Method for detecting tension of linear member for structure, linear member for structure, and sensor installation jig for linear member

Abstract

To provide a tension detecting method for a linear member for a structure capable of detecting tension of the linear member for the structure with less time and effort, and to provide the linear member for the structure and a sensor installing jig for the linear member.SOLUTION: In a tension detecting method for a linear member for a structure, the strain variation of a cable 1 for a bridge is measured by an optical fiber type strain sensor 2 in a strain fluctuation measuring step. In a variation frequency distribution detection step, the distribution of a variation frequency of the strain caused by vibration A in a direction perpendicular to an axis of the cable 1 for the bridge is detected on the basis of the variation of the strain. In a natural frequency specifying step, the natural frequency of the cable 1 for the bridge is specified on the basis of the variation frequency distribution of the strain. In a tension calculation step, the tension of the cable 1 for the bridge is calculated on the basis of the natural frequency.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a tension detection method for detecting tension acting on a linear member of a structure such as a bridge, a linear member for use in the structure, and a sensor installation jig for a linear member for installing a sensor on the linear member. [Background technology]

[0002] A conventional method for measuring the tension of a cable in a cable-stayed bridge involves installing a vibration sensor on the cable and calculating the tension of the cable based on the information detected by the vibration sensor when an impact is applied to the cable (see Patent Document 1). This method involves frequency analysis of the vibration detected by the vibration sensor, and determining the bending stiffness of the cable under tension from multiple natural frequencies present in a frequency range where the influence of the bending stiffness of the cable is dominant. The cable tension is then calculated based on the bending stiffness and natural frequencies thus determined.

[0003] As a vibration sensor that can be used in the above-mentioned method of measuring cable tension, an acceleration type vibration sensor that has a built-in vibrator and electrically detects the acceleration acting on the vibrator and outputs the frequency of the vibration is widely used.

[0004] Meanwhile, a PC steel strand with a built-in strain-measuring optical fiber has been proposed to measure the tension of PC (Prestressed Concrete) steel strands, which are used to construct prestressed concrete structures and reinforce concrete by applying compressive force to the concrete (see Patent Document 2). This PC steel strand with optical fiber has a strain-measuring optical fiber twisted into the steel strand that constitutes the core material, and is configured so that the strain generated by the tension acting on the PC steel strand is directly measured by the strain-measuring optical fiber. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 3313028 Publication [Patent Document 2] Japanese Patent Application Publication No. 2019-70593 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the conventional cable tension measurement method, the vibration sensor must be installed and removed each time the tension is measured, and the vibration sensor must be firmly fixed to the cable to accurately measure vibration. Therefore, the installation and removal of the vibration sensor is time-consuming and labor-intensive. Furthermore, acceleration-type vibration sensors, which are widely used as vibration sensors, have a built-in vibrator, which is a moving part, and therefore have relatively poor weather resistance and are not suitable for long-term measurements.

[0007] On the other hand, the conventional PC steel strand with optical fiber directly measures the strain in the extension direction of the PC steel strand caused by tension using a strain-measuring optical fiber. Because the strain in the extension direction of the PC steel strand is very small, the strain-measuring optical fiber needs to be firmly fixed to the steel strand in order to detect even small strain values. For this reason, the PC steel strand with optical fiber is manufactured by twisting the optical fiber wrapped in a resin filler with the stranded wire. Therefore, the PC steel strand with optical fiber is disadvantageous in that it is time-consuming to manufacture. Furthermore, the axial strain value of the PC steel strand measured with the strain-measuring optical fiber is affected by the temperature at the measurement position, so temperature compensation for the strain is necessary. Therefore, the PC steel strand with optical fiber has the disadvantages of being complicated in structure and requiring complex analysis for detecting tension, since the temperature-measuring optical fiber is twisted together with the steel strand.

[0008] Therefore, an object of the present invention is to provide a tension detection method for a linear member for a structure that can be implemented with little effort. Also, an object of the present invention is to provide a tension detection method for a linear member for a structure that can be implemented over a long period of time. Another object of the present invention is to provide a linear member for a structure that can detect tension with a relatively simple structure. Another object of the present invention is to provide a linear member for a structure that can detect tension through a relatively simple analysis. [Means for solving the problem]

[0009] In order to solve the above problems, the tension detection method for a linear member for a structure of the present invention is a tension detection method for detecting a tension acting on a linear member used in a structure, a strain fluctuation measuring step of measuring a strain fluctuation of the linear member using an optical fiber strain sensor; a fluctuation frequency distribution detection step of detecting a distribution of fluctuation frequencies of strain caused by vibration of the linear member in the axis-perpendicular direction based on the fluctuation of strain measured in the strain fluctuation measurement step; a natural frequency specifying step of specifying a natural frequency of the linear member based on the strain fluctuation frequency distribution detected in the fluctuation frequency distribution detecting step; a tension calculation step of calculating a tension of the linear member based on the natural frequency identified in the natural frequency identification step; It is characterized by having:

[0010] According to the above configuration, in the strain fluctuation measuring step, strain fluctuations of the linear member are measured using an optical fiber strain sensor. Here, optical fiber strain sensors that measure strain based on the physical characteristics of light transmitted through an optical fiber can be widely used. Furthermore, the strain fluctuations of the linear member to be measured are changes in strain over time, and the value of the strain itself does not need to be measured with high precision. Next, in the fluctuation frequency distribution detecting step, a distribution of strain fluctuation frequencies caused by vibrations of the linear member in the direction perpendicular to the axis is detected based on the strain fluctuations measured in the strain fluctuation measuring step. Here, the distribution of strain fluctuation frequencies can be obtained by, for example, performing a fast Fourier transform on the measured values ​​of the optical fiber strain sensor. Next, in the natural frequency identifying step, the natural frequency of the linear member is identified based on the strain fluctuation frequency distribution detected in the fluctuation frequency distribution detecting step. Thereafter, in a tension calculation step, the tension of the linear member is calculated based on the natural frequency identified in the natural frequency identification step. Various known calculation formulas can be used to calculate the tension of the linear member based on the natural frequency.

[0011] According to the tension detection method for a structural linear member having the above configuration, since the object to be measured in the strain fluctuation measurement process is the strain fluctuation of the linear member, the optical fiber strain sensor only needs to be installed on the structural linear member to an extent that the strain fluctuation can be measured, and the sensor does not need to be firmly fixed, as is the case when measuring the strain value itself. Therefore, the structure for fixing the optical fiber strain sensor to the structural linear member can be simplified. As a result, when the optical fiber strain sensor is retrofitted to the structural linear member for measurement, the measurement effort can be reduced. Furthermore, when the optical fiber strain sensor is pre-installed in the structural linear member, the structural linear member equipped with the optical fiber strain sensor can be easily manufactured. Furthermore, since the object to be measured in the strain fluctuation measurement process is the strain fluctuation of the linear member and the strain value itself does not need to be measured with high accuracy, the measurement value of the optical fiber strain sensor does not need to be temperature corrected. Therefore, since there is no need to measure the temperature in the strain fluctuation measurement process, the measurement effort can be reduced. Furthermore, since there is no need to consider temperature when calculating tension in the tension calculation process, analysis can be performed with less effort. Furthermore, since an optical fiber strain sensor is used in the strain variation measurement process, the optical fiber strain sensor has high durability, allowing for longer-term detection of the tension of a structural linear member than conventional methods that use an acceleration-type vibration sensor to measure the vibration of a linear member. In the present invention, the term "structure" refers to civil engineering structures such as bridges, tunnels, dams, water gates, retaining walls, levees, and tanks, as well as buildings with roofs and columns or walls. Furthermore, the term "linear member" refers to wire ropes or PC steel strands made of stranded wires, parallel cables made of bundled wires, round bars, square bars, etc., and the material thereof is not limited. Furthermore, the linear member may be formed from multiple materials, such as a core material and a protective material covering the core material.

[0012] In one embodiment of the tension detection method for a linear member for a structure, the strain fluctuation measurement process measures strain fluctuations using a plurality of optical fiber strain sensors arranged near the circumferential surface of the linear member at angles of 90° or 180° to each other in cross section.

[0013] According to the above embodiment, strain fluctuations are measured using multiple optical fiber strain sensors arranged near the circumferential surface of a linear member at 90° or 180° angles from each other in cross section. These strain fluctuations caused by loads acting in the axial direction of the linear member are identical, but those caused by vibrations in the direction perpendicular to the axis of the linear member are different. Therefore, strain fluctuations caused by vibrations can be detected with high accuracy based on the measurements of the multiple optical fiber strain sensors.

[0014] In one embodiment of the tension detection method for a linear member for a structure, the fluctuation frequency distribution detection process detects strain fluctuations caused by vibrations perpendicular to the axis of the linear member based on the difference between measurement values ​​from two of the optical fiber strain sensors.

[0015] According to the above embodiment, by calculating the difference between the measured values ​​from the two optical fiber strain sensors, it is possible to eliminate strain fluctuations caused by loads acting in the axial direction of the linear member and extract only strain fluctuations caused by vibrations perpendicular to the axis of the linear member.

[0016] In one embodiment of the method for detecting tension in a linear member for a structure, the optical fiber strain sensor is built into the linear member in advance.

[0017] According to the above embodiment, by using an optical fiber strain sensor pre-installed in a linear member of a structure, the tension of the linear member can be detected with little effort. Furthermore, by using an optical fiber strain sensor pre-installed in a linear member of a structure, the tension of the linear member can be measured over a long period of time. For example, the tension of a linear member can be detected for various purposes over a long period of time, from construction management during the manufacture of a structure to maintenance management after the structure is completed.

[0018] In one embodiment of the method for detecting tension in a linear member for a structure, the optical fiber strain sensor is retrofitted to the surface of the linear member via a jig that amplifies the strain of the linear member.

[0019] According to the above embodiment, the tension of a linear member of a structure can be detected with high accuracy by retrofitting the optical fiber strain sensor to the surface of the linear member of the structure via a jig that amplifies the strain of the linear member. Also, by retrofitting the optical fiber strain sensor with a jig to a linear member of a structure that is not previously equipped with a tension detection function, it becomes possible to detect the tension of the linear member.

[0020] In one embodiment of the method for detecting tension of a linear member for a structure, the optical fiber strain sensor is attached to the surface of the linear member with adhesive tape.

[0021] According to the above embodiment, the optical fiber strain sensor can be attached to the surface of a linear member of a structure using adhesive tape, thereby making it possible to detect the tension of the linear member with little effort. Here, since the object to be measured by the optical fiber strain sensor is the change in strain of the linear member, fixing the sensor using adhesive tape makes it possible to perform measurements sufficient for detecting the tension of the linear member.

[0022] In one embodiment of the method for detecting tension of a linear member for use in a structure, the optical fiber strain sensor is an FBG sensor.

[0023] According to the above embodiment, by using an FBG (Fiber Bragg Grating) sensor as the optical fiber strain sensor, it is possible to measure the strain fluctuation of a linear member of a structure with good accuracy.

[0024] A linear member for a structure according to another aspect of the present invention is a linear member used in a structure to bear tension, comprising: A core material that transmits tension; a protective container having both ends fixed near the surface of the core material; an FBG sensor that is housed in the protective container, and has both sides of a strain detection unit sandwiched between them fixed to the protective container near fixed portions at both ends of the protective container, and detects a change in strain of the linear member; a protective material that covers the core material and the protective container in which the FBG sensor is housed; It is characterized by having:

[0025] According to the above configuration, a linear member used in a structure to bear tension includes a core material that transmits tension, and both ends of a protective container are fixed near the surface of the core material. The protective container can be formed, for example, in the shape of an elongated cylinder extending along the core material or an elongated rectangular parallelepiped. An FBG sensor that detects strain fluctuations in the linear member is housed within the protective container. Both sides of the FBG sensor, sandwiching a strain detection unit therebetween, are fixed to the protective container near the fixing portions at both ends of the protective container. The core material and the protective container housing the FBG sensor are covered with a protective material. With this configuration, strain fluctuations in the linear member can be measured using the FBG sensor. Furthermore, since the FBG sensor is used and is protected by the protective container and protective material, tension can be detected stably over a long period of time.

[0026] A linear member for a structure according to another aspect of the present invention is a linear member used in a structure to bear tension, comprising: A core material that transmits tension; a protective container having fixed portions whose both ends are fixed near the surface of the core material, and an expandable portion provided between the fixed portions and formed to be expandable in the longitudinal direction; an FBG sensor that is housed in the protective container, and has both sides of a strain detection unit sandwiched between them fixed to the protective container at positions inside the fixed portions at both ends of the protective container, and detects variations in strain of the linear member; a protective material that covers the core material and the protective container in which the FBG sensor is housed; It is characterized by having:

[0027] According to the above configuration, a linear member used in a structure to bear tension includes a core material that transmits tension, and both ends of a protective container are fixed near the surface of the core material. The protective container can be formed, for example, in the shape of an elongated cylinder extending along the core material or an elongated rectangular parallelepiped. An FBG sensor that detects strain fluctuations in the linear member is housed within the protective container. Both side portions of the FBG sensor, sandwiching a strain detection unit therebetween, are fixed to the protective container at positions inside the fixed portions at both ends of the protective container. The protective container has an expandable portion between the positions where the both sides of the FBG sensor are fixed. The expandable portion may be composed of multiple cylindrical members formed to slidably overlap each other, or may be composed of a bellows-shaped member. Furthermore, the protective container does not have an expandable portion formed in the portion between the fixed portion fixed near the surface of the core material and the positions where the both sides of the FBG sensor are fixed, so that displacement of the fixed portion is transmitted to the both sides of the FBG sensor without substantially changing. The core material and the protective container in which the FBG sensor is housed are covered with a protective material. With this configuration, the FBG sensor can measure strain fluctuations in the linear member. Furthermore, since both side portions of the FBG sensor sandwiching the strain detection unit are fixed to the protective container at positions inside the fixed portions at both ends of the protective container, the strain in the linear member is amplified in accordance with the ratio between the distance between the fixed portions at both ends of the protective container and the distance between the both side portions of the FBG sensor. Therefore, strain fluctuations in the linear member can be measured with good accuracy. Furthermore, since the FBG sensor is used and is protected by the protective container and protective material, tension can be detected stably over a long period of time.

[0028] A linear member for a structure according to another aspect of the present invention is a linear member used in a structure to bear tension, comprising: A core material that transmits tension; An FBG sensor attached to the surface of the core material with adhesive tape; a protective material that covers the core material and the adhesive tape to which the FBG sensor is attached; It is characterized by having:

[0029] According to the above configuration, a linear member used in a structure to bear tension includes a core material that transmits tension, and an FBG sensor is attached to the surface of this core material with adhesive tape. The core material and the adhesive tape with the FBG sensor attached are covered with a protective material. With this configuration, the FBG sensor can measure fluctuations in strain in the linear member. To detect the tension in the linear member, the FBG sensor simply measures fluctuations in strain caused by vibrations in the direction perpendicular to the axis of the linear member. Therefore, a linear member for a structure capable of detecting tension can be obtained with a relatively simple structure in which an FBG sensor is attached to a core material with adhesive tape and protected with a protective material.

[0030] In one embodiment, a connector for connecting to the FBG sensor is provided near an end of the linear member for use in a structure.

[0031] According to the above embodiment, by connecting an FBG sensor measuring device to a connector provided near the end of the linear member for a structure, strain fluctuations can be measured quickly and with little effort using the FBG sensor built into the linear member.

[0032] A sensor installation jig for a linear component for a structure according to another aspect of the present invention is a sensor installation jig for installing an FBG sensor on a linear component for a structure to which tension is applied, the FBG sensor detecting a variation in strain caused by vibration of the linear component in a direction perpendicular to the axis of the linear component, two fixing members fixed to the surface of the linear member at a predetermined interval in the axial direction of the linear member; two extension members whose base ends are fixed to the two fixing members, extend parallel to the linear member, and whose tip ends extend in directions approaching each other; two sensor fixing bodies provided at the tips of the two extension members, respectively, for fixing both sides of the strain detection part of the FBG sensor; It is characterized by having:

[0033] According to the above configuration, a sensor installation jig for installing an FBG sensor on a linear member for a structure in which tension is applied is used to detect strain fluctuations caused by vibrations of the linear member in a direction perpendicular to its axis with the FBG sensor and to detect the tension of the linear member based on the detected information. In this sensor installation jig, two fixing members are fixed to the surface of the linear member at a predetermined distance in the axial direction of the linear member. Two extension members are respectively installed on these fixing members, extending parallel to the linear member and with their tips extending in directions approaching each other. Sensor fixing bodies are respectively provided at the tips of these two extension members, fixing both sides of the strain detection unit of the FBG sensor. With this configuration, the strain of the linear member is amplified in accordance with the ratio between the distance between the two fixing members and the distance between the two sensor fixing bodies. Therefore, by using this sensor installation jig to retrofit an FBG sensor to a linear member, it is possible to measure the strain fluctuations of the linear member with good accuracy, and as a result, it is possible to detect the tension acting on the linear member with good accuracy. [Brief explanation of the drawings]

[0034] [Figure 1] 1 is a schematic diagram showing how a tension detection method for a linear member for a structure according to an embodiment of the present invention is applied to a cable of a bridge. FIG. [Figure 2] FIG. 2 is a flow chart showing a method for detecting tension in a linear member for use in a structure according to an embodiment of the present invention. [Figure 3] 1 is a cross-sectional view showing an end portion of a linear member for use in a structure according to a first embodiment of the present invention. [Figure 4A] 1 is a cross-sectional view schematically showing a linear member for use in structures according to a first embodiment. [Figure 4B] 1 is a longitudinal cross-sectional view schematically showing a linear member for use in structures according to a first embodiment. [Figure 5A] FIG. 4 is a cross-sectional view schematically showing a linear member for use in structures according to a second embodiment. [Figure 5B] FIG. 4 is a longitudinal cross-sectional view schematically showing a linear member for use in structures according to a second embodiment. [Figure 6] FIG. 10 is a cross-sectional view schematically showing a linear member for use in structures according to a third embodiment. [Figure 7] FIG. 10 is a cross-sectional view schematically showing a linear member for use in structures according to a fourth embodiment. [Figure 8] FIG. 10 is a longitudinal cross-sectional view schematically showing a linear member for use in structures according to a fifth embodiment. [Figure 9] FIG. 13 is a longitudinal cross-sectional view schematically showing a protective container and an optical fiber strain sensor provided in a linear member for use in a structure according to a sixth embodiment. [Figure 10A] FIG. 13 is a plan view showing an optical fiber strain sensor installed on a linear member for a structure in a method for detecting tension in a linear member for a structure according to a seventh embodiment. [Figure 10B] FIG. 13 is a side view showing two optical fiber strain sensors installed on a linear member for a structure in a method for detecting tension in a linear member for a structure according to a seventh embodiment. [Figure 10C] FIG. 13 is a cross-sectional view showing two optical fiber strain sensors installed on a linear member for a structure in a method for detecting tension in a linear member for a structure according to a seventh embodiment. [Figure 11A] 13 is a plan view showing an optical fiber strain sensor installed on a linear member for a structure in a method for detecting tension in a linear member for a structure according to an eighth embodiment. FIG. [Figure 11B] FIG. 13 is a side view showing two optical fiber strain sensors installed on a linear member for a structure in the method for detecting tension of a linear member for a structure according to the eighth embodiment. [Figure 11C] 13 is a cross-sectional view showing two optical fiber strain sensors installed on a linear member for a structure in a method for detecting tension in a linear member for a structure according to an eighth embodiment. FIG. [Figure 12A] FIG. 13 is a plan view showing an optical fiber strain sensor installed on a linear member for a structure in a method for detecting tension of a linear member for a structure according to a ninth embodiment. [Figure 12B] FIG. 13 is a side view showing two optical fiber strain sensors installed on a linear member for a structure in a method for detecting tension in a linear member for a structure according to a ninth embodiment. [Figure 12C]FIG. 13 is a cross-sectional view showing two optical fiber strain sensors installed on a linear member for a structure in a method for detecting tension in a linear member for a structure according to a ninth embodiment. [Figure 13A] 13 is a plan view showing an optical fiber strain sensor installed on a linear member for a structure in a method for detecting tension in a linear member for a structure according to a tenth embodiment. FIG. [Figure 13B] FIG. 13 is a side view showing two optical fiber strain sensors installed on a linear member for a structure in the method for detecting tension in a linear member for a structure according to the tenth embodiment. [Figure 13C] FIG. 20 is a cross-sectional view showing two optical fiber strain sensors installed on a linear member for a structure in a method for detecting tension in a linear member for a structure according to a tenth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0035] The present invention will be described in detail below with reference to the illustrated embodiments.

[0036] 1 is a schematic diagram showing how a tension detection method for a linear member for a structure according to an embodiment of the present invention is applied to a cable for a bridge. The tension detection method for a linear member for a structure according to this embodiment detects the tension acting on a cable as a linear member used in a bridge as a structure.

[0037] As shown in FIG. 1, in this embodiment of the method for detecting tension in a linear structural member, an optical fiber strain sensor 2 is installed in a bridge cable 1. The optical fiber strain sensor 2 is preferably an FBG sensor. The optical fiber strain sensor 2 may be pre-installed in the bridge cable 1 or may be retrofitted to the bridge cable 1 when measurements are performed. The optical fiber strain sensor 2 measures strain fluctuations caused by vibrations that displace the bridge cable 1 in a direction perpendicular to its axis, as indicated by arrow A. When an FBG sensor is used as the optical fiber strain sensor 2, the FBG sensor has a strain detection unit 2a in which a diffraction grating is formed at a predetermined position in the core of an optical fiber. The FBG sensor detects strain based on changes in the wavelength of light reflected by the diffraction grating of the strain detection unit 2a. The optical fiber strain sensor 2 is connected to an FBG measurement instrument 3.

[0038] The FBG measuring instrument 3 transmits an optical signal to the optical fiber strain sensor 2 and receives reflected light from the strain detecting unit 2a, and outputs information about the strain detected by the strain detecting unit 2a based on the difference between the transmitted and reflected light. The FBG measuring instrument 3 is connected to a personal computer 4, which stores the information about strain received from the FBG measuring instrument 3 and analyzes the information about strain to detect the tension of the bridge cable 1.

[0039] FIG. 2 is a flow chart showing a tension detection method for a linear member for a structure according to this embodiment. As shown in the flow chart of FIG. 2, in this tension detection method for a linear member for a structure according to this embodiment, first, strain fluctuations in a bridge cable 1 are measured using an FBG sensor as an optical fiber strain sensor 2 (step S1). When measuring strain fluctuations in a bridge cable 1, strain fluctuations may be generated by applying vibrations to the bridge cable 1 using a vibrator or the like. Alternatively, strain fluctuations caused by constant micro-vibrations that occur in the bridge cable 1 due to various loads, such as wind loads and live loads, may be measured. Here, the strain fluctuations measured by the optical fiber strain sensor 2 overlap strain fluctuations caused by load fluctuations acting in the axial direction of the bridge cable 1 and strain fluctuations caused by vibration A in the direction perpendicular to the axis of the bridge cable 1. An example of a load acting in the axial direction of the bridge cable 1 is the load of a vehicle traveling on the deck supported by the bridge cable 1.

[0040] Next, the strain measurement value obtained by the optical fiber strain sensor 2 is subjected to FFT (Fast Fourier Transform) processing to detect the distribution of the strain fluctuation frequency (step S2). This FFT processing may be performed by either the FBG measuring instrument 3 or the personal computer 4.

[0041] Thereafter, the natural frequency of the bridge cable 1 caused by the vibration A in the transverse direction is identified based on the distribution of the strain fluctuation frequency detected in step S2 (step S3). The natural frequency of the bridge cable 1 caused by the vibration A in the transverse direction can be identified based on the frequency band of the strain fluctuation frequency distribution. Alternatively, the natural frequency may be identified based on the results of taking the difference between the strain fluctuations measured simultaneously at multiple different circumferential positions on the bridge cable 1 and performing FFT processing on this difference to detect the frequency distribution. Here, the bridge cable 1 experiences overlapping strain fluctuations caused by the load acting in the axial direction and strain fluctuations caused by the vibration A in the transverse direction of the bridge cable 1, and of these, the strain fluctuations caused by the axial load are the same at all positions on the bridge cable 1. Therefore, by taking the difference between the multiple strain fluctuations measured simultaneously at different circumferential positions of the bridge cable 1, it is possible to cancel out the strain fluctuations caused by the axial load and extract only the strain fluctuations caused by the vibration A perpendicular to the axis of the bridge cable 1.

[0042] Thereafter, the tension acting on the bridge cable 1 is calculated based on the natural frequency of the bridge cable 1 identified in step S3 (step S4). The following relational expression (1) can be used to calculate the tension of the bridge cable 1 using the natural frequency.

number

number

[0043] The calculation of the tension of the bridge cable 1 based on the above formulas (1) and (2) is performed by a personal computer 4. Here, the formula for calculating the tension of the bridge cable 1 using the natural frequency is not limited to the above formulas (1) and (2), and various formulas proposed as the so-called vibration method can be used.

[0044] As described above, in the tension detection method for a linear member for a structure according to this embodiment, the optical fiber strain sensor 2 measures strain fluctuations in the bridge cable 1, identifies the natural frequency of the bridge cable 1 from the strain fluctuation measurement results, and detects the tension of the bridge cable 1 based on the identified natural frequency. Therefore, the optical fiber strain sensor 2 does not need to accurately measure the strain value of the bridge cable 1; it need only be attached to the bridge cable 1 with enough strength to measure strain fluctuations. That is, unlike conventional PC steel strands with optical fiber, which aim to detect the tension of the PC steel strands from the axial strain value of the PC steel strands, there is no need to wrap the strain-measuring optical fiber in a filler and twist it tightly to the steel strands to match the strain of the strain-measuring optical fiber to the strain of the steel strands. Therefore, in the method for detecting tension in a linear member for a structure of this embodiment, an optical fiber strain sensor 2 can be used that is easily attached to a bridge cable 1 with a simple fixing structure, and as a result, the preparation and work for detecting tension can be made easier than before.

[0045] In the method for detecting tension in a linear member for a structure according to this embodiment, although the optical fiber strain sensor 2 may be retrofitted to the bridge cable 1 to measure strain fluctuations, it is preferable to use a sensor-embedded bridge cable 1 that already has the optical fiber strain sensor 2 built in, as this simplifies the measurement process. A sensor-embedded bridge cable 1 that already has the optical fiber strain sensor 2 built in can be formed, for example, by covering the outer periphery of a core material with a protective tube, inserting the optical fiber strain sensor 2 inside the protective tube, and then injecting a fixing resin such as an adhesive into the protective tube. Because a bridge cable 1 with a built-in optical fiber strain sensor 2 can be used with such a simple structure, bridge cables 1 capable of detecting tension can be manufactured at low cost.

[0046] FIG. 3 is a cross-sectional view showing the end of a bridge cable serving as a linear structural member according to a first embodiment of the present invention, to which the tension detection method for a linear structural member of the present invention can be applied. The sensor-embedded bridge cable 11 of the first embodiment is pre-installed with an optical fiber strain sensor 2. This sensor-embedded bridge cable 11 is used to suspend and support girders and other components on bridges such as cable-stayed bridges. This sensor-embedded bridge cable 11 includes a core member 14 for transmitting tension. This core member 14 is formed of multiple parallel-arranged wires made of high-tensile zinc-plated steel. The outside of this core member 14 is covered with a protective film 15 that serves as a protective material to protect the core member 14. The protective film 15 is preferably formed of polyethylene, but may be formed of other materials. An outer tube 16 having a core material 14 and a protective film 15 inserted therein is provided at the end of the sensor-embedded bridge cable 11, and a socket portion 12 is provided at the tip side of this outer tube 16 for connecting the end of the sensor-embedded bridge cable 11 to other components such as a girder.

[0047] In the sensor-embedded bridge cable 11 of this embodiment, an optical fiber strain sensor 2 is arranged along the outer surface of the core material 14. This optical fiber strain sensor 2 is an FBG sensor, and has a strain detection unit 2a, which is made up of a diffraction grating formed on the tip side of an optical fiber extending over the entire length. This strain detection unit 2a is arranged in a position that contacts the outer surface of the core material 14 and is covered with a protective film 15. The portion of the optical fiber strain sensor 2 that is closer to the base end than the strain detection unit 2a extends along the surface of the core material 14 toward the socket unit 12 and is drawn out near the tip of the socket unit 12. A connector 20 is provided at the base end of the optical fiber strain sensor 2 for connection to an FBG measurement instrument 3.

[0048] FIG. 4A is a cross-sectional view of a sensor-embedded bridge cable 11 according to the first embodiment, showing a cross section of the strain detection unit 2a of the optical fiber strain sensor 2. FIG. 4B is a longitudinal cross-sectional view of the sensor-embedded bridge cable 11, showing a cross section of the strain detection unit 2a of the optical fiber strain sensor 2. In FIGS. 4A and 4B, the shape of the gap between the core material 14 and the protective film 15 and the shape of the protective film 15 are shown only for illustrative purposes; the actual shapes of the gap and the protective film 15 are different. As shown in FIGS. 4A and 4B, the optical fiber strain sensor 2 can be embedded in the sensor-embedded bridge cable 11 with a simple structure: the optical fiber strain sensor 2 is disposed in contact with the outer peripheral surface of the core material 14, and the exterior of the core material 14 and the optical fiber strain sensor 2 are covered with the protective film 15. By measuring strain fluctuations in the sensor-embedded bridge cable 11 using the optical fiber strain sensor 2 built in with such a simple structure, it is possible to detect the distribution of fluctuation frequencies caused by vibrations in the transverse direction of the sensor-embedded bridge cable 11, identify the natural frequency, and detect the tension of the sensor-embedded bridge cable 11. Here, the sensor-embedded bridge cable 11 is preferably arranged so that the strain detection unit 2a of the optical fiber strain sensor 2 is located at the upper end of the sensor-embedded bridge cable 11 in the cross section transverse to the axis. The strain detection unit 2a of the optical fiber strain sensor 2 located at the upper end of the sensor-embedded bridge cable 11 can effectively measure strain fluctuations caused by the predominant vertical vibrations that occur in the sensor-embedded bridge cable 11. Alternatively, the strain detection unit 2a of the optical fiber strain sensor 2 may be arranged so that it is located at the lower end of the sensor-embedded bridge cable 11 in the cross section transverse to the axis. The strain detection unit 2a of the optical fiber strain sensor 2 may be placed at another position depending on the vibrations occurring in the sensor-embedded bridge cable 11. The number of optical fiber strain sensors 2 placed in the sensor-embedded bridge cable 11 is not limited to one, and may be two or more.Furthermore, when two or more optical fiber strain sensors 2 are arranged in the sensor-embedded bridge cable 11, it is preferable to arrange them at an angle of 90° or 180° from each other in a cross section perpendicular to the axis of the sensor-embedded bridge cable 11.

[0049] Furthermore, the sensor-embedded cable for bridges 11 of the first embodiment is provided with a connector 20 for the optical fiber strain sensor 2 in the socket portion 12 at the end of the sensor-embedded cable for bridges 11, so that an operator can easily and quickly measure strain by simply connecting the FBG measurement device 3 to the connector 20. Here, the optical fiber strain sensor 2 may be drawn out to the outside of the cable 11 from the end face of the socket portion 12, or may be drawn out to the outside of the cable 11 from a position other than the socket portion 12. Furthermore, the position of the strain detection portion 2a of the optical fiber strain sensor 2 may be at the end or the center in the axial direction of the cable 11, and can be placed at any position.

[0050] Fig. 5A is a cross-sectional view of a sensor-embedded bridge cable 21 as a linear member for a structure of the second embodiment, and is a cross-sectional view that schematically shows a cross section at the strain detection unit 2a of the optical fiber strain sensor 2. Fig. 5B is a longitudinal cross-sectional view of the sensor-embedded bridge cable 21, and is a longitudinal cross-sectional view that schematically shows the peripheral portion of the strain detection unit 2a of the optical fiber strain sensor 2. In Figs. 5A and 5B, the shape of the gap between the core material 14 and the tape fixing portion 23, the shape of the tape fixing portion 23, and the shape of the gap between the tape fixing portion 23 and the outer tube 22 are shown only schematically, and the actual shapes of each gap and the tape fixing portion 23 will differ.

[0051] The sensor-embedded bridge cable 21 of the second embodiment has an optical fiber strain sensor 2 built in. As shown in FIGS. 5A and 5B , the sensor-embedded bridge cable 21 of the second embodiment is formed by housing a core member 14 inside a jacket tube 22. The sensor-embedded bridge cable 21 includes a core member 14 made of parallelly arranged wires, an optical fiber strain sensor 2 arranged in contact with the outer circumferential surface of the core member 14, and a tape fixing portion 23 arranged to surround the core member 14 and a portion near the strain detection portion 2a of the optical fiber strain sensor 2. The tape fixing portion 23 is formed by winding a resin adhesive tape. The core member 14, the optical fiber strain sensor 2, and the tape fixing portion 23 are housed inside the jacket tube 22. A rust-preventive layer 24 formed by injecting an anticorrosive agent is formed as a protective material between the core member 14, the optical fiber strain sensor 2, the tape fixing portion 23, and the jacket tube 22. In this way, a sensor-embedded bridge cable 21 with an optical fiber strain sensor 2 built in can be configured with a simple structure in which the optical fiber strain sensor 2 is fixed to the outer peripheral surface of the core material 14 with the tape fixing portion 23. The optical fiber strain sensor 2 built in with such a simple structure can measure strain fluctuations in the sensor-embedded bridge cable 21, identify the natural frequency based on the strain fluctuations, and detect the tension of the sensor-embedded bridge cable 21. Here, the tape fixing portion 23 is not limited to being formed by wrapping adhesive tape around the entire circumference of the core material 14, but may also be formed by attaching adhesive tape to a portion of the circumference of the core material 14. The shape of the tape fixing portion 23 is not particularly limited as long as the optical fiber strain sensor 2 is attached to the core material 14 with adhesive tape.

[0052] 6 is a cross-sectional view of a sensor-embedded cable for bridges 31 as a linear member for structure of the third embodiment, and is a cross-sectional view that schematically shows a cross section at the strain detection portion 2a of the optical fiber strain sensor 2. The sensor-embedded cable for bridges 31 of the third embodiment differs from the bridge cable 11 of the first embodiment in that it is provided with a fixing member 26 for the optical fiber strain sensor 2. In the sensor-embedded cable for bridges 31 of the third embodiment, components that are similar to those of the bridge cable 11 of the first embodiment are designated by the same reference numerals, and detailed descriptions thereof will be omitted.

[0053] The sensor-embedded bridge cable 31 of the third embodiment has an optical fiber strain sensor 2 pre-installed, and this optical fiber strain sensor 2 is installed on the surface of the core material 14 via a fixing member 26. The fixing member 26 is a strip-like plate with a width wider than the diameter of the optical fiber strain sensor 2, and can be made of resin or metal. The core material 14 and the optical fiber strain sensor 2 attached to the core material 14 via the fixing member 26 are covered with a protective film 15. The sensor-embedded bridge cable 31 of the third embodiment can effectively measure strain fluctuations using the optical fiber strain sensor 2 that contacts the core material 14 via the fixing member 26.

[0054] 7 is a cross-sectional view of a sensor-embedded cable for bridges 41 as a linear member for a structure according to the fourth embodiment, and is a cross-sectional view that schematically shows a cross section at the strain detection portion 2a of the optical fiber strain sensor 2. The sensor-embedded cable for bridges 41 of the fourth embodiment differs from the sensor-embedded cable for bridges 21 of the second embodiment in that it is provided with a fixing member 26 for the optical fiber strain sensor 2. In the sensor-embedded cable for bridges 41 of the fourth embodiment, components that are the same as those in the sensor-embedded cable for bridges 21 of the second embodiment are designated by the same reference numerals, and detailed descriptions thereof will be omitted.

[0055] The sensor-embedded bridge cable 41 of the fourth embodiment has an optical fiber strain sensor 2 pre-installed, and the optical fiber strain sensor 2 is attached to the surface of the core material 14 via a fixing member 26. The fixing member 26 is a strip-shaped plate with a width wider than the diameter of the optical fiber strain sensor 2, and can be made of resin or metal. The core material 14 and the optical fiber strain sensor 2 attached to the core material 14 via the fixing member 26 are covered with a tape fixing portion 23 near the strain detection unit 2a. The sensor-embedded bridge cable 41 of the fourth embodiment can effectively measure strain fluctuations using the optical fiber strain sensor 2 that contacts the core material 14 via the fixing member 26.

[0056] FIG. 8 is a longitudinal cross-sectional view of a sensor-embedded bridge cable 51 as a linear member for a structure according to the fifth embodiment, and is a longitudinal cross-sectional view schematically showing the peripheral portion of the strain detection unit 2a of the optical fiber strain sensor 2. The sensor-embedded bridge cable 51 of the fifth embodiment is a sensor-embedded cable that incorporates the optical fiber strain sensor 2. This sensor-embedded bridge cable 51 differs from the bridge cable 11 of the first embodiment in that the optical fiber strain sensor 2 is housed in a protective container, and the protective container that houses the optical fiber strain sensor 2 is embedded in a protective material that covers the core material 14. In the sensor-embedded bridge cable 51 of the fifth embodiment, components that are similar to those of the bridge cable 11 of the first embodiment are designated by the same reference numerals, and detailed description thereof will be omitted.

[0057] As shown in FIG. 8, a sensor-embedded cable for bridges 51 according to the fifth embodiment includes a core material 14, a protective container 36 that is disposed near the surface of the core material 14 and houses an optical fiber strain sensor 2, and a protective layer 35 that serves as a protective material covering the core material 14 and the protective container 36. FIG. 8 is a schematic diagram in which the dimensions of the core material 14 are understated relative to the dimensions of the protective container 36 and the protective layer 35. The protective container 36 is a cylindrical tube made of stainless steel and houses the optical fiber strain sensor 2 approximately coaxially. The protective container 36 is disposed near the surface of the core material 14 and is embedded in the protective layer 35 in a state parallel to the core material 14. Two fixing portions 37A and 37B that are fixed to the protective layer 35 are provided on the outer surface of the protective container 36. These fixing portions 37A, 37B are formed as protrusions projecting radially outward from the protective container 36 and engage with the protective layer 35, thereby immobilizing the protective layer 35 in the axial direction. Optical fiber fixing bodies 38A, 38B for fixing the optical fiber strain sensor 2 are provided on the inner surface of the protective container 36 at axial positions substantially identical to the axial positions at which the fixing portions 37A, 37B are provided. These two optical fiber fixing bodies 38A, 38B are disposed on either side of the strain detection unit 2a of the optical fiber strain sensor 2, respectively, and fix the fixing portion of the optical fiber strain sensor 2 so as not to move in the axial direction. The protective layer 35 is formed of polyethylene. In this sensor-embedded bridge cable 51, the protective container 36 is disposed on the surface side of the core material 14, and then a molten polyethylene material is applied to form the protective layer 35, which covers the core material 14 and the protective container 36 and embeds the protective container 36 inside. Here, the protective layer 35 may be formed of other materials as long as it has the function of covering and protecting the core material 14 and the protective container 36 .

[0058] In a sensor-embedded bridge cable 51 of the fifth embodiment, strain fluctuations caused by axial loads and vibrations in the direction perpendicular to the axis are measured by the detecting unit 2a of the optical fiber strain sensor 2. Here, the strain ε0 measured by the detecting unit 2a can be expressed as ε0 = ΔL / L0 when a displacement of ΔL occurs with respect to the distance L0 between two optical fiber fixtures 38A and 38B shown in Fig. 8. In the sensor-embedded bridge cable 51 of this embodiment, the optical fiber strain sensor 2 is housed in a protective container 36 fixed to the protective layer 35 by fixing units 37A and 37B, and is fixed by optical fiber fixtures 38A and 38B fixed to the protective container 36. Therefore, strain fluctuations occurring in the sensor-embedded bridge cable 51 can be measured stably and with good accuracy.

[0059] 9 is a vertical cross-sectional view schematically showing a protective container 43 and an optical fiber strain sensor 2 provided in a sensor-embedded cable for bridges as a linear member for structure of the sixth embodiment. The bridge cable of the sixth embodiment differs from the sensor-embedded cable for bridges 51 of the fifth embodiment in that the protective container that houses the optical fiber strain sensor 2 is formed to be expandable and contractible, and in the installation positions of the optical fiber fixtures 38A, 38B of the optical fiber strain sensor 2. In the sensor-embedded cable for bridges of the sixth embodiment, components similar to those of the sensor-embedded cable for bridges 51 of the fifth embodiment are designated by the same reference numerals, and detailed description thereof will be omitted.

[0060] The sensor-embedded cable for bridges of the sixth embodiment is a sensor-embedded cable in which an optical fiber strain sensor 2 is pre-installed. As shown in FIG. 9 , the protective container 43 of the sensor-embedded cable for bridges of the sixth embodiment has two fixed containers 43a, 43a formed from cylindrical tubes, arranged with their end faces facing each other and a predetermined distance apart. A sliding tube 43b formed from a cylindrical tube and having an inner diameter slightly larger than the outer diameter of the fixed container 43a is fitted to cover the outer surfaces of the ends of these two fixed containers 43a, 43a. The portion between these fixed containers 43a and fitted onto the sliding tube 43b corresponds to the expandable portion of the protective container. Optical fiber fixers 38A, 38B are provided on the inner surfaces of the fixed containers 43a, 43a of the protective container near the opposing end faces, respectively, to fix both sides of the strain detection unit 2a of the optical fiber strain sensor 2. Furthermore, fixing portions 44A and 44B for fixing the fixed containers 43a and 43a to the protective layer 35 are provided on the outer surfaces of the fixed containers 43a and 43a of the protective container at axial positions farther from each other than the optical fiber fixers 38A and 38B. This protective container 43 is disposed on the surface side of the core material 14, and is covered together with the core material 14 by the protective layer 35, as in the fifth embodiment.

[0061] In the sensor-embedded bridge cable of the sixth embodiment, strain fluctuations due to axial loads and vibrations in the transverse direction are measured by the detecting unit 2a of the optical fiber strain sensor 2. Here, when a displacement of ΔL occurs with respect to the distance L1 between the two optical fiber fixed bodies 38A and 38B shown in FIG. 9, the strain ε1 measured by the detecting unit 2a can be expressed as ε1 = ΔL / L1. Here, this displacement ΔL is the same as the displacement occurring with respect to the distance L2 between the fixed parts 44A and 44B of the fixed container, so the strain ε2 occurring between these fixed parts 44A and 44B can be expressed as ε2 = ΔL / L2. Therefore, the relationship ε1 = ε2 · (L2 / L1) holds between the strain ε1 measured by the detecting unit 2a and the strain ε2 actually occurring in the sensor-embedded bridge cable, which is the strain occurring between the fixed parts 44A and 44B. As described above, the sensor-embedded bridge cable of the sixth embodiment can amplify and measure strain by providing the optical fiber fixtures 38A, 38B, which are arranged inside the expandable protective container 43 at a distance smaller than the distance between the fixing parts 44A, 44B of the protective container, and fixing the optical fiber strain sensor 2 thereto. That is, the displacement between the fixing parts 44A, 44B of the protective container is transmitted by using the parts of the fixing container 43a that are closer to each other than the fixing parts 44A, 44B as displacement transmitting means, and is measured by the detecting part 2a between the optical fiber fixtures 38A, 38B, thereby amplifying the strain measured by the optical fiber strain sensor 2. As a result, strain fluctuations occurring in the sensor-embedded bridge cable can be measured stably and with good accuracy.

[0062] In the sixth embodiment, the protective container 43 has an expandable portion formed between the end faces of the two fixed containers 43a by the sliding tube 43b fitted onto the end of the fixed container 43a, but the expandable portion may have another structure. For example, the expandable portion may be formed by a bellows-like member or a flexible cylindrical body connected between the end faces of the fixed containers 43a.

[0063] FIG. 10A is a plan view showing a bridge cable 61 to which an optical fiber strain sensor 2A has been retrofitted in the seventh embodiment of the tension detection method for a linear member for a structure. FIG. 10B is a side view showing a bridge cable 61 to which first and second optical fiber strain sensors 2A and 2B have been retrofitted, and FIG. 10C is a cross-sectional view of the bridge cable 61 to which the first and second optical fiber strain sensors 2A and 2B have been retrofitted. The tension detection method for a linear member for a structure of this embodiment differs from the tension detection method using the bridge cable 11 of the first embodiment in that the first and second optical fiber strain sensors 2A and 2B are retrofitted to the surface of the bridge cable 61. In the seventh embodiment, the same components as those in the first embodiment are designated by the same reference numerals, and detailed description thereof will be omitted.

[0064] In the seventh embodiment, a tension detection method for a linear member for a structure is performed by attaching two optical fiber strain sensors 2A, 2B with adhesive tape 53 to the surface of an existing bridge cable 61, which has a core material 14 covered with a protective film 15. These first and second optical fiber strain sensors 2A, 2B are FBG sensors, and a predetermined axial range including the strain detection unit 2a is covered with adhesive tape 53 and attached to the surface of the bridge cable 61. As shown in Figures 10B and 10C, the first and second optical fiber strain sensors 2A, 2B are arranged at an angle of 180° at the upper and lower ends of the bridge cable 61 in a cross section perpendicular to the axis.

[0065] In the seventh embodiment of the tension detection method for a linear member for a structure, first and second optical fiber strain sensors 2A and 2B are attached to a bridge cable 61, and then the first and second optical fiber strain sensors 2A and 2B measure the vibration of the bridge cable 61 to perform a strain fluctuation measurement process. This is followed by a fluctuation frequency distribution detection process, in which a distribution of strain fluctuation frequencies caused by the vibration of the bridge cable 61 in the transverse direction is detected based on the strain fluctuations measured by the first and second optical fiber strain sensors 2A and 2B in the strain fluctuation measurement process. Here, the difference between the measurements by the first and second optical fiber strain sensors 2A and 2B is calculated to cancel out the strain fluctuation caused by the load acting in the axial direction of the bridge cable 61. This allows only the strain fluctuation caused by the vibration of the bridge cable 61 in the transverse direction to be extracted. By performing FFT processing on the extracted strain fluctuations caused by the vibrations of the bridge cable 61 in the direction perpendicular to its axis, it is possible to detect the distribution of the fluctuation frequencies of the strain caused by the vibrations of the bridge cable 61 in the direction perpendicular to its axis. After this, a natural frequency identification step is performed to identify the natural frequency of the bridge cable 61 based on the strain fluctuation frequency distribution of the bridge cable 61. Based on the identified natural frequency, the tension of the bridge cable 61 is calculated in a tension calculation step.

[0066] As described above, the tension detection method for a linear member for a structure according to the seventh embodiment can detect the tension of a bridge cable 61 through the simple process of attaching two optical fiber strain sensors 2A, 2B to the bridge cable 61 with adhesive tape 53. Furthermore, the first and second optical fiber strain sensors 2A, 2B are disposed at the upper and lower ends of the bridge cable 61, spaced apart by an angle of 180° in the cross section perpendicular to the axis, and the difference between the measurements of the first and second optical fiber strain sensors 2A, 2B is calculated, thereby extracting only the vibration of the bridge cable 61 in the direction perpendicular to the axis. Therefore, the natural frequency of the bridge cable 61 can be determined with high accuracy, and the tension of the bridge cable 61 can be easily detected with high accuracy.

[0067] FIG. 11A is a plan view showing the state in which an optical fiber strain sensor 2A is retrofitted to a bridge cable 61 in the method for detecting tension in a linear member for a structure of the eighth embodiment. FIG. 11B is a side view showing the state in which first and second optical fiber strain sensors 2A and 2B are retrofitted to a bridge cable 61, and FIG. 11C is a cross-sectional view of the bridge cable 61 to which the first and second optical fiber strain sensors 2A and 2B are retrofitted. The method for detecting tension in a linear member for a structure of this embodiment differs from the method for detecting tension in a linear member for a structure of the seventh embodiment in that the optical fiber strain sensors 2A and 2B are retrofitted to the surface of the bridge cable 61 using an attachment jig. In the eighth embodiment, the same components as in the seventh embodiment are designated by the same reference numerals, and detailed description thereof will be omitted.

[0068] In the eighth embodiment, a tension detection method for a linear member for a structure involves retrofitting two optical fiber strain sensors 2A, 2B to the upper and lower ends of an existing bridge cable 61 using mounting jigs 54A, 54B. The first optical fiber strain sensor 2A, which is attached to the upper end of the bridge cable 61, is attached using the first mounting jig 54A. This first mounting jig 54A has first and second cable fixing members 55A, 55B, which are arranged on both sides of the strain detection unit 2a and contact the surface of the bridge cable 61. First and second optical fiber fixing bodies 56A, 56B, which fix and support the optical fiber portion of the first optical fiber strain sensor 2A, are arranged on the surfaces of these cable fixing members 55A, 55B, respectively, that are farther from the bridge cable 61. A second mounting jig 54B for mounting a second optical fiber strain sensor 2B to the lower end of a bridge cable 61 has first and second cable fixing members 55A, 55B and first and second optical fiber fixing bodies 56A, 56B similar to those of the first mounting jig 54A. The first cable fixing member 55A of the first mounting jig 54A and the first cable fixing member 55A of the second mounting jig 54B are fastened to each other with a fixing band 57 and fixed to the surface of the bridge cable 61. The second cable fixing member 55B of the first mounting jig 54A and the second cable fixing member 55B of the second mounting jig 54B are fastened to each other with a fixing band 57 and fixed to the surface of the bridge cable 61. The fixing band 57 can be formed of a resin lashing belt, bundling belt, or bundling band, or a metal strip may also be used.

[0069] In the method for detecting tension in a linear member for a structure of the eighth embodiment, similarly to the sixth embodiment, first and second optical fiber strain sensors 2A and 2B are attached to a bridge cable 61, and then the optical fiber strain sensors 2A and 2B measure the vibration of the bridge cable 61 to perform a strain fluctuation measurement process. Then, a fluctuation frequency distribution detection process is performed to detect the distribution of strain fluctuation frequencies caused by the vibration of the bridge cable 61 in the transverse direction based on the difference between the strain fluctuations measured by the first and second optical fiber strain sensors 2A and 2B in the strain fluctuation measurement process. Then, an FFT process is performed on the strain fluctuations caused by the vibration of the bridge cable 61 in the transverse direction to detect the distribution of strain fluctuation frequencies. Then, a natural frequency identification process is performed to identify the natural frequency of the bridge cable 61, and a tension calculation process is performed to calculate the tension of the bridge cable 61 based on the identified natural frequency.

[0070] In this way, the method for detecting tension in a linear member for a structure of the eighth embodiment allows the first and second optical fiber strain sensors 2A, 2B to be easily fixed to the bridge cable 61 using the first and second mounting jigs 54A, 54B, and the strain fluctuations of the bridge cable 61 to be measured, thereby making it possible to detect strain fluctuations caused by vibrations in the direction perpendicular to the axis of the bridge cable 61. As a result, the tension of the bridge cable 61 can be easily detected with good accuracy.

[0071] FIG. 12A is a plan view showing a bridge cable 61 to which an optical fiber strain sensor 2A has been retrofitted in the ninth embodiment of the tension detection method for a linear member for a structure. FIG. 12B is a side view showing a bridge cable 61 to which first and second optical fiber strain sensors 2A and 2B have been retrofitted, and FIG. 12C is a cross-sectional view of the bridge cable 61 to which the first and second optical fiber strain sensors 2A and 2B have been retrofitted. The tension detection method for a linear member for a structure of this embodiment differs from the tension detection method for a linear member for a structure of the eighth embodiment in that the structure of the mounting jig for retrofitting the optical fiber strain sensors 2A and 2B to the surface of the bridge cable 61 is different. In the ninth embodiment, the same components as those in the eighth embodiment are designated by the same reference numerals, and detailed description thereof will be omitted.

[0072] In the ninth embodiment, a tension detection method for a linear member for a structure retrofits first and second optical fiber strain sensors 2A and 2B onto the surface of an existing bridge cable 61 using first and second mounting jigs 62A and 62B, respectively. These mounting jigs 62A and 62B are amplification-type mounting jigs that amplify the strain measurements of the first and second optical fiber strain sensors 2A and 2B. The first amplification-type mounting jig 62A mounts the first optical fiber strain sensor 2A to the upper end of the bridge cable 61. This first amplification-type mounting jig 62A is disposed on both sides of the strain detection unit 2a and has cable fixing members 63A and 63B that contact the surface of the bridge cable 61. The base ends of two extension members 64A and 64B that extend parallel to the bridge cable 61 and approach each other are fixed to these cable fixing members 63A and 63B. The extension members 64A, 64B are preferably formed from a rod-shaped material, but their cross-sectional shapes are not particularly limited. Optical fiber fixtures 65A, 65B are provided at the tips of the two extension members 64A, 64B, respectively, to secure both sides of the strain detection unit 2a of the first optical fiber strain sensor 2A. A second amplified mounting jig 62B, which mounts the second optical fiber strain sensor 2B to the lower end of the bridge cable 61, includes cable fixing members 63A, 63B similar to those of the first amplified mounting jig 62A, extension members 64A, 64B, and optical fiber fixtures 65A, 65B. The first cable fixing member 63A of the first amplified mounting jig 62A and the first cable fixing member 63A of the second amplified mounting jig 62B are fastened together with a fixing band 57 and fixed to the surface of the bridge cable 61. In addition, the second cable fixing member 63B of the first amplifying type mounting jig 62A and the second cable fixing member 63B of the second amplifying type mounting jig 62B are fastened to each other by a fixing band 57 and fixed to the surface of the bridge cable 61.

[0073] The amplifying type mounting jig 62A used in the method for detecting tension of a linear member for a structure according to the ninth embodiment can amplify the strain of the bridge cable 61 measured by the detecting unit 2a of the optical fiber strain sensor 2. 10When a displacement of ΔL occurs with respect to the distance L3 between the two optical fiber fixing bodies 65A and 65B shown in FIG. 12B, ε 10 =ΔL / L3. Here, since this displacement ΔL is the same as the displacement occurring with respect to the distance L4 between the two cable fixing members 63A and 63B, the strain ε occurring between these cable fixing members 63A and 63B can be expressed as 20 is ε 20 =ΔL / L4. From these, the strain ε measured by the detector 2a can be expressed as 10 and the strain occurring between the cable fixing members 63A and 63B, which is the strain actually occurring in the bridge cable 61, ε 20 Between 10 =ε 20 The relationship (L4 / L3) holds.

[0074] As described above, the amplifying type mounting jig 62A used in the tension detection method for a linear member for a structure of the ninth embodiment has the base ends of two extension members 64A, 64B fixed to cable fixing members 63A, 63B that contact the surface of the bridge cable 61, and both sides of the strain detection unit 2a of the optical fiber strain sensor 2A fixed by optical fiber fixers 65A, 65B provided at the tips of these extension members 64A, 64B, thereby amplifying and measuring the strain of the bridge cable 61. That is, the displacement between the cable fixing members 63A, 63B is transmitted by the extension members 64A, 64B, which function as displacement transmission means, and is measured by the detection unit 2a between the optical fiber fixers 65A, 65B, thereby amplifying the strain measured by the optical fiber strain sensor 2. Both the first amplifying type mounting jig 62A and the second amplifying type mounting jig 62B can exhibit this strain amplification effect in the same way. As a result, the fluctuation of strain occurring in the bridge cable 61 can be measured stably with good accuracy.

[0075] FIG. 13A is a plan view showing a bridge cable 61 to which an optical fiber strain sensor 2A has been retrofitted in a method for detecting tension in a linear member for a structure according to the tenth embodiment. FIG. 13B is a side view showing a bridge cable 61 to which first and second optical fiber strain sensors 2A and 2B have been retrofitted, and FIG. 13C is a cross-sectional view of the bridge cable 61 to which the first and second optical fiber strain sensors 2A and 2B have been retrofitted. The method for detecting tension in a linear member for a structure according to the present embodiment differs from the method for detecting tension in a linear member for a structure according to the ninth embodiment in that the fixing structure of the mounting jig for retrofitting the optical fiber strain sensors 2A and 2B to the surface of the bridge cable 61 is different. In the tenth embodiment, the same components as those in the ninth embodiment are designated by the same reference numerals, and detailed description thereof will be omitted.

[0076] In the method for detecting tension in a linear member for a structure of the tenth embodiment, first and second amplifying type mounting jigs 62A, 62B similar to those of the ninth embodiment are used to retrofit first and second optical fiber strain sensors 2A, 2B to an existing bridge cable 61. That is, the first and second amplifying type mounting jigs 62A, 62B include cable fixing members 63A, 63B that contact the surface of the bridge cable 61, two extension members 64A, 64B whose base ends are fixed to the cable fixing members 63A, 63B, and two optical fiber fixers 65A, 65B that are provided at the tips of the extension members 64A, 64B and fix both sides of the strain detection unit 2a of the optical fiber strain sensor 2A. In the tenth embodiment of the tension detection method for a linear member for a structure, the first and second amplifying mounting jigs 62A, 62B are fixed to each other by metal fixing rings 73, 73 instead of the fixing band 57. These fixing rings 73, 73 are attached to the bridge cable 61 by arranging two semicircular metal belts, each having a cable fixing member 63A, 63B inserted through its center, so as to surround the surface of the bridge cable 61, and fixing flanges formed on the ends of the bolts to each other with bolts 74. The fixing rings 73, 73 allow the first and second optical fiber strain sensors 2A, 2B to be firmly fixed to the surface of the bridge cable 61. As a result, strain fluctuations occurring in the bridge cable 61 can be measured stably and with good accuracy by the retrofitted first and second optical fiber strain sensors 2A, 2B.

[0077] In the tension detection methods for linear members for structures of the seventh to tenth embodiments, the optical fiber strain sensors 2A, 2B are placed at the upper and lower ends of the bridge cable 61, but they may be placed at other positions as long as they are different positions in the vertical direction. Also, the optical fiber strain sensors 2A, 2B are placed at an angle of 180° in the cross section perpendicular to the axis of the bridge cable 61, but they may be placed at other angles, for example, at an angle of 90°. Also, the number of optical fiber sensors 2 placed on the bridge cable 61 may be one.

[0078] Furthermore, in each of the above embodiments, an FBG sensor is used as the optical fiber strain sensor 2, but an optical fiber strain sensor that applies other strain measurement principles, such as a Brillouin scattering optical fiber sensor, may also be used.

[0079] In the above embodiments, the bridge cables 1, 61 and the sensor-embedded bridge cables 11, 21, 31, 41, and 51 have a core 14 formed by arranging multiple wires in parallel. However, the shape of the cable core is not limited to this, as long as it transmits tension. For example, the present invention can also be applied to cables having a core formed of steel strands, such as galvanized PC steel strands. In this case, the optical fiber strain sensor 2 does not need to be twisted into the steel strands; it can simply be placed on or near the surface of the core, parallel to the extension direction of the core.

[0080] In addition, in the above embodiments, the tension of a cable serving as a linear member used in a bridge structure is detected. However, various types of bridges, such as cable-stayed bridges and Nielsen-Lohse bridges, are applicable. Furthermore, the linear members include those used to suspend deck slabs via girders, as well as prestressing steel wires, prestressing steel strands, and prestressing steel rods used to provide tension in prestressed concrete. The present invention is particularly effective when measuring the tension of prestressing steel wires and prestressing steel rods in prestressed concrete with an exterior-type external cable reinforcement structure. Furthermore, the structure is not limited to a bridge, but includes various civil engineering structures, architectural structures, and the like. Furthermore, the linear members are not limited to cables, but include a wide range of linear members used to provide tension, such as steel wires and steel rods.

[0081] The present invention is not limited to the above-described embodiments, and many modifications can be made by a person having ordinary skill in the art within the technical concept of the present invention. [Explanation of symbols]

[0082] 1,61 Bridge cables 11, 21, 31, 41, 51 Sensor-embedded bridge cables 2, 2A, 2B Optical fiber strain sensor 2a Strain detection part of optical fiber strain sensor 3 FBG measuring instrument 4. Personal Computers 12 Socket part 14 Core material 15 Protective film 16,22 outer tube 20 Connectors 23 Tape fixing part 24 Anti-rust layer 26 Fixing member 35 Protective layer 36,43 Protective container 37A, 37B, 44A, 44B Protective container fixing part 38A, 38B, 56A, 56B, 65A, 65B Optical fiber fixing body 43a Fixed container 43b Sliding tube 53 Adhesive Tape 54A, 54B Optical fiber strain sensor mounting fixture 55A, 55B, 63A, 63B Cable fixing member 57 Fixing band 62A, 62B Amplified mounting fixture for optical fiber strain sensors 64A,64B Extension member 73 Retaining ring 74 volts

Claims

1. A tension detection method for detecting tension acting on a linear member used in a structure, comprising: a strain fluctuation measuring step of measuring a strain fluctuation of the linear member using an optical fiber strain sensor installed on the linear member; a fluctuation frequency distribution detection step of detecting a distribution of fluctuation frequencies of strain caused by vibration of the linear member in the axis-perpendicular direction based on the fluctuation of strain measured in the strain fluctuation measurement step; a natural frequency specifying step of specifying a natural frequency of the linear member based on the strain fluctuation frequency distribution detected in the fluctuation frequency distribution detecting step; a tension calculation step of calculating a tension of the linear member based on the natural frequency identified in the natural frequency identification step; A method for detecting tension in a linear member for a structure, comprising:

2. 2. The method for detecting tension of a linear member for a structure according to claim 1, The strain fluctuation measuring process is a method for detecting tension in a linear member for a structure, characterized in that the strain fluctuation is measured using a plurality of optical fiber strain sensors arranged near the peripheral surface of the linear member at angles of 90° or 180° to each other in cross section.

3. 3. The method for detecting tension of a linear member for a structure according to claim 2, The method for detecting tension in a linear member for a structure is characterized in that, in the fluctuation frequency distribution detection process, fluctuations in strain caused by vibrations perpendicular to the axis of the linear member are detected based on the difference between the measurement values ​​obtained by two of the optical fiber strain sensors.

4. 2. The method for detecting tension of a linear member for a structure according to claim 1, A method for detecting tension in a linear member for a structure, characterized in that the optical fiber strain sensor is built into the linear member in advance.

5. 2. The method for detecting tension of a linear member for a structure according to claim 1, A method for detecting tension in a linear member for a structure, characterized in that the optical fiber strain sensor is retrofitted to the surface of the linear member via a jig that amplifies the strain of the linear member.

6. 2. The method for detecting tension of a linear member for a structure according to claim 1, A method for detecting tension in a linear member for a structure, characterized in that the optical fiber strain sensor is attached to the surface of the linear member with adhesive tape.

7. 7. The method for detecting tension of a linear member for a structure according to claim 1, A method for detecting tension in a linear member for a structure, wherein the optical fiber strain sensor is an FBG sensor.

8. A linear member used in a structure to bear tension, A core material that transmits tension; a protective container having both ends fixed near the surface of the core material; an FBG sensor housed within the protective container, with both sides of a strain detection unit sandwiched between them fixed to the protective container near fixed portions at both ends of the protective container, and detecting strain fluctuations caused by vibrations of the linear member in the axis-perpendicular direction; a protective material that covers the core material and the protective container in which the FBG sensor is housed; A linear member for a structure, comprising:

9. A linear member used in a structure to bear tension, A core material that transmits tension; a protective container having fixed portions whose both ends are fixed near the surface of the core material, and an expandable portion provided between the fixed portions and formed to be expandable in the longitudinal direction; an FBG sensor housed within the protective container, with both sides of a strain detection unit sandwiched between them fixed to the protective container at positions inside fixed portions at both ends of the protective container, and which detects strain fluctuations caused by vibrations of the linear member in the axis-perpendicular direction; a protective material that covers the core material and the protective container in which the FBG sensor is housed; A linear member for a structure, comprising:

10. A linear member used in a structure to bear tension, A core material that transmits tension; an FBG sensor attached to a surface of the core material with adhesive tape to detect strain fluctuations caused by vibrations in the direction perpendicular to the axis; a protective material that covers the core material and the adhesive tape to which the FBG sensor is attached; A linear member for a structure, comprising:

11. The linear member for a structure according to any one of claims 8 to 10, A linear member for use in a structure, characterized in that a connector for connecting to the FBG sensor is provided near an end portion.

12. A sensor installation jig for installing an FBG sensor on a linear member for a structure to which tension is applied, the FBG sensor detecting a change in strain caused by vibration of the linear member in a direction perpendicular to the axis, two fixing members fixed to the surface of the linear member at a predetermined interval in the axial direction of the linear member; two extending members whose base ends are fixed to the two fixing members, extend parallel to the linear member, and whose tip ends extend in directions approaching each other; two sensor fixing bodies provided at the tips of the two extension members, respectively, for fixing both sides of the strain detection portion of the FBG sensor; A sensor installation jig for a linear member for a structure, comprising:

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