5. Summary and conclusions
In this paper, the results of an experimental program on slender dual steel tubular columns filled with normal and ultra-high strength concrete were presented. This work provided novel results to this research field since the number of similar experimental campaigns available in the literature is scarce. The influence on the load-bearing capacity of different parameters was studied. The parameters were the concrete strength (normal strength and ultra-high strength concrete), the location of the concrete with higher compressive strength (inner core or outer ring) and the ratio between the inner and outer steel tube thicknesses. For this purpose, 14 specimens were tested: 12 CFDST columns divided into two series (Series 1: NSC outer ring and Series 2: UHSC outer ring) together with the two corresponding CFST columns which served as a reference. The load-deflection curves were extracted and the maximum load achieved during the experiment was registered. From the test results it was found that the trends observed for both series were different, confirming the influence of the type of concrete placed in the outer ring. Two coefficients were also employed in the analysis, the concretesteel contribution ratio (CSCR) and the inner concrete contribution ratio (ICCR). After the study of this ratio, it was observed that for those columns with NSC outer ring, a double-tube column with the thick tube in the inner position can reach almost the same buckling load than a double-skin CFST column with the thick steel tube in the outer position and without inner concrete. Besides, it results also noteworthy the reduced effect presented by the UHSC in the inner core in comparison with NSC. With respect to the columns with UHSC in the outer ring, the main conclusion drawn was that the maximum load is independent on the steel tubes thicknesses distribution. In these cases, the behaviour is dominated by the UHSC outer ring. In addition, it was observed that the CSCR of the double-skin columns of this series is close to 1, which proves that it is possible to obtain the same load-bearing capacity than a CFST column but with less weight. Another aspect detected through the analysis of the ICCR values was the efficiency of filling the inner tube in those specimens with NSC outer ring with respect to those with UHSC outer ring, where this action has minor effect. Finally, the design rules in Eurocode 4 were assessed by means of the tests results. Considering the limited cases of this work, it can be initially inferred that the method in EC4 Part 1.1 for members in axial compression provides unsafe results for evaluating the buckling resistance of CFDST columns. Nevertheless, adopting an initial small eccentricity (e = 5 mm) produces considerably different results, being conservative in this case. Further tests are needed for evaluating the accuracy of EC4 method in a more reliable manner.
