| With the development of urban rail transit in our country,more and more subway lines will cross the vibration-sensitive regions,and the application of damping track structure is inevitable.Therefore,a variety of vibration reduction track are derived.Among them,the Long Elastic Sleeper Track(LEST)is a new type of vibration reduction track structure,which has been gradually applied to the construction of urban rail transit.However,the vibration reduction performances and dynamic characteristics of LEST are not yet clear;the relative theoretical research is scarce;and there is still no research based on field dynamic tests.LEST of Beijing Subway Line 15 was taken as the research background in this paper.Based on the finite element theory and the theory of vehicle track coupling dynamics,and combined with the modality test and the on-site dynamic tests,the dynamic characteristics of the LEST were studied.The main work and conclusions are as follows:(1)The modality test,vibration transmission test of long elastic sleeper and field dynamic tests in subway tunnel were carried out.The first ten modes of long elastic sleeper were tested.By comparing with the model test results,the accuracy of the finite element model and the parameters were verified.The vibration transmission test of single sleeper has verified the vibration reduction effect of the elastic cushion.From the analysis of the vibration transmission characteristics of LEST,it can be concluded that the lower the stiffness of the cushion is,the lower the resonance frequency of the track structure is,and the lower the peak value of the admittance curve in the high frequency ranges is,which is more conducive to vibration reduction.According to the field dynamic test,the vibration reduction effect of LEST was quantified from time domain and frequency domain.Under the test conditions,the LEST can be reduced by 10.52dB at the bottom of the tunnel wall,and 9.6db can be reduced at 1.5m of the tunnel wall.Under test conditions,the application of LEST can reduce the vibration level of the bottom of tunnel wall by 10.52dB,while 9.60dB at point of 1.5m height on tunnel wall.(2)A vehicle-track-tunnel-soil coupling dynamic analysis model was established.The models of three kinds of track structure in subway tunnel,the common ballastless track,Low Vibration Track(LVT)and LEST were established.The dynamic responses of three kinds of track structures are calculated with the train speed of 80km/h.The results show that,compared with the common ballastless track,LVT and LEST can significantly reduce the structural vibration.LEST can reduce the vibration acceleration of the tunnel wall to a greater extent,which shows that LEST is superior to LVT in reducing the impact of the subway vibration environment.(3)Based on the established dynamic model,the influences of parameters such as stiffness and damping ratio of elastic cushion and train speed on the dynamic characteristics of LEST were studied.Research shows that with the decrease of the cushion stiffness,the wheel-rail vertical force increases gradually,and dynamic displacements of rails and sleepers increase gradually,while changes of derailment coefficient,wheel unloading rate and lateral displacement of rails are not obvious.The vibration accelerations of rails and sleepers gradually increase,and the vibration of the track bed and tunnel wall gradually decrease.It is indicated that the smaller cushion stiffness is more beneficial to the vibration reduction effect.With the increase of the cushion damping ratio,the wheel-rail force,the dynamic displacement and the vibration acceleration decrease,which shows that the higher damping ratio is more conducive to vibration reduction.When the train speed is different,the dynamic indexes increase with the increase of train speed.Therefore,the LEST design should consider the various factors,such as elastic cushion stiffness,the cushion damping ratio and train speed etc.(4)From the angle of wheel-rail forces,dynamic displacement,reaction force of fasteners and structure acceleration,the influence of fastener failure,unsupported sleepers,and cushion aging on the dynamic performances of LEST.The research shows that when the fastener failure occurs,it has little influence on vehicle dynamic behaviors and the safety indexes.The fastener failure has a great influence on reaction force of fasteners within 3 sets of adjacent fasteners.If a set of fasteners fail completely,reaction force of adjacent fasteners will increase by 33.77%.Fastener failure has a great influence on the dynamic displacements of rails and sleepers.At he total failure position,rail lateral displacements increases by 49.18%,rail vertical displacements increases by 1.64 times,sleeper vertical displacement increases by 43.27%.Besides,fastener failure will increase rail,the adjacent sleepers vibration acceleration,seriously affecting track stability.The more number of failure fasteners,the more obvious the impact on each index.In the aspect of unsupported sleeper and cushion aging disease,the greater the unsupporting area,the greater the unsupporting capacity and the larger the cushion aging stiffness are,the greater the influence on the dynamic displacement of LEST and the vibration acceleration of subway structures are.Therefore,the relevant disease should be focused on to put an end to related diseases.(5)A new method of vibration reduction is put forward,which comined the LEST with damped rails,vibration absorber fastener,and damping concrete.The dynamic response analysis shows that:the damped rails can significantly reduce the vibration acceleration of rail and sleeper of LEST,and has less influence on other dynamic indexes.The lower the stiffness of fastener is,the better the damping effect is,while the dynamic displacements of rails and sleepers are enlarged to some extent.The vibration reduciton effect of the damping concrete track bed on the tunnel wall is obvious,which has less influence on other dynamic indexes.Compared to common concrete track bed,the vibration level of point of 1.5m on the tunnel wall can be reduced by 11.12dB. |
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