1 overview
The rapid development of transportation in China provides a good opportunity for the construction of roads and bridges. Long-span suspension bridges, cable-stayed bridges, arch bridges and continuous rigid-frame bridges with different structural forms have been built one after another. At present, the total number of bridges in service has reached more than 800,000, which indicates that China's bridge technology has entered the advanced ranks in the world. However, the environment in which the bridge is located is relatively harsh, which is affected by many man-made and natural disasters. The lack of bridge management and limited manpower and material resources make the aging and damage of the bridge more serious. In order to meet the traffic needs, make full use of existing bridges and serve the society safely, it is necessary to evaluate bridges, especially those that have been in disrepair for a long time, understand the working conditions of bridges and predict their carrying capacity.
This includes bridge quality inspection, structural check and calculation, and load test when necessary, which is called bridge inspection and evaluation. The purpose is to understand all kinds of diseases of bridges, obtain important data such as stress (strain), deformation, displacement or settlement of key parts, and determine the causes of diseases, actual bearing capacity and remaining life of bridge structures through calculation, analysis and research, so as to provide basis for bridge maintenance. Usually, the inspection items of bridge structure include: bridge deck system inspection, reinforced concrete and prestressed concrete beam or steel beam inspection, masonry and reinforced concrete arch and buildings on the arch inspection, bridge bearing inspection, bridge substructure inspection, bridge hydrology and treatment structure inspection, structural crack inspection and so on. The bridge structure inspection is based on the relevant specifications, design basis or completion data of the bridge structure, and the main control sections and weak parts of the bridge structure can also be inspected according to the inspection results to evaluate the bearing capacity and applicable conditions of the bridge structure. Bridge load test is a scientific test for direct load test of bridge structure. Based on the conclusion of bridge detection and structural calculation, the bridge load test is carried out to obtain the measured data, and the bearing capacity of the bridge is analyzed and evaluated.
Detection of cable force of cable-stayed cable
The structure of cable-stayed bridge is mainly composed of three parts. The dead load and live load on the main beam of cable-stayed bridge are transmitted to the pier or abutment through cables. Cable force detection includes cable force detection, anchorage zone detection, tower top displacement detection, main girder elevation measurement, daily change tracking observation of typical parts, etc. The detection of cable force is one of the characteristics of this structure with flexible members. By accurately measuring the cable force, the stress state of the whole bridge structure can be fully grasped. The detection methods of cable force of cable-stayed bridge after completion include frequency method, magnetic flux method and fiber grating method. The sensor used in the fiber grating method is to irradiate the side of the fiber with ultraviolet light or write by other methods, so that the refractive index in this range changes periodically along the axial direction of the fiber, and then the change of external physical quantity is sensed by the shift of the periodically changing grating reflection wavelength. This measurement technology is characterized by small volume, high linearity, good repeatability, strong electromagnetic interference and corrosion resistance, and fast absolute measurement and response speed. It is an ideal high-precision structural health monitoring technology. But at present, this method has not been widely used and popularized, so the price is too high.
Magnetic flux method is a nondestructive method to measure cable force and monitor the corrosion degree of stay cables. The use of this method is to put the magnetic flux ring as a sensor on the inclined cable in advance, and calculate the cable force according to the relationship between cable force and magnetic flux by measuring the change of magnetic flux. The sensor material used in the magnetic flux method is electromagnetic and consists of two layers of coils, so it will not affect any mechanical and physical characteristics of the cable, and is hardly affected by other interference factors except temperature, so it has higher accuracy than other detection methods. The disadvantage is that it is not suitable for cable measurement without embedded sensor. In addition, sensors and test instruments are very expensive. There are usually hundreds or even hundreds of stay cables of long-span cable-stayed bridges. If each cable is equipped with a magnetic flux sensor, the cost is too high. At present, there is a new type of magnetic flux sensor, which consists of two half rings. When testing the cable force, it can be buckled outside the stay cable anytime and anywhere, which can greatly reduce the testing cost. However, the half-ring synthetic magnetic flux sensor has low sensitivity and instability, and is still in the development stage, so it has no practical engineering value.
The frequency method is used to detect the cable force, that is, the random vibration signal of the cable under artificial or environmental excitation is picked up by the acceleration sensor, and the real-time time time domain diagram is obtained. Then the time domain diagram is transformed into the frequency spectrum diagram of the stay cable by FFT, and the natural frequencies of each order of the stay cable are determined. According to the corresponding relationship between cable force and natural frequency, the measured cable force is obtained. Frequency method is an indirect method to measure cable force, and its accuracy depends on highly sensitive vibration pickup technology and the precise relationship between cable force and frequency. When testing, the acceleration sensor is simply fixed on the stay cable, which can detect the stress of single or multiple cables at the same time. Because there is no need to embed sensors, it is not only suitable for bridges under construction, but also suitable for bridge completion detection and long-term monitoring, especially for old bridges without other sensors embedded in advance, which is almost the only choice. There is no embedded acceleration sensor, which can be reused, with low cost and good accuracy. It is the most widely used cable force detection method today. Using vibration frequency method to find cable force can ensure the safety of stay cables. Because the actual cable force of the cable-stayed bridge is only about 40% of the ultimate strength of the stay cables, as long as the stay cables are not corroded and the anchorage zone is not loose or damaged, there is generally no problem with the stay cables. However, it is far from enough to fully understand the working state of stay cables. The existing research work points out that factors such as stiffness, sag, elevation, wind, rain and snow have effects on natural frequency. In order to correctly grasp the cable tension, we should also consider eliminating these factors.
Displacement detection of the top of No.3 cable tower
The traffic load on the deck of the cable-stayed bridge, the weight of the main girder and the weight of various facilities are all borne by the cables transmitted to the pylon. The pylon of cable-stayed bridge has no other constraints except the rigid consolidation of the root and foundation. All the cable forces of the dead weight of the pylon are the loads of the pylon. The function of cable force is along the axis of cable, and its horizontal component is to make the tower move horizontally. Usually, the balance of cable tower is maintained by using the cable forces on both sides of the tower axis symmetry. Due to the unevenness of building materials and errors in the construction process, it is difficult to ensure the complete symmetry of cable forces on both sides of the bridge tower, which leads to the deviation of the bridge tower. Therefore, the positioning of the top of the tower is an important measure to ensure the deviation of the tower body. Especially considering the slenderness ratio of the cable tower, although it has certain rigidity, it is still a thin rod. According to the structural analysis, the cable tower can be regarded as a cantilever member, and the displacement of the cable tower is the most significant.
The influence of cable force change on the horizontal displacement of cable tower should not be underestimated. In addition, both steel towers and concrete towers are greatly affected by temperature. Most temperatures in China, such as the difference between winter and summer, the difference between day and night, and the difference between yin and yang during the day, will have a temperature effect on cable towers. Further considering the influence of wind, rain and snow, the top of the cable tower actually keeps swinging. Using existing measuring instruments, such as total station, GPS and other equipment, it is no problem to detect the displacement of the top of the cable tower. However, only by trying to eliminate these comprehensive factors, the measured displacement of the cable tower is the real stress state of the cable tower.
The existing research shows that in the actual detection, the tower top can be continuously tracked and measured in a typical period of time, and environmental data such as temperature, wind force and wind direction can be monitored at the same time. If possible, it is best to detect the cable force of the corresponding stay cables, and then summarize the relationship between the tower top displacement and these factors, and finally give the real detection value of the tower top displacement.
4 Simulation calculation during the operation of the structure
The application of bridge structure simulation technology is more and more extensive, and it has become an indispensable and important link in bridge engineering design, construction monitoring and detection. The simulation calculation of cable-stayed bridge is to establish a complete finite element simulation model that can fully and correctly reflect the authenticity of bridge structure, and make calculation and analysis according to the structural characteristics and mechanical characteristics of cable-stayed bridge to replace some actual work and reduce some actual engineering workload.
In the process of establishing the simulation model of cable-stayed bridge, the choice of calculation mode and calculation theory should accurately simulate the spatial position, size, material characteristics, connection form and load action of load-bearing members. Then the structural effect of the whole bridge is analyzed and calculated on a large scale, and a more detailed, accurate and reliable analysis result is obtained. In the process of modeling, the reasonable selection and division of elements and the correct simulation of boundary conditions are the keys to truly reflect the actual state of the bridge. The comparative analysis between the structural theoretical calculation results based on the finite element simulation model and the actual inspection results of cable-stayed bridges can verify each other, find out the existing errors, provide the basis for future revision and more accurate modeling, and provide guidance for future inspection work, so as to achieve the purpose of replacing part of cable-stayed bridges' inspection work.
The simulation calculation of cable-stayed bridge in the process of structural operation should not only consider the correct modeling, but also consider the dynamic factors of cable-stayed bridge. In the normal use of cable-stayed bridge, due to the influence of load and environmental factors, the elevation of main girder and the position of cable tower are not fixed. Therefore, in the simulation calculation, the coordinates of each element node in the finite element analysis should be adjusted according to the actual working state. In addition, with the passage of time, the material will gradually age and damage, and the decline of material properties should also be considered in the analysis. These reasons require that the simulation calculation must be combined with the actual detection to truly and accurately reflect the stress state of the cable-stayed bridge.
Table 1 is the result of five height measurements of a cable-stayed bridge from 1997 to 2002. According to the idea of dynamic modeling, in the finite element data calculated in these five time periods, the coordinates of node 17 should be substituted according to the actual detection value, which is the real structural size of the bridge.
In addition, the dynamic parameters to be considered in the modeling process are cable tower (especially the coordinates of the top of the tower) and cable force. Only by synthesizing these factors can we ensure that the conclusions drawn from simulation analysis are feasible and meaningful.
5 research prospect
Through the detection of the actual stress state of cable-stayed bridge, it can provide scientific basis, accumulation and necessary technical data for the safety and reliability of bridge use, maintenance and reinforcement. In addition, the design and calculation theory of bridge structure can be further improved and developed by establishing the health file database of cable-stayed bridge.
With the development of science and technology, the design and construction of various bridges and the performance of building materials are constantly improving, but safety is still a problem that cannot be ignored. For example, the life of stay cables is one of the keys to the safety of cable-stayed bridges. People are most concerned about how to prevent the corrosion of stay cables, as well as the structural damage and fatigue resistance of anchorage zone. In recent years, although the anti-corrosion measures of stay cables have been continuously improved, and the design and construction methods of anchorage zone structures have also been improved, the bridge structure will still experience wind and rain for a long time. Therefore, only through cable force detection can we know that the safety of stay cables is the most reliable. Similarly, the inspection of the whole structure is still an important measure to ensure the safety of the bridge in the future.
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