TESTING STUDY ON DYNAMIC PROPERTIES OF LONGITUDINAL CONTINUOUS SLAB TRACK AND SIMPLY-SUPPORTED BRIDGES ON HIGH-SPEED RAILWAY
Wenshuo LIU1，2，Gonglian DAI1，2，Zhiwu YU1，2，Lvjun LONG1，Xiangyu LIU1
1School of Civil Engineering，Central South University，Changsha 410075
2National Engineering Laboratory for High Speed Railway Construction，Changsha 410075
Abstract：Based on the experimental testing on dynamic properties of longitudinal continuous slab track and 32 m standard prestressed simply-supported bridge on a newly-built high-speed railway，the dynamic characteristics of track-bridge system under the CRH380AM train were obtained and analyzed.The dynamic responses of each structure layer under different running velocities were discussed，including the acceleration，displacement，natural frequencies，vibration modes，damping and so on.Through the field testing，obvious decaying law of vertical and transverse vibration of rail-track plate-bottom plate-bridge deck-piers was proved，and some valuable references were provided for the study and design of high-speed railway bridges.
Keywords：high-speed railway，dynamic property，field testing，longitudinal continuous slab track.
Email：liuwenshuo@csu.edu.cn
1 Introduction
Bridges are the important part of the high-speed railway（HSR）system，occupying a large proportion in the mileage of the HSR line.With the increase of the train’s running speed，the vibration of the train and the track intensifies and the interaction between the train and the railway structure escalates[1，2].To ensure the safety and running comfort of the HSR，the vibration of the bridge caused by high-speed running train has become a hot issue，on which a lot of theoretical and experimental researches have been carried out at home and abroad[3-6].
The CRTSⅡ slab ballastless track，used in Beijing-Tianjin intercity railway for the first time，has spread out more than 5000 kilometers in China.This new type of track is now widely adopted in Beijing-Shanghai，Beijing-Guangzhou，Hangzhou-Changsha high-speed railways，etc.The main characteristic of this type of track is that the rail，track slab and the bottom slab are all longitudinally continuous.
As to the CRTSⅡ ballastless track，only a few theoretical and experimental studies have been conducted，and further research on the vibration property，deformation characteristics and vibration transmission law of the track-bridge system need to be carried out urgently.
2 Field Testing Object and Scheme
An extra-long bridge consisting of 12×32 m simply-supported beams was chosen in the dynamic response testing，with the whole length of 405.98 m（as shown in Figure 1）.All of the beams are standard 32 m prestressed concrete simply-supported beams which are commonly applied in high-speed railway.Adopting the single box and single cell section，the beam height is 3.05 m，the deck width is 12.0 m，the whole length of each beam is 32.6 m，the calculation span is 31.5 m，and the beam spacing is 4.5 m.Besides，six surface drainage modes are used in the bridge.
Figure 1 Elevation and planar drawing of testing bridge
Figure 2 Layout of dynamic testing scheme
Figure 3 Scenes of testing bridge
Figure 4 CRTSⅡ track on the bridge
Figure 5 Standard section of testing bridge
In this dynamic experiment，three spans were selected as the testing objects，including the 1#、6# and 12# beam（shown in Figure 2）.Acceleration sensors were installed on the rail，track slab，bottom slab，bridge deck，top of pier or abutment，and ground.Relative displacement transducers were located on the bottom slab，bridge deck and top of pier.In order to obtain the absolute displacement of bridge，supporting frames were erected on the treated ground under the bottom of bridge in the position of midspan.Displacement sensors were installed on the supporting frames to measure the absolute displacement.In the total，188 sensors were used in this field testing，including 120acceleration sensors，59 displacement transducers and 9 strain gages.
The running train adopted in this testing was CRH380AM train，which is developed for comprehensive detection in high-speed railway by CRRC Qingdao Sifang Co.，Ltd.This new high-speed bullet train has 6 carriages，with the whole length of 153.5 m.During the process of dynamic testing，we have obtained in total 22sets of data of the track-bridge system when the CRH380AM train running over the testing sites at the speed of 200-370 km/h.Taking the 6#beam as the example，the dynamic responses of the system of selected sites were analyzed as follows.
3 Dynamic Response and Property
3.1 Displacement response
To ensure the running safety and comfort，the smoothness of the high-speed railway lines are strictly limited[7-9]，so the deformation of bridge structure must meet certain requirements in relative codes.
Relative displacement sensors were located to obtain the relative displacement between two structure layers，such as the track slab-bottom slab displacement，bottom slab-bridge deck displacement，and girder-pier displacement.Absolute displacement sensors were installed on the supporting frames to measure the absolute displacement of the bottom of beam at the mid-span.
When the CRH380AM train travelled at the speed of 200-370 km/h，the time-history curve of the midspan deflection of 32 m simply supported bridge（6# span）under the train is shown in Figure 6.From Table 1，it is indicated that the maximum deflection in the midspan is 0.5717 mm（less than 20.375 mm limited），so the bridge stiffness is adequate to meet relative regulations[10].
Figure 6 Time-history curve of absolute displacement of the midspan
Table 1 Absolute displacement of mid-span
3.2 Acceleration response
Taking the 6# span as the example，the vertical and transverse accelerations of different structural layers of the track-bridge system at the location of midspan and beam ends are shown in Figure 8-Figure 11.
Figure 8 Vertical acceleration maximum in the midspan
Figure 9 Vertical acceleration maximum at the beam end
Figure 10 Transverse acceleration maximum in the midspan
Figure 11 Transverse acceleration maximum at the beam end
Table 2 Vertical and transverse acceleration amplitudes in the 6# span
From Table 1 and Figure 7-Figure 10，it is shown that the acceleration response of the rail-track slab-bottom slab-pier-ground has an obvious decline trend，and vibrations caused by running train gradually decrease from top to down.
Comparing the responses between the mid-span and beam ends，it is found that different layers have different regulations.For example，the rail’s accelerations in the midspan are always larger than that of beam ends，while the track slab’s accelerations at beam ends are larger than that of midspan.
In addition，the dynamic properties of the track-bridge system are influenced by several factors，including the track irregularity，atmospheric temperature，vehicles，etc.The relation between dynamic responses and the train speed isn’t obvious，and the maximum accelerations appear when the train passing the bridge at the speed of 340-350 km/h.
3.3 Natural vibration frequency and impact factor
Natural vibration frequencies are significant indexes for describing dynamic characteristics of bridges[7]，and the impact factor is an important parameter for designing of bridges[9].We can obtain the vertical and transverse natural vibration frequencies of testing bridges，by the analysis of residual vibration when the train running away from the bridge（Residual Vibration Method）[7，11].Through the analysis of time history curves of the mid-span deflection，the impact factors of 32 m simply-supported bridge are obtained and shown in Table 3.
Table 3 Natural vibration frequency and dynamic factor of beams under the CRH380AM
4 Conclusions
Based on dynamic field testing and analysis of 32 m standard high-speed railway bridge，some conclusions can be drawn as follows：（1）Vertical and transverse vibration of rail-track plate-bottom plate-bridge deck-piers decay obviously from top to down；（2）The stiffness of bridge meet relative regulations in design codes，and the maximum deflection of 0.5717 mm appears when the train speed is 300 km/h；（3）The frequency and impact factor obtained by field experiment and numerical analysis coincide to each other well.
Acknowledgement
The authors wish to acknowledge the support and motivation provided by China Postdoctoral Science Foundation（No.2015M570687）.
References
[1]Gao L.，Xin T.，Xiao H.，Qu C.，2012.Comparative Study on Dynamic Characteristics of Different Sleeper Types on Bridge in High-speed Railway[J].Journal of Tongji University，40（1）：68-72.
[2]Xia C.Y.，Lei J.Q.，Zhang N.，Xia H.，2012.Dynamic Response of High-speed Railway Bridge Subjected to Collision Load and Train Running Safety Analysis[J].Engineering Mechanics，29（12）：101-120.
[3]Xia H.，Zhang N.，Zhang H.R.，2003.Experimental Study of a Prestressed Trough Bridge for High-speed Railway[J].Engineering Mechanics，20（6）：99-105.
[4]Xia H.，Zhang N.，Gao R.I.，et al.，2007.Experimental Study on a Railway Bridge and High-speed Trains[J].Engineering Mechanics，24（9）：166-172.
[5]Liu P.H.，Yao J.C.，Yin J.，et al.，2013.Experimental Study on Dynamic Behaviors of 200-250 km/h High-speed Railway Bridges[J].China Civil Engineering Journal，46（3）：96-102.
[6]Yang Y.Q.，Yao J.C.，Meng X.，et al.，2013.Experimental Study on Dynamic Behaviors of Bridge for 300-350 km/h High Speed Railway[J].China Railway Science，34（3）：14-19.
[7]Ladislav F.，2007.Dynamics Mechanics of Railway Bridge[M].Beijing：Science Press.
[8]Xiang J.，2006.Study on Mechanism and Analysis Theory of Train Derailment[D].Changsha：Central South University.
[9]Zheng J.，2008.China High-speed Railway Bridge[M].Beijing：Higher Education Press.
[10]TB 10621，2014.Code for Design of High Speed Railway[S].Beijing：National Railway Bureau.
[11]Xia H.，Guo W.W.，Zhang N.，2006.Analysis of Resonance Mechanism and Conditions of Train-bridge System[J].Journal of the China Railway Society，28（5）：52-58.
ICRE2016-International Conference on Railway Engineering