Fiber-optic systems for technical monitoring of construction structures.
Authors:
Egorov F.A., Ph.D. physics and mathematics sciences; Pospelov V.I., Ph.D. technical sciences; Bykovsky V.A.; Neugodnikov A.P.
In parallel with the rapid growth of construction technologies, the business sector represented by companies that specialize in automation of building management is actively expanding.
At the same time, the main goals pursued by building automation systems are resource conservation, safety and comfort.
Obviously, of the goals presented, safety ranks highest in importance.
And here a paradox arises in the very approach to ensuring safety, since the primary system — the system for monitoring the structural state, or more precisely, monitoring the stress-strain state of the building — is, as a rule, not represented in the “gentleman’s set” of control systems.
Basic monitoring system
The need to create monitoring systems for construction structures today does not require evidence or justification. Moreover, in Russia and abroad there is an active process of development of various technical monitoring systems for construction structures.
The main question that can be defined as the most important in this area is why the existing monitoring systems have not become a tool that reliably and effectively monitors destructive changes in structures?
The question is far from simple, and the answer to it should be sought in a complex of reasons.
We list the main aspects influencing the development of such systems:
• technical — determination of the physical and technical principles on which the measuring system is based ;
• technological – development of methods and methods for the production of components, system installation and operation;
• economic – optimization of the price parameters of the system.
Obviously, the main reason for the lack of a standard monitoring system lies in the fact that most systems being developed are limited to considering one or a few control parameters.
In this case, the control of specific parameters is based on sensors of various types.
Since any measuring system necessarily has two main components: a physical quantity converter and an electronic processing unit, the lack of uniformity in each of these parts greatly increases the diversity of monitoring systems and, as a consequence, reduces the possibility of creating a standard system.
Information from a large number of different types of measuring systems requires the creation of a complex processing system.
On the other hand, system automation specialists develop increasingly complex technologies without worrying about optimizing physical control procedures.
At the same time, specific systems are created for various tasks, designs and control and measurement conditions.
As a result, a mass of independent systems of different operating principles appears; when a new task occurs, a new system appears, etc.
Therefore, it is necessary to create a monitoring system in the basic version, which should provide control of the main parameters responsible for the most common causes of potential accidents.
If necessary, the basic version should be expandable, both in the number of control points and in the types of controlled structures, as well as in the list of controlled parameters.
If necessary, the basic monitoring option should be able to be supplemented with control and measuring equipment based on other physical principles.
The set goal can be achieved by solving the following problems (Table 1).
| Substantive formulation of the problem. Net Web portal (Starwood information platform and systems) | Technical statement of the problem |
| Diversity building structures, technologies and projects | Development of the most unified monitoring system |
| Different types of existing sensors and control and measuring systems, disparate from the point of view of automation, signal processing and system diagnostics. | Definition of a basic (reference) monitoring system based on a single physical principle, supplemented, if necessary, by other types of control and measuring systems |
| The dominance of reinforced concrete elements in the modern construction industry | Development of a monitoring system with sensors that can be installed inside a reinforced concrete product, while having high stability and accuracy |
| Possibilities for rapid modification of the monitoring system for various tasks | The monitoring system must have maximum flexibility of the measuring unit, combined with the versatility of the electronic unit. This is realized in the case of using sensors of a single physical principle |
Monitoring system based on fiber-optic sensors “Monitoring-Center”
Table 2. Technical characteristics fiber-optic sensors «Monitoring-Center».
| Parameter | Fiber optic ground pressure sensor | Fiber optic strain sensor</td > |
| Range of measured parameters | 0х10 kgf/cm2 | Relative strain — 0h2•10-2 |
| Measurement error | 2% | 1.5% |
| Sensitivity threshold </td > | 0.2 kgf/cm2 | 10 μ |
| Power consumption of the signal meter | No more than 2 W | No more than 2 W |
| Operating temperature | -20…+60°C | -30…+60°C |
| Corrosion resistance | yes | yes |
| Humidity during operation | 0…100% | 0…100% |
| Service life | At least 10 years | At least 10 years |
| Overall dimensions of the sensor housing | Diameter – 180 mm, thickness – 20 mm | 60×44×14 mm |
| Availability of power supply in the primary converter |
absent </u > | absent |
To achieve the goal of developing an optimal construction monitoring system , defined by a list of tasks, an instrumentation system based on fiber-optic sensors is proposed.
Let us formulate the basic principles of this system.
The basic sensor used in the monitoring system is a fiber-optic strain sensor.
The sensor has several design options, allowing it to be poured into a reinforced concrete structure or mounted on the surface of building elements.
Installation of sensors at points of potential sources of destruction (heavy loads, moments) is regulated at the project stage.
Control can be carried out as in during installation and during operation of the structure.
The electronic signal processing unit receives constant information about the state of the structure at internal and external control points.
Comparison of this information with design data on an ongoing basis allows us to draw conclusions about the “health” of the structure.
In this case, the analysis is carried out by comparing the results of numerical modeling of the state of the structure with actually measured data, which are included in the calculation.
The result obtained allows us to understand how the building as a whole was deformed. It is important to note that information is obtained only from local deformations, and conclusions can be drawn from changes in the building as a whole.
Additionally, the system contains a fiber-optic temperature sensor, the miniature size of the sensitive element of which allows it to be mounted in the most inaccessible places, including, again, inside reinforced concrete structures.
For critical objects or in cases where additional control is necessary, the number of sensors is increased, the system is supplemented with other sensors (both fiber-optic and traditional).
The electronic signal transmission and processing unit used in the system has a unified structure.
Signal transmission can be carried out both through fiber-optic communication channels and through existing electrical networks (which does not require additional work on the equipment of communication channels), as well as in a wireless format.
A monitoring system based on fiber-optic sensors has the important property of clarity of the physical principle, which is extremely important from the point of view of large-scale implementation.
In addition, fiber optic sensors are an example of the safest sensors that guarantee explosion and fire safety, since they do not contain electrical circuits or signals. In addition, fiber-optic sensors are not affected by electromagnetic fields and do not induce them.
Fiber-optic construction monitoring systems, having high accuracy and “unpretentiousness” in terms of stability, durability and operating mode in difficult operating conditions ( for example, the possibility of embedding in reinforced concrete structures), in many cases they have no competition as a tool for monitoring the level of safety.
Such systems protect houses and structures from possible emergency events, without making a difference between man-made or natural causes of their origin.
The operator’s console receives a signal that the specified boundaries of controlled deformation or temperature have been exceeded, and then experts analyze the information received about the “health” of the building, predict the development of the situation and make recommendations for developing the best solution.
A recorded emergency event at the very beginning of development is the real safety that a fiber-optic construction monitoring system creates.
Implementation of a fiber-optic monitoring system at a high-rise facility
At the moment, the monitoring system described above based on fiber-optic sensors is being installed at a multifunctional complex under construction in Moscow.
The customer of the monitoring system was the MonArch Concern for a multifunctional business sports and recreational complex, which included an office building, a hotel and a shopping center.
The installation of the monitoring system is carried out by “Monitoring-Center”.
The monitoring system is a proprietary development of Monitoring-Center, and the sensors are certified and mass-produced. Scientific guidance is provided by the head of the Department of Soil Mechanics, Foundations and Foundations of MGSU, Professor Z.G. Ter-Martirosyan.
As an object of control, the designers identified the most complex block of the multifunctional complex — an office block, the height of the above-ground part of which is 33 floors, and the underground part includes 3 floors.
As a result of a thorough analysis of the geotechnical parameters of the soil in combination with a complex design solution, it was determined to install 125 sensors on the office block according to the following scheme:
• 24 ground pressure sensors along the base of the foundation (type “M” sensors)
• 21 sensors for reinforcement deformation in the foundation (F type sensors)
• 80 strain sensors in vertical elements (type “B” sensors), consisting of:
• in 10 pylons, 2 sensors each on the “minus” 3rd floor;
• in 10 pylons with 2 sensors each on the “minus” 1st floor;
• in 10 pylons with 2 sensors each on the 3rd floor;
• in 10 pylons with 2 sensors each each on the 18th floor.
Fiber-optic ground pressure sensors are positioned in such a way that 2 monitoring tasks can be solved:
• local pressure control at a given point;
• global control of pressure distribution over the foundation slab.
Fiber-optic strain sensors installed in the foundation slab, also, according to the decision of the scientific supervisor of the monitoring, must solve 2 problems: local and global.
As a local monitoring tool, the strain gauge records the degree of elongation or compression of the reinforcement, which, after normalizing to the sensor base, can be converted into stresses using Hooke’s law and compared with the calculated values.
Based on the data obtained as a result of regular registration sessions, we can give an overall assessment of the situation as satisfactory. In general, reinforcing bars operate in the mode “prescribed” by calculations; analysis of individual data for each sensor gives a good correlation with the standards.
The pressure distribution along the base of the foundation provides interesting information about how the slab is loaded.
These data require detailed analysis. But there is already some correlation with strain sensor data.
First of all, we cannot exclude the influence of a wall in the ground, which certainly puts pressure on the structure of the building being erected, and the resulting one, being laid out in the vertical and normal directions, can generate additional forces both in terms of deformation of the slab and in terms of pressure on the ground.
Today, development work on the creation of a fiber-optic sensor for wind loads on facades is being completed at the Monitoring Center research laboratory.
Upon completion of this work, after finalizing the design documentation and carrying out the appropriate patent and certification activities, the authors believe to offer the construction sector a complete line of fiber-optic measuring systems.
Based on such a line of measuring systems, it will be possible to create the most complete and effective monitoring system for the main technical parameters of high-rise and multifunctional buildings.