Keywords

1 Introduction

Steel bars may greatly increase the ductility of concrete filled steel tubular (CFST) in addition to increasing the final bearing capacity of composite components. The compressive strength of hollow steel tube concrete components is around 10% higher than that of conventional poured steel tube concrete components as a result of the steam health preservation procedure used in their fabrication [1]. Researchers both domestically and internationally have conducted a great deal of experimental work recently to enhance the mechanical characteristics of CFST members. In 2016, Hamidian et al. [2] conducted axial compression test study on 15 specimens fitted with spiral reinforcement. The results showed that the spiral reinforcement design outperformed conventional concrete-filled steel tube concrete columns. The test results are compared with ACI 318-11 and EC4-1994 to indicate a substantial improvement in the performance of concrete-filled steel tube columns after yielding. The test results and EC4 are well-aligned, and a conservative estimate of ACI has been made.

In 2018, Hasan et al. [3] tested the mechanical performance of reinforced concrete filled steel tube columns under axial loads and compared the performance of composite columns made of no more than two alternative configurations of steel bars welded into steel tubes and embedded in concrete. Because of the constraint effect between the stirrup and the steel tube, a new axial ultimate load model was proposed in order to accurately predict the member’s ultimate bearing capacity. In 2020, Fujiang Xia et al. [4] showed through tests that the configuration of steel bars in the member. Instead of increasing the wall thickness of steel tubes, it is preferable to enhance the mechanical characteristics of concrete columns welded steel tube. In 2021, Chen Zongping [5] carried out eccentric compression tests on 18 specimens by adjusting the spiral steel bar diameter, longitudinal bar diameter and other parameters, and suggested the optimal steel blending design scheme by parameter analysis.

In 2022, Yuan [6] et al. conducted axial compression test research on concrete filled square steel tube columns with built-in spiral steel bar constraints, which proved that increasing the volume of HSS spiral bars improved the ultimate bearing capacity of components better than improving the ultimate bearing capacity of components by increasing the external steel tube of the same volume. In 2015, Lu et al. [7] carried out an experimental investigation on the RC column reinforced by SCC filled square steel tube under eccentric compression, and developed a design formula to compute the ultimate strength of the reinforced column under eccentric load. In 2020, Li [8] et al. conducted that the reinforcement of damaged RC square columns with a square steel tube sandwich can significantly improve the stiffness and load-bearing capacity of RC columns and significantly improve the ductility of the members by studying the reinforcement of damaged RC square columns with a square steel tube sandwich.. In 2018, Yang et al. [9] proposed a reinforced hollow steel tube high-strength concrete column to improve the mechanical properties of the member while reducing the self-weight of the member, while to a certain extent reducing the wet work at the construction site..

Using ABAQUS finite element software, the author simulates the functioning state of reinforced hollow high strength concrete filled square steel tube components under eccentric load, taking into account that in practical applications, hollow components are typically in an eccentric state under wind load, seismic action, or the entire structural system. The effects of parameters such as eccentricity, slenderness ratio, and steel content on the mechanical properties of components are studied, which provides a theoretical basis for practical engineering applications.

2 Model Design

In this article, a total of 20 eccentrically compressed mid-length members are designed. The members are often made of steel tube, sandwich concrete, and PHC column concrete. Spiral steel bars, prestressed tendons, and HRB400 steel bars are used in the construction of the tube columns.

Table 1 displays the different member parameters in detail. The major focus of this article is on the relationship between mechanical qualities of components and steel yield strength, eccentricity, steel content, and slenderness ratio. Figure 1 depicts the components’ cross-sectional structure.

Table 1 Design parameters of specimens
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Fig. 1
A cross-sectional illustration of a component that has a deformed bar, prestressed tendon, stirrup, P H C column, steel tube, and sandwich concrete.

Schematic diagram of the component section

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3 Numerical Model Building

3.1 Selection of Material Constitutive Relation

The sandwich concrete and PHC column concrete make up the middle-long column concrete portion of the reinforced hollow high strength concrete filled square steel tube. The concrete plastic damage model of ABAQUS is used for the concrete [10]. The stress state is analogous to a three-dimensional load since it is jointly restrained by the steel tube. The concrete uniaxial stress–strain model modified by Liu Wei is adopted for the stress state and constitutive relationship [11]. The prestress of prestressed tendon is applied by cooling technique [12], the steel tube employs the low carbon steel five-fold line model, and the steel bar uses the two-fold line model [13].

3.2 Establishment of Finite Element Model

The concrete used for the tube column is reinforced with spiral, prestressed tendon, and HRB400 steel bars. Tie restrictions are used in PHC column concrete and sandwich concrete. Sandwich concrete and steel tube are designed to use hard contact in the vertical plane, and the Coulomb friction model is used in the tangential direction. Then, as illustrated in Fig. 2, bind the tube string, sandwich concrete, and steel tube end surfaces to the end plate. The loading method is displacement loading, with the rotation of the top and bottom of the column set to zero in the X, Z direction. Next, set the displacement in the X, Y direction of the column’s top and the X, Y, Z direction of the column’s base to zero. For steel tubes, PHC columns, and sandwich concrete, C3D8R solid units are utilised. For steel bars, truss units are used.

Fig. 2
An illustrated diagram of the model-building process. The layers, reinforced skeleton, P H C column, sandwich concrete, and steel tube with C 2 D 8 R on top form a reinforced hollow tube concrete column by binding and hard contact. Displacement loading is on the top with N 1. N 2 is at the bottom.

Model building process

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3.3 Validation of Finite Element Model

When the finite element analysis results are compared to the experimental data from the literature [14], it is discovered that the ultimate bearing capacity of the finite element simulation components is 3.15% lower than the ultimate bearing capacity of the test. Simultaneously, the deflection of the mid-height corresponding to the ultimate bearing capacity of the components is 0.33 mm different from the test. Figure 3 shows that the finite element analysis curve is in good agreement with the trend of the test curve, demonstrating the model’s stability.

Fig. 3
A multiline graph for component load plots N u in kilonewtons versus delta in millimeters. 2 lines for test and finite element analysis increase steeply till 5.5 millimeters and then decrease gradually. A photograph of the component under load is inset.

Component load–deflection curve of middle section in literature [14]

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4 Finite Element Analysis Results

4.1 Analysis of the Whole Process of Force

Elastic phase (OA): Fig. 4 shows that when the load increases, the cross-sectional deflection of the elastic stage elements also increases. The components are in a full-section compression condition, and the curves are roughly linearly coupled at this point. Since the stress states of the steel tube, sandwich concrete, and PHC column concrete are all different at this point, the steel tube does not have a significant restraint impact on the concrete.

Fig. 4.
A multiline graph for E L R H C F S T 4 member plots N u versus delta. 4 curves labeled sandwich concrete, P H C column concrete, steel tube, and composite column increase, peak between 6 and 8 millimeters, and then decrease gradually. Points A to D are marked along the composite column peak.

ELRHCFST-4 member load-medium section deflection curve

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Elastoplastic stage (AB): The element enters the elastic–plastic stage as it proceeds from characteristic point A to characteristic point B; at this point, the steel tube and concrete continue to support the majority of the load. The steel tube on the compression side commences to gradually enter the yield stage when it achieves characteristic point B.

Plastic strengthening stage (BC): The component undergoes the plastic strengthening stage from characteristic point B to characteristic point C at this point, and the sandwich concrete and PHC column concrete share the bulk of the internal force. At this stage, as the load increases, so does the growth rate of the segment deformation in the component.

Descending stage (CD): The maximum bearing capacity of the member has been attained at characteristic point C. The stiffness of the part continues to decrease when the load is applied. The sandwich concrete is currently being crushed in a portion of the segment on the compression side, and the PHC column concrete is also gradually gaining its functional capacity at this point.

4.2 Effect of Configuration Reinforcement on the Mechanical Properties of Components

According to Fig. 5, the ultimate bearing capacity of components constructed with HRB400 is enhanced by 3.46% when compared to components without HRB400. In accordance with the maximum bearing capacity, the middle section diverts by 0.43 mm more. Figure 5 shows that the ductility of the HRB400-configured components has also been greatly enhanced. Concrete structures made of hollow steel tubes are more resilient to elastoplastic deformation than components without steel bars.

Fig. 5
A multiline graph for H R B 400 plots N u versus delta. 2 curves labeled E L R H C F S T 8, and E L R H C F S T 4 increase, peak between 5.5 and 6 millimeters, and then decrease gradually. The curve for E L R H C F S T 8 is at the top. The area between declining curves is shaded.

The influence of HRB400 on the mechanical properties of components

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5 Parameter Analysis

5.1 Effects of Eccentricity

The load-mid-heigth section deflection curves for components with various eccentricities are shown in Fig. 6. The final bearing capacity of the components reduces by 10.9%, 10.1%, 9.8%, 9.2%, 8.6%, and 8.1%, respectively, as the eccentricity goes from 0.2 to 0.8. The ultimate bearing capacity of the component constantly declines at a rate proportional to the increase in eccentricity. Figure 7 depicts the variation in elastic stiffness for the various eccentricity-related components. It is evident that both the component’s elasticity and eccentricity both drop from 0.8 to 0.2. The stiffness rose by 11.6%, 19.6%, 20.2%, 23.3%, 31.6%, and 50.5%. The elastic stiffness of the component increases as eccentricity decreases, and the growth rate of the elastic modulus of the component likewise increases.

Fig. 6
A multiline graph for eccentricity plots N u versus delta. 7 curves labeled E L R H C F S T 1 through 7 increase, peak between 4 and 7 millimeters, and then decrease gradually. The curve for E L R H C F S T 7 is at the top followed by E L R H C F S T 1, 2, 3, 4, 5, and 6.

Effect of eccentricity on load-mid-section deflection curve

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Fig. 7
A bar graph plots E A in kilonewton per millimeter versus e. 7 bars for elastic stiffness have a decreasing trend between (0.2, 5299.8), (0.3, 3520.9), (0.4, 2675.5), (0.5, 2168.9), (0.6, 1804.3), (0.7, 1509.1), and (0.8, 1352.1).

Effect of eccentricity on initial stiffness

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5.2 Effect of Steel Strength

Figure 8 illustrates the mid-heigth section deflection curve corresponding to the segments of the steel tube under different yield strengths. It can be seen that the increase in the yield strength of the steel has no obvious effect on the elastic stiffness of the components. The ultimate bearing capacity of the components increased by 12.64%, 3.08%, 2.55%, and 3.24%, respectively, while the yield in steel strength varied from 235 to 460 MPa. Figure 9 depicts the ultimate bearing capacity of the component steel yield strength at various eccentricities. According to the investigation, the lower the eccentricity under the same conditions, the more noticeable the improvement in steel yield strength on the ultimate bearing capacity of the component.

Fig. 8
A multiline graph for steel tube yield strength plots N u versus delta. 5 curves labeled E L R H C F S T 9, 4, 10, 11, and 12 increase, peak at around 6 millimeters, and then decrease gradually. The curve for E L R H C F S T 12 is at the top followed by E L R H C F S T 11, 10, 4, and 9.

Effect of steel tube yield strength on load-medium section deflection curve

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Fig. 9
A grouped bar graph for steel tube yield plots N u versus e with bars for E L R H Q 235, 355, 390, 420, and 460. The bars have an overall decreasing trend with the highest values at e 0.2, followed by 0.4, 0.6, and 0.8. The bars for E L R H Q 460 are the highest, followed by 420, 390, 355, and 235.

Effect of steel tube yield strength on bearing capacity

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5.3 Effect of Steel Content

Figure 10 shows the load-mid-heigth section deflection curves for steel tube members with various wall thicknesses. With an increase in steel content, the member’s elastic stiffness gradually rises, but elastic stiffness is also impacted by the addition of steel. Because the confinement effect of the steel tube on the concrete is improved by increasing the wall thickness of the steel tube, it is not particularly significant. The ultimate bearing capacity of the member rises by 5.67%, 5.14%, 5.25%, and 4.85%, respectively, while the steel composition varies from 0.061 to 0.112. The maximum bearing capacity of components with various steel contents at various eccentricities is shown in Fig. 11. It may be inferred that the greater the eccentricity of the components under the same parameters, the greater the effect of increasing the wall thickness of the steel tube on enhancing the components’ ultimate bearing capacity.

Fig. 10
A multiline graph for steel content plots N u versus delta. 5 curves labeled E L R H C F S T 13, 4, 14, 15, and 16 increase, peak at around 6, and then decrease gradually. The curve for E L R H C F S T 16 is at the top followed by E L R H C F S T 15, 14, 4, and 13.

Effect of steel content on load-to-middle section deflection curve

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Fig. 11
A grouped bar graph for steel content plots N u versus e with bars for E L R H t 5, 6, 7, 8, and 9. The bars have an overall decreasing trend with the highest values recorded at eccentricity 0.2, followed by 0.4, 0.6, and 0.8. The bars for E L R H t 9 are the highest followed by 8, 7, 6, and 5.

Effect of steel content on bearing capacity

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5.4 Effect of Slenderness Ratio

The load-mid-heigth section deflection curves for members with different slenderness ratios are shown in Fig. 12. The effect of increasing the slenderness ratio on the elastic stiffness of the part becomes more obvious. The maximum bearing capacity of the member varies when the slenderness ratio increases from 17.32 to 41.57. The respective forces decreased by 1.55%, 1.64%, 2.94%, and 6.34%. The maximum carrying capacity of components with various length-to-slenderness ratios at various eccentricities is shown in Fig. 13. The analysis inferred that the carrying capacity of components with various lengths and slender ratios decreases under the same conditions at various eccentricities.

Fig. 12
A multiline graph for slenderness ratio plots N u versus delta. 5 curves labeled E L R H C F S T 4, 17, 18, 19, and 20 increase, peak between 5 and 30 millimeters, and then decrease gradually. The curve for E L R H C F S T 20 is at the top followed by E L R H C F S T 19, 18, 17, and 4.

Effect of slenderness ratio on load-medium section deflection curve

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Fig. 13
A grouped bar graph for slenderness ratio plots N u versus e with bars for E L R H L 2000, 2400, 2800, 3200, and 3600. The bars have an overall decreasing trend with the highest values recorded at eccentricity 0.2, followed by 0.4, 0.6, and 0.8. The bars for E L R H L 2000 are highest at each point.

Effect of slenderness ratio on bearing capacity

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6 Conclusion

  1. (1)

    During the loading process, the different components of the reinforced hollow high strength concrete filled square steel tube mid-long column can work well together. The concrete of the PHC column can still play a significant role after the ultimate bearing capacity when the steel tube initially reaches the yield strength and exits the working state.

  2. (2)

    When all other parameters kept constant, the change in eccentricity or slenderness ratio has the greatest impact on the elastic stiffness of the member. The change in steel content ratio has a minor effect on the elastic stiffness of the member, while the change in steel yield strength has almost no effect on the elastic stiffness of the member.

  3. (3)

    When the eccentricity is small and all other parameters are kept constant, the effect of changing the steel yield strength or steel content on the final bearing capacity of the member is more obvious than the effect of altering the slenderness ratio.

Innovation

This innovative reinforced hollow concrete-filled steel tube composite member not only enhances the functionality of previous hollow concrete-filled steel tube components, but also, to a certain extent, expands the range of applications for hollow components, offering a certain benchmark for practical engineering.