Research On New System Of Multi-tower Cable-stayed Bridge And Internal Force Of Creep

The cable-stayed bridge is highly redundant complex structure,while multi-tower cable-stayed bridges are less rigid than double-tower cable-stayed bridges,so the overall structure is more flexible.The main reason is that the middle tower has no end anchors to limit its displacement,resulting in a small stiffness of the tower a nd a low overall stiffness,which in turn leads to a significant increase in the deflection of the main girder.This shortcoming has also become the primary reason for limiting the practical application of multi-tower cable-stayed bridges.When the composite beam is applied to the cable-stayed bridge,the creep effect is obvious due to the large axial force,and the secondary internal force of creep is the focus of its design.In order to solve the above two difficulties,this paper proposes a new system and uses new materials to provide practical solutions through calculations,tests,and analysis.The main tasks are as follows:(1)To solve the problem of insufficient rigidity of the middle tower in the multi-tower cable-stayed bridge,this paper proposes a new three-tower cable-stayed bridge scheme with unique size fans,aiming at improving the overall stiffness of the multi-tower cable-stayed bridge and controlling the full bridge cost at the same time.That is,since the support stiffness provided by the side tower cable is much larger than that of the middle tower cable,the range of the main span supported by the side tower cable is expanded,and the range of the main span supported by the middle tower cable is reduced.This scheme is similar in appearance to three "fans" of different sizes,so it is called unique size fans cable-stayed bridge system.Through MIDAS,a three-tower cable-stayed bridge with a main span of 2×600m is calculated and compared.From the aspects of stiffness,internal force and economy,the new scheme has a strong advantage over the traditional scheme.(2)When the unique size fans scheme is used in the three tower cable-stayed bridge,the accumulation of the horizontal component of the stayed cable on the main girder increases correspondingly due to the increase of the large fan of the side tower.When the composite beam is used,the shrinkage and creep problem is also prominent..The steel-UHPC main girder is considered here to overcome this problem,so it is necessary to study the creep characteristics of UHPC.In order to analyze the creep characteristics of UHPC and the influence of creep on the composite section,four columns were designed and tested for creep observation,including two full-section UHPC columns and two steel-UHPC composite section columns.The curing conditions of each type of the column are divided into high temperature steam curing and natural curing.The load applied to the four columns by prestressing will cause the UHPC to undergo creep under the continuous load,and the strain of each column will be continuously observed for about 1 year.This test focuses on the creep strain of UHPC under different curing conditions and the stress redistribution of the steel-UHPC composite section due to creep.Then,by comparing the calculated value with the experimental value,it provides a basis for the design and stress redistribution analysis of the composite beam.(3)The factors affecting the creep of UHPC include many aspects.At present,there are many calculation theories about the creep of concrete.The steel-UHPC combined section test column was theoretically analyzed by the effective modulus method adjusted by age,and the stress redistribution law of the compostie section was explored.The nominal creep coefficient of the combined section,the stress and strain increment of steel and UHPC were obtained.In the end,the theoretical value is compared with the experimental value to test its rationality.(4)When the steel-UHPC composite beam is used for the cable-stayed bridge,the diagonal cable will generate a large horizontal component force on the main beam to cause the UHPC bridge panel to creep.This paper proposes an unique size fans cable-stayed bridge scheme for the optimization of the stiffness of multi-tower cable-stayed bridges.In this scheme,the middle tower is reduced and the side tower is enlarged,which leads to an increase in the number of stay cables of the side tower,and the horizontal component force generated by the main beam at the side to wer is also increased.Based on the finite element model of cable-stayed bridge which was used in multi tower cable-stayed bridge stiffness analysis,considering the construction process and the time-varying characteristics of UHPC,the influence of UHPC shrinkage and creep on the long-term performance of steel-UHPC composite girder in cable-stayed bridge is discussed.In order to simplify the analysis process of the time-varying behavior of cable-stayed bridges,the calculation and analysis according to the age-adjusted elastic modulus method is adopted.