Dynamic Parameter Identification And Serviceability Assessment For The Long-Span Footbridges With Tuned Mass Dampers

In recent years,with the wide application of light-weight and high-strength materials,the greatly progress of structural design methods and the increasing demand for urban construction and tourism development in China,the long-span footbridges have been widely constructed.In 2000,the Millennium Bridge in London had to be closed due to the human induced vibration,which attracted wide attention in the field of bridge engineering.Moreover,353 people died due to human induced vibration trampling at Diamond bridge in Cambodia in 2010,which showed the importance of mitigation of human induced vibration.Tuned mass damper(TMD)is an effective means to mitigate wind-induced vibration and human-induced vibration and improve pedestrian serviceability.Accurate identification of dynamic characteristics of footbridge structure and optimization of TMD parameters are extremely important to ensure the effective performance of TMD.Therefore,this paper focuses on the TMD-based vibration reduction and serviceability of footbridges.Research on modal parameter identification of footbridges via dynamic tests,parameter optimization of multiple tuned mass dampers(MTMD),and performance evaluation of TMD for vibration reduction are carried out.Also,the comfortability assessment of footbridges caused by wind-induced vibration is take into consideration.The major work of this dissertation is presented as follows:(1)Research on modal parameter identification method of footbridge and the corresponding dynamic test is carried out.The modal parameters identification methods of footbridges under different loads are summarized,including environmental vibration,free vibration and forced vibration.The advantages and disadvantages of these methods are given.Based on the measured data of environmental vibration and crowd load,the dynamic parameters of two large-span footbridges are identified and compared with the results of finite element model.(2)Based on the additional known mass,the modal mass identification method of the footbridge is established,which can identify the effective modal mass of the structural damping mode,and lay the foundation for the optimal design of TMD.The method assumes that the modal shape and structural stiffness do not change with or without the additional mass during the load test.The modal mass of the original structure is identified on the basis of the frequency variation of the relevant structural modes causing by loading and unloading various additional mass.The numerical simulation of three types of bridge,i.e.beam bridge,cable-stayed bridge and suspension bridge,is carried out.For the beam bridge and cable-stayed bridge,the frequency and mode shape with or without additional mass are unchanged,and the method can accurately identify their modal mass of the relevant modes.However,for the suspension bridge,the proposed method cannot identify the modal mass of the relevant modes.The main reason for this case is pointed out.(3)The optimization of parameters of MTMD and its performance evaluation for vibration reduction are carried out.In this paper,research on parameter optimization of TMD system under harmonic load with several subsystems,whose frequency is identical,is conducted.The results show that the optimal parameters of each TMD subsystem are the same as those of a set of TMDs.That is,the mass ratio of MTMD with the same parameters must be determined by the total weight of TMD.Based on the TMD prototype installed in the Liuyang River Han Bridge for vibration reduction,the characteristics of the TMD prototype with the same design parameters are further tested.The frequency discreteness of each sub TMD prototype is very small,but the measured damping ratio has some discreteness.(4)Based on the parameter identification of the pedestrian bridge-TMD coupling system,a method is proposed for TMD debugging and vibration reduction efficiency evaluation.The method first identifies the controlled modal and TMD modal parameters via dynamic test of bridge-TMD coupling system.Then,on the basis of the identified results,one can debug the frequency and damping ratio of the TMD using the identified parameters,construct the frequency response function of bridge-TMD coupling system,and evaluate the performance TMD using a predefined vibration reduction rate.The effectiveness and reliability of the proposed approach is verified by numerical example.The results show that the method is suitable for the debugging and evaluation of TMD vibration reduction of pedestrian girder bridges.(5)Based on the model wind tunnel test and theoretical analysis,research on serviceability evaluation of footbridges under vortex-induced vibration is carried out.By comparing with the existing pedestrian comfort standards,an amplitude limit-standard of footbridge under vortex-induced vibration is proposed on the basis of pedestrian comfortability.The amplitude limits of vortex-induced vibration for low-order and high-order modes of long-span footbridges are analyzed.For the modes larger than 1Hz,the amplitude limit is less than 5mm.The cross-section of the main girder of footbridges should be optimized through aerodynamic measures to avoid vortex-induced vibration.Based on the investigation of Liuyang River Han Bridge,the problem of vortex-induced resonance is studied.The amplitudes of vortex-induced resonance under different structural damping ratios are compared.