Firstly, the development and theory of bridge health monitoring system.
1. Monitoring system
In the middle and late 1980s, various bridge health monitoring systems were established one after another. For example, in Britain, sensors are laid on foyle Bridge, a 522m-m-long three-span continuous steel box girder bridge with variable height, to monitor the vibration, deflection and strain response of the main girder under the action of vehicles and wind loads during the bridge operation, and to monitor the ambient wind and structural temperature field. This system is one of the earliest and more perfect monitoring systems, which realizes real-time monitoring, real-time analysis and data network sharing. Typical bridges for establishing health monitoring system are Skarnsundet cable-stayed bridge in Norway (main span 530m) [2], Sunshine Skyway cable-stayed bridge in the United States with main span 440m, Great Belt East suspension bridge in Denmark with main span 1624m[3] and Flintshire single-tower cable-stayed bridge with main span 194m[4]. And the Confederate Bridge in Canada. Since 1990s, China has also established structural monitoring systems of different scales on some large and important bridges, such as Qingma Bridge, Jishuimen Bridge and Tingjiu Bridge in Hong Kong, Xupu Bridge in Shanghai and Jiangyin Yangtze River Bridge in the Mainland [6~8].
Judging from the monitoring objectives, functions and system operation of the established monitoring systems, these monitoring systems have the following characteristics:
(1) Generally, sensing devices that measure various responses of structures obtain various records reflecting structural behaviors;
(2) In addition to monitoring the state and behavior of the structure itself, it is also necessary to monitor the environmental conditions of the structure (such as wind, vehicle load, etc.). ) is also monitored, recorded and analyzed; At the same time, it tries to establish the "fingerprint" of the structure through the dynamic response of the bridge under the action of normal vehicles and wind load, and develop real-time structural integrity and safety evaluation technology;
(3) Monitor the structural status continuously or intermittently after operation, and strive to obtain continuous and complete bridge structural information. Some bridge monitoring sensors start to work at the bridge construction stage to monitor the construction quality.
(4) The monitoring system has the ability of fast and large-capacity information collection, communication and processing, and realizes data sharing on the network.
These characteristics make the health monitoring of long-span bridges different from the traditional bridge detection process. In addition, it should be pointed out that the object of bridge health monitoring is no longer limited to the structure itself: the working conditions of some important auxiliary facilities have also been included in the scope of long-term monitoring (such as stay cable vibration control device [4], etc.). ).
2. Theoretical research
For more than ten years, the research of bridge health monitoring theory has mainly focused on structural integrity assessment and damage identification. Due to the in-depth research and application of integrity assessment technology based on vibration information in aerospace, machinery and other fields, this kind of technology has been widely studied as the most important integrity assessment method in civil structures except nondestructive testing technology "1, 7,9 ~11". Another reason why people devote themselves to the study of integrity evaluation method based on vibration measurement is that in the process of bridge operation, structural vibration information can be obtained by environmental vibration method, so this method has the potential of real-time monitoring.
Structural integrity assessment methods can be divided into three categories: pattern recognition method, system identification method and neural network method "1". Structural modal parameters are often used as fingerprint features of structures, and are also the main input information of system identification methods and neural network methods. In addition, the evaluation methods based on static responses such as structural strain modes and strain curvatures also show their respective damage detection capabilities to varying degrees [10]. However, although some integrity assessment techniques have been successfully applied to some simple structures, they cannot be reliably applied to complex structures. The main reasons that hinder the application of this technology include: ① the uncertainty in structure and environment and the influence of non-structural factors; ② The measurement information is incomplete; (3) the measurement accuracy is not enough, and the measurement signal is noisy; ④ The redundancy of bridge structure is large, and the measurement signal is insensitive to local damage of the structure.
In addition, from the evaluation method, the current safety evaluation of long-span bridges basically follows the conventional grading evaluation method of small and medium-sized bridges, which is a qualitative and superficial safety evaluation mainly focusing on structural appearance and normal service performance.
Second, the new concept of bridge health monitoring
The basic connotation of bridge health monitoring is to monitor and evaluate the structural state of the bridge, trigger early warning signals for the bridge under special climate, traffic conditions or serious abnormal bridge operation conditions, and provide basis and guidance for bridge maintenance and management decisions. To this end, the monitoring system monitors the following aspects:
The physical and mechanical state of the bridge structure under normal environment and traffic conditions;
Working conditions of important non-structural components (with bearings) and auxiliary facilities (such as vibration control elements) of the bridge;
Durability of structural members;
Environmental conditions of the bridge; Wait a minute.
Different from the traditional detection technology, large-scale bridge health monitoring not only requires fast and large-capacity information collection and communication capabilities, but also strives to monitor the overall behavior of the structure in real time and intelligently evaluate the state of the structure.
However, bridge structural health monitoring is not only to monitor and evaluate the structural state. Due to the mechanical and structural characteristics of large-scale bridges (especially cable-stayed bridges and suspension bridges) and their specific environment, it is very difficult to fully grasp and predict the mechanical characteristics and behavior of structures in the bridge design stage. The design of long-span cable-stayed bridge depends on theoretical analysis. The dynamic performance of the bridge is predicted by wind tunnel and shaking table simulation tests, and its dynamic safety is verified. However, structural theoretical analysis is often based on idealized finite element discrete model, and analysis is often based on many assumptions. The simulation of bridge wind environment and ground motion in wind tunnel or shaking table test may not be completely consistent with the real bridge site environment. Therefore, it is of great significance to verify the theoretical model and calculation hypothesis of the bridge through the dynamic and static behavior of the actual structure obtained from the bridge health monitoring. In fact, some important bridges abroad emphasize the use of monitoring information to verify the structural design when establishing health monitoring systems.
The far-reaching significance of bridge health monitoring information feedback in structural design lies in that structural design methods and corresponding codes and standards may be improved; Moreover, the understanding of the real behavior of the bridge under various traffic conditions and natural environment and the reasonable modeling of environmental loads are the basis for realizing the virtual design of the bridge in the future.
It should also be noted that bridge health monitoring will not only bring a monitoring system and reflection on specific bridge design, but also become a "field laboratory" for bridge research. Although the research achievements in the field of wind and earthquake resistance of bridges and the emergence of new materials and technologies have promoted the development of bridges, there are still many unknowns and assumptions in the design of long-span bridges, and there are also many problems to be studied in the design of super-long-span bridges. At the same time, the in-depth research and development of bridge structure control and health assessment technology still need structural field test and investigation. Bridge health monitoring provides a new opportunity for the unknown problems in bridge engineering and the research of super-long-span bridges. The information obtained from bridge structure and its operating environment is not only a supplement to theoretical research and laboratory investigation, but also can provide the most authentic information about structural behavior and environmental laws. In addition, the development and application of bridge vibration control and health assessment technology need field test and investigation.
To sum up, the health monitoring of large bridges is not only a new technology of traditional bridge detection and structural evaluation, but also given the significance of structural monitoring and evaluation, design verification and research and development.
Thirdly, the design of health monitoring system.
1. Design standard of monitoring system
It can be seen from the arrangement of measuring points of two large-scale bridge health monitoring systems that there are great differences in monitoring items and scales between the two monitoring systems. In addition to environmental factors such as bridge type and bridge site, this difference is mainly due to the different investment of each monitoring system and/or the purpose of establishing each system (or the functional requirements for the system). Therefore, the design of bridge monitoring system actually follows some criteria intentionally or unintentionally.
Obviously, the design of monitoring system should first consider the purpose and function of establishing the system. The significance of three aspects of bridge health monitoring mentioned above is also the purpose and function of bridge health monitoring. For a specific bridge, the purpose of establishing a health monitoring system can be bridge monitoring and evaluation, design verification or even research and development; It can also be two or even all. Once the purpose of establishing the system is determined, the monitoring items of the system can be basically determined. In addition, the scale of each monitoring item in the monitoring system and the determination of the sensing instruments and communication equipment used need to consider the limit of investment. Therefore, when designing the monitoring system, it is necessary to analyze the cost-benefit of the monitoring system scheme. Cost-benefit analysis is the premise of establishing an efficient and reasonable monitoring system.
According to the functional requirements and cost-benefit analysis, the monitoring items and measuring points can be designed to the required range, and the hardware facilities of the system can be intelligently selected and installed. Therefore, functional requirements and benefit-cost analysis are two criteria for designing bridge health monitoring system.
2. Monitoring projects
Different functional objectives require different monitoring items. Most of the monitoring items of long-span bridge monitoring system are mainly structural monitoring and evaluation, and some also take into account structural design verification, and some even focus on bridge problems [5]. Literature [12] Based on a large number of disease investigation and detection analysis of many cable-stayed bridges in operation in China, the representative monitoring items for condition monitoring and evaluation of cable-stayed bridges are put forward.
If the monitoring system has the function of structural design verification, it needs to obtain more information needed for structural system identification. Therefore, for long-span bridges with residual bearings, more sensors should be arranged in the tower, stiffening beam and cable/cable to obtain more detailed structural dynamic behavior and verify the dynamic analysis model and response prediction in structural design. In addition, sensors must be installed on brackets, stops and some connecting parts to obtain information reflecting their force transmission and constraint conditions.
At present, one of the purposes of some monitoring systems is to develop structural integrity and safety assessment technology. Although the monitoring system combined with bridge research is rare, some systems also have special monitoring items for research. The monitoring items related to theoretical research can be determined according to the nature of the problem to be studied. Judging from the current development of bridge engineering, with the help of bridge health monitoring, the following problems can be further studied or demonstrated.
Wind resistance: including observation of wind field characteristics, structural behavior in natural wind field and wind stability.
Seismic aspects: including the study of the temporal and spatial variation of ground motion, soil-structure interaction, traveling wave effect and the influence of multi-support excitation on structural response. It is of great significance to establish restoring force model by monitoring the strain, deformation and acceleration of pier top and pier bottom for seismic analysis of bridges.
The overall behavior of the structure: including the study of the nonlinear characteristics of the structure under the action of strong wind and strong ground motion, and the influence of the change of environmental conditions at the bridge site on the dynamic characteristics and static (internal force distribution and deformation) of the structure. This is very important for developing the overall evaluation method based on monitoring data.
Structural local problems: such as boundary, connection conditions, fatigue problems such as steel beam weld fatigue, failure mechanism of composite beam interface (including shear key), etc. The vibration, vibration reduction and local failure mechanism of cables and suspenders of cable-supported bridges are also worthy of further observation and study.
Durability: There are still many problems in the durability of bridge structures that need further study. Special attention should be paid to the corrosion and rust of cables and suspenders.
Foundation: The use of large-diameter piles also brings some design problems, and it is not reasonable to directly apply the original calculation method to medium-diameter piles. With the help of large-scale bridge monitoring system, it is also the need of design department to investigate the deformation law of large-diameter piles and study the bearing capacity of piles.
Four. abstract
(1) Bridge structural health monitoring is not only a simple improvement of traditional bridge detection technology, but also uses modern sensing and communication technology to monitor the structural response and behavior in various environmental conditions at the operation stage of the bridge in real time, and obtain all kinds of information reflecting the structural state and environmental factors, so as to analyze the structural health status and evaluate the reliability of the structure, and provide scientific basis for the management and maintenance decision of the bridge. At the same time, structural health monitoring of large-scale bridges is of great significance for verifying and perfecting structural design theories and methods, developing and realizing various structural control technologies, and deeply studying unknown problems of large-scale bridges. Therefore, health monitoring has opened up a new space for the development of bridge engineering.
(2) The significance of large-scale bridge health monitoring reflects the concern of people engaged in bridge maintenance management, design consultation and theoretical research. The design of monitoring system should be based on functional requirements and benefit-cost analysis. In addition, the design of monitoring system should be optimized and analyzed, and the very important communication problems in system implementation should be considered.
(3) For long-span cable-stayed bridges and suspension bridges, the integrity assessment is only a part of the structural safety state assessment, and the safety state of the bridge structure cannot be explained only through the integrity assessment. At the same time, the mechanical characteristics of long-span bridges determine that their safety assessment is different from that of conventional small and medium-sized bridges in concept and method.
(4) The purpose of safety state assessment of long-span bridge structure is to control the risk of bridge operation and support the decision-making of disaster reduction. Therefore, combined with the safety evaluation of the bridge health monitoring system, it should be possible to evaluate the basic state and structural behavior of the bridge structure through the obtained monitoring data. Identify the structural damage and changes of key parts regularly or after accidental events (such as earthquakes), and objectively and quantitatively evaluate the bearing capacity, wind resistance and seismic capacity of bridge structures in each stage of their life cycle.