3.1. Failure Morphology
The failure morphology of the samples is presented in
Figure 3. The eccentric-loaded columns showed obvious buckling and lateral deflection, which is evident in
Figure 3. However, the characteristics of the failure modes of the CFST columns under an eccentric load included bending and buckling [
38]. Moreover, columns undergoing axial compression displayed evident local bulking and rupture of the steel tube. When wall thickness varied from 1.2 mm to 3.0 mm, the numbers of the bulking are reduced and its locations are transferred from the ends to the middle position. Similarly, the buckling and lateral deflection is also transferred from the end to the middle position.
Figure 4 presents the relationship between the load and deformation. The load varies with displacement which is displayed in
Figure 4a. It firstly increases gradually to the peak value and then the descend phase decreases rapidly when the columns undergo axial compression, but decrease gently when the columns undergo eccentric compression. Therefore, compared with columns subjected to an axial compressive load, columns under eccentric compression display better ductility. The curves present a similar development trend under axial or eccentric compression with different wall thicknesses. Nevertheless, the ultimate capacity is increased with increasing wall thickness. The load–longitudinal strain curves were derived from the relationship between the load and the average strain along the axial direction of 5 to 8; as shown in
Figure 4b, the strain almost proximately increases linearly with the load until about 75 percent of the peak load. Then, the load varies nonlinearly with the strain. All the load–strain curves present a similar variation trend. However, the axial strains increase faster than the transverse strains.
Figure 4c displays load–lateral deflection curves of columns undergoing eccentric compression, which demonstrate that the relationship between the load and lateral deflection has a similar development tendency. Lateral deflections increase slowly before the peak load and then decrease slowly during the whole eccentric loading process.
3.2. Theory Calculation Formulas for Ultimate Capacity of Design Codes
To explore the feasibility of predicting the columns’ ultimate capacity using the design codes, calculation formulas for the ultimate capacity of six design codes including JCJ 01-89 [
49], CECS 28 [
50], DL/T 5085 [
51], DBJ l3-51-2003 [
52], AISC-LRFD-1999 [
53], and BS 5400 [
54] are summarized as follows. The prediction of the ultimate capacity was calculated using these codes; after that, a comparison between these values and the experimental results was conducted.
The calculation formula for the ultimate capacity of the code JCJ 01-89 [
49] is expressed as Equation (1):
where
is the increase coefficient of the concrete strength,
represents the steel tube’s design compressive strength,
is the concrete’s design compressive strength,
stands for the steel tube’s cross-sectional area,
is the cross-sectional area of the concrete,
is the reduction coefficient of eccentric compression, and
is the correction of
.
The formula for the ultimate capacity of the code CECS 28:90 [
50] is expressed as Equation (2):
where
is the reduction coefficient of the slenderness ratio.
The ultimate capacity of the code DL/T 5085-1999 [
51] is calculated using Equations (3)–(13). If
,
If
,
where
represents the moment coefficient, and its value is in accordance with code GB 50017-2003 [
55], and
is the flexural plasticity development coefficient of the component in the sectional area: if
≥ 0.85,
= 1.4; otherwise,
= 1.2. The standard confinement coefficient
is calculated by Equation (5):
where
is the steel tube yield strength.
stands for the composite design compressive strength, and it is calculated using Equation (6):
where
and
are coefficients, which are determined as
where
is the design confinement coefficient, which can be calculated using Equation (9):
where
represents the Euler critical load, which is determined by Equation (10):
where
stands for the total sectional area, and
is the composite modulus of elasticity and is computed using Equation (11):
where
is the proportional limit, and
is the strain under the proportional limit. They can be calculated using Equations (12) and (13), respectively:
where
is the standard composite axial strength.
The ultimate capacity of the code DBJ l3-51-2003 [
52] is calculated using Equations (14)–(21). If
,
If
,
where
is the stability coefficient, and
is a coefficient, which can be calculated using Equation (16):
is the flexural modulus of the sectional area; for the circular steel tube, it can be calculated using Equation (17):
where
represents the steel tube’s diameter.
,
,
, and
are coefficients; they are calculated using Equations (18)–(21):
The ultimate capacity of the code AISC-LRFD-1999 [
53] is calculated using the following equations. If
,
If
,
where
stands for the design axial force,
is the design bending moment, both
and
are the coefficient, and their values are 0.85 and 0.9, respectively.
stands for the axial ultimate load, and it can be calculated using Equation (24):
where
is the critical force; it can be computed as follows:
where
represents the slenderness ratio; it is expressed as
where
stands for the effective length coefficient,
stands for the gyration radius, and
is the equivalent yield strength, which is expressed as Equation (28):
represents the equivalent modulus of elasticity, which is expressed as Equation (29):
where
represents the steel tube’s modulus of elasticity, and
is the concrete’s modulus of elasticity.
is the flexural capacity, which is determined using Equation (30):
where
is the plastic bending modulus, which can be calculated as follows:
The ultimate capacity formulas of the code BS 5400 [
54] are expressed as follows:
where
stands for the design axial force, and
is the bending moment.
is the ultimate bearing capacity under axial load, which can be calculated as follows:
where
and
represent partial coefficients of steel and concrete, respectively; their values are 1.1 and 1.5, respectively.
stands for steel yield strength after reduction, which can be calculated as follows:
is the concrete strength when subjected to triaxial compression, which is expressed as Equation (36):
is the flexural capacity and is expressed as follows:
is the stability coefficient, which can be calculated as follows:
where
and
are the modulus of elasticity of steel and concrete, respectively.
and
stand for the inertia moment of the steel tube and concrete, respectively.
and
can be calculated as follows:
Pacheco et al. suggest that the conversion coefficient of 150 mm cubic to cylinder with a diameter of 150 mm for recycled concrete is 0.77 [
56]. Additionally, the conversion coefficient of 150 mm cubic to prism with dimensions of 150 mm × 150 mm × 300 mm for recycled concrete is 0.67 [
57,
58].
3.3. Comparison of the Ultimate Load
The ultimate capacities of the columns undergoing eccentric and axial compression from calculated results using different design codes and experimental results are represented by N
uc and N
ue, respectively, in
Table 4.
Table 5 compares N
uc and N
ue.
Table 6 shows the statistical characteristics of the comparison results. A comparison of the ultimate capacities from the experimental results and calculated results using different specifications is displayed in
Figure 5.
It can be seen from
Table 4 that the experimental ultimate capacities of A-N-T1, A-N-T2, E-N-T1, and E-N-T2 are 1331.0, 1627.8, 759.0, and 1065.8 kN, respectively. Moreover, the ranges of the calculated ultimate capacities of A-N-T1, A-N-T2, E-N-T1, and E-N-T2 using different design codes are 809.5 kN~1117.0 kN, 1028.3 kN~1582.4 kN, 267.3 kN~711.3 kN, and 465.0 kN~978.6 kN, respectively. This clearly demonstrates that the calculated ultimate capacities are correspondingly lower than those of the experimental results.
Table 5 gives the ratio ranges of calculated ultimate capacities to tested results corresponding to A-N-T1, A-N-T2, E-N-T1, and E-N-T2. The ratios range from 0.61 to 0.84, 0.63 to 0.97, 0.35 to 0.94, and 0.44 to 0.92, respectively. This indicates that the calculated ultimate capacity is conservative and safe. Moreover, the calculated ultimate capacities using the codes AISC-LRFD and JCJ 01-89 are the lowest and the highest, respectively. Thus, AISC-LRFD and JCJ 01-89 are the most conservative and the most accurate for the ultimate capacity calculation. Similarly, the ultimate capacity prediction results of the CFST columns were 25% conservative using code AISC-LRFD [
8]. The different specifications predicted the ultimate capacity with different precision due to variations in the considered factors, such as the confinement effect of the steel tube on the concrete, the reduction coefficients of materials, and the eccentricity. For example, ACI [
59] ignores the contribution of the confinement effect of the steel tube on the concrete, which can improve the ultimate capacity. Additionally, EC4 [
60] takes into account the confinement effect, but it provides relatively conservative predictions [
61].
Table 6 displays the statistical properties of the comparison between the ultimate capacities of the experimental results and calculated results. The average value and variation coefficient of the ratio of the calculated ultimate capacities to experimental ultimate capacities (N
uc/N
ue) of A-N-T1 and A-N-T2 are within the range of 0.62~0.91 and 0.02~0.09, respectively. Additionally, the average value and variation coefficient of N
uc/N
ue of the eccentric compression columns E-N-T1 and E-N-T2 are within the range of 0.42~0.93 and 0.01~0.24, respectively. This implies that the decrease ratio of calculated ultimate capacity to experimental ultimate capacity of the eccentric compression columns is larger than that of the axial compression columns.
The ratios of the average value of N
uc/N
ue of the columns under eccentric load to the average value of N
uc/N
ue of the axial-loaded columns are in the range of 0.67~1.18. This indicates that the predicted ultimate capacity of the eccentric compression columns using design codes is smaller than that of the axial compression columns.
Figure 5 compares the ultimate capacities of the tested results and the calculated results using different design codes. It indicates that the predicted ultimate capacities are correspondingly smaller than those of the experimental results.