In the long-term use of bridges, various structural damages will inevitably occur. Bridge inspection is to evaluate the bridge according to the actual situation, which has the following two important meanings:
First, through a comprehensive, detailed and in-depth on-site inspection of the service condition, defects and damages of the bridge, the nature, location, severity and development trend of the defects and damages are clarified, the causes of the defects and damages are found, and the influence of the defects and damages on the service performance and bearing capacity of the bridge is analyzed and evaluated, so as to provide timely and targeted first-hand information for the design of bridge maintenance and reinforcement foundation reconstruction.
Secondly, through the comprehensive inspection of bridges, the current technical data of bridges are systematically collected and technical data are accumulated, which provides necessary conditions for enriching the bridge database, strengthening the scientific management of bridges and improving the technical level of bridges; Through the reasonable design of detection methods and the arrangement of long-term monitoring equipment, the bridge health monitoring system is gradually established to ensure the long-term safe operation of the bridge, so as to give full play to its best economic and social benefits.
2. Bridge detection foundation
Bridge inspection is mainly based on the standards and specifications issued by People's Republic of China (PRC), Ministry of Communications and Ministry of Construction, and the relevant technical standards are as follows:
(1) The Measures for Quality Evaluation of Highway Engineering is submitted to Highway 446;
(2) Technical Specification for Highway Maintenance (JTJ 73-96);
(3) Technical Standard for Highway Engineering (JTGB 01-2003);
(4) Appraisal Method for Bearing Capacity of Old Highway Bridges (Trial) Bridge Design Code (CJJ11-93);
(5) Design Load Standard for Urban Bridges (CJJ77-98)JTJ073.
Three types of bridge detection
There are three kinds of bridge inspection: periodic inspection, periodic inspection and special inspection. Regular inspection refers to the observation of the bridge by road section inspectors or bridge maintenance personnel under various weather conditions, with the purpose of ensuring the normal function of the bridge structure, making the structure timely maintained and urgently handled, and making reports on some major problems. Regular inspection depends on experienced full-time bridge inspection engineers, mainly by visual inspection, supplemented by necessary tools, conventional measuring instruments, cameras and other equipment. , judge the cause of the disease on the spot, make a quality score, estimate the scope and method of maintenance, or put forward suggestions to restrict traffic, which is a comprehensive inspection of the bridge structure quality on a regular basis. Special inspection is a comprehensive inspection, strength measurement and defect measurement of bridges based on certain physical and chemical nondestructive testing methods for various special reasons, aiming at finding out the clear cause, degree and scope of damage and analyzing the consequences caused by damage and the dangers that potential defects may bring to the structure.
4 the content of bridge detection
4. 1 detection point
Bridge inspection can usually determine the main points of investigation according to the bridge type. For example, the inspection points of beam bridge are: mid-span cracks and deflection; End oblique crack; The state of the connecting parts of the main girder; The appearance quality of parts and so on. The inspection points of arch bridge include: arch ring vault crack, pier displacement and so on. Through visual inspection, the causes of bridge diseases can be analyzed and judged.
4.2 Test content
Bridges can generally be divided into superstructure, substructure and accessory structure. The superstructure mainly refers to the main girder in the beam bridge; The substructure includes piers, abutments, foundations, caps and piles. Subsidiary structures include bridge deck pavement, 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.
The inspection contents of bridge superstructure include the concrete strength of hollow slab beam, the carbonation depth of hollow slab beam concrete, the crack status and distribution law of beam, etc. The detection contents of bridge substructure include the crack status and distribution law of pier and abutment, weathering, peeling, cracking, dislocation, subsidence, horizontal displacement or rotation of frame pier. The detection contents of bridge deck system include bridge deck pavement, sidewalk, railing, drainage facilities and expansion joints. The inspection is mainly carried out from the following aspects: the plane pavement has no cracks, peeling, sunken water, pits, waves and bulges; Whether sidewalks and kerbs are peeled off or damaged; Whether the railing system is damaged, loose, cracked, bent downward, arched upward, skewed and cracked in the concrete of the member due to impact; The drainage facilities on the official surface are free from damage, blockage and leakage; Whether the expansion joint is damaged, whether the function is normal, etc. The test contents of bridge bearings include whether the bearing function is in good condition and whether the components are complete and clean; Whether the concrete of base, beam bottom and roller shaft is damaged; Whether the seat plate and tooth plate are welded off; Whether there are fractures, dislocations and cavities; Whether the rubber bearing is aging, deformed or invalid.
4.3 detection steps
The first is bridge investigation and calculation, mainly data collection. 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 materials include historical special vehicles, traffic conditions and maintenance materials.
Secondly, the bearing capacity check. After understanding the overall characteristics of the bridge, some necessary inspections should be carried out. The principle of checking calculation is related technical specifications, but it should be noted that some specific parameters should be based on the actual bridge, and the reduction should be reduced, and some favorable factors should be considered when necessary. Through checking calculation, we can judge whether the safety bearing capacity of the bridge structure meets the design requirements, and evaluate the construction quality and operation status of the bridge. For the old bridge, we can tap its bearing potential, and put forward suggestions for strengthening or rebuilding the existing bridge that can not meet the requirements.
5 bridge detection method
5. 1 rebound detection method. Rebound method is a method to detect the compressive strength of concrete by rebound instrument. Rebound instrument is a mechanical nondestructive testing instrument. Because there is a certain relationship between the compressive strength of concrete and its surface hardness, the hammer of the rebound instrument is hit on the concrete surface by a certain elastic force, and its rebound height (rebound value read by the rebound instrument) is also proportional to the surface hardness of concrete. The rebound method is simple in equipment, convenient in operation and fast in testing, and is widely used in on-site direct measurement.
5.2 Dynamic test method. The dynamic load test of bridge detection is the basic test item of dynamic measurement and evaluation method, which mainly includes the test and analysis of structural dynamic characteristics and dynamic load response. The main part of the measurement is the control section of the dynamic stress and deformation of the component with the greatest structural dynamic effect. Generally speaking, the test items mainly include modal parameters test of bridge dynamic characteristics (frequency, vibration mode and damping ratio) and dynamic response test of bridge (dynamic deflection, dynamic stress, acceleration and impact coefficient). The measurement structure of bridge natural frequency is relatively simple, only the first-order frequency of the structure is needed, and the second, third and higher-order frequencies should be considered in the complex dynamic analysis of the structure. 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.
The impact coefficient is usually measured by measuring the dynamic strain or 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 is 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.
5.3 Load test. Bridge dynamic load test refers to the test of applying dynamic load such as traffic load or other dynamic load to the bridge structure to measure the dynamic characteristics of the structure, so as to judge the impact and vibration of the bridge structure under the dynamic load. Usually, the strain, deflection and crack of the beam are measured by vehicle load, and the actual bearing capacity of the bridge is judged according to the comparative analysis of the test results and theoretical calculation values. This load test is non-destructive. According to the nature of test load, it is usually divided into static load test and dynamic load test. The former reflects the structural performance of the bridge under static load, while the latter reflects the dynamic performance of the bridge structure. Static load test is a test to test the static strain, static displacement and cracks of the structure by applying a static load at a specified position on the bridge, so as to infer the working state and service capacity of the bridge structure under the load. The general analysis and evaluation work mainly includes the evaluation of the working condition of the structure, the strength and stability of the structure, the foundation and foundation, the stiffness requirements of the structure, cracks and so on.
6 concluding remarks
Bridge detection is a very detailed work, and there are many methods for bridge detection. Inspectors should not only have rich practical field experience, but also have a solid theoretical foundation as a guide. Only by fully combining theory with practice, coupled with the tacit cooperation between commanders and testers, can we do a good job in testing, obtain satisfactory data and make accurate evaluation.
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