Analytical Methods For Preliminary Design Of Multispan Suspension Bridges

Multispan suspension bridges have become one of the most competitive structures of long-span bridge construction,due to its superior crossing capability and reduction in the dimensions of bridge components.However,lacking in vertical stiffness,multispan suspension bridges will suffer large vertical deformation if they are in the most undesired loading scenario.Increasing the stiffness of middle tower is an effective approach to increase the vertical stiffness of multispan suspension bridges,but may significantly reduce the anti-slip safety between the main cable and saddle.The application of double-cable structures is another approach to increase the bridge's vertical stiffness,which also faces the problems of anti-slip safety between the main cable and saddle,deficient theoretical analysis and unknown practical saddle structures.Additionally,the complexity of traditional design process of multispan suspension bridges also restricts the development of the structure.It is urgent that simplified analysis methods for preliminary design of multispan suspension bridges be developed.Aimed at traditional multispan suspension bridges and double-cable multispan suspension bridges,the following studies are carried out in this thesis:(1)By transforming multispan suspension bridges into the equivalent spring-model,the horizontal stiffness and forces of the main cables in the most undesired loading scenario are figured out.Based on the deformation compatibility condition of the main cables and towers,the deformation and cable tensions of the structure are worked out.The theoretical models of the two kinds of multispan suspension bridges are then put forward according to the analytical results mentioned above.By comparing the results of the theoretical models and the finite element models in the software MIDAS/Civil,the validity of the theoretical models is conducted.(2)Due to the defects of the anti-slip safety coefficient(K)mentioned in the specifications,a new assessment method of anti-slip safety between the main cables and saddle(the anti-slip safety coefficient K_n)is adopted,which is more reasonable and has clear physical significance.Moreover,the required value of the K_n is proposed.The deformation of the structure and the anti-slip safety coefficient K_n are the key controlling factors of the design of multispan suspension bridges.(3)To solve the problem of unknown saddle structures in the double-cable suspension bridge,an innovative saddle structure is proposed,with its assembly procedure and arrangement method of cables described.Additionally,the frictional resistance between the main cables and saddle is carried out through mechanical analyses.(4)By analyzing the proposed theoretical models of the two kinds of multispan suspension bridges,the conclusion is reached that the key design parameters of multispan suspension bridges are the sag-to-span ratios of the cables in main spans and the stiffness of the middle tower.Besides,the weight of the girders also has a great influence on the structure.The design of traditional multispan suspension bridges is determined by both the structural stiffness and the anti-slip safety performance between the main cables and the saddle.When the sag-to-span ratios of the cables in main spans decrease,the vertical stiffness of the bridge decreases and the lower limit of the middle tower stiffness rises.The anti-slip safety between the main cables and the saddle and the upper limit of the middle tower stiffness decrease first and increase then with the decrease of the main cable's sag-to-span ratios.The increase of the main span length and the weight of the girders helps increase the ratio of dead load to live load in the bridges and make the influence of live load decline,which can improve the mechanical behavior of the structure and enlarge the range of the middle tower stiffness significantly.The design of double-cable multispan suspension bridges is only determined by the anti-slip safety performance between the main cables and the saddle,for the reason that it has a huge structural stiffness and is not restricted by the middle-tower-effect within a certain range of middle tower stiffness.However,the double-cable structure has a severer problem of anti-slip safety between the main cables and the saddle than the single-cable one.The more difference between the sags of top and bottom cables,the worse anti-slip safety performance between the main cables and the saddle is.Although the increase of the main span length and the weight of girders helps increase the ratio of dead load to live load in the bridges,the anti-slip safety performance between the main cables and the saddle is not improved much because of the increase of dead load redistribution.Only when the length of main span exceeds 4,000 meters,can the influence of live load decline remarkably and the range of middle tower stiffness increase a lot.The summaries above can provide guidances on the preliminary design of multispan suspension bridges.