| Longitudinal coupled ballastless track,represented by CRTS? slab track,is one of the most widely used track structures in China.The track structure is exposed to ever-changing natural environment all the year round and bears the effects of cyclical temperature changes and temperature gradients.In the high temperature weather of summer,due to the temperature pressure,there is slab upwarp in some sections.Slab upwarp will obviously affect the geometry of the track and reduce the comfort and safety of driving.The research about the formation mechanism,influencing factors and prevention and control measures of the upwarp on the longitudinal coupled track is not systematical and perfect.In this paper,around the problem of slab upwarp,combining the structural characteristics and load forms of CRTS? slab track and based on static and dynamic criteria of structural stability and other related theory,the model is systematically establishedthe and upwarp stability of slab under temperature pressure is analyzed.On this basis,the in-depth study on the upwarp mechanism,influencing factors and control measures is carried out.1.Established the differential equation method for the calculation of the stability of the longitudinally connected ballastless track,and the influence of the subject parameters is analyzed.The slab is regarded as an infinite long beam on the rigid foundation and the boundary conditions before and after upwarp are clarified.The upwarp curve of the slab is derived according to the deflection differential equation of beam.Based on the static criterion of structural stability,under the condition of small deformation,it is assumed that the waveforms before and after the upwarp on the slab meeting the upwarp curve.Principle of energy standing is used to derive the relationship between longitudinal temperature pressure and the upwarp sagittal under different initial upwarp sagittal.The differential equation method for the calculation of the stability of the longitudinal coupled ballastless track is proposed.This method modifies the Taylor/Gan method and is more rigorous in mechanics.Compared with the hypothetical deformation method,it is more accurate and can better describe the upwarp behavior of slab.According to the differential equation method,regardless of the release of temperature pressure,the equilibrium path of slab includes three stages of holding,expanding and destabilizing.Considering the release of temperature force,if the initial upwarp sagittal is small,the equilibrium path includes four stages of holding,expanding,destabilizing and strengthening,if the initial upwarp sagittal is larger,the equilibrium path only includes two stages of holding and strengthening.The influence of the subject parameters of slab is analyzed by differential equation method.The result shows that for every 1GPa increase of the elastic modulus,the holding limit is reduced by 0.8?.For every 5mm increase of the thickness slab,the holding limit is reduced by 1.6??2.7?.For every increase of slab gravity load of 6.25 k N/m,the holding limit is reduced by about 1.7? ? 2.7?.2.The finite element analysis model of upwarp instability of slab is established,and the main causes and influencing factors of slab upwarp are clarified.According to upwarp characteristics of slab,the finite element model of upwarp stability of slab is established.The arc length method is used to calculate the equilibrium path of slabupwarp,which is in good agreement with the differential equation method.The effects of temperature rise,initial upwarp sagittal,dummy joint,positive and negative temperature gradient,rail on upwarp stability of slab are analyzed.The results show that: the initial upwarp and longitudinal temperature stress is the main influencing factor of upwarp stability.The greater the initial upwarp sagittal,the worse the stability.If the limit of maximum upwarp displacement of slab is 2mm,in the case of the maximum temperature rise of 60?,when the initial upwarp of slab is greater than 22 mm,there is the possibility of over-upwarp.Dummy joint reduces the bending stiffness of slab and reduces its stability.The effect of positive and negative temperature gradients and dummy joint on the upwarp stability of slab are small,which can be ignored during the study.Rail can increase the upwarp stability of slab,and upwarp stability of slab is reduced after considering the temperature rise of rail.3.The finite element model of broad-narrow joint with damage is established,and the influence of typical damage of broad-narrow joint on the upwarp stability of slab was analyzed.Aiming at the breakage of narrow joint and crack at the interface between broad joint and narrow joint,the influence on upwarp stability of slab is studied.The results show that the breakage of narrow joint is an important factor in upwarp stability of slab.Under the temperature load,the breakage of narrow joint can lead to large eccentric compression,which reduces the upwarp stability of slab.When the narrow joint is completely breakaged,the maximum upwarp displacement is 8.44 mm with the temperature rise is 60 ?.When the elastic modulus of the narrow joint is higher than 0.1 times of its design value or the breakage height is less than 70 mm,the maximum upwarp displacement of slab at 60? is less than 2mm and the breakage of narrow joint has little effect on the stability of slab at this time.It is recommended that the damage should be repaired in time when it exceeds this limit.The overall damage of broad-narrow joint and crack at the interface between broad joint and narrow joint have less influence on the stability of slab.4.The detail finite element model of broad-narrow joint is established,and the damage behavior of broad-narrow joint under temperature pressure is studied.Based on cohesive zone theory and damaged plasticity theory of concrete,a detail finite element model for calculating the damage of broad-narrow joint is established.The influence of the elastic modulus and the width of narrow joint on the damage is analyzed,and the causes of typical damage of broad-narrow joint in site are explained.The stress characteristics of broad-narrow joint under longitudinal temperature stress are analyzed,witch further explain the cause of the damage from the perspective of force.The results show that the broad-narrow joint are in the state of vertical tension and longitudinal compression under temperature rise,which is the main reason for the damage of broad-narrow joint.Breakage of narrow joint is a kind of progressive compressive damage caused by insufficient strength of concrete at broad-narrow joint.Under the existing conditions,if the overall temperature rises above 20?,the narrow joint have a risk of breakage.Crack at interface between broad joint and narrow joint is a kind of abrupt tension damage caused by unequal width of broad joint and narrow joint.Under the existing conditions,when the overall temperature rises is 5?,the interface will be completely crack and this damage is difficult to avoid.Increasing the strength of broad-narrow joint can improve the longitudinal stress but cannot improve the vertical stress effectively,hence,the compression damage can be reduced effectively,but not for tension damage.Increasing the width of narrow joint can improve the longitudinal stress and vertical stress so that the compression and tension damage of broad-narrow joint can be effectively reduced.In order to reduce the damage of broad-narrow joint and improve the stability of slab,it is suggested that the strength of broad-narrow joint is equal to that of slab and the width of broad joint and narrow joint is equal.5.The vertical vibration model of slab under longitudinal temperature stress is established,and the frequency characteristics of slab under temperature load are analyzed.Based on the dynamic criterion of structural stability,the finite element model of vertical vibration of slab is established.Firstly,the natural frequency characteristics of slab during the upwarp process are studied,and effects of temperature rise,temperature gradient,dummy joint and foundationstiffness are analyzed.Then the influence of the length of under-slab and temperature rise on the vibration frequency of slab is studied,and the influence degree of these two factors is clarified.The results show that natural frequency of slab decreases gradually with the increase of longitudinal temperature compressure in slab,and approaches 0 when the temperature rise reaches the critical temperature.Temperature gradient,dummy joint and the decrease of foundation stiffness will reduce natural frequency of slab,but they have little effect on the critical temperature of slab instability.The length of under-slab has almost the same influence on each order vibration frequency of slab,and the temperature rise has a greater influence on the first three-order vibration frequency,and has less influence on the fourth to sixth-order vibration frequencies.The effect of the length of under-slab on the vibration frequency of slab is much greater than the temperature rise.It is recommended that when the vertical vibration frequency of slab is lower than 2 Hz,it should be rectified in time.6.Summarize the control measures of upwarp stability of slab and propose an optimization scheme of pin anchoring.Brushing heat-insulating coatings,structural optimization,and the remediation measures such as pin anchoring,bottom-injection and slab stress relaxation were systematically summarized.The heat-insulating coating can reduce the daytime temperature of slab by 7?9? and the nighttime by 2?4?.The temperature gradient can be reduced by more than 30?/m during the day and 8?/m?10?/m at night.The equal width of broad joint and narrow joint and equal strength of broad-narrow joint and slab can reduce the vertical tensile force at the interface of the broad joint and narrow joint,which can effectively reduce the damage of broad-narrow joint.Aiming at the pin anchoring measures,the upwarp stability of slab and the stress under different schemes of the pin are analyzed.Pin anchoring can not reduce the damage of narrow joint and the narrow joints still have the possibility of complete damage.The number of anchor points on one slab has less effect on the stability of the slab and the force of the pin.The farther the first anchor point is from the broad-narrow joint of the damage,the worse the stability of the slab but the smaller the force of the pin.The number of pins at the same anchor point has less effect on the stability of the track plate,but increasing the number of pins can significantly improve the force.For the stability of the integrated slab and the force and durability of the pin,it is recommended to use 3 pins for anchoring at the second sleeper next to the broad-narrow joint. |
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