US2783024A - Bar gripping means for use in the production of prestressed concrete - Google Patents

Description

Feb. 26, 1957 D. H. LEE ,78

BAR GRIPPING MEANS FOR USE IN THE PRODUCTION OF PRESTRESSED CONCRETE Filed April 1, 1954 3 Sheets-Sheet 1 4 TTOK/YEYS D. H. LEE 2,783,024 IN THE PRODUCTION ONCRETE 3 Sheets-Sheet 3 I I Illn 2 (if (III I QMM 4 v R l h t x Q w .FN \w///////////// B w \N Feb. 26, 1957 BAR GRIPPING MEANS FOR USE OF PRESTRESSED C Filed April 1, 1954 1 L fi United States Patent BAR GRIPPING MEANS FOR USE IN THE PRO- DUCTION OF PRESTRESSED CONCRETE Donovan Henry Lee, London, England Application April 1, 1954, Serial No. 420,431

Claims priority, application Great Britain May 12, 1953 Claims. (Cl. 254-29) This invention relates to the prestressing of concrete and in particular to means for gripping the prestress bars or the like for the purpose of tensioning them.

The problems involved in devising gripping means for anchoring and holding prestress bars under the high tensile forces employed in prestressed concrete work do not merely comprise the provision of a device strong enough to resist the tensile forces and to grip the bars so that they do not slip, but also the provision of a gripping device which can be used either for post-tensioning or pretensioning, in the former case having ready means of transferring the thrust onto the concrete prior to removing the jack. Unless a gripping device, of the type with which the present invention is concerned, is also a relatively cheap item to produce, it is of little value commercially however effective it is in carrying out its job. In this case the usefulness is increased by the gripping means being adaptable to suit various diameters of bars by the use of different Wedges.

It is therefore the main object of the present invention to provide a gripping device for prestress bars in the manufacture of prestressed concrete which are highly eflicient as to their bar gripping and tension sustaining properties but are also economic to manufacture and use and are suitable for application in a variety of different circumstances and with varying types of apparatus, more particularly different forms of jack.

The gripping means now to be described is applicable to both pretensioning and post-tensioning techniques, that is to say it is not dependent on whether the concrete has been poured and has hardened before the bars, or the like, are tension'ed. This invention is primarily applicable to bars With deformed surface but can if desired be used on plain round bars.

In accordance with known practice in the making of prestressed concrete members the steel is tensioned, either before or after the concrete has hardened. In the former case (pretensioning), steel. thrust plates or the like are held apart by any desired form of strutting while the concrete is placed and sets round the bar or bars. In the latter case (post-te'nsioning) the bar when tensioned and gripped reacts directly against the ends of the hardened concrete.

In accordance with this invention, a bar to be tensioned by either method of prestressing is gripped by a plurality of grooved or serrated wedges retained in a steel or other metal collar with a sloping or tapering internal surface which may, for example, be square tapering or conical, the collar being threaded on its external face to engage by screwing a further threaded collar so that the latter may be turned to occupy any desired position along the threading of the wedge retaining collar and being further co-axiall'y threaded radially inwardly of said external face threading to provide for the direct or indirect attachment of a co-axi'al jack shaft. This further adjustable collar may have sufiicient axial length to the extent to make contact with the beforementioned steel thrust plate or the 'end of the hardened concrete member or alternatively a cylindrical metal distance piece, for example a sleeve or short length of pipe, may be interposed.

The inner or smaller diameter co-axial threading on the Wedge retaining collar may be externally facing on a portion of the collar of reduced diameter or may be internal threading on a cylindrical recess or bore in the collar, the said inner threading being locatable remote from the face of the concrete member, when the gripping means is in situ.

The steel bar held by the gripping device is tensioned by any suitable means, such as a hydraulic jack, the shaft of which is connected either directly to the gripping means by the engagement of threads on the jack shaft with the aforesaid internally facing inner threads on the gripping means where such is provided, or indirectly by means of an intervening threaded sleeve having threaded engagement with the inner externally facing threads on the gripping member and threaded on cotter-and-slot engagement with the jack shaft.

As the bar is stretched in the manner described, the adjustable collar on the outermost threading on the gripping means is screwed forward to maintain contact with the face of the concrete member or thrust plate and when the desired tension or stretch is obtained the jacking means may be disconnected, the tension in the bar being taken by the collar on the steel thrust plate or the hardened concrete member as the case may be, While the concrete hardens in the case of pretensioning, or while grout injected along the length of the duct in which the bar is placed hardens in thecase of post-tensioned concrete.

In the accompanying drawings:

Figure l is a sectional side elevation of one form of gripping means, according to the present invention,

Figure 2 is a section on the line 2-2 of Figure 1,

Figure 3 is a diagrammatic plan view of the general assembly, and I Figures 4 to 6 are each sectional side elevations of modified forms of the gripping means, shown in Figure 1.

In carrying the-invention into effect according to one convenient mode by way of example, as applied to the construction of a prestressed concrete member by the post-tensioning method,.a 'prestress bar L having ribshaped surface deformations 2,- is located in a duct 3 formed in the concrete member 4, see Figure l. The bar 1 is anchored in a gripping device, the subject of the present invention, indicated generally at 5, see Figure 3, to the opposite end of which a jack 6 is connected by its shaft 7 as described hereafter. The jack 6 is braced against the end face '8 of the concrete member 4 by legs 9 and has the usual jack operating means, hydraulic or mechanical (not shown). The arrangement shown in Figure 3 is diagrammatic only to show the general arrangement and application of the gripping mean-s according to the present invention.

The bar 1 is gripped in the gripping device 5' by wedges 10, which as shown may be provided with grooves 11 to accommodate the deformations 2 and provide inter-'- lock while at the same time enabling the wedges to havemajor surface contact with the surfaceproper 1a of the bar, or the wedges may have serrated faces (not shown) which cut into the deformations and surface of the The wedges 10 may be two in number, as shown in Figure 2, and be contained in a tapered hole 12 of rectangular cross section in a wedge collar or block 13 (hereinafter referred to as the wedge collar 13), or may be more than two in number and contained in a hole of conical form (not shown) in which case the wedges may preferably be substa'ntially' a' lit to the circular form of the hole. The Wedge collar 13 is externally of cylindrical form and is threaded concentrically with its axis (as described in' detail hereafter). The threading of smaller diameter, indicated at 14 is formed on a portion of the wedge collar 13 of reduced diameter compared with that of the other threaded portion, indicated at 15 and is located at the end of the wedge collar 13 which will be disposed remote from the face of the concrete member. This threading 14 (hereinafter referred to as the inner threading 14) is for the purpose of connecting the gripping device to the jack, directly or indirectly to the jack shaft 7. Thus an adaptor 16, in the form of a pipe or sleeve, is threaded at one end over the inner threading 14 and at its other end has threaded connection with the jack shaft 7, if necessary through an externally and internally threaded collar 17.

The threading 15 on the larger diameter (hereinafter referred to as the outer threading 15) is for the purpose of engaging an internally threaded thrust collar or sleeve 18 which can be adjusted axially lengthwise of the wedge collar 13, and of the bar 1 as the bar is tensioned so as to maintain its bearing on the face 8 of the concrete member (or hearing plate 19), where such is provided, as shown in Figures 1, 2 and 3. The bearing plate 19 may have a rubber or other resilient backing 20 interposed between it and the face 8 of the concrete member and may be large enough also to provide a bearing for the thrust of the jack through the jack legs 9, as shown particularly in Figures 2 and 3. The bearing plate 19 may further be thick enough to have a grouting hole 21 formed therein to allow for the force of grout in the duct space 3 around the bar 1 in the concrete member 4. To prevent the escape of grout outwardly, the bearing plate 19 may have a rubber or other washer 22 located in a recess 23 aroundthe axis of the bar 1, which is held in position and tightened against the bar by a gland ring 24 threaded into the recess 23.

In use the procedure is to place the bearing plate 19 and backing 20 over the end of the bar, fit the washer 22 and tighten the gland ring 24. The wedges 10 are then placed on the bar and secured thereon with the wedge collar 13, close to the bearing plate and with the thrust collar 18 screwed back. The jack 6 is then placed in position and connected to the wedge collar 13 by the adaptor 16 (or in the alternative ways hereinafter described) with its legs 9 bearing on the bearing plate 19.

When the jack 6 has caused the bar 1 to be fully tensioned and extended (or during this process) the thrust collar 19 is screwed up and tightened against the bearing plate 19, so that when tension is relieved from the jack and the latter removed the bar 1 is held in tension and extension until the bar duct 3 is grouted (for example through the thrust plate grouting hole 21) and the grout has hardened.

The thrust collar 18 is then screwed back, the jack 6 being re-applied if necessary, to make unscrewing easier, and the gripping device 5 removed entirely by dismantling, after which the projecting end of the bar 1 may be cut off.

For use in the production of prestressed concrete members by the pretensioning method, the procedure dc scribed above can be used where the moulds themselves are sufficiently strong to resist the reaction to tension of the prestress bar or bars. Further no provisions for grouting are required since the concrete bonds directly with the bar or bars and usually the bearing plate 19 can be omitted as the end of the mould is normally such as to be able to take the thrust from the jack itself and thrust collar 18 unaided.

When manufacturing pretensioned concrete by the socalled long line method, the gripping devices 5 according to the present invention, may be placed at one or both ends of each prestress bar and extensions or draw bars may be used to connect the gripping devices at one end with the fixed abutment of the pretensioning bed and at the other end with the thrust plate to which the jack or jacks apply the tension to the bars in known manner. Theextensions or draw bars are therefore attached to 4 the gripping devices, according to the present invention, in the same or similar ways as the jack may be attached, as described above and hereafter, but the thrust collar 18 is not needed and therefore the outer threading 15 on the wedge collar 13 is not employed.

The application of the gripping device, according to the present invention, to the production of prestressed concrete by the pretensioned method has been described herein to show pretensioning and post-tensioning methods and is therefore doubly useful.

According to a modified form of a gripping device described with reference to Figures 1 and 2, the inner threading by which the jack is connected to the gripping device may be formed on an inner recessed surface as opposed to an externally facing surface of reduced diameter. Thus, see Figure 4, the wedge collar 13a is provided with a co-axial recess or bore 30, which is internally threaded at 14a to engage a jack adaptor sleeve or pipe i6a connected to the jack shaft 7 as previously described. In order to support the portion 31 of the wedge collar 13a against outward radial pressure auxiliary threads are provided on the outer surface of the wedge collar to engage the thrust collar 18 in continuation of the outer threading 15, a portion 32 being relieved of thread ing to reduce the interthread friction.

According to a still further modification shown in Figure 5, inner threads 14a may again be provided on a recess or bore 30 to engage with a jack adaptor pipe or sleeve 1612, the wedge collar 1311 being provided with a long continuous outer threading 15b on its external periphery which engages a thrust collar 18a of very short axial extent compared with the thrust collar 18 previously described. in order to bridge the gap between the short thrust collar 18a and the face 8 of the concrete member (or the thrust plate 19) a plain intervening member 18b is provided as a distance piece therebetween.

The previously described method of coupling the inner threading 14 or 14a to the jack adaptor sleeve or pipe may comprise providing a jack adaptor sleeve or pipe 16b having therein a slot 33 through which a cotter 34 can pass to engage a slot 35 in the jack shaft 7a.

1 claim:

1. A device for stressing reinforcing bars in concrete that comprises a collar member adapted to surround an end of a reinforcing bar, a tapered inner surface on said collar, tapered bar-encircling wedges adapted to fit between said collar and the reinforcing bar to grasp said bar, a threaded outer surface on said collar, an internally threaded tension-retaining sleeve received on the threaded portion of said collar and serving to retain the tension in said reinforcing bar after it is applied by the. device, a second threaded surface on said collar of lesser diameter than the first, and a threaded tension member received upon said second threaded portion and serving to adjustably connect said collar to a tension-producing means.

2. A device for stressing reinforcing bars in concrete as claimed in claim 1, wherein a pair of wedges are provided which are substantially flat on all but their barcontacting surfaces and are accommodated in the wedge collar in a tapering aperture of corresponding cross section.

3. A device for stressing reinforcing bars in concrete as claimed in claim 1, wherein the plurality of wedges have arcuate outer surfaces in cross-section so as to gether to make up a circular cross section and are accommodated in the wedge collar in a tapering apertureof circular cross-section.

4. A device for stressing reinforcing bars in concrete as claimed in claim I, wherein the bar-engaging surfaces of the wedges are grooved to conform with rib-shaped deformations provided on the prestress bar so as to provide interlock and major surface contact between the wedges and the bar surface proper.

5. A device for stressing reinforcing bars in concrete,

5 as defined in claim 1, wherein the tension member is connected to the tensioning device shaft by a slot and cotter arrangement.

6. A device for stressing reinforcing bars in concrete in accordance with claim 1, wherein the sleeve threaded on to said outer threaded surface of the wedge collar is of sufiicient axial length to extend from a wedge collar to said thrust supporting surface associated with the conerete. Y 7; A device for stressing reinforcing bars in concrete, as defined in claim 1, wherein the sleeve threaded on to said outer threaded surface of the wedge collar is of short axial length and a non-threaded extension is provided between the sleeve and a thrust supporting surface associated with the concrete.

8; A device for stressing reinforcing bars in concrete, as defined in claim 1, in which the second threaded surface on the wedge collar is an external surface of reduced diameter.

9. A device for stressing reinforcing bars in concrete, as defined in claim 1, in which the second threaded surface on the wedge collar is an internal surface of lesser diameter than the threaded outer surface.

10. A device for stressing reinforcing bars in concrete, as defined in claim 1, in which the threaded tension member is disposed coaxially with the bar to be tensioned and is connected to a jacking device for applying tension to the reinforcing rod.

References Cited in the file of this patent UNITED STATES PATENTS 1,597,071 Eubanks Aug. 24, 1926 1,606,789 Hooley Nov. 16, 1926 FOREIGN PATENTS 951,592 France Apr. 18, 1949

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