| Crack is one of the most common diseases of bridge structure,whose existence will lead to the change of the bridge structural stiffness,furthermore,the dynamic characteristics of bridge will be changed,which provides a theoretical basis for bridge crack diagnosis technology based on dynamic characteristics.The dynamic load test of bridge structure has many advantages,such as short test time,no need to close traffic,strong objectivity,and can find hidden diseases and so on.Therefore,the technology of bridge damage diagnosis based on dynamic characteristics has been paid more and more attention.At present,the technology of bridge damage diagnosis on the basis of dynamic characteristics are based on the difference between theoretical values and test values of dynamic characteristics?or process indexes based on dynamic characteristics?.Therefore,the accurate calculation of dynamic characteristics of bridge structures with disease is of great significance.In the past few decades,a variety of structural damage identification methods based on dynamic characteristics have been developed,which have been proved to be effective by numerical simulations or laboratory experiments.However,the application of these methods to practical projects has encountered problems such as inaccurate results and sometimes even erroneous conclusions.This is not because of the problem of damage identification method itself,but that the variation of dynamic characteristics of bridge structure caused by external factors not only lies in the same order of magnitude as the variation of dynamic characteristics caused by damage,but also even completely covers it up.Vehicle is one of the main external factors leading to the variation of dynamic characteristics of bridge structures.The bridge frequency measured under the vehicle action is actually the vibration frequency of the vehicle-bridge coupling system?also known as the loaded frequency in engineering?,which takes the bridge vibration as the main vibration form.The previous work of the research group shows that there is a great difference between the loaded frequency and the natural frequency of the bridge.There is always the function of passing vehicles on the bridges in the bridge health monitoring,and the vehicle is often used as the excitation source to excite the bridge?common vehicle bump test and moving vehicle test?in the dynamic load test of medium and small span bridge structure.Therefore,the effect of vehicle must be taken into account in the calculation of dynamic characteristics of bridge structures with cracks.Many bridge structures can be simplified as beam structures although the bridge structure is a complex mechanical system.High-span ratio is often used to characterize the influence degree of shear effect on mechanical properties in engineering.The beam-type structure is divided into two categories:Euler-Bernoulli beam-like structure and Timoshenko beam-like structure according to High-span ratio.The beam structure can be regarded as Euler-Bernoulli beam structure when high-span ratio is small?the shear effect is negligible?.Euler-Bernoulli beam structure is a widely used form of bridge structure,such as long span continuous beam bridge,simply supported beam bridge,continuous rigid frame bridge and so on.The beam structure with large high-span ratio is regarded as Timoshenko beam structure,which is also very common in the field of bridge engineering.Large high-span ratio main beam,a lid beam of bridge and some other members of the bridge structure can be considered as Timoshenko beams.For example,Dr.Dezeng Jiang and Professor Ronghui Wang of South China University of Technology regard main ribs on both sides of a cable-stayed bridge with slanted pylon as Timoshenko beam.Consequently,it can be considered that both Euler-Bernoulli beam structure and Timoshenko beam structure are very common in bridge engineering.In conclusion,the dynamic characteristic analysis method of cracked beam bridge structure under the vehicles is established,and the research on the effect of vehicle and crack on the dynamic characteristics of beam bridge structure can provide a powerful theoretical support for bridge crack damage diagnosis based on dynamic characteristics,which has important theoretical significance and engineering value.This paper relies on the National Natural Science Foundation of China“Research on Natural Frequency Analysis Method for the Small and Medium Span Beam Bridges Considering the Coupling Action of Vehicles”and Science and Technology Project of Transportation Department of Jilin Province“Research on safety evaluation and strategy of repair and reinforcement on highway bridge in seasonal frozen region based on dynamic reliability”,to discuss the influence of vehicles and cracks on the dynamic characteristics of beam bridges.In this paper,Euler-Bernoulli beam structure and Timoshenko beam structure are taken as research objects,and the vehicle is simplified as spring-mass system,the following research work is carried out:In terms of Euler-Bernoulli beam structure1?The analytical solution of the mode shape function of the first kind of non-uniform beam(moment of inertia satisfy I?7?x?8??28?a1?7?1?10?bx?8?r?10?4and mass per unit length satisfy ??? and the power series solution of the mode shape function of the second kind of non-uniform beam?moment of inertia and mass per unit length obey polynomial variation?are derived,and then the calculation method of the dynamic characteristics of these two kinds of non-uniform beam structure is formed,which provides theoretical support for solving the dynamic characteristics of cracked beam structure carrying spring-mass system.The accuracy and reliability of the method are verified by finite element method and engineering entity test.2?Firstly,the calculation formula of local flexibility coefficient of multi I-section is derived based on the basic principle of local flexibility formation,which is suitable for many kinds of common cross sections in bridge structures.Secondly,a group of basis functions are constructed by means of the mode shape function of the uniform beam,and the accurate solution of the dynamic characteristics of the uniform beam structure with an arbitrary number of cracks is obtained.A convenient method for solving the dynamic characteristics of two kinds of non-uniform beam structures is developed by using the massless torsion spring to simulate the cracks and by means of the transfer matrix method.The method is verified by laboratory experiment and finite element method,and the influence of crack location and depth on the dynamic characteristics of beam structure is discussed by numerical examples.3?A method for solving the dynamic characteristics of cracked beam structures carrying spring-mass system is established based on the theoretical model for the analysis of dynamic characteristics of cracked beam structures in this paper.Taking a practical two-span continuous box beam bridge as the research object,the effect of vehicle on the natural frequency of beam bridge structure is analyzed,and the comprehensive effect of vehicle and crack on the natural frequency of beam bridge structure is investigated.In terms of Timoshenko beam structure4?The undetermined coefficient transfer equation of non-uniform Timoshenko beam expressed in the form of transfer matrix is derived,meanwhile,the massless extension and torsion spring are used to simulate the crack,integrating spring-mass system position and shear force at support,moment equilibrium condition and deformation continuous conditions,the method for solving the dynamic characteristics of cracked Timoshenko beams carrying spring-mass system is formed,and the present references and finite element method are used to verify the method in this paper.The effects of spring-mass system parameters and crack parameters on the dynamic characteristics of two-span non-uniform Timoshenko beam are explored. |
PDF Full Text Request