Stress measurement method and stress measurement apparatus

Abstract

The present invention provides a stress measurement method and a stress measurement device that can accurately measure the stress generated inside a target member into which prestress is introduced. [Solution] A stress measurement method for evaluating prestress based on strain generated inside a target member 30 into which prestress is introduced in one direction, comprising: an installation step of installing a measuring optical fiber 10 for detecting strain generated in the target member 30 in the target member 30; and a calculation step of calculating stress based on the strain. In the installation step, a first detection unit 11 for detecting strain along the direction of prestress introduction in the target member 30 and a second detection unit 12 for detecting strain along at least one of two orthogonal directions substantially perpendicular to the introduction direction are installed in the target member 30. In the calculation step, stress is calculated based on the strain along the introduction direction, the strain along at least one of the two orthogonal directions, the creep strain generated in the target member 30, and the specifications of the target member 30.

Description

[Technical Field]

[0001] This invention relates to a stress measurement method and a stress measurement apparatus. [Background technology]

[0002] Patent Document 1 describes a technique for removing creep strain when calculating effective stress from the detected strain. This technique involves attaching a first strain gauge in the x-direction and a second strain gauge in the y-direction to the side surface of a reinforced concrete member to which stress is applied in the x-direction, cutting out a concrete piece that includes the side surface to which the first and second strain gauges are attached, detecting strain in the x-direction via the first strain gauge and strain in the y-direction via the second strain gauge. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2008-268123 [Overview of the project] [Problems that the invention aims to solve]

[0004] In concrete structures where prestress is applied in a predetermined direction, if the prestress decreases over time, there is a risk of reduced rigidity, increased deflection, and rusting of internal reinforcing steel and PC steel due to cracking. If the prestress is insufficient, measures such as reinforcement will be necessary. Therefore, it is desirable to accurately confirm whether the prestress is being properly maintained.

[0005] Generally, the relationship between stress and strain in concrete structures can be described three-dimensionally by a matrix that follows elastic theory, where stress is the product of the elastic modulus and the strain. The strain in concrete structures includes not only elastic strain, but also inelastic strains such as temperature strain (responding to temperature changes), drying shrinkage strain (responding to drying shrinkage), and creep strain (responding to creep phenomena).

[0006] Since the sum of these inelastic strains is assumed to be several times greater than the elastic strain, it is necessary to subtract the inelastic strains from the elastic strain in concrete structures. However, directly detecting these strains is not practically easy. Furthermore, the conventional techniques described above substitute drying shrinkage strain and creep strain with estimated values ​​from prediction formulas, but these still contain errors, and there is room for improvement in the accuracy of stress measurement.

[0007] The present invention aims to provide a stress measurement method and a stress measurement device that can accurately measure the stress generated inside a target member into which prestress is introduced. [Means for solving the problem]

[0008] (1) A stress measurement method according to one aspect of the present invention is a stress measurement method for evaluating prestress based on strain generated inside a target member into which prestress is introduced in one direction, comprising: an installation step of installing a detection unit for detecting strain in the target member; a detection step of detecting strain generated in the target member using the detection unit; and a calculation step of calculating stress based on the detected strain, wherein in the installation step, a first detection unit for detecting strain along the direction of prestress introduction in the target member and a second detection unit for detecting strain along at least one of two orthogonal directions substantially perpendicular to the introduction direction in the target member are installed as detection units in the target member, and in the calculation step, stress is calculated based on strain along the introduction direction, strain along at least one of the two orthogonal directions, creep strain generated in the target member, and specifications of the target member.

[0009] In a stress measurement method according to one aspect of the present invention, in the installation step, not only a first detection unit that detects strain along the direction of prestress introduction in the target member, but also a second detection unit that detects strain along at least one of two orthogonal directions substantially perpendicular to the introduction direction in the target member is installed on the target member. In the calculation step, the stress is calculated using not only the detected strain value along the introduction direction, but also the detected strain value along at least one of the two orthogonal directions. Here, the inventors investigated how inelastic strain occurs in two orthogonal directions substantially perpendicular to the introduction direction in a target member into which prestress is introduced in one direction, and applied the assumptions that hold true for how inelastic strain occurs to the three-dimensional relationship between stress and strain, and discovered that it is possible to omit the direct detection of temperature strain and drying shrinkage strain among the inelastic strains. Temperature strain and drying shrinkage strain among the inelastic strains are also included in the strain along the two orthogonal directions. Furthermore, temperature strain and drying shrinkage strain can be considered to be substantially equivalent in the introduction direction and the two orthogonal directions. In this case, by using not only the detected strain along the introduction direction but also the detected strain along at least one of the two orthogonal directions, it becomes possible to perform calculations that cancel out the effects of temperature strain and drying shrinkage strain included in both. Therefore, according to one aspect of the present invention, the stress measurement method can accurately measure the stress generated inside the target member into which prestress is introduced, compared to the case where only the strain along the introduction direction is used as the detected strain.

[0010] (2) In the calculation step of (1) above, the stress may be calculated assuming that the target member is an isotropically elastic body and that the temperature strain, drying shrinkage strain, and creep coefficient are equal in the introduction direction and the two orthogonal directions, respectively. In this case, the effects of temperature strain and drying shrinkage strain can be more reliably canceled out.

[0011] (3) Specifically in (2) above, the stress may be calculated in the calculation step based on the following equations (1) to (5).

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[0012] (4) In any one of (1) to (3) above, the installation step may involve installing a measuring optical fiber as a detection unit on the target member, installing a light source capable of outputting measurement light toward the measuring optical fiber, and in the calculation step, calculating the strain along the introduction direction and the strain along at least one of the two orthogonal directions based on the scattered light and measurement light generated by the measuring optical fiber. In this case, by using a measuring optical fiber as a detection unit, the detected value of the strain along the introduction direction and the detected value of the strain along at least one of the two orthogonal directions can be detected as a linear distribution along the measuring optical fiber.

[0013] (5) In the above (4), the first detection unit and the second detection unit may be configured with a single measuring optical fiber during the installation process. In this case, the strain detection value along the introduction direction and the strain detection value along at least one of the two orthogonal directions can be detected with a single measuring optical fiber, and the calculation can be performed with a single measuring instrument.

[0014] (6) Another aspect of the present invention is a stress measuring device for evaluating prestress based on strain generated inside a target member into which prestress is introduced in one direction, and comprises a detection unit installed on the target member for detecting strain generated in the target member, and a calculation unit for calculating stress based on the detected strain, wherein the detection unit has a first detection unit for detecting strain along the direction of prestress introduction in the target member, and a second detection unit for detecting strain along at least one of two orthogonal directions substantially perpendicular to the introduction direction in the target member, and the calculation unit calculates stress based on the strain along the introduction direction, the strain along at least one of the two orthogonal directions, the creep strain generated in the target member, and the specifications of the target member.

[0015] In another embodiment of the present invention, the stress measuring device includes a detection unit that detects strain along the direction of prestress introduction in the target member, and a second detection unit that detects strain along at least one of two orthogonal directions substantially perpendicular to the introduction direction in the target member. The calculation unit calculates the stress based on the strain along the introduction direction, the strain along the orthogonal direction, the creep strain generated in the target member, and the specifications of the target member. Here, the inventors investigated how inelastic strain occurs in two orthogonal directions substantially perpendicular to the introduction direction in a target member into which prestress is introduced in one direction, and by applying the assumptions that hold true for how inelastic strain occurs to the three-dimensional relationship between stress and strain, they discovered that it is possible to omit the direct detection of temperature strain and drying shrinkage strain among the inelastic strains. Temperature strain and drying shrinkage strain among the inelastic strains are also included in the strain along the two orthogonal directions. Furthermore, thermal strain and drying shrinkage strain can be considered to be approximately equivalent in the introduction direction and the two orthogonal directions. In this case, by using not only the detected strain along the introduction direction but also the detected strain along at least one of the two orthogonal directions, it becomes possible to perform calculations that cancel out the effects of thermal strain and drying shrinkage strain included in both. Therefore, according to one aspect of the present invention, the stress generated inside the target member into which prestress is introduced can be measured with greater accuracy compared to the case where only the strain along the introduction direction is used as the detected strain value. [Effects of the Invention]

[0016] According to the present invention, it is possible to accurately measure the stress generated inside a target member into which prestress is introduced. [Brief explanation of the drawing]

[0017] [Figure 1] This is a schematic diagram illustrating a stress measurement device according to an embodiment. [Figure 2] This is a flowchart to explain the stress measurement method. [Figure 3] This is a schematic cross-sectional view of the target member showing an example of the arrangement of the first detection unit and the second detection unit. [Figure 4] This is a schematic diagram of the target component showing other arrangement examples of the first and second detection units. [Figure 5] This is a schematic diagram of the target component showing other arrangement examples of the first and second detection units. [Modes for carrying out the invention]

[0018] Embodiments of the present invention will be described below with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant descriptions are omitted. Hereinafter, "optical fiber" means optical fiber cables and optical fiber sensors.

[0019] Figure 1 is a schematic diagram illustrating a stress measurement device according to an embodiment. As shown in Figure 1, the stress measurement device 1 is a device for evaluating prestress based on the strain generated inside a target member. The target member 30 is a concrete structure in which prestress is introduced in a predetermined one direction, and examples include prestressed concrete (PC) structures such as bridge girders and deck slabs. The stress measurement device 1 may be permanently placed on the target member 30 to continuously obtain the strain generated in the target member 30. Alternatively, the stress measurement device 1 may be temporarily attached to the target member 30 at each measurement timing and removed after measurement.

[0020] The stress measuring device 1 is installed on the target member 30 and includes a measuring optical fiber 10 (detection unit) that detects the strain occurring in the target member 30. The measuring optical fiber 10 functions as a sensor for measuring the strain or temperature change occurring in the target member 30. As the measuring optical fiber 10, for example, an optical fiber core is used, which has a predetermined length in total and has protective material (e.g., cladding and coating) attached to the optical fiber strands.

[0021] The measuring optical fiber 10 has such conformability between the optical fiber strands and protective material that it experiences strain similar to that of the target member 30. When strain occurs in the target member 30, strain corresponding to that of the target member 30 also occurs in the measuring optical fiber 10. The stress measuring device 1 measures the strain and temperature change that occurs in the measuring optical fiber 10 and measures the internal stress of the target member 30 based on the measured strain and temperature change. The measuring optical fiber 10 may include multiple measurement sections at predetermined distances in order to measure strain and temperature change at multiple locations. A measurement section is a section in which strain and temperature change are detected as a linear distribution.

[0022] The stress measurement device 1 includes a measuring instrument 20. The measuring instrument 20 incidents measurement light onto a measurement optical fiber 10 and obtains the light intensity of the scattered light generated in the measurement optical fiber 10. The measuring instrument 20 has a light source 21, an optical circulator 22, and a light receiving unit 23, which are, for example, built into a portable housing of the measuring instrument 20.

[0023] The light source 21 is a light source capable of outputting measurement light (e.g., pulsed light) toward the measurement optical fiber 10. For example, a laser diode may be used as the light source 21. The light source 21 repeatedly outputs measurement light at a predetermined period.

[0024] The optical circulator 22 is connected to the light source 21, the measurement optical fiber 10, and the light receiving unit 23, respectively. The optical circulator 22 outputs the measurement light from the input light source 21 toward the measurement optical fiber 10, and also outputs the scattered light generated in the measurement optical fiber 10 toward the calculation unit 24 and then to the light receiving unit 23.

[0025] In the measurement optical fiber 10, scattered light is generated at various positions along its extension direction. The scattered light generated in the measurement optical fiber 10 is input to the optical circulator 22 and output from the optical circulator 22 to the processing unit 24 and then to the light receiving unit 23. Examples of scattered light include Rayleigh scattered light and Brillouin scattered light. As an example of the measuring instrument 20, for example, a TW-COTDR that utilizes Rayleigh scattered light, or a PPP-BOTDR or BOTDR that utilizes Brillouin scattered light can be used.

[0026] The light receiving unit 23 receives scattered light output by the optical circulator 22. The light receiving unit 23 also receives measurement light from the light source 21. The light receiving unit 23 may include, for example, a balance detector. The light receiving unit 23 may also include an AD converter that performs digital sampling for transmission to the calculation unit 24. The specific configuration of the light receiving unit 23 is not particularly limited, as long as the calculation unit 24 can obtain the scattered light intensity.

[0027] The arithmetic unit 24 is a computer comprising, for example, a CPU (Central Processing Unit), ROM (Read Only Memory), and RAM (Random Access Memory). Based on the measurement light from the light source 21 and the scattered light output by the optical circulator 22, the arithmetic unit 24 performs, for example, an analysis of the scattered light intensity (spectrum) or wavelength for each frequency, and calculates the strain and temperature change at each position along the longitudinal direction of the measurement optical fiber 10. Alternatively, measurements may be performed using Rayleigh scattered light and Brillouin scattered light separately, and the strain and temperature change may be detected separately by solving a system of equations using the respective measurement results.

[0028] In this embodiment, the measuring optical fiber 10 includes a first detection unit 11 that detects strain along the direction of prestress introduction in the target member 30, and a second detection unit 12 that detects strain along at least one of two orthogonal directions substantially perpendicular to the introduction direction in the target member 30.

[0029] The first detection unit 11 is, for example, a part of the measuring optical fiber 10, and corresponds to the portion of the measuring optical fiber 10 arranged along the direction of prestress introduction in the target member 30. The second detection unit 12 is, for example, a part of the measuring optical fiber 10, and corresponds to the portion of the measuring optical fiber 10 arranged along at least one of two orthogonal directions substantially perpendicular to the direction of prestress introduction in the target member 30.

[0030] The second detection unit 12 may be positioned in only one of the two orthogonal directions. In this case, for example, when the introduction direction is the x direction, the second detection unit 12 may be positioned in only one of the y or z directions. The second detection unit 12 may be positioned in both orthogonal directions. In this case, for example, when the introduction direction is the x direction, the second detection unit 12 may be positioned in both the y and z directions.

[0031] Several specific examples of the arrangement of the first detection unit 11 and the second detection unit 12 will be described. Figure 3 is a schematic cross-sectional view of a target member showing an example of the arrangement of the first detection unit and the second detection unit. As shown in Figure 3, an example of the target member 30 is a test specimen that simulates a PC beam of a prestressed concrete (PC) structure. The target member 30 is designed so that prestress is introduced with its longitudinal direction as the introduction direction. The target member 30 has PC steel members 31, axial reinforcement bars 32, and shear reinforcement bars 33, and deformed reinforcement bars are arranged in two orthogonal directions perpendicular to the introduction direction.

[0032] PC steel members 31 are steel members used to introduce prestress into a prestressed concrete structure. The PC steel members 31 are arranged along the axial direction (longitudinal direction) of the target member 30. For example, three strands of PC steel (e.g., 1S15.2) are used as PC steel members 31, and they are arranged in a straight line using a sheath with a nominal diameter of 35 mm. A tensioning force of 180 kN / member may be introduced into the PC steel members 31 using a post-tensioning method. After the introduction of prestress, grout material may be filled around the PC steel members 31 to integrate the PC steel members 31 with the concrete.

[0033] The axial reinforcement bars 32 are arranged along the axial direction of the target member 30. The axial reinforcement bars 32 extend parallel to the PC steel members 31 within the target member 30.

[0034] The shear reinforcement bars 33 are stirrups arranged at intervals along the cross-section of the target member 30. The shear reinforcement bars 33 have a rectangular shape when viewed from the axial direction of the target member 30. If the introduction direction is the x-direction, and the two orthogonal directions approximately perpendicular to the introduction direction are the y-direction and z-direction, then one pair of opposite sides of the rectangle of the shear reinforcement bars 33 extends along the y-direction, and the other pair of opposite sides extends along the z-direction.

[0035] In the target member 30, prestress is applied from both axial sides by the PC steel bars 31, compressing the target member 30. Incidentally, although the axial reinforcement bars 32 are subjected to compressive force due to the prestress, most of the compressive force is received by the concrete cross section of the target member 30, so the compressive force received by the cross section of the axial reinforcement bars 32 can be almost ignored.

[0036] The measuring optical fiber 10 is embedded inside the target member 30. The first detection unit 11 detects strain along the direction of prestress introduction of the target member 30, and in the example of Figure 3, it is fixed to the surface of the axial reinforcement bar 32 using adhesive. The second detection unit 12 detects strain along two orthogonal directions perpendicular to the introduction direction of the target member 30, and in the example of Figure 3, the optical fiber core is covered with rubber having an uneven surface and is intermittently fixed to the shear reinforcement bar 33 with cable ties.

[0037] Figure 4 is a schematic diagram of a target member showing other arrangement examples of the first and second detection units. As shown in Figure 4, the first detection unit 11 is installed to detect strain along the prestress introduction direction of the target member 30A. The first detection unit 11 is positioned as part of an extended portion along the introduction direction. The second detection unit 12 is installed to detect strain along one of two orthogonal directions perpendicular to the prestress introduction direction of the target member 30A. The second detection unit 12 is positioned as part of an extended portion along one of the two orthogonal directions perpendicular to the introduction direction.

[0038] Specifically, the measuring optical fiber 10 has a first detection unit 11 arranged linearly along the introduction direction inside the target member 30A, a second detection unit 12 arranged linearly along one of two orthogonal directions perpendicular to the introduction direction, and a curved portion 13 connecting the first detection unit 11 and the second detection unit 12. The first detection unit 11 and the second detection unit 12 are arranged to be perpendicular to each other at a common point inside the target member 30A, for example. The measuring optical fiber 10 extends inside the target member 30A in a continuous line, and the second detection unit 12 and the curved portion 13 are continuous, forming an S-shape.

[0039] Figure 5 is a schematic diagram of a target member showing other arrangement examples of the first and second detection units. In the example in Figure 5, there is a region inside the target member 30B where the temperature change strain and the drying shrinkage strain can be considered uniform, and the measuring optical fiber 10 is arranged in this region.

[0040] The measuring optical fiber 10 has a first detection unit 11 arranged linearly along the introduction direction inside the target member 30B, a second detection unit 12 arranged linearly along one of two orthogonal directions perpendicular to the introduction direction, and a connecting unit 14 connecting the first detection unit 11 and the second detection unit 12. The first detection unit 11 and the second detection unit 12 do not orthogonal at a common point inside the target member 30B, but detect strain at different positions.

[0041] In the example described above, the first detection unit 11 and the second detection unit 12 share a single measurement optical fiber 10. However, the example is not limited to this, and one of the first detection unit 11 and the second detection unit 12 may be a measurement optical fiber provided separately from the measurement optical fiber 10.

[0042] The calculation unit 24 calculates stress based on the detected strain along the introduction direction, the detected strain along at least one of two orthogonal directions substantially perpendicular to the introduction direction, the creep strain generated in the target member 30, and the specifications of the target member 30. In this embodiment, the stress calculation by the calculation unit 24 of the stress measuring device 1 will also be described as a stress measurement method.

[0043] The stress measurement method is a method of measuring stress based on the strain generated inside the target member 30. Figure 2 is a flowchart illustrating the stress measurement method. As shown in Figure 2, the stress measurement method comprises an installation step S11, a detection step S12, and a calculation step S13. The stress calculation process of the calculation unit 24 corresponds to the calculation step S13 of the stress measurement method.

[0044] In the stress measurement method, first, in S11, an installation process is performed. In the installation process S11, a measuring optical fiber 10 is installed in the target member 30 as a detection unit for detecting strain. In the installation process S11, for example, a first detection unit 11 and a second detection unit 12 are constructed using one measuring optical fiber 10. For example, as in the arrangement example of the measuring optical fiber 10 described above, the measuring optical fiber 10 is embedded inside the target member 30.

[0045] In installation step S11, the light source 21 and the like are installed by installing the measuring instrument 20 in a designated measuring room or the like. Then, one or both ends of the measuring optical fiber 10 are drawn out from the target member 30 and connected to the measuring instrument 20, for example, via an optical coupler (not shown) which serves as the output terminal of the measuring instrument 20. In installation step S11, the calculation unit 24 is installed in a designated measuring room or the like.

[0046] In detection step S12, strain and temperature changes occurring in the target component 30 are detected using the measuring optical fiber 10. To this end, in detection step S12, first, measurement light is output from the light source 21 towards the measuring optical fiber 10. For example, the measurement light is repeatedly output from the light source 21 at a predetermined period while modulating the wavelength and frequency.

[0047] In detection step S12, reference data that serves as the basis for stress measurement is acquired as time-series data at a reference time, for example, shortly after the installation of the target member 30. The reference data includes data on scattered light generated by the measuring optical fiber 10 received by the light receiving unit 23 and the measurement light from the light source 21 received by the light receiving unit 23 at the reference time. Reference data is acquired for each of the multiple measurement points of the measuring optical fiber 10. The reference data may be acquired later than shortly after the installation of the target member 30.

[0048] In detection step S12, for example, when measuring the target member 30 during a predetermined periodic inspection, measurement data corresponding to the strain and temperature change at the time of measurement is acquired as time-series data. The measurement data includes data of scattered light generated by the measurement optical fiber 10 received by the light receiving unit 23 and measurement light from the light source 21 received by the light receiving unit 23 at the time of measurement. Measurement data is acquired for each of the multiple measurement points of the measurement optical fiber 10.

[0049] In calculation step S13, the detected strain and temperature change are calculated. For example, a Fourier transform is performed on the time-series data of the measurement optical fiber 10 to identify the positional information on the measurement optical fiber 10 for the reference data and measurement data of the obtained scattered light intensity. The time difference between the time when the measurement light was incident and the time associated with the reference data and measurement data is calculated. Using this time difference and the speed of light, the time associated with the reference data and measurement data is converted into positional information. Then, the intensity distribution of the scattered light is obtained by associating the reference data, measurement data and positional information. Furthermore, the scattered light intensity is divided into intervals between multiple measurement points on the measurement optical fiber 10, and an inverse Fourier transform is performed to obtain a spectrum. The degree of interval division here corresponds to the spatial resolution. Then, by calculating the cross-correlation coefficient for the scattered light intensity spectrum of the obtained intervals, the frequency shift amount for each interval is obtained, and the strain for each interval, which is proportional to this frequency shift amount, is obtained.

[0050] In calculation step S13, stress is calculated based on the detected strain. In calculation step S13, the strain along the introduction direction and the strain along at least one of the two orthogonal directions are calculated based on the scattered light and measurement light generated by the measurement optical fiber. By performing the processing of calculation step S13 for the entire length of each of the multiple measurement points of the measurement optical fiber 10, the distribution of the detected strain values ​​can be obtained.

[0051] In calculation step S13, stress is calculated based on the strain along the introduction direction, the strain along at least one of the two orthogonal directions, the creep strain occurring in the target member, and the specifications of the target member. In calculation step S13, for example, the internal stress of the target member 30 is calculated based on the following equations (1) to (5).

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[0052] Furthermore, when detecting strain along only one of the two orthogonal directions, one should select the equation (2) or (3) corresponding to that direction. When detecting strain along both orthogonal directions, both equations (2) and (3) may be used.

[0053] ε y,t ,ε z,t For each temperature strain, since temperature is originally a scalar quantity and has no directionality, it can be assumed that it is equal in the direction of introduction and in two orthogonal directions. In this case, ε y,t ,ε z,t This can be calculated, for example, as the product of β and ΔT. Here, β is the coefficient of linear expansion of iron, and ΔT is the temperature change at the position of the second detection unit 12. ΔT may also be the temperature change detected by the measuring optical fiber 10.

[0054] p d For example, if the target member 30 is a bridge deck member, then p d =A sd / (b d ×s d The floor plate reinforcement ratio can be expressed as follows: A sd This refers to the cross-sectional area (mm²) of the reinforcement bars in the girder width direction of the deck slab (orthogonal reinforcement). 2 ) and b d This is the thickness of the floor slab (mm), and s d This is the spacing (mm) of the reinforcement bars in the girder width direction of the deck slab.

[0055] p w For example, if the target member 30 is a bridge girder member, p w =A ss / (b w ×s w The shear reinforcement ratio can be expressed as ). Here, A ssThis is the cross-sectional area (mm²) of one set of stirrups (shear reinforcement bars). 2 ) and b w This is the width (mm) of the girder cross-section web, and s w This is the spacing between the stirrups (mm).

[0056] In calculation step S13, in order to calculate the internal stress of the target member 30 based on the above equations (1) to (5), first, the total strain ε in each direction is calculated using the strain detection value obtained from the measuring optical fiber 10. x,total , ε y,total , ε z,total Next, we calculate the temperature strain ε from the product of β and ΔT. y,t , ε z,t We find the ε x,total , ε y,total , ε z,total The creep coefficients φ and p d and p w , as well as ν and E corresponding to the specifications of the target member 30. c , E s By applying the above equations (2) to (5), we obtain the elastic strain ε x,e , ε y,e , ε z,e We obtain the elastic strain ε. x,e , ε y,e , ε z,e and ν and E c By applying this to equation (1) above, the stress σ in the x direction (introduction direction) x We seek.

[0057] By following the above procedure, the stress σ inside the target member 30 is x (Stress) can be calculated. Also, the stress σ inside the target member 30 can be calculated. x By using the calculation results and comparing them with reference data, it becomes possible to evaluate prestress by confirming whether prestress is being properly maintained.

[0058] By the way, in calculation step S13, assuming that the target member 30 is an isotropically elastic body, and that the temperature strain, drying shrinkage strain, and creep coefficient are equal in the introduction direction and the two orthogonal directions, the stress σ x The above equations (1) to (5) can be derived to calculate (stress).

[0059] If concrete is considered an isotropic elastic body, the relationship between stress and strain in an elastic body is given by the elastic modulus E c Using and Poisson's ratio ν, it can be expressed as shown in equations (6) and (7) below.

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[0060] Since shear stress τ is not considered here and can be ignored, the relationship between normal stress σ and elastic strain can be expressed as shown in equations (8) and (9) below. Here, the subscript e is added to the elastic strain ε to distinguish it from inelastic strain (temperature strain, drying shrinkage strain, and creep strain) which will be discussed later.

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[0061] If the target member 30 is a bridge, considering that in the case of a deck slab, there is usually little or no reinforcement in the width direction of the deck slab, and in the case of the upper part of a girder, there is usually little or no reinforcement in the width direction of the upper part, σ y =σ z We can set = 0. In this case, ε y,e ,ε z,e is, ε x,e This is related to the following equation (10).

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[0062] Using the measurement optical fiber 10 installed on the target member 30, the subscript of the detected value of the strain ε measured is total denoted as, and the subscript of the temperature strain ε is t denoted as, and the subscript of the drying shrinkage strain ε is sh denoted as, and the subscript of the creep strain ε is cr denoted as, then the relationships of the following formulas (11a) to (11c) exist.

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[0063] The stress degrees along the two orthogonal directions correspond to the reaction forces of the tensile stresses of the reinforcing bars along which the measurement optical fiber 10 is arranged in their respective directions. If the subscript of this stress degree σ is re denoted as, then the relationships of the following formulas (12a) and (12b) exist.

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[0065] The stress σ along the two orthogonal directions in equations (12a) and (12b) above y,re , σ z,re Using this, the total strain ε along each of the two orthogonal directions y,total and ε z,total The stress σ y,re , σ z,re and temperature strain ε y,t ,ε z,t And, the coefficient of linear expansion β and can be calculated from. That is, the subscript of the elastic strain ε due to rebar restraint along two orthogonal directions is re Therefore, the stress σ in equations (12a) and (12b) above y,re , σ z,re Using the three-dimensional stress-strain relationship in equation (9) above, the elastic strain ε due to reinforcement restraint is calculated for each of the two orthogonal directions. y,re , ε z,re This can be expressed as shown in equations (13a), (13b), (14a), and (14b) below.

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[0066] If there is nothing to restrain the elongation due to the Poisson effect, σ y,re , σz,re We can assume that = 0. In this case, the elastic strain ε due to reinforcement restraint is y,re , ε z,re = 0.

[0067] The subscript of the elastic strain ε due to prestress is pe Therefore, the elastic strain ε due to prestress y,pe , ε z,pe and elastic strain ε due to rebar restraint y,re , ε z,re The sum of these is the elastic strain ε in each direction. y,e , ε z,e We can set it to be equal to , and equations (8) and (9) above become equations (15) and (16) below.

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[0068] Creep strain ε y,cr , ε z,cr For this, an estimated value can be used, calculated from the stress in both orthogonal directions, which is determined from the reaction force of the tensile force on the reinforcing bars in the stirrups or slabs as described above, and the creep coefficient φ (or other creep evaluation formulas, etc.). Since the age, loading start age, and elapsed time are the same in the three directions (introduction direction and the two orthogonal directions), the creep coefficient φ can be assumed to be the same value in all three directions. In this case, the elastic strain ε due to creep y,cr , ε z,cr This is expressed by equations (17) and (18) below.

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[0069] Furthermore, assuming that there is no difference in temperature strain and drying shrinkage strain depending on the direction, the following equations (20) and (21) are obtained.

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[0070] Using the relationships described above, the y and z components of equation (16) are given by equations (22a), (22b), (23a), and (23b), respectively.

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[0071] Stress due to prestress σ x Temperature strain and drying shrinkage strain ε in the same direction x,t +ε x,sh This is the ε on the left side of equation (22b) above. y,t +ε y,sh , or the ε on the left side of equation (23b) z,t +ε z,sh Using the fact that it is equal to, the x component of equation (16) above (equation (24) below) can be expressed as, for example, the ε of equation (23b) above.z,t +ε z,sh Substituting this (in equation (25) below), we obtain equation (26) below.

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[0072] Using the relationship between equations (12a) and (12b) above,

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[0073] If you want to detect strain along only one of two orthogonal directions, you can select the equation corresponding to that direction from equations (22a), (22b), (23a), and (23b). Similarly, if you use the y-component relation, you will get equation (30) below.

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[0074] By applying either equation (29) or equation (30) above to equation (31) below, which is a modified version of equation (8), the stress σ in the x direction can be obtained. x That is, the equation representing the stress due to prestress is obtained as the following equation (32), which corresponds to the above equation (1).

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[0075] Furthermore, when detecting strain along both orthogonal directions, the above equations (22a), (22b), (23a), and (23b) may be used by employing the equations corresponding to both directions and using the average value of the obtained values.

[0076] According to the stress measurement device 1 and stress measurement method described above, not only is a first detection unit 11 installed on the target member 30 to detect strain along the direction of prestress introduction in the target member 30, but a second detection unit 12 is installed on the target member 30 to detect strain along at least one of two orthogonal directions substantially perpendicular to the introduction direction in the target member 30. The stress is calculated using not only the detected strain value along the introduction direction but also the detected strain value along at least one of the two orthogonal directions. Here, as described above, the inventors investigated how inelastic strain occurs in two orthogonal directions substantially perpendicular to the introduction direction in a target member 30 into which prestress is introduced in one direction, and applied the assumptions that hold true for how inelastic strain occurs to the three-dimensional relationship between stress and strain, and discovered that it is possible to omit the direct detection of temperature strain and drying shrinkage strain among the inelastic strains. Temperature strain and drying shrinkage strain among the inelastic strains are also included in the strain along the two orthogonal directions. Furthermore, the thermal strain and drying shrinkage strain can be considered to be approximately equivalent in the introduction direction and the two orthogonal directions. In this case, by using not only the detected strain along the introduction direction but also the detected strain along at least one of the two orthogonal directions, it becomes possible to perform calculations that cancel out the effects of the thermal strain and drying shrinkage strain included in both directions. Therefore, the stress measurement device 1 and stress measurement method can measure the stress generated inside the target member 30 into which prestress is introduced with greater accuracy compared to the case where only the strain along the introduction direction is used as the detected strain value.

[0077] According to the stress measurement device 1 and stress measurement method, stress is calculated assuming that the target member 30 is an isotropic elastic body, and that the temperature strain, drying shrinkage strain, and creep coefficient are equal in the introduction direction and the two orthogonal directions, respectively. This makes it possible to more reliably cancel out the effects of temperature strain and drying shrinkage strain.

[0078] According to the stress measurement device 1 and stress measurement method, a measurement optical fiber 10 is installed on the target member 30 as a detection unit, a light source 21 capable of outputting measurement light toward the measurement optical fiber 10 is installed, and based on the scattered light generated by the measurement optical fiber 10 and the measurement light, the strain along the introduction direction and the strain along at least one of the two orthogonal directions are calculated. Thus, by using the measurement optical fiber 10 as a detection unit, it is possible to detect the detected strain along the introduction direction and the detected strain along at least one of the two orthogonal directions as a linear distribution along the measurement optical fiber 10.

[0079] According to the stress measurement device 1 and stress measurement method, a first detection unit 11 and a second detection unit 12 are configured using a single measuring optical fiber 10. This allows for the detection of strain along the introduction direction and strain along at least one of two orthogonal directions using a single measuring optical fiber 10, and the calculations can be performed using a single measuring instrument 20.

[0080] Although embodiments of the present invention have been described above, the present invention is not limited to the above embodiments and can be implemented in various forms.

[0081] In the above embodiment, stress was calculated based on equations (1) to (5) above, but the invention is not limited to this example. Furthermore, the stress was calculated assuming that the target member 30 is an isotropic elastic body, and that the temperature strain, drying shrinkage strain, and creep coefficient are equal in the introduction direction and the two orthogonal directions, but the invention is not limited to this example. For example, if the target member 30 is not an isotropic elastic body, the stress may be calculated by adding appropriate correction terms to equations (1) to (5) above or by making other modifications. If the temperature strain, drying shrinkage strain, and creep coefficient are not equal in the introduction direction and the two orthogonal directions, the stress may be calculated by adding appropriate correction terms to equations (1) to (5) above or by making other modifications.

[0082] In the above embodiment, a measuring optical fiber 10 was installed on the target member 30 as the detection unit, but the invention is not limited to this example. For example, a strain gauge or the like may be used as the detection unit.

[0083] In the above embodiment, the first detection unit 11 and the second detection unit 12 were configured using a single measuring optical fiber 10, but the invention is not limited to this example. For example, the first detection unit and the second detection unit may be configured using multiple measuring optical fibers, or the first detection unit and the second detection unit may be configured using multiple strain gauges.

[0084] In the above embodiment, a bridge with girders and a deck slab was given as an example of a concrete structure to which prestressing is introduced, but the invention is not limited to this example. For example, the target members include arch bridges, floor slabs, beams and columns of high-rise buildings, liquid storage tanks, grain silos, offshore structures, parking structures, gravity dams, and other public and public-interest facilities and structures. [Explanation of Symbols]

[0085] 1... Stress measuring device, 10... Optical fiber for measurement (detection unit), 11... First detection unit, 12... Second detection unit, 21... Light source, 24... Calculation unit, 30, 30A, 30B... Target member, S11... Installation process, S12... Detection process, S13... Calculation process, ν... Poisson's ratio.

Claims

1. A stress measurement method for evaluating prestress based on the strain generated inside a target member into which prestress is introduced in one direction, Installation step of installing a detection unit for detecting strain on the target member, A detection step of detecting the strain occurring in the target member using the detection unit, The system comprises a calculation step for calculating the stress based on the detected strain, In the installation step, a first detection unit for detecting the strain along the direction of introduction of the prestress in the target member, and a second detection unit for detecting the strain along at least one of two orthogonal directions substantially perpendicular to the introduction direction in the target member are installed as the detection unit in the target member. A stress measurement method comprising the calculation step of calculating the stress based on the strain along the introduction direction, the strain along at least one of the two orthogonal directions, the creep strain generated in the target member, and the specifications of the target member.

2. The stress measurement method according to claim 1, wherein in the calculation step, the stress is calculated assuming that the target member is an isotropically elastic body, and that the temperature strain, drying shrinkage strain, and creep coefficient are equal in the introduction direction and the two orthogonal directions, respectively.

3. The stress measurement method according to claim 2, wherein the calculation step involves calculating the stress based on the following equations (1) to (5). [Math 1] [Math 2] [Math 3] [Math 4] [Math 5] however, The x-direction is the introduction direction, The y-direction is one of the two orthogonal directions mentioned above. The z-direction is the other of the two orthogonal directions mentioned above. σ ｘ This is the stress along the x-direction in the target member, E ｃ This is the elastic modulus of the concrete of the target member, ν is the Poisson's ratio of the concrete of the target member, ε ｘ，ｅ , ε ｙ，ｅ , ε ｚ，ｅ These are elastic strains along the x, y, and z directions, respectively. ε ｘ，ｔｏｔａｌ , ε ｙ，ｔｏｔａｌ , and ε ｚ，ｔｏｔａｌ These are the total strains along the x, y, and z directions, respectively. ε ｘ，ｐｅ ,ε ｙ，ｐｅ ,ε ｚ，ｐｅ are, respectively, elastic strains due to prestresses along the x, y, and z directions, ε ｙ，ｒｅ , ε ｚ，ｒｅ These are the elastic strains due to the restraint of the reinforcing bars along the y and z directions, respectively. ε ｙ，ｔ , ε ｚ，ｔ These are temperature strains along the y and z directions, respectively. p ｄ This is the ratio obtained by dividing the cross-sectional area of ​​the orthogonal reinforcement bars in the target member by the spacing between the orthogonal reinforcement bars and the thickness of the target member. p ｗ This is the ratio obtained by dividing the cross-sectional area of ​​the shear reinforcement bars in the target member by the spacing between the shear reinforcement bars and the width of the target member. E ｓ This is the elastic modulus of iron, E ｃ This is the elastic modulus of concrete, φ is the creep coefficient of concrete.

4. In the installation step, a measuring optical fiber is installed as the detection unit on the target member, and a light source capable of outputting measurement light toward the measuring optical fiber is installed. The stress measurement method according to claim 1 or 2, wherein the calculation step calculates the strain along the introduction direction and the strain along at least one of the two orthogonal directions based on the scattered light generated by the measuring optical fiber and the measurement light.

5. The stress measurement method according to claim 4, wherein in the installation step, the first detection unit and the second detection unit are configured with a single optical fiber for measurement.

6. A stress measuring device for evaluating prestress based on the strain generated inside a target member into which prestress is introduced in one direction, A detection unit is installed on the target member and detects the strain occurring in the target member, The system comprises a calculation unit that calculates the stress based on the detected strain, The detection unit includes a first detection unit that detects the strain along the direction of introduction of the prestress in the target member, and a second detection unit that detects the strain along at least one of two orthogonal directions substantially perpendicular to the introduction direction in the target member. A stress measuring device in which the calculation unit calculates the stress based on the strain along the introduction direction, the strain along at least one of the two orthogonal directions, the creep strain occurring in the target member, and the specifications of the target member.

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