- مبلغ: ۸۶,۰۰۰ تومان
- مبلغ: ۹۱,۰۰۰ تومان
Concrete filled double skinned steel tubes (CFDST) are proved to have good structural performance in terms of strength, stiffness, ductility and fire resistance. Long CFSDT columns find application in elevated corridors, bridge piers and also in buildings. However, the behaviour of CFDST long columns is still not fully understood and there is limited research in this area. In this paper, axial capacity equations for long column CFDST sections are proposed based on strength super-position method of design. Column capacity computed using the proposed equation is validated through experimental studies conducted by the authors (for columns having L/D ratio of 20) as well as additional tests reported in literature. Tests were conducted on CFDST, Concrete Filled Steel Tube (CFST) and Concrete Filled Hollow Single skinned Steel Tube (CFHSST) cross-sections. Parameters considered in the test include (i) length of the column, (ii) shape of the inner tube, and (iii) absence of inner tube. Results from the test viz., (a) load carrying capacity, (b) load vs. axial deformation curves, and (c) load vs. lateral deflection curves, have been reported. Test result shows that the contribution of inner tube on the axial capacity of long column is less than the predicted value, as the column undergoes elastic buckling prior to yielding. A reduction factor is proposed to account for the reduced contribution of inner steel tube, and it is applied as a correction to the initially proposed equations. The results from proposed capacity equation are compared with experimental results and are found to be in good agreement. It is concluded that the long column axial capacity equation specified for CFST in AISC-360 and EC4 could be extended for CFDST sections after incorporating the new reduction factor.
4. Summary and conclusions
Axial compression test is carried out to study the long column behaviour of CFDST. Among the five selected cross-sections, one is CFST (benchmark specimen), two are CFDST, with variation in the inner tube shape and the remaining two are CFHSST. The axial load versus axial deformation curves indicate that the behaviour of all the specimens are similar till the buckling point, later on, in the post-buckling phase, the cross-sections show distinct behaviour owing to the difference in their cross-sectional yielding pattern. The axial load versus lateral deflections are also plotted, and it reverberates the phenomenon which was observed in axial deformation curves. The design procedure of CFST column specified in EC4 and AISC-360 is extended for CFDST by modifying certain parameters. The axial capacity values from these modified code equations and from design formulae proposed by Han et al. (2009)  is compared with the experimental results of the present work and Romero et al. (2015) . The analytical results are found to be un-conservative, and inconsideration of the delay in yielding of inner tube is found to be the reason for this error. A new reduction factor is incorporated in the modified AISC-360 and EC4 equations for CFDST column design. The final equation is in good agreement with the test results. The following are some of the conclusions drawn based on this limited research.