Research On Structural Stiffness And Stay Cables Overlap Proportion Of Multi-tower Cable-stayed Bridge With Crossed Cables

Multi-tower cable-stayed bridge have more than two main spans,compared with the traditional twin towers cable-stayed bridge,it has the advantages of great spanning capacity,low construction difficulty and low cost.But the lack of necessary constraints such as the end anchor cable with center tower or auxiliary pier,cause the structural stiffness insufficient,which is the key factor restricting its development.For that reason,predecessors proposed a series of conceptual design schemes with cable constraint.With the construction of Queensferry Crossing in British,this paper carries out systematic study on setting crossed cables in midspan to improve the stiffness of multi-tower cable-stayed bridge.The main work includes the following several aspects:(1)In order to explore the action mechanism of crossed cables,an analytic formula for calculating the longitudinal restraint stiffness of the middle tower of multi-tower cable-stayed bridge with crossed cables by considering the effect of tower and beam based on the principle of deformation coordination was deduced,and carry out a numerical example.The results show that the errors between the formula in this paper and finite element are within 2%~8%,it can meet the requirements of the conceptual design of multi-tower cable-stayed bridge.When the tower is in the vertical displacement,the weight of the beam segment is reallocated in the cross.This makes the mid-tower cross tension increase and the side-tower cross tension reduce.The action mechanism of crossed cables is that they can make the beam deflection decrease,thus increasing the overall stiffness of the cable-stayed bridge.The overall stiffness of multi-tower cable-stayed bridge rises as the number of crossed cables increasing,the displacement of mid-tower reduction up to 51% after set 10 pairs crossed cables.The change of the stiffness of the tower's effect on longitudinal restraint stiffness of the middle tower for multi-tower cable-stayed bridge is greater than the main beam,but the crossed cables play more important roles in increasing the stiffness of the structure.(2)In order to study the influence degree and regularities of structural rigidity with crossed cables,two kinds of setting schemes of crossed cables are presented.The finite element analysis of 11 contrast schemes is established by changing the quantity of the crossed cables.The results show that tower displacement,the force of mid-tower,beam stress,beam deformation and the cable force change values in mid-span are significantly reduce under unbalanced load,at the same time,the side-tower force are increase.The role of crossed cables are just like a connection which increase stress of members which are not involved to bear the load or to participate in the weaker.The peak value of the internal force of the main bearing component is weakened.Thus reducing the deformation of the structure,the whole bridge stress is more balanced,and the material of each component can be fully utilized.(3)In order to give a reasonable range of the proportion of crossed cables based on double indexes,this paper analysis the efficiency of the cables for structure stiffness and the economy of the cables for the cable quantity.And combined with the existing specification and the data of existing bridges,ascertaining the limit value of two indexes.Seting cable-stayed bridge with different proportion of crossed cables into three regions.In each region,there is a difference between the stiffness of structure and the amount of cables.From this we can determine the optimal scheme.The results show that the optimal scheme is the crossed cables setted by adopting the method of scheme 2,and the reasonable range of the proportion of crossed cables is 15%~35%.If we have to use the scheme 1,we must control the reasonable range of the proportion of crossed cables within 16% so as to ensure the economic of structure.