RU2808937C1 - Method for determining absolute strains and stresses with mechanical strain gauge - Google Patents

Abstract

FIELD: construction.

SUBSTANCE: invention relates to assessing the technical condition of building structures and can be used to determine mechanical stresses, for example, in the frames of buildings and structures, structural elements of bridges. The essence of the invention is to determine mechanical stresses in structural elements, a mechanical strain gauge with a removable strainmeter is installed on them. It is used to determine the absolute deformations that occur in structural elements from the application of an external load. Based on absolute deformations, stresses in structural elements are calculated, allowing to judge their load-bearing capacity (strength and stability).

EFFECT: increasing the reliability of measurements and determination of parameters of the stressed state of structures.

1 cl, 23 dwg

Description

The invention relates to the field of assessing the technical condition of building structures and can be used to determine mechanical stresses, for example, in the frames of buildings and structures, structural elements of bridges. The essence of the invention is that to determine mechanical stresses in structural elements, a mechanical strain gauge with a removable strainmeter is installed on them. It is used to determine the absolute deformations that occur in structural elements from the application of an external load. Based on absolute deformations, stresses in structural elements are calculated, allowing one to judge their load-bearing capacity (strength and stability). Measurements using a mechanical strain gauge are independent of temperature. When using a mechanical strain gauge with a removable strainmeter, which has special tips that allow it to be securely fixed in the measuring device, and the strainmeter scale is rearranged in such a way that the zero report is transferred to the middle of its scale, which allows you to measure both compressive and tensile absolute deformations and obtain stresses in structural elements.

The technical result of the invention is to increase the reliability of measurements and determination of parameters of the stressed state of structures, which affects their reliability.

The invention relates to the field of assessing the technical condition of building structures and can be used to determine mechanical stresses, for example, in the frames of buildings and structures, structural elements of bridges.

There is a well-known mechanical strain gauge of the Hugenberger system, developed at the beginning of the 20th century, which makes it possible to determine absolute deformations in structural elements from external loads acting on it and is the closest prototype to our invention. It uses the principle of an unequal lever to increase small deformations of the top layer of the tested element to movements of the end of the arrow visible to the naked eye. It consists of a fixed and movable prism, rigidly connected to the lever, a horizontal rocker arm, which transmits movements to a arrow attached to the fixed lever, which records the absolute deformations of the structural element. The base of the strain gauge can be changed within the range of 20-250 mm using a special extension included with the device. There is a mirror on the scale of the device, which serves to achieve a constant position of the observer’s eye at different readings. When taking a reading, the image of the arrow in the mirror is combined with the arrow; in this case, the observer’s gaze is constantly perpendicular to the instrument scale, which reduces the error when taking a reading.

The disadvantages of the system for recording absolute deformations using a mechanical Hugenberger strain gauge include:

1. mechanical Hugenberger strain gauges require careful installation of the prisms (without excessive pressure, but with a sufficiently tight connection to the structure), they must be handled with care; during measurements, you must not touch the devices to avoid moving them from their original position;

2. they allow you to obtain absolute deformations only from the moment of their installation and do not allow you to obtain the full stress in the structural element;

3. to measure deformation (stress), the device must be constantly located on the structural element under study;

4. The proposed measurement system gives reliable results only in laboratory conditions.

There is a known method for measuring absolute deformations using an electromechanical Aistov strain gauge (author's certificate No. 34797 in 1934), which is an elastic steel element attached to the tested structure with embedded parts, and deformed together with it. A strain gauge using a micrometer screw and a dial attached to it for reading the angle of rotation when closing the electrical signal circuit, characterized in that a plate is used to measure the deformation, tightly fastened to a prism, for example, by means of a frame, and which serves to establish contact when screwing the micrometer screw. The resulting absolute strains are measured using ohmic resistance wire strain gauges. The electromechanical strain gauge of the Aistov system, when carrying out measurements, is deformed together with the structural element under study and converts the deformation into a variable electrical parameter, which is recorded by a recording installation, and the installation scale is calibrated in units of deformation.

The disadvantages of the system for recording absolute deformations using the Aistov electromechanical strain gauge include:

1. such a measurement system allows you to obtain absolute deformations only from the moment of their installation and does not allow you to obtain the full stress in a structural element for measuring deformation (stress);

2. the device must always be located on the structural element under study;

3. Taking readings on the disk scale by rotating it manually increases the measurement error.

4. Visual readings on the scale of each device require additional staff and time. The danger of damage to strain gauges during the collapse of a structure requires their timely removal and eliminates the possibility of conducting observations until the moment of destruction.

5. the need to use a sound signal and the impossibility of taking readings without touching the device.

6. the proposed measurement system gives reliable results only in laboratory conditions;

7. the use of ohmic resistance sensors in this measurement system adds the disadvantages inherent in these sensors (the influence of temperature errors, etc.);

8. the need to use recording equipment and a large number of wires to connect the ohmic resistance sensor to it.

9. possibility of accidental damage to connecting wires with recording equipment

There is a known method for measuring absolute deformations using string strain gauges (author's certificate No. 308314, published 07/01/1971, Bulletin No. 21). A string strain gauge for measuring deformations, containing a cover, a string with string holders fixed on the surface of the test part, and a string oscillation exciter made in the form of an electromagnetic coil with a ferromagnetic core, characterized in that, in order to simplify the design and manufacturability of the strain gauge, the cover is made of plastic with a coil reinforced inside it with a core located perpendicular to the string.

Unlike previous prototypes, they are a more reliable and durable sensor for measuring strains and stresses directly on structural elements. String strain gauges consist of a solid cylindrical body and two anchors, between which a tensioned string is mounted. To excite the string with an electromagnetic field pulse and create an alternating EMF from its own oscillations, an electromagnetic head installed in the middle of the string is used. The deformation of the element under study is transmitted to the string through the anchors, changing its tension and frequency of natural vibrations. Based on the changed period of oscillation of the string, using the individual calibration dependence of the elongation of the string on the frequency of its oscillations, the magnitude of the relative deformations of the strain gauge base is determined. To excite vibrations, a device placed next to the string is used in which the vibrations of the string induce an alternating current of the same frequency, which is noted by a special recording device. To eliminate the influence of temperature on the obtained measurement results, use a special compensation device, which is placed next to the study site, so that deformations do not affect it,

The disadvantages of this measurement method include:

1. the device must always be located on the structural element under study;

2. use of a compensation device in connections to eliminate the influence of temperature on the string sensor;

3. the need to use recording equipment and a large number of wires to connect the string strain gauge to the recording equipment.

4. the possibility of accidental damage to connecting wires with recording equipment.

The proposed method for measuring absolute strains (stresses) eliminates all the disadvantages inherent in the prototypes described above.

The method for measuring absolute strains (stresses) is implemented as follows.

To obtain the values of the stress state parameters, a mechanical strain gauge with a removable strain gauge based on a dial indicator is used, which is adjusted in such a way that the beginning of its scale is shifted to the middle of the measurement scale. This makes it possible to obtain values of absolute deformations (stresses), both in compressed and tensile structural elements, directly during measurements. The measurements obtained by a mechanical strain gauge are independent of the effects of temperature and are automatically included in the measured stresses.

Disclosure and implementation of the method with examples

Measurement of stress and absolute strain is carried out using a mechanical strain gauge with a removable strain gauge (Fig. 2). It consists of three angles welded to the structural element at the measurement location. The two extreme corners are located at a distance from each other (the base of the measuring device), and the third serves to support the rod of the mechanical strain gauge. The strain gauge consists of a rod in the form of a solid rod, which has a thread on one side, with the help of which and two nuts it is attached to one of the extreme corners and a bolt, which is attached with two nuts to the other extreme corner. The rod and bolt have pressed balls at their ends. To measure stresses and absolute deformations, a removable strainmeter equipped with special tips is inserted between the bolt and the rod.

Specialists from third-party organizations attempted to measure stresses without using a removable strainmeter, but using a caliper equipped with an electronic measurement scale. These attempts were unsuccessful due to the impossibility of reliable installation of the rod elements on the balls of the rod and the bolt of the mechanical strain gauge

Examples of using a mechanical strain gauge to determine stresses in structural elements

The proposed measurement system has been tested at several significant facilities in our country. It allows you to obtain reliable and reliable data on the stresses that arise in structural elements, including the increased level of responsibility provided for by the federal law of our country FZ-384 “Technical Regulations on the Safety of Buildings”.

As examples of stress determination, we give the use of this stress determination system at the following objects:

1. Long-span BSA covering of the Luzhniki stadium in Moscow.

2. Long-span transformable covering of the Gazprom Arena stadium in St. Petersburg.

3. Spacer system for the foundation pit of a multi-storey complex near the Kursky railway station in Moscow.

4. Radio-television tower under construction in Vladikavkaz.

Examples of using the stress measurement system at the Luzhniki Stadium are shown in Fig. 9, at the Gazprom Arena stadium, in fig. 10,11, on the expansion pit system in Fig. 12,13 and on the radio-television tower under construction in Vladikavkaz in Fig. 14.15,

Submitted drawings:

Fig. 1. General view of the caliber

Fig. 2. Calibration of the strainmeter using a gauge

Fig. 3. Mechanical strain gauge diagram

Fig. 4. Node 1

Fig. 5. Node 2

Fig. 6. Node 3

Fig. 7. Node 4

Fig. 8. Node 5

Fig. 9. Node 6

Fig. 10. Removable strain gauge Front view

Fig. 11. Removable strain gauge Side view

Fig. 12. Calibration of the strain gauge strain gauge before measurement

Fig. 13. Installation of a strain gauge in a mechanical strain gauge

Fig. 14. Measurement of strains (stresses) with a mechanical strain gauge

Fig. 15. Removing the strain gauge from a mechanical strain gauge

Fig. 16. Removable strain gauge

Fig. 17. Measurement of deformations (stresses) on the coating elements of the Grand Sports Arena of the Luzhniki Stadium

Fig. 18. Mechanical strain gauge installed on the internal contour of the coating of the Gazprom Arena stadium in St. Petersburg"

Fig. 19. Measurement of deformations (stresses) on elements of the transformable coating of the Gazprom Arena stadium in St. Petersburg"

Fig. 20. Measurement of deformations (stresses) on the elements of the expansion pit system of the Chkalov multifunctional complex in Moscow

Fig. 21. Measurement of deformations (stresses) on one of the elements of the expansion pit system of the Chkalov multifunctional complex in Moscow

Fig. 22. Installation of a mechanical strain gauge on one of the elements of a radio and television tower under construction in Vladikavkaz

Fig. 23. Measurement of deformations (stresses) on one of the elements of a radio and television tower under construction in Vladikavkaz

Legend

1 – node 1 2 – node 2 3 – node 3 4 – node 4 5 – node 5 6 – node 6 7 – penetration weld points

Claims (1)

A method for determining absolute strains and stresses with a mechanical strain gauge, which consists in the use of a mechanical strain gauge, characterized in that the strain gauge is prepared by installing three corners, and on two of them a bolt with a pressed-in ball is installed and a rod made of a round rod, where a ball is pressed into the end of the rod, while the third corner supports the rod, and the strainmeter based on the dial indicator is installed between the balls and tips are installed at the ends of the dial indicator, and the zero mark of the indicator is shifted to the center of the scale, then the strainmeter at the end of the measurements is removed from the strain gauge, while the strainmeter is calibrated before each measurement .