| The method of precast segmental construction began to evolve 40 years ago. The principle of precast segmental construction lies in that concrete segments are assembled as a whole structure to bear load through pressure acted on the segments by post-tensioning tendons. Precast segmental construction may adopt two methods generally: balanced cantilever construction and span-by-span construction.The principal advantages of precast segmental construction may be summarized as follows:1. As the girders are fabricated segmentally, there are reliable qualities and small ventures during superstructure construction. Furthermore, geometrical sizes of superstructures are easier to perform.2. The prefabrication of superstructure segments can be processed while the substructure is under construction. So it will speed up the erection of bridges and shorten the time of construction.3. The construction mechanization is relatively high, so precast segmental construction is superior to other methods of construction in the long or long-span bridges.4. As a result of the maturity of the concrete and the application of prestress after long-time maintenance, effects of concrete creep and long-term loss of prestress can be minimized.5. Precast segmental construction can minimize the impact on natural environment and residential community. Interference with existing traffic under bridges during construction is significantly reduced.Precast segmental structure with external tendons (PSBGET) closely accords with the development of prestressing technique and modern construction technology. As a safe, economic and practical construction method, it has been widely accepted and utilized in the bridge engineering all over the world. The combination of the external prestressing technique and precast segmental construction technology in the engineering is a good way to settle the conflict between the quality and the speed of bridge construction.As PSBGET has evolved remarkably in the past 20 years, the construction technology of PSBGET is relatively mature in other countries now. But in our country, its utilization is still of initial stage. In the last few years, it has been used in the field of highway bridges and civic bridges. However, in the field of railroad bridges, the comprehensive studies of this technology have not begun. So it's necessary to analyze and study the mechanical characteristics and failure states of PSBGET in high-speed railways, with the requirements of railroading in our country taken into consideration. Based on the research project of "experimental study on the key techniques of PSBGET in the railway bridges" and the preliminary designs of Jing-hu high-speed railway bridges, mechanical characteristics of PSBGET are analyzed and studied in this thesis.The finite element models of PSBGET are established in the thesis by utilizing finite element software, ANSYS. Owing to the joints in the precastsegmental box girders, the key problem is how to simulate the interactions between the joints. In the thesis, 3D finite element, models (FEM) of the precast segmental box girders are established with contact elements, which may simulate the interactions between the joints. The influences of material nonlinearity of concrete and geometric nonlinearity of structure are also taken into account in the computed models. 3D-FEM analyses of B.GRabbat and K.Sowlat's experimental model and CEBTP (France) experimental model have proved that the computed models can actually reflect the mechanical characteristics of PSBGET.Based on the simulating results of simply-supported box girders (39.1m)and continuous box girders (2 X 40m), the paper emphatically analyzes the full mechanical characteristics of PSBGET and the bend failure states of the right section. The influence of different areas and eccentricities of external tendons on ultimate bearing capacity and ductility is studied in detail. The differences of ultimate bearing capacity and the mechanical characteristics between the se...
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