With the development of highway bridges in China, more and more highways and bridges have been built, and many existing bridges have gradually entered the maintenance stage. Relevant experts believe that the bridge will enter the aging period after more than 25 years of use. According to statistics, 40% of the total number of bridges in China already belong to this category, all of which are "aging" bridges. And with the passage of time, its number is still growing, and bridge managers pay more and more attention to the maintenance of bridges. In order to meet the requirements of the continuous development of highway transportation load, make full use of the existing highway bridges and make them continue to serve highway transportation safely, it is necessary to identify the bridges according to the requirements of the Technical Specification for Highway Maintenance issued by the Ministry of Communications.
At the same time, new materials, new processes and new structural forms are increasingly adopted. In order to accumulate engineering experience in this field, it is necessary for us to do some inspection work. Of course, those bridges that have diseases due to reasons such as rushing the construction period, using inferior materials and improper construction methods need to be identified to ensure their safe operation.
Bridge structure appraisal mainly includes calculation, appearance inspection and load test of existing bridges. Through calculation and appearance inspection, the service condition of bridge structure can be basically determined. However, there are often some differences between theoretical deduction and actual structural characteristics, especially the identification of bearing capacity is still inseparable from load test.
2. Bridges that need to be tested
Bridges that need to be inspected have various reasons, most of which are old bridges, because the information of old bridges is relatively scarce and it is relatively difficult to manage; Under special circumstances, some new bridges also need to be inspected, which mainly includes the following factors:
(1) Bridges lacking design and construction data.
(2) Bridges with poor construction quality that do not meet the design requirements.
(3) After the bridge was built and operated for a period of time, serious diseases were found, which affected its bearing capacity.
(4) The construction quality of the bridge is good and the operation is good, but it is hoped to improve its allowable bearing capacity.
(5) Bridges that need to pass vehicles with special loads exceeding the design standards.
There are also some bridges that need to be tested for some special reasons, such as to obtain certain scientific research data and so on. At present, some super-large bridges need not only short-term inspection, but also long-term health monitoring, which will not be described here.
3. Bridge investigation and calculation
Detection is to evaluate the bridge according to the actual situation, so the main work in the early stage is to start with the existing state and characteristics, have a general grasp of the entity to be detected, and clarify the direction of the later work, that is, investigation and calculation.
3. 1 data collection
The scope of data collection mentioned here is wider than usual, including not only design data, but also construction data and related maintenance, repair and reinforcement data.
There are many details involved in data collection, such as calculations, design drawings, revised drawings, geological data, etc. Construction materials include as-built drawings, as-built instructions, material test data, construction records and completion acceptance data. At each stage; Other maintenance and repair materials include historical models, loads, traffic conditions, maintenance materials, etc.
There are also some information related to the natural environment or natural disasters (floods, earthquakes, frozen soil, mudslides, etc.). ) If necessary, information of relevant departments should also be collected.
3.2 Appearance inspection and disease analysis
Appearance inspection is a very important work in bridge detection. Usually, if there is a disease, there will be some symptoms. Through visual inspection, the causes of these diseases can be analyzed and judged, and corrective measures can be put forward, which is helpful to clarify the focus of the next work.
Appearance inspection requires grasping the key points and striving for comprehensiveness.
3.2. 1 Determine the investigation focus according to the stress characteristics.
Usually, the main points of investigation can be determined according to the type of bridge. For example, the inspection points of beam bridge are: cracks and deflection in the mid-span area; End shear joint; The state of the connecting parts of the main girder; The appearance quality of parts and so on. For example, the inspection points of arch are: cracks between the lower edge of arch ring vault and the upper edge of arch foot; Arch axis coordinates; Displacement of pier and so on. Cable structure also has the quality of cable and anchor and so on.
3.2.2 Check the parts one by one.
Bridges can generally be divided into superstructure, substructure and accessory structure. The superstructure mainly refers to the main girder in the beam bridge, including the main arch rib, arch wave, stay cable, wind brace and so on. In the arch, it varies according to different structural forms; The substructure includes piers, abutments, foundations, caps and piles. Subsidiary structures include bridge deck pavement, sidewalks, kerbs, railings, expansion joints, etc.
Each part has its own stress characteristics, and the disease also has some * * * characteristics. If it is not a routine disease, it is necessary to study it carefully and find out the cause. While finding out the causes of conventional diseases, we should evaluate them according to their damage degree, and then we need to reinforce or replace parts to maintain normal operation.
3.3 Investigation of Material Characteristics
Material is the most basic element of the component, and it is also the basis to ensure whether the component can work normally.
With the diversification of bridge types and the continuous development of new technologies, more and more materials will be applied to bridge structures, but at present, steel bars and concrete are the most basic and widely used. Generally, the strength of steel is based on the relevant data of design and construction, and no inspection is needed. When the quality of steel is in doubt or the data is unclear, necessary measures should be taken to intercept samples for material test. The strength of concrete will change with time, and larger bridges usually have test blocks to determine the strength at the same time. For bridges without test blocks, the strength of concrete can be estimated by rebound instrument and ultrasonic flaw detector, and if necessary, test pieces can be drilled on the structure for material test.
3.4 Calculation of Bearing Capacity
After understanding the overall characteristics of the bridge, we need to do some checking work, and the principle of checking calculation is related technical specifications. It should be noted that the relevant parameters should be based on the actual bridge and can be reduced, and some favorable factors can be considered when necessary. Bridges that can't meet the requirements can be rebuilt by checking calculation, and useful bridges need further identification.
4. Static test
4. 1 Preparation before test
It is very complicated to carry out the load test, so it is necessary to make full preparations before the test, which is directly related to the success or failure of our test and whether our test can obtain ideal data, otherwise the next work can only be in vain.
4. 1. 1 Select the test hole (or pier)
The selection of test holes mainly considers the following conditions:
(1) The calculated stress of this hole (or pier) is the most unfavorable;
(2) The quality of the tunnel (or pier) project is poor, with many defects or serious diseases;
(3) The hole (or pier) is convenient for scaffolding, setting measuring points or loading during the test.
The selection of test holes is very important, which is related to whether our test can accurately reflect the performance of this part of the structure and the whole bridge structure, and needs rich field test experience.
4. Determination of1.2 Loading Scheme
After preparing the test holes, we must carefully plan the loading scheme in order to obtain representative test values on the limited test holes. On the premise of determining the bearing capacity of the bridge, the arrangement of loading projects should highlight the key points and should not be too much. Static load test generally has one or two main internal force control sections, in addition, several additional internal force control sections can be set according to the specific conditions of the bridge. The scheme should also determine the appropriate efficiency coefficient according to the inspection situation and the current situation of loading equipment.
Sometimes we choose more than one test hole, which requires us to have a clear mind. How to use limited equipment to meet the test requirements of each control section of the test and how to organize personnel to carry out the test efficiently and orderly are all factors that need to be fully considered in the scheme.
4. 1.3 Other site coordination preparations
Load testing is a process of combining theory with practice, and it also involves many other events. For example, scaffolding is usually needed to set up measuring points, vehicles need to be prepared in advance for experiments, traffic control needs to be coordinated by relevant departments on site, instruments need necessary lighting and stable power supply, as well as the safety guarantee and contact equipment for field staff, and so on.
The content of the field test is numerous and complicated, so the preparation work should be considered as comprehensively as possible to ensure that our test can be carried out in an orderly manner as planned.
4.2 Main data and test methods obtained in the test
4.2. 1 Determine the data to be obtained according to the test purpose.
As a whole structure, the bridge involves many details, so we can't cover everything. According to the preliminary visual inspection and appraisal work, the parts to be measured and the parameters to be obtained should be made clear to reduce unnecessary labor.
Clarifying the purpose helps us grasp the key points in our work, determines what test data we need to obtain, and ensures the reliability of key test data.
4.2.2 Data to be monitored under normal circumstances
Usually, old bridges need to be evaluated mainly through the mechanical characteristics of the superstructure. Monitoring the stress and strain of the control section under various working conditions of test load is the main content, and some parameters in normal use, such as deflection and crack width, are also the main test contents of the old bridge.
The stress and strain measuring points are arranged in the main internal force control section and the additional control section in the above scheme stage. At the same time, some representative measuring points should be selected on each control section according to the section form of the superstructure to reflect the stress state of the whole control section. For some special structures, such as cable structures, it is generally necessary to carry out cable force test.
In addition, it is necessary to monitor some diseases in individual parts according to the actual appearance inspection, such as the settlement of bearings, the dislocation of clapboards and the development of shear joints. Because the destruction of some non-main structures or auxiliary structures will also have a great influence on the test results of the main structure, and it may also be the main reason for the unfavorable stress of the main structure.
4.2.3 Testing instruments and methods
The instruments commonly used in the experiment include strain gauge, deflection gauge, level gauge, total station, dial gauge and so on. Sometimes, due to the limitation of field conditions, we can't directly use instruments to test the parameters we want, which requires our testers to have certain experience and take certain measures to measure the data we want with existing instruments.
Of course, with the continuous development of science and technology, the instruments that can be used in the experiment are more and more advanced and the operation is more and more convenient. Anyone who can accurately test the parameters we want can use them boldly.
4.3 Implementation and on-site organization arrangement of load test
Field testing is the last and most critical work. It is a test of our preparation work, and whether we can get accurate data is closely related to the success of the field test.
4.3. 1 Reasonable division of labor
Bridge testing involves a certain range in space and time, and everyone's task is also very heavy during the testing process, which requires us to carefully plan and arrange before the test, make clear the task of each tester at each stage, be orderly, and ensure that the test is carried out as planned.
4.3.2 Control and command of test flow
There should be experienced commanders in the field test, and other cooperating personnel should absolutely cooperate with the commanders. Commanders are required to be very clear about the whole test process, and to know what to do at that stage, to read the data, to make some preliminary judgments in time according to the feedback data, and to constantly adjust the field test process.
Loading shall be carried out in strict accordance with the hierarchical loading method determined in the scheme. When the control load condition is not reached, but the data is found to be beyond the normal allowable range, the commander should stop the test process to ensure the safety of the testers.
4.3.3 Site preparation and temporary response measures
Because there are many unknown factors in the old highway bridge, it is difficult to predict the possible problems in the field test. For example, if it is found that the loaded vehicles are different from the requirements of the scheme, the parking spaces can be adjusted to meet the requirements of the efficiency coefficient. During the test, if the tester finds any abnormal situation, he should inform the commander and other testers in time, and then make a judgment before proceeding with the follow-up work.
5. Dynamic testing
The dynamic load test of bridge detection is the basic test item of dynamic test and evaluation method. In order to meet the needs of engineering application, the combination of theoretical analysis and experimental test is a necessary means to solve the bridge vibration problem, an important link in bridge detection, and provides important data parameters for the evaluation of bridge service state and bearing capacity. The content of dynamic load test in bridge detection is mainly the test and analysis of structural dynamic characteristics and dynamic load response, and the main part of measurement is the dynamic stress and dynamic deformation control section of the component with the greatest structural dynamic effect. Generally speaking, the test items mainly include:
(1) Modal parameter test of bridge dynamic characteristics (frequency, vibration mode and damping ratio);
(2) Dynamic response test of bridge (dynamic deflection, dynamic stress, acceleration and impact coefficient).
5. 1 test instrument
The test instruments of dynamic load test mainly include test sensor, signal amplifier, optical oscilloscope, tape recorder and digital signal processor. According to the performance of the instrument and the characteristics of the sensors used, different test systems can be selected.
5.2 Excitation method of bridge dynamic load test
The excitation method of bridge dynamic load test should be based on the structural type and stiffness of the bridge, and the method with good effect and easy implementation should be selected. There are three commonly used methods: natural vibration method, * * * vibration method and pulsation method.
5.3 Dynamic Load Test Data Analysis and Evaluation
The dynamic characteristics of bridge structure are related to the inherent characteristics of the structure itself, such as structural form stiffness, mass distribution, material characteristics, etc., and have nothing to do with other conditions such as load. Modal parameters of bridge are the basic characteristics of the whole structural vibration system and the necessary parameters for structural dynamic analysis. The results can not only be used to analyze the stress of the structure under dynamic load, but also provide an important index for evaluating the bearing capacity of the bridge.
Determination of natural frequency
For simple structures, only the first frequency of the structure is needed, and for complex structural dynamic analysis, the second, third and higher frequencies need to be considered. The natural frequency of the bridge can be directly determined by the peak value, time-history curve or its autocorrelation diagram on the power spectrum recorded by the test system. The dynamic stiffness of load-bearing structure can also be calculated from the fundamental frequency.
damp
The damping characteristics of bridge structures are generally expressed by logarithmic attenuation rate δ or damping ratio D, which can be obtained from the vibration attenuation curve in time domain signals. In addition, the damping can also be calculated by the half-power bandwidth method from the power spectrum diagram, which can be completed by the general test system software.
Pattern shape
The fundamental frequency of general bridge structure is an important parameter in dynamic analysis. According to different structural forms, the layout of sensor measuring points is determined through theoretical analysis.
There are usually two ways to measure vibration modes. One is to use multiple sensors to measure the vibration mode. The other is to use sensors to change the position measurement. In this case, a reference point is needed, and the test is cumbersome. If it is used under limited conditions, the first method should generally be adopted.
impact factor
The impact coefficient μ defined in the bridge code is the ratio of impact force to vehicle load. For structures in linear elastic state, the ratio of load effect caused by dynamic load to load effect caused by static load is 1+μ. Therefore, the impact coefficient is usually measured by measuring the dynamic strain or dynamic deflection of the structure.
Before the test, resistance strain gage's displacement gauge or strain gauge is arranged in the middle of the beam (or at the maximum displacement and strain), and it is connected with the computer through the dynamic strain gauge. During the test, the loaded vehicle passes through the measuring point at a certain speed and records the real-time signal of its output strain changing with time. Generally speaking, the output strain results at various vehicle speeds should be tested and recorded for analysis and comparison.
Generally speaking, when the bridge span L = 30 ~ 70m, the natural frequencies of vehicles and bridges are close, and it is easy to produce * * * vibration, and the impact coefficient under the action of single vehicle is particularly large; The impact coefficient increases with the decrease of damping ratio. The smaller the damping ratio, the more obvious the impact coefficient is affected by the bridge. The impact coefficient of prestressed concrete beam bridge is greater than that of reinforced concrete beam bridge with the same span, so we should pay attention to this point in the test in order to better analyze the test results of impact coefficient.
In fact, the measured vehicle impact coefficient is not only related to the structure itself, but also related to the properties of the tested vehicle, the smoothness of the road surface, the speed and other factors. Vehicle load itself is a vibration system with mass. When it runs on the bridge, it will produce vehicle-bridge coupling vibration with the bridge. Due to the complexity of vehicle dynamics, the discreteness of bridge damping and the randomness of deck unevenness, the impact coefficient measured by different tests of the same bridge is not the same.
6. Conclusion
Bridge inspection is a complex and meticulous work, which requires not only rich practical field experience, but also a solid theoretical foundation as guidance. Only when theory and practice are fully combined, coupled with the tacit cooperation between the commander and the experimenter, can we do a good job in testing and obtain satisfactory data, and only in this way can we make an accurate evaluation.
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