JP7752995B2 - Method for detecting tension of linear member for structure, linear member for structure, and sensor installation jig for linear member - Google Patents

Description

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 linear members for installing a sensor on the linear member.

A conventional method for measuring the tension in a cable of a cable-stayed bridge involves installing a vibration sensor on the cable and calculating the tension in 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 vibrations detected by the vibration sensor, and determining the bending stiffness of the cable under tension from multiple natural frequencies that exist in a frequency range where the influence of the cable's bending stiffness is dominant. The cable tension is then calculated based on the bending stiffness and natural frequencies thus determined.

A commonly used vibration sensor that can be used to measure the tension of the cable is an acceleration vibration sensor, which has a built-in vibrator and electrically detects the acceleration acting on the vibrator and outputs the frequency of the vibration.

Meanwhile, a PC steel strand with a built-in optical fiber for strain measurement 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 measurement optical fiber twisted into the steel strand that makes up the core material, and is configured so that the strain caused by the tension acting on the PC steel strand is directly measured by the optical fiber for strain measurement.

Patent No. 3313028 JP 2019-70593 A

However, with the above-mentioned conventional method of measuring cable tension, a vibration sensor must be installed and removed each time tension is measured, and the vibration sensor must be firmly fixed to the cable to accurately measure vibration. This has the disadvantage of being time-consuming and labor-intensive to install and remove the vibration sensor. Furthermore, acceleration-type vibration sensors, which are commonly used as vibration sensors, have a built-in vibrator, which is a moving part, and therefore are relatively poorly weather-resistant, making them unsuitable for long-term measurements.

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 must 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 with the strand while it is wrapped in a resin filler. 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 location, so temperature compensation for the strain is necessary. For this reason, the PC steel strand with optical fiber has the temperature-measuring optical fiber twisted together with the steel strand, resulting in a complex structure and complicated analysis for detecting tension.

The present invention aims to provide a method for detecting tension in a linear member for a structure that can be implemented with minimal effort. It also aims to provide a method for detecting tension in a linear member for a structure that can be implemented over a long period of time. It also aims to provide a linear member for a structure that can detect tension with a relatively simple structure. It also aims to provide a linear member for a structure that can detect tension through relatively simple analysis.

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;
The method is characterized by further comprising a tension calculation step of calculating a tension of the linear member based on the natural frequency identified in the natural frequency identification step.

According to the above configuration, in the strain fluctuation measurement process, the strain fluctuation of the linear member is measured using an optical fiber strain sensor. Here, optical fiber strain sensors that measure strain based on the physical characteristics of light transmitted through optical fibers can be widely used. Furthermore, the strain fluctuation of the linear member being measured refers to the change 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 detection process, the distribution of strain fluctuation frequencies caused by vibrations perpendicular to the axis of the linear member is detected based on the strain fluctuation measured in the strain fluctuation measurement process. Here, the distribution of strain fluctuation frequencies can be determined by, for example, performing a fast Fourier transform on the measured values of the optical fiber strain sensor. Next, in the natural frequency identification process, the natural frequency of the linear member is identified based on the strain fluctuation frequency distribution detected in the fluctuation frequency distribution detection process. Then, in the tension calculation process, the tension of the linear member is calculated based on the natural frequency identified in the natural frequency identification process. Various known calculation formulas can be used to calculate the tension of the linear member based on the natural frequency.

In the above-described tension detection method for a structural linear member, since the object of measurement in the strain fluctuation measurement process is the strain fluctuation of the linear member, the optical fiber strain sensor need only be installed on the structural linear member to the extent that strain fluctuations can be measured; 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, a structural linear member equipped with an optical fiber strain sensor can be easily manufactured. Furthermore, since the object of measurement 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 precision, the measurement value of the optical fiber strain sensor does not need to be temperature corrected. Therefore, since there is no need to measure temperature in the strain fluctuation measurement process, the measurement effort can be reduced. Furthermore, since temperature does not need to be taken into consideration when calculating tension in the tension calculation process, analysis can be performed with minimal effort. Furthermore, since an optical fiber strain sensor is used in the strain fluctuation measurement process, the optical fiber strain sensor's high durability allows for longer-term detection of the tension of a structural linear member than conventional methods that use acceleration-type vibration sensors to measure the vibration of linear members. In this invention, "structures" refer 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, "linear members" refer to wire ropes or PC steel strands made of stranded wires, parallel cables made of bundled wires, round bars, square bars, etc., and are not limited by material. Furthermore, linear members may be made of multiple materials, such as those including a core material and a protective material covering the core material.

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

In the above embodiment, strain fluctuations are measured using multiple optical fiber strain sensors arranged near the circumferential surface of the 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, while those caused by vibrations 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.

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 the measured values from two of the optical fiber strain sensors.

According to the above embodiment, by calculating the difference between the measurements 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.

In one embodiment of the method for detecting tension in a linear member for a structure, the optical fiber strain sensor is pre-installed in the linear member.

According to the above embodiment, the tension of a linear member of a structure can be detected with minimal effort by using an optical fiber strain sensor that is pre-installed in the linear member of the structure. Furthermore, by using an optical fiber strain sensor that is pre-installed in the linear member of the 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 a variety of long-term purposes, from construction management during the manufacture of a structure to maintenance management after the structure is completed.

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

According to the above embodiment, by retrofitting an optical fiber strain sensor to the surface of a linear member of a structure via a jig that amplifies the strain of the linear member, it is possible to detect the tension of the linear member with high accuracy. Furthermore, by retrofitting an optical fiber strain sensor using a jig to a linear member of a structure that is not previously equipped with a tension detection function, it is possible to detect the tension of the linear member.

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

According to the above embodiment, the optical fiber strain sensor can be attached to the surface of a linear component of a structure using adhesive tape, allowing the tension of the linear component to be detected with minimal effort. Here, the object measured by the optical fiber strain sensor is the strain fluctuation of the linear component, so fixing it using adhesive tape allows for measurements sufficient to detect the tension of the linear component.

In one embodiment of the method for detecting tension in a linear structural member, the optical fiber strain sensor is an FBG sensor.

In the above embodiment, by using an FBG (Fiber Bragg Grating) sensor as an optical fiber strain sensor, strain fluctuations in linear components of a structure can be measured with high accuracy.

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 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 a change in strain of the linear member;
The sensor is characterized by including a protective material that covers the core material and the protective container in which the FBG sensor is housed.

According to the above configuration, a linear member used in a structure to support 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 the strain detection unit, are fixed to the protective container near the fixed 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. This configuration allows the FBG sensor to measure strain fluctuations in the linear member. Furthermore, the use of an FBG sensor and its protection by the protective container and protective material allows for stable tension detection over a long period of time.

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 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 in the linear member;
The sensor is characterized by comprising the core material and a protective material that covers the protective container in which the FBG sensor is housed.

According to the above configuration, a linear member used in a structure to support 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, 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 section between the positions where both sides of the FBG sensor are fixed. The expandable section may be composed of multiple cylindrical members that are slidably overlapped with each other, or may be composed of a bellows-shaped member. Furthermore, the protective container does not have an expandable section between the fixed portion fixed near the surface of the core material and the positions where both sides of the FBG sensor are fixed, allowing displacement of the fixed portion to be transmitted to both sides of the FBG sensor without substantially changing it. The core material and the protective container housing the FBG sensor are covered with a protective material. This configuration allows the FBG sensor to measure strain fluctuations in a linear member. Furthermore, the FBG sensor is fixed to the protective container at both ends of the protective container, with both sides of the strain detection unit sandwiched between them at positions inside the fixed portions at both ends of the protective container. This amplifies the strain in the linear member in accordance with the ratio between the distance between the fixed portions at both ends of the protective container and the distance between the two ends of the FBG sensor. Therefore, strain fluctuations in the linear member can be measured with high accuracy. Furthermore, by using an FBG sensor and protecting it with a protective container and protective material, tension can be detected stably over a long period of time.

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 a surface of the core material with adhesive tape;
The device is characterized by comprising the core material and a protective material that covers the adhesive tape to which the FBG sensor is attached.

According to the above configuration, a linear member used in a structure to support tension has 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 a linear member, the FBG sensor simply measures fluctuations in strain caused by vibrations 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 the FBG sensor is attached to the core material with adhesive tape and protected with a protective material.

In one embodiment, the linear component for structures has a connector near its end for connecting to the FBG sensor.

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

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 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;
The FBG sensor is characterized by comprising 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.

According to the above configuration, a sensor installation jig for installing an FBG sensor on a linear member of a structure under tension uses the FBG sensor to detect strain fluctuations caused by vibrations of the linear member in the direction perpendicular to its axis, and detects 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 attached to each of these fixing members, extending parallel to the linear member and with their tips approaching each other. A sensor fixing body is provided at the tip of each 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 fluctuations in the strain of the linear member with high accuracy, and as a result, it is possible to detect the tension acting on the linear member with high accuracy.

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. 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. 1 is a cross-sectional view showing an end portion of a linear member for use in structures according to a first embodiment of the present invention. 1 is a cross-sectional view schematically showing a linear member for use in structures according to a first embodiment; 1 is a longitudinal cross-sectional view schematically showing a linear member for use in structures according to a first embodiment; FIG. 10 is a cross-sectional view schematically showing a linear member for use in structures according to a second embodiment. FIG. 10 is a longitudinal cross-sectional view schematically showing a linear member for use in structures according to a second embodiment. FIG. 10 is a cross-sectional view schematically showing a linear member for use in structures according to a third embodiment. FIG. 10 is a cross-sectional view schematically showing a linear member for use in structures according to a fourth embodiment. FIG. 10 is a longitudinal cross-sectional view schematically showing a linear member for use in structures according to a fifth embodiment. 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. 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 seventh embodiment. FIG. 13 is a side view showing two optical fiber strain sensors installed on a linear member for a structure in the seventh embodiment of the method for detecting tension in a linear member for a structure. FIG. 13 is a cross-sectional view showing two optical fiber strain sensors installed on a linear member for a structure in the seventh embodiment of the method for detecting tension in a linear member for a structure. FIG. 13 is a plan view showing an optical fiber strain sensor 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. 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. FIG. 13 is a cross-sectional 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. 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. FIG. A side view showing two optical fiber strain sensors installed on a linear member for a structure in the ninth embodiment of the method for detecting tension in a linear member for a structure. A cross-sectional view showing two optical fiber strain sensors 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. 10. FIG. 11 is a plan view showing an optical fiber strain sensor installed on a linear member for a structure in the method for detecting tension of a linear member for a structure according to the tenth embodiment. 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 of the tenth embodiment. A cross-sectional 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 of the tenth embodiment.

The present invention will now be described in detail with reference to the illustrated embodiments.

Figure 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 bridge cable. The tension detection method for a linear member for a structure according to this embodiment detects the tension acting on a cable, which is a linear member used in a bridge, which is a structure.

As shown in FIG. 1, this embodiment of the method for detecting tension in a linear structural member involves installing an optical fiber strain sensor 2 in a bridge cable 1. The optical fiber strain sensor 2 is preferably an FBG sensor. This 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 measuring instrument 3.

The FBG measuring device 3 sends an optical signal to the optical fiber strain sensor 2 and receives reflected light from the strain detection unit 2a. Based on the difference between the sent and reflected light, it outputs information about the strain detected by the strain detection unit 2a. The FBG measuring device 3 is connected to a personal computer 4, which stores the strain information received from the FBG measuring device 3 and analyzes the strain information to detect the tension in the bridge cable 1.

2 is a flow diagram showing a tension detection method for a linear structural member according to this embodiment. As shown in the flow diagram of FIG. 2, in this embodiment, the strain detection method for a linear structural member first measures strain fluctuations in the bridge cable 1 using an FBG sensor as the optical fiber strain sensor 2 (step S1). When measuring strain fluctuations in the bridge cable 1, strain fluctuations may be generated by vibrating the bridge cable 1 using a vibrator or the like. However, 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 also 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.

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

Then, based on the strain fluctuation frequency distribution detected in step S2, the natural frequency caused by the vibration A in the transverse direction of the bridge cable 1 is identified (step S3). The natural frequency caused by the vibration A in the transverse direction of the bridge cable 1 can be identified based on the frequency band of the strain fluctuation frequency distribution. Alternatively, the natural frequency can be identified based on the results of taking the difference between the strain fluctuations measured simultaneously at multiple different circumferential positions of 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 axial load 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 identical at all positions on the bridge cable 1. Therefore, by taking the difference between multiple strain fluctuations measured simultaneously at different circumferential positions on the bridge cable 1, it is possible to cancel out strain fluctuations caused by axial loads and extract only strain fluctuations caused by vibration A perpendicular to the axis of the bridge cable 1.

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.
Here, T is the tension of the bridge cable 1, w is the unit weight of the bridge cable 1, g is the gravitational acceleration, fn is the nth-order natural frequency, L is the length of the bridge cable 1, n is the order of the natural frequency, and C is a constant. This constant C can be calculated using the following relational expression (2).
Here, E·I is the bending rigidity of the bridge cable 1.

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 so-called vibration methods can be used.

In this way, according to the tension detection method for a linear member for a structure of 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 measurement optical fiber in a filler and twist it tightly to the steel strands to match the strain of the strain measurement optical fiber to the strain of the steel strands. Therefore, the tension detection method for structural linear members of this embodiment can use an optical fiber strain sensor 2 that can be easily attached to a bridge cable 1 using a simple fixing structure, making the preparation and work required for tension detection easier than ever before.

In the method for detecting tension in a linear structural member of this embodiment, although an optical fiber strain sensor 2 may be retrofitted to a bridge cable 1 to measure strain fluctuations, using a sensor-embedded bridge cable 1 with an optical fiber strain sensor 2 already built in is preferable because it simplifies the measurement process. A sensor-embedded bridge cable 1 with an optical fiber strain sensor 2 already 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 an optical fiber strain sensor 2 built in can be used with such a simple structure, bridge cables 1 capable of detecting tension can be manufactured at low cost.

Figure 3 is a cross-sectional view showing the end of a bridge cable as a first embodiment of a linear structural member 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 has an optical fiber strain sensor 2 pre-installed. This sensor-embedded bridge cable 11 is used to suspend and support girders, etc., on bridges such as cable-stayed bridges. This sensor-embedded bridge cable 11 has a core material 14 that transmits tension, and this core material 14 is formed by arranging multiple strands made of high-tensile zinc-plated steel in parallel. The outside of this core material 14 is covered with a protective film 15 that serves as a protective material to protect the core material 14. The protective film 15 is preferably formed of polyethylene, but may be formed of other materials. The end of the sensor-embedded bridge cable 11 is provided with a jacket tube 16 into which a core material 14 and a protective film 15 are inserted, and a socket portion 12 is provided at the tip of this jacket tube 16 for connecting the end of the sensor-embedded bridge cable 11 to another component such as a girder.

In the sensor-embedded bridge cable 11 of this embodiment, the 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 end of the optical fiber extending along its 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 closer to the base end than the strain detection unit 2a extends along the surface of the core material 14 toward the socket portion 12 and is pulled out near the tip of the socket portion 12. A connector 20 is provided at the base end of the optical fiber strain sensor 2 for connection to the FBG measuring device 3.

Figure 4A is a cross-sectional view of the first embodiment of the sensor-embedded bridge cable 11, showing a cross section of the strain detection unit 2a of the optical fiber strain sensor 2. Figure 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 Figures 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 Figures 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 positioned in contact with the outer peripheral surface of the core material 14, and the outside 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 fluctuating 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. Preferably, the sensor-embedded bridge cable 11 is 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 predominant vertical vibrations 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 also be placed in other positions 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. When two or more optical fiber strain sensors 2 are placed in the sensor-embedded bridge cable 11, it is preferable to place them at angles of 90° or 180° from each other in the cross section perpendicular to the axis of the sensor-embedded bridge cable 11.

Furthermore, the sensor-embedded bridge cable 11 of the first embodiment is equipped with a connector 20 for the optical fiber strain sensor 2 in the socket portion 12 at the end of the sensor-embedded bridge cable 11. Therefore, simply connecting the FBG measuring device 3 to the connector 20 allows the operator to easily and quickly measure strain. The optical fiber strain sensor 2 may be pulled out from the end face of the socket portion 12 to the outside of the cable 11, or may be pulled out from a position other than the socket portion 12 to the outside of the cable 11. Furthermore, the strain detection portion 2a of the optical fiber strain sensor 2 may be located at either the end or the center in the axial direction of the cable 11, and can be placed at any position.

Figure 5A is a cross-sectional view of a sensor-embedded bridge cable 21 as a linear component for structures according to the second embodiment, showing a cross section at the strain detection unit 2a of the optical fiber strain sensor 2. Figure 5B is a longitudinal cross-sectional view of the sensor-embedded bridge cable 21, showing a cross section at the strain detection unit 2a of the optical fiber strain sensor 2. In Figures 5A and 5B, the shape of the gap between the core material 14 and the tape fixing unit 23, the shape of the tape fixing unit 23, and the shape of the gap between the tape fixing unit 23 and the outer tube 22 are shown only schematically; the actual shapes of the gaps and the tape fixing unit 23 will differ.

The sensor-embedded bridge cable 21 of the second embodiment has an optical fiber strain sensor 2 pre-installed. As shown in Figures 5A and 5B, the sensor-embedded bridge cable 21 of the second embodiment is formed by housing a core material 14 within a jacket tube 22. This sensor-embedded bridge cable 21 includes a core material 14 made of parallelly arranged wires, an optical fiber strain sensor 2 arranged in contact with the outer surface of the core material 14, and a tape fixing portion 23 arranged to surround the core material 14 and the 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 material 14, the optical fiber strain sensor 2, and the tape fixing portion 23 are housed within the jacket tube 22. A rust-preventive layer 24 made by injecting an anti-rust agent is formed as a protective material between the core material 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 incorporating an optical fiber strain sensor 2 can be constructed with a simple structure in which the optical fiber strain sensor 2 is fixed to the outer 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.

Figure 6 is a cross-sectional view of a third embodiment of a sensor-embedded bridge cable 31 as a linear component for structures, showing a cross section of the strain detection section 2a of the optical fiber strain sensor 2. The sensor-embedded bridge cable 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 bridge cable 31 of the third embodiment, components 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.

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-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 by 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.

Figure 7 is a cross-sectional view of a fourth embodiment of a sensor-embedded bridge cable 41 as a linear component for structures, showing a cross section of the strain detection section 2a of the optical fiber strain sensor 2. The fourth embodiment of the sensor-embedded bridge cable 41 differs from the second embodiment of the sensor-embedded bridge cable 21 in that it is provided with a fixing member 26 for the optical fiber strain sensor 2. In the fourth embodiment of the sensor-embedded bridge cable 41, components similar to those of the second embodiment of the sensor-embedded bridge cable 21 are designated by the same reference numerals, and detailed descriptions thereof will be omitted.

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 installed on 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.

Figure 8 is a vertical cross-sectional view of a fifth embodiment of a sensor-embedded bridge cable 51 as a linear component for structures, showing a schematic view of the area surrounding the strain detection unit 2a of the optical fiber strain sensor 2. The fifth embodiment of the sensor-embedded bridge cable 51 is a sensor-embedded cable incorporating 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 housing the optical fiber strain sensor 2 is embedded in a protective material that covers the core material 14. In the fifth embodiment of the sensor-embedded bridge cable 51, components 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.

As shown in Figure 8, the fifth embodiment of the sensor-embedded bridge cable 51 comprises a core material 14, a protective container 36 that is positioned near the surface of the core material 14 and houses the optical fiber strain sensor 2, and a protective layer 35 that serves as a protective material covering the core material 14 and protective container 36. Figure 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 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 embedded in the protective layer 35, positioned parallel to the core material 14 near the surface of 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 both sides 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 that 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 from other materials as long as it has the function of covering and protecting the core material 14 and protective container 36.

In the 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 the two optical fiber fixtures 38A, 38B shown in Figure 8. In the sensor-embedded bridge cable 51 of this embodiment, the optical fiber strain sensor 2 is housed in the protective container 36 fixed to the protective layer 35 by fixing units 37A, 37B, and is fixed by the optical fiber fixtures 38A, 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.

Figure 9 is a vertical cross-sectional view schematically showing the protective container 43 and optical fiber strain sensor 2 provided in a sixth embodiment of a sensor-embedded bridge cable serving as a linear component for structures. The bridge cable of the sixth embodiment differs from the sensor-embedded bridge cable 51 of the fifth embodiment in that the protective container housing the optical fiber strain sensor 2 is formed to be expandable and contractible, and in the installation positions of the optical fiber fixators 38A, 38B of the optical fiber strain sensor 2. In the sixth embodiment of the sensor-embedded bridge cable, components similar to those of the sensor-embedded bridge cable 51 of the fifth embodiment are designated by the same reference numerals, and detailed descriptions will be omitted.

The sixth embodiment of the sensor-embedded bridge cable is a sensor-embedded cable with an optical fiber strain sensor 2 already built in. As shown in Figure 9, the protective container 43 of the sixth embodiment of the sensor-embedded bridge cable consists of two fixed containers 43a, 43a formed from cylindrical tubes, arranged with their end faces facing each other and spaced 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 fixators 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. Additionally, 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 fixing bodies 38A and 38B. As in the fifth embodiment, this protective container 43 is positioned on the surface side of the core material 14 and is covered together with the core material 14 by the protective layer 35.

In the bridge cable with built-in sensors of the sixth embodiment, strain fluctuations caused by axial loads and vibrations in the direction perpendicular to the axis are measured by the detection unit 2a of the optical fiber strain sensor 2. Here, when a displacement of ΔL occurs with respect to the distance _L1 between two optical fiber fixed bodies 38A and 38B shown in Fig. 9, the strain _ε1 measured by the detection 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 . From these, the relationship _ε1 = ε2·(L2/L1) holds between the strain ε1 measured by the detecting unit _2a and the strain _ε2 actually generated in the sensor-embedded bridge cable, which is the strain generated between the fixed units _44A and _44B . As described above, the sensor-embedded bridge cable of the sixth embodiment can amplify and measure strain by fixing the optical fiber strain sensor ₂ to the optical fiber fixators 38A and 38B, which are disposed inside the expandable protective container 43 at a distance smaller than the distance between the fixed units 44A and 44B of the protective container. That is, the displacement between the fixed units 44A and 44B of the protective container is transmitted by using the parts of the fixed container 43a that are closer to each other than the fixed units 44A and 44B as displacement transmitting means, and is measured by the detecting unit 2a between the optical fiber fixators 38A and 38B, thereby amplifying the strain measured by the optical fiber strain sensor 2. As a result, strain fluctuations occurring in bridge cables with built-in sensors can be measured stably and with good accuracy.

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

Figure 10A is a plan view showing the bridge cable 61 retrofitted with an optical fiber strain sensor 2A in the seventh embodiment of the tension detection method for linear structural members. Figure 10B is a side view showing the bridge cable 61 retrofitted with first and second optical fiber strain sensors 2A and 2B, and Figure 10C is a cross-sectional view of the bridge cable 61 retrofitted with the first and second optical fiber strain sensors 2A and 2B. The tension detection method for linear structural members 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, components similar to those in the first embodiment are designated by the same reference numerals, and detailed descriptions are omitted.

In the seventh embodiment, the tension detection method for a linear structural member involves attaching two optical fiber strain sensors 2A, 2B with adhesive tape 53 to the surface of an existing bridge cable 61, whose core material 14 is 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 positioned at an angle of 180° at the upper and lower ends of the bridge cable 61 in a cross section perpendicular to the axis.

In this 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 the 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 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 frequency fluctuations 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 process is performed to identify the natural frequency of the bridge cable 61 based on the frequency distribution of the strain fluctuations of the bridge cable 61. Based on the identified natural frequency, the tension of the bridge cable 61 is calculated in a tension calculation process.

In this way, the seventh embodiment of the tension detection method for linear structural members 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 positioned at the upper and lower ends of the bridge cable 61, separated by an angle of 180° in the cross section perpendicular to the axis, and the difference between the measurements of these first and second optical fiber strain sensors 2A, 2B is taken, 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.

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

In the eighth embodiment of the method for detecting tension in a linear structural member, two optical fiber strain sensors 2A, 2B are retrofitted 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 a 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 respectively arranged on the surfaces of these cable fixing members 55A, 55B, which are located away from the bridge cable 61. The second mounting jig 54B, which mounts the second optical fiber strain sensor 2B on the lower end of the 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 fastening 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 fastening band 57 and fixed to the surface of the bridge cable 61. The fastening band 57 can be formed of a resin lashing belt, bundling belt, or bundling band, but a metal strip may also be used.

In the eighth embodiment of the tension detection method for a linear member for a structure, similar to the sixth embodiment, first and second optical fiber strain sensors 2A and 2B are attached to the bridge cable 61, and then the vibration of the bridge cable 61 is measured using the optical fiber strain sensors 2A and 2B to perform a strain fluctuation measurement process. This is followed by a fluctuation frequency distribution detection process, in which the distribution of strain fluctuation frequencies caused by the vibration of the bridge cable 61 in the transverse direction is detected 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. This strain fluctuation caused by the vibration of the bridge cable 61 in the transverse direction is subjected to FFT processing to detect the distribution of strain fluctuation frequencies. This is followed by a natural frequency identification process to identify the natural frequency of the bridge cable 61, and then a tension calculation process to calculate the tension of the bridge cable 61 based on the identified natural frequency.

In this way, the eighth embodiment of the tension detection method for a linear structural member 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, measuring strain fluctuations in the bridge cable 61 and detecting strain fluctuations caused by vibrations perpendicular to the axis of the bridge cable 61. As a result, the tension in the bridge cable 61 can be easily detected with good accuracy.

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

In the ninth embodiment of the method for detecting tension in a linear structural member, first and second optical fiber strain sensors 2A and 2B are retrofitted to 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 arranged 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, 64B, which extend parallel to the bridge cable 61 and approach each other, are fixed to these cable fixing members 63A, 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 fixers 65A, 65B are provided at the tips of the two extension members 64A, 64B, respectively, to fix both sides of the strain detection unit 2a of the first optical fiber strain sensor 2A. The second amplifying type mounting jig 62B, which mounts the second optical fiber strain sensor 2B to the lower end of the bridge cable 61, has the same cable fixing members 63A, 63B as the first amplifying type mounting jig 62A, the extension members 64A, 64B, and the optical fiber fixers 65A, 65B. The first cable fixing member 63A of the first amplifying type mounting jig 62A and the first cable fixing member 63A of the second amplifying type mounting jig 62B are fastened together with a fixing band 57 and fixed to the surface of the bridge cable 61. Furthermore, 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 together with a fixing band 57 and fixed to the surface of the bridge cable 61.

The amplifying mounting jig 62A used in the tension detection method for a linear member for a structure of the ninth embodiment can amplify the strain of a bridge cable 61 measured by the detecting unit 2a of the optical fiber strain sensor 2. When a displacement of ΔL occurs with respect to the distance _L3 between two optical fiber fixed bodies 65A, 65B shown in Figure 12B, the strain _ε10 measured by the detecting unit 2a can be expressed as _ε10 = ΔL/ _L3 . Here, this displacement ΔL is the same as the displacement occurring with respect to the distance _L4 between two cable fixing members 63A, 63B, so the strain _ε20 occurring between these cable fixing members 63A, 63B can be expressed as _ε20 = ΔL/ _L4 . From these, the relationship ε ₁₀ = ε _{20 · (L 4 /L 3 ) holds between the strain ε 10 measured} by the detection unit 2 a and the strain ε ₂₀ that occurs between the cable fixing members _63A , _63B and that actually occurs in the bridge cable 61.

In this way, the amplifying mounting jig 62A used in the ninth embodiment of the method for detecting tension in a linear structural member 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. The optical fiber fixing members 65A, 65B provided at the tips of these extension members 64A, 64B fix both sides of the strain detection unit 2a of the optical fiber strain sensor 2A, thereby amplifying and measuring the strain in the bridge cable 61. In other words, the displacement between the cable fixing members 63A, 63B is transmitted by the extension members 64A, 64B, which function as a displacement transmission means, and measured by the detection unit 2a between the optical fiber fixing members 65A, 65B, thereby amplifying the strain measured by the optical fiber strain sensor 2. This strain amplification effect can be achieved in both the first amplifying mounting jig 62A and the second amplifying mounting jig 62B. As a result, strain fluctuations occurring in the bridge cable 61 can be measured stably and with good accuracy.

Figure 13A is a plan view showing the state in which an optical fiber strain sensor 2A is retrofitted to a bridge cable 61 in the tension detection method for a linear member for a structure of the tenth embodiment. Figure 13B is a side view showing the state in which first and second optical fiber strain sensors 2A, 2B are retrofitted to a bridge cable 61, and Figure 13C is a cross-sectional view of the bridge cable 61 to which the first and second optical fiber strain sensors 2A, 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 ninth embodiment in that the fixing structure of the mounting jig for retrofitting the optical fiber strain sensors 2A, 2B to the surface of the bridge cable 61 is different. In the tenth embodiment, the same components as in the ninth embodiment are designated by the same reference numerals, and detailed description thereof will be omitted.

In the tenth embodiment of the method for detecting tension in a linear member for a structure, first and second amplified mounting jigs 62A, 62B similar to those in 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 amplified 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 fixing bodies 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 structural member, the first and second amplifying mounting jigs 62A, 62B are secured 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 placing two semicircular metal belts, each with a cable fixing member 63A, 63B inserted through its center, around the surface of the bridge cable 61. The flanges formed on the ends of the bolts are fastened to each other with bolts 74. These fixing rings 73, 73 firmly secure the first and second optical fiber strain sensors 2A, 2B to the surface of the bridge cable 61. As a result, strain fluctuations occurring in the bridge cable 61 can be measured stably and accurately by the retrofitted first and second optical fiber strain sensors 2A, 2B.

In the tension detection methods for linear structural members 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. Furthermore, 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, such as an angle of 90°. Furthermore, the number of optical fiber sensors 2 placed on the bridge cable 61 may be just one.

In addition, 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.

In addition, 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, but 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 with cores formed from 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, and can simply be placed on or near the surface of the core, parallel to the extension direction of the core.

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

The present invention is not limited to the embodiments described above, and many modifications are possible within the technical scope of the present invention by those skilled in the art.

REFERENCE SIGNS LIST 1, 61 Bridge cable 11, 21, 31, 41, 51 Sensor-embedded bridge cable 2, 2A, 2B Optical fiber strain sensor 2a Strain detection section of optical fiber strain sensor 3 FBG measuring instrument 4 Personal computer 12 Socket section 14 Core material 15 Protective film 16, 22 Outer tube 20 Connector 23 Tape fixing section 24 Anti-rust layer 26 Fixing member 35 Protective layer 36, 43 Protective container 37A, 37B, 44A, 44B Fixing section of protective container 38A, 38B, 56A, 56B, 65A, 65B Optical fiber fixing body 43a Fixing container 43b Sliding tube 53 Adhesive tape 54A, 54B Mounting jig for optical fiber strain sensor 55A, 55B, 63A, 63B Cable fixing member 57 Fixing band 62A, 62B Amplifying type mounting jig for optical fiber strain sensor 64A, 64B Extension member 73 Fixing ring 74 Bolt

Claims (12)

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 method for detecting tension in a linear member for a structure, comprising a tension calculation step of calculating the tension in the linear member based on the natural frequency identified in the natural frequency identification step. 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. 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. 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. 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. 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. 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. 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 linear member for a structure, comprising: a protective material that covers the core material and the protective container in which the FBG sensor is housed. 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 linear member for a structure, comprising the core material and a protective material that covers a protective container in which the FBG sensor is housed. 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 linear member for a structure, comprising the core material and a protective material that covers the adhesive tape to which the FBG sensor is attached. 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. 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;
A sensor installation jig for a linear member for a structure, characterized in that it comprises two sensor fixing bodies that are respectively provided at the tips of the two extension members and that fix both sides of the strain detection portion of the FBG sensor.