Study On Phonate Mechanism And Characteristics Of Acoustic Radiation Of Elevated Railway Box-girder Bridge

The development of China’s rail transit has made remarkable achievements.Meanwhile,more challenges are subsequently aroused.In the new development period,the rail transit must satisfy the social and economic development concepts,such as environment protection,intelligence and sustainability.Therefore,the requirements for safety,comfort,environmental protection and reliability will be sequentially improved.The urban rail transit(URT)and high-speed railway(HSR)have a close relation with people’s daily routines,which bring great convenience to the people’s travel.However,the problems concerning environmental protection have become an important issue,among which the noise problem is of particular concern.For the elevated line in rail transit,the bridge radiates vibrational noise during train passing which furtherly enhances the overall noise level.And the simply-supported box-girder concrete bridges have been widely applied in rail transit of China.Therefore it is a necessity to carry out a systematic research into the mechanism and characteristic of structural noise radiated by the box-girder bridge in rail transit.The accurate dynamics result plays a key role in the reliability of the noise radiation prediction.Thus an elaborate dynamics model for the train-track-bridge dynamic interaction is adopted to calculate the bridge vibrations.Due to the fact that the acoustic mechanism is more complex than the vibration principle,it is necessary to study its acoustic radiation mechanism in detail before the noise reduction design.It is only possible to put forward the reasonable and effective noise reduction measures for the bridge without blindness in the design.The research on the mechanism of the noise reduction measure can provide a theoretical guidance for the proposal and optimization of the noise reduction measure.The installation of sound barriers on the elevated bridge will inevitably change the acoustic radiation of the entire structure.The study on the noise radiation of the elevated bridge will become more comprehensive and more practical with considering the structural acoustic radiation of the sound barriers.The following works are carried out in this dissertation with taking the simple supported box-girder bridge as the research objective.A prediction model for the vibrational noise radiated by the bridge is firstly established based on the train-track-bridge dynamic interaction theory and the acoustic boundary element theory.In the dynamics model,the train is a multi-rigid-body system of 35 degrees of freedom and the track is modeled by finite element method.The sound radiation mechanism as well as the acoustic radiation characteristics of the elevated bridge for HSR and URT,including the infrasound waves below the audibility threshold,is subsequently implemented using the prediction model.Then based on the acoustic radiation mechanism and characteristics of the box-girder bridge,the design of low-noise bridge and the noise reduction measures are studied and the involved noise reduction mechanisms are deeply explored.Finally,the whole acoustic radiation of the bridge installed with the sound barriers is studied systematically.Through these researches,the findings are summarized as follows:Firstly,the acoustic radiation of the box-girder bridge is mainly concentrated on the low frequencies.The box-girder bridge can also radiate strong infrasound wave,even below the frequency band of the audible threshold which has an adverse influence on humanity and ecosystem.Thus the research on acoustic radiation of box-girder bridge should not only be limited on the noise,but also pay more attention on infrasound of radiation.Moreover,the box-girder bridge can only radiate acoustic waves when a train passes,thus sudden noise can be applied to evaluate the acoustic radiation.Currently the acoustic evaluation caused by rail transit mainly depends on the relevant specification of living areas of surrounding residents.Secondly,the main principles relevant to acoustic radiation of box-girder bridges mainly contain resonance effect,modal acoustic radiation efficiency theory,acoustic waveguide management theory and violent vibration aroused by external excitations.Based on the acoustic radiation performed by box-girder bridge,the main frequency of acoustic radiation can be divided into two sections with different mechanism.The acoustic radiation of the box girder bridge in the lower frequency band is closely related to its own characteristics.Under the influence of the resonance effect,the modal acoustic radiation efficiency or the formation of the acoustic cavity of the box girder bridge cavity,small vibration might also possess large acoustic radiation capability.The acoustic radiation of the box girder bridge in the main vibration frequency band,it is mainly caused by the severe vibration under the external excitation,and there are multiple normal frequencies of the fluid motion in the box-girder bridge cavity.Besides,high-order waves can be formed in the box-girder bridge cavity under the external excitation,and accordingly the box-girder bridge becomes a sound chamber to enhance the structural noise of box-girder bridge radiation.In general,the acoustic radiation capability caused by the characteristics of the box-girder bridge might even higher than those by the violent vibration of the external excitation.Thirdly,according to the sound radiation mechanism and characteristic of box-girder bridge of rail transit,the more reasonable and practical low-noise bridge design and vibration and noise reduction measures of box-girder bridge are more effectively carried out.The relevant design are achieved by changing the natural vibration characteristics of the girder bridge,the vibration distribution of the external excitation and the vibration response of the box-girder bridge,avoiding the influence of the resonance effect,suppressing the acoustic radiation of box-girder bridge and combining vibration reduction to reduce the acoustic radiation.Finally,the sound barrier is installed on the box-girder bridge of the rail transit,and the sound barrier itself becomes a sound source due to its vibration caused by the vehicle,which will interact with the acoustic radiation of the box-girder bridge to affect the acoustic radiation of the overall elevated rail-line structure.Besides,the sound barrier can significantly change the acoustic radiation distribution of the elevated structures.