3.2. Axial behaviour of slender CFS

3.2. Axial behaviour of slender CFSTs (series II)3.2.1. Failure modesThe failure modes of slender CFST columns in Series II werecharacterised by local and global buckling. Typical failure modes of slender columns are shown in Fig. 11. The failure of Specimen SL-500–70was initiated by concrete crushing and local buckling. For SpecimenSL-2000–70 with the highest slenderness ratio, a significant lateral deformation was observed prior to the ultimate load. The global bucklinggradually developed and resulted in a plastic hinge at the mid-heightof the specimen with an increase in the axial compressive load. Meanwhile, both the local and global buckling failure was observed for Specimens SL-1000–70 and SL-1500–70, which possessed intermediatemember slenderness ratios.Concrete crushing was confirmed by a loud sound during each test,together with a sudden drop in the load-shortening curves. Evidentstrength reduction resulting from the concrete crushing could beobserved in Fig. 12.3.2.2. Load-shortening and load-lateral deflection curvesLoad-shortening curves of the tested slender specimens are illustrated in Fig. 12 and the ultimate loads are tabulated in Table 6. Compared with Specimens EC-0-70 and EC-0-100, Specimens SL-500–70and SL-500–100 had similar structural behaviour. This indicated thatthe nominal section capacity was still reached for CFST columns with amember slenderness ratio (Le/r) around 14. In addition, the ultimate