SHAKE TABLE TESTS AND NUMERICAL MODELING OF SEISMICALLY ISOLATED RAILWAY BRIDGES

In the implementation of seismic isolation system to railway bridges, the basic requirements are as follows. The first is to secure the running safety of trains under occasional earthquake (Level 1) ground motion. For this purpose, the isolation bearings should work almost like fixed hinges not to allow large displacement. The second is to protect bridge piers from severe damage under rare earthquake (Level 2) ground motion. For this purpose, the isolation bearings should play their roles effectively to add flexibility and damping capability to railway bridge systems. It is also expected to come back to the original position after severe earthquake motion is over, to maintain the function of the train operation. To satisfy above mentioned requirements, accurate estimation of dynamic behavior of the seismically isolated bridge structures is necessary. Especially, complicated constraining force from the railway track to the bridges girder has to be accurately investigated and modeled. For this purpose, shake table tests of partial models of railway bridge structures are conducted subjected to different types and levels of earthquake ground motion. The direct measurement of the constraining force to the girder is almost impossible, so, in this study, it is detected from the equilibrium of dynamic motion of the system using displacement and acceleration data of the tests. The nonlinear and non-symmetric constraining force is found first time in this field. The numerical models are proposed based on the experimental results. The numerical simulation of the isolated bridge structures with the proposed models agrees with experimental results fairly well, suggesting that the proposed models can be used for structural design process for new construction and seismic retrofit of existing railway bridges.