| Long-span composite box bridges are characterized by their innovative configuration, economical construction and favorable mechanics. As assumptions and simplifications are usually introduced in general analysis methods, for the design of this type of bridges, simulation analysis methods are inevitably needed. This thesis, with Yi-Chang Yangtze Railway Bridge (YCYRB) as an engineering background, is focused deeply on the computational theory and its application with respect to simulation analysis techniques. Firstly, based on the generalized variational method, a new plate displacement model with independent interpolation functions of linear displacement from angular rotation is presented. Accordingly, a Plate Segment Element Variational Method (PSEVM) and Plate Segment Element Method (PSEM) are established for the first time which could be applied in the simulation analysis of composite box bridges. Furthermore, a program for PSEM is developed and its efficiency and precision is verified by a numerical model and a Plexiglas's physical model test. Secondly, by using PSEM, the static and dynamic characteristics of YCYRB are studied thoroughly. Many proposals for the improvement of transverse flexible norm stress in box girder are presented and a corresponding parameters analysis is completed. The mechanics of bridges composed by stiff girder and tender arch are investigated deeply. At last, the anti-seismic properties of YCYRB are discussed generally in this thesis. The main research work is listed as follows:1. New computation theories for box girder are presented based on the assumptions of plate element.At first, based on the presumptions of plate element, new geometrical equations are proposed in this thesis according to deformation characteristics of box girder. And stiffness matrices for web and top/bottom plate of box girder are derived on the basis of variational method. To simulate accurately the influence of diaphragms on mechanics of box girder and guarantee continuity of displacement models between elements, independent interpolation functions of angular rotation from linear displacement are established and corresponding diaphragm element formulations are developed based on generalized variational method. Thus an Analytic Method based on Plate element (AMP) for box girder is presented. Generally speaking, the precision and efficiency of AMP is better than traditional plate element because of its appropriate and compatible displacement models. Moreover, the influence of diaphragms on mechanics of box girder could be reflected more accurately than existed methods. To guarantee the continuity of displacement models between AMP and other traditional elements, some possible constraint equations are discussed in this thesis.2. A new PSEM is proposed which could be applied in the simulation analysis of composite box bridges with box girder.Firstly, Degrees Of Freedom (DOF) of AMP along with their essence are analyzed and the choice of these DOFs is made according to their significations. Furthermore, with the application of generalized variational method, a new displacement model for PSEM is built which adopts only the DOFs of traditional plate elements. The PSEM displacement model, besides the more accurate reflection of box girder's mechanics as well as the same precision and efficiency compared to ASM, is characterized by its independent interpolation functions of angular rotation from that of linear displacement. Thus SSEM has better adaptiveness than AMP when applied in simulation analysis of box girder. Moreover, a substitute shear strain model is introduced to avoid shear lock when the exact numerical integration of stiffness matrix is performed. Thus the PSEVM is established and its corresponding PSEM is proposed. Finally the program for SSEM is developed and verified in this thesis. Such advantages as high efficiency and better precision of PSEM make its application in the simulation analysis of long span composite box bridges more convenient than AMP.3. A simulation analysis on the static characteristics of YCYRB is preformed.For the innovative configuration of YCYRB, an entire simulation model is built and analyzed based on the PSEM program developed in this paper. Firstly, a plane analysis considering the influence of construction phases is performed to achieve the primary properties of YCYRB at the time when the box-girder is closed up. Further, taking it as the initial state of spatial simulation analysis, the behavior and characteristics of YCYRB under deck load as well as live load is investigated thoroughly. These are mainly about the manner of load transmission with respect to webs of the box girder, the shear lag of top/bottom plate of box girder and the rules of transverse normal stress due to flexural deformation. Besides these, to improve the disadvantages of transverse stress state according to design project, several substitute projects are proposed and corresponding design parameters analysis are performed. Thus the mechanics of diaphragm is revealed and the behavior and effect of diaphragm influence on box girder are compared thoroughly.4. A simulation analysis on the dynamic characteristics of YCYRB is preformed.At fist, the mass matrix for SSEM is derived. And to achieve quickly partial eigenvalues from large-scale characteristic dynamical equations, Ritz-Vector Superposition Method is introduced, thus the application of PSEM in the dynamic properties analysis of composite box bridges is. practical. And then the dynamic characteristics analysis of YCYRB is preformed in this paper. These includes the comparison of results between SSEM and traditional chine-like models, the influence analysis of deck mass and different pile-soil models on dynamic properties of YCYRB.5. The anti-seismic properties of YCYRB are investigated.Firstly, the preliminary anti-seismic properties of YCYRB are discussed, an artificial earthquake wave is synthesized according to the site-soil properties of YCYRB and the choice of parameters relating to anti-seismic analysis is made. Successively, the earthquake response of YCYRB is calculated separately by response spectrum method and time-history method. When taking into account the influence of multi-excitation due to traveling-wave effect which depends on the velocity of earthquake wave near the site soil, Large Mass Method (LMM) is introduced. Thus the time history analysis of YCYRB under earthquake action is more convenient than others methods. At last, by the comparison of results from different methods as well as different directions of earthquake action, the anti-seismic properties of YCYRB's components such as beams, arches and piers are discussed generally in this thesis.6. Some helpful advice for the design of composite box bridges with stiff girder and tender arch is made in this thesis.By the simulation analysis of YCYRB, the mechanics of this type of bridges composed by stiff girder and tender arch is revealed generally and some helpful advice for the design is made suitably. These include the choice of improvement projects for transverse bending stress, the computing methods for dynamic properties as well as the unfavorable positions of YCYRB under seismic action. The work in this thesis could be useful for the safety of YCYRB in construction and operation stages. And the proposed PSEM could be applied in the simulation analysis of long span composite box bridges. As a result, Practical merits and wide applications could be achieved in the future.
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