Evaluation of necessary capacity and development of dynamic design method for unseting prevention cables of bridges with shock absorbing devices

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A rational design method of an unseating prevention cable is not established yet especially for that with a shock absorber. This research aims to clarify a dynamic property of an unseating prevention cable with a shock absorber considering its deformation limit and nonlinearity. The results show that the shock absorber can be designed according to the law of conservation of energy.Then, the design method for the unseating prevention cable was studied for the continuous girder bridges. Once a continuous girder bridge becomes unseated at one pier, the girder is likely to collapse due to bending or shear forces. In this study, the necessary capacity of an unseating prevention cable to prevent the girder of a continuous girder bridge from collapsing was investigated using static and dynamic analysis. The static analysis verified that an unseating prevention cable designed using the present design procedure had sufficient capacity to resist the excessive bending moment and shear force caused by falling of the girder. The dynamic analysis showed that a part of the girder yields due to the large bending moment if a low stiffness cable is used as the unseating prevention cable.Further, the relative velocity during collision between the adjacent bridges was estimated in order to estimate the collision force. The elasto-plastic response analyses were carried out using 2-degree-of freedom model. The parametric study was conducted on the influence of the relative velocity of type of input motion, ground classifications, damping ratio and collision spring. As compared to elastic responses, the relative velocity was reduced in about half and it ranged from 0.3 to 2.5m/sec. Finally, it is clarified that the ground classifications, differential ratio of natural period and collision spring strongly affect on the relative velocity during pounding girders.