Stay cables of cable-stayed bridges, main cables of suspension bridges, suspenders and tie bars of tied arch bridges are the key stress components of these bridge systems, as are prestressed anchorage systems of other civil engineering structures, such as anchor cables and anchor rods used for structural reinforcement. The stress magnitude and distribution changes of the above-mentioned stressed members directly reflect the health status of the structure, so it is of great significance to monitor the stress status of these members and analyze and evaluate their safety on this basis.
In bridge monitoring, it is usually divided into three categories according to the purpose:
1) construction monitoring, bridge load test monitoring and health monitoring in operation stage. Construction monitoring is to monitor the state of bridge structure at the critical stage of construction, so as to ensure the safety and quality of bridge construction and find the hidden dangers of the bridge;
2) The completed bridge load test is to check the bearing capacity, stress and deformation state of the bridge before operation, and the test values obtained from the load test are archival materials reflecting the initial state of the structure.
3) Health monitoring in operation stage means that with the increase of bridge service life, the material performance is aging and the structural performance is degraded. The monitoring system monitors the deformation, stress, strain and other data of each node in real time, and compares them with the reference state, so as to evaluate the actual safety state of the bridge structure.
The monitoring contents of the bridge structure safety monitoring system include:
Monitoring of stress distribution and temperature distribution in steel box girder section;
Partial stay cable tension;
Displacement of main span steel box girder;
Stress change of bridge caused by temperature change
Monitoring bridge vibration and stress caused by traffic load and wind load;
Displacement and stress change of main tower
As a kind of strain sensor, fiber grating is applied to bridge health and safety monitoring system, and the strain sensors are arranged in bridge structures such as piers, box girders and suspended cables. It is found in the project that the measured values of strain sensors arranged in the box girder can be applied to the monitoring of bridge heavy-duty vehicles. When heavy vehicles pass through the bridge, the box girder at the stressed position will have a large elastic deformation response. By measuring the strain of box girder, the measurement parameters such as axle load, wheelbase, speed and total weight of heavy vehicle can be calculated. The monitoring system will run at two different sampling frequencies. The general system will monitor the structural health, and the strain sensor will run at a low sampling frequency, with a typical value of 0. 1 ~ 1 Hz. For the strain sensor in the box girder, when the sensor located at the front end of the incoming flow direction detects abnormal strain, the sampling frequency increases, with a typical value of 50 ~ 200 Hz. The sensor in the middle of the bridge will detect the details of the strain, and according to these values, the vehicle parameters such as axle load, wheelbase, speed and total weight can be calculated. At the same time, the image recording system is started to take images of overloaded vehicles at this time for future management. The license plate can also be recognized in real time by image recognition software and displayed on the information board of the lower bridge to guide heavy vehicles into the management area.