Main methods and sensor selection for bridge structural health monitoring

1. Preamble

The monitoring of bridge structure state parameters is not short-term manual monitoring, but long-term automatic monitoring. It requires that once the sensor is installed in the measured part of the structure and the power supply and signal lines are connected, the sensor system can automatically collect structural parameters for a long time without human intervention. Therefore, the requirements for the long-term stability, reliability, environmental adaptability and anti-interference ability of the sensor for the monitoring of bridge structure state parameters are much more stringent than the requirements of conventional bridge structure construction monitoring and load testing, and the connection and network distribution of the sensor system are also much stricter than those of construction monitoring and load testing. For these requirements, different parameters have different specific requirements. 

TwoStrain monitoring

Stress is a key intrinsic indicator that reflects the stress and failure conditions of the structure. The main purpose of monitoring structural stress is to study the internal force distribution, local structure and connection response under various loads through the monitoring of internal forces in the control parts and key parts of the structure, so as to provide a basis for structural damage identification, fatigue life assessment and structural condition assessment. Stress and strain have a definite relationship, and the elastic modulus of the bridge structure can be calculated by the strain. Therefore, strain monitoring of bridge structures is necessary. 

For skillful strain monitoring, it has different characteristics compared with strain monitoring in construction. Generally speaking, the shrinkage caused by concrete solidification is the largest part of the structural deformation of the concrete ±, and this process is basically completed after 28 days of concrete construction. Within two years after prestressing and tensioning W, most of the creep of cocoagulation is basically completed. At this time, the strain of the coagulation ± is reflected as the load strain and the temperature strain, and the range of change is small, so the strain measurement of the bridge requires the sensor to be small in the range but high in measurement accuracy. 

At present, the main materials of bridge structures are steel structure and cohesive ± structure. For steel structures dominated by W steel, strain sensors only need to be installed on the surface of the structure. However, for on-site concrete structures, strain sensors can be embedded in the concrete structure during construction or installed on the surface of the structure after construction. Due to the solidification shrinkage and creep of the mud solidification structure, the measurement range of the embedded strain sensor is required to be larger than that of the surface-mounted sensor. Although there are various strain monitoring methods such as resistance strain gauges, differential strain gauges, steel string strain gauges, piezomagnetic strain boxes, and fiber optic strain sensors, the sensors that can actually form products and can be applied to strain monitoring of bridge structures are still very limited. At present, the main sensors mainly suitable for strain monitoring of bridge structures include differential strain gauges, steel string strain gauges, fiber grating strain sensors, and fiber optic seedling strain sensors. 

ThreeDeformation monitoring

Deformation is the external manifestation of structural stress and failure, and is the most intuitive key indicator to reflect structural safety. Different parts of the delicate beam structure have different deformation forms, and their monitoring characteristics and requirements are also different. Although the current bridge structure deformation monitoring methods include total station, GPS, laser, and image method, they are very different in terms of performance, price, etc., and the characteristics of different bridge structures and different parts are also very different, so there is no universal bridge structure deformation monitoring method, and different monitoring methods can only be selected according to the different requirements of different bridge structures and different parts. Table 2.2 below shows the deformation forms of different bridge types: 

Table 1 Main deformation forms and requirements of bridge structures

FourTemperature and humidity sensors

Through the monitoring of the temperature field distribution of bridges, it can provide an original basis for the calculation and analysis of temperature influence in the design, and compare and quantitatively analyze the changes of the working conditions of bridges under different temperature states, such as bridge deformation and stress changes, which is of positive significance for the verification and improvement of bridge design theory. 

Temperature and humidity monitoring are mainly the environmental W of the bridge and the temperature field monitoring of the structure itself, so its measurement range is the ultimate temperature and humidity of the environment where the bridge is located. In the south, it is generally -10°C~70°C, 30RH~95RH, while in the north, it is generally -40°C~50°C, 10RH~90RH. The measurement accuracy is 0.5°C and 1RH. Although there are many different types of temperature sensor products such as thermocouples and thermistors on the market, almost most of them can only be used in indoor environments, and it is difficult to meet the harsh working environment where the bridge is located, and its long-term stability generally does not meet the requirements. Therefore, humidity and humidity sensors are key to long-term reliability in harsh working environments in the field. 

FiveVibration monitoring

When the overall performance of the structure changes, some modal parameters (such as frequency, mode, etc.) that reflect its internal characteristics will also change accordingly. Vibration monitoring is to monitor the vibration of the structure under external excitation and grasp the internal dynamic characteristics of the structure, so as to analyze the damage and safety assessment of the structure. The damage and safety reduction of bridge structure are mainly due to the cumulative result of fatigue damage of the main components and structures of the bridge, while the structural fatigue damage of the bridge is mainly due to the alternating stress under the action of dynamic load. The vibration monitoring of the delicate beam can examine the fatigue response of the structure, and then the safety and reliability of the structure. The dynamic response of the main girder is often related to the strong seismic source that causes the vibration of the structure, so the vibration monitoring of the structure can not only identify the dynamic characteristic parameters of the structure, but also realize the recording of the fluctuating load history of the main beam structure. 

Vibration monitoring is generally measured by accelerometer sensors. Since the inherent vibration characteristics of cable towers, main beams, main cables, cranes^ and cable-stayed cables are different, the sensor selection is based on the technical performance of the sensor (frequency range, sensitivity, sampling characteristics, etc.). Due to the extremely low natural frequency of large bridges, the low-frequency response of the acceleration sensor is extremely high, and the low-frequency response is generally required to reach 0.01Hz to 0.1Hz. Therefore, the cut-off frequency of the low-frequency response of the acceleration sensor is the most important indicator, and its main indicators are sensitivity, long-term reliability, signal transmission distance, environmental adaptability, etc.

Although there are many different types of acceleration sensors on the market, such as piezoelectric acceleration sensors, differential capacitive acceleration sensors, piezoresistive acceleration sensors, and force balance acceleration sensors, most of the low-frequency responses do not meet the requirements. In addition, the output signal of the accelerometer is an analog signal, so its signal output line must use a high-quality single-core shielded cable, and the transmission distance of the signal generally cannot exceed 100 meters without special signal processing technology. Long-term reliability in harsh environments in the wild is another key factor. 

6. Monitoring of cable-stayed cable force

The ideal state of cable-stayed bridges is in the form of compression of the main tower and tension of the cable. The compression of the main tower is formed by the reasonable transmission of static and dynamic loads by symmetrically distributed diagonal cables. The main performance of cable-stayed cables mainly depends on the cable-stayed cable, the monitoring of cable-stayed cable force, and the size distribution and change of the cable-stayed tension force, so as to fully understand the stress and deformation state of the main tower, main beam and the overall structure. The cable force state of the cable-stayed cable is an important indicator to measure whether the bridge is in normal operation, so in order to ensure structural safety, it is necessary to master the cable force change law of the cable. How to accurately measure the cable force has always been a topic of concern to relevant scientific research departments. Commonly used measurement methods are divided into two categories: direct method and indirect method. The former has pressure gauge measurement method, electrical measurement method, etc., which are usually used for monitoring and measurement during the construction process; The latter, such as the vibration frequency method, is mostly used for monitoring after bridge.

the basic principle of vibration frequency method; 

The development of signal processing technology has promoted the progress of cable-stayed cable force monitoring and analysis. In the early days, cable-stayed bridges had a small span and small cable force, and a first-order standing wave could be formed by applying artificial excitation to the cable, and the fundamental frequency of the cable could be obtained by using a general frequency tester. With the emergence of large-span and extra-large-span cable-stayed bridges, the cable-stayed cable force reaches thousands of N or even tens of thousands of N; The dead weight of the rope generally exceeds 50kg/m, sometimes up to lOOkg/m. In this case, it is difficult to obtain the ideal vibration signal if the artificial excitation is not possible. At the same time, it is impossible to pre-calibrate such a large component to obtain the relationship between frequency and cable force. Therefore, a vibration method for measuring the frequency of the cables is proposed by using the characteristics of random vibration when the cables change with the environment.

The oblique cable is regarded as a chord vibrating in a plane, and the displacement at point X is u(x,t) as shown in Fig. 2.1, and the motion of a microsegment △x is analyzed. According to the assumption that the vibrating string is completely soft, the tension T1 and T2 at point X and point x+△x are the tangent direction of the string, and the angle between them and the X axis is θ1 and θ2.