The collapse behavior of cold-formed steel composite beam structure

The innovation of cold-formed steel increases along with the development of sustainable development principles that prioritize eco-friendly, practical, and economical materials. This material, being recyclable, practical, lightweight, and durable, also has the disadvantage of a thin profile thickness, so that it is prone to buckling. In this study, an experimental study was conducted related to cold-formed steel composite beams with concrete material that also contained cavities to reduce the weight of the beams. The specimens are concrete and have a double canal profile of C80.30.9 with a profile thickness of 0.75mm. The connections are made using SDS 12x20 screw connections with a spacing of 150, 200, and 300 mm. The beam span is subjected to concentrated load until the structure collapses. From this research, it was found that collapse from the flexural beam is caused by cracks due to bending moments or bending collapse. The depth of the crack that occurs beyond the cold-formed steel inside the concrete.


Introduction
Construction innovations have evolved from environmentally friendly materials to long-term, efficient materials such as steel.Steel is a popular construction material globally, reducing cement consumption and CO2 emissions, promoting green building and sustainable construction.Cold-formed steel (CFS) is gaining popularity in green structures due to its speedy construction, higher strength, dimensional stability, recycled material use, easy assembly, competitive construction, and cost savings [1].However, its use can lead to design problems like buckling strength, torsional rigidity, web crippling, ductility, connections, fire resistance, and corrosion [2].It is suitable for mass construction and can achieve substantial economies in the building industry [3].Its versatile nature allows for the forming of almost any section geometry, making it a popular choice in steel construction [4].It used as primary structural members in small-to-medium-rise buildings, contributing to sustainability and environmental friendliness by reducing environmental impact and increasing energy efficiency [5].
However, CFS has weaknesses due to its thin profile thickness, causing potential bending failure.To overcome this weaknesses, composite structures are being used, which combine two materials to withstand loads.Composite steel-concrete systems are cost-effective for multi-story buildings due to efficient use of steel and concrete, reduced disadvantages, and enhanced construction speed through reduced propping and formwork installation [6].
Studies on CFS behavior, including numerical and experimental studies on axial, flexural, and connection performance, have been conducted to improve design problems.Screw connections are commonly used due to the thinness of CFS sections [7][8][9].The study tested the capacity of cold-formed steel against tensile loads to predict failure mechanisms in connection sections.Canal type specimens with three screw configurations were tested.Results showed tilting and pull-out failure are common failure modes, increased maximum load by 6.67% and 10.81% for screws, and 300 percent greater average extension [9].In addition, the study shows that torsional restraint significantly impacts beam resistance, with maximum stress decreasing with increased rotational stiffness and the resistance of the beam increasing with rotational stiffness [8].Another research examined the static strengths and loaddeformation behavior of S900 and S960 steel tubular X-joints, revealing that current design provisions are insufficient for accurate predictions, emphasizing the need for improved design provisions [7].
Several studies conduct in experimental and numerical studies related to the investigation of coldformed steel composite structure [10][11][12][13].A study analyzing CFS composite beams reinforced with diagonal web rebars found that beams with fly ash concrete encased webs have twice the moment resisting capacity of plain beams [11].The study examined the impact of longitudinal stiffeners on the behavior of square cold-formed steel-concrete composite columns.Results showed that the axial compressive strength of both sections is nearly equal, with stiffened columns showing higher reduction factors [12].In addition, CFS section flexural performance in a composite beam system was also analyzed, demonstrating its feasibility in small and medium-sized buildings and lightweight industrial composite constructions, and eliminating the need for Hot Rolled Steel (HRS) sections [13].
On the other hand, shear connectors in cold-formed steel-composite beams are crucial for structural integrity to prevent crack formation and increase load-carrying capacity by 60% [14].The study found that a composite beam with five connectors is more ductile and has a ductility factor of 14, confirming the findings from ANSYS software.The composite model's properties were determined by testing the tensile strength of CFS and the compressive strength of concrete [15].
This research aims to study the collapse behavior of CFS composite beams with two profiles combined into one by modeling the presence of a cavity in the middle of the beam and variations in joint distance.The cavity aims to reduce the weight of the structure, which will be able to change the behavior of the structure because of the existing load.

Methods
In this study, the cylindrical test specimen used was 15 cm in diameter and 30 cm high, and the hollow composite concrete test specimen measured 15 x 20 x 72 cm.For cold-formed steel hollow composite beams, double C 80 x 30 x 9 x 0.75 mm by using screw connections on cold-formed steel profiles.The spacing between joints used in cold-formed steel profiles is 150 mm, 200 mm, and 300 mm.In addition, cold-formed steel test specimens for tensile tests have dimensions of 20 x 230 mm.
The CFS used an 80.30.9 channel profile with a thickness of 0.75 mm and was arranged into a double channel profile with screw connections, with the quality of G550 on the market as shown in Figure 1.

Tensile test specimens
To find out the mechanical properties of cold-formed steel, tensile tests were done.The 0.2% proof stresses that were found were between 323 and 795 MPa.The research involved the development of three cold-formed steel specimens.The results, which can be seen in Figure 4, gave detailed information on the specimens and the tests that were done based on ASTM E8/E8M-16a.According to this standard, for tensile tests of sheet-type specimens with thicknesses between 0.13 mm to 19 mm which is exactly 0.75 mm, the dimensions, including a gauge length, a total height, and a total length, will be determined.The result of failed tensile test specimens is shown in Figure 5.

Results and Discussions
The discussion focused on the tensile test results, compression test results, load-deflection curve, and collapse modes for each specimen.

Tensile stress and compressive stress
From the experimental data, it was found that the tensile strength of the CFS is 525.57MPa as shown in Table 1.Test results of CFS available on the Indonesian market indicate that the data obtained is in accordance with the rules used as a reference, i.e.SNI 7971:2017 which is refer to AS/NZS 4600:2005.According to this guidance, the minimum strength of CFS complying with AS 1397 with quality specification is 550 MPa.The average compressive strength of concrete is a measure of its durability and strength is 26 MPa, which is larger than the initial calculation related to normal concrete, which is shown in Table 1 and Figure 6, respectively.

Load vs deflection
Different deflection magnitudes are visible in the average maximum load that beams of the same value can withstand.Based on SNI 2847:2019, the deflection limit for simple beams is L/360, or 2 mm.The graph for this deflection is shown in Figure 7. CFS structure's screws can impact connection deflection, potentially failing during shear and bending, while other screws may remain elastic [16].Self-drilling screws are widely used in cold-formed steel structures, and their load-deflection response before failure needs to be thoroughly investigated [17].The study tested cold-formed steel's capacity against tensile loads to predict connection section failure [9].It found tilting and pull-out failure as the most common failure modes for all screw connections, using canal type specimens.
According to this research, the test specimens with a screw distance of 150mm exhibit the lowest average deflection value.Therefore, a smaller distance between the screw joints results in a reduced deflection value.The beams with a screw spacing of 300 mm exhibit the greatest deflection value, which is 140% higher than the deflection observed in beams with a screw spacing of 150 mm.
The beam with a 150mm screw distance was the test specimen that could handle the most weight of 2.6 tons.This is true even though there was not a big difference in permit deflection between the three variations, which shows that screw spacing does not significantly effect on load control.6 managing structural damage [18].The depth of the crack that occurs beyond the cold-formed steel inside the concrete, as can be seen in Figure 8.
The flexural test results show that composite beams with different screw spacings exhibit bending collapse or crack due to bending moment, indicating that screw distance doesn't affect beam crack pattern due to applied bending load.However, the entire beam's crack pattern is a bending crack of varying width, with the widest crack width occurring in beams with 300mm screw spacing.

Conclusions
The experimental results revealed that the smallest deflection occurred at 150 mm screw distance, while the largest occurred at 300 mm.The largest beam stiffness value was obtained at 150 mm screw distance.
The crack pattern was a bending crack of varying width.

Figure 6 .
Figure 6.Compressive stress of the concrete (a) Load vs deflection (b) Detail of the load at the deflection limit of 2 mm

Figure 7 .
Figure 7. Load vs deflection result for the composite beam at the deflection limit3.3.Collapse ModesFrom the experimental data, it was found that collapse from the flexural beam is caused by cracks due to bending moments or bending collapse.The assessment of fracture distribution plays a pivotal role in

Figure 8 .
Figure 8. Collapse mechanism for composite beam

Table 1 .
Tensile strength of the CFS