HK1175221B - Method and system for equally tensioning multiple strands - Google Patents

Description

Method and system for equally tensioning a plurality of strands

Technical Field

The present invention relates to the field of tensioning structural cables, and in particular to tensioning structural cables such that the overall tension on the cable is equally distributed across the cable strand components.

Background

Prestressed cables are used in many structural applications and in particular for reinforcing concrete structures by maintaining concrete compression cables. In many applications, the amount of compression applied to the concrete is not critical, and it may be sufficient that the compression is well above a specified minimum, while the cable tension is well below its breaking tension.

However, there are applications where the rebar must be tensioned to a high gauge and within narrow tolerances. For example, these applications include concrete pressure vessel shells in nuclear power plants or in oil/gas storage facilities. The integrity of these vessels is largely dependent on the tension in the post-tensioned (PT) tendons, and it is therefore critical for the builder of these installations to be able to demonstrate that the stressed tendons are tensioned to within prescribed tolerances.

For example, a typical PT cable or rebar may be constructed of 55 strands fed through a conduit and tensioned from one or both ends of the conduit using hydraulic jacks. The containment vessel may be of cylindrical or spherical configuration with the conduit following a curved path in the concrete. Once the PT strands are stressed to the required tension, tapered wedges are typically used to anchor the strands to the anchor plate. After installation and tensioning is complete, periodic checks need to be made throughout the life of the equipment to ensure continued integrity of the strands within the conduit and to ensure that the tension in the strands is still within prescribed tolerances. In making these checks, the tendon forces may be measured using the so-called lift-off technique (lift-off technique) in which a jack is used to raise the end anchor. The force required to move the anchor will give an indication of the tension in the twisted wire bundles making up the PT cable. For a combined tensioning system, lift-off may be performed at any point until the time of pulping; this technique may be performed at any time for the unbonded rebar.

One difficulty with tensioning the steel reinforcement within the conduit is the friction between the strands and the conduit wall and between the strands themselves. These friction effects may lead to a non-uniform or variable force distribution between the strands and/or along the length of the individual strands during and after the tensioning operation. This problem is particularly prevalent in applications that use very long strands and in non-straight conduits where the strands are subjected to not only longitudinal forces, but also lateral forces that push the strands together and/or against the conduit wall. For example, in a circular cross-section conduit that passes through pressurized concrete along a curved path, as the slack is taken up, the loosely distributed strands will be pulled inward so that all of the strands eventually experience lateral forces and lateral movement along the radius of curvature of the conduit path.

It is known in the prior art to perform tensioning of PT bars in two stages (a first pre-tensioning equalisation stage and a subsequent main tensioning stage): during a first pre-tensioning equalisation phase, the strands are individually tensioned so that all strands are stressed to a relatively low equal tension; during the subsequent main tensioning phase, the strands are raised (jack) as a group to the desired tension.

European patent application EP0421862 describes such a method of tensioning a plurality of strands to achieve equal tension on all strands. The method of EP0421862 involves tensioning the reference strand to a desired tension. This is performed by using hydraulic jacks to apply pressure to the reference strand, while load cells are used to measure the tension on the strand. The other strands are then stressed to the force given by the reference load cell. It is assumed that: although the individual stresses on the individual strands will be slightly less as more strands are tensioned, the individual stresses will be equal after stressing.

An alternative method is described in european patent application EP0544573, in which a plurality of strands are pre-tensioned to about 10% of their final tension using a plurality of small jacks (one for each strand). The individual pretensioning jacks are supplied by the same pressure source, so it is assumed that: once the initial tensioning stage is complete, all of the slack in all of the strands has been taken up and all of the strands are tensioned to the same tension.

The initial tensioning phase in EP0544573 and EP0421862 is therefore designed to achieve a relatively low identical tension for all strands. Once this stage is complete, the strands are tensioned up to their desired full tension using a single large hydraulic jack that tensions all of the strands together. It is assumed that: since all strands are at the same equal tension at the beginning of the main hoisting operation and since all strands are assumed to be identical in material, the tension will remain equal during the main hoisting operation. Another assumption made is that: once the strands have been fully tensioned and anchored, the tension in the individual strands will still be equal.

As mentioned above, the methods described in the prior art make a large number of assumptions about the uniformity of the behaviour of the strands during and after tensioning. In practice, however, the strands are not identical, and the tendency of the strands to change relative to each other and their surroundings means that they are subjected to different forces during the tensioning operation. In particular, individual strands may become tangled or snagged between other strands or between other strands and the conduit wall. If such a blockage occurs, the tension in the strand is likely to be unevenly distributed along the length of the strand. This may have the result that: even if the external appearance is that the tension in the strands is within a specified or expected range, the tension in one or more strands may be locally outside of specified safety or operating tolerances.

If the tension distribution in the strands is particularly uneven, this may result in some of the strands being stressed beyond their operating range during the main tensioning phase and the strands may break or become overstrained. In some cases, such mechanical failure may occur without being detected, in which case the main lifting operation will be performed in connection with a group of strands, including weakened or broken strands. In order to achieve the desired tension overall on a wire bundle containing one or more mechanically weakened strands, the tension in each individual strand will be greater than specified or expected. In this way, the tension may appear to be within tolerance, while the individual strands may inadvertently be tensioned beyond their specified limits.

Different situations may also arise when the strand becomes constrained at two or more points along its length during the tensioning phase. During the initial tensioning phase, the ends of the strands will be tensioned to the desired tension, but there may be a length of strand between the two constrained points where the tension is much lower than at the ends of the strands. In such a case, the subsequent main lifting operation may have an unpredictable effect on the tension profile in the strand of interest. If one or the other of the constrained points becomes loose during the second tensioning phase, the tension in that portion of the strand, and thus the tension over the entire length of the strand, may change abruptly.

Strands may also suffer from mechanical weakness (weakness) caused by abrasion or material imperfections. Such mechanical weakness may lead to sudden failure (breakage) or gradual stretching (creep or yielding), either of which may lead to dangerous tension loss in the grouped strands as a whole. If a fault occurs during the main tensioning phase, the remaining strands will be overloaded to compensate for the weakness in the broken strands. This is a significant problem where the strands are to be stressed to near their maximum operating stress (near their yield stress).

These effects are likely to manifest themselves during the main tightening phase, as this is when most significant changes and movements occur within and between strands. However, such strand failure or movement may also occur late in the life of the equipment, either spontaneously or as a result of some stress event. For this reason, periodic checks will be performed to verify that the tension on the strand bundle is still within tolerance. However, such inspection is generally performed only on the rebar strand as a whole. Inspecting individual strands is generally not a viable option, but in some cases it is possible to perform lift measurements on each strand individually.

Disclosure of Invention

The present invention aims to provide a method and system that solves the above and other problems with the prior art.

To this end, the present invention provides a method of tensioning a plurality of strands, the method comprising: the first step is as follows: arranging a plurality of individual first tension sensing members to determine individual tensions in individual strands; the second step is as follows: individually tensioning the individual strands to a common first tension amount; the third step: determining a first individual tension measurement value for each strand using a plurality of individual first tension sensing members when each strand is tensioned to the same common first tension amount; the fourth step: the first individual tension measurements determined by the plurality of first tension sensing members are calibrated to a first amount of tension. The first amount of tension may be any amount of tension at which it can be determined that all individual jacks have completed tensioning of the slack in the individual strands, or it may be a predetermined amount of tension, such as 10% or 15% of the specified final tension, for example.

According to a variant of the method of the invention, the method further comprises a fifth step of: tensioning the plurality of strands to a second tensioning amount; and a sixth step of: a second individual tension measurement is determined for each individual strand using the individual first tension sensing members when the strands are tensioned to a second amount of tension. The second amount of tension may be any selected amount of tension during the tensioning process, or it may be a predetermined amount of tension, such as 50% or 100% of the desired final tension, for example.

Providing separate tension sensing members (such as load cells, for example) on each strand allows the installer to monitor the tension evolution in each individual strand during and/or after the first and/or second tensioning steps. Any sticking or snagging or breaking or unequal strand loading can therefore be detected as it occurs, rather than at the next inspection. The calibration step, since it is performed when all the strands are tensioned to the first tension, enables to normalize the tension value detected by the load cell to calibrate to the value of the first tension.

According to another variant of the method of the invention, the method comprises a seventh step of: arranging a second tension sensing member to determine a combined tension on the plurality of strands, and an eighth step of: the combined tension is compared to the individual tension measurements detected by the first tension sensing member. For example, the separate tension sensing member may be a magnetic load sensor (magneto).

According to another variant of the method of the invention, the method comprises a ninth step: the individual tension sensing members are removed after the strands have been tensioned. Alternatively, a plurality of individual load cells may be arranged such that they continue to provide individual tension values for the individual strands after the strands have been tensioned.

The present invention also provides a system for tensioning a plurality of structural strands, the system comprising: the apparatus comprises a first tensioning member for individually tensioning each strand to a common first amount of tension, a common tensioning member for tensioning a plurality of strands to a second amount of tension, a plurality of individual tension sensing members arranged to detect individual tension measurements for each strand, and a first calibration member for calibrating the individual tension measurements with respect to the first amount of tension.

According to another variant of the system according to the invention, the individual tensioning members comprise one or more individual hydraulic jacks, the or each individual hydraulic jack being arranged to tension one strand.

According to another variant of the system according to the invention, the individual tensioning members comprise a plurality of individual hydraulic jacks supplied by a common pressure source or by independent sources at a common pressure.

According to another variant of the system according to the invention, the individual tensioning members comprise individual hydraulic jacks which can be displaced to tension one strand in turn step by step. The separate tension sensing member may be a magnetic load cell.

According to another variant of the system according to the invention, the individual tension sensing members are arranged in one or more common planes orthogonal to a longitudinal axis, the longitudinal axis being substantially parallel to the tensioning direction of the strand.

According to another variant of the system according to the invention, the individual tension sensing members are arranged such that they can be held in place in order to measure the individual tensions in the individual strands once the strand tensioning has been completed.

According to another variant of the system according to the invention, the individual tensioning members and the common tensioning member are identical.

According to another variant of the invention, the system comprises a common tension sensing member for determining a common tension on the plurality of strands and a second calibration member calibrating the individual tension measurement values determined by the individual tension sensing members with respect to the common tension determined by the common tension sensing member.

The invention has been described with reference to tensioning a strand in a conduit. However, the same technique can also be applied to a stranded wire not confined to a conduit, such as a pull cable. Indeed, the invention may be implemented for stressing any collection of strands.

Drawings

The invention will now be described with reference to the accompanying drawings, in which:

figure 1 shows a schematic cross-sectional view of a first embodiment of the invention using an array of individual tensioning jacks.

Figure 2 shows a schematic cross-sectional view of an array of load cells mounted under tension around a strand, the load cells being arranged in a single plane.

Fig. 3 shows the same load cell array depicted in fig. 2 in a top view.

Figure 4 shows a schematic cross-sectional view of an array of load cells mounted under tension around a strand, the load cells being offset in three planes.

Fig. 5 shows the same load cell array depicted in fig. 4 in a top view.

Figure 6 shows a schematic cross-sectional view of a second embodiment of the invention using a single tensioning jack.

Figure 7 shows a schematic cross-sectional view of a variant of the invention using a second jack to provide the claimed tightening stage.

Fig. 8 and 9 show schematic cross-sectional views of an array of individual jacks used in the same unit and another variation of the invention that claims a tight jack.

Fig. 10a to 10c show a calibration procedure used in various embodiments and variants of the invention.

FIG. 11 depicts a chart showing the verification steps used in various embodiments of the present invention.

Fig. 12 depicts an overall curve (displacement curve) showing, for example, the strand tension distribution within a PT stress rebar.

Detailed Description

The accompanying drawings are provided to aid the understanding of the present invention and should not be taken as limiting the scope of the invention, which is defined by the appended claims. The use of the same reference symbols in different drawings is intended to indicate the same or corresponding features.

Fig. 1 shows a schematic cross-sectional view of an apparatus according to an example of a first embodiment of the invention. A plurality of strands 1 are shown emerging from the structure 5 to be stressed. The strands 1 may be fed through a conduit, or the strands (e.g. in the case of a guy cable) may be suspended in the free space between anchor points of the structure 5 to be tensioned. The strands 1 are inserted through the structure 5 and through the anchor block 30, the anchor block 30 having a separate anchor element 32, the anchor element 32 comprising, for example, a tapered wedge which grips the strands 1 due to the tension in the strands 1. When the strands 1 are tensioned, the anchoring elements 32 prevent the strands 1 from returning in the direction of the concrete structure 5 while allowing removal of the conduit and away from the concrete structure 5.

To minimize friction and drag, the strands 1 are preferably fed through a path in the structure such that each strand 1 remains in approximately the same position in the strand at all times as it passes through the structure, and such that the strands are aligned with corresponding openings in the equal tension jack 10 and anchor block 30 at each end of the conduit.

The strands 1 are fed through the load cell array 20 such that each strand 1 passes through a separate load cell 22. The load cell 22 may be, for example, a magnetic load cell (magnetic load cell) that measures changes in the electromagnetic properties of the steel strand 1 as the tension in the strand 1 changes. Other types of load cells 22 may be used depending moderately on the device geometry and the material of the strand 1. The load cells 22 are typically pre-calibrated for the particular type of strand 1 being used, or for a range of types of strands, and they can be immediately recalibrated in the field once the strands are in place in the load cell array ready for tensioning.

The arrangement of the load cells 22 in the array 20 can be better understood by referring to fig. 3 and 5, which will be described in more detail below. There is a corresponding arrangement of anchoring elements 32 in the anchor block 30 and jacks 11 in the jack array 10.

The strands are also fed into individual jacks 11 in an equal tension jack unit 10, the jack unit 10 being positioned against the load cell array unit 22 in preparation for the start of tensioning. The number of individual jacks may be any suitable number. The tension harness may comprise, for example, 55 strands arranged in a compact layout similar to the arrangement of load cells shown in fig. 3 or 5; in this case, the jack array 10 may also include 55 jacks. The cross-sectional view of the seven strands in fig. 1 is intended to correspond to the cross-section of the array of 55 strands in this example. The array of 55 strands is shown in fig. 3 and 5, which shows a preferred layout of 55 load cells for mating a 55 strand bundle. Fig. 3 and 5 will be discussed later herein.

The equal tension jack 10 comprises a plurality (e.g. 55) of individual hydraulic jacks 11, the hydraulic jacks 11 being operated from the same pressure source 12, 13. One jack 11 is provided for each strand 1. The individual jacks 11 may each be hydraulic stroke jacks, for example, which are capable of tightening a variable amount of slack in the strand 1 by repeatedly pulling a particular strand 1 back and then back forward to perform another pulling stroke until the tension on the strand 1 reaches the hydraulically generated force on the corresponding hydraulic jack piston. All hydraulic jacks 11 are substantially identical in that they are all supplied by the same pressure source 12 and 13 (e.g. which supplies a tension stroke and a return stroke respectively), which means that all jacks 11 are effective to pull the individual strands 1 to the same tension.

The operation of the tensioning assembly depicted in fig. 1 is as follows: first, the strands 1 are individually tensioned to a certain tension by the individual jacks 11. Then, when all slack has been taken up and the strand 1 has been stressed to the same common tension, the hydraulic pressure in the equal tension jack 10 at that point will be recorded as a reference against which the individual load cells 22 will be calibrated. It is known that the hydraulic pressure can be highly accurate (by measurement and/or calculation) and, therefore, since the dimensions and mechanical characteristics of the individual jacks, such as the friction between the piston and the cylinder, can also be accurately known (each jack can be individually pre-calibrated so as to map the hydraulic pressure with respect to the force exerted by the jack), the expected pressure value in each jack 11, and therefore the tension in each strand 1, can be accurately calculated and compared with the corresponding tension measurement actually obtained by the individual load cell 22 mounted on the strand 1 concerned. The readings from the different load cells 22 will inevitably vary slightly from one another; for example, in the case of a magnetic load cell, the variation may be caused by a change in temperature or a difference in mechanical and electromagnetic properties of the steel strand.

At this stage in the tensioning process, all strands 1 are at the same tension and the load cell 22 has been recalibrated to that tension. The strands are also held substantially at that tension by the anchoring elements 32 in the anchor block 30 so that an equal tension jack can be removed immediately if necessary, while leaving the strands under tension, anchored by the anchor block 30, and leaving the load cell array 20 in position adjacent to the anchor block 30. It is noted that anchor block 30 may include a spring or other biasing element for biasing the tapered wedge into its locked configuration, thereby blocking return movement of strand 1 such that there is insignificant return movement and/or loss of tension in strand 1 when the lifting stress is removed.

Unless the above equal tensioning procedure achieves the desired tension in the strand, a second tensioning operation will then need to be performed on the strand 1 in order to stress the strand to the desired tension. Depending on the tensioning capacity of the individual jacks, this can be performed using the same equal tension jack 10 as used to perform the equal tensioning phase. However, the individual jacks 11 more typically have limited tensioning capability, whereas tensioning the strand 1 to its desired tension would require more powerful jacks, such as long stroke jacks, for example. In this case, the equal tension jack 10 is removed from the strand 1, leaving the load cell array 20 in place, and then a more powerful jack may be fitted to the strand and the load cell array 20. Since the load cell array is not moved or disturbed during this process, the accuracy of the calibration performed at the end of the equal tensioning process is maintained. Subsequently, a second main-compression phase may be performed, in which the precisely calibrated load cell array is in place to measure and monitor the individual tensions in the strands while main-compression is performed. In this way, any unintended changes in tension can be detected as they occur and isolated from the particular affected strand. Such an unexpected change may indicate a material failure in the strand, such as a break or premature yielding, for example, or such an unexpected change may indicate a sudden change (a drop in tension or an unexpected rapid increase in tension) following immersion into the type described above. The calibrated load cell array may also be used to show that the tension distribution across the various strands remains within acceptable tolerances during and after the main stressing process. If no significant difference is detected between the outputs of the individual load cells while the main tensioning is being performed, and/or when the strand 1 has been fully tensioned, this can serve as evidence advising that tensioning has passed without falling into any of the aforementioned problems or friction problems. If a significant difference between the output values of the individual load cells 22 is detected, it can on the other hand be assumed that these may be indications of unsatisfactory tensioning, and a decision can be made whether the magnitude of the change permits the release of the tension and the reinstallation of the strand 1. The use of separate load cells for each individual strand means that the tension distribution across the strand is accurately and completely known, rather than requiring statistical analysis or estimation from a set of sample measurements.

The individual load cells 22 should preferably be as similar to each other as possible, especially when it is desired to monitor the strand response characteristics in the strand 1 in the tension range. This is so that once the load cells 22 have been calibrated to a first known tension (i.e. after all slack has been taken up and all individual jacks 11 have taken up the strand 1 to its relatively low level of first tension), all of the load cells 22 produce a similar load/output response characteristic during the main stress phase, and the result is that the difference between the load cell outputs can be used to represent the difference between the strand tensions.

According to an improvement of the method according to the invention, the tension measurement made for each strand can also be verified or validated by comparing the individual measurement results or a brief function of the individual measurement results, such as a sum, average or other statistical function, with the measurement results of the combined forces on all strands 1. Such combined or bulk force (global force) measurements may be made using load cells arranged to measure the stress exerted by the main (e.g. long stroke) jack used in the main pressure build-up phase. Alternatively, the overall stress measurement on the strand may be derived from a hydraulic measurement in the main jack (in the case of a hydraulic jack) using a pre-calibration conversion to a tension value, or by theoretical calculations based on the geometry and dimensions of the hydraulic jack.

The above verification/validation steps may be performed at any point of the tensioning process, where the second tension value may be accurately measured or calculated separately from the tension measurements made during the equal tensioning phase (e.g., where a second jack or other tensioning member is used). In fact, the load cell array remains in place throughout both compression phases and is accurately calibrated to at least one accurately known tension value, giving continuous reliable tension measurements through both compression phases. It is noted that the first pressing phase may be considered as an equal tension pressing phase, wherein the strands are pressed to the same tension, while the second pressing phase is an equal elongation phase, wherein pressing occurs by equally extending the length of the strands.

Fig. 2-5 show cross-sectional schematic views and top schematic views of two examples of load cell arrays 20, such as the array depicted in fig. 1. Figures 2 and 4 represent cross-sections through the load cell arrays of figures 3 and 5 along axes a-a and B-B, respectively.

In both cases, one load cell 22 will be provided for each strand 1 whose tension is to be measured. The load cells 22 are preferably magnetic load cells, such as those known as magnetic flux sensors (electromagnetic sensors) or magnetoelastic sensors (magnetoelastic sensors), which are typically implemented as two inductive windings around the strand 1. The two windings are not independently labeled in the figure. In use, an electrical pulse is applied to one winding and the resulting inductive pulse is measured across the other winding. The magnetic permeability of the steel in the strand changes with the amount of tension in the steel, so that the amount of inductive signal transmission also changes with increasing tension. It is noted that the permeability of steel also depends on the temperature of the material, and that the load cell measurements are corrected or compensated for to account for temperature fluctuations. Temperature sensors may be built into each load cell, for example, and temperature measurement information may be output along with tension measurement information. Alternatively, each load cell may be provided with its own temperature correction means (e.g., a computing circuit) that may be pre-calibrated to allow for temperature correction to be performed at the load cell such that each load cell may output a temperature corrected tension value.

It should be noted that other forms of load cells may be used in place of the magnetic flux load cells, such as ultrasonic strain gauges, capacitive strain gauges, and the like.

Fig. 2 and 3 show an array of 55 such load cells arranged in a single plane. The load cell 22 is shielded from external electromagnetic fields by the shroud 21. The leads 24 provide power to the load cell 22 and contain output signal leads for communicating output values from the load cell to an external monitoring or processing device. Such a planar arrangement of load cells 22 is possible if the load cells 22 each have a diameter that can fit into the single planar array 20 shown.

For larger load cells, or to reduce the overall diameter of the load cell array, arrangements such as the examples shown in figures 4 and 5 are preferred. The alternative load cells 22a, 22b and 22c are arranged to be offset in different planes so that the strands 1 can be held closer together than would be allowed if the load cells were arranged in a single plane as in fig. 4 and 5. In the example shown, the load cells covered by the different covers 22a, 22b and 22c are arranged in three different planar arrays 20a, 20b and 20c, respectively. However, these arrangements are given as examples only, and other offset arrangements are envisaged to suit the geometry or arrangement of a particular load cell.

The load cell array 20 is shown in FIG. 1 as being removably mounted adjacent to an anchor block 30, which is outside of the structure being tensioned. This allows the load cell array 20 to be removed once tensioning has been completed and the individual strand tensions have been demonstrated to be within tolerance. However, in a variant of the invention, the load cell 22 may be positioned on the side of the anchor block 30 remote from the jack (this arrangement is not shown in the figures). In this case the load cells 22 are used in the same way as in the method described above, except that when tensioning is complete and the jack has been removed, the load cells 20, 22 remain in place on the strands, with the result that the tension in the individual strands 1 can be measured at any time after the jack has been removed. In this configuration, once tensioning and anchoring is complete, there is no change in the mechanical or electromagnetic characteristics in the vicinity of each load cell 22, so any change detected in the load cell output signal can be assumed to be the result of a change in the tension of the strand 1 around which the load cell 22 is fitted. Thus, the load cells 20, 22 may remain in place and be used to monitor the tension in the strand 1 continuously or intermittently as desired. The monitoring may be performed in a comparative mode, i.e. monitoring the relative output value of the load cells 22 and detecting the difference between the tension values in the individual strands 1, or in an absolute mode, wherein the change in the output value is tracked over time for the individual strands 1 or the collective strand bundle or both. If a "lift-off" tension measurement or any other suitable tension check is subsequently performed in order to verify the overall tension in the strand bundle, the data from this measurement can be used to recalibrate the overall absolute value of the load cell array 20, or to re-verify the measurement data provided by the load cells. The individual load cells 22 can also be normalized (normalized) to a new measurement at this point, if desired, by assuming an even distribution of force across all strands 1 or by maintaining the same force distribution that existed prior to the lift test. Any unexpected change in the reading from a particular load cell will then indicate a change in the overall tension in the strand(s) as monitored by the particular load cell.

Fig. 6 shows an alternative embodiment of the invention, in which the individual pressing of the individual strands 1 is performed by a single jack, which is driven by hydraulic pressure at the connections 12 and 13 to press one strand 1. The jack 14 may be moved from strand to strand until all strands have been tensioned to an equal tensioning pressure. Since the compression of one strand may affect the tension in the other strands, the compression of the individual strands may be repeated as often as necessary until it is determined that all strands have been tensioned to the same hydraulic pressure. This equal tensioning method can be used when a perfectly equal tension jack as shown in fig. 1 is not available on site. However, the principle remains the same as for an equal tension jack; all strands are individually tensioned to the same tension. Calibration of the load cells 22 in the load cell array 20 may then be performed in the manner described above.

Fig. 7 shows a cross-sectional and schematic view of a long stroke equal elongation jack 40 that may be used to perform a second main pressing operation in a single stroke or multiple lifting strokes. The jack 40 may be replaced by an equal tension jack 10 as described previously. The strands 1 are anchored by anchor blocks 30 to resist movement of the strands in a direction opposite to the direction of applied pressure. The second anchoring member 50 grips the strand 1 so that the jack piston 41 can be hydraulically retracted within the jack main cylinder 42, thereby applying the prevailing tension to the strand. When this main compression occurs, the tension in each strand is monitored by the load cells 22 in the load cell array 20. Jack 40 may incorporate a universal load cell as described above for measuring the tension across all of the strands being tensioned. Alternatively, the overall or combined tension may be derived from hydraulic pressure applied to jack 40.

Fig. 8 and 9 show how the individual jack arrays 10 (equal tension jacks) of fig. 1 can be incorporated into a larger jack 40. In this case, the equal tension jack 10 will not be removed to be able to fit the main pressure jack 40, and both the equal tension and main pressure operations can be performed using a single piece device. When using this type of jack, the aforementioned equal tension load cell calibration step is performed once all slack in the strands has been tightened by the equal tension jack 10 and all strands have been tightened to the same tension. Fig. 8 shows the master jack 40 in its activated position, e.g., when an equal tension and/or load cell calibration step is being performed, while fig. 9 shows the master jack 40 in its retracted position at the end of its pull stroke. As a result of a further development of the device shown in fig. 8 and 9, a second anchor block similar to the anchor block 50 shown in fig. 7 may be mounted behind the equal tension jack 10 (i.e. above the equal tension jack 10 as viewed in fig. 8 and 9). This second anchor block (not shown) will be used for more tension than can be borne by the individual anchoring members in the individual jacks of the equal tension jack 10.

Fig. 10a to 10c and 11 show how load cell calibration may be performed throughout the pressing phase. In each graph, the S-axis represents the tension measurement output from the load cell, while the F-axis represents the hydraulic pressure applied to (or the tension applied to) the individual jacks for the respective strands, which can be derived from the hydraulic pressure in the individual jacks.

First in the laboratory, a conventional calibration is performed, for example with respect to a "known" reference force. This produces a calibration curve of the load cell output S versus the actual applied force F. This calibration curve for a single load cell is shown in figure 10 a. In FIG. 10a, the S-axis represents the output reading of the load cell being calibrated, while the F-axis represents the actual force applied to the test steel used to calibrate the load cell.

In the field, the pressure process is inevitably different from the original laboratory calibration situationThe conditions (mechanical and magnetic properties of the steel, temperature, etc.) occur and the calibration curve will need to be adjusted to account for these conditions. Prior art methods of calibrating load cells are limited to zeroing the load cell output under zero load conditions and to accommodating temperature variations. The present methods and systems of the invention improve these methods by matching the laboratory calibration curve for the load cell to a set of true measurements for each individual strand. The more values measured, the more accurately the curve can be matched to the measurement data. A diagram of this matching process is shown in FIG. 10b, which shows two points F with labels₀-S₀And F₁-S₁Original calibration curve (solid line). S₀And S₁Respectively, at an applied pressure tension F₀And F₁Is read from the expected tension of the load cell. On the other hand, S'₀And S'₁Respectively represent at a tension F₀And F₁At the actual measured tension value (actual force F) indicated by the load cell₀And F₁Known or calculated from the pressure applied to the respective hydraulic jack). The dashed line is a slightly shifted and rotated version of the original calibration curve, moved so that it coincides with the actual measurement data. By matching the curve with the actual measurement results for each load cell during the equal tensioning phase, it is often possible to more accurately simulate the individual tension in the strands to a tension above the pressing range of the individual jack 11 when each individual tension value for each individual strand can be measured. In this way, load cell readings at these higher tensions are often more accurate, even though individual tension readings from individual load cells may not be independently corroborated at these higher tensions.

Fig. 10c shows another improvement of the calibration process. In this case, F₁To achieve the tension at the first amount of tension (i.e., once all slack has been taken up by the equal tensioning jack and all strands are under the same tension). Continue to press alone to exceed F by using equal tensioning jacks 10₁Another separate load cell measurement may be at a known force F'₁、F''₁And the like. These other measurements can be used to more accurately match the calibration curve to the actual situation.

Fig. 11 shows another modification of the calibration method. The calibration curve for a particular load cell has been matched during an equal tensioning phase, at F₂A set of verification steps is performed. In this step, the combined tension on the strand being tensioned is compared to a function of the individual measurements from the individual load cells, and the expected value S of the load cell reading is derived₂. The result can simply be a cross check that the sum of the individual load cell readings equals the expected total value S₂. Alternatively, the calibration curve may be further matched to include point S₂。

S'₂It may be a simple average calculated by dividing the combined tension by the number of readings, or it may be a more sophisticated mathematical function. It is noted that it is also advantageous that this verification step can be performed during equal tension phases, so as to give the individual load cell outputs an initial cross calibration with the tension measuring members used to measure the combined tension in all strands.

In practical situations, the tension in the individual strands does not remain exactly the same during the main pressing operation. After equal tensioning operations, slight differences in shape, material or orientation will inevitably lead to deviations in the individual tensions due to the pressing process. Fig. 12 shows an example of how strand tension (F-axis) may be distributed among the number of strands (N-axis). F₄And F₅Representing a difference amount that can be used to give an indication of the spread of tension values in the set of strands. Prior art systems use this difference calculation to determine whether the tension profile falls within a specified acceptable range for a particular post-tensioning application. However, the statistical analysis may not preclude one or more strands from being overstressed (exceeding the maximum stress F)₆Which may represent, for example, 95% of the yield stress or fracture (below the minimum stress F)₃) The possibility of (2).

Rather, the method of the present invention makes these statistical analyses redundant, as it allows the installer to exceed the purely stated probability and instead prove that all strands are individually within the specified tension tolerance.

The above description has focused on tensioning one end of a set of strands. However, in some installations it may be advantageous to tension a set of strands from both ends. This can further reduce the effects of being trapped in the problems and friction problems described earlier. The strands may be tensioned using two jack assemblies (one at each end of the strand bundle). In this case, the two tensioning processes in the two jacks can be carried out simultaneously, or one after the other, or alternately step by step. To effect equal tension calibration, it is preferred that calibration of the load cells in the two jack assemblies is performed simultaneously after the slack has been tightened and after the two jacks have tensioned the strands to the first tension. It is also possible that the two separate sets of jacks are driven by the same pressure source, or at least at the same pressure, as this will minimise the amount of movement of the strands within the conduit. Having two sets of load cells, especially if all strands are calibrated to the same tension, makes the tension monitoring system still more sensitive to the above mentioned friction effects. Not only are the output values of the load cells of one array compared to each other and the changes in the load cell values over time monitored, it is also currently possible to compare the load cells of one array to the corresponding load cells (i.e., the same strand) in the other array. For example, a strand that will become contracted somewhere along its length will be at a higher tension at one end than the other, and this difference can be detected by comparing two load cells at the ends of the strand. Comparisons between two load cells or between two arrays of load cells may also be used to corroborate measurements obtained by other types of comparisons as mentioned.

Claims (15)

1. A method of tensioning a plurality of strands (1), the method comprising:

the first step is as follows: arranging a plurality of individual first tension sensing members (20, 22) to determine individual tensions in the individual strands (1),

the second step is as follows: tensioning the individual strands (1) individually to a common first tensioning force (F)₁)，

The third step: when each strand (1) is tensioned to the first tension (F)₁) Using the plurality of individual first tension sensing members (20, 22) to determine for each strandFirst individual tension measurement (S) of the wire (1)₁)，

The fourth step: the first individual tension measurement values (S) to be determined by the plurality of first tension-sensing members (20, 22)₁) Calibrated to said first tension (F)₁)。

2. The method of claim 1, further comprising:

the fifth step: tensioning the plurality of strands (1) to a second tensioning amount (F)₂) And an

A sixth step: when the strand (1) is tensioned to the second tension amount (F)₂) Using the individual first tension-sensing members (20, 22) to determine a second individual tension measurement (S) for each individual strand (1)₂)。

3. The method of claim 2, further comprising:

a seventh step of: arranging a second tension sensing member to determine a combined tension on the plurality of strands (1), an

An eighth step: comparing the combined tension with a second individual tension measurement (S) detected by the first tension-sensing member (20, 22)₂) And (6) comparing.

4. The method of claim 1, wherein the separate first tension sensing member (22) is a magnetic load cell.

5. A method according to claim 3, characterized in that the method comprises a ninth step of: removing the separate first tension sensing member (20, 22) after the strand (1) has been tensioned.

6. The method according to one of claims 1 to 4, wherein the plurality of individual first tension sensing members (22) are arranged such that they continue to provide individual tension values for the individual strands (1) after the strands (1) have been tensioned.

7. A system for tensioning a plurality of structural strands (1), the system comprising:

separate tensioning members for individually tensioning the strands (1) to a common first tension amount (F)₁)，

A common tensioning member for tensioning the plurality of strands (1) to a second amount of tension (F)₂)，

Said system being characterized in that it comprises:

a plurality of individual tension sensing members (20, 22) arranged to detect individual tension measurements (S) for each of the strands (1)₁) And an

A first alignment member for aligning with respect to the first amount of tension (F)₁) To calibrate the individual tension measurements.

8. A system according to claim 7, wherein the separate tensioning member comprises one or more separate hydraulic jacks, the or each separate hydraulic jack being arranged to tension a strand (1).

9. A system according to claim 8, wherein the separate tensioning members comprise a plurality of separate hydraulic jacks supplied by a common pressure source (12, 13) or separate sources at a common pressure.

10. System according to claim 8, characterized in that the separate tensioning member comprises a separate hydraulic jack which is displaceable to tension one strand (1) step by step in turn.

11. The system of one of claims 7 to 10, wherein the separate tension sensing member (22) is a magnetic load cell.

12. The system according to one of claims 7 to 10, wherein the individual tension sensing members (22) are arranged in one or more common planes orthogonal to a longitudinal axis, which is parallel to a tensioning direction of the strand (1).

13. System according to one of the claims 7-10, wherein the individual tension sensing members (22) are arranged such that they can be held in place in order to measure the individual tensions in the individual strands (1) once the tensioning of the strands has been completed.

14. The system of one of claims 7 to 10, wherein the individual tension members and the common tension member are identical.

15. System according to one of claims 7 to 10, comprising means for determining the common tension (F) on the plurality of strands₁，F₂) A common tension sensing member, and

a second calibration means for calibrating the common tension (F) determined by the common tension sensing means with respect to the common tension₁，F₂) To calibrate the individual tension measurement values (S) determined by the individual tension sensing members (22)₁，S₂)。