Seismic design of unseating prevention system for bridges

日本所有的公路桥梁都安装了地震脱空预防系统，以防止桥梁在地震中倒塌。在日本公路桥梁的现行设计中，规定系统的必要强度为桥梁恒载反作用力的1.5倍。也就是说，不考虑桥梁系统的动力响应，仅以恒载来规定承载力。近年来，随着日本的重要课题--抗震防脱系统的出现，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，如电缆的能力，而研究的设计程序的防脱系统本身仍然有限。由于在实际地震中很难评估系统的运行情况，因此，桥梁防脱系统的设计是基于许多假设下的理论考虑。在本研究中，为了防止桥梁在强烈地震作用下发生脱空，将桥梁的速度响应考虑在内，对桥梁连接装置的设计进行了扩展。连接电缆的需求强度和横截面积是基于能量守恒考虑得出的。连接索的需求能力也被定义为最坏的情况下，梁从桥墩上福尔斯。根据减震器的变形极限和拉索的刚度，安装具有最佳刚度的减震器，可以减小拉索上的应力和所需的横截面积。

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.