JP7846597B2 - Displacement measurement structure - Google Patents

Description

This invention relates to a displacement measurement structure.

Conventionally, a displacement measurement structure for measuring the relative displacement between structures constituting a connected structure is known, as described in Patent Document 1 below. This displacement measurement structure consists of an optical fiber cable connected to an OTDR measuring device, which is stretched between structures as it passes through a constricted groove and catches on protrusions provided on each structure. When the structures are displaced relative to each other, the optical fiber cable stretches between the protrusions or bends upon contact with the wall of the constricted groove, thereby detecting the displacement between the structures. It has been proposed that this displacement measurement structure can detect displacement in any direction: left-right, up-down, or front-back.

Patent No. 03725513

However, this displacement measurement structure cannot capture the relative displacement in complex cases where relative displacements occur simultaneously in the left-right, up-down, and front-back directions of structures, or where relative rotational displacements occur between structures. The present invention aims to provide a displacement measurement structure capable of detecting relatively complex relative displacements between structures in connected structures.

The present invention is as follows:

[1] A displacement measuring structure for measuring the relative displacement between structures in a connected structure formed by butting the joint surfaces of the structures together, comprising at least three optical fibers including free-length portions extending across the connecting portion between the structures, wherein each free-length portion is fixed at one end to a fixed point on one of the structures and at the other end to a fixed point on the other structure, and elastically expands and contracts in accordance with the change in distance between the two fixed points.

[2] The connecting structure is columnar in shape with a circumferential surface extending in a direction perpendicular to the joining surface, and three free-length portions are installed along one outer surface which is part of the circumferential surface and forms a plane perpendicular to the joining surface, and the relative displacement between the structures in a plane parallel to the outer surface is measured based on the expansion and contraction of each of the free-length portions, as described in [1].

[3] The displacement measuring structure according to [1] or [2], wherein the connecting structure is shaped like a rectangular prism, and the outer surface is one of the four outer surfaces of the rectangular prism perpendicular to the joint surface.

[4] The connecting structure is columnar in shape with a circumferential surface extending in a direction perpendicular to the joining surface, and the three free-length portions are arranged along the circumferential surface such that not all three free-length portions lie on the same plane, and the relative translational displacement between the structures in a direction perpendicular to the joining surface and the relative rotational displacement between the structures around two axes parallel to and perpendicular to the joining surface are measured based on the expansion and contraction of each of the free-length portions, as described in [1].

[5] A displacement measurement structure according to any one of [2] to [4], comprising an optical fiber holding portion provided on each of the one structure and the other structure so as to protrude out of the circumferential surface, wherein one end of the free length portion is fixed to the optical fiber holding portion of one structure, and the other end of the free length portion is fixed to the optical fiber holding portion of the other structure.

According to the present invention, a displacement measurement structure capable of detecting relatively complex relative displacements between structures in a connected structure can be provided.

This is a perspective view of the sewer culvert that the displacement measurement structure according to the embodiment is intended to measure. This is a schematic side view showing one of the strain sensor sections of the displacement measurement structure. This is a perspective view showing a first embodiment of the displacement measurement structure. (a) is a plan view showing a model of three free-length sections installed along the outer surface, and (b) is a plan view showing the state after relative displacement between the box culverts in the said model. This is a perspective view showing a second embodiment of the displacement measurement structure. This is a perspective view showing a third embodiment of the displacement measurement structure. This is a perspective view showing a fourth embodiment of the displacement measurement structure. (a) and (b) are plan views illustrating various modified examples of the displacement measurement structure.

[First Embodiment]
The first embodiment of the displacement measurement structure according to the present invention will be described in detail below with reference to the drawings. As shown in Figure 1, the connected structure to be measured by the displacement measurement structure of this embodiment is an existing sewer box culvert 3 buried underground. The sewer box culvert 3 has a rectangular prism shape with a hollow waterway 3a formed inside, and is formed by connecting a plurality of box culverts 5 that have a similar rectangular prism shape in the longitudinal direction. The box culverts 5 are, for example, about 3 m in height, about 3 m in width, and about 25 m in length. Adjacent box culverts 5 are connected by butting the joint surfaces 7, 7 at their longitudinal ends, forming a joint 9 as a connecting part. In this joint 9, a reinforced concrete section 11 is formed so as to surround the joint surfaces 7, 7.

In the sewer culvert 3, the box culverts 5 adjacent to each other, separated by a joint 9, may be displaced relative to each other due to the action of external forces or contraction/expansion caused by temperature changes, potentially causing the joint 9 to open. It should be assumed that the box culverts 5 themselves do not undergo deformation. Possible external forces include forces caused by ground movement due to construction work near the sewer culvert 3, forces acting on the sewer culvert 3 due to the weight of the sewage flowing through the waterway 3a, and forces caused by ground subsidence near the sewer culvert 3.

In the following, when distinguishing between two box culverts 5 adjacent to each other with respect to the joint 9 described, one will be referred to as box culvert 5A and the other as box culvert 5B. Also, as shown in the figure, a Cartesian coordinate system is set with the center of the joint surfaces 7, 7 of box culverts 5A and 5B as the origin, with the X-axis in the width direction of the sewer box culvert 3, the Y-axis in the longitudinal direction, and the Z-axis in the height direction, and X, Y, Z, etc. are used in the explanation. In this embodiment, the Z-axis is the vertical axis and the XY plane is the horizontal plane. Furthermore, regarding each component of the relative displacement of box culvert 5B with respect to box culvert 5A,
Let _Δx be the translational displacement in the X direction.
The translational displacement in the Y direction is _Δy .
The translational displacement in the Z direction is _Δz .
Rotational displacement around the X-axis is _θx ,
Rotational displacement around the Y-axis is θ _y ,
Let θ _z be the rotational displacement around the Z-axis.

To detect the relative displacement between the box culverts 5 as described above, a displacement measurement structure 1 (see Figure 3) is constructed at each joint 9 along the outer circumferential surface 13 of the sewer culvert 3. The displacement measurement structure 1 is not shown in Figure 1. Since the sewer culvert 3 is rectangular prism-shaped, the outer circumferential surface 13 of the sewer culvert 3 is composed of four rectangular prism outer surfaces 13c, 13d, 13e, and 13f perpendicular to the joint surface 7. Of these, outer surface 13c is the horizontal plane forming the upper surface of the sewer culvert 3.

(Strain sensor section)
The displacement measurement structure 1 (see Figure 3) constructed in each joint 9 is equipped with a plurality of strain sensor units 15 as shown in Figure 2. Figure 2 is a schematic side view showing one strain sensor unit 15. As shown in the figure, the strain sensor unit 15 has one optical fiber cable 17. The optical fiber cable 17 may be, for example, an optical fiber core in which a thermoplastic resin layer is coated around the optical fiber strands. The strain sensor unit 15 also has two optical fiber holding units 19A and 19B that hold and guide the optical fiber cable 17. The optical fiber holding unit 19A is provided on the outer surface 13c of the box culvert 5A, and the optical fiber holding unit 19B is provided on the outer surface 13c of the box culvert 5B. These optical fiber holding units 19A and 19B protrude vertically out of the plane from their respective outer surfaces 13c to a height exceeding the height of the reinforcing concrete section 11.

The optical fiber cable 17 of the strain sensor unit 15 extends from the outer surface 13c of the box culvert 5A to the outer surface 13c of the box culvert 5B. The optical fiber cable 17 may be fixed on the outer surface 13c. While extending from the box culvert 5A to the box culvert 5B, the optical fiber cable 17 is bent so as to be lifted out of the plane of the outer surface 13c as it passes through the optical fiber holding sections 19A and 19B. Between the optical fiber holding sections 19A and 19B, the optical fiber cable 17 extends away from the outer surface 13c, crossing over the reinforced concrete section 11. As described above, in order to bend the optical fiber cable 17 in a smooth path near the joint 9, the optical fiber holding sections 19A and 19B are provided with slope-shaped guide sections 21. The slope-shaped guide section 21 forms an inclined surface that guides the optical fiber cable 17 and is inclined to gradually move away from the outer surface 13c as it approaches the joint 9. The shape of the sloped guide section 21 is set to ensure a bending radius that does not impede the measurement performance of the optical fiber cable 17 (for example, a bending radius of more than twice the diameter of the optical fiber cable).

The optical fiber cable 17 is fixed to the optical fiber holding section 19A at least at the end of the optical fiber holding section 19A on the joint section 9 side. Similarly, the optical fiber cable 17 is fixed to the optical fiber holding section 19B at least at the end of the optical fiber holding section 19B on the joint section 9 side. Hereinafter, the fixing point of the optical fiber cable 17 at the end of the optical fiber holding section 19A on the joint section 9 side will be referred to as "fixing point 18A," and the fixing point of the optical fiber cable 17 at the end of the optical fiber holding section 19B on the joint section 9 side will be referred to as "fixing point 18B." The optical fiber cable 17 may also be bonded to the optical fiber holding sections 19A and 19B at locations other than fixing points 18A and 18B. The portion of the optical fiber cable 17 from fixing point 18A to fixing point 18B is a free-length portion 23 that extends across the joint section 9 without interfering with the box culverts 5A and 5B or the reinforced concrete section 11.

The strain sensor section 15 is constructed with pre-tensioned tensile force applied to the free length portion 23. Due to this pre-tension, the free length portion 23 extends linearly from the end of the optical fiber holding section 19A on the joint 9 side to the end of the optical fiber holding section 19B on the joint 9 side. In this structure, the free length portion 23 is fixed at one end 23a to the fixing point 18A of the optical fiber holding section 19A, and at the other end 23b to the fixing point 18B of the optical fiber holding section 19B.

Furthermore, the strain sensor section 15 has a protective tube 27 through which the free-length portion 23 is inserted into the hollow section. The protective tube 27 is made of, for example, a PVC pipe with an appropriate inner diameter that allows the optical fiber cable 17 to pass through. The protective tube 27 is fixed to the reinforced concrete section 11 via, for example, a pipe clamp material 27a. One end of the protective tube 27 is located close to the fixing point 18A, and a soft gap-filling material 29 is interposed between the one end of the protective tube 27 and the fixing point 18A. Similarly, the other end of the protective tube 27 is located close to the fixing point 18B, and a soft gap-filling material 29 is also interposed between the other end of the protective tube 27 and the fixing point 18B.

With this structure, when the box culverts 5A and 5B are displaced relative to each other, the optical fiber holding sections 19A and 19B are also displaced relative to each other. This causes a change in the distance between the fixed points 18A and 18B, and the free-length portion 23 expands and contracts freely and elastically within the hollow section of the protective tube 27 in response to this distance change. At this time, the sliding resistance between the free-length portion 23 and the protective tube 27 is negligibly small. Furthermore, because the gap-filling material 29 deforms, the relative displacement between the optical fiber holding sections 19A and 19B is not hindered by the protective tube 27.

The end of the optical fiber cable 17 of the strain sensor unit 15 is connected to the measuring instrument 31. Furthermore, an analysis device 33 is connected to this measuring instrument 31. The measuring instrument 31 incidents pulsed light onto the optical fiber cable 17 and receives various scattered light returning from various positions along the longitudinal direction of the optical fiber cable 17. It then transmits information regarding the intensity and wavelength of the received scattered light to the analysis device 33. Examples of the scattered light include Rayleigh scattered light and Brillouin scattered light. As examples of the measuring instrument 31, for instance, an OTDR (Optical Time Domain Reflectometer) utilizing Rayleigh scattered light or a BOTDR (Brillouin Optical Time Domain Reflectometer) utilizing Brillouin scattered light can be used.

The analysis device 33 is a computer composed of, for example, a CPU (Central Processing Unit), ROM (Read Only Memory), and RAM (Random Access Memory). Based on the principle that the intensity and wavelength of scattered light depend on the strain applied to the optical fiber cable 17, the analysis device 33 analyzes the intensity and wavelength of scattered light at each position along the longitudinal direction of the optical fiber cable 17. Through this analysis, the analysis device 33 acquires the strain occurring at each position along the longitudinal direction of the optical fiber cable 17, for example, at intervals of several centimeters.

In this way, the analysis device 33 can obtain the strain occurring at each position of the free length portion 23 of the optical fiber cable 17 at intervals of several centimeters. Based on this strain information and the initial length (known) of the free length portion 23, the amount of expansion or contraction of the free length portion 23 can be obtained through calculations. Specifically, for multiple measurement points on the free length portion 23, the sum of the products of the strain at each measurement point and the interval between measurement points is the amount of expansion or contraction of the free length portion 23. Alternatively, for multiple measurement points on the free length portion 23, the product of the average value of the strain at each measurement point and the length of the free length portion 23 is the amount of expansion or contraction of the free length portion 23.

As described above, the strain sensor unit 15 can acquire the amount of expansion or contraction of the free length portion 23. That is, it can acquire the amount of expansion or contraction of the distance between the fixed point 18A and the fixed point 18B. Furthermore, as mentioned earlier, since the strain sensor unit 15 is constructed with pre-tension in the tensile direction applied to the free length portion 23, the amount of reduction in the distance between the fixed point 18A and the fixed point 18B can be acquired by observing the shortening of the free length portion 23 from its initial state.

More precisely, the intensity and wavelength of scattered light in the optical fiber cable 17, as described above, depend not only on the strain but also on the ambient temperature around the optical fiber cable 17. Therefore, a temperature correction mechanism may be provided to remove the influence of temperature from the scattered light information of the optical fiber cable 17 acquired by the analysis device 33. As an example of such a temperature correction mechanism, the strain sensor unit 15 may have a temperature-measuring optical fiber cable (not shown) installed near the free-length portion 23, separate from the optical fiber cable 17. For example, near the optical fiber holding sections 19A and 19B, a protective tube (not shown) similar to the protective tube 27 is installed along the outer surface 13c at a position lower than the height of the optical fiber holding sections 19A and 19B. The temperature-measuring optical fiber cable is then housed freely in the hollow portion of this protective tube, so that the temperature-measuring optical fiber cable is not affected by the relative displacement between the box culverts 5A and 5B. Furthermore, the temperature-measuring optical fiber cable is connected to the measuring instrument 31 and the analysis device 33, similar to the optical fiber cable 17.

As described above, the temperature measurement optical fiber cable is not affected by the relative displacement between the box culverts 5A and 5B. Therefore, the scattered light information from the temperature measurement optical fiber cable does not include the effect of the relative displacement between the box culverts 5A and 5B, but does include the effect of the temperature near the free length portion 23. Thus, the analysis device 33 can perform a calculation to remove the effect of temperature by comparing the scattered light information from the optical fiber cable 17 with the scattered light information from the temperature measurement optical fiber cable, thereby obtaining the accurate strain of the optical fiber cable 17.

The length of the free length portion 23 is set based on the relative displacement between the assumed fixed points 18A and 18B. That is, the maximum measurable elongation in the free length portion 23 is expressed as the product of the maximum measurable strain in the optical fiber cable 17 and the length of the free length portion 23. Therefore, the length of the free length portion 23 is set such that the above product is greater than the assumed maximum elongation between fixed points 18A and 18B. This prevents excessive strain from occurring in the free length portion 23 within the assumed relative displacement range, thus obtaining accurate expansion and contraction amounts. Similarly, the maximum measurable shortening in the free length portion 23 depends on the tensile pretension applied to the free length portion 23 beforehand. Therefore, the strength of this pretension should also be set based on the assumed relative displacement between fixed points 18A and 18B.

(Displacement measurement structure)
Figure 3 is a perspective view showing a displacement measurement structure 1 constructed in one joint 9. As shown in the figure, the displacement measurement structure 1 in one joint 9 is equipped with three strain sensor units 15 as described above. These three strain sensor units 15 are the same size and have the same configuration, and are all mounted on the outer surface 13c and arranged parallel to each other. That is, the displacement measurement structure 1 is equipped with three parallel free-length portions 23 that are installed along the outer surface 13c and extend in the Y direction. The three free-length portions 23 are all positioned at the same Y-direction position and are all the same length. After such a displacement measurement structure 1 is constructed along the outer surface 13 of the sewer culvert 3, the sewer culvert 3 is backfilled into the ground.

Figure 4(a) is a plan view showing a model of three free-length sections 23 installed along the outer surface 13c, and Figure 4(b) is a plan view showing the state after relative displacement between box culvert 5A and box culvert 5B in the same model. As shown in the figures, the first free-length section 23 is represented as "free-length section 23h", the second free-length section 23 as "free-length section 23j", and the third free-length section 23 as "free-length section 23k". The distance in the X direction between the fixed point 18A of free-length section 23h and the fixed point 18A of free-length section 23j is _Dh , and the distance in the X direction between the fixed point 18A of free-length section 23j and the fixed point 18A of free-length section 23k is _Dk . The amount of expansion and contraction of free-length section 23h is _Lh , the amount of expansion and contraction of free-length section 23j is _Lj , and the amount of expansion and contraction of free-length section 23k is _Lk .

This displacement measurement structure 1 is
(a) Box culvert 5B does not twist around the Y-axis relative to box culvert 5A (or twisting around the Y-axis can be ignored).
(b) Box culvert 5B is displaced only in the horizontal plane (XY plane) relative to box culvert 5A (or displacements other than those in the horizontal plane can be ignored).
It is used in environments such as the following. In other words, it is used under the assumption that θ _x = 0, θ _y = 0, and Δ _z = 0.

Under the above assumptions that _θx = 0, _θy = 0, and _Δz = 0, the following equations (1), (2), and (3) hold true based on the relative positions of the fixed points 18A and 18B.

Furthermore, as mentioned above, the expansion and contraction amounts _Lh , _Lj , and _Lk of each free length portion 23h, 23j, and 23k can be obtained by the analysis device 33 using the three strain sensor units 15, and _Dh and _Dk are known. Therefore, by solving the equations represented by formulas (1), (2), and (3) calculated by the analysis device 33, _Δx , _Δy , and _θz can be obtained.

As described above, the displacement measurement structure 1 allows us to determine the relative displacement between the box culverts 5A and 5B in the joint 9, including the translational displacement in the X direction ( _Δx ), the translational displacement in the Y direction ( _Δy ), and the rotational displacement around the Z axis ( _θz ). In other words, the relative displacement between the box culverts 5A and 5B in a plane parallel to the outer surface 13c (in the horizontal plane) can be determined.

Furthermore, the analysis device 33 can determine the amount of opening in the joint 9 based on the relative displacement between the box culverts 5A and 5B, as determined above. The analysis device 33 can also evaluate the integrity of the joint 9 by comparing the determined amount of opening with a predetermined threshold value. Additionally, if, for example, a rubber expansion joint is used in the joint 9, the analysis device 33 can evaluate the integrity of the expansion joint by comparing the amount of opening with the design value of the expansion joint.

Furthermore, since the strain sensor unit 15 has a protective tube 27 through which the free-length portion 23 is inserted, direct contact of the free-length portion 23 with soil is avoided. Therefore, the possibility of the expansion and contraction of the free-length portion 23 being hindered by the soil is low, and smooth expansion and contraction of the free-length portion 23 is ensured. Moreover, even if the optical fiber cable 17 gets wet, it does not affect strain measurement, so no special waterproofing measures for the free-length portion 23 are necessary.

[Second Embodiment]
Next, a second embodiment of the displacement measurement structure will be described. In the displacement measurement structure 92 shown in Figure 5, the three strain sensor units 15 described above are installed on the outer surface 13f instead of the outer surface 13c. That is, the three free-length portions 23 are installed along the outer surface 13f. This displacement measurement structure 92 is,
(a) Box culvert 5B does not twist around the Y-axis relative to box culvert 5A (or twisting around the Y-axis can be ignored).
(c) Box culvert 5B is displaced only in the vertical plane (YZ plane) relative to box culvert 5A (or displacements other than those in the vertical plane can be ignored).
It is used in environments such as the above. In other words, it is used under the premise that _Δx = 0, _θy = 0, and _θz = 0. And, as with the first embodiment, the equations obtained by swapping X and Z in equations (1), (2), and (3) are valid, so _θx , _Δy , and _Δz can be obtained by solving these equations using the calculations of the analysis device 33.

[Third Embodiment]
Next, a third embodiment of the displacement measurement structure will be described. In the displacement measurement structure 93 shown in Figure 6, the three strain sensor units 15 described above are installed on both the outer surface 13c and the outer surface 13f. That is, the displacement measurement structure 93 has a total of six strain sensor units 15 and a total of six free-length portions 23. Three free-length portions 23 are installed along the outer surface 13c, and three free-length portions 23 are installed along the outer surface 13f.

If three of the six free-length portions 23 installed along the outer surface 13c are used, then, as described in the first embodiment, _Δx , _Δy , and θz are measured (hereinafter referred to as "measurement 1"), assuming that _θx = 0, θy = 0, and _Δz = 0 (hereinafter referred to as "assumment 1 _" ). Furthermore, if three of the free-length portions 23 installed along the outer surface 13f are used, then, as described in the second embodiment, _θx = 0, _θy = 0, and _θz = 0 (hereinafter referred to as "assumment 2"), and _θx , _Δy , and _Δz are measured (hereinafter referred to _as "measurement 2").

An example of how to operate this displacement measurement structure 93 is as follows. First, both measurement 1 and measurement 2 are performed using the measuring instrument 31 and the analysis device 33. Then, based on these measurement results, the analysis device 33 determines whether the displacement in the horizontal plane or the displacement in the vertical plane is dominant in relation to the relative displacement between the box culverts 5A and 5B. That is, it determines which of the above assumptions 1 and 2 is more valid. If assumption 1 is deemed more valid, the result of measurement 1 is adopted; if assumption 2 is deemed more valid, the result of measurement 2 is adopted. Note that the user may also perform the judgment of the validity of assumptions 1 and 2 as described above.

[Fourth Embodiment]
Next, a fourth embodiment of the displacement measurement structure will be described. In the displacement measurement structure 94 of this embodiment, the three free-length portions 23 are installed along the outer surface 13 such that not all three free-length portions 23 are located on the same plane. For example, some of the three free-length portions 23 are installed along the outer surface 13c, and the other free-length portions 23 are installed along any of the outer surfaces 13d, 13e, or 13f. Alternatively, each of the three free-length portions 23 may be installed along different outer surfaces 13c to 13f.

As one specific example of this configuration, in the displacement measurement structure 94 shown in Figure 7, two of the three strain sensor units 15 mentioned above are installed on the outer surface 13c, and one is installed on the outer surface 13f. This displacement measurement structure 94 is
(a) Box culvert 5B does not twist around the Y-axis relative to box culvert 5A (or twisting around the Y-axis can be ignored).
(d) The box culvert 5B does not shift in the X and Z directions relative to the box culvert 5A (or the shift in the X and Z directions can be ignored).
It is used in such environments. In other words, it is used under the premise that _Δx = 0, _θy = 0, and _Δz = 0. Then, following the aforementioned formulas (1), (2), and (3), a predetermined equation including the expansion and contraction amounts of the three free length portions 23 can be set, and by solving this equation using the analysis device 33, _θx , _Δy , and _θz can be obtained.

The present invention can be implemented in various forms, including the embodiments described above, by making various modifications and improvements based on the knowledge of those skilled in the art. Furthermore, it is possible to construct modified versions by utilizing the technical matters described in the embodiments described above. The configurations of each embodiment may be used in appropriate combinations.

In this embodiment, all free-length portions 23 extend in the Y direction, but the direction of extension of the free-length portions 23 is not limited to this and may be inclined with respect to the Y direction. For example, the three free-length portions 23 in the first to third embodiments may be arranged in the manner shown in Figure 8(a) or Figure 8(b). Figures 8(a) and (b) are plan views modeling three free-length portions 23 installed along the outer surface 13c and/or outer surface 13f. In the configuration of Figure 8(a) or Figure 8(b), two optical fiber holding portions 19A are provided on the box culvert 5A side, and two optical fiber holding portions 19B are provided on the box culvert 5B side. One of the optical fiber holding portions 19A is shared by two strain sensor portions 15. That is, two optical fiber cables 17 pass through one of the optical fiber holding portions 19A, and one end 23a of the two free-length portions 23 is fixed at a fixed point 18A. Similarly, one of the optical fiber holding sections 19B is shared by two strain sensor sections 15. That is, two optical fiber cables 17 pass through one of the optical fiber holding sections 19B, and the other ends 23b of the two free-length sections 23 are fixed at fixed points 18B.

In the configuration shown in Figure 8(a) or Figure 8(b), a predetermined equation including the expansion and contraction amounts of the three free-length portions 23 can be set in accordance with the aforementioned formulas (1), (2), and (3), and ( _Δx , _Δy , _θz ) and/or ( _θx , _Δy , _Δz ) can be obtained by calculation by the analysis device 33. The configuration shown in Figure 8(a) or Figure 8(b) is preferable in that it can reduce the number of optical fiber holding portions 19A and 19B.

Furthermore, in the first to fourth embodiments, the number of free-length portions 23 used in the calculation of relative displacement is not limited to three, but may be four or more. In this case, by using the expansion and contraction amounts of four or more free-length portions 23 in the calculation, the calculation results become redundant, and the reliability of the measurement results is improved.

Furthermore, while the first to fourth embodiments described examples where a reinforced concrete section 11 is formed in the joint section 9, the displacement measurement structures 1, 92, and 93 can also be applied to sewer culverts without a reinforced concrete section 11. In this case, the height of the optical fiber holding sections 19A and 19B of the strain sensor section 15 can be reduced. Alternatively, the optical fiber holding sections 19A and 19B may be omitted.

Furthermore, in the first to fourth embodiments, the displacement measurement structures 1,92, 93, and 94 are constructed along the outer circumferential surface 13 of the sewer culvert 3. However, the displacement measurement structures 1,92, 93, and 94 may also be constructed along the wall surface (inner wall surface of the hollow section) on the channel 3a side of the sewer culvert 3. Moreover, the displacement measurement structures 1,92, 93, and 94 are not limited to sewer culverts but can be applied to other culverts. Furthermore, the displacement measurement structures 1,92, 93, and 94 are not limited to culverts with a rectangular cross-section as in the first to fourth embodiments, but can also be applied to culverts with a circular cross-section, for example. Finally, the displacement measurement structures 1,92, 93, and 94 are not limited to culverts but can be applied to various connecting structures formed by butting the joint surfaces of structures together.

1, 92, 93, 94…Displacement measurement structure; 3…Sewer box culvert (connecting structure); 5, 5A, 5B…Box culvert (structure); 7…Joint surface; 9…Joint section (connecting section); 13…Outer surface; 13c, 13d, 13e, 13f…Outer surface (outer surface of rectangular prism); 17…Optical fiber cable; 19A, 19B…Optical fiber holding section; 18A, 18B…Fixed point; 23, 23h, 23j, 23k…Free length section; 23a…One end; 23b…Other end.

Claims (3)

A displacement measuring structure for measuring the relative displacement between structural members in a columnar connecting structure, which is formed by butting the joint surfaces of structural members together and has a circumferential surface extending in a direction perpendicular to the joint surfaces ,
The system includes at least three optical fibers, each containing a free-length portion that extends across the connecting portion between the aforementioned structures.
The three free-length portions are arranged along the circumferential surface such that not all three of the free-length portions lie on the same plane.
Each of the aforementioned free length portions is
It is fixed at one end to a fixing point on one side of the structure, and at the other end to a fixing point on the other side of the structure.
A displacement measuring structure is configured to elastically expand and contract by the same amount as the change in the linear distance between one fixed point and the other fixed point , thereby enabling the measurement of the relative translational displacement between the structures in a direction perpendicular to the joint surface and the relative rotational displacement between the structures around two axes parallel to and perpendicular to the joint surface, based on the expansion and contraction of each of the free length portions . In a connected structure where the joint surfaces of the structures are butted together and connected,
The system includes at least three optical fibers, each containing a free-length portion that extends across the connecting portion between the aforementioned structures.
Each of the aforementioned free length portions is
It is fixed at one end to a fixing point on one side of the structure, and at the other end to a fixing point on the other side of the structure.
A displacement measurement method for measuring the relative displacement between structures using a displacement measurement structure that elastically expands and contracts by the same amount as the amount of change in the straight-line distance between one fixed point and the other fixed point ,
The connecting structure is columnar in shape, having a circumferential surface extending in a direction perpendicular to the joining surface.
The three free-length portions are arranged along the circumferential surface such that not all three of the free-length portions lie on the same plane.
A displacement measurement method in which, based on the expansion and contraction of each of the free length portions, the relative translational displacement of the structures in a direction perpendicular to the joint surface and the relative rotational displacement of the structures around two axes parallel to the joint surface and perpendicular to each other are measured. Each of the aforementioned structures, one and the other, is provided with an optical fiber holding portion that protrudes out of the circumferential surface,
The one end of the free length portion is fixed to the optical fiber holding portion of the other structure.
The displacement measurement structure according to claim 1, wherein the other end of the free length portion is fixed to the optical fiber holding portion of the other structure.