Fundamental Study on Analytical Evaluation of Modal Damping in Cable-supported Bridges

在斜拉桥的抗风设计中，模态结构的阻尼力是必不可少的，但通常是经验假定的。虽然通过现场振动试验收集了大量的阻尼比数据，但对试验数据进行分析的理论研究工作还不多见。本研究的主要目的是提出一种斜拉桥模态阻尼力的分析方法，并通过对其进行阻尼力分析来展示其潜力。主要研究成果如下：1.利用文献调研建立的桥梁振型阻尼力数据库，对斜拉桥的振型阻尼力特性进行了研究。结果表明，斜拉桥具有明显的振型相关特性，而悬索桥没有这种特性。2.结合现场振动试验数据，对Tsurumi Tsubasa大桥进行了详细的减振分析。不仅对主梁、塔架和拉索的内耗进行了半解析估算，还对气动阻尼力、支座摩擦力和拉索阻尼器引起的阻尼力进行了半解析计算。提出了基于振型阻尼力能量表示的阻尼力分析方法，并将其应用于三种不同类型的大跨度斜拉桥：池口组合箱梁桥、横滨海湾桁架桥和单平面斜拉索体系的鹤足桥。计算了各大桥不同振型的振型阻尼比，并与实测值进行了比较。通过这些减振分析，也显示了分析评价减振特性的巨大潜力。

The modal structural damping is indispensable but is usually assumed empirically for the wind resistant design of cable-supported bridges. Although many data of damping ratio have been collected through field vibration tests, there haven't been many theoretical research works for analyzing the experimental data. The main objectives of this study are to propose an analytical method for the estimation of the modal damping in cable-stayd bridges and to show its potential by conducting the damping analyzes.The main research results are summarized as follows.1.The modal damping characteristics of cable-supported bridges were discussed by using the database of modal damping which was completed by the literature survey. It was found that the mode-dependent damping characteristics is peculiar to cable-stayd bridges but is not identified in suspension bridges.2.The damping analysis of Tsurumi Tsubasa Bridges was conducted in detail with the data of field vibration test. Not only the internal damping in the girder, the towers and the cables, but also the aerodynamic damping, the support friction and the damping due to cable dampers, were estimated semi-analytically. As a result, the contribution of each damping source to the modal damping was clearly shown.3.The analytical method for the damping evaluation based on the energy representation of modal damping was proposed and applied to three different types of long-spanned cable-stayd bridges ; Ikuchi Bridge with composite box-girders, Yokohama Bay Bridge with truss girder and Tsurumi Tsubasa Bridge with the stay-cable system in single plane. The modal damping ratios for different modes of each bridge were evaluated and compared with the field-measured values. The high potential of the analytical evaluation of the damping characteristics was also shown through these damping analyses.