Dynamic Detection Technique And Damage Identification Method For Beam Bridges With Small And Medium Span

Bridge structure is one of the most important parts in national basic infrastructure.in other words, bridge structure is the throat of the transportation syetem.Bridges not only need huge investment of the country ,but also are infrastructure their service with life that usually lasts hundred years.They have great influence on national economic development and people's life, property,and safety.With the rapid increasing volume of traffic and the prolongation of bridge service period, damages appear in quite a lot of bridge structures,which results in performance degradation,and the function is hard to meet the requirements of transportation safety.If damage detection and monitoring of bridge structure are improper, accidents may occur,such as bridge collapse and death of people,and it even bring the country tremendous economic loss.It's not allowed to use the method of reconstruction or structural members replacement for damaged bridges in china at present.Even if in american whose economic is developed,it will cost millions of dollars to improve all the defective bridges to current safety standard, let alone reconstruction. American are unable to stand such huge expenses.Because bridge structural damage and function reduction are usually a gradual altering process.Regular detection or monitoring is needed for damaged bridges,and it's also necessary to evaluate its usage status,in order to mining bridge existing bearing capacity,prolong service life and finally achieve the goal of saving money.Statistical datas show that the number of chinese highway bridges is already close to 340 thousand,while 70% of them are beam bridges with small and medium span.Because of the large number of the beam bridges with small and medium span,it is unreasonable ,from economic angle,to use expensive monitoring system for real-time monitoring in bridges of this kind.It is also infeasible in fact.Therefore,finding a kind of technology of rapid detection and evaluation of usage status for small and medium span bridges is needed.Compared with other traditional detection methods,because bridge dynamic detection technique has many advantages,as follows:(1) It can discovery damages or defects on the internal structure and some parts difficult to approach;(2) Human factors have little influence on diagnosis results,and objectivity and accuracy of the results are relatively high;(3) Detection time is short,and even it's unnecessary to close traffic sometimes;(4) It needs less detection workload and less detection cost.So bridge dynamic detection technique can meet requirements of rapid detection for Chinese small and medium span bridges.It can be said that, it has important strategic significance to maintenance and safe use for Chinese small and medium span bridges to carry out bridge dynamic detection technique for small and medium span bridges.Bridge dynamic detection technique is a technique of multi-subjects.It mainly includes bridge exciting method in structural dynamic field, signal sampling and processing technique in communication field, modal analysis technique in system identification subject and damage identification technique which is dominated by mathematics field.Although, domestic and foreign scholars did research about bridge dynamic detection technique in various aspects.But in view of multi-subjects and complexity, theoretical system of bridge dynamic detection technique is not exactly formed.Even more, effective technical standard and regulations is not determined.Therefore,it's very necessary to carry out the research about bridge dynamic detection technique and damage identification method of beam bridges with small and medium span.Combined with transportation technology project of Jilin University named'Rapid Evaluation Technology and System Development of Bearing Capacity of Concrete Beam Bridges',and national high technology research and development plan (863 project ) named'Research of large range road disaster parameter monitoring and identification and early warning system in seasonal frozen region', several aspects of research work have been carried out as follows:1. Based on vehicle-bridge coupled vibrations,using alembert principle and euler-bernoulli beam hypothesis, theoretical model about dynamic response calculation in the whole process of jumping excitation is established. Calculation method of limited vehicle jumping height is also clarified theoretically,and it provides theoretical guarantee for vehicle jumping experiment.2. Based on modal synthesis analysis method, an analysis model of three-dimensional space vehicle-bridge coupled vibrations in the whole process of vehicle excitation is established. Based on the model of vehicle-bridge coupled vibrations,influence law of vehicle parameters ,deck smoothness, vehicle speed and acting position of vehicles to impact effect is analysed.3. Under the condition of without considering tire stiffness, multiple theoretical models of interaction of vehicle-simply supported girder bridge system are established.Using modal analysis method and free vibration theory, bridge frequency under vehicle load can be solved.And analytical expression of bridge frequency under vehicle load with single vehicle load is given.Because of without considering tire stiffness when deriving the model,this calculation model is only suitable for the vehicles with relatively large tire stiffness.4. Under the condition of considering tire stiffness, multiple theoretical models of interaction of vehicle-simply supported girder bridge system are established.Using modal analysis method and free vibration theory, bridge frequency under vehicle load can be solved.And analytical expression of bridge frequency under vehicle load with single vehicle load is given.Based on derived theoretical calculation model, influence law of vehicle position, suspension stiffness, car body weight, tire weight and tire stiffness to the change value (the difference between bridge frequency under vehicle load and natural frequency) of bridge frequency under vehicle load.5.Based on the analytical expression of bridge frequency under vehicle load with single vehicle load, the analytical expression between bridge frequency under vehicle load (test frequency) and natural frequency is established.This expression can be used to calculate bridge natural frequency from measured frequency under vehicle load.6.A method of bridge structure dynamic parameter identification only by using free attenuation response of measuring point displacement is presented.The method can be used to identify three bridge structure dynamic parameters,such as frequency, vibration mode and damping ratio,and identification accuracy is relatively high. It ensures the stability by using cubic spline curve to carry out the pretreatment of displacement response during identification process7. A damage identification method based on sensitivity matrix for beam bridge is established.The damage identification method can be used to identify the damage of beam bridges with small and medium span more precisely. It can avoid partial differential calculation during solving sensitivity process and improve the practicality of the identification method by using matrix perturbation theory to form dynamic parameter sensitivity matrix. Using modern mathematical methods,the statistical characteristics of this damage identification method is analyzed,and the uncertainty of damage degree identified is also determined.