Prestress detection device and method for steel bars or steel strands used in bridge construction

Abstract

The present invention relates to the technical field of bridge prestressing tensioning measurement, and in particular to a detection device and method for prestressing of steel bars or steel strands used in bridge construction. The device comprises a main body, a pedometer mechanism arranged on the main body for detecting the length of the steel bars, and a clamping mechanism for cooperating with the pedometer mechanism to clamp the steel bars, wherein the pedometer mechanism and the clamping mechanism enable the detection device to be suspended on the steel bars and move along the steel bars. The data detected by the present invention is accurate and direct, eliminating the interference of other error factors, and is applied to tensioning judgment and result analysis, which can achieve a high correspondence between the upper supply degree of the prestressed plate and the design value, and is significantly superior to the construction results obtained by the existing prestressing measurement method, and has obvious advantages.

Description

Technical Field

[0001] The invention relates to the technical field of bridge prestressing tension measurement, and in particular to a detection device and a detection method for prestressing of steel bars or steel strands used in bridge construction. Background Art

[0002] Prestressed concrete bridges are also called prestressed steel strand / reinforced concrete bridges. They are bridges built with prestressed concrete in their span structures. This type of bridge uses the reaction force of the pre-tension of steel strands / reinforcements or steel wires (cables) to pre-compress the concrete before loading. There is no tensile stress during the operation phase (called fully prestressed concrete), or there is tensile stress but no cracks or cracks are controlled within the allowable width (called partially prestressed concrete). In order to obtain prestress, the steel strands / reinforcements must be tensioned before pouring concrete. The prestressed tensioned components are subjected to compressive stress, which in turn causes them to undergo certain deformations to cope with the loads on the structure itself, including the load of the component's own weight, wind load, snow load, and earthquake load. The tensioning methods mainly include pre-tensioning and post-tensioning. Pre-tensioning is more common. Pre-tensioning is to tension the steel strands / reinforcements first, then pour concrete, and when the concrete reaches the specified strength, relax the ends of the steel strands / reinforcements.

[0003] The main reason for the widespread cracks in the webs of prestressed concrete box girder bridges is the failure of vertical prestressed tendons, which seriously affects the durability of such bridges, preventing them from reaching their service life and causing huge social and economic losses. The failure of vertical prestressed tendons is not a problem with the prestressed threaded steel strand / rebar anchoring system, but because there is no effective means to detect the tension of threaded steel strands / rebars, resulting in workers' arbitrary tensioning and anchoring of threaded steel strands / rebars, and the failure of vertical prestressed tendons or the failure of tension to meet the design requirements are common.

[0004] To calculate the elongation of the steel strand / rebar tensioning, you should first master the calculated length of the prestressed steel strand / rebar tensioning. The design control tension force refers to the tension of the steel strand / rebar before the anchor after the prestressing is completed. Therefore, when calculating the theoretical elongation of the prestressed steel strand / rebar tensioning, the calculated length of the steel strand / rebar is the length of the steel strand / rebar between the anchor points at both ends of the steel strand / rebar. However, during the prestressing, the controlled tension force of the steel strand / rebar is controlled at the jack tool anchor. Generally, the length of the steel strand / rebar between the anchor points at both ends of the steel strand / rebar, plus the working length of the steel strand / rebar in the jack, is used as the calculated length of the theoretical elongation of the prestressed steel strand / rebar tensioning.

[0005] The current measurement method can only measure the piston stroke of the tensioning jack to calculate the tensioning elongation of the steel strand / rebar, but at the same time, the anchor retraction during the entire tensioning process of the steel strand / rebar should also be subtracted. Although this method is simple and direct, it is very inaccurate and can only obtain the approximate value of the required elongation. However, in actual construction, the fixing conditions of each steel strand / rebar are unlikely to be exactly the same. If some steel strands / rebars are not clamped, invalid tensioning will occur, which cannot be discovered. Invalid tensioning of individual steel strands / rebars will make the obtained stress lower than the expected requirement, thus causing the above-mentioned problems in later use.

[0006] On the other hand, the steel strands / rebars are heavy and slender, and inevitably sag in the middle after being fixed at both ends. On the one hand, it is impossible to achieve the ideal straight line and fixation at both ends. On the other hand, the sag of each steel strand / rebar between the two anchor points cannot be consistent, and there may even be a huge difference. Under the limited tensioning distance, these differences cause huge potential interference to the prestressing, so that the actual performance of the prestressed concrete obtained during construction may be far from the design performance, which lays a huge hidden danger for later maintenance. Summary of the invention

[0007] In view of the deficiencies in the prior art, the present invention measures the actual length of the steel bars, thereby providing a steel bar or steel rope prestressing detection device and method for bridge construction, which can accurately detect the elongation of the prestressed steel strand / steel bar.

[0008] The technical solution adopted by the present invention to achieve the above-mentioned purpose is: a prestressed steel bar or steel strand detection device for bridge construction, including a main board body, a pedometer mechanism arranged on the main board body for detecting the length of the steel bar, and a clamping mechanism for cooperating with the pedometer mechanism to clamp the steel bar, the pedometer mechanism and the clamping mechanism enable the detection device to be suspended on the steel bar and move along the steel bar.

[0009] The present invention adopts a more in-depth technical solution to achieve the above-mentioned purpose. According to the walking direction of the detection device when working, the pedometer mechanism and the clamping mechanism are arranged in a manner of clamping the steel bars and walking along the steel bars at the same time.

[0010] The present invention adopts a more in-depth technical solution to achieve the above purpose.

[0011] The step counting mechanism is provided with two groups, which are respectively arranged at two ends of the main body plate in the horizontal direction;

[0012] Or, the pedometer mechanism includes a rotatable pedometer wheel disposed on the main body plate, and a code disc for counting fixed coaxially with the pedometer wheel;

[0013] Or, the pedometer wheel and the code disc are respectively arranged on two sides of the main body plate;

[0014] Alternatively, the pedometer wheel is provided with a circle of arc grooves along the circumference thereof, which are adapted to the steel bars or steel strands.

[0015] The present invention adopts a more in-depth technical solution to achieve the above purpose. The main body plate is provided with a fixed shaft, the fixed shaft is connected with a shaft sleeve through a bearing sleeve, and the pedometer wheel is connected to the code disc through the shaft sleeve;

[0016] Alternatively, proximity switches are provided on the outside of the two pedometer mechanisms in the horizontal direction and at the widest ends of the main body, respectively, for contacting the steel bars / steel strands fixed anchor points at both ends to terminate counting and travel.

[0017] The present invention adopts a more in-depth technical solution to achieve the above purpose. A mounting plate is provided at the bottom of the main board, a mounting opening is provided on the main board, and the clamping mechanism is provided inside the mounting opening.

[0018] Alternatively, the clamping mechanism includes a lifting block and an adjustment block which are arranged in the mounting opening and slide up and down along the mounting opening. The lifting block is located above the adjustment block. The lifting block and the adjustment block are connected and cooperated with each other through a compression spring. A vertical adjustment screw is fixed on the bottom surface of the adjustment block. The adjustment screw passes through the mounting plate and the height of the fixed adjustment block relative to the mounting opening is adjusted by a nut.

[0019] The present invention adopts a more in-depth technical solution to achieve the above purpose. The two ends of the adjustment block are provided with a card slot, and the card slot clamps the side wall of the installation opening to achieve up and down sliding;

[0020] Alternatively, two support rods are longitudinally fixed in the installation opening, and the lifting block, the compression spring and the adjustment block are sequentially sleeved on the two support rods from top to bottom.

[0021] The present invention adopts a more in-depth technical solution to achieve the above purpose. The lifting block is provided with a power wheel, and a stepper motor is installed on the power wheel. The output end of the stepper motor is coaxially fixedly connected with the power wheel;

[0022] Alternatively, the power wheel and the stepping motor are respectively arranged on both sides of the lifting block, and the power wheel and the pedometer wheel are located on the same side of the main body plate.

[0023] The present invention adopts a more in-depth technical solution to achieve the above purpose. A control board is installed on the mounting plate, and the stepping motor and the proximity switch are connected to the control board through a line;

[0024] Or, the control panel is provided with a speed regulating knob for controlling the rotation speed of the stepping motor;

[0025] Or, the control panel is equipped with a display screen for directly displaying the step counting data;

[0026] Or, a battery box is provided at the bottom of the mounting plate, and batteries are placed in the battery box to provide power for the control board and the stepper motor;

[0027] Or, the control panel is provided with a USB port for transmitting data;

[0028] Alternatively, a handle is provided in the middle of the main board body.

[0029] The present invention also includes a detection method for a prestressed steel bar or steel strand detection device for bridge construction,

[0030] (1) Adjust the distance between the pedometer mechanism and the clamping mechanism, and hang the detection device on one end of the steel bar or steel strand at the fixed anchor point so that the steel bar or steel strand is clamped between the pedometer mechanism and the clamping mechanism;

[0031] (2) Start the motor, and the detection device moves along the steel bar or steel strand from one end to the other end of the steel bar or steel strand. The display screen shows the number of circles of the two pedometer wheels or displays the length of the measured steel bar or steel strand through an integrated algorithm;

[0032] (3) The number of circles of the two pedometer wheels before and after the steel bar or steel strand is tensioned is measured, and the elongation of the steel bar or steel strand is calculated.

[0033] The present invention adopts a more in-depth technical solution to achieve the above purpose.

[0034] (1) Adjust the adjusting nut, adjust the position of the adjusting block in the installation opening by adjusting the screw, push the lifting block to move under the elastic force of the compression spring, and then adjust the distance between the power wheel and the pedometer wheel, clamp the steel bar or steel strand with the fixed anchor point between the arc groove of the two pedometer wheels and the power wheel, so that the detection device is suspended on one end of the steel bar or steel strand;

[0035] (2) Start the stepper motor and the power wheel rotates. Under the action of friction, the detection device moves from one end of the steel bar or steel strand to the other end. At the same time, the two pedometer wheels rotate with the movement of the detection device. The display screen shows the number of circles of the two pedometer wheels or the length of the measured steel bar or steel strand through an integrated algorithm.

[0036] The length calculation formula is:

[0037] L=N×C+D

[0038] L: The length measured from one end of the steel bar or strand to the other end

[0039] C: The circumference of the bottom of the arc groove of the pedometer wheel;

[0040] D: The length between two proximity switches.

[0041] N: the number of circles walked by each pedometer wheel;

[0042] (3) Detect the number of circles of the two pedometer wheels before and after the steel bar or steel strand is tensioned, or display the length of the measured steel bar or steel strand through an integrated algorithm, and calculate the elongation of the steel bar or steel strand when tensioned;

[0043] Among them, the calculation formula for the tension elongation ΔL is:

[0044] ΔL= (L1+ L2) / 2-(L1'+L2') / 2

[0045] L1 and L2: the length of the steel bar / strand between the fixed anchor points measured by two pedometer wheels before tensioning;

[0046] L1' and L2': The length of the rebar / strand between the fixed anchor points after tensioning, measured by two pedometer wheels.

[0047] The beneficial effects of the steel bar or steel strand prestress detection device and detection method for bridge construction of the present invention are:

[0048] The data detected by the present invention is accurate and direct, eliminating the interference of other error factors, and is applied to tensioning judgment and result analysis. It can make the upper supply degree of the prestressed plate correspond to the design value to a high degree, which is significantly superior to the construction results obtained by the existing prestressed measurement method and has obvious advantages. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 is a schematic diagram of the main structure of an embodiment of the present invention;

[0050] Figure 2 is a schematic side view of the structure of an embodiment of the present invention;

[0051] Figure 3 is a rear view structural schematic diagram of an embodiment of the present invention;

[0052] Figure 4 is a bottom view structural schematic diagram of an embodiment of the present invention;

[0053] The components in the figure are marked as follows:

[0054] 1-main body, 2-proximity switch, 3-pedometer wheel, 4-mounting port, 5-lifting block, 6-adjustment block, 7-adjustment screw, 8-power wheel, 9-stepping motor, 10-compression spring, 11-mounting plate, 12-control board, 13-battery box, 14-display screen, 15-speed knob, 16-handle, 17-code disk, 18-steel bar, 19-arc groove, 20-nut, 21-support rod. DETAILED DESCRIPTION

[0055] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments: Example

[0056] like Figure 1-4 As shown, a prestressed steel bar or steel strand detection device for bridge construction includes a main board body 1 similar to a T-shaped surface, the main board body 1 is a vertical extension surface, and the upper part of the main board body 1 extends to both sides to form the widest end of the main board body 1, defined as the butt end, and also includes a pedometer mechanism arranged on the main board body 1 for detecting the length of the steel bar, and a clamping mechanism for cooperating with the pedometer mechanism to clamp the steel bar 18, the pedometer mechanism and the clamping mechanism enable the detection device to be suspended on the steel bar 18 and move along the steel bar.

[0057] The step counting mechanism is provided with two groups, which are respectively arranged at two ends of the main body plate 1 in the horizontal direction;

[0058] The pedometer mechanism includes a fixed shaft fixed on the edges of both sides of the main board 1, a shaft sleeve is connected to the fixed shaft through a bearing sleeve, a rotatable pedometer wheel 3 is arranged on the edge position of the main board 1 near the collision end, and a code disk 17 for counting is coaxially fixed with the pedometer wheel 3, and the code disk 17 is used for counting; the pedometer wheel 3 and the code disk 17 are connected through a shaft sleeve; the pedometer wheel 3 and the code disk 17 are respectively arranged on both sides of the main board 1; the pedometer wheel 3 is provided with a circle of arc grooves 19 adapted to steel bars or steel strands along the circumferential side, so that the two pedometer wheels 3 can straddle the steel bars or steel strands.

[0059] The two pedometer mechanisms are provided with proximity switches 2 at the outer sides in the horizontal direction and at the contact ends of the main body 1, respectively, for contacting the steel bars / steel strands fixed anchor points at both ends to terminate the counting and travel.

[0060] A mounting plate 11 is provided at the bottom of the main board 1, and the mounting plate 11 extends horizontally. A mounting opening 4 is provided near the lower part of the main board 1, and the mounting opening 4 is rectangular. The clamping mechanism is provided inside the mounting opening.

[0061] The clamping mechanism includes a lifting block 5 and an adjusting block 6 which are arranged in the installation opening 4 and slide up and down along the installation opening 4. Both ends of the lifting block 5 and the adjusting block 6 are provided with a card slot, and the card slot clamps the side wall of the installation opening 4 to achieve up and down sliding; the lifting block 5 is located above the adjusting block 6, and the lifting block 5 and the adjusting block 6 are connected and matched by a compression spring 10, and the compression spring 10 has an elastic force to separate the lifting block 5 and the adjusting block 6. The lifting block 5 is provided with a power wheel 8, and the power wheel 8 is installed with a stepping motor 9, which is used to provide driving force for the whole device to walk along the steel bar or steel strand. The output end of the stepping motor 9 is coaxially fixedly connected with the power wheel 8; the power wheel 8 and the stepping motor 9 are respectively arranged on both sides of the lifting block 6, and the power wheel 8 and the pedometer wheel 3 are located on the same side of the main body plate 1. A vertical adjustment screw 7 is fixed on the bottom surface of the adjustment block 6, and the adjustment screw 7 passes through the installation plate 11, and adjusts the height of the fixed adjustment block 6 relative to the installation opening 4 through a nut 20. The rope body is clamped by the compression spring 10, and the clamping force can be adjusted by adjusting the screw 7. Two support rods 21 are longitudinally fixed in the installation opening 4, and the lifting block 5, the compression spring 7 and the adjustment block 6 are sequentially sleeved on the two support rods 21 from top to bottom.

[0062] A control board 12 is installed on the mounting plate 11, and the stepper motor 9 and the proximity switch 2 are connected to the control board 12 through a line; the proximity switch 2 is used to feedback the termination signal. A battery box 13 is provided at the bottom of the mounting plate 11, and a battery is placed in the battery box 13 to provide power for the control board 12 and the stepper motor 9; the battery is used to supply power and counterweight at the same time. A display screen 14 is installed on the control board 12 for directly displaying the step counting data; the displayed data includes the number of turns respectively recorded by the two step counting wheels 3, and the algorithm can also be integrated to directly display the measured length of the steel bar or steel strand.

[0063] The present invention also includes a detection method for a prestressed steel bar or steel strand detection device for bridge construction: by using the device, walking along the steel bar or steel strand, the actual length of each steel bar before and after tensioning can be measured, thereby obtaining the tensioning elongation ΔL:

[0064] (1) Adjust the adjusting nut 20, adjust the position of the adjusting block 6 in the installation opening 4 by adjusting the screw rod 7, push the lifting block 5 to move under the elastic force of the compression spring 10, and then adjust the distance between the power wheel 8 and the pedometer wheel 3, clamp the steel bar or steel strand of the fixed anchor point between the arc groove 19 of the two pedometer wheels 3 and the power wheel 8, and hang the detection device on one end of the steel bar or steel strand;

[0065] (2) The stepper motor 9 is started, and the power wheel 8 rotates. Under the action of friction, the detection device moves from one end of the steel bar or steel strand to the other end. At the same time, the two pedometer wheels 3 rotate with the movement of the detection device. The display screen displays the number of circles of the two pedometer wheels 3 or displays the length of the measured steel bar or steel strand through an integrated algorithm.

[0066] The length calculation formula is:

[0067] L=N×C+D

[0068] L: The length measured from one end of the steel bar or strand to the other end

[0069] C: The circumference of the bottom of the arc groove of the pedometer wheel;

[0070] D: The length between two proximity switches.

[0071] N: the number of circles walked by each pedometer wheel;

[0072] C and D are fixed values, so the above calculation method does not involve other equipment and error parameters. What is obtained is the direct length of the steel bar or steel strand, eliminating interference factors, and the data is very accurate.

[0073] Through the above simple formula, it can be obtained that the length before tensioning includes L1 and L2, and the length after tensioning includes L1' and L2', and the above data can be directly displayed on the display screen.

[0074] The two pedometer wheels can also eliminate the problem of invalid errors in measuring walking. When slipping occurs while walking, there will inevitably be obvious differences in the number of circles obtained by the two pedometer wheels and the measurement results. When there is an obvious difference, it means that slipping occurs, which is invalid data and should be remeasured.

[0075] (3) Detect the number of circles of the two pedometer wheels before and after the steel bar or steel strand is tensioned, or display the length of the measured steel bar or steel strand through an integrated algorithm, and calculate the elongation of the steel bar or steel strand when tensioned;

[0076] Among them, the calculation formula for the tension elongation ΔL is:

[0077] ΔL= (L1+ L2) / 2-(L1'+L2') / 2

[0078] L1 and L2: the length of the steel bar / strand between the fixed anchor points measured by two pedometer wheels before tensioning;

[0079] L1' and L2': The length of the rebar / strand between the fixed anchor points after tensioning, measured by two pedometer wheels. Example

[0080] like Figure 1-4 As shown, the same points as Example 1 are not repeated here, and the difference lies in that:

[0081] When the difference between L1 and L2 is greater than 1%, re-measure L1 and L2. When the difference between L1' and L2' is greater than 1%, re-measure L1' and L2'. Example

[0082] like Figure 1-4 As shown, the same points as Example 1 are not repeated here, and the difference lies in that:

[0083] The control panel 12 is provided with a speed regulating knob 15 for controlling the rotation speed of the stepping motor 9 . Example

[0084] like Figure 1-4 As shown, the same points as Example 1 are not repeated here, and the difference lies in that:

[0085] The control panel 12 is provided with a USB port for transmitting data. Example

[0086] like Figure 1-4 As shown, the same points as Example 1 are not repeated here, and the difference lies in that:

[0087] A handle 16 is provided in the middle of the main board body 1 .

[0088] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable ordinary technicians in the field to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made based on the essence of the content of the present invention should be included in the protection scope of the present invention.

Claims

1. A device for detecting prestress of steel bars or steel strands for bridge construction, characterized by: It includes a main body, a pedometer mechanism arranged on the main body for detecting the length of the steel bar, and a clamping mechanism for cooperating with the pedometer mechanism to clamp the steel bar, wherein the pedometer mechanism and the clamping mechanism enable the detection device to be suspended on the steel bar and walk along the steel bar; The step counting mechanism is provided with two groups, which are respectively arranged at two ends of the main body plate in the horizontal direction; The pedometer mechanism comprises a rotatable pedometer wheel disposed on the main body plate, and a code disc for counting coaxially fixed to the pedometer wheel; The pedometer wheel and the code disc are respectively arranged on both sides of the main body plate; The pedometer wheel is provided with a circle of arc grooves along the circumference thereof, which are adapted to the steel bars or steel strands; The main body plate is provided with a fixed shaft, the fixed shaft is connected with a shaft sleeve via a bearing sleeve, and the pedometer wheel is connected to the code disc via the shaft sleeve; The two pedometers are provided with proximity switches at the widest ends of the main body and outside the two pedometers in the horizontal direction, respectively, for contacting the steel bars / steel strands fixed anchor points at the two ends to terminate the counting and travel; The clamping mechanism includes a lifting block and an adjusting block which are arranged in the mounting opening and slide up and down along the mounting opening. The lifting block is located above the adjusting block. The lifting block and the adjusting block are connected and matched by a compression spring. A vertical adjusting screw is fixed to the bottom surface of the adjusting block. The adjusting screw passes through the mounting plate and is adjusted by a nut to fix the height of the adjusting block relative to the mounting opening. Both ends of the lifting block and the adjusting block are provided with a clamping groove, and the clamping groove clamps the side wall of the installation opening to achieve up and down sliding; Two support rods are longitudinally fixed in the installation opening, and the lifting block, the compression spring and the adjustment block are sequentially sleeved on the two support rods from top to bottom; The lifting block is provided with a power wheel, the power wheel is equipped with a stepping motor, and the output end of the stepping motor is coaxially fixedly connected with the power wheel; The power wheel and the stepping motor are respectively arranged on both sides of the lifting block, and the power wheel and the step counting wheel are located on the same side of the main body plate.

2. The prestressed steel bar or steel strand detection device for bridge construction according to claim 1 is characterized by: According to the walking direction of the detection device when it is working, the pedometer mechanism and the clamping mechanism are arranged in a manner of clamping the steel bars and walking along the steel bars at the same time.

3. The device for detecting prestress of steel bars or steel strands for bridge construction according to claim 1, characterized in that: A mounting plate is provided at the bottom of the main board body, a mounting opening is provided on the main board, and the clamping mechanism is arranged inside the mounting opening.

4. The device for detecting prestress of steel bars or steel strands for bridge construction according to claim 1, characterized in that: A control board is installed on the mounting plate, and the stepping motor and the proximity switch are connected to the control board through lines.

5. The device for detecting prestress of steel bars or steel strands for bridge construction according to claim 4, characterized in that: The control panel is provided with a speed regulating knob for controlling the rotation speed of the stepping motor; The control panel is provided with a display screen for directly displaying the step counting data; A battery box is provided at the bottom of the mounting plate, and batteries are placed in the battery box to provide power for the control board and the stepper motor; The control panel is provided with a USB port for transmitting data; A handle is provided in the middle of the main board body.

6. The detection method of the prestress detection device for steel bars or steel strands for bridge construction according to any one of claims 1 to 5, characterized in that: (1) Adjust the adjusting nut, adjust the position of the adjusting block in the installation opening by adjusting the screw, push the lifting block to move under the elastic force of the compression spring, and then adjust the distance between the power wheel and the pedometer wheel, clamp the steel bar or steel strand with the fixed anchor point between the arc groove of the two pedometer wheels and the power wheel, so that the detection device is suspended on one end of the steel bar or steel strand; (2) Start the stepper motor and the power wheel rotates. Under the action of friction, the detection device moves from one end of the steel bar or steel strand to the other end. At the same time, the two pedometer wheels rotate with the movement of the detection device. The display screen shows the number of circles of the two pedometer wheels or the length of the measured steel bar or steel strand through an integrated algorithm. The length calculation formula is: L=N×C+D L: The length measured from one end of the steel bar or strand to the other end C: The circumference of the bottom of the arc groove of the pedometer wheel; D: The length between two proximity switches; N: the number of circles walked by each pedometer wheel; (3) Detect the number of circles of the two pedometer wheels before and after the steel bar or steel strand is tensioned, or display the length of the measured steel bar or steel strand through an integrated algorithm, and calculate the elongation of the steel bar or steel strand when tensioned; Among them, the calculation formula for the tension elongation ΔL is: ΔL= (L1+ L2) / 2-(L1'+L2') / 2 L1 and L2: the length of the steel bar / strand between the fixed anchor points measured by two pedometer wheels before tensioning; L1' and L2': The length of the rebar / strand between the fixed anchor points after tensioning, measured by two pedometer wheels.

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