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

目前还没有形成一套合理的防脱索设计方法，尤其是带有减振器的防脱索。本研究的目的是在考虑其变形极限和非线性的情况下，阐明带有减振器的防脱层缆索的动力特性。结果表明，减振器可以根据能量守恒定律进行设计。然后，研究了连续梁桥防脱层索的设计方法。一旦一座连续梁桥在一个桥墩出现下座，由于弯曲或剪力作用，该梁很可能会坍塌。本文采用静力和动力分析方法，研究了连续梁桥防脱层索防止主梁倒塌的必要能力。静力分析表明，按该设计方法设计的防脱索具有足够的抗弯剪能力，可以抵抗下沉引起的过大弯矩和剪力。动力分析表明，采用低刚度索作为防脱索时，由于弯矩较大，部分主梁发生屈服，并估算了相邻桥梁碰撞过程中的相对速度，从而估算出撞击力。采用二自由度模型进行弹塑性响应分析。对输入运动类型、地面分类、阻尼比和碰撞弹簧对相对速度的影响进行了参数研究。与弹性响应相比，相对速度降低了约一半，变化范围为0.3~2.5m/s。最后阐明了地基分类、固有周期差分比和碰撞弹簧对撞梁过程中相对速度的强烈影响。

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.