| In China,with the urbanization rapid development,the urban traffic congestion becomes more prominent,which has been an urgent problem for the urban development.The urban rail transit(URT)is an effective measure to relieve this problem.Unfortunately,the URT often brings about environmental vibration and noise pollution.For example,when the subway train runs in the vibration-sensitive region,the moving train induced vibrations often greatly plague the working and living environment of urban residents and even cause damage to nearby buildings in the long term.In order to mitigate these vibrations,a lot of vibration reduction measures have been developed,in which the vibration damping track is commonly used.In addition,the noise problem caused by URT not only affects the riding comfort,but also has a great impact on the sound environment in the area along the railway.The elevated URT usually give rise to more severe noise problem,due to the fact that the moving train also causes the bridge structure to radiate vibrational noises.Therefore,this thesis based on the theories of train-track-bridge dynamic interaction,the acoustic boundary element methods and structural acoustics,focuses on the URT with typical vibration damping track and carries out the studies on the low-frequency acoustic characteristics of structural noise in elevated URT due to moving train.Modelling methods for noise radiation of the track structure on the bridge and the characteristics and mechanism of low-frequency vibration and structural noise of the typical vibration damping track are studied in detail.The vibration and sound characteristics of the box-girder bridge paved with vibration damping track are also studied,and the commonality and differences in the vibration and sound characteristics between the track and the bridge are analyzed.The effects of pavement rule and the reflection of the rigid bridge surface on the sound radiation of the vibration damping track are investigated.The results show that the theoretical model established in this thesis can effectively predict the low-frequency acoustic characteristics of structural noise in elevated URT due to moving train.Through the comparison of calculation results from different numbers of track slabs on single-span bridge,it is necessary for the study on the acoustic characteristics of the track structure on bridge to establish the full-scale acoustic model of all slabs on the single-span bridge.The slabs of steel-spring floating-slab track(FST)have densely distributed vibrations with small amplitude in the frequency range below 150 Hz,and the vibration shapes are mainly indicated as vertical bending and thus the radiated sound pressure distribution in the free field is regular.The vibration shapes are highly consistent with the modal shapes in the frequency range below 150 Hz,and the vibration frequencies are closed to the natural frequencies.Hence,the resonance phenomenon happens,resulting in that the small vibration can cause great sound radiation.The frequency range of 200~250Hz is the vibration dominant range for the slab of the steel-spring FST,and the violent vibration can motivate strong ability of sound radiation.However,in this frequency range,the vibration shapes of the slab mainly belong to the local vibration,and hence,the radiated sound pressure distribution in the free field is irregular.The vibration and sound characteristics of the two types of the slabs are similar.In the frequency range below 50 Hz,the acoustic radiation characteristics of the short slab are stronger than those of the long slab,and even in the same sound field,the sound radiation contributions of different short slabs are distinctly different.The vibrations of the ladder-sleeper track are mainly concentrated in the positions of the sleepers and rail pads,and the acoustic characteristics are the most significant in the range of 200~250Hz,and besides the vibration reduction effect of the vertical damping pad is more significant when the vibration is violent.L-type support has a great influence on the propagation of sound radiated by the sleepers and rail pads,which not only changes the sound radiation law of ladder sleepers,but also affects the acoustic pressures at different positions in the sound field.The vibration and sound of the box-girder bridge are concentrated in the frequency range below 250 Hz,which is lower than that of the track structure.Both the double-line bridge paved with steel-spring FST and the single-line bridge paved with ladder track have the densest vibration in the frequency range below 150 Hz,and the sound radiation capacity is the most at frequencies below 50 Hz,mainly because the resonance effect and sound radiation efficiency greatly enhance the sound radiation capacity of the bridge.The sound radiation rules of these two types of bridges are very regular in the lower frequency range,and they become irregular with the frequency increasing.Under the external excitation action,these two types of bridges can form the entire vertical bending vibration and the local(m,n)vibrations of the top plate,which have a great influence on the sound radiation of the box-girder bridge,and the regularity and the occurrence probability of these vibrations are gradually reduced from the single-line bridge paved with ladder-sleeper track to double-line bridge paved with FST of long slabs,and to double-line bridge paved with FST of short slabs.In general,the sound radiation contributions of all plates of single-line box-girder bridge are significantly stronger than those of double-line box-girder bridge.For the single-line bridge,the sound radiation contributions of two web plates are more balanced.While,for the double-line bridge,the sound radiation contribution of the web plate at side of the running train is significantly stronger than that at the other side.The overall sound radiation of the single-line bridge is stronger than that of the double-line bridge.The noise reduction design of elevated URT box-girder bridge can mainly focuses on the frequency range of below 50 Hz,while the others frequencies also need to be considered for the single-line bridge paved with ladder track,due to its frequency range of sound radiation is wider.The rigid bridge surface can obviously change the sound radiation path of the track structure and the sound pressures in the sound field,and thus the sound pressures in the horizontal and far field are greatly enhanced.At the position in the horizontal and 25 m form the track center,the overall sound pressure induced by the steel-spring FST with short slabs and the ladder-sleeper track are increased about 26.2dB/L and 6.3dB/L,respectively.Comparison with the ladder-sleeper track,the sound field above the FST is more concentrated,and the overall sound pressures above the FST with short slab and the long slabs are increased about 0.8dB/L and 0.5dB/L,respectively.The overall sound pressures above the ladder-sleeper track is conversely decreased,due to the conjunct influences of the shielding effect of the L-type support and the reflection effect of the rigid bridge surface. |
PDF Full Text Request