Steelreinforced concretefilled steel tubular columns under axial and lateral cyclic loading
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Abstract
SRCFT columns are formed by inserting a steel section into a concretefilled steel tube. These types of columns are named steelreinforced concretefilled steel tubular (SRCFT) columns. The current study aims at investigating the various types of reinforcing steel section to improve the strength and hysteresis behavior of SRCFT columns under axial and lateral cyclic loading. To attain this objective, a numerical study has been conducted on a series of composite columns. First, FEM procedure has been verified by the use of available experimental studies. Next, eight composite columns having different types of cross sections were analyzed. For comparison purpose, the base model was a CFT column used as a benchmark specimen. Nevertheless, the other specimens were SRCFT types. The results indicate that reinforcement of a CFT column through this method leads to enhancement in loadcarrying capacity, enhancement in lateral drift ratio, ductility, preventing of local buckling in steel shell, and enhancement in energy absorption capacity. Under cyclic displacement history, it was observed that the use of crossshaped reinforcing steel section causes a higher level of energy dissipation and the moment of inertia of the reinforcing steel sections was found to be the most significant parameter affecting the hysteresis behavior of SRCFT columns.
Keywords
Compressive strength Hysteresis behavior SRCFT columns Finiteelement analysis Composite actionIntroduction
Composite columns have higher strength and ductility efficiency due to composite action between steel and concrete core. In this type of composite column, the concrete can be plain such as (CFT) or reinforced concrete with steel bar (RCFT). Current studies (Xiamuxi and Hasegawa 2012; Endo et al. 2000; Hua et al. 2005; Xiamuxi and Hasegawa 2011) show that RCFT columns have better performance regarding moderate and severe earthquake excitations, higher toughness, and ductility in comparison with CFT columns.
Hamidian et al. (2016) have investigated the axial compressive behaviors of concretefilled steel tube columns reinforced with different spiral pitch spacing. They found that the rate of pitch spacing has an important role on the postyield behavior of the reinforced concretefilled steel tube. The results show that as the rate of pitch spacing decreases, the postyield behavior of SRCFTs improves. In addition, the effectiveness of the pitch spacing rate on the postyield behavior of a SRCFT column is more than the thickness rate of the steel tube. Wang et al. (2004) investigated the strength and ductility of crossshaped steelreinforced concretefilled tube (SRCFT) columns subjected to axial compressive loads. The results showed that composite columns had higher strength, energy absorption capacity, and ductility performance due to the composite action between steel tube, reinforcing steel section, and concrete. Chang et al. (2012) present a numerical study of cyclically loaded crossshaped steelreinforced concretefilled tube (SRCFT) and the mechanical performance of SRCFT columns under cyclic loading. They found that the presence of the section steel could carry the lateral load and reduce the tensile zone of the concrete section. The structural steel section could provide a confinement effect on the concrete core and increase the loadcarrying capacity and postpeak strength of SRCFT columns. Lai and Ho (2016) mentioned that the composite action could not be fully developed because of various dilatation attributes of concrete and steel tube in the elastic stage. In addition, due to the inelastic outward buckling of steel tube, CFT columns might suffer serious degradation. Qin and Xiao (2013) have been conducted a research on concretefilled steel tube columns subjected to cyclic lateral force. They found that the ratio of diameter to thickness and the material properties strongly affect the seismic behavior of CFT columns. Better performance could be observed for CFT columns with smaller tube diameter to thickness ratio and higher material strengths.
The reinforcing steel section has a significant role in improving the strength of SRCFT columns and due to the lack of previous researches, this paper presents the mechanical and hysteretic behavior of different types of SRCFT columns under compressive axial and lateral cyclic loading.
Finiteelement modeling of SRCFT columns
Understanding the structural behavior of SRCFT columns and caring out a comparative investigation, geometric, and material nonlinear finiteelement analyses based on the commercial FE package ANSYS® User’s Manual (2005) have been undertaken under axial and lateral cyclic loading. In this paper, the effect of various types of reinforcing steel sections and interaction between steel and concrete surface has been investigated.
Characteristics of finiteelement modeling
For modeling of various components of SRCFT column, ANSYS® User’s Manual (2005) elements and capabilities are as follows.
The concrete was modeled using a special concrete element SOLID 65. This element is an eightnode solid brick element that has crushing (compressive) and cracking (tensile) capabilities. For modeling of steel tube and reinforcing steel profile, a 3D solid element SOLID 45 was used. The element has plasticity, creep, swelling, stress stiffening, large deflection, and large strain capabilities. CONTAC52 (for the modeling of gap between steel and concrete) represents two surfaces that might maintain or break physical contact or might slide relative to each other. SHELL43 (for the modeling of the rigid plate for load applying) is well suited to model linear, warped, and moderately thick shell structures. The element has plasticity, stress stiffening, creep, large strain, and large deflection capabilities.
Material characteristics
For multilinear isotropic properties, stress–strain relation of concrete was defined based on modified Popovic model (1973) for unconfined concrete and Belarbi and Hsu’s model (1994) was used for modeling the concrete tensile behavior. The model presented by Mander et al. (1988) for confined concrete is a simplified version of the Karsan and Jirsa (1969) model that also models the capability of concrete to carry some tensile stresses. The envelope curve is assumed to be given by the Popovic equation, which also accounts for the effect of confinement.
For the unconfined concrete element, the elastic modules (Ex = 3E4 MPa), the Poisson’s ratio (ν_{ xy } = 0.2), the values for the ultimate tensile strength (f_{ r } = 3.72), and ultimate compressive strength (f_{c} = 40 MPa) are the properties of isotropic material, as shown in Fig. 2. Considering Fig. 3, the behavior of steel is characterized with an initial linear elastic portion of stress–strain relationship with a modulus of elasticity, 2E5 MPa and up to the yield stress f_{y} (ST 37 with F_{ u } = 370 N/mm^{2}), is equal to 240 MPa, followed by a strain plateau of varying length (strain = 0.015) and a following region of strain hardening.
Verification of finiteelement modeling under axial loading
Geometric and material properties of test specimen under axial loading.
After Wang et al. (2004)
Specimen  Shape  H  D  t  Steel property  Concrete property  

f _{y}  E _{s}  f_{c}′  E _{c}  
NSA1  Cir.  465  166  2.7  288  20,700  29.6  33,490 
Verification of finiteelement modeling under lateral cyclic loading
Geometrical and material properties for the test specimen under lateral cyclic loading.
After Chang et al. (2012)
Specimen ID  Section steel  Steel tube  Concrete properties  

I steel  A_{s} (mm^{2})  f_{y1} (MPa)  f_{y2} (MPa)  D × t (mm)  f_{cu} (MPa)  n  
HC121  112  3570  314  269  218 × 4  74.3  0.5 
Numerical results of SRCFT specimens under axial loading

The column is not slender and all nodes are restrained at the columns’ base and top.

The internal steel reinforcement was selected from among DIN standard profiles and dimension. In addition, geometric parameters of CFT specimens controlled by the help of BS 54001 (1990) and EC4 (1994) codes.

The units of length and force are in millimeter and Newton units, respectively.

The columns are considered as fixedend columns.

Modulus of elasticity of steel is in the form of E_{s} = 2E5 MPa.

The yield strength of steel is in the form of f_{y} = 240 MPa.

The compressive strength of concrete is in the form of f_{c}′ = 40 MPa.

The concrete modulus of elasticity of concrete is in the form E_{c} = 3E4 MPa.

Length of specimens, L = 6000 mm.

Area percentage of steel reinforcement ratio to the total crosssectional area = ρ_{s}.

Circular SRCFT column reinforced with IPB (120, 160, 200) steel section, 2 IPE steel section, and crossshaped steel section, respectively = CB (120, 160, 200), C 2 IPE, CCross.

Square SRCFT column reinforced with IPB (120, 160, 200) steel section, 2 IPE steel section, and crossshaped steel section, respectively = SB (120, 160, 200), S 2 IPE, SCross.

Circular CFT column and square CFT column, respectively = CCFT, SCFT.
Specifications of SRCFT columns
Specimens  Shape of section  Dimensions of steel tubes (mm)  Type of steel section  Steel tube thickness (mm)  ρ_{s} (%)  

First group  1. CCFT  Circular  300  –  6  – 
2. CB 120  Circular  300  IPB 120  6  4.81  
3. CB 160  Circular  300  IPB 160  6  7.68  
4. CB 200  Circular  300  IPB 200  6  11.03  
5. C 2 IPE  Circular  300  2 IPE  6  11.03  
6. CCross  Circular  300  Cross  6  11.03  
Second group  7. SCFT  Square  266 × 266  –  5.34  – 
8. SB 120  Square  266 × 266  IPB 120  5.34  4.81  
9. SB 160  Square  266 × 266  IPB 160  5.34  7.68  
10. SB 200  Square  266 × 266  IPB 200  5.34  11.03  
11. S 2 IPE  Square  266 × 266  2 IPE  5.34  11.03  
12. SCross  Square  266 × 266  Cross  5.34  11.03 
Effect of ratio of reinforcing steel section
Therefore, variation of reinforcing steel ratio had a significant effect on the performance of SRCFT columns under axial loading in such a way that load reduction in SRCFT columns with lower ρ is more than the states which had higher reinforcing ratio due to larger crosssectional area of the reinforcing steel profiles.
It shows that increase in loadcarrying capacity of column after postbuckling level is more obvious and more tangible. In addition, it is absorbed that increase in loadcarrying capacity of circular SRCFT columns is superior to that of Square sections. The significant point is that, at square CFT columns, load buckling occurs on the steel shell at axial deformation about 28 mm. On the other hand, by reinforcing CFT section with the reinforcing steel section, loadcarrying capacity of column increases and local buckling of the steel shell does not occur. In addition, the column is able to bear more deflections.
Effect of types of reinforcing steel section
Figure 13 demonstrates that under axial loading, at small deflections, reinforcing steel section shape does not have any impact on the strength of SRCFT columns. However, at large deflections, the effect of reinforcing steel section appears gradually. In addition, it is observed that SCross, SB 200, and S 2 IPE sections have the highest impacts on the increment of SRCFT columns’ strength, respectively.
Effect of interaction between steel and concrete
To get a better realization of the interaction mechanism, three types of circular SRCFT columns (CCross, CB 200, and C 2 IPE) and square SRCFT columns (SCross, SB 200, and S 2 IPE) from each group have been analyzed. The specimens were loaded under axial compression. The geometric and material specifications of the specimens are illustrated in Table 3. For this purpose, first, reinforcing steel section (St.steel) and CFT section have been separately analyzed. Then, the superposed curves, achieved from the addition of the St.steel section and CFT section, have been compared with the curves achieved from the analyses of SRCFT specimens.
Considering the curves of SRCFT and St.Steel + CFT achieved from the analysis of C 2 IPE specimen, it is obvious that at the linear section of the curves, the interaction between steel and concrete has not exhibited a higher impact on the loadcarrying capacity of columns. After this point, this mechanism is very efficient, because the concrete prevents the local buckling of reinforcing steel section and interface friction between the reinforcing steel section and the concrete surface will cause more transition of contact stress between two surfaces, and therefore, the axial load carrying of specimen will be increased.
Considering Fig. 14, it is obvious that the degradation of axial strength at SRCFT column was initiated at a deformation equal to 20 mm, whereas this quantity for St.Steel + CFT specimen is approximately 12 mm; thus, the important role of contact stress in increment of axial strength can be perceived.
Numerical results of SRCFT specimens under lateral cyclic loading
Geometric and material properties of specimens
Specimen  Shape  D  L/D  Steel reinforcement section  Area of steel reinforcement (mm^{2})  t  Area of concrete (mm^{2}) 

CFT  Circle  538  5.57  –  –  14  204,282 
CCross  Circle  538  5.57  Cross IPE  7810  14  204,282 
C 2 IPE  Circle  538  5.57  2 IPE  7810  14  204,282 
CB 200  Circle  538  5.57  IPB 200  7810  14  204,282 

The columns are considered as fixedend columns.

E_{s} = 2E5 MPa, f_{y} = 240 MPa, f_{c}′ = 40 MPa, E_{c} = 3E4 MPa, L = 3000 mm.
The total crosssectional area of the outer steel tube is 23,047 mm^{2} and the crosssectional area of concrete is 204,282 mm^{2}. In addition, the area of steel reinforcement is 7810 mm^{2}. It is noticeable that the area of crossshaped section for the reinforcing steel section is equal to IPB 200 standard steel profile.
Hysteretic behavior of SRCFT columns
It is observed that the maximum shear force in cycle No. 8 in CFT specimen is 1180 kN where as this quantity for any of SRCFT specimens is more than 1298 kN. Figure 19 clearly shows that SRCFT specimens have the lower degradation of load at the further cycles. However, in SRCFT columns, the reinforcing steel section will not be buckled before reaching the steel wall to the yield stress. Thus, the confinement effect of concrete has been increased.
Considering Fig. 17, the degradation of strength in CFT column was began suddenly at 4% of lateral displacement, whereas the degradation of shear strength has inclined slope at SRCFT column. At the end of loading, crushing of concrete and local buckling of steel tube was observed for CFT column, while those were prevented at SRCFT column because of good effect of reinforcing steel section.
Comparison of the hysteresis loops of SRCFT columns
By considering the specimens, it is observed that CCross, CB 200, and C 2 IPE sections (which both have the same crosssectional area) have the higher moment of inertia, respectively. Figure 18 shows that CCross, CB 200, and C 2 IPE sections have the higher amount of shear strength, respectively. Therefore, it is concluded that shear strength capacity of SRCFT columns has a positive correlation with the moment of inertia of reinforcing steel section under cyclic loading.
In the envelope curves of hysteresis loops, it is observed that CFT column loses its strength in upper cycles due to the concrete crushing of steel shell. However, this phenomenon is not occurs in SRCFT columns because of the effect of reinforcing steel section in the concrete core confinement and the prevention of quick crushing of concrete.
Comparison of energy dissipation
Energy dissipation is a very important parameter to describe the hysteretic performance of columns. The area enclosed by each cycle of displacement has been considered as the energy dissipated from the column at the same cycle.
Comparison of important parameters of SRCFT columns
Specimen  Maximum shear strength (kN)  Ratio  Energy dissipated (kN m)  Ratio  Stiffness plastic (N/mm)  Ratio 

CCFT  1180  1.00  842  1.00  413  1.00 
CB 200  1298  1.10  1753  2.08  427.3  1.03 
C 2 IPE  1438  1.21  1983  2.35  434.5  1.05 
CCross  1471  1.24  2184  2.59  477.83  1.15 
Considering the maximum shear strength parameter, CCross specimen showed differences about 24% higher than that of the CFT specimen. Considering the energy dissipation parameter, SRCFT specimen reinforced with crossshaped steel section had almost 159% improvement in comparison with the circular CFT column. For plastic stiffness, CCross specimen exhibited a minimum value of 15% increase against the circular CFT specimen. It is obvious from the findings provided in Table 5 that SRCFT columns have higher shear strength, absorbing energy capacity, and more appropriate ductility characteristics compared to CFT columns. Furthermore, columns CCross, C 2 IPE, and CB 200 showed the best performance with regard to stiffness, the maximum shear strength, and energy absorption capacity, respectively.
Summery and conclusions
 1.
Variation of reinforcing steel ratio had a significant effect on the performance of SRCFT columns under axial loading in such a way that load reduction in SRCFT columns with lower ρ is more than the states which had higher reinforcing ratio. By the increase of ρ_{s} parameter, loadcarrying capacity of section increases. This increment at CFT circular section is considerably higher than that of CFT square section.
 2.
The effect of types of reinforcing steel section from the viewpoint of loadcarrying capacity and stiffness, and it is resulted that CCross specimen has a better performance compared to that of CB 200 and C 2 IPE. The influence of higher moment inertia of CCrossreinforcing steel profiles, the confinement effect of concrete was the main reasons of better behavior and helped to delay the formation of internal splitting cracks along the steel profiles length.
 3.
Loadbearing capacity of SRCFT columns compared to the superposed load bearing of reinforcing steel section and CFT columns increases because of the positive effects of interaction between steel and concrete in SRCFT columns. The effects of composite action in C 2 IPE specimens are considerably higher than that of CB 200 and CCross specimens.
 4.
It is found that the presence of the section steel can carry the lateral load and reduce the tensile zone of the concrete section. As a result, SRCFT columns have higher stiffness and peak lateral load than common CFT columns even with the same geometrical and material parameters. The section steel can also enhance the deformation ability of a SRCFT column. The flanges and webs of the reinforcing steel section can also give some confining effect on the core concrete.
 5.
By the investigation of envelope curves of hysteresis loops of SRCFT columns, it is observed that reinforcing CFT column by a steel section causes less degradation of load at large deflections, increase of energy absorption capacity, and shows the appropriate behavior of them under lateral cyclic loading.
 6.
Considering the results, CCross, C 2 IPE, and CB 200 sections have the higher amount of moment of inertia, higher shear strength, and energy absorption capacity, respectively. Therefore, the moment of inertia in the reinforcing steel section was found to be the most significant parameter on the hysteresis behavior of SRCFT columns.
Design recommendation

The reinforcing steel cross section is offered for the design of SRCFT columns under axial loading. For the design of SRCFT columns under cyclic loading, reinforcing steel section with the maximum moment of inertia is proposed. In general, considering the symmetry property of crossshaped steel section, this section has a better performance compared to the other sections for axial and seismic loading. Therefore, crossshaped steel section is proposed for the reinforcement of CFT columns.
Notes
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