Investigation of double-skinned square steel-concrete composite columns with in-built square cores

ABSTRACT

Double-skinned steel-concrete composite columns are famous nowadays in the construction industry because of their structural advantages. The analysis of the performance of double-skinned composite columns with two steel skins of the outer and inner tube, in addition to an in-built steel core in-filled with concrete, was attempted. Steel skins can serve multiple functions, notably defining the geometry of the concrete column and preventing cracks from tensile pressure. This article consists of the research work, numerical and experimental investigation, of the behaviour of Double-Skinned Square Composite Columns (DSSCC) with square cores in-filled with concrete under axial compressive load. The square steel tubes are of size 150 and 50 mm with 6 and 3 mm thickness of outer and inner tubes, respectively, and a height of 500 mm characterized by an inner core of 1 mm thickness. The steel tube considered in the current numerical study (using Abaqus 6.14) is of grade Fe250, Fe350 and Fe415, and in-filled concrete is of grade M20, M25, M30, M35, M40 and M45. The steel tube considered in the present experimental study is of grade Fe250, in addition to the steel cores, which are made of grade A1008 cold-formed steel. The average compressive strength of the concrete used in an experimental study, after 28 days of curing, is measured as 26.07, 32.89 and 40.29 N/mm². The current study was performed to find the axial compressive behaviour, ultimate load, load versus vertical and horizontal deflection behaviour and corresponding stress and strain value and failure modes. Stiffness, ductility ratio and energy absorption capacity were determined from the observed test values. The results show that increasing concrete compressive strength improves the load-carrying capacity of the column. The experimental and numerical results were discussed and validated.

Keywords:
Composite columns; Double-skinned; Axial load; Concrete strength; Steel core performance; Cold-formed steel; Square cores

1. INTRODUCTION

Composite construction combines the strength of concrete and steel to maximize the efficiency of composite structural elements. The superior strength-to-weight ratio of steel enables the creation of smaller and lighter foundations. When combined with concrete, these columns effectively reduce wobble and lateral deflections in building structures. Particularly in seismic zones, hollow columns show promise due to their low self-weight and high flexural rigidity, offering potential benefits such as decreased labour requirements, shorter construction time and reduced formwork without compromising construction quality [¹]. The structural effectiveness of CFDST (Concrete-Filled Double Steel Tubular) columns, which incorporate an inner steel tube, outperforms that of traditional CFST (Concrete-Filled Steel Tubular) columns [², ³]. Because of its superior strength and rigidity compared to traditional reinforced concrete or steel columns, concrete-filled tubes are frequently used in the construction of high-rise structures [⁴]. The concept of CFDST columns emerged as an innovative structural element consisting of two steel tubes arranged concentrically and filled with concrete in between. Incorporating an inner steel tube within a CFST column it enables a decrease in structural weight without compromising on bearing capacity. These columns offer several advantageous characteristics, such as remarkable fire resistance, strong local and global stability and enhanced ductility, attributed to the combination of concrete infill and steel skin in their construction [⁵]. In this type of column design, the materials synergize optimally; the steel tube enhances structural integrity by confining the concrete, while the concrete core reinforces the column’s load-bearing capability and mitigates the risk of localized buckling in the steel tube [⁶]. Local bucking or yielding failure is the most common cause of failure in Hybrid double-skinned composite steel columns. It has been discovered that using European codal provisions (Euro code) as a design strategy yields more consistent outcomes that are closer to experimental values [⁷]. For over two decades, scientists across the globe have dedicated themselves to understanding the dynamics of CFDST columns. With advancements in computer technology and the widespread availability of finite element (FE) software such as ANSYS and Abaqus, there has been a notable increase in thorough numerical analyses exploring the behaviour of CFDST columns. These investigations cover various aspects, including material properties, both local and global imperfections, as well as diverse loading and boundary conditions [⁸]. In recent years, many researchers have investigated double-skin composite columns under various materials and findings [⁹,¹⁰,¹¹,¹²]. CFDST columns consist of two steel tubes arranged concentrically, with a space filled with concrete between them, creating a concrete-filled double-skin steel tube (CFDST) column involves enclosing concrete within two steel tubes. These tubes can take on different cross-sectional shapes, including a square outer tube with a square inner tube, a circular outer tube with a square inner tube, or a square outer tube with a circular inner tube [¹³]. Moreover, designers have the option to make the internal and external cross-section shapes of the tubes identical [¹⁴]. Previous studies have demonstrated that the double-section column design is especially effective in providing extremely high levels of stiffness, ductility, and strength. Finding a way from concept to design is the challenge at hand, which calls for a lot of comparison testing on traditional systems [¹⁵]. Parametric analysis was utilized to explore how various factors influence the torsional capacity of CFDST (Concrete Filled Double-Skin Tubular) columns. These factors include the hollow ratio, yield strength, and thickness of the inner steel tube, as well as the strength of the concrete, yield strength, and steel ratio of the outer steel tube [¹⁶]. Accurate material behaviour specifications are essential for numerical modelling. While existing techniques adequately predict the behaviour of concrete cores confined within circular tubes, they may not apply to square composite columns. This discrepancy arises from variations in the concrete confinement process between the two shapes [¹⁷]. The addition of concrete within the inner steel tube significantly increases the lateral deformation capacity of double-skin tubular columns with a bigger inner steel tube to a level that is more than in companion double-skin tubular columns with a smaller hollow inner steel tube [¹⁸]. Steel can be used with smaller, lighter foundations due to its less weight and higher strength. The building frame can readily restrict the sway and lateral deflections attributable to the subsequent concrete construction. Due to the hollow column’s low self-weight and strong flexural rigidity, its application in seismic zones appears promising. It decreases labour needs, building time, and formwork requirements while maintaining construction quality [¹]. The experimental analysis demonstrated that the ultimate strength of the column was primarily influenced by the section hollow ratio and the strength of the concrete [¹⁹]. The model was employed to conduct finite element analysis (FEA) on the circular, round, octagonal, hexagonal, and rectangular segments of the CFST stub columns [²⁰]. By harnessing cutting-edge computer modelling and simulation technology, structural engineers have at their disposal robust tools to forecast the behavior of high-strength rectangular double-skin concrete-filled steel tubular (RDCFST) short columns. These columns are assembled using rectangular steel sections of different classifications [²¹].

2. METHODS AND MATERIALS

The methodology employed in this study involves Finite Element Analysis (FEA) utilizing the software Abaqus 6.14 3D Experience (2015) [²²], with a primary focus on validation. The process begins with finite element modelling (FEM), encompassing geometric and material definitions, along with the creation of model instances and incorporation of interaction modules. Subsequently, the model undergoes meshing to discretize the geometry, followed by the application of boundary conditions and loads representative of the real-world scenario. The analysis is then executed, and the results are interpreted. The experimental phase involves the casting of column specimens and subsequent testing under controlled monotonic load conditions. The results obtained from both FEA and experimental testing are thoroughly discussed, allowing for a comparative study to ascertain the accuracy and reliability of the FEM in predicting the behaviour of the element under consideration.

3. NUMERICAL INVESTIGATION USING ABAQUS

FEM-based software, Abaqus 6.14 3D Experience (2015), is employed in the analysis of columns. It addresses certain issues, such as altered boundary conditions, dynamic non-linear problems and multi-purpose issues. In the current numerical investigation, a DSSCC tube with square cores is designed by parts, assembled, and provided surface-surface together interaction and mesh.

3.1. Material properties

The characteristic properties of Fe250, Fe350 and Fe415 are shown in Table 1. Compressive behaviour and tensile behaviour of concrete of grades M20, M25, M30, M35, M40 and M45 are shown in Figure 1(a) and 1(b) respectively.

Table 1
Material properties of steel.

Fe250		Fe350		Fe415
YOUNG’S MODULUS (MPa)	POISSON’S RATIO	YOUNG’S MODULUS (MPa)	POISSON’S RATIO	YOUNG’S MODULUS (MPa)	POISSON’S RATIO
199817	0.30	200000	0.31	200000	0.33
YIELD STRESS (MPa)	PLASTIC STRAIN	YIELD STRESS (MPa)	PLASTIC STRAIN	YIELD STRESS (MPa)	PLASTIC STRAIN
200	0	270	0	270	0
246	0.0235	300	0.0025	300	0.0025
264	0.024	330	0.0075	330	0.0075
–	–	350	0.0125	350	0.0125
–	–	–	–	370	0.0175
–	–	–	–	400	0.04

Figure 1
Behaviour of concrete (a) compressive, (b) tensile.

3.2. Numerical modelling

The numerical modelling of DSSCC tubes with in-built concrete-infilled square cores provides valuable insights into their structural behaviour under monotonic loading conditions. The analysis results can be used for design improvements, optimize material usage, and ensure the safety and reliability of the advanced structural elements in construction applications. The numerical model of the DSSCC tube with square cores and its concrete-infilled model are shown in Figure 2. The description of specimens is mentioned in Table 2.

Figure 2
Numerical model of DSSCC tube (a) built-in square cores, (b) concrete-infilled built-in square cores.

Table 2
Details of the numerical model.

		OUTER TUBE			INNER TUBE
COLUMN REFERENCE	fy (MPa)	S (mm)	te (mm)	S/te	S (mm)	ti (mm)	S/ti	L (mm)	fc’ (MPa)	S/S	ti/te
NM1	250	146	6	24.3	48	3	16	500	20	0.33	0.5
NM2		146	6	24.3	48	3	16	500	25	0.33	0.5
NM3		146	6	24.3	48	3	16	500	30	0.33	0.5
NM4		146	6	24.3	48	3	16	500	35	0.33	0.5
NM5		146	6	24.3	48	3	16	500	40	0.33	0.5
NM6		146	6	24.3	48	3	16	500	45	0.33	0.5
NM7	350	146	6	24.3	48	3	16	500	20	0.33	0.5
NM8		146	6	24.3	48	3	16	500	25	0.33	0.5
NM9		146	6	24.3	48	3	16	500	30	0.33	0.5
NM10		146	6	24.3	48	3	16	500	35	0.33	0.5
NM11		146	6	24.3	48	3	16	500	40	0.33	0.5
NM12		146	6	24.3	48	3	16	500	45	0.33	0.5
NM13	415	146	6	24.3	48	3	16	500	20	0.33	0.5
NM14		146	6	24.3	48	3	16	500	25	0.33	0.5
NM15		146	6	24.3	48	3	16	500	30	0.33	0.5
NM16		146	6	24.3	48	3	16	500	35	0.33	0.5
NM17		146	6	24.3	48	3	16	500	40	0.33	0.5
NM18		146	6	24.3	48	3	16	500	45	0.33	0.5

3.3. Load versus displacement relationship

The load versus displacement relationship is a crucial aspect of structural analysis, providing insight into the performance and behaviour of structural elements under load. This relationship helps in understanding the stiffness, strength, and ductility of materials and structural systems. The load versus displacement behaviour is shown in Figure 3, and the results of the numerical investigation are given in Table 3.

Figure 3
Load versus displacement (a) Fe250-Horizontal, (b) Fe250-Vertical, (c) Fe350-Horizontal, (d) Fe350-Vertical, (e) Fe415-Horizontal, (f) Fe415-Vertical.

Table 3
Results of numerical investigation.

COLUMN REFERENCE	GRADE OF STEEL	GRADE OF CONCRETE	ULTIMATE LOAD (kN)	VERTICAL DISPLACEMENT (mm)	HORIZONTAL DISPLACEMENT (mm)
NM1	Fe250	M20	2103	1.20	0.78
NM2	Fe250	M25	2180	1.23	0.79
NM3	Fe250	M30	2243	1.25	0.84
NM4	Fe250	M35	2387	1.31	0.89
NM5	Fe250	M40	2550	1.42	1.02
NM6	Fe250	M45	2632	1.44	1.07
NM7	Fe350	M20	2241	1.21	0.79
NM8	Fe350	M25	2324	1.24	0.81
NM9	Fe350	M30	2390	1.27	0.83
NM10	Fe350	M35	2504	1.33	0.93
NM11	Fe350	M40	2679	1.40	1.04
NM12	Fe350	M45	2775	1.47	1.05
NM13	Fe415	M20	2301	1.18	0.81
NM14	Fe415	M25	2372	1.22	0.83
NM15	Fe415	M30	2462	1.24	0.86
NM16	Fe415	M35	2565	1.32	0.92
NM17	Fe415	M40	2732	1.43	1.03
NM18	Fe415	M45	2834	1.46	1.06

The ultimate load capacity and displacement of DSSCC steel tubes of grade Fe250, Fe350, and Fe415 with steel square cores and concrete-infill of concrete grade M20, M25, M30, M35, M40 and M45 are analysed using FEA. The results showed that the increase in ultimate load capacity of DSSCC, when the grade of concrete increased from M20 to M45, is 25, 24 and 23% for steel tubes of grade Fe250, Fe350, and Fe415, respectively. The change in vertical displacements of DSSCC for steel tubes of higher grades compared to that of lower ones is identified to be marginal. Similar observations were made with horizontal displacements also. The use of steel tubes of higher grades enhanced the ultimate load capacity (ranging from 6.61 to 9.76% for Fe350 and Fe415 grade steel tubes compared to that of Fe250) but also minimized deformations thus improving structural integrity and durability. The current study highlighted the improved compressive behaviour of DSSCC columns of higher steel and concrete grades, emphasizing the importance of material selection in optimizing structural performance. Further studies will show that increased concrete’s compressive strengths can improve its load-bearing capability. An experimental investigation was subsequently conducted to evaluate the performance of steel, which is available in the market, in structural applications.

3.4. Load versus strain relationship

The load versus strain relationship of columns provides insights into the deformation characteristics of selected material under the given loading conditions. This relationship is critical for understanding the material’s elastic and plastic behaviour, ultimate strength, and failure mechanisms. It is shown in Figure 4.

Figure 4
Load versus Strain behavior (a) Fe250-Steel, (b) Fe250-Concrete, (c) Fe350-Steel, (d) Fe350-Concrete, (e) Fe415-Steel, (f) Fe415-Concrete.

The ultimate load capacity and strain characteristics of DSSCC steel tubes of grade Fe250, Fe350, and Fe415 with steel square cores and concrete-infill of concrete grade M20, M25, M30, M35, M40 and M45 are evaluated using FEA. The strain on the steel tube of DSSCC varied from 0.00132 to 0.00263, 0.00112 to 0.002579, and 0.001166 mm to 0.003536 mm for Fe250, Fe350, and Fe415 grades. Similarly, the strain on concrete of DSSCC varied from 0.00085 to 0.001731, 0.00062 to 0.00242, and 0.00083 to 0.00222 for Fe250, Fe350, and Fe415 grade steel tubes. Higher grades of steel resulted in higher ultimate loads, with Fe415 achieving the highest loads for all concrete grades. Additionally, higher-grade steel generally exhibited lower strains, suggesting better performance. However, the strain on concrete varied and did not consistently decrease with higher steel grades, indicating a complex interaction between steel and concrete grades. Using higher grades of steel, such as Fe350 and Fe415, not only increases the ultimate load capacity but also influences the strain behaviour of both steel and concrete, highlighting the importance of material selection in structural design.

4. EXPERIMENTAL INVESTIGATION

A total of three DSSCCs with different grades of concrete were selected for this experimental study and analysed (ACI 318, 2002; ENV 1994-1-1, 2004; AISC, 2010) [²³,²⁴,²⁵]. The experimental program includes testing three specimens of outer and inner steel tubes of square shape, with in-filled square steel cores cast with concrete of three grades. The dimensions of outer and inner square steel tubes, along with square steel cores, are kept constant for the three specimens.

4.1. Material properties of specimen

Hot rolled square steel tubes serve as both the outer and inner tubes of DSSCC. The steel tubes (of average yield strength of 250 N/mm²) consist of a gap in between, and this inner area comprises square mild-steel cores. The infill material for all test specimens comprises standard-strength concrete. Concrete cubes with dimensions of 150 mm × 150 mm × 150 mm are cast, compacted, cured, and then examined in accordance with the procedures specified in IS:516-1959 (2006) [²⁶]. The average compressive strength of the concrete specimens under consideration for the experimental study of DSSCC, after 28 days of curing, is found to be 26.07, 32.89, and 40.29 N/mm².

4.2. Preparation of the specimen

The standard steel tubes arrived at their customary length before being cut and machined along their edges to meet the required 0.5 meters in length. Following this, one end of each tube was affixed to a 3 mm thick base plate through welding, leaving the other end open for concreting. This task should be carried out properly in accordance with the design, and it is a time-consuming process during fabrication. For testing, strain gauges were applied to both the inner and outer surfaces of the steel tubes prior to their attachment to the baseplate. This step was crucial for accurately measuring strain in both the steel and concrete components during testing procedures. Before the concrete was poured into the columns, both steel tubes were securely welded to the baseplate to ensure consistent spacing between the inner and outer tubes. Figure 5(a,b) depicts the fabricated specimen, and Figure 5(c) exhibits the finished specimen infused with concrete. Following preparation, these specimens were allowed to rest at room temperature for 24 hours before undergoing a 28-day curing process. Before being placed in the loading frame, the ends of the specimens were suitably prepared. To promote uniform distribution of load across the cross-section during loading, a 3 mm-thick plate was welded onto the top side of the column. Furthermore, the specimen was coated with brown paint to aid in the visual assessment of deformations during loading.

Figure 5
Fabricated specimen (a) side view, (b) top view, (c) concrete-infilled specimen, (d) experimental testing setup.

4.2.1. Test loading, instrumentation and setup

The evaluations of specimens subjected to axial compressive force are examined at SRM University in Chennai. The specimens were securely placed within a 3MN loading frame featuring fixed-fixed boundary conditions. Specialized endplates, hardened adequately, were utilized to distribute the load to the specimens while preventing deformation effectively. The experimental setup, as illustrated in Figure 5(d), was carefully configured. Dial indicators were utilized to gauge both the axial deformation and lateral displacement of the specimens when subjected to stress. To evaluate lateral deformation, two indicators were strategically positioned: one at the loading end and another at the midpoint. Furthermore, strain gauges were carefully installed to gauge strain values, ensuring consistent loading throughout the cross-section. The upper end of the specimen experienced direct application of axial compressive force. Loading continued until the occurrence of overall buckling, marked by a sudden decrease in load accompanied by a sharp rise in lateral deflection, indicating the yield of the cross-section. Both strain gauge measurements and LVDT measurements of deformations were conducted manually for accuracy and precision.

5. RESULTS AND DISCUSSIONS

The results and discussions show a detailed analysis of the experimental findings with practical implications. By meticulously interpreting the data, discussing failure modes, and comparing various specimens, this section provides valuable insights into the behaviour and performance of columns for further design practices.

5.1. Experimental results

The experimental results show the relationship between load and displacement. The critical points where significant changes occur are identified and analyzed to understand the material’s performance and structural integrity.

5.1.1. Load variation with displacement

The experimental results highlight the correlation between load and displacement in structural specimens. Key points of significant change with ultimate load capacities and displacement are identified and analyzed. These observations provide valuable insights into the material and structural behaviour under load. The experimental results are given in Table 4 and graphically represented in Figure 6.

Table 4
Results of experimental investigation.

COLUMN REFERENCE	GRADE OF STEEL	COMPRESSIVE STRENGTH OF CONCRETE (MPa)	ULTIMATE LOAD (kN)	VERTICAL DISPLACEMENT (mm)	HORIZONTAL DISPLACEMENT (mm)
ES1	Fe250	26.07	2090	0.98	0.68
ES2	Fe250	32.89	2350	1.39	1.29
ES3	Fe250	40.29	2600	1.81	1.29

Figure 6
Load versus vertical and horizontal displacement of DSSCC (a,d) M20, (b,e) M30, (c,f) M40.

Load versus vertical displacement data of M20 grade of concrete was noted. From the results, at 0.06 mm vertical displacement, the load is 100 kN. At 0.58 mm vertical displacement, the load increases to 1800 kN. There is a significant increase in load between 0.06 mm and 0.58 mm of vertical displacement. As the vertical displacement further increases to 0.62 mm, the load rises to 2000 kN. At 0.72 mm of displacement, the load reaches 2090 kN. There’s a noticeable increase in the load after 0.62 mm, but it plateaus after 0.72 mm. The load remains relatively constant, oscillating around 2000 kN, up to 0.98 mm of vertical displacement. The load versus vertical displacement curve shows that the structural system exhibits both linear and nonlinear behaviour. The initial linear segment, seen from 0.06 mm to 0.58 mm, could be attributed to the elastic deformation of the steel tubes and cores. After this point, nonlinear behaviour is observed, likely due to plastic deformation, material yielding, or buckling of components within the structure. The load versus horizontal displacement data of M20 grade concrete was obtained from the experimental findings. Initially, the load and horizontal displacement are directly proportional, indicating elastic deformation. However, as the load increases, plastic deformation begins to occur. The data shows that as the load continues to increase, the displacement increases at a higher rate, which is indicative of material yielding and deformation of the inner core. A significant increase in the load is observed at a load value of 1800 kN and a displacement of 0.3 mm. This is likely the yield point, where the material starts to undergo plastic deformation. Beyond this point, the increase in load results in a less substantial increase in displacement, indicating the onset of post-yield behaviour. The load-displacement curve becomes less steep after the yield point, showing that the inner core has undergone plastic deformation. The displacement continues to increase with increasing load, indicating the ductile behaviour of the steel core. The highest load observed in this study is 2090 kN, which occurs at a displacement of 0.45 mm. This represents the maximum load the structure can bear before a potential structural failure.

The early section of the load versus vertical displacement graph for M30 grade concrete demonstrates a nonlinear correlation between the applied load and the vertical displacement. Initially, the specimens showed a linear load-displacement response, indicating elastic behaviour. Beyond a certain point, the load-displacement curve started to exhibit nonlinearity, marking the onset of plastic deformation. At higher loads, the displacement increased significantly, indicating deformation and potential instability. The load versus horizontal displacement data of the M30 grade of concrete was obtained from the experimental findings. Up to a load of 900 kN, the horizontal displacement is minimal, indicating the elastic range of deformation. Beyond 900 kN, the displacement increases progressively and shows nonlinear behaviour. Notably, the most substantial horizontal displacement is observed beyond 1800 kN, signifying the onset of significant structural distress. At the highest load of 2350 kN, the composite column ultimately failed.

The load versus vertical displacement data of M40 grade concrete was obtained from the experimental findings. The data shows that as the load increases, the displacement also increases, which is typical behaviour for such structures. The relationship between load and displacement is non-linear, indicating that the composite system undergoes gradual deformation before reaching a critical point. The initial linear portion of the graph shows that the composite system can withstand small to moderate loads without significant deformation. As the load approaches the ultimate limit, the vertical displacement increases more rapidly, indicating that the structure is approaching its capacity. The load versus horizontal displacement data of M40 grade concrete was obtained from the experimental findings. The initial load ranges from 0 to 700 kN and does not result in any noticeable horizontal displacement. This indicates the tubes’ ability to support the load without significant deformation. As the applied load increases beyond 700 kN, the horizontal displacement remains minimal, indicating the structural integrity and stiffness of the square steel tubes and the cold-formed steel core. The relationship between load and displacement appears to be relatively linear during this range. A non-linear response is observed as the load exceeds 1300 kN, with a more pronounced increase in horizontal displacement compared to the previous load increments, indicating that the structure may undergo plastic deformation or other structural changes under extreme loads. In this finding, the peak load obtained is 2600 kN.

5.1.2. Load and strain relationship

The experimental results show the relationship between load and strain, highlighting the material’s response to applied forces. The points of transition from elastic to plastic behaviour are critical for understanding the material’s limits and for designing structures that can withstand anticipated loads without failing. These findings provide essential insights for improving material selection and structural design.

5.1.2.1. Strain on concrete

As per the test results, the load versus strain behaviour of concrete (as shown in Figure 7(a)) of DSSCC (of concrete M20 grade), the relationship between applied load and strain appears to be linear initially at lower load values (up to approximately 800 kN). This indicates that the material exhibits elastic behaviour within this specific range, implying that it reverts to its initial form once the applied force is lifted. The linear relationship can be described by Hooke’s law, where strain is directly proportional to stress (load per unit area). Beyond the 800 kN load threshold, the data shows a clear deviation from linearity. The strain increases more rapidly for each additional unit of load. This behaviour is indicative of the material undergoing plastic deformation or yielding. In the context of structural design, this non-linear region represents a limit beyond which the material may not return to its original shape once the load is removed, potentially causing permanent deformation or failure. The data also reveals that the composite column eventually reaches an ultimate load of around 2090 kN, at which point the strain experiences a steep increase. This abrupt increase may signify the point at which the material is on the verge of structural failure. As per the results of load versus strain behaviour of DSSCC of concrete of M30 grade, the relationship between applied load and strain on the concrete appears to be non-linear. Initially, for lower loads, the strain increases slowly, but as the load continues to increase, the strain increases at a faster rate. The material exhibits non-linear behaviour, which is common in many structural materials. As per the results of load versus strain behaviour of DSSCC of concrete of M40 grade, the relationship between load and strain appears to be linear at the initial stages of loading (up to approximately 0.0002 strain), indicating that the structure behaves elastically. This is a favourable property in terms of structural design, as it shows that the columns can handle loads without undergoing significant deformation. A yield point is observed around 0.0002 strain beyond the linear region. This signifies the onset of plastic deformation in the material, where it can no longer return to its original shape after unloading. The composite columns show plastic behaviour, which may provide ductility in a real-world structural application. The columns continue to withstand increased loads up to 0.0016 strain, indicating significant load-bearing capacity. This property of material is crucial in structural engineering applications, as it shows that columns can support substantial loads before experiencing failure.

Figure 7
Load versus strain (a) concrete, (b) steel, Failure of DSSCC specimens (c) M20, (d) M30, (e) M40.

5.1.2.2. Strain on steel

The load versus strain on the steel of DSSCC of M20 grade concrete was noted. At the initial levels of loading, the data shows that the strain increases gradually as the load increases. This indicates the linear elastic behaviour of the steel and composite column within this load range. As the load continues to increase, at a certain point, the strain starts to increase at a faster rate. This indicates that the material is shifting from linear elastic to non-linear region. Beyond a certain load value, it was observed that the strain increases significantly with a relatively small increase in load. This indicates the stage of plastic deformation, where the material will not return to its original shape. As per the test results of load versus strain on the steel of DSSCC of M30 grade of concrete, the strain on the steel remains relatively low and gradually increases at the lower end of the load spectrum (0 to 0.0001 strain). This indicates that at lower loads, the material remains in its elastic region; it deforms elastically and returns to its original shape once the load is removed. This behaviour is typical of materials with linear elastic properties. As the load exceeds a certain level and continues to increase, the strain on steel starts to exhibit non-linear behaviour. This indicates that since the material is nearing its yield point, a stage is reached at which it begins to undergo plastic deformation. This type of deformation is permanent, and the material does not regain its initial shape once the external force is withdrawn. This change in behaviour may indicate that the material is undergoing a phase transition, possibly from elastic to plastic deformation. As per the test results of load versus strain on steel of DSSCC of M40 grade of concrete, in the initial part of the curve, up to approximately 0.0014 strain, the response is linear. This indicates that the column is in its elastic range, where the deformation is directly proportional to the applied load. This is a desirable behaviour for structural components as it allows for deformation without permanent damage. At around 0.0014 strain, there is a noticeable deviation from the linear portion of the curve. This point represents the material’s yield strength. In this instance, the cold-formed steel cores and Fe250 steel tubes are probably at their yield point. Beyond this point, the column begins to undergo plastic deformation. After reaching the yield point, the column undergoes plastic deformation, where it continues to carry an increasing load while the strain increases rapidly. This indicates that the column is undergoing significant permanent deformation. The curve reaches a peak load at approximately 0.00145 strain. Determining the maximum load a column can bear before its load-bearing capacity diminishes significantly is crucial for evaluating the structural soundness of a composite column. Once the peak load is attained, the column’s ability to carry loads begins to decrease due to factors like local buckling, material softening and stress redistribution within the column. This diminishing capacity culminates in the ultimate load capacity, which marks the threshold beyond which the column cannot sustain any further load and often results in catastrophic failure. Experimental data depicting the strain on steel are visually represented in Figure 7(b) for reference.

The failure type observed in all DSSCC specimens of M20, M30 and M40 grade of concrete is shown in Figure 7(c), (d) and (e), respectively. All steel tubes are experiencing local buckling only. DSSCC of M20 grade concrete alone has undergone failure at joints (welding), in addition to local buckling.

5.2. Stiffness, ductility ratio and energy absorption capacity

The Equivalent Elastic-Perfectly Plastic (EEEP) curve is a representation used in structural engineering to approximate the behaviour of materials beyond their elastic limit. It helps in understanding the load-deformation behaviour of structural components like columns, beams, and frames which is facilitated through analysis. Composite columns, often comprised of steel and concrete, require the construction of an EEEP curve tailored to the specific material properties and structural behaviour of the composite system. Stiffness, ductility ratio and energy absorption capacity were calculated using this method, and the formulae given in equations 1, 2 and 3 are based on the earlier research [²⁷, ²⁸].

$$\text{Stiffness}=0.4{\text{V}}_{\max }{\text{/Δ}}_{0.4\text{vmax}}$$ (1)

$$\text{Ductility ratio}={\text{Δ}}_{\text{peak}}{\text{/Δ}}_{\text{e}}$$ (2)

$$\text{Energy absorption capacity = Energy under the curve to failure}$$ (3)

whereas,

• 0.4V_max – 40% of the peak load

• Δ0.4_vmax – Measured displacement

• Δ_peak – Measured displacement at peak load

• Δ_e – Yield displacement

EEEP curve was plotted as shown in Figure 8(a), (b) and (c) for DSSCC of M20, M30 and M40 grade concrete, respectively, and the results of the EEEP curve analysis were presented in Figure 9(a), (b) and (c) for DSSCC of M20, M30 and M40 grade concrete respectively.

Figure 8
EEEP curve for experimental results (a) M20, (b) M30, (c) M40.

Figure 9
EEEP curve results from (a) stiffness, (b) ductility ratio, and (c) energy absorption capacity.

5.2.1. Stiffness

Stiffness is a crucial parameter in the design and analysis of structural elements. Composite columns must be designed to withstand various types of loads while maintaining structural integrity and stability. The stiffness of the column affects its behaviour under axial loads, bending moments, and lateral forces such as wind or seismic loads. Double-skinned composite columns with square cores typically exhibit enhanced stiffness compared to single-material columns. The combination of different materials with distinct mechanical properties allows for the optimization of stiffness and load-bearing capacity. In this context, the stiffness of the composite column is influenced not only by the properties of concrete but also by an infilled core of steel in the column. The elastic modulus reflects the material’s ability to deform elastically under stress. In composite columns, it determines the extent of deformation and the distribution of loads among different materials. Higher elastic modulus values indicate greater resistance to deformation and higher stiffness. However, it’s essential to consider not only the elastic modulus but also other factors such as strength, durability, and constructability in structural design. Double-skinned composite columns, combining concrete and steel, provide enhanced stiffness and load-bearing capacity, effectively resisting various loads, with optimal design requiring consideration of strength, durability, and constructability.

5.2.2. Ductility ratio

When designing structural components, ductility ratio is an essential factor, particularly in composite columns where the behaviour is complicated because of the interplay of many materials. The ductility ratio is an important factor in assessing the structural integrity and resilience of double-skinned square composite columns with square cores under different loading scenarios. The ratio of ultimate displacement to yield displacement is known as the column’s ductility ratio. It essentially indicates how much deformation a column can undergo before failure, which is crucial for ensuring that the structure can withstand severe loading conditions without collapse. Higher ductility ratios generally imply better structural performance and resilience against earthquakes and other dynamic loads. From the current test data, it is evident that ES3 has the highest ductility ratio, followed by ES1 and ES2. This implies that the column made of M40 grade concrete (ES3) exhibits the highest level of ductility among the three grades considered. In contrast, the M30 grade (ES1) ranks second, and the M20 grade (ES2) demonstrates the least ductility. The column with M40 grade concrete shows the highest ductility ratio, indicating superior deformation capacity before failure. This grade of concrete is suitable for applications where high structural resilience and ductility are required, such as in earthquake-prone regions or structures subjected to dynamic loads. While not as ductile as M40 grade, M30 grade concrete still offers a considerable level of ductility, making it suitable for a wide range of structural applications. It strikes a balance between strength and ductility, providing adequate structural performance in various loading scenarios. The column constructed with M20 grade concrete exhibits the lowest ductility ratio among the three grades considered. While it may suffice for structures with less stringent requirements, such as low-rise buildings or non-seismic regions, it may not be suitable for applications where higher ductility and resilience are essential. Several factors influence the ductility ratio of composite columns, including material properties, cross-sectional geometry, reinforcement detailing, and loading conditions. The grade of concrete significantly affects the ductility ratio, with higher-grade concretes generally offering better ductility and resilience against deformation.

5.2.3. Energy absorption capacity

The ability of a material or structure to dissipate energy in response to loads or external forces is known as energy absorption capacity. In structural engineering, it is an essential parameter, especially for evaluating the robustness and suitability of columns and beams under dynamic loads such as collisions or earthquakes. The concrete’s compressive strength has an impact on a composite column’s ability to absorb energy. Better energy absorption is anticipated from higher-grade concrete columns (M40 is higher in the current study) because of its superior structural qualities. Comparing the values, ES2 and ES3 have a substantially greater capacity for energy absorption, about 3.4 to 3.6 times greater, than ES1 indicating that the column functions better in such circumstances. Concrete classes like M20, M30, and M40 represent the compressive strength of the concrete mix. The concrete gets stronger and more resilient as the grade rises from M20 to M40, which may have a favourable effect on the composite column’s ability to absorb energy. Higher-grade concrete usually absorbs more energy since it can tolerate higher strains and deformations before failing. The column’s geometric characteristics, such as its aspect ratio and cross-sectional dimensions, are also important in determining how much energy it can absorb. Owing to the increased material volume available to dissipate energy, columns with bigger cross-sectional areas often demonstrate higher energy absorption. With higher energy absorption values, columns ES2 and ES3 clearly surpass ES1, which has lesser energy absorption according to the data interpretation. This demonstrates that, in comparison to M20 grade columns, columns constructed with M30 and M40 grade concrete have higher energy absorption capacity. In practice, better structural performance and robustness under extreme loading situations, such as earthquakes or impact events, are implied by composite columns’ higher energy absorption capacity. Engineers and designers can use this knowledge to improve the way structures are designed and increase their resistance to unanticipated forces.

6. RESULTS OF VALIDATION

The validation results involve comparing experimental data with numerical simulations conducted using Abaqus 6.14 3D Experience (2015) to assess the accuracy and reliability of the simulation results for columns.

6.1. Load versus displacement relationship

Any difference between the experimental and numerical curves is analyzed to identify potential sources of error, such as inaccuracies in the material properties, boundary conditions, or modelling assumptions. The numerical results are given in Table 5, graphically represented in Figure 10 and are compared with the experimental results for validation.

Table 5
Results of numerical investigation.

COLUMN REFERENCE	GRADE OF STEEL	GRADE OF CONCRETE	ULTIMATE LOAD (kN)	VERTICAL DISPLACEMENT (mm)	HORIZONTAL DISPLACEMENT (mm)
NS1	Fe250	26.07	2119	1.22	0.78
NS2	Fe250	32.89	2286	1.29	0.87
NS3	Fe250	40.29	2554	1.43	1.04

Figure 10
Numerical results (a) load versus horizontal displacement, (b) load versus vertical displacement.

The numerical results show that using higher-grade concrete significantly enhances the load-bearing capacity of reinforced concrete columns. However, the relationship between concrete grade and strains in steel and concrete shows some complexities. While both steel and concrete strains generally increase with the concrete grade, the variations in steel strain between NS2 and NS3 indicate that other factors, such as the distribution of stresses, confinement effects, and interaction between materials, might play a role. These findings underscore the importance of considering both the grade of materials and the structural interactions when designing steel-concrete composite structures. High-grade concrete improves performance but requires careful analysis of strain behaviour to ensure the overall integrity and safety of the structure.

6.2. Load versus strain relationship

The validation results for the load versus strain relationship involve comparing experimental data with numerical simulations to assess the accuracy. The numerical results of load versus strain are given in Figure 11.

Figure 11
Load versus strain (a) concrete, (b) steel.

The analysis demonstrates the influence of concrete grade on the structural behaviour of columns under vertical loading conditions. As expected, higher-grade concrete DSSCC exhibits increased ultimate load capacity and greater displacement capability, indicating both enhanced strength and flexibility. These results can be valuable for optimizing structural designs and selecting appropriate concrete grades based on specific requirements.

6.3. Comparison of results

By observing experimental results, it is found that the maximum load attained by DSSCC of M20 grade concrete is 2090 kN, the difference being 1.37% only from numerical results (2119 kN). Similarly, for DSSCC of M30 and M40 grade concrete, the difference is found to be 2.72 and 1.77%, respectively. The applied load P is depicted together with the deflections of the specimens at the centre in Figure 6. With the applied force P up to the yield point, the vertical and horizontal deflection of the composite column increased linearly, and the deformation of the vertical dimension of the column was more compared to the horizontal one while applying load. For every 100 kN increment in load, there was a change in vertical displacement. The robust outer tube (of D/t = 24.3) effectively managed the compression behaviour of the columns, ensuring that shear failure modes were arrested. Additionally, this thicker outer tube endowed the composite columns with a dominant characteristic. Conversely, the thinner outer tube (of D/t = 41.72) resulted in the manifestation of shear cracks within the concrete [¹]. This illustrates the effective composite action facilitated by the thicker outer tube, allowing the concrete and steel components to synergize effectively. Understanding the failure patterns of the inner steel tubes is essential in analysing the behaviour of double-skin columns. In such structures, the use of a narrower inner steel tube (of approximately D/t = 16) significantly compromises performance. Severe local buckling of the inner steel tube inhibits its yield performance, rendering it ineffective. Moreover, this level of local buckling jeopardizes the performance of the surrounding concrete shell. Consequently, both the inner steel tube and the shell concrete will be unable to function optimally in this scenario [¹]. Therefore, the robust inner steel tube must sufficiently satisfy the performance requirements for double skin columns and possesses the strength necessary to mitigate the adverse effects of premature local buckling [²⁹], the ultimate load for the specimen [of outer tube (120 mm × 3 mm), inner tube (32 mm × 3 mm) and height 360 mm] for concrete compressive strength 37.4 MPa is 1054 kN. While comparing the result with the square core, it shows a 55% increased strength as per experimental results. From the earlier study [³⁰], the ultimate load for the specimen [of the outer tube (114.3 mm × 6.11 mm), inner tube (60.3 mm × 2.52 mm)] for concrete compressive strength 42.87 MPa and steel-545 MPa] is 1681.98 kN. While comparing the result with the square core, it shows a 35.3% increased strength as per experimental results.

7. CONCLUSION

The load versus strain curve of the double-skinned square composite column with square cores exhibits typical behaviour for steel and composite structures, including an elastic region, yield point, plastic deformation, peak load, and post-peak behaviour. Fe250 steel tubes and cold-formed steel cores used in the column design appear to provide adequate load-carrying capacity. However, further analysis is required to determine the structural safety and potential improvements. The composite columns exhibit a linear elastic region, showing that they can withstand loads without undergoing significant deformation up to a certain point. The columns demonstrate plastic behaviour beyond the yield point, which may provide ductility in structural applications, allowing them to absorb energy during extreme loads. The ultimate load capacity of the columns is notable, making them suitable for applications where high load-bearing capacity is required. These findings make double-skinned square composite columns a promising structural solution, offering a combination of strength and ductility for various construction projects. The focal point of this study underscores the substantial influence of concrete compressive strength on the load-bearing capacity and structural integrity of composite columns. Through this research, a discernible correlation was unveiled between the characteristic compressive strength of concrete, ranging from 20 to 40 MPa, and the heightened load-bearing capacity and resilience of the structure. This shows that in a practical scenario, opting for concrete with higher compressive strength can offer advantages in bolstering the performance of such composite columns. Additionally, employing double-skinned columns featuring steel outer and inner tubes serves not only to delineate the geometry of the concrete column but also effectively mitigates cracks caused by tensile pressures. This dual function is essential in enhancing the durability and safety of structural elements. A comparative analysis shows that circular tube designs outperform square tube designs in terms of strength, with a notable increase of 55 and 35.3% respectively, demonstrating the benefits of composite action in structural applications. The comparison and validation of the experimental results with numerical simulations strengthen the reliability of the findings and the applicability of this innovative construction method. This holistic approach, combining experimentation and numerical modelling, contributes to a better understanding of the behaviour of DSSCC. In summary, the current research signifies the potential of DSSCC with square cores as a viable and advantageous construction technique. It underscores the importance of concrete compressive strength and the effectiveness of steel skins in improving load-bearing capacity and structural resilience. These findings can serve as a valuable reference for engineers and architects looking to optimize the design and construction of robust and efficient composite columns in various applications within the construction industry.

8. FUTURE STUDIES

• Studies can be conducted to investigate the behavior of composite columns under combined axial and lateral loads and compare the performance of high-strength concrete with standard concrete in terms of energy absorption, ductility, and axial load capacity which would provide insights into more efficient designs [³¹].

• Future research could focus on optimizing the composition and hybridization of fibers (such as steel and polypropylene) in concrete-filled steel tubular (CFST) columns to enhance structural performance, specifically focusing on the compressive strength and ductility under different loading conditions [³²].

• Future studies which involve validating FEA models with experimental results for cold-formed steel and composite columns can be done. This would focus on capturing failure modes, load-deflection characteristics, and identifying potential improvements in FEA methods [³³].

9. BIBLIOGRAPHY

• ^[1] HASTEMOGLU, H., ERKAN., “Behaviour of double skinned composite columns with concrete filled tubular columns”, Journal of Architectural Engineering Technology, v. 6, n. 2, pp. 194, 2017. doi: http://doi.org/10.4172/2168-9717.1000194.
  » https://doi.org/10.4172/2168-9717.1000194
• ^[2] AHMED, M., LIANG, Q.Q., PATEL, V.I., et al., “Nonlinear analysis of rectangular concrete-filled double steel tubular short columns incorporating local buckling”, Engineering Structures, v. 175, pp. 13–26, 2018. doi: http://doi.org/10.1016/j.engstruct.2018.08.032.
  » https://doi.org/10.1016/j.engstruct.2018.08.032
• ^[3] AHMED, M., LIANG, Q.Q., PATEL, V.I., et al., “Local-global interaction buckling of square high-strength concrete-filled double steel tubular slender beam-columns”, Thin-walled Structures, v. 143, pp. 106244, 2019. doi: http://doi.org/10.1016/j.tws.2019.106244.
  » https://doi.org/10.1016/j.tws.2019.106244
• ^[4] LIEW, J.Y.R., XIONG, D.X., “Effect of preload on the axial capacity of concrete-filled composite columns”, Journal of Constructional Steel Research, v. 65, n. 3, pp. 709–722, 2009. doi: http://doi.org/10.1016/j.jcsr.2008.03.023.
  » https://doi.org/10.1016/j.jcsr.2008.03.023
• ^[5] IBANEZ, C., PIQUER, A., HERNANDEZ-FIGUEIRIDO, D., et al., “Experimental analysis of concrete-filled double-skin tubular columns subjected to eccentric loads”, Eurosteel, v. 1, n. 2, pp. 2138–2146, 2017. doi: http://doi.org/10.1002/cepa.259.
  » https://doi.org/10.1002/cepa.259
• ^[6] LOPES, R.F.R., RODRIGUES, J.P.C., “Behaviour of restrained concrete-filled square double-skin and double-tube hollow columns in case of fire”, Engineering Structures, v. 216, pp. 110736, 2020. doi: http://doi.org/10.1016/j.engstruct.2020.110736.
  » https://doi.org/10.1016/j.engstruct.2020.110736
• ^[7] NADAGOWDA, P.R.P., BR, N., KUMAR, N.S., “Experimental study on hybrid double skinned steel tubular columns (SCC in Filled) subjected to monotonic loading”, SSRG International Journal of Civil Engineering, v. 5, n. 5, pp. 19–24, 2018. doi: http://doi.org/10.14445/23488352/IJCE-V5I5P104.
  » https://doi.org/10.14445/23488352/IJCE-V5I5P104
• ^[8] AYOUGH, P., SULONG, N.H.R., IBRAHIM, Z., et al., “Nonlinear analysis of square concrete-filled double-skin steel tubular columns under axial compression”, Engineering Structures, v. 216, pp. 110678, 2020. doi: http://doi.org/10.1016/j.engstruct.2020.110678.
  » https://doi.org/10.1016/j.engstruct.2020.110678
• ^[9] WANG, F.-C., ZHAO, H.-Y., “Experimental investigation on blast furnace slag aggregate concrete filled double-skin tubular (CFDST) stub columns under sustained loading”, Structures, v. 27, pp. 352–360, 2020. doi: http://doi.org/10.1016/j.istruc.2020.05.046.
  » https://doi.org/10.1016/j.istruc.2020.05.046
• ^[10] TZIAVOS, N.I., GKANTOU, M., THEOFANOUS, M., et al., “Behaviour of grout-filled double-skin tubular steel stub-columns: numerical modelling and design considerations”, Structures, v. 27, pp. 1623–1636, 2020. doi: http://doi.org/10.1016/j.istruc.2020.07.021.
  » https://doi.org/10.1016/j.istruc.2020.07.021
• ^[11] XIONG, Z., CAI, Q., LIU, F., et al., “Dynamic performance of RAC-filled double-skin tubular columns subjected to cyclic axial compression”, Construction & Building Materials, v. 248, pp. 118665, 2020. doi: http://doi.org/10.1016/j.conbuildmat.2020.118665.
  » https://doi.org/10.1016/j.conbuildmat.2020.118665
• ^[12] RAZA, A., EL OUNI, M.H., KHAN, Q.U.Z., et al., “Seismic behavior of Double-Skin tubular E-waste concrete columns transversely confined with steel and GFRP tubes”, Composite Structures, v. 282, pp. 115076, 2022. doi: http://doi.org/10.1016/j.compstruct.2021.115076.
  » https://doi.org/10.1016/j.compstruct.2021.115076
• ^[13] HASSANEIN, M., KHAROOB, O.F., GARDNER, L., “Behaviour and design of square concrete-filled double-skin tubular columns with inner circular tubes”, Engineering Structures, v. 100, pp. 410–424, 2015. doi: http://doi.org/10.1016/j.engstruct.2015.06.022.
  » https://doi.org/10.1016/j.engstruct.2015.06.022
• ^[14] TIWARY, A.K., “Experimental investigation into mild steel circular concrete-filled double skin steel tube columns”, Journal of Constructional Steel Research, v. 198, pp. 107527, 2022. doi: http://doi.org/10.1016/j.jcsr.2022.107527.
  » https://doi.org/10.1016/j.jcsr.2022.107527
• ^[15] EKMEKYAPAR, T., ALWAN, O.H., HASAN, H.G., et al., “Comparison of classical, double skin and double section CFST stub columns: experiments and design formulations”, Journal of Constructional Steel Research, v. 155, pp. 192–204, 2019. doi: http://doi.org/10.1016/j.jcsr.2018.12.025.
  » https://doi.org/10.1016/j.jcsr.2018.12.025
• ^[16] LU, G.-B., ZHOU, X.-H., WANG, Y.-H., et al., “Numerical investigation on circular concrete-filled double skin steel tube columns under torsion”, Structures, v. 37, pp. 17–31, 2022. doi: http://doi.org/10.1016/j.istruc.2021.12.069.
  » https://doi.org/10.1016/j.istruc.2021.12.069
• ^[17] AYOUGH, P., SULONG, N.H.R., IBRAHIM, Z., “Analysis and review of concrete-filled double-skin steel tubes under compression”, Thin-walled Structures, v. 148, pp. 106495, 2020. doi: http://doi.org/10.1016/j.tws.2019.106495.
  » https://doi.org/10.1016/j.tws.2019.106495
• ^[18] IDRIS, Y., OZBAKKALOGLU, T., “Behavior of square fiber reinforced polymer-high-strength concrete-steel double-skin tubular columns under combined axial compression and reversedcyclic lateral loading”, Engineering Structures, v. 118, pp. 307–319, 2016. doi: http://doi.org/10.1016/j.engstruct.2016.03.059.
  » https://doi.org/10.1016/j.engstruct.2016.03.059
• ^[19] GUO, Z., CHEN, Y., WANG, Y., et al., “Experimental study on square concrete-filled double skin steel tubular short columns”, Thin-walled Structures, v. 156, pp. 107017, 2020. doi: http://doi.org/10.1016/j.tws.2020.107017.
  » https://doi.org/10.1016/j.tws.2020.107017
• ^[20] DING, F., WANG, W., LU, D., et al., “Study on the behavior of concrete-filled square double-skin steel tubular stub columns under axial loading”, Structures, v. 23, n. Feb, pp. 665–676, 2020. doi: http://doi.org/10.1016/j.istruc.2019.12.008.
  » https://doi.org/10.1016/j.istruc.2019.12.008
• ^[21] RIZWAN, M., LIANG, Q.Q., HADI, M.N.S., “Fiber-based computational modeling of rectangular double-skin concrete-filled steel tubular short columns including local buckling”, Engineering Structures, v. 248, pp. 113268, 2021. doi: http://doi.org/10.1016/j.engstruct.2021.113268.
  » https://doi.org/10.1016/j.engstruct.2021.113268
• ^[22] ABAQUS, “Abaqus 6.14 analysis user’s guide”, http://www.3ds.com/products-services/simulia/products/abaqus/, accessed in December, 2015.
  » http://www.3ds.com/products-services/simulia/products/abaqus/
• ^[23] AMERICAN CONCRETE INSTITUTE, Building Code Requirements for Reinforced Concrete 2002, Farmington Hills, ACI, 2002.
• ^[24] BRITISH STANDARD INSTITUTION, ENV 1994-1-1 Eurocode 4: Design of Composite Steel and Concrete Structures—Part 1.1: General Rules and Rules for Buildings, London, UK, British Standard Institution, 2004.
• ^[25] AMERICAN INSTITUTE OF STEEL CONSTRUCTION, Load and Resistance Factor Design Specification for Structural Steel Buildings 2010, AISC, Chicago, 2010.
• ^[26] BUREAU OF INDIAN STANDARDS, IS 516-1959: Method of Test for Strength of Concrete, New Delhi, IS, 2006.
• ^[27] HASSANEIN, M.F., PATEL, V.I., BOCK, M., “Behaviour and design of hexagonal concrete-filled steel tubular short columns under axial compression”, Engineering Structures, v. 153, pp. 732–748, Dec. 2017. doi: http://doi.org/10.1016/j.engstruct.2017.10.010.
  » https://doi.org/10.1016/j.engstruct.2017.10.010
• ^[28] EKMEKYAPAR, T., HASAN, H.G., “The influence of the inner steel tube on the compression behaviour of the concrete filled double skin steel tube (CFDST) columns”, Marine Structures, v. 66, pp. 197–212, 2019. doi: http://doi.org/10.1016/j.marstruc.2019.04.006.
  » https://doi.org/10.1016/j.marstruc.2019.04.006
• ^[29] SINHA, A., GUPTA, R., “Strain distribution in OSB and GWB in wood-frame shear walls”, Journal of Structural Engineering, v. 135, n. 6, pp. 666–675, Jun. 2009. doi: http://doi.org/10.1061/(ASCE)0733-9445(2009)135:6(666).
  » https://doi.org/10.1061/(ASCE)0733-9445(2009)135:6(666)
• ^[30] MERCIMEK, O., GHOROUBI, R., ANIL, O., et al., “Strength, ductility, and energy dissipation capacity of RC column strengthened with CFRP strip under axial load”, Mechanics Based Design of Structures and Machines, v. 51, n. 2, pp. 961–979, 2023. doi: http://doi.org/10.1080/15397734.2020.1860772.
  » https://doi.org/10.1080/15397734.2020.1860772
• ^[31] MARUTHAI, S.M., PALANISAMI, S., “Assessment of axial load carrying capacity of fully encased composite columns: comparative study with different codes”, Matéria (Rio de Janeiro), v. 29, n. 3, pp. e20240303, 2024. doi: http://doi.org/10.1590/1517-7076-rmat-2024-0303.
  » https://doi.org/10.1590/1517-7076-rmat-2024-0303
• ^[32] KARUPPANAN, K., GOVINDASAMY, V., “Behaviour of hybrid fibre reinforced concrete-filled steel tubular beams and columns”, Matéria (Rio de Janeiro), v. 25, n. 1, pp. e-12558, 2020. doi: http://doi.org/10.1590/s1517-707620200001.0883.
  » https://doi.org/10.1590/s1517-707620200001.0883
• ^[33] SANTAPHAN, S., BASKAR, R., “Experimental investigation on cold-formed steel structural frame made with lipped channel column and beam elements subjected to lateral loading”, Matéria (Rio de Janeiro), v. 29, n. 3, pp. e20240323, 2024. doi: http://doi.org/10.1590/1517-7076-rmat-2024-0323.
  » https://doi.org/10.1590/1517-7076-rmat-2024-0323

Publication Dates

• Publication in this collection
  27 Jan 2025
• Date of issue
  2025

History

• Received
  18 Oct 2024
• Accepted
  28 Nov 2024