2106947A - Pre-stressed structural member

Info

Publication number: GB2106947A
Application number: GB08138772A
Authority: GB
Inventors: Abraham Behar
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-09-17
Filing date: 1981-12-23
Publication date: 1983-04-20
Also published as: IT8223129A0; JPS58106044A; IT1152533B; DE3151783A1

Abstract

In order to prevent radial expansion and axial retraction of the pre-stressing steel 4 in a body of cast concrete A when the tension in the steel is released, a generally cylindrical barrel 20 containing a wedge 26 is permanently fitted on the steel adjacent to the surface B of the concrete body. The wedge 26 has an external frusto-conical surface 28 converging towards the body of concrete, which engages an internal frusto-conical surface 24 of the barrel 20, and an internal cylindrical surface 30 which grips the steel. Adjustable means such as a nut 36 is provided engaging the wedge 26 and the barrel 20 to prevent relative movement between them after the release of tension in the steel 4. The wedge and barrel assembly may alternatively be positioned within a recess in the surface of the cast body and may be attached to a plate in surface-to-surface engagement with the body. <IMAGE>

Description

SPECIFICATION Apparatus for pre-stressing concrete structural member The invention provides, in a pre-tensioned cast concrete body, means for preventing radial expansion of the steel within the body and its retraction into the body on release of tension on the steel, and therefore the elimination of a development length zone within the cast body, by the provision of interfitting barrel and wedge means surrounding the steel at the location of its entrance into the cast body, together with means for preventing relative movement between the wedge means and the barrel after release of tension on the steel.

Fig. 1 is a schematic view showing the conventional means used in forming a cast, pre-stressed concrete member, illustrating the development length zone resulting from the use of such conventional means; Fig. 2 is a view which is similar to part of Fig. 1 showing one positioning of the barrel and wedge means in accordance with the invention; Fig. 3is an enlarged sectional view showing one end of a cast concrete body and disclosing an embodiment of the means provided by the invention; Figs. 4 and 5 are views which aresimilarto Fig. 3 and show further embodiments of the invention, and Figs. 6, 7 and 8 show embodiments of the invention in which the cast body is provided with an end plate.

In casting pre-stressed cast concrete structures, such as beams, it is conventional to provide wedge means external to the piece to be cast for holding the tensioned steel from movement after tension is removed from the strand, which may be a wire, cable, rod or the like and which is referred to hereinafter, as in the industry, by the term "the steel".

The release of tension on the steel causes two phenomena to occur, the first being that the embedded steel tends to expand in diameter, and the other being that the steel tends to move into the cast concrete in a direction longitudinal to the steel.

These effects result in weakening the concrete in a zone surrounding the steel and extending inwardly from the external surface of the cast member for a distance which may be a large multiple of the diameter of the steel, this weakened zone being known in the industry as the "development length".

In Fig. 1 of the drawings there is shown, for purpose of illustration of the prior art, the conventional assembly of parts used in the manufacture of pre-tensioned pre-cast structural members. Such an assembly includes a form 2 within which the concrete body A is cast and into which body a tensioned steel strand 4 is passed and protrudes from the other end of the body.Outside each end of the form there is provided means for holding the tensioned steel, which comprises a wedge assembly including barrel 6 which rests on a fixed member 8 and has an internal frusto-conical surface 10 on which wedge means 12 is positioned in surrounding relation to the steel. Atensioning force in the direction of the labeled arrows of Fig. 1 is applied to the steel, the concrete is cast within the form, and after the concrete is fully set the tension is released by cutting the steel between fixed member 8 and the cast body.

The purpose of the wedge means is to hold the steel in tension until the cast body has set.

It is often desirable or necessary, for example when two cast members are to be connected end-to-end, to attach an end plate 14 to the cast body, and because of the existence of the development length zone additional means must be provided for securing the end plate to the body. This is conventionally done by welding one or more elongated rods 16 to the inner surface of the end plate, each rod having a length greater than the depth of the development length zone, as shown in Fig. 1.

It has therefore been the object of this invention to provide means by the use of which the development length zone is prevented in the manufacture of cast concrete bodies, such as structural members, beams, columns, piles and the like.

In accordance with the invention, and as shown in Fig. 2, the assembly of barrel and wedge means is positioned in surrounding relation to the steel at or adjacent the location of the entrance of the steel into the body of concrete A, and means are provided for connecting the wedge means to the barrel to prevent relative movement between them when tension on the steel is released.

A first embodiment of the invention is disclosed in Fig. 3, in which the steel 4 enters the cast body of concrete A at the surface B of the body. A barrel 20 rests on surface B and has a threaded outercylindric- al surface 22 and an internal frusto-conical surface 24 which converges toward the body of concrete and defines an inner space within which is positioned wedge means 26 which has an outer frusto-conical surface 28 which lies in surface-to-surface engagement with the inner surface 24 of the barrel. The wedge means also has a cylindrical interior surface 30 which surrounds and tightly engages the steel.

Means are provided by the invention for preventing relative movement between the wedge means and the barrel after adjustment of the wedge means into engagement with the steel and, most particularly, after release of tension on the steel. In the embodiment being described the outer surface 22 of the barrel is connected to the internal threads 32 on the skirt 34 of a nut 36 which at its upper, or outer, end has an inturned annular flange 38 which extends across the upper, or outer, end surface of the barrel and overlies the upper, or outer, end of the wedge means.

With the parts in place as described, tension is applied to the steel and concrete is poured within a conventional form (not shown in Fig. 3) and allowed to set. The tension on the steel is now released by cutting the steel external to the barrel and wedge assembly. As stated, the effect of this on the steel is to cause it to tend to expand radially within the cast body and to tend to move into the cast body, with the adverse results described above, including the formation of the development length zone. However, the apparatus provided by the invention prevents these adverse results.The enlargement of the steel cannot take place because of the engagement of the abutting inclined surfaces of the wedge means and the barrel, and the resitance of the flange 38 of nut 36 to any movement of the wedge means outwardly in the direction of the steel prevents any loosening of the wedge means within the barrel which might be caused by a component parallel to the steel of the radial pressure of the wedge means on the barrel and which, if it occurred, would permit retraction of the steel into the cast body.

As described above, in accordance with the invention the wedge and barrel assembly is positioned at or adjacent the entry of the steel into the cast body, and in Fig. there is disclosed an embodiment of the invention in which it is positioned within a recess 40 in the end of the cast body A. The barrel 42 is positioned within the recess with its external surface preferably in surface-to-surface engagement with the wall of the recess, its lower surface in engagement with the bottom of the recess, and its inner cylindrical surface 44 and its inner frusto-conical surface 46 surrounding the steel 4. The wedge means 48 is positioned within the barrel, also surrounding the steel, with its external frusto-conical surface in surface-to-surface engagement with the corresponding inner surface of the barrel.The upper end of the barrel is externally threaded at 50 to receive the internal threads on the skirt 52 of a nut 54 which has an inwardly extending flange 56 which surrounds the steel and overlies the upper, or outer, end of the wedge means and is moved into engagement with the wedge means before tensioning the steel, thereby preventing any relative movement between the barrel and wedge means, particularly after release of tension on the steel.

A further embodiment of the invention is disclosed in Fig. 5 in association with a cast concrete body A having surface 60 through which steel 4 extends into the cast body. A barrel 62 has its one end in engagement with surface 60 (or with a plate engaging the surface) and is provided at its inner end adjacent the cast body with an internal frusto-conical surface 64 which converges toward the cast body.

Wedge means 66 are positioned within the barrel in surrounding relation to the steel and engage frustoconical surface 64. At its outer end the barrel is provided with an internal cylindrical threaded surface 66 within which a threaded nut 68 is received, also in surrounding relation to the steel, the parts being so constructed and arranged that the nut may be screwed inward into engagement with the outer end of the wedge means to lock the wedge means to the barrel.

As described above, it is often desirable or necessary to attach an end plate to the cast structural member, and in Figs. 6, 7 and 8 there are disclosed embodiments of the invention including such end plates. Thus, in Fig. 6 the end plate 14 has integrally formed thereon a cylindrical flange 70 which extends inwardly of the form 2 from the inner surface of the plate and which is positioned concentrically with an opening 72 in the end plate through which the steel 4 extends, it being understood that any number of such openings and surrounding flanges may be formed in the end plate at each of the form. The flange 70 is internally threaded, as shown, for threaded connection to a cylindrical barrel 74 which is provided with a central opening therethrough which, adjacent the end plate 14, has a frusto-conical surface 76 with its larger end adjacent the end plate.

Within the central opening in the barrel there are provided a plurality of wedge members 80 which are positioned in surrounding arrangement in close engagement with the steel with their outer surfaces in engagement with the frusto-conical surface 76 of the barrel, the larger ends of the wedges being adjacent the plate 14.

In the embodiment disclosed in Fig. 7 a threaded opening 80 is provided in end plate 14, and within this opening and in the general plane of the plate there is threaded a cylindrical plug 82 which has a central un-threaded opening through which the steel passes into the cast body and which extends within the body below the inner surface of the end plate, thus forming an interior abutment surrounding the steel. A barrel 84 is internally threaded at its upper end for adjustable attachment to the external threads of the plug 82, and below these threads, or internally of the form, the barrel is provided with an internal frusto-conical surface 86 which is concentric with the steel and plug 82 and has its larger end adjacent the inner end of the plug.A plurality of wedge segments 88 are provided within the barrel and have external surfaces corresponding to the internal frusto-conical surface of the barrel.

In the embodiment of the invention disclosed in Fig. 8 the upper end of the barrel 90 is externally threaded for adjustable connection within an inter nallythreaded opening 92 in plate 14, and is also internally threaded to receive an externally threaded plug 94 which has a central opening through which the steel 4 passes. The assembly of wedges 96 surrounds the steel within the barrel and co-operates with the frusto-conical internal surface 98 of the barrel in the same manner as in other forms of the invention.

It is important to note that in all forms of the invention, the wedges cannot become loose after the parts are adjusted prior to pouring concrete, because the barrel and wedges are moved into engagement with a fixed part, after which the barrel, wedges and fixed part form a rigid assembly which prevents expansion of the steel and retraction of the steel into the cast concrete after release of the external means which applies tension to the steel.

Claims

1. A pre-stressed cast concrete body comprising: (a). a body of concrete having a surface, (b). a tensioned steel extending through the surface into the body, (c). a generally cylindrical barrel positioned adjacent the surface having an internal frusto-conical surface surrounding the steel and converging to ward the body of concrete, (d). wedge means within the barrel having an external frusto-conical surface converging toward the body of concrete and engaging the internal surface of the barrel, and (e). adjustable means engaging the wedge means and the barrel preventing relative movement between them.

2. A pre-stressed cast concrete body according to claim 1, in which the barrel and wedge means are positioned outside the cast body with the barrel in contact with the cast body.

3. A pre-stressed cast concrete body according to claim 1, in which the cast body has a recess therein opening into said surface thereof, and the barrel and wedge means are positoned within the recess.

4. A pre-stressed cast concrete body according to claim 1, in which a plate is positioned in engagement with the surface of the cast body in surrounding relation to the steel, and the barrel is positioned in engagement with the outer surface of the plate.

5. A pre-stressed cast concrete body according to claim 1, in which the barrel is externally threaded, and comprising in addition a nut having a skirt part internally threaded to the barrel and having at its one end an inturned annular flange which overlies and engages the larger end of the wedge means to prevent relative movement of the wedge means with respect to the barrel in the direction of the steel.

6. A pre-stressed cast concrete body comprising: (a). a body of concrete having a surface, (b). a tensioned steel extending through the surface into the body, (c). a generally cylindrical barrel positioned adjacent the surface having an inner internal frusto-conical surface surrounding the steel and converging toward the body of concrete, and an outer internal cylindrical threaded surface, (d). wedge means within the barrel having an external frusto-conical surface converging toward the body of concrete and engaging the internal surface of the barrel, and (e). a nut threaded to the outer internal cylindrical surface of the barrel and engaging the outer end of the wedge means.

7. A pre-iensioned, pre-stressed, pre-cast concrete structural member comprising a body of cast concrete having at least one surface, a plate in surface-to-surface engagement with the surface, a tensioned steel extending through the plate and into the body, and means within the body connected to the plate and operable to resist transverse expansion of the steel and movement of the steel longitudinally into the concrete body, said means comprising wedge means at least partially surrounding and engaging the steel, and means for adjustably connecting the wedge means to the plate.

8. The structural assembly according to claim 7, in which the last named means comprises a barrel having a frusto-conical interior surface engaging the corresponding surface of the wedge means, and an adjustable connection between the barrel and the plate.

9. The structural assembly according to claim 7, in which the means within the concrete body comprises an annular flange integrally formed with the plate and extending inwardly therefrom into the cast body in surrounding relation to the steel and being integrally threaded, an externally threaded barrel threadedly connected to the flange within the cast body and having an internal frusto-conical surface with its large end adjacent the plate, and wedge means within the barrel in surrounding relation to the steel and having its wedge surface engaging the corresponding surface of the barrel.

10. The structural assembly according to claim 7, in which the means within the cast body comprises a threaded opening in the plate, a cylindrical plug externally screw threaded into the opening and having a central opening through which the steel extends, a barrel having internal screw threads at one end threaded to the plug and having a frusto-conical internal surface concentric with the plug and extending in the direction internally of the cast body from the plug, and wedge means within the barrel in surrounding relation to the steel and having its wedge surface engaging the corresponding surface of the barrel.

11. The structural assembly according to claim 7, in which the means within the cast body comprises a threaded opening in the plate through which the steel extends, a barrel surrounding the steel and having internal and external screw threads at its one end and being connected within the plate opening by its external thread, an annular plug threaded into the end of the barrel and positioned generally in the plane of the plate and surrounding the steel, the barrel having a frusto-conical internal surface concentric with the plug and extending in the direction internally of the cast body from the plug, with its largest surface adjacent the plate and the plug, and wedge means within the barrel in surrounding relation to the steel and having its wedge surface engaging the corresponding surface of the barrel and its largest end engaging the plug when the parts are in operative positions and relations.

12. A pre-stressed cast concrete body which is substantially as described with reference to Figures 2and3,4or5.

13. A pre-stressed cast concrete body which is substantially as described with reference to Figures 6,7or8.