Study On Super Long-Span Bridge With Lightweight FRP Cables

With the increment of the bridge span, the disadvantages of traditional steel cables have become more obvious, such as heavy self-weight, large sag effect, and low load-carrying efficiency, which restricts the increase of the spanning ability of the super long-span bridges. Meanwhile the fatigue degradation and corrosion of the traditional steel cable seriously affect the durability. Fiber Reinforced Polymer (FRP) materials have been applied in civil engineering during the past 30 years due to their superior mechanical and chemical properties, such as the high strength-to-weight ratio, corrosion and fatigue-resistance, and ease of tailoring. As one of the most effective application of the FRP material, FRP cables can take advantages of FRP materials, and avoid the disadvantages induced by anisotropy of the FRP material. When applying as cables in super-long span bridge, FRP material not only can increase the span of the bridge, but also improve the long-term performance. Therefore FRP cables can provide an effective way to realize super long-span bridge with both short-term and long-term superior behavior.The application of the FRP cable in super long-span bridge can realize the lightweight of the bridge. However, few studies have been made on the mechanical behavior of the lightweight study of the super long-span bridges with FRP by now, especially the vibration study induced by lightweight of FRP cables. Due to the complex machanism, the vibration characteristics and damping properties of the FRP cable should be studied based on vibration experiment. Semi-theory and semi-experience method is adopted in the current studies on the dynamic performance of the FRP cables because of lacking vibration testes. Besides, it is necessary to optimize the structural design of the long-span bridge in order to improve the mechanical performance and economic performance of the whole bridge. A series of investigations were conducted focusing on the above problems caused by the lightweight of the super long-span bridge with FRP cable. The main research works and conclusions are listed as follows.(1) Reduced-scale cable models were designed based on similarity criteria, selecting the two typical stayed cables of Su-tong Bridge as the prototype. The in-plane and out-of-plane vibration experiments of model cables were conducted according to the method of free vibration decaying. The results of the natural frequencies indicate that the model cables can well represent the dynamic properties of real cables. In addition, the analysis of damping ratios of different material cables show that the modal damping ratios of in-plane vibration of CFRP and BFRP cables are much larger than those of traditional steel cables. Therefore the vibration damping characteristics can be improved by the lightweight with FRP cables. Moreover the analysis results also indicate that the modal damping ratios of out-of-plane vibration are inversely proportional to the natural frequencies. The equation for modal damping ratios of in-plane vibration was modified based on the energy dissipation theory and the mode shape curve (MSC) method. The coefficients of dynamic strain damping energy of CFRP and BFRP cables were derived by the modified equation. According to the experimental results, it is verified that the coefficients of dynamic strain damping energy can be used to evaluate the vibration damping characteristics of different material cables.(2) A new type FRP cable with self-damping function is designed according to the different dynamic characteristic of these two portions of cables. The model vibration experiments were conducted to study the vibration characteristics and damping properties of the FRP cable with self-damping function. The experiment results show that the modal damping ratios of the FRP cable with self-damping function can increase with the vibration amplitude of the cable. It is proved that the FRP cable with self-damping function can dissipate vibration energy intelligently with respect to the amplitude of vibration. Based on the modified equation for modal damping ratio and viscoelastic damping theory, the calculation method of the modal damping ratio of the FRP cable with self-damping function was proposed. The results of theoretic calculation are well coincident with the test results, which indicates the proposed calculation method can effectively predict the modal damping ratio of the FRP cable with self-damping function.(3) The reasonable spans of different FRP cables were determined based on the key parametric study by means of the analytical solutions. The design strategy of cable-stayed bridge with hybrid arrangement of FRP cables was proposed. Then the static and dynamic behaviors of cable-stayed bridge with hybrid arrangement of FRP cables were evaluated with a case study of the cable-stayed bridge with a main span of 2038m. The results show that the bridge design scheme with hybrid arrangement of FRP cables can satisfy the design requirement of the long-span cable-stayed bridge. Moreover the hybrid arrangement of FRP cables can increase the natural frequencies of the entire bridge. Besides the seismic safety and aerodynamic stability of the long-span cable-stayed bridge can be improved by the hybrid arrangement of FRP cables. In addition, the evaluation results of the life-cycle cost of the different material stayed cable indicate that the cable-stayed bridge with hybrid arrangement of FRP cables offers an optimum economic performance.(4) According to the similarity criterion and the reasonable span of different FRP cables, the design scheme of the model bridge was proposed based on the prototype of the Su-tong Bridge. And the main structural members of the model bridge, including tower, girder, stayed cable and piers, are detailed designed in detail according to the requirement of the stiffness similarity and mass similarity. Construction analysis of the model bridge is conducted according to backward-calculation method.(5) The static and dynamic behaviors of the entire suspension bridge with steel and different FRP cables were analyzed using the finite element method, which takes the Messina Bridge with a main span of 3300 m as the prototype. The results indicate that the application of FRP cables in long-span suspension bridge benefits the spanning ability, improves the load-carrying efficiency and the efficiency of material utilization of the cable. Due to the lightweight of FRP cables, the natural frequencies and the aerodynamic stability of large-span suspension bridges with FRP cables can be improved. Furthermore, the seismic responses of the long-span suspension bridge with FRP cables were reduced owing to the better energy dissipation behavior of FRP cables. According to the analysis of the maintenance cost of the Forth Road Bridge (the first internal inspection strength evaluation acoustic monitoring and dehumidification of the main cables), life-cycle costs of the long-span suspension bridge with different material cables were evaluate. The life-cycle cost results indicate that FRP cable can effectively improve the economic performance of the entire bridge due to their better anti-corrosion properties and fatigue resistance performances.Finally, all the achievements and innovations of the study are concluded and some suggestions for furture study are proposed.