Study On Rational Structural System Of Multi-pylon Cable-stayed Bridges

Multi-pylon cable-stayed bridge is a new type of bridge developed on the basis of conventional cable-stayed bridges,because of its beautiful appearance,superior spanning ability,and economical cost,it has been widely used all over the world,there are many applications in construction,especially in bridge projects across rivers and seas.The notable feature of the multi-pylon cable-stayed bridge is that the middle pylon except for the side pylon are not fixed by end anchor cables,and the middle pylon is prone to relatively large horizontal displacement,and different structure systems will directly affect the load paths and change the mechanical behavior of the multi-pylon cable-stayed bridge structure,affect the overall stiffness of the bridge.The reasonable structural system of multi-tower cable-stayed bridges with different spans,different numbers of pylons,and different functions,as well as the reasonable matching relationship between the longitudinal stiffness,form and structural system of the middle tower,are studied through the following aspects.The research content and results have the following aspects.(1)Through the collection of relevant materials and the reading of literature,basing on the actual engineering projects of many multi-pylon cable-stayed bridges that have been built and under construction,rail-cum-road three-pylon cable-stayed bridge and highway three-pylon cable-stayed bridge with six main spans of 300 m,500m,700 m,900m,1100 m,and 1300 m has been built,the basic model scheme of a four-pylon,five-pylon,and six-pylon cable-stayed bridge with three spans of 300 m,500m,and 700 m has been built,carried out the calculation and analysis of the dead load state and the load state of the operation stage.(2)Taking the rail-cum-road and highway three-pylon cable-stayed bridge with the main span of 300 m to 1300 m as the research objects,analyzed the structural internal force and displacement of the five structural systems of semi-floating system,three-pylon longitudinal full constraint system,middle pylon longitudinal constraint system and middle pylon elastic cable constraint system,and pylon-beam pier consolidation system under live loads,longitudinal wind loads and temperature loads.Research indicates: the semi-floating system lacking longitudinal restraint has poor resistance to longitudinal loads,the longitudinal displacement of the bridge tower and the internal force along the bridge are reduced by setting the longitudinal restraint support.For long-span rail-cum-road three-pylon cable-stayed bridge,the design method of constraining the girder and the pylon with horizontal elastic cables is easier to implement.(3)Analyzed the matching relationship between the main span of the three-pylon cable-stayed bridge,the use function,the structural system,the stiffness of the middle pylon,and the form of the middle pylon,furthermore,it proposes the rigidity of the middle pylon and the form of the middle pylon suitable for different spans and different structure systems.Research indicates: different spans,different use functions,and different structural systems are applicable to different tower stiffness and tower forms,when adopting middle pylon longitudinal constraint system,longitudinal space towers are recommended for rail-cum-road three-pylon cable-stayed bridges with a span greater than 700 m,the space tower is recommended when the span of the highway three-pylon cable-stayed bridge is 1300 m and above.When adopting the semi-floating system,longitudinal space towers are recommended for rail-cum-road three-pylon cable-stayed bridges with a span greater than 500 m,the space tower is recommended when the span of the highway three-pylon cable-stayed bridge is1100 m and above,but the semi-floating system has a large displacement of the beam end of the girder under various load conditions,and as the span increases,the displacement of girder increases significantly,and excessive displacement of girder is not conducive to the setting of expansion joints,so the three-pylon cable-stayed bridge with a large span needs to be carefully selected for this system.The three-pylon longitudinal full constraint system will generate a large temperature-induced internal force due to the constrained longitudinal deformation of the main girder under temperature load,and increasing with the main span,temperature internal force increases rapidly,so it is not recommended to use this system for a three-tower cable-stayed bridge with conventional pier heights.(4)Taking highway four-pylon cable-stayed bridge,highway five-pylon cable-stayed bridge and highway six-pylon cable-stayed bridge with a main span of 300 m,500m and700 m as the research object,study the influence of structural systems with different numbers of longitudinal restraints on the mechanical behavior of multi-pylon cable-stayed bridges with more than three towers,summarize the minimum mid-tower stiffness values that a multi-tower cable-stayed bridge with different numbers of pylons needs to meet under different spans and different structural systems.Research indicates: only adding a longitudinal restraint at the middle pylon cannot reduce the vertical deflection of the main girder of cable-stayed bridges with more than three towers,the main beam of the structural system with only one longitudinal constraint will produce large longitudinal deformation under temperature load,so this system is not suitable for multi-pylon cable-stayed bridges with more than three towers.When more than three-pylon cable-stayed bridge adopts a semi-floating system without longitudinal restraints,the rigidity of the middle tower needs to be the largest.When semi-floating system is used for multi-pylon cable-stayed bridge with a large span,it is necessary to consider the problem of excessively large expansion joints caused by large displacement of the main girder under various load conditions.When adopting the middle-pylon full restraint system,more longitudinal restraints of the pylon-beam can greatly reduce the vertical deflection of the girder under live load.If you refer to the Jiashao bridge and set a rigid hinge in the middle of the span,longitudinal deformation of the main beam is not restricted under temperature load.Besides,it can also effectively reduce the temperature internal force and reduce the minimum rigidity of the central tower required by the full restraint system,reduce the scale of the middle tower and foundation and control the cost in a reasonable range.