Seismic design of unseating prevention system for bridges

日本所有的公路桥都安装了防震脱座系统，以防止地震时桥梁倒塌。在目前日本公路桥设计中，规定该体系的必要强度为桥梁主梁自重反力的1.5倍。换句话说，容量仅根据恒载规定，未考虑桥梁体系的动力响应。随着地震失座预防系统作为一个重要问题在日本的出现，近年来进行了大量的研究。然而，大多数研究都集中在桥梁倒塌的具体因素，如索容量，而对防脱座系统本身的设计过程的研究还很有限。由于在实际地震中难以对桥梁失座系统的运行情况进行评估，因此在设计桥梁失座系统时，考虑了许多假设条件，从理论上进行了考虑。有必要确保系统在地震中充分发挥作用。在本研究中，将强震作用下防止桥梁梁脱臼的连接装置设计扩展到考虑桥梁的速度响应。基于能量守恒的考虑，导出了连接电缆的需求强度和截面积。在梁从桥墩脱落的最坏情况下，确定了连接索的需求能力。以减振器的变形极限和索刚度为基础，选择最优刚度的减振器，既能减小索的受力，又能减小所需的横截面积。

All road bridges in Japan have been fitted with a seismic unseating prevention system to prevent bridge girders from falling during an earthquake. In the current design of Japanese highway bridges, the necessary strength of the system is prescribed to be 1.5 times the reaction force for the dead load of the bridge girder. In other words, the capacity is stipulated in terms of the dead load alone, with no consideration of the dynamic response of the bridge system.With the emergence of the seismic unseating prevention system as an important issue in Japan, much research has been conducted in recent years. However, most studies have focused on specific factors in bridge collapse such as cable capacity, while research on the design procedure for the unseating prevention system itself remains limited. The bridge unseating prevention system has been designed based on theoretical considerations under many assumptions because it is difficult to evaluate the operation of the system during an actual earthquake. It is necessary to ensure that the system functions adequately during earthquakes.In this study, the design of connecting devices for preventing bridge girders from becoming unseated during strong seismic motion is extended to include consideration of the velocity response of the bridge. The demand strength and cross-sectional area of the connecting cable are derived based on conservation of energy considerations. The demand capacity of the connecting cable is also defined for the worst case that the girder falls from the pier. The installation of shock absorber with optimum stiffness based on its deformation limit and the cable stiffness is found to reduce both the stress on the cable and the required cross-sectional area.