1. The task of bridge field test
1. 1 Check the quality of bridge design and construction. For some newly built large and medium-sized bridges or specially designed bridges, many new problems will inevitably be encountered in the design and construction process. In order to ensure the bridge construction, many new problems will inevitably be encountered in the design and construction process. In order to ensure the quality of bridge construction, monitoring is often needed during the construction process. After completion, the site load test is generally carried out, and the test results are the main technical data and basis for evaluating the quality of bridge engineering.
1.2 to judge the actual bearing capacity of the bridge structure. Many bridges built in the early years of our country have low design load levels, which are difficult to meet the needs of today's traffic. In order to strengthen and transform, it is necessary to determine the actual bearing capacity of the bridge through experiments. Sometimes, due to special reasons (such as super-heavy vehicles crossing the bridge or accidental damage to the structure, etc. ), the bearing capacity of the bridge should be determined by test method.
1.3 verifies the design theory and method of bridge structure. New structures, new materials and new technologies in bridge engineering are constantly innovating. The in-depth study of some theoretical problems and the application practice of a new method and new material often require measured data from field tests.
Experimental study on natural vibration characteristics of bridge structure and structural dynamic response under dynamic load. For the dynamic response of some bridges under dynamic load, the wind-resistant stability of long-span flexible structures and the seismic performance of bridge structures in earthquake areas, it is necessary to understand the natural vibration characteristics and dynamic response of bridge structures through actual measurement.
2. It is necessary to observe some major bridge structural systems.
2. 1 beam bridge.
(1) simply supported beam. Mainly observe the mid-span deflection, section stress (or strain) and fulcrum settlement. In addition, the deflection of a quarter of the span and the stress of the inclined section of the fulcrum are observed.
(2) continuous beam. Mainly observe the midspan deflection, midspan and fulcrum section stress (or strain). Additional observation of deflection and section stress (or strain), fulcrum section angle, fulcrum settlement and fulcrum oblique section stress at l/4 span.
(3) Cantilever beam (including the cantilever part of T-shaped steel structure) mainly observes the deflection and rotation angle of the cantilever end, the stress and rotation angle of the root or fulcrum section of the fixed end, and the stress of the control section of the T-shaped rigid frame bridge pier. In addition, the mid-span deflection of cantilever, local stress of support and pier top displacement (horizontal and vertical displacement, rotation angle) are observed.
2.2 Arch bridge. Mainly observe the deflection and stress at the mid-span and span 1/4, the stress at the arch foot, the deflection and stress at the span, the deflection and stress at the control section of the arch e building, and the displacement and rotation angle of the pier top.
2.3 Rigid frame bridge (including frame, inclined leg rigid frame and rigid frame-arch composite system) mainly observes the deflection and stress of the midspan section, as well as the stress, displacement and rotation angle of the section just near the node. In addition, the stress, displacement and rotation angle of column foot section, displacement and rotation angle of pier top are observed.
2.4 Suspended cable structures (including cable-stayed bridges and deck suspension bridges) mainly observe the maximum deflection of the main girder, the torsional displacement under eccentric load and the stress of the control section, the horizontal displacement at the top of the pylon and the cable force. Supplementary observation of the deflection of the connection between the steel cable and the beam, the stress of the bottom section of the tower column and the tension of the anchor cable.
The main part of the above-mentioned bridge system is the part that must be observed in the general static load test. A schematic diagram of the structure should be drawn on the scheme, indicating the location, total number and quantity of measuring points.
3. Test process
3. 1 Initial reading of static load. The initial reading of static load refers to the no-load reading at the beginning of the experiment, not the reading of the debugging instrument in the preparation stage. For newly-built bridges, jacking is often needed before initial reading (generally, some heavy vehicles slow down several times on the bridge). From the initial reading, the whole test system began to run, and the measuring, reading and recording personnel entered the site to perform their duties.
3.2 Loading. Arrange the load according to the parking line delineated on the bridge, and arrange special personnel to direct the parking of vehicles.
3.3 readings after stabilization. The deformation and internal force of the structure need a stable process. The length of this process is different for different structures, and it is generally based on the stability of the strain value or deflection value of the control point. As long as the reading fluctuation value is within the precision range of the test instrument, it is considered that the structure is in a phase-by-phase stable state and can measure the reading.
3.4 Unloading reading zero. At the end of a working state, the load falls off the bridge. Every measuring point has to read back the zero value, and there must be a stable process.
3.5 Check the data. During the static load test, the main working conditions shall be repeated at least 1 time. During the test, we must always pay attention to the data of several control points, and reload the test once there is a problem (poor data rules or instrument failure, etc.). ) I found them all. This method of field data inspection can avoid big errors in measured data.
4. On-site dynamic load test of bridge
The dynamic load test can be carried out together with the static load test or independently.
The general content of on-site dynamic load test is to measure the deflection and strain of bridge structure under the dynamic action of vehicles. The instruments used are more and more complicated than those used in static test, and the test requirements are higher than those of static test. Especially for dynamic deflection test, in addition to the fixed support of small and medium-sized bridges, the photoelectric deflection meter can also be used for large and medium-sized bridges.
The main differences between dynamic load test and static load test are as follows.
4. 1 instrument debugging. All instruments and equipment should have been debugged in the preparation stage, and the specific recording method should be considered. If dynamic resistance strain gauge is used, the gain and calibration range must be determined according to the estimated strain, and the recording speed and amplitude must be adjusted. If the computer dynamic data acquisition system is used for direct sampling and storage, the gain, calibration value and other conditions are not much different.
4.2 Vehicle control. It is necessary to control the speed, position and time of getting on and off the bridge. It is necessary to assist the driver to accurately control the driving speed and pay attention to the driving route every time on the bridge. For some long-span bridges, it is also necessary to determine the position information of vehicles when they travel to various sections.
4.3 Test records.
(1) sports car. The purpose of sports car test is to judge the dynamic response of bridge structure at different vehicle speeds (such as dynamic increment and time-history curve of displacement or stress), and then analyze the relationship between dynamic response and vehicle speed. Specify the speed of each gear for the vehicle, require the vehicle to drive at a constant speed on the bridge, and record the whole process of dynamic response. If the speed of the sports car is quite slow, the process curve recorded by the dynamic measuring instrument is the internal force influence line or deflection influence line corresponding to the position of the measuring point.
(2) brake. When the vehicle is driving at a certain speed, it suddenly brakes at the current position and records the dynamic increment when braking.
(3) jumping (crossing obstacles). Generally, an obstacle is set at the position of the characteristic road section on the bridge to simulate the unevenness of the road surface (arched wooden boards are ideal). The dynamic increment of the structure is measured when the vehicle passes through the obstacle at different speeds.
The above three different car dealership situations can be single car or multi-car.
4.4 Make records. In the dynamic load test, there are many influencing factors, so we should pay attention to master the main contents under various working conditions. If it is required to record the whole process of structural dynamic response, the emphasis should be placed on the integrity of the recorded signal; When determining the power increment, it is necessary to record the peak value of the response signal and some signals near it.
5. Detection test of single simply supported beam
The most widely used beams in bridge engineering are simply supported reinforced concrete beams and simply supported prestressed concrete beams. The test of a single simply supported beam is generally a static load test. The preliminary preparation and test method of static load test of real bridge are also suitable for single beam test. This paper introduces some basic contents of prestressed concrete single beam test.
5. 1 tensile stress test of prestressed concrete beam. Engineers and technicians are more concerned about the stress in prestressed concrete beams before and after the tensioning construction of prestressed steel bars. Because for prestressed concrete beams, the difference or consistency between design calculation and actual construction is directly related to the prestressed quality of beams. In practical testing, the active end, passive end and mid-span of prestressed beams are often taken to measure the stress values before and after tensioning.
5.2 Loading method. Static loading of single beam usually adopts reaction frame with jack equipment, and other loading methods are also adopted where there is no reaction frame equipment. The general principle of load, failure load and actual test requirements is that the load grade difference should not be too large, especially before and after cracking of prestressed concrete beams.
The duration of each loading or unloading depends on the time required for the structural displacement to reach the stability standard. It is required that the structural displacement in the previous load stage is stable before entering the next load stage. Generally speaking, if the minimum resolution of the measuring instrument is less than the load of the same level, the structural displacement is considered to be stable in phase.
5.3 Crack resistance test. It is very important to determine the cracking load of prestressed concrete beams, which can be judged according to the variation law of strain readings of steel bars and concrete at the lower edge. For example, in the relationship curve between concrete strain and load at the lower edge of the beam, the cracking load can be determined by the inflection point of the curve; In the relationship curve between stress and load of steel bar at the lower edge of beam, the load corresponding to the significant change of curve slope is cracking load.
5.4 Determination of ultimate bearing capacity. The normal section failure criteria of reinforced concrete beams and ordinary reinforced prestressed concrete beams are controlled as follows: (1) The tensile stress of the lower edge reinforcement reaches the yield strength; (2) The compressive stress of the upper edge concrete reaches the ultimate compressive strength (which is generally impossible in practice) or the compressive strain reaches the ultimate compressive strain.
For some beams controlled by shear compression (tension) failure, the determination standard of their ultimate bearing capacity is still unclear. It is very important to detect and evaluate the damage of bridge structure in modern times. Knowing detailed data can judge whether it is necessary to maintain normal use, and every detail is crucial. Zhong Da Consulting will explain to you the modern detection and evaluation of bridge structural damage.
In recent years, with the development of transportation, the importance of bridges is increasing day by day. However, with the increase of automobile traffic and heavy vehicle traffic and the influence of human external forces and natural disasters on the bridge environment, the deterioration of existing bridges is more serious. In order to ensure the functionality and safety of these bridges, it is necessary to carry out damage detection and safety assessment on their health conditions.
1 Damage detection method for highway bridges
In recent decades, bridge researchers at home and abroad have put forward various detection methods for different types of damage and aging of old and new bridges. Generally speaking, the damage detection of bridge structure is divided into local detection method and overall detection method.
1. 1 local detection technology
Local detection technology focuses on the target part of the structure. Generally, nondestructive testing technology is used as a tool to detect local damage of structures, which can accurately locate, explore and even quantitatively analyze structural defects. The following focuses on nondestructive testing technology:
Traditional nondestructive evaluation (NDE) technology has made great progress. At present, there are dozens of ultrasonic detection, infrared detection, acoustic emission, spontaneous potential detection, impact echo detection, magnetic detection, R or X-ray detection, optical interference, pulse radar, vibration test and analysis. Except vibration test analysis, most nondestructive testing techniques belong to local testing methods. There are still some unfavorable factors in the application of some nondestructive testing technologies in bridge structures. For example, R or X-ray inspection can only detect concrete within a certain thickness range, which has certain requirements for inspection space and certain radioactive risks; Although ultrasonic testing has a good effect on steel structure, it is not accurate enough to detect anisotropic materials such as concrete, and the cost of testing equipment is high. Infrared detection can be used for remote and rapid detection and diagnosis, but the detection cost is high, which has an impact on traffic flow. The local inspection method requires people to search on the carpet, which is time-consuming and laborious and has poor reliability. However, for a large number of small and medium-sized bridges, from the technical and economic considerations, manual inspection is still an important and realistic technical management means. The future direction is to expand the application scope of advanced detection technology and actively research and apply small-scale detection instruments with high automation. The traditional inspection methods can generally monitor the appearance and some structural characteristics of the bridge, and can reasonably judge the damage of the local key structural members and nodes of the bridge. However, it is difficult to comprehensively reflect the overall health status of the bridge and make a systematic evaluation of the safety and remaining life of the bridge structure. Scholars at home and abroad generally agree and devote themselves to the research of nondestructive testing methods, that is, experimental modal analysis combining system identification, vibration theory, vibration testing technology, signal acquisition and other interdisciplinary technologies. At present, the whole detection technology has achieved positive results in some local areas. A more realistic damage detection method may be a combination of overall damage location and local refinement detection.
1.2 integral detection technology
1.2. 1 overall inspection is to grasp the actual working state of the whole structure as a whole, and can continuously or intermittently check the safety state of the structure, and can be used to guide the location of suspected damage parts and evaluate the damage degree, thus improving the inspection efficiency. The whole detection method can be divided into static detection method and dynamic detection method.
1) Static test method is to test the bridge statically when it is out of service.
Load test, which measures the static parameters related to the performance of the bridge structure, such as the deformation, deflection, strain and cracks of the bridge under static load. By analyzing these parameters, the static bearing capacity of the whole bridge can be directly judged, and the strength, stiffness and crack resistance of the structure can be obtained.
2) Dynamic detection method (vibration-based test identification method) is adopted to detect the bridge structure.
Dynamic load test and dynamic performance of structure are the basis for judging the running state and bearing capacity of bridge. The method is to excite the tested structural system, determine the mechanical characteristics of the structure from the input and output of the system through vibration testing, data acquisition, signal analysis and processing, and identify the damage according to the dynamic characteristics of the structural system.
1.2.2 current situation of integral detection technology
For special and important long-span bridges, in recent years, people have devoted themselves to the research of overall damage diagnosis and evaluation methods. Real-time monitoring and fault diagnosis technology has been widely used in aerospace, military and mechanical industries in developed countries, and many technologies are very mature. However, due to the complexity and particularity of large-scale civil engineering structures and materials, it is not appropriate to directly evaluate the state of the whole structure with a single dynamic parameter index by imitating mechanical vibration modal technology. At the same time, there are still many problems when other technologies are applied to civil engineering structures, especially bridge structures, such as the optimal arrangement of sensors and the identification of structural dynamic fingerprint changes.
The research on the overall health monitoring system of bridge structure is expected to change the passive situation that structural faults can not be found in time in the past, and to know the overall working state of the structure in time is one of the future development directions. However, this involves three aspects: a, the collection of working parameters; B, identifying and processing the working parameters to obtain the working state information of the bridge; According to the working state information, the bridge health state evaluation is given.
At present, most of the work focuses on the former, while the latter two are still in the stage of theoretical and practical exploration. Generally speaking, it is still very difficult.
1) the optimal arrangement of sensors. The structural damage detection first involves the signal acquisition technology. In the research and practice of structural damage detection, sensors are a research hotspot. Large-scale bridge structure monitoring system generally includes various types and a large number of sensors, such as the permanent health monitoring system installed on the Tsing Ma Bridge in Hong Kong, including more than 700 anemometers, accelerometers, strain gauges, displacement meters, thermometers, levels and speedometers. A large number of sensors form a sensor group, which brings the study of optimal sensor arrangement. The number and position of sensors in the structure have an important influence on the quality and deviation of model parameter estimation. However, for a large structure like a bridge, it is impossible to obtain the complete modal data of the structure, and only a part of the modes with all degrees of freedom can be obtained through measurement. In addition, this process will inevitably introduce errors and make damage detection more difficult. Therefore, considering the influence of cost, it is the first key link of damage detection to determine the optimal or near-optimal sensor type, number and location, so as to realize the optimal collection of information from a limited number of sensor systems. At present, some optimization algorithms have been proposed, such as the minimum criterion of off-diagonal elements of MAC matrix and genetic algorithm. Tsinghua University Mutu System used the generalized genetic algorithm (1997) to optimize the optimal position of the sensor group of Qingma Bridge. Practice has proved that the algorithm is feasible, and it can obtain global optimization or approximate optimization.
2) Bridge damage identification method
A dynamic fingerprint method
Dynamic fingerprint method is to judge the real state of the structure by analyzing the changes of dynamic fingerprint related to the dynamic characteristics of the structure. Commonly used dynamic fingerprints include frequency, vibration mode, modal curvature, strain mode, transfer function, power spectrum, modal assurance criterion (MAC), coordinate modal assurance criterion (COMAC), energy transfer ratio (ETR) and so on. The methods of testing dynamic characteristics by single item include frequency ratio method, modal difference method, strain modal method, curvature modal method and so on. There are many methods to test dynamic characteristics, such as flexibility difference matrix, stiffness difference matrix, uniform load deformation-curvature method, energy damage fingerprint, energy quotient difference fingerprint and so on. Use other methods to test the response, such as FRF waveform fingerprints, including WCC, ATM, SAC and other pointers. A large number of model and actual structural tests show that the measured structural frequency is accurate, but not sensitive to local damage; Modal shapes, especially high-order modes, are sensitive to local stiffness changes, but it is difficult to measure them accurately. The dynamic fingerprints related to vibration such as MAC and COMAC all have similar problems, but the modal curvature and strain mode are too small to be effectively distinguished in low-amplitude vibration testing. Some indexes, such as ETR and modal strain energy, can effectively determine the location or development of damage. However, these indicators are very sensitive to noise and are easily overwhelmed by noise. At present, the existing research shows that the dynamic fingerprint method is successful for the simple model structure in the laboratory, but the result is not ideal when applied to the actual structure. It can be said that the ability of dynamic parameter method to identify structural damage is still very limited so far. The successful application of dynamic fingerprint technology may depend on the research of new comprehensive damage index and the development of testing technology.
B model correction method
The model updating method mainly uses the directly or indirectly measured data to continuously update the stiffness distribution of the structural model through conditional optimization constraints, so as to obtain the information of structural stiffness change and realize structural damage identification and location. The finite element model updating methods for nondestructive evaluation include modal flexibility method, optimal matrix updating method, sensitivity matrix updating method, characteristic structure configuration method, measured stiffness change method and comprehensive modal parameter method. Due to technical reasons, usually only some low-order modes of the structure are used for finite element model updating. But in fact, only the modes corresponding to higher frequencies are sensitive to the damage location of the structure, and the low-order modes have no obvious contribution to determining the damage location, but increase the calculation workload. The disadvantages of this method are that the complete modal set of the structure cannot be obtained, the signal-to-noise ratio in the measurement is low, and it is difficult to give enough correction information from the test data, which leads to the uniqueness of the solution.
Artificial neural network method
Rajagopalan et al. (1996) discussed two applications of artificial intelligence (AI) in the field of nondestructive testing and evaluation. They think that KBS and An Zaiai can be properly applied to NDE. Artificial neural network is a simplification, abstraction and simulation of human brain neural network in neural network research. Neural network has collective computing ability, adaptive learning ability, strong fault tolerance and robustness, and can be associated, integrated and popularized.
Some researchers believe that the traditional damage assessment algorithm is based on accurate mathematical modeling, but the performance of complex structures has not reached the level of accurate understanding; Neural network method can save the damaged and undamaged modes of the structure, and can learn by itself. Damage can be identified through comparative analysis.
In recent years, artificial neural network has been successfully applied to filtering, spectrum estimation, signal detection, system identification and pattern recognition. Neural network recognition method can solve the shortcomings of high noise interference and pattern loss in traditional pattern recognition. The artificial neural network method and wavelet analysis technology are used to preprocess the bridge monitoring signal and extract the damage characteristics. Because of the incompleteness of the test data obtained by bridge structure damage detection, neural network method can use limited data for training and use incomplete data for identification, which can better solve the system identification problem caused by nonlinearity and uncertainty without mathematical model. At present, neural network (BP), radial basis function neural network (RBF) and self-organizing neural network (ART) based on error back propagation are applied to structural damage identification. The main limitation of artificial neural network method lies in the acquisition of training data set, and its accuracy depends largely on the completeness of training data set.
3) System response identification under environmental excitation
The excitation technology in the structural vibration test can adopt excitation equipment such as launching rockets, explosions, artificial earthquakes or other excitation means. Using special excitation equipment or artificial excitation in bridge structures often needs to close traffic or cause structural damage, while using heavy excitation equipment often increases the cost of system identification. There are many advantages to identify the structural system by using the natural environment excitation such as vehicles, pedestrians, wind and their combinations acting on the bridge structure: there is no need to interrupt the traffic flow and arrange expensive equipment, which is convenient and time-saving.
In fact, the input of environmental excitation cannot be accurately known, so the identification of environmental excitation system is a system identification method that only knows the signal output but not the signal input, which is a challenge to the traditional system identification method. However, the environmental excitation response is generally small in amplitude, random, susceptible to noise and large in data, which requires some special identification techniques. The identification methods proposed by foreign scholars for different purposes include: peak method based on power spectral density, ARMA model based on discrete time data, natural excitation technology, random subspace method and so on. Ren Weixin compared the peak method in frequency domain with the random subspace method in time domain, and applied it to a 15-story reinforced concrete building and a steel arch bridge. The results show that PP method is simple, fast and practical, but it can't identify the structural damping, and the modal identification accuracy is not high. However, SSI method has a large amount of calculation, but high recognition quality; Therefore, it is suggested that PP should be used to check the data and identify the dynamic characteristics of the structure, and then SSI method should be used for further analysis to ensure the correctness of the results.
4) Expert system
Damage diagnosis and evaluation of structures need not only profound theoretical foundation, but also rich expert experience. Knowledge-based expert system brings together experts' knowledge, breaks through the limitation of time domain, and makes damage diagnosis and evaluation gradually intelligent and automatic. At present, there have been attempts in the application and development of expert system in bridge damage assessment and maintenance countermeasures. Expert system generally integrates fuzzy theory to adapt to the ability of dealing with uncertain information. Because expert system is a symbol-based reasoning system, it has explanatory function, but it is difficult to acquire knowledge, while artificial neural network has learning ability, but it does not have explanatory ability. The combination of expert system and artificial neural network to establish an intelligent diagnosis system for structural damage shows a good development prospect.
2 Bridge structural safety assessment and life prediction
The process of bridge structure from normal to abnormal, which leads to defects, is called cracking process or damage process. The purpose of damage detection is to objectively evaluate the bridge, so as to guide the traffic of vehicles, provide scientific basis for the maintenance and reasonable and effective reinforcement of the bridge, and reasonably predict the development trend and remaining life of the bridge.
2. 1 safety assessment of bridge structure
Bridge safety assessment is divided into two levels: preliminary assessment and detailed assessment. Preliminary assessment can quickly screen out the safety degree of a large number of bridges, and then the competent department will decide whether detailed assessment is needed according to the importance of the bridges.
1).
According to the items that affect the earthquake resistance, load resistance and scour resistance of the bridge, the score of each item is evaluated by filling in a table, and then the total score is obtained comprehensively, so as to judge whether the earthquake resistance, load resistance and scour resistance of the evaluated bridge are sufficient or questionable or insufficient.
2) Detailed evaluation.
According to the actual situation of the bridge and the latest relevant design specifications, the seismic resistance and bearing capacity of the bridge are calculated through detailed structural analysis. Bridges that show insufficient safety after detailed evaluation should be reinforced immediately, and the information obtained from bridge safety evaluation should be used as an important reference for reinforcement.
2.2 Life prediction
The service life or durability of bridge structure refers to the time when the bridge in service still has the intended use function under normal use and maintenance conditions. Before life prediction, it is necessary to know what the expected function of the structure is and how to judge the functional failure of the structure, that is, the definition of limit state, which is the key to the life prediction and residual life evaluation of the structure. The service life of bridge structure is related to many factors, such as material properties, detailed structure, service state, deterioration mechanism and so on, and the interaction of many factors is difficult to quantify. At present, there are various forecasting methods. At present, the commonly used methods are empirical prediction method, mathematical model prediction method and life prediction random method.
3. Conclusion
Damage detection and assessment of bridge structures involves many disciplines such as structure, communication, computer and management science, among which the latest technologies such as system theory, information theory, cybernetics and nonlinear science are widely used. Generally speaking, it is still in the initial exploration stage. With the further cross and synchronous development of various disciplines, the new discipline of bridge structure health monitoring and evaluation will be greatly developed. A comprehensive decision-making system integrating long-term real-time or regular online automatic monitoring of bridges, health assessment (including rapid assessment after catastrophic natural or man-made disasters), traffic management and maintenance decision-making will also be realized as soon as possible.
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