JP2001004463A - Method for measuring stress acting on structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for measuring an acting stress by which boring of a structure required for releasing stress is minimized. SOLUTION: Optical fibers 2, 3, 4 are attached onto the surface 1A of a concrete structure 1 and one end parts 2A, 3A, 4A of the optical fibers are bored 5 along with the concrete structure 1. Variation of strain is then measured from variation in the optical characteristics of the optical fibers 2, 3, 4 caused by boring and then a stress acting on the concrete structure 1 is determined.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring a stress acting on an actual structure such as concrete.

[0002]

2. Description of the Related Art For example, when measuring the stress acting on a concrete actual structure, conventionally, three wire strain gauges for measuring concrete strain are used, and the stress state is measured on the surface of the concrete structure. Paste the other one at a 45 ° angle to one of the other so that the two are at right angles, drill a hole near the intersection of the three wire strain gauges and concrete The acting stress was released, and the acting stress on the concrete structure was calculated from the change in the amount of strain acting on the wire strain gauge.
The angle at which the wire strain gauge is attached may be any angle as long as the principal stress can be calculated from the three measured values.

Further, a method has been adopted in which stress is released by over-coring with a size surrounding the three attached wire strain gauges, and the applied stress is calculated from a change in the measured strain. I have.

[0004]

The base length of currently commercially available wire strain gauges is 37 m.
m (in the case of a gauge length of 30 mm), and it is necessary to make a hole with a diameter of about 100 mm in order to accurately measure strain due to stress release. For this reason, according to the above-mentioned conventional method, there is a problem that the size of the hole is increased due to the problem of the size of the base constituting the wire strain gauge, which adversely affects the structure.
Drilling holes of such a size is not only complicated, but also considering the adverse effect on the structure, the number of measurement points is limited, and data for evaluating the stress state acting on the structure is also required. And the measurement accuracy tends to be low. Furthermore, since the measurement becomes impossible when the wire strain gauge is cut, there is a disadvantage that the measurement can be performed only once.

An object of the present invention is therefore to provide a method for measuring the stress acting on a structure, which makes it possible to solve the above-mentioned problems in the prior art.

[0006]

According to the present invention, an optical fiber cable is attached to the surface of a structure to be measured, and a part of the optical fiber cable is drilled together with the structure to be measured. Alternatively, a groove-shaped cut is provided together with the structure to be measured to release the stress, and the stress acting on the structure to be measured is determined from the change in the optical characteristics of the optical fiber cable caused by the release of the stress. A method is proposed.

When a hole or a groove is cut in the structure to be measured, the optical fiber cable is physically deformed due to the stress released by the drilling or the groove, and the optical characteristics change in accordance with the deformation of the optical fiber cable. become. The change in the optical characteristics of the optical fiber cable is measured, for example, by measuring the signal power of the reflected light by injecting coherent light from one end of the optical fiber cable, and measuring the bending loss due to the deformation of the optical fiber cable to obtain the structure to be measured. Can be detected. If the direction of the stress applied to the structure to be measured is known in advance, one optical fiber cable may be attached along the direction. However, if the direction of applying the stress is unknown, the optical fiber cable may be used. May be attached in a rosette shape, for example.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a diagram showing a procedure for measuring a stress acting on an existing concrete structure 1 by the method of the present invention. First, as shown in FIG. 1A, optical fiber cables 2, 3, and 4 are attached to a surface 1A of a concrete structure 1 whose stress state is to be measured.
If the direction of the stress acting on the concrete structure 1 is known in advance, only one optical fiber cable needs to be attached corresponding to the direction. However, if the direction of the acting stress is unknown, FIG. As shown in (A), the optical fiber cables 2 and 3 are attached to the surface 1A so as to be perpendicular to each other, and the remaining optical fiber cables 4 form an angle of 45 ° with respect to each of the optical fiber cables 2 and 3. On the surface 1A. At this time, the optical fiber cables 2, 3, and 4 are attached so that the one ends 2A, 3A, and 4A of the optical fiber cables 2, 3, and 4 approach each other near the corner 1B. The sticking angle may not be the above angle as long as the principal stress can be calculated.

Next, as shown in FIG. 1B, the optical fiber cables 2, 3A, 4A are placed near the respective ends 2A, 3A, 4A of the optical fiber cables 2, 3, 4 using a hand drill or a disc sander. A part of 4 is drilled together with the concrete to form a drilled hole 5, whereby the stress acting on the concrete structure 1 is released. Here, since the diameters of the optical fiber cables 2, 3, and 4 are all about several mm, even if the diameter of the hole 5 is as small as about 10 mm, it is sufficient for detecting a change in strain due to stress release. That is, the hole or the notch groove can be smaller than before. In FIG. 1B, a part of the optical fiber cables 2, 3, and 4 was drilled together with the concrete structure 1, but together with a part of the optical fiber cables 2, 3, and 4, a groove was formed on the surface 1A of the concrete structure 1. May be formed so that the stress acting on the concrete structure 1 is released.

When the stress acting on the concrete structure 1 is released in this way, each of the optical fiber cables 2, 3, and 4 has a strain corresponding to the direction of the stress acting on the concrete structure 1. Changes occur. In the present embodiment, the tensile stress acting on the concrete structure 1 is released, and each optical fiber cable 2,
3 and 4 illustrate examples in which the shrinkage of the concrete structure 1 causes a local (local) change in the axial strain amount.

FIG. 2A is an enlarged view of the optical fiber cable 2 in the state of FIG. 1B. One end 2A of the optical fiber cable 2 is cut simultaneously with the drilling of the concrete structure 1 so that the cut surface 2X
Is formed, and a concrete structure 1
The axial strain change portion (unstrained portion) 2Y for the contraction of the concrete structure 1 due to the release of the stress acting on the concrete structure 1 is formed.

The optical transmission characteristic of the optical fiber cable 2 at the strain change portion 2Y in the axial direction is measured by the optical measuring device 10, so that the stress acting on the optical fiber cable 2, that is, the stress acting on the concrete structure 1 is reduced. Measure.

The optical measuring device 10 has a light source 11 for outputting coherent light and a light receiver 12. The light source 11 is optically connected to the other end 2 B of the optical fiber cable 2 via an optical path 13. A half mirror 14 is provided in the middle of the light path 13, and the light reflected from the optical fiber cable 2 and returned is incident on the light receiver 12.

Now, when the test pulse light PL is output from the light source 11, the test pulse light PL enters the optical fiber cable 2 from the other end 2B of the optical fiber cable 2 through the optical path 13. A part of the incident test pulse light PL is reflected as the backscattered light SL while propagating in the optical fiber cable 2 and returns to the other end 2B of the optical fiber cable 2. At this time, the axial distortion changing section 2Y
In this case, the transmission loss of light becomes large, and when the light pulse passes through the light pulse, the power of the light signal is greatly attenuated. At the cut surface 2X of the optical fiber cable 2, Fresnel reflection by the test pulse light PL is generated, and a large level of reflected light is generated.

Since the speed Vt at which the test pulse light PL travels in the optical fiber cable 2 is known in advance, the instantaneous instantaneous backscattered light SL after the test pulse light PL enters the optical fiber cable 2 is received by the light receiver. 12, the level change of the backscattered light SL generated at each point of the optical fiber cable 2 is measured by the analyzer 15 as a continuous distribution characteristic. This makes it possible to measure how the level of the reflected light changes at each point of the optical fiber cable 2. If there is no discontinuity in the optical fiber cable 2, the light transmission loss in the optical fiber cable 2 is uniform, and the intensity of the reflected light is the other end 2B.
It is proportional to the distance from.

In FIG. 2B, the horizontal axis is the distance L from the other end 2B of the optical fiber cable 2 to the cut surface 2X, and the vertical axis is reflected at each point in the length direction of the optical fiber cable 2. 9 is a graph showing the power level PW of the backscattered light SL returned. If the optical fiber cable 2 does not have the axial distortion change section 2Y, the power level PW becomes a linear function of the distance L. However, since the optical fiber cable 2 has the axial distortion changing section 2Y, the level of the optical signal passing through the axial distortion changing section 2Y is greatly attenuated. In FIG. 2B, the axial strain change unit 2
It can be seen that a distortion loss occurs at a portion corresponding to Y.

By measuring the amount of the strain loss in the analyzer 15, the stress acting on the optical fiber cable 2, that is, the value of the stress acting on the concrete structure 1 can be estimated. As described above, there is a section in which strain is completely released from the stress flow field very close to the drilled hole, and the feature of the present invention is to detect a change there.

The optical fiber cable 2 has been described above. The same measurement is performed for the other optical fiber cables 3 and 4, and the respective optical fiber cables 2, 3, and 4 are measured.
By combining the stresses acting on the concrete structure 1, the stress acting on the concrete structure 1 can be measured.

As described above, after attaching the optical fiber cable to the surface 1A of the concrete structure 1, one end of the optical fiber cable is cut along with the concrete structure 1 by drilling. Is measured as the light transmission loss, the stress acting on the concrete structure 1 can be measured. Therefore, unlike the case of the conventional wire strain gauge, the hole or the notch groove can be small, and the damage to the proof stress of the concrete structure 1 can be minimized. Further, drilling or the like is performed again on the side close to the optical measurement device 10, and the resulting distortion of the optical fiber cable can be measured as described above. Measurement data can be collected. Since the size of the holes or grooves to be formed in the concrete structure 1 for measurement can be small, even if the number of measurement points is increased, the adverse effect on the concrete structure 1 is small, and the damage to the concrete structure 1 is increased. The measurement accuracy can be improved without performing.

In the above embodiment, the case where the stress acting on the concrete structure is measured has been described. However, the present invention is not intended only for the concrete structure, and is not limited to the concrete structure. The stress acting on all other materials can be similarly measured using an optical fiber cable.

[0022]

According to the present invention, as described above, the optical fiber cable is attached to the surface of the structure to be measured, and one end thereof is cut together with the structure to be measured by drilling or groove-like cutting. Sometimes, the amount of change in the axial strain generated in the optical fiber cable is measured as light transmission loss, so unlike the case of measurement using a conventional wire strain gauge, a hole or a notch groove is small,
Damage to the proof stress of the structure to be measured can be minimized. Furthermore, after the optical fiber cable is once cut, it is cut again, and the resulting distortion of the optical fiber cable can be measured.
Many measurement data can be collected using the optical fiber cable. Thus, the size of the holes or grooves to be formed in the structure to be measured for measurement can be small, so that even if the number of measurement points is increased, the adverse effect on the structure to be measured is small, and The number of measurement points can be increased and the measurement accuracy can be improved without increasing the damage of the measurement.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram for explaining a procedure for measuring a stress acting on an existing concrete structure by a method of the present invention.

FIG. 2 is an explanatory diagram for explaining a method for measuring an acting stress from a change in light transmission characteristics due to distortion of the optical fiber cable shown in FIG. 1;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Concrete structure 1A Surface 2, 3, 4 Optical fiber cable 2A, 3A, 4A One end 2X Cutting surface 2Y Axial strain change part 5 Drilling hole PL Test pulse light SL Backscattered light P Bending loss

Claims (1)

[Claims] 1. A method for measuring a stress acting on a structure, comprising attaching an optical fiber cable to a surface of a structure to be measured, and drilling a part of the optical fiber cable together with the structure to be measured. Or a groove-shaped cut is provided together with the structure to be measured to release the stress, and the stress acting on the structure to be measured is obtained from the change in the optical characteristics of the optical fiber cable caused by the release of the stress. A method for measuring a stress acting on a structure, characterized in that: