| Concrete filled steel tube (CFST) truss girder structure is a new type of composite structure, which has a series of advantages such as high bearing capacity, great stiffness, light weight, large spanning ability, convenient construction, and less usage of cement and concrete. Therefore, its technical and economic advantages are obvious and the application prospect is broad. Mechanical properties of concrete-filled in chord are extremely important in the structure, because of the complex stress state of tension-bending and compression-bending. Due to the fact that steel fiber can significantly improve the flexural strength of concrete, chord filling with steel fiber reinforced micro-expansive concrete (SEC) gives full play to the overall mechanical performance advantage. The paper was supported by the research project of the Ministry of Communications " Study on the technology of middle span CFST truss continuous beam bridge "(2009318000105), and relied on the first all-tube truss structure continuous beam bridge in the world—Ganhaizi Bridge, in which the composition, structure and properties of steel fiber reinforced micro-expansive concrete filled steel tube(SE-CFST) were studied, and the research results were applied in the project. Specific research work and the main conclusions are as follows:The design and preparation method of SE-CFST was put forward. And the impact of expansive agent and steel fiber on working performances and mechanical properties of concrete are studied. While the volume deformation laws of SEC under the confined conditions of steel tube were also ascertained. Moreover, the antifreeze technologies of SE-CFST were discussed, and the frost resistance enhancement mechanism of steel tube confined to SEC was studied. C60steel fiber reinforced micro-expansive and self-compacting concrete was prepared to meet the mechanical properties and construction requirements of Ganhaizi Bridge, and the test results showed that the concrete in tube reached the dense packing state and the structure service were in good condition.Based on the push-out test, load-slip curves of SE-CFST were analyzed, the degradation model of interface bond strength between core concrete and tube wall was studied, the steel ratio, interfacial bond length and the impact of steel fiber content on interfacial bond strength were investigated, and the interfacial bond strength calculation method was put forward. The results showed that:the expansion and deformation of concrete were limited by steel fiber, which reduced the interfacial bond strength of SE-CFST. But, its interfacial bond strength raises about1-3times than of the ordinary CFST. Steel ratio was the main influencing factor of the interfacial bond strength; the higher steel ratio, the stronger the constraint effect, and the higher the interfacial bond strength. And the length of the interface had little impact on the interfacial bond strength.The influence law of the steel ratio and steel fiber content on the bearing capacity, deformation form and failure characteristics of SE-CFST under axial tension load have been discussed. And axial tensile performance differences between SE-CFST and hollow steel tube were investigated. The results showed that, radial shrinkage of the steel tube was effective limited by core concrete, radial stiffness of cross section of CFST was significantly intensified, axial tensile capacity increased about15%-37%than that of hollow tube; and the lower the steel ratio, the stronger the increase. Internal cracks propagation and extension of core concrete were prevented by steel fiber. Therefore, the number of through seam was reduced, and constraint of concrete on the steel tube enhanced. Initial cracking load of SE-CFST increased relatively with the increase of steel fiber content, however, yield load improved not obvious. The simplified calculation method for axial tensile capacity of SE-CFST member was proposed, comparisons between the predicted results and test ones were carried out, and good agreements were achieved.The effect of steel ratio and steel fiber content on the flexural capacity, deformation form, strain distribution and failure characteristics of SE-CFST members under bending load were studied. The results showed that, the failure mode of steel tube transformed from plastic collapse into convex destruction, because of the filling of concrete. Flexural capacity of CFST component enhanced about80%～290%than that of hollow tube component, and the lower the steel ratio, the greater the enhancement. Stress concentration of compression zone of CFST specimen receded obviously compared to that of hollow steel tube, and the deformation of cross section conforms to the plane section assumption, basically. The mixing of steel fiber in core concrete was effective in improving the work performances and increased the yield load. The calculation formula for flexural capacity of SE-CFST member was deduced, and its results have a better approach to the test values. The formulas proposed in this paper for calculating the capacity of SE-CFST members subject to axial tension and bending load can provide reference for the design of CFST truss girder.The influences of the steel fiber reinforced micro-expansive concrete filled in the chord of steel tube truss girder on the capacity, deflection deformation, strain distribution and failure mode were researched. The results showed that the uniformity of its cross section strain distribution significantly improved, due to the filling of concrete in chord. And the overall work performances of the truss girder greatly enhanced, bending stiffness and capacity distinctively increased. Damage of hollow steel tube truss girder was mainly due to the plastic failure at nodes, and the flexure deformation of the structure is not obvious. However, when the chord filled with concrete, the damage of the specimen was as follows:if a higher steel ratio and a lower thickness ratio of web to chord were obtained, its destruction was owned to webs buckling, while the overall bending deformation was significant. Otherwise, when it comes to a lower steel ratio and a higher thickness ratio of web to chord, the failure of specimen mainly due to the punching failure at bottom chord node, and the whole bending deformation was unconspicuous. The bending capacity of CFST truss girder still depended on the strength of the chord node. |
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