given cycle using the values of peak load Pp and corresponding displacement Δp  measured for the   cycle.

values compared with their normal concrete counterparts. The cross-sectional type also affects the ductility of specimens under cyclic lateral loading. Since the circular stainless steel tube   tends

to provide better confinement to its core concrete and local buck- ling in square CFSST normally occurs earlier than that in circular CFSST, the circular CFSSTs correspondingly have higher ductility coefficients (μ) than their square counterparts. For example, under a high axial load level of 0.6, the μ value of the circular section with normal concrete (CN-6) is 4.58 while that of its square counterpart (SN-6) is only 2.44.

Energy dissipation ability is another important index to evaluate the seismic performance of a member or structure. In this paper, the cumulative energy (E) was calculated from the P − Δ hysteretic curve as the area enclosed by the hysteretic hoops. The cumulative energy (E) versus relative lateral displacement (Δ=Δy) curves are given in Fig. 13, and the values of total energy dissipated (Etotal) are given in Table 1 for all specimens. In general, the maximum dis- sipated energy of a circular specimen is higher than that of the square counterpart. The reason is attributed to the fact that local buckling occurred earlier for the square columns, thus reducing their deformation capacity when compared with the circular col- umns. For example, the displacement at failure Δu is 41.4 mm for circular specimen CN-6, whereas the corresponding displace- ment for square specimen SN-6 is only 27.7 mm. Results in Table 1 also indicate that the total energy dissipated (Etotal) for a CFSST specimen decreases with the increase of the axial load level due to reduced deformation capacity and/or reduced load-carrying capacity. Compared with the specimens made with normal con- crete, the total dissipated energy for the specimens made with RAC decreases by 7.8–17.0%. This may be attributed to the lower quality of RAC. The residual mortar attached to the recycled ag- gregate particles has a relatively lower modulus of elasticity, which might accelerate the damage (cracking and/or crushing) of the core RAC (Yang et al. 2009). Despite this, RAC can still be considered as an alternative choice to replace normal concrete in infilling

stainless steel tube without causing obvious strength and ductility loss for CFSST columns. The combined use of RAC and the stain- less steel tube produces a very environmentally friendly type of composite construction (Tam et al.  2014).

Strain Analysis and Axial Shortening

Fig. 14(a) presents the positions of the strain gauges on the bottom surface of the steel tube, where Strain 1, Strain 3, and Strain 4 are the extreme fiber longitudinal strains measured from gauges 10, 100, and 200 mm away from the stub edge, respectively; whereas Strain 2 is the transverse strain measured from a strain gauge 10 mm away from the stub edge. Figs. 14(b–e) give the lateral load

(P) versus strain (ε) relations of typical circular Specimen CN-3 and Figs. 14(f–i) present those of typical square Specimen SN-3, respectively, in which the yield strain (εy) of steel is also marked in the graphs. It can be seen that the P − ε relations showed a linear range in the initial loading stage and the load decreased nearly lin- early in the elastic unloading stage with comparable stiffness. After the yielding of the steel, the P − ε curves began to exhibit nonlinear characteristics and residual strain was observed upon unloading. Comparing Strains 1, 3, and 4, it is clear that the strain at a location closer to the stub developed more significantly than that far from the stub, due to the fact that the largest flexural deformation oc- curred at the mid-span of the specimen. From the P − ε relation for Strain 2, it was observed that the transverse strain increased rapidly after the occurrence of the steel local  buckling.