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1. (WO1979000856) A DEVICE FOR TENSIONING A REINFORCING STRAND
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A DEVICE FOR TENSIONING A REINFORCING STRAND
Reinforcing strands in concrete structures are often subjected to a tensioning action during the erection. On those occasions when the reinforcing strands are located in passages in the concrete structure, and the casting of the latter is' completed before tensioning is applied to the strands, it will, especially with long strands, and with strands which do not run rectilinearly , be found that friction against the walls of the passages causes a very uneven stress distribution along the length of the individual strand. The tension is highest at the end where the pulling force is applied, and the tension will be reduced in the direction away from said end.
In order to bring about a levelling of the stress distribution along the strand the inventor has previously proposed to terminate the static, stepwise tensioning by a sudden stress change, which causes a shock wave to pass along the strand. During a short moment of time this shock will release the local frictional grips between the passage walls and the strand.
When this activity is terminated the passage is filled with concrete slurry so the strand will be firm- . ly bonded to the structure.
This shock wave has been caused by a hydraulic ram, sometimes subjected to the action of a pressuri-zed gas, stored in a container. A shock wave for causing the desired stress relieving in the strand has a very short duration - it is a question of parts of a second -and experience has shown that it is very difficult to accelerate the means for jerking the strand. The rapid sequence will further make it. necessary to use short conduits and large flow areas.
A substantial simplification of the handling is obtainable according to the invention by using a device for tensioning the strands and being provided with means adapted to supply the cylinder with gas obtained by the burning of powder or some similar substance, A satisfactory tensioning of a strand will comprise a static tensioning, usually performed in severa steps, followed by at least one jerk.
For the static tensioning a double acting ra of arbitrary type may be used, and the aim of the present invention is, in the first hand, to propose a device for causing the jerk, which in a simple manner may be devised so it can be used for the static tensioning also.
According to a development of the invention the device may include an accumulator, at least one portion of which forms an axial extension of its cylinder. A support determining the inward movement of the piston into the cylinder is then formed as a fixed annular flang between the cylinder and the accumulator, and a cooperat-ing slave piston is adapted to form a temporary seal between the cylinder and the accumulator, said slave piston turning a smaller surface towards the accumulator, than towards the cylinder of the ram.
A connecting duct having a governing valve may be provided for occasionally communicating the accumulator with a space between the support flange and the slave piston.
The device may alternatively include a stati nary piston and an enclosing cylinder, which at one end is formed with means for the attachment of the ends of a number of strands, while its other end includes a bottom structure comprising at leat one loading chamber, which may be closed by a breech mechanism.
The cylinder is preferably provided with a number of exhaust ports in its shell, located at a distan ce from the bottom structure corresponding to the desired stroke.
The exhaust ports may be arranged in series at varying distances from the bottom structure, and a closure means is provided to select one series, while closing the remaining ports, to make possible a variation of the stroke length.

The cylinder may be provided with a firing mechanism communicating with the combustion chamber by a passage directed towards the end of the powder charge protruding into the combustion chamber.
Three embodiments of the invention will below be described with reference to the accompanying drawings, in which
Figure 1 shows a device, actuated by powder gas, and
adapted to be used for the static tensioning as
well as for the jerk,
Figure 2 shows a modified embodiment of the arrangement
according to Figure 1 , and
Figure 3 shows a device where the cylinder is moveable
in relation to the stationary piston.
In Figure 1 10 denotes a portion of concrete structure, which is to be reinforced by a reinforcing strand 11, which extends through a passage 12 in the structure. It is presupposed that the opposite end of the strand is firmly anchored to the structure.
The tensioning is caused by a device, generally denoted by 13 » and provided with supporting legs for engagement with the concrete structure. The device comprises a ram I k , which may be used for the static tensioning, as well as for the jerk.
The ram h includes a cylinder 15» 16 and a piston 17 operating therein, the piston being mounted upon a tubular piston rod 18. This encloses the end of strand 11 and passes sealingly through end plates, outwardly defining the ram 1 proper and an accumulator part 1 - Ob-viously the strand may be lengthened outside of the concrete structure by an extension member, the latter then passing through the ram. Piston rod 18 has a sufficient length to maintain its sealing contact with the end plates during the stroke movements of piston 17*
Beyond the path of the piston rod there is a first locking member 19» by means of which the strand 11 can be obtained after a tensioning step, while piston 17 is returned in preparation for the next step. There are several kinds of suitable locking members, so no detailed description is considered necessary.
A second locking member 20 is attachable to strand 11, or possibly upon.piston rod 18, outside of the ram. The second member 20 locks strand 11 before piston 1 performs its active stroke, and will retain the strand so the first locking member 19 may be set free. When the piston has performed its stroke the second member 20
(with the end of the strand) has been pushed a correspond ing distance away from the ram.
Locking members 19 and 20 will thus be close and opened, respectively, in step with the extension of the strand's end away from the concrete structure.
The end of piston rod 18 extending out of ra 1 is provided with a stop 21 , which determines how far piston 17 m y be retracted into the cylinder. The stop 21 may be designed in such a manner that it becomes adjust-able in the axial direction of the rod.
The portion of ram 1*. located between piston 17 and the supporting legs is formed as a pressure chamber, which will be charged by gas obtained by the burning of powder, or some other suitable substance. A breech lock mechanism 22 is mounted at the wall of chamber 15» The charge may be formed as powder bags, cartridges 23 *r fuse like units having a suitable size and composition for causing the desired result. It may be ignitied by means of a firing pin 2k , or in any other suitable manner.
The size and the shape of the charge is selected so the gases, during an ordinary tensioning stroke provides a moderate piston speed. The return stroke of the piston may be obtained in any suitable manner, not shown in the drawing. For the jerking action a rapidly burning powder charge is used, producing gases which provide a predetermined pressure sequence. By using a suit- able charge it is possible to obtain not only the pulling force desirable on each occasion, but also a desired shape of the pressure curve.
For the sake of simplicity the drawing shows a single strand only, but it is evident that the device may be adapted for simultaneously tensioning a number of parallel strands located in the same passage.
Figure 2 shows a more sofisticated arrangement of the basic design illustrated in Figure 1. Chamber 1 s here formed as an accumulator and between the latter and cylinder 16 of the ram there is an annular support flange 25. which replaces stop 21 at the embodiment in
Figure 1 , and which will limit the inward movement of piston 17*
This support flange has such an extension in the radial direction, that a passage 26, having a large flow area, will be provided. This passage may be closed by a stepped slave piston 27 » which cooperates with piston 17 of the ram during the jerking action only. The slave pis-ton 27 turns it bigger end face towards the ram cylinder 16, and its smaller face towards accumulator cylinder 15» and is furthermore adapted to sealingly engage support flange 25 » against which it is normally forced by the pressure in the ram cylinder.
For the sake of simple understanding the space between piston 27 and ram piston 17 is termed the plus side (a supply of pressurized fluid here will tension the
strand) , while the space at the opposite side of the ram piston is termed the minus side (a supply of pressurized fluid here will cause the piston to return from its active stroke) .
As chamber 15 here is formed as an accumulator it is possible, by burning a certain amount of powder, to obtain a volume of gas, which is sufficient to perform one or several strokes. The gas pressure may be augmented by burning a further charge.
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< WiPO ^ A conduit 28 and branch conduits 29 > 30 extending therefrom communicate accumulator chamber 15 with the plus and the minus sides of ram cylinder 16. A multiple way valve 31 governs the flow of gas from the accu-mulator chamber to the ram in such a manner that, its piston 17 will be moved forwards and backwards, as desired,-for performing the static tensioning.
A further conduit 32 communicates the accumu lator chamber 15 with the space between support flange 25 and slave piston 27. Gas flow through conduit 32 is governed by means of a valve 33- During the static tensioning conduit 3 between the valve 33 and said space is open to the atmosphere in order to ensure a contact pressure and sealing between flange 25 and slave piston 27.
When valve 33 is opened gas will low from the accumulator to the space at the back of the slave piston 27 and lifts the same away from support flange 25* The pressurized gas in the accumulator can then act against the full area of the slave piston, and there is a rapid flow of gas through passage 16. The slave piston 27 will bring the ram piston 17 along, and this movement is, in the manner above described, transferred to the strand, which will be subjected to a sudden jerk, which like a wave travels along the strand and brings about the desired equalization of local stress accumulations in the latter.
Figure 3 shows a modified embodiment of a device, in which the cylinder is movable in relation to a stationary piston.
Each passage in the concrete structure may hold a number of parallel strands, and Figure 3 shows a portion of a concrete structure 50 , where each passage 51 encloses a number of strands 52.
Each passage 51 1S terminated by a support 53 mounted flush with the wall face and provided with an inwardly tapering opening. The individual strands 5 are

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Z° lead outwards along the wall of said opening, and may be locked against this wall by means of a conical wedge 5^, which will be driven into the support after each tensioning operation. Care must be taken that sufficiently long ends of the strand protrude from the support 53 to make possible a satisfactory attachment to the tensioning device.
A support ring 55 is mounted at the outward face of the support 53 » and a piston member 56, comprising a head 7 and a piston body 58, is attached to the support ring 55.
The head 57 has a bigger circumference than the piston body 58 and is provided with a number of grooves 59 through which the ends of strands _j1 may pass freely. An axially displaceable plunge 60 is fitted in the piston head and is biased by a spring 61 towards wedge 5 .
During a tensioning of strands 52, wedge 5^ will be pulled free, and will compress spring 61 byway of the plunge. When the tensioning is terminated spring 61 and plunge 60 will force wedge 5^ into the support opening, thereby locking the strands in their new position, 'instead of using a spring 61 for acting upon the plunge, it is possible to use a pressure fluid, which may be governed in known manner so the action upon the wedge will be more precise and forceful.
The piston body 58 is enclosed by a cylinder 62 , which at its end turned towards the head 57 of the piston member is formed as a radially outwards directed flange 63• This is provided with a number of through bores 64 for the strand ends, which will be individually locked in relation to the flange by suitable members 65 of
arbitrary known type.
An axially directed loading chamber 67 is formed in the bottom structure 66 outwardly defining cylin-der 62. A stick of powder 68 or a cartridge may be fitted into this chamber, which is then closed by a breech mechanism 69 of suitable known type.

In the starting position a combustion chambe 70 will remain between the bottom structure 66 and the en face of the piston member, and the stick of powder 68 will reach into this combustion chamber. An ignition mechanism 71. containing a firing cartridge 7 is attached to the cylinder, and communicates with the combustion chamber by way of a passage 73 » directed towards the stick of powder. Hereby an ignition of the stick of powder will occur at the end of the stick projecting into the combus-tion chamber.
In order to facilitate use of charges of different magnitude a number of parallel chambers may be pro vided in the bottom structure of the cylinder.
The firing cartridge 72 may be ignited in any suitable manner, for instance by way of an electrically activated firing pin 7k . The issuing hot jet of gas will sweep around the combustion chamber and ignite the stick of powder 68.
Safety devices (not shown) will prevent ini-tiation of firing cartridge 72 if the stick of powder has not been properly lodged and/or if the breech mechanism' 69 has nβ*t been safely closed.
A number of exhaust ports 75 are provided in the envelope wall of cylinder 62. When these openings, during an outward movement of the cylinder, have passed sealing means 76 at the outward end of piston member 58 the gases will be exhausted to the atmosphere, and the out ward movement of the piston will cease. The ports may be arranged in series at different distances from the bottom structure. A mechanism, for instance a slidable ring 77. is then provided, by means of which one series may be kept open, while the other ports are closed, whereby a variation of the stroke length is obtainable.
By selecting the size and the composition of -the powder charge it is possible to use a device of this type for the initial tensioning as well as for the final jerk.

After an outward stroke the cylinder will be brought back to its initial position by suitable means of known type (not shown). With vertically running strands the device will be mounted vertically, and when the breech mechanism 69 has been opened and residues of powder or a possible protecting cartridge for the stick of powder have been removed hardly any force will be needed to push the cylinder' downwards.
When a horizontal positioning is desirable the device will be hung in an ear 78 attachable to a chain or the like, but also with this arrangement small force will be needed to push the cylinder inwards.
The drawings are very schematic and only intended to illustrate the various functions. The shape and size of the individual components may therefore, within the scope of the appended claims, vary in the practical embodiment .
Means for governing the pressure, safety valves and other monitoring equipment, as well as means with the embodiments according to Figures 1 and 2 for causing and controlling the gas expulsion from the minus side of the ram, during the jerk will be included, but have not been shown.
On certain occasions it may be desirable to limit the stroke of piston during the jerk. This may be arranged by enclosing an amount of gas in the minus part of the ram, to act as a pneumatic bumper. Ideal pressure sequences during the jerk will be obtained by selecting the composition and the size of the powder charge.
After the initial static tensioning it may on one occasion be desirable that the pressure - in spite of the increasing volume of the plus space in the ram -xapidly rises and then is maintained substantially
constant, or rises rectilinearly to a novel value. On an other occasion it may be desirable that the pressure first rapidly rises to a high value, and then gradually sinks, substantially to the pressure prevailing at the end of the static tensioning. The time alotted for pressure increase and for pressure reduction, respectively, will depend upon local condition.