CFDST columns subjected to cyclic loading

Nakanishi, Kitada, and Nakai (1999) experimentally examined, under strong (simulated) earthquakes, the ultimate strength and ductility of cantilever short columns of the following members: (i) steel SHS, (ii) steel SHS fully filled with concrete, (iii) double skin steel tubes made of SHS as the outer tube and CHS as the inner tube where the annulus between the tubes was partly filled with concrete in the lower part, (iv) concrete-filled double skin steel tubes made of SHS as the outer tube and CHS as the inner tube, and (v) concrete-filled double skin tubes made of steel SHS as the outer tube and plastic CHS as the inner tube. The specimens were made of compact mild steel sections and the welding method between the flange and web plates was groove welding with full penetration. The average concrete compressive strength

Chapter 2: Literature Review 29 was 11.8 MPa. The axial load was considered to be 15% of squash force. They reported that ultimate strength of the SHS column coincides almost with the theoretical fully plastic state, while the ultimate strengths of the four composite specimens are much greater than the theoretical fully plastic state composite column. Additionally, their results showed that concrete-filled double skin tube columns have the best performance under cyclic loading.

Hsu and Lin (1999) investigated the behaviour of cantilever short columns of form CFST with SHS as the outer tube and CFDST with SHS as the inner and outer tubes subjected to axial and cyclic bending loads. The outer tubes were made of mild steel section, fabricated by fillet-welding four plates, of diffident thickness to study the variations in behaviour between members with compact and con-compact plates. Moreover, the inner tube sections were made of mild steel compact plates. The average concrete strength was 20.6 MPa. The axial load was considered to be 10% of specimens compressive yield strength to take into account the effect of superstructure weight. During the test, they observed local buckling of the outer tubes close to the fixed support which resulted in crumbling of concrete. However, due to the existence of concrete core, the specimens did not show significant deterioration in strength until the buckled plates fractured. Therefore, it was concluded that the concrete, even after crushing, can effectively maintain the columns stability. They reported that the contribution of concrete to the member performance was more significant for members with higher steel width-to-thickness ratios. The results showed that the strength of the CFDST columns is higher than the corresponding CFST columns. The improvement in strength reached up to 45% for CFDST with non-compact outer tube, while this was 31% for CFDST with compact outer tube.

Further to these studies, Han, Huang, Tao, and Zhao (2006) conducted a series of experiments to evaluate the response of simply supported CFDST beam-columns under constant axial load and cyclically increasing flexural loading. They considered specimens with SHS outer tube and CHS inner tube as well as specimens with CHS as both outer and inner tubes. The tubes were fabricated from mild steel sheet with compact plates cut from the sheet, tack welded into a circular or square shape and then welded with a single bevel butt weld. The average concrete strength was 38.9 MPa and 58.1 MPa for the specimens with circular and square sections, respectively. Apart from the cross-sectional geometry, the test design parameters were the axial load level, n (i.e., the ratio of the axial load applied to the axial compressive capacity of a column) from 0 to 0.6, and hollowness ratio, χ from 0 to 0.77. Results showed that the failure

30 Chapter 2: Literaure Review features of CFDST specimens under cyclic loading are very similar to those of CFST columns.

The typical failure modes of outer tube, inner tube, and concrete core as observed are shown in Figure 2-6.

(a) Outer tube failure

b) Concrete core failure

(c) Inner tube failure

Figure 2-6: Typical failure mode of outer tube, inner tube and concrete core of a square CFDST column subjected to cyclic loading (Han et al., 2006)

The authors reported that after the steel reached its yield strain, an outward bulge formed close to the stub at the compression face of the CFDST column on both sides of the stub. The bulge also formed on the other face of the column when the lateral load was reversed. The bulge then grew with increasing lateral displacement until the bulge formed a complete ring on each side of the stub. The column finally failed due to tensile fracture at the bulge location, accompanied with a sudden drop in the lateral load bearing capacity. Inspection of the columns showed an inward indent for the inner steel tube and crushing of the sandwiched concrete. The results showed that all the CFDST columns behaved in a ductile manner; however, the ductility and energy dissipation ability of columns with a CHS as the outer skin were higher than those of the specimens with an SHS. It was found that the axial load levels influence the ductility of the specimens. Generally, the ductility of the specimens decreases with the increase in the axial load level. Additionally, it was observed that the lateral strength generally increases with an increase in axial load level up to a point (n=0.3 in their tests). However, it decreases with further

Chapter 2: Literature Review 31 increase in axial load level. It was also found that the hollowness ratio has a moderate effect on the behaviour of the lateral load-lateral displacement response of the CFDST columns.

Generally, the ductility of the columns decreases with the increase of hollowness ratio.

Recently, Han, Huang, and Zhao (2009) developed a mechanics model for CFDST beam-columns subjected to constant axial load and cyclically increasing flexural loading. Based on the verified theoretical model, they performed a parametric analysis to assess influence of the changing axial load level, nominal steel ratio, strength of outer steel tube, strength of inner steel tube, strength of concrete, width-to-thickness ratio of inner steel tube, and hollowness ratio on the lateral load-lateral deflection responses of CFDST columns. The results of the parametric study revealed that the stiffness of the curves in the elastic stage increases significantly as the nominal steel ratio increases, or as the slenderness ratio decreases. The axial load level, strength of outer steel tube, strength of inner steel tube, strength of concrete, and width-to-thickness ratio of inner steel tube have a moderate effect on the stiffness in the elastic stage. Moreover, stiffness in the elastic stage increases considerably as hollowness ratio increases for the area of the concrete section decreases. The stiffness in the descending stage increases as hollowness ratio increases for the area of inner steel increases. It was also found that the ultimate lateral load increases as the nominal steel ratio, strength of outer steel tube or concrete strength increases, or the slenderness ratio decreases. The ultimate lateral load increases with the axial load level changes from 0 to a point (n=0.2 in this study); however, further increase in the axial load level reduces the ultimate lateral load. Strength of inner steel tube and width-to-thickness ratio of inner steel tube have a moderate effect on the lateral load-lateral deflection curves.