关于斜拉桥的中英文翻译.doc

Study on nonlinear analysis of a highly redundant cable-stayed bridge1AbstractA comparison on nonlinear analysis of a highly redundant cable-stayed bridge is performed in the study. The initial shapes including geometry and prestress distribution of the bridge are determined by using a two-loop iteration method, i.e., an equilibrium iteration loop and a shape iteration loop. For the initial shape analysis a linear and a nonlinear computation procedure are set up. In the former all nonlinearities of cable-stayed bridges are disregarded, and the shape iteration is carried out without considering equilibrium. In the latter all nonlinearities of the bridges are taken into consideration and both the equilibrium and the shape iteration are carried out. Based on the convergent initial shapes determined by the different procedures, the natural frequencies and vibration modes are then examined in details. Numerical results show that a convergent initial shape can be found rapidly by the two-loop iteration method, a reasonable initial shape can be determined by using the linear computation procedure, and a lot of computation efforts can thus be saved. There are only small differences in geometry and prestress distribution between the results determined by linear and nonlinear computation procedures. However, for the analysis of natural frequency and vibration modes, significant differences in the fundamental frequencies and vibration modes will occur, and the nonlinearities of the cable-stayed bridge response appear only in the modes determined on basis of the initial shape found by the nonlinear computation.2. IntroductionRapid progress in the analysis and construction of cable-stayed bridges has been made over the last three decades. The progress is mainly due to developments in the fields of computer technology, high strength steel cables, orthotropic steel decks and construction technology. Since the first modern cable-stayed bridge was built in Sweden in 1955, their popularity has rapidly been increasing all over the world. Because of its aesthetic appeal, economic grounds and ease of erection, the cable-stayed bridge is considered as the most suitable construction type for spans ranging from 200 to about 1000 m. The worlds longest cable-stayed bridge today is the Tatara bridge across the Seto Inland Sea, linking the main islands Honshu and Shikoku in Japan. The Tatara cable-stayed bridge was opened in 1 May, 1999 and has a center span of 890m and a total length of 1480m. A cable-stayed bridge consists of three principal components, namely girders, towers and inclined cable stays. The girder is supported elastically at points along its length by inclined cable stays so that the girder can span a much longer distance without intermediate piers. The dead load and traffic load on the girders are transmitted to the towers by inclined cables. High tensile forces exist in cable-stays which induce high compression forces in towers and part of girders. The sources of nonlinearity in cable-stayed bridges mainly include the cable sag, beam-column and large deflection effects. Since high pretension force exists in inclined cables before live loads are applied, the initial geometry and the prestress of cable-stayed bridges depend on each other. They cannot be specified independently as for conventional steel or reinforced concrete bridges. Therefore the initial shape has to be determined correctly prior to analyzing the bridge. Only based on the correct initial shape a correct deflection and vibration analysis can be achieved. The purpose of this paper is to present a comparison on the nonlinear analysis of a highly redundant stiff cable-stayed bridge, in which the initial shape of the bridge will be determined iteratively by using both linear and nonlinear computation procedures. Based on the initial shapes evaluated, the vibration frequencies and modes of the bridge are examined.3. System equations3.1. General system equationWhen only nonlinearities in stiffness are taken into account, and the system mass and damping matrices are considered as constant, the general system equation of a finite element model of structures in nonlinear dynamics can be derived from the Lagranges virtual work principle and written as follows:Kjbj-Sjaj= Mq”+ Dq3.2. Linearized system equationIn order to incrementally solve the large deflection problem, the linearized system equations has to be derived. By taking the first order terms of the Taylors expansion of the general system equation, the linearized equation for a small time (or load) interval is obtained as follows: Mq”+Dq +2Kq=p- up3.3. Linearized system equation in staticsIn nonlinear statics, the linearized system equation becomes2Kq=p- up4. Nonlinear analysis4.1. Initial shape analysisThe initial shape of a cable-stayed bridge provides the geometric configuration as well as the prestress distribution of the bridge under action of dead loads of girders and towers and under pretension force in inclined cable stays. The relations for the equilibrium conditions, the specified boundary conditions, and the requirements of architectural design should be satisfied. For shape finding computations, only the dead load of girders and towers is taken into account, and the dead load of cables is neglected, but cable sag nonlinearity is included. The computation for shape finding is performed by using the two-loop iteration method, i.e., equilibrium iteration and shape iteration loop. This can start with an arbitrary small tension force in inclined cables. Based on a reference configuration (the architectural designed form), having no deflection and zero prestress in girders and towers, the equilibrium position of the cable-stayed bridges under dead load is first determined iteratively (equilibrium iteration). Although this first determined configuration satisfies the equilibrium conditions and the boundary conditions, the requirements of architectural design are, in general, not fulfilled. Since the bridge span is large and no pretension forces exist in inclined cables, quite large deflections and very large bending moments may appear in the girders and towers. Another iteration then has to be carried out in order to reduce the deflection and to smooth the bending moments in the girder and finally to find the correct initial shape. Such an iteration procedure is named here the shape iteration. For shape iteration, the element axial forces determined in the previous step will be taken as initial element forces for the next iteration, and a new equilibrium configuration under the action of dead load and such initial forces will be determined again. During shape iteration, several control points (nodes intersected by the girder and the cable) will be chosen for checking the convergence tolerance. In each shape iteration the ratio of the vertical displacement at control points to the main span length will be checked, i.e., The shape iteration will be repeated until the convergence tolerance, say 10-4, is achieved. When the convergence tolerance is reached, the computation will stop and the initial shape of the cable-stayed bridges is found. Numerical experiments show that the iteration converges monotonously and that all three nonlinearities have less influence on the final geometry of the initial shape. Only the cable sag effect is significant for cable forces determined in the initial shape analysis, and the beam-column and large deflection effects become insignificant.The initial analysis can be performed in two different ways: a linear and a nonlinear computation procedure. 1. Linear computation procedure: To find the equilibrium configuration of the bridge, all nonlinearities of cable stayed bridges are neglected and only the linear elastic cable, beam-column elements and linear constant coordinate transformation coefficients are used. The shape iteration is carried out without considering the equilibrium iteration. A reasonable convergent initial shape is found, and a lot of computation efforts can be saved.2. Nonlinear computation procedure: All nonlinearities of cable-stayed bridges are taken into consideration during the whole computation process. The nonlinear cable element with sag effect and the beam-column element including stability coefficients and nonlinear coordinate transformation coefficients are used. Both the shape iteration and the equilibrium iteration are carried out in the nonlinear computation. NewtonRaphson method is utilized here for equilibrium iteration.4.2. Static deflection analysisBased on the determined initial shape, the nonlinear static deflection analysis of cable-stayed bridges under live load can be performed incrementwise or iterationwise. It is well known that the load increment method leads to large numerical errors. The iteration method would be preferred for the nonlinear computation and a desired convergence tolerance can be achieved. Newton Raphson iteration procedure is employed. For nonlinear analysis of large or complex structural systems, a fulliteration procedure (iteration performed for a single full load step) will often fail. An incrementiteration procedure is highly recommended, in which the load will be incremented, and the iteration will be carried out in each load step. The static deflection analysis of the cable stayed bridge will start from the initial shape determined by the shape finding procedure using a linear or nonlinear computation. The algorithm of the static deflection analysis of cable-stayed bridges is summarized in Section 4.4.2.4.3. Linearized vibration analysisWhen a structural system is stiff enough and the external excitation is not too intensive, the system may vibrate with small amplitude around a certain nonlinear static state, where the change of the nonlinear static state induced by the vibration is very small and negligible. Such vibration with small amplitude around a certain nonlinear static state is termed linearized vibration. The linearized vibration is different from the linear vibration, where the system vibrates with small amplitude around a linear static state. The nonlinear static state qa can be statically determined by nonlinear deflection analysis. After determining qa , the system matrices may be established with respect to such a nonlinear static state, and the linearized system equation has the form as follows:MAq”+ DAq+ 2KAq=p(t)- TAwhere the superscript A denotes the quantity calculated at the nonlinear static state qa . This equation represents a set of linear ordinary differential equations of second order with constant coefficient matrices MA, DA and 2KA. The equation can be solved by the modal superposition method, the integral transformation methods or the direct integration methods.When damping effect and load terms are neglected, the system equation becomesMAq” + 2KAq=0This equation represents the natural vibrations of an undamped system based on the nonlinear static state qa The natural vibration frequencies and modes can be obtained from the above equation by using eigensolution procedures, e.g., subspace iteration methods. For the cable-stayed bridge, its initial shape is the nonlinear static state qa . When the cable-stayed bridge vibrates with small amplitude based on the initial shape, the natural frequencies and modes can be found by solving the above equation.4.4. Computation algorithms of cable-stayed bridge analysisThe algorithms for shape finding computation, static deflection analysis and vibration analysis of cable-stayed bridges are briefly summarized in the following.4.4.1. Initial shape analysis1. Input of the geometric and physical data of the bridge.2. Input of the dead load of girders and towers and suitably estimated initial forces in cable stays.3. Find equilibrium position(i) Linear procedure Linear cable and beam-column stiffness elements are used. Linear constant coordinate transformation coefficients ajare used. Establish the linear system stiffness matrix K by assembling element stiffness matrices. Solve the linear system equation for q (equilibrium position). No equilibrium iteration is carried out.(ii) Nonlinear procedure Nonlinear cables with sag effect and beam-column elements are used. Nonlinear coordinate transformation coeffi- cients aj; aj， are used. Establish the tangent system stiffness matrix 2K. Solve the incremental system equation for q. Equilibrium iteration is performed by using the NewtonRaphson method.4. Shape iteration5. Output of the initial shape including geometric shape and element forces.6. For linear static deflection analysis, only linear stiff-ness elements and transformation coefficients are used and no equilibrium iteration is carried out.4.4.3. Vibration analysis1. Input of the geometric and physical data of the bridge. 2. Input of the initial shape data including initial geometry and initial element forces.3. Set up the linearized system equation of free vibrations based on the initial shape.4. Find vibration frequencies and modes by sub-space iteration methods, such as the Rutishauser Method.5. Estimation of the trial initial cable forcesIn the recent study of Wang and Lin, the shape finding of small cable-stayed bridges has been performed by using arbitrary small or large trial initial cable forces. There the iteration converges monotonously, and the convergent solutions have similar results, if different trial values of initial cable forces are used. However for large cable-stayed bridges, shape finding computations become more difficult to converge. In nonlinear analysis, the Newton-type iterative computation can converge, only when the estimated values of the solution is locate in the neighborhood of the true values. Difficulties in convergence may appear, when the shape finding analysis of cable-stayed bridges is started by use of arbitrary small initial cable forces suggested in the papers of Wang et al. Therefore, to estimate a suitable trial initial cable forces in order to get a convergent solution becomes important for the shape finding analysis. In the following, several methods to estimate trial initial cable forces will be discussed.5.1. Balance of vertical loads5.2. Zero moment control5.3. Zero displacement control5.4. Concept of cable equivalent modulus ratio5.5. Consideration of the unsymmetryIf the estimated initial cable forces are determined independently for each cable stay by the methods mentioned above, there may exist unbalanced horizontal forces on the tower in unsymmetric cable-stayed bridges. Forsymmetric arrangements of the cable-stays on the central (main) span and the side span with respect to the tower, the resultant of the horizontal components of the cable-stays acting on the tower is zero,