An Investigation into the Static and Dynamic Mechanical Behaviors of Self-anchored Suspension Bridges

• Main Authors: Tsai, Jeng-Lin, 蔡政霖
• Other Authors: Kou, Chang-Huan
• Format: Others
• Language: zh-TW
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/60452433622991991730

博士 === 中華大學 === 土木與工程資訊學系(所) === 98 === The suspension bridge is characterized by its long span and elegant structural shape. Traditional suspension bridges often span over long distances with the main cables earth-anchored at two ground structures near the abutments. As the length of the span decreases, the cost to anchor the main cables rises and eventually reaches a point where the construction cost of the suspension bridge significantly exceeds that of other forms of bridges. In fact, if the main cables of the suspension bridge are directly anchored onto the main girder, great savings on the anchorage can be achieved. Suspension bridges with this type of anchorage are referred to as a self-anchored suspension bridge. The self-anchored suspension bridge is increasingly favored in construction due to its aesthetic appearance, economic benefits, and strong adaptability to the local topographical and geological conditions. It has become a very competitive candidate for bridges that are used to cross rivers, or a small bridge with a short span in urban areas. A number of self-anchored bridges have already been constructed abroad. Even though the only difference between the self-anchored suspension bridge and the traditional earth-anchored suspension bridge is that the locations of the main cables’ anchorages are different, the load-bearing characteristics of the structure and the construction method are subjected to significant changes. The self-anchored suspension bridge does not only inherit the mechanical characteristics of the traditional suspension bridge, but also possesses its own unique static and dynamic features due to the large axial force in the main girder. In order to gain a deeper understanding of these features, this study analyzes and discusses the responses of the self-anchored suspension bridge under static loading, its ultimate load-bearing capacity, and its responses under seismic loading. The main contents and outcomes of this research are as follows. (1)Analyze and discuss what effects the breakage of hangers will have on the response of the self-anchored suspension bridge under static loading at various locations: The results indicated that when the hanger broke, the cable force in the adjacent hangers increased noticeably. Moreover, the breakage of the hanger at the mid-span of the main span had the largest influence on the maximum negative bending moment. When the hanger that lies in the vicinity of the tower broke, evident changes in the internal forces of the self-anchored suspension bridge and changes in joint displacements were observed. (2)Analyze and discuss the ultimate load-bearing capacity of the self-anchored suspension bridge and its failure process: The influences of what the changes in strength or stiffness of substructures and the influences of broken hangers will have on the ultimate loading-bearing capacity and the failure process of the bridge was investigated. The results showed that changes in the strength or stiffness of different substructures had effects on the ultimate load-bearing capacity of the self-anchored suspension bridge to various extents. While the ultimate load-bearing capacity was clearly affected by the breakage of some hangers, the concurrent breakage of a number of these hangers could lead to the immediate failure of the bridge. (3)Analyze and discuss earthquake responses of the self-anchored suspension bridge by performing response spectrum analysis and time history analysis. In addition, the influences of geometric changes, the changes in the stiffness of substructures, and the influences of the broken hangers on the earthquake responses are also studied. The analysis results indicate that the self-anchored suspension bridge also keeps some of dynamic characteristics of the earth-anchored suspension bridges, which has relatively long low-level natural vibration periods, large vertical displacements, possible beam lowering, and possible damage of the expansion joints. Moreover, the change in the stiffness of the main girder had the largest influence on the vibration frequency, the internal forces in the direction along the main girder under earthquake events, and on the joint displacements. The change in the stiffness of main cables had the largest effect on the internal structural forces and joint displacements under vertical earthquake actions. (4)Analyze and discuss the ductile earthquake responses of the self-anchored suspension bridge through time history analysis and study the earthquake responses after the installation of the metal dampers: The effects of geometric and stiffness changes of substructures and the broken hangers on the bridge’s earthquake responses are also studied. The results suggested that by considering the ductile earthquake responses, the seismic loading on the bridge can be lowered, and consequently, the seismic resistance of the self-anchored suspension bridge was improved. However, joint displacements were observed to have increased remarkably. Furthermore, metal dampers can consume a large amount of seismic energy. As a result, the bending moment at the base of the tower was reduced noticeably, and the inflections of the main girder and the tower could be better controlled. This therefore reduces the possibility of beam lowering and causing damage to the expansion joint devices.