The Effects Of The Separation And Cavity On The Mechanical Properties Of CFST

Concrete-filled steel tube (CFST) material has been widely applied in construction of arch bridges, due to its high strength, light weight, preferable seismic performance, excellent plasticity and ductility, ease of erection, fast construction, etc. However, different degrees of separation between concrete and steel tube for some inevitable factors, such as the poor quality of mixed core concrete, concrete has not pumped sufficiently, temperature change at the outside andshrinkage or creep of the filled concrete. Therefore, it is very essential to analyze the separation and cavity effects on mechanical property of CFST structure. The main object of this research is the four-tube CFST laced structure which connected by stiffening steel plate.Firstly, based on the hypothesis of complete separation between steel tube and core concrete, the separation effects on the structural stiffness and stress distribution of single tube, dumbbell and four-tube CFST rib in elastic stage were theoretically researched. The results of theoretical analysis were proved to be correct by using MIDAS/CIVIL to analyze some relevant examples. Based on the ABAQUS solid element model, the separation effects of parameters including separation ratio, eccentricity ratio, slenderness ratio and steel ratio on the mechanical properties of four-tube CFST laced member in detail were analysed. Finally,a four-tube CFST laced arch bridge with a span of76.5m was employed as an example, based on ANSYS beam element model, to analyze the the separation and cavity effects of parameters including separation ratio, the way of load and the separation place on the structure ultimate bearing capacity. Meanwhile the effects of factor which contains separation ratio and steel ratio on the stiffness of four-tube CFST laced rib was also discussed in this research. Furthermore, based on the test of separation single tube CFST member which has been done, the finite element model which has been established in this paper results show that establishment of FE model is correct and the constitutional relationship utilized is reasonable..Some conclusions of this paper are drawn as follows: (1) In elastic stage, the stiffness of single tube CFST arch rib after complete separation doesn’t deteriorate and the stress distribution is not affected; while the results of dumbbell and four-tube CFST rib are changed obviously;(2) According to experimental results of separation single tube structure, the FE model established in this paper is reasonable;(3) With the increase of separation ratio, the bearing capacity of members reduce significantly for the four-tube CFST laced structure, but ductility of members is not affected so much; influence of eccentricity ratio, slenderness ratio on the bearing capacity of members will become more sensitive; with the increase of steel ratio, the adverse effect of separation will be reduced;(4) For the four-tube CFST arch bridge, the separation and cavity will significantly reduce the bearing capacity of the bridge, but the structure still possess good plasticity and ductility; the influence of separation on the bearing capacity of bridge is also different under different load forms; the separation place which has lager axial force will lead to a greater effect on the bearing capacity of structure; the increase of steel ratio can reduce the reduction effect on arch rib.