| Extradosed cable-stayed bridge is a kind of competitive bridge with excellent economic and aesthetic effect,which mechanical performance between continuous system girder bridge and conventional cable-stayed bridge. The research on the structural system of it has an important practical significance.From the perspective of structural analysis,it analyzes the problems which exist in the design theory of the extradosedcable-stayed bridge. The main research results are incluled in this paper:(1)The origin and the development of the extradosed cable-stayed bridge was firstly described. It pionted out that the cable/girder live load ratio could reflect structural effect of extradosed cable-stayed bridge under the lane load. Using the analogy girder method, under the lane load for cable/girder live load ratio formulation was deduced, which could apply to extradosed cable stayed bridge with single tower and double-tower or multi-tower. The relationship of between the structural parameters and cable/girder live load ratio was studied in this paper, and it was proved that the cable /girder live load ratio can reflect the character of cable-tower-girderand supporting condition, so it facilities the designerto grasp the structural behaviors of extradosed cable-stayed bridge from the macro, and to select reasonable structural parameters in the conceptual design stage.(2)Based on load balance theory and cable membrane theory, the axial force and bending moment for main girder or the axial force for tower werededuced under dead load; based on elastically supported continuous girder theory, the bending moment and deformation for main girder or tower were deduced under the live load; vertical displacement for main girder was deduced with considering deformation for elastic cable, tower longitudinal and elastic compression etc.( 3) To calculate frequency of extradosed cable-stayed bridge conveniently,the extradosed cable-stayed bridge with double-towers was taken as research object, while the tower is rigidly connected with the girder and supported by the pier or with high pier. Frequency formulas for first symmetric and anti-symmetric vertical vibration modes or longitudinal vibration mode of it were induced by the Rayleigh method, and nominal unity mass anti-bending rigidity concept was proposed. Finally, the presented theoretical formulas were validated by the engineering project, and the application condition was discussed. The results indicate that constraint condition has a great influence on frequency, which should be correctly considered in frequency calculation.The error between values calculated by the proposed formula and the FEM meets with the requirement of conceptual design. The proposed formulas can be appliedto the extradosed cable-stayed bridge, and vertical frequency formula in the wind-resistant design specification cannot be applied to the former bridges.(4)In order to study for longitudinal stiffness of middle tower in multi-span extrodosed cable stayed bridge, the cable was treated as spring element in considering its constraint to the main tower. Conservation of energy principle was applied to deduce expression for longitudinal stiffness of middle tower without considering the main tower of elastic compression.The longitudinal deformation of main tower under live load causes the longitudinal bending of main tower, vertical bending of main girder and elastic elongation of cable. In the above process, the longititude work of cable is equal to the increment of the gravitational potential energy for main girderand the increment of the elastic potential energy forcable.According the above principle, it could induce the formula of restraint on cable for the tower longitudinal stiffness. The results indicated that the relationship of between the equivalent spring stiffness and cable linear stiffness is linear; and the relationship of between the equivalent spring stiffness and the angle of the cable is nonlinear. The recommended formula has a high precision and offer to selected reasonable parameters for multi-span cable stayed bridge in conceptual design stage.(5) In order to study for main girder vertical deformation in multi-span extrodosed cable-stayed bridge under live load, multi-span extrodosed cable-stayed bridge is equivalent to multi-span elastic support continuous girder.Based on structural feature of multi-span extrodosed cable-stayed bridge, formulas of eleastic supported rigidity coefficient for itwere deduced. It carries on the parametric study, and discusses the tower/girder rigidity for cable-stayed system.(6)Aiming at the mechanical behavior of prestressed concrete extradosed cable-stayed bridge, a new method based on cable/girder weight matrix was put forward to optimize the cable force. The method followed the idea that the normal stress on the top and bottom of main girder section were controlling condition and the feasible region of the stress and dead moment were deduced. The design strength of the code for the highway bridge was subtracted the live load envelope, which can meet the stress feasible region of girder section. Unit-load method was put forward, and the least square principle was adopted to optimize cable and prestressed force. The method was materialized with program and the optimization theory was examined on a real bridge.(7) According to the data of domestic and board continuous system girder bridge, conventional cable-stayed bridge and extradosed cable-stayed bridge, the indexs of concrete,reinforcing steel bars and prestressed tendons of girder were fitted, and then the econimical span for the above three kinds of bridgewas fitted;it pointed out that large span of extradosed cable-stayed bridge was carry out by structural system measures,comonent system measures and material measures;finally,according to approximativeestimate formulas, it pointed out limition span for the extradosed cable-stayed bridge from the view of mechanical performance.The finite element model was established and the comparative with the above formulaswere made to verify its effectiveness. |
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