Loading Protocol

The tests were performed under cyclic loading and terminated when the applied lateral load decreased to a value smaller than 85% of the peak load. The axial compressive load (N0) was applied at the beginning of the test and was kept constant during the whole process of testing. The cyclic lateral load (P) was applied to the midspan of the column according to the ATC-24 code (ATC 1992), where the loading history consists of a force-control stage and a displacement-control stage, as shown in Fig. 4. The force-control stage was performed at the load levels of 0.25Pu, 0.5Pu, and 0.7Pu, respectively, where Pu is the estimated lateral load capacity. Two cycles were imposed at each load level in this stage. During the displacement-control stage, the loading was applied as increments in displacement at the levels of 1Δy, 1.5Δy, 2Δy, 3Δy, 5Δy, 7Δy, and 8Δy, where Δy (¼ 0.7Pu=Ksec, where Ksec is the secant stiff- ness corresponding to a load of 0.7Pu) is the estimated yield dis- placement. Three cycles were imposed at the displacement levels of 1Δy, 1.5Δy, and 2Δy, and two cycles were imposed at each of the higher displacement levels.

No slippage of the reaction blocks relative to the laboratory floor was observed until the column significantly bowed after reaching the failure load.

Fig. 4. Loading protocol

Experimental  Results  and Discussion

Experimental  Observations

Fig. 5(a) shows all the specimens after testing, which demonstrated obvious flexural deformation. Generally, the columns failed in an overall buckling mode, while significant local buckling was ob- served near the stub at the midspan as well. For Specimen CN-6 with a high axial load level of 0.6, its overall buckling shape is quite different from that of other specimens. This may be due to the dif- ference in the distribution of initial  imperfections.

For the circular specimens, the stainless steel tube near the mid- span developed slight outward buckling parallel to the plane of applied lateral loading when the imposed lateral displacement attained 3–5Δy. At the incremental displacement of 6–7Δy, local buckling became significant and a pronounced outward bulge formed close to the midspan. Upon further loading, the bulge grew rapidly until a complete ring-formed outward buckle developed in the stainless steel tube. For the square specimens, local buckling of the steel section appeared earlier than in the circular counterpart test. Buckling was first observed at the incremental displacement of 1.5–2Δy, and an outward buckle in all four component plates had formed near the mid-span as  the  displacement attained 3–4Δy. Most of the square specimens failed when steel fracture occurred at the corners near the midspan where large outward local buckles had formed.

Figs. 5(b and c) present the failure modes of typical specimens. Obvious outward local buckling was formed at both circular and square stainless steel tube. In two square CFSST  specimens (SN-0 and SN-3), a steel fracture was found in the square steel tube, as shown in Fig. 5(c). The appearance of the concrete core after testing is also shown in Figs. 5(b and c). It can be seen that the concrete was crushed at the locations of the outward buckles of the steel tube. At other locations, the core concrete remained intact after testing. Generally, the failure mode of RAC infilled specimens was very similar to that of the specimens with normal core concrete. Compared with conventional carbon steel CFST specimens tested by Han and Yang (2005) and Han et al. (2006), it seems that there is no obvious difference in terms of test observations and fail- ure modes. However, less-obvious tensile fracture was observed for CFSST specimens after cyclic loading, especially for the circular section. Moreover, the attained lateral displacements of the CFSST columns at failure are much higher than those of the CFST counter- parts (Han and Yang 2005; Han et al. 2006). For instance, under the same axial load level of 0.6, the failure lateral displacement of the CFSST column (CN-6) in current tests reached around 41.4 mm, while that of the corresponding CFST specimen (SC2-4) reported in Han and Yang (2005) was around 23.8 mm. This is attributed to the fact that the stainless steel material shows much higher ductility