Research On The Monitoring And Safety Evaluation Of The Service Status Of Long-span Railway Steel Truss Arch Bridge

The condition of the service performance of long-span railway steel arch truss bridges directly relates to the safety of high-speed trains during their operation period. It is still one challenge in the research field of high-speed railway bridges about how to utilize the monitoring data from the actual service circumstances to real-time monitor the service performance of the steel arch truss bridges and to give safety evaluation. In this paper, the research object is the lateral steel arch truss of the Nanjing Dashengguan Yangtze Bridge. Using the long-term monitoring data of temperature field,train loads, axial strain and longitudinal displacements, the method of safety evaluation on the service performance of long-span railway steel arch truss bridges is deeply researched under the actions of temperature field and the train loads. During the research, some new methods are put forward including calculating the standard values of gradient temperature in member sections,building the 3-D mathematical models of the longitudinal static displacements from bearings,identifying and real-time monitoring the degrading bearings. The main work and research fruits are as follows:1. Long-term monitoring and analysis of the temperature field and train loads. Using the monitoring data of temperature field and parameters of train loads from the Nanjing Dashengguan Yangtze Bridge, the monitoring time-dependent trends of gradient temperatures are deeply studied from the cross sections of the top chord, the diagonal web member, the bottom chord and the bridge deck chord in the steel arch truss, and the probability statistics models of gradient temperatures are built and the standard values of the gradient temperatures are furthermore calculated. Besides, the time-dependent trends of the loading locations, the quantity of carriages and the moving speed in the train loads are studied in detail under the actual operation circumstances.2. Long-term monitoring and analysis of axial strains in the truss members under the actions of temperature field and train loads. The monitoring time-dependent trends of axial static strains and axial static dynamic strains in the top chord, the diagonal web member, the bottom chord and the bridge deck chord in the steel arch truss are deeply studied, the influence of uniform temperature at night and gradient temperature at daytime on the axial static strains in the four members are furthermore researched, and finally the mathematical model between axial static strain and temperature field are built. The time-dependent trends and spatial distribution trends of dynamic load factors from the top chord, the diagonal web member, the bottom chord and the bridge deck chord are specifically investigated, and the influence of loading locations, the quantity of carriages and moving speed on the axial dynamic strain of each member in the truss are finally made clear.3. Long-term monitoring and analysis of longitudinal displacements in bearings under the actions of temperature field and train loads. The monitoring time-dependent trends of longitudinal static displacements and the cumulative values of small-amplitude dynamic strains in bearings are deeply studied, and furthermore the correlations between longitudinal displacements and temperature filed(the whole structural temperature and the gradient temperatures between members) are deeply researched, and the spatial correlations between longitudinal displacements of different bearings are revealed, from which the mathematics model of multivariable linearization regression is used to describe the location-varying and time-varying longitudinal displacements in the main girder. The influence of the quantity of trains on the cumulative distances of dynamic displacements in bearings is thoroughly studied, and the probability statistics characteristics of the cumulative distances under single train load is deeply researched. Finally, the cumulative distances of bearings under many train loads are simulated and verified using the Monte Carlo method.4. The method of safety evaluation on the service performance of the steel truss arch bridge. The static performance of the steel truss arch bridge is real-time monitored using the static strain residuals, and the static performance of the steel truss arch bridge under the designed service life is evaluated using the standard values of static strains. The failure probability of the monitoring dynamic load factors over the designed values in the codes is calculated, and it is suggested that the dynamic performance of the steel truss arch bridge is real-time monitored by tracking the time-dependent trend of the failure probability. The standard values are calculated utilizing the e probability statistics models of dynamic load factors and then compared with many country codes.The method of identifying and real-time monitoring the degrading bearings is raised using the mathematics model of longitudinal displacements in bearings, and the failure probability of the cumulative values of small-amplitude dynamic strains in bearings over the wearing limit is calculated to judge whether the bearings reach the wearing limit in their service life.