The health monitoring of large bridge structure is actually a multi-parameter monitoring (including temperature, stress, displacement, dynamic characteristics, etc.). The so-called health monitoring technology of large-scale bridge structure refers to the real-time online measurement of various reactions of large-scale bridge structure during operation by using some test components, test systems and test instruments set in key parts of large-scale bridge structure, and through a series of programs such as on-site acquisition of test data, remote transmission of data and instructions, data storage and processing, structural safety state evaluation and early warning. The safety situation of large bridge structure is analyzed, and its ability to bear static and dynamic loads and structure are evaluated.
The health monitoring technology of large bridge structure involves many cross fields. With the rapid development of modern detection technology, computer technology, communication technology, network technology, signal analysis technology and artificial intelligence, the health monitoring technology of large bridge structures is developing towards real-time, automation and networking. At present, the health monitoring system, which contains a variety of detection contents, can monitor the state of large bridges in real time, and integrates remote communication and evaluation control, has become the frontier of the development of health monitoring technology for large bridges.
The health monitoring technology of large-scale bridge structure mainly includes the overall design technology of monitoring system, sensors and their optimal layout technology, automatic data acquisition and transmission technology, structural simulation analysis technology, health diagnosis and structural safety assessment technology.
Second, the overall design technology of large-scale bridge structure health monitoring system
Large-scale bridge structural health monitoring system is a comprehensive monitoring system integrating structural monitoring, system identification and structural evaluation. Various advanced test instruments and equipment are usually used to monitor the response of large bridges under various external stimuli (including traffic loads and environmental loads); Then, the monitoring information is processed, and the structure is diagnosed and the damage state of the structure is analyzed by combining the knowledge of structural model. Finally, the health status of large-scale bridge structures is evaluated, and scientific bridge maintenance and maintenance strategies are determined. The monitoring contents generally include
1) The response of physical and mechanical properties of large-scale bridge structure under normal environment and traffic conditions, including internal force (stress), deformation, natural frequency, vibration mode, concrete carbonation, steel corrosion, etc.
2) the working conditions of important non-structural components (such as bearings) and ancillary facilities of large bridges;
3) Environmental conditions of the bridge, etc.
The structural health monitoring of large bridges uses modern sensing and communication technology to monitor the structural response and behavior of large bridges under various environmental loads in real time. For the monitoring system design of a specific large bridge, the corresponding monitoring method, content, scale and monitoring effect are different due to its own structural characteristics and monitoring emphasis, but generally the following design criteria should be followed:
1, system function requirements
Different functional objectives require different monitoring items. The monitoring items of long-span and large-scale bridge structure monitoring system are mostly based on structural monitoring and evaluation. If the monitoring system has the function of structural design verification, it needs to obtain more information needed for structural system identification. Generally speaking, for long-span cable-supported bridges, it is necessary to arrange more sensors in the tower, stiffening beam and cable to obtain more detailed structural dynamic behavior and verify the dynamic analysis model and response prediction in structural design.
In addition, sensors should be installed on brackets, stops and some connecting parts to obtain information reflecting their force transmission and constraint conditions. Therefore, the function of large-scale bridge structure health monitoring system should consider the following main aspects:
1) structural integrity: including studying the nonlinear characteristics of the structure under the action of vehicle-bridge interaction, strong wind and strong ground motion, and the influence of the change of bridge site environmental conditions on the dynamic characteristics and static (internal force distribution and deformation) of the structure.
2) Structural local problems: such as boundary, connection conditions, fatigue of steel beam weld, etc. Failure mechanism of composite beam interface: vibration local damage mechanism of cable (cable) and suspender of cable-supported bridge.
3) Anti-seismic: including the temporal and spatial variation of ground motion at each site, structural interaction and the influence of multi-support excitation on structural response, etc. By monitoring the strain, deformation and acceleration of pier top and bottom, the seismic analysis of large bridges is carried out.
4) Wind resistance: it includes observing the characteristics of wind field, the behavior of the structure in natural wind field and wind resistance stability.
In addition, attention should also be paid to structural durability, deformation law of foundation and bearing capacity of pile foundation.
2. Benefit/cost analysis
The design of monitoring system should first consider the purpose and function of establishing the system. For a specific large-scale bridge, the purpose of establishing a structural health monitoring system can be to monitor and evaluate the large-scale bridge, to verify the design, or even to develop. Once the purpose of establishing the system is determined, the monitoring items of the system can be basically determined and the design requirements of its functions can also be determined. However, in the process of monitoring system design, the scale of each monitoring project and the determination of sensing instruments and communication equipment used need to consider the investment limit, so the cost/benefit analysis of monitoring system scheme must be carried out when designing monitoring system. According to the functional requirements and cost/benefit analysis, the number of monitoring items and measuring points is designed to the required range, so as to optimize the selection and installation of system hardware facilities.
Third, the sensor and its optimal layout technology
The selection of sensors mainly considers the following factors: the selection of sensor type, the accuracy, resolution, frequency response and dynamic range of sensors; The layout position of the sensor and its influence on the surrounding dynamic environment, the influence of measurement noise, etc.
Sensors used in health detection and monitoring of large bridges are mainly used to measure parameters such as acceleration, speed, displacement and strain. Because of the huge size of large bridges, the natural frequency is often very low, and the response level of the structure is usually very small, so the sensor is required to have the characteristics of wide frequency response range, good low frequency response and large measurement range. Traditional sensors include piezoelectric force sensor, acceleration sensor, impedance sensor, strain gauge and so on. They have been widely used in the measurement of various engineering structures, so I won't repeat them here.
At present, the emerging sensors mainly include fatigue life wire, piezoelectric material sensor, carbon fiber, semiconductor material and optical fiber sensor.
Optical fiber sensor is an advanced sensor with the vigorous development of optical fiber communication technology, and it is the most ideal material for long-term monitoring. Its main performance features include:
1) has dual functions of perception and transmission; Anti-electromagnetic interference, electrical insulation, corrosion resistance, intrinsically safe and reliable, and good durability; High sensitivity; Light weight, small size, flexibility and little influence on the measured medium;
2) it is convenient for multiplexing and networking, and it is beneficial to form telemetry network and optical fiber sensing network with existing optical communication technology;
3) Wide measuring range. It can measure temperature, pressure, strain, stress, flow, speed, current, voltage, liquid level, liquid concentration, composition and so on.
Fourthly, the overall design of large-scale bridge structure health monitoring system.
Modern large-scale bridge structural health monitoring technology is not only a simple improvement of traditional large-scale bridge detection technology, but also uses modern sensing and communication technology to monitor the structural response and behavior of large-scale bridges in various environmental conditions in real time, and obtain all kinds of information reflecting the structural state and environmental factors, so as to analyze the structural health state and evaluate the structural reliability, and provide scientific basis for the management and maintenance decision of large-scale bridges.
1 Establishment of monitoring system
2 design principles of monitoring system
1) Principal and auxiliary principles of purpose and function
The design of monitoring system should be based on the purpose and function of establishing the system. After the purpose of establishing health monitoring system is determined, the monitoring items and instrument system of the system can be basically determined. Generally speaking, the main purpose of establishing a large-scale bridge health monitoring system is to master the operational safety of the structure, so the design of the health monitoring system should first consider the monitoring principle of structural safety, which is the key monitoring content that can be related to structural safety, and other purposes are auxiliary.
2) Function and cost optimization principle
The cost of health monitoring system is usually relatively large, which generally consists of three parts: structural simulation analysis cost, instrument system cost and processing software cost. The cost of structural simulation analysis is generally small, but it is of great significance. Instrument system is the main part of the cost of health monitoring system. The more monitoring items and sensors, the more comprehensive the monitoring information and the higher the system cost. On the contrary, it will reduce the system cost, but at the same time, it may also reduce the effectiveness of monitoring data because of insufficient monitoring information. Therefore, in order to make the system cost more reasonable, we must optimize the function and cost, and get the maximum effective monitoring information with the minimum investment. The cost of information processing software, whose main function is to explain, store, transmit and preliminarily evaluate a large amount of information,
This part of the cost is relatively small.
3) the principle of systematicness and reliability
Monitoring analysis, simulation calculation and engineering experience are organically combined. Only by applying the principle of system analysis can the measuring points and monitoring items be combined with each other and the monitoring efficiency of the whole system be improved. The most basic requirement of monitoring system is reliability, and the reliability of the whole system depends on the reliability of various instruments, the overall layout of monitoring network and design, and the cooperation in construction.
4) The principle of giving priority to key points and giving consideration to all aspects.
Key components refer to the areas, deformation sensitive areas and key parts that may cause structural damage due to various reasons, and these key components must be monitored emphatically. But we should also consider comprehensiveness and monitor the structural integrity, such as the overall safety monitoring of the foundation.
5) The principle of combining real-time monitoring with regular monitoring.
After determining the monitoring items according to the monitoring purpose, function and cost optimization, the principle of setting real-time monitoring and regular monitoring separately should be considered. Due to different monitoring projects, some projects do not need long-term real-time monitoring, but their monitoring frequency is much higher than manual monitoring. At this time, regular monitoring can be considered to reduce the maintenance cost and data processing pressure in the later period.
Traffic is the economic lifeline of a country, and large bridges are the throat of traffic. The construction and maintenance of large bridges is an important part of a country's infrastructure construction, and it is also a sign of economic development and technological progress. This paper briefly analyzes the design of large-scale bridge health system, hoping to help peers.
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