Analysis On The Behavior Of The Steel-concrete Composite Structure Of The Hybric Girder Of A Long-span Cable-stayed Bridge

The girger of long-span cable-stayed bridges generally is a hybric girder which consists of prestressed concrete(PC) box girder in the side span and steel box girder in the mid-span. The steel-concrete composite structure in the transition zone between steel box girder and PC box girder is a critical component for force transmission, stiffness transition and the continuity of the girder. With complex structure and stiffness mutation, the composite structure is prone to stress concentration. There is no specific clause in the current code for the design of the steel-concrete composite structure in China and foreign countries. So, it is significant to analyse the behavior of the steel-concrete composite structure of the hybric girder. In this paper, a study is carried out with field test and finite element analysis on the steel-concrete composite structure of the hybric girder of Jiang Shun Bridge, which is a long-span hybrid girder cable-stayed bridge and located at the cross intersection of Guangzhou-Foshan expressway and the West river. The research work and conclusions are as follows:(1) Several concrete shrinkage and creep models are compared to consider the concrete shrinkage and creep effects on the measured strain and stress of concrete. As the PC box girder in the side span of Jiang Shun Bridge adopts a kind of C55 concrete added fly ash, a coefficient for consideration of both shrinkage and creep effect of this kind of concrete, are introduced to modify the CEB-FIP(2010) model. The revised CEB-FIP(2010) model is used to estimate the shrinkage and creep of concrete in the field test on the steel-concrete composite structure of Jiang Shun Bridge.(2) A field test was conducted in the steel-concrete composite structure of the girder of Jiang Shun Bridge. The stress distribution and development trends in the steel-concrete composite structure, and the neighbouring strengthened part of the steel box girder and the PC box girder were investigated. It was observed that an obvious shear lag effcet exists in the strengthened part of steel box girder. The stress along the longitudinal direction of the bridge in this part around the outside web of the steel box girder is greater than other parts. Due to the influence of shear lag effect of the strengthened part of the steel box girder the steel-concrete composite structure also exhibits a obvious shear lag effect. On the top of the strengthened part the steel box girder, T-shaped stiffening ribs bear large axial forces, which indicates that T stiffening ribs play an important role in force transfer and dispersion. For the strengthened part of of steel box girder, as well as the steel-concrete composite structure and the neighbouring part of PC box girde, the overall stress levels are low, which shows that structural design of the girder is reasonable, and the steel-concrete composite structure has adequate safety margins.(3) A finite element model for Jiang Shun Bridge is established with the softwore MIDAS/Civil, and a three-dimension finite element model for the steel-concrete composite structure is established with software ANSYS Workbench. Under the action of dead load in the completed condiction of the bridge, the internal forces on the two end sections of joint part, composed of the strengthened part of of steel box girder, the steel-concrete composite structure and the neighbouring PC box girde, drawn from the whole bridge model, are added to the local finite element model. The stress distribution obtained from local finite element model is consistent with the measured stress distribution, which indicates that the simplification of the local finite element model and the assumption of boundary conditions are reasonable.(4) The most unfavorable load combinations for the steel-concrete composite structure under the condiction of a finished state the bridge was obtained by using the whole bridge element model. Under the action of this unfavorable load combinations, a study on the steel-concrete composite structure is carried out to analysis the load bearing behavior of this structure. The results are as follows: 1) The exist significant stress concentration at the joint of U-shaped stiffning ribs and bearing plate, at the intersection between T stiffning ribs and bearing plate, and at the intersection among the web plate of the strengthened part of the steel box girder, the bearing plate and the bottom plate of the steel-concrete composite structure. 2) The stresses in concrete along longitudinal directions in the steel-concrete composite structure exhibit an apparent layered distribution along the vertical direction, and the stress of section of solid concrete beam in this steel-concrete composite structure is similar to the hollow box section. 3) The stress transition in longitudinal directions in the concrete in the steel-concrete composite structure is smooth, which shows that the axial force transfer between concrete and steel structure is reliable. 4) The max shear force of shear studs is located next to the sidewalk. When the overall stress level of the steel-concrete composite structure is low, shear force born by shear studs is lower than the shear force born by PBL(perfobond rib shear connector), but shear studs can prevent the strip between steel and concrete effectively.