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To better understand the mechanical behavior of cold-formed steel (CFS) quadruple-limb built-up box-section (QBB) columns associated with flexural buckling, 12 axial compression tests with different slenderness ratios and screw spacing were conducted. The test results showed that, for a slender specimen (L=2,700 and 3,300 mm), when the screw spacing was reduced from 600 to 300 mm, the load-bearing capacity increased by about 6%–10%. When the screw spacing was reduced from 600 to 150 mm, the load-bearing capacity increased by about 14.6%–17.5%. Arranging end fastener groups can enhance the strength of the column, but it may be cumbersome and uneconomical. A finite-element model (FEM) was developed and calibrated against the test results. A series of parametric analyses was carried out to investigate the effects of the screw spacing and shear stiffness on the elastic buckling load and ultimate load-bearing capacity. Finally, a practical design method was proposed to evaluate the column's elastic buckling load and load-bearing capacity associated with flexural buckling. Compared with the simulation and test results, the proposed design method was accurate at predicting the load-bearing capacity of the specimens, with a relative difference of less than 6%. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive study on the flexural buckling behavior of Cold-formed steel (CFS) back-to-back built-up columns with Σ-section (CFS-BBC-Σ), where compression tests and numerical simulations were conducted to investigate the influence of slenderness ratio and fastener arrangements. The material characterization, shearing resistance behavior of the fastener, and imperfection measurements were also performed. The test and simulation results revealed that reducing the fastener spacing and increasing the shearing-resistance stiffness could enhance the load-bearing capacity. The elastic global buckling load (P cr) of CFS-BBC-Σ was determined using the energy method and verified against FEM simulations. The proposed energy method is an explicit expression that demonstrates high accuracy and ease of operation. Then, the P cr was introduced into AISI-S100 specifications as an alternative to the modified slenderness ratio method (MSR) for calculating the load-bearing capacity (P u) of CFS-BBC-Σ. Reliability analysis revealed that the proposed method provides more accurate and safe predictions for P u than the original MSR in AISI-S100 specifications. • Fifteen compression tests were conducted on cold-formed steel back-to-back built-up columns with different lengths and fastener arrangements. • Numerical models were developed and calibrated against the tests, followed by 84 parametric analyses to investigate the influence of fastener arrangements. • The theoretical global buckling load of the cold-formed steel back-to-back built-up columns was derived based on the energy method and verified against the numerical simulation. • A design method was proposed to evaluate the load-bearing capacity of the cold-formed steel back-to-back built-up columns, and it was assessed by reliability analyses. [ABSTRACT FROM AUTHOR]

This paper presents a study on the behavior of cold-formed steel built-up box-shape columns with longitudinal stiffeners (CFS-BCL) under axial compression. The specimens are fabricated using a pair of channel sections and lipped channel sections with longitudinal stiffeners, which are expected to enhance the local buckling resistance of the web. Sixteen specimens were prepared for compression tests, with varying lengths and screw arrangements. Prior to the tests, initial geometric imperfections of the specimens, material properties, and shearing-resistance behavior of the screw fasteners were measured. The results indicate that screw arrangements have a significant effect on simply supported specimens; reducing the screw spacing from 300 mm to 100 mm can increase load-bearing capacity by up to 11%. Finite element models were developed using ABAQUS and validated against test results. The parametric analysis revealed that screw arrangements had minimal impact on the local buckling capacity of specimens, with an increase in capacity of <8%. However, global buckling behavior was significantly affected by screw arrangements, resulting in an increase in capacity of up to 20% for slender columns. The test and analytical results were compared with the Direct Strength Method (DSM), and indicate that the DSM is conservative when the non-dimensional slenderness ratio λ g and λ l is higher than 1.2 and 1.4, respectively. The coefficients of the DSM were modified based on the test and simulation results, and its predictive accuracy was found to be highly increased. • Sixteen compression tests were conducted on cold-formed steel box-shaped built-up columns with longitudinal stiffeners. • Numerical models were developed and calibrated against the tests, and parametric analyses were conducted to investigate the influence of screw arrangements. • The Direct Strength Method was calibrated using both test and parametric analysis results. [ABSTRACT FROM AUTHOR]

Box-shape cold-formed steel built-up columns (CFS-QBC) are popular in multi-story CFS buildings. This paper conducted experimental research, numerical simulation, and parametric analyses to investigate its fire resistance. Twelve slender columns failing in global buckling were tested in a transient fire condition, where different initial load ratios and screw spacing were considered. The results showed that the specimen's critical temperature decreased with the screw spacing, and the screw spacing could significantly affect the deformation modes; Besides, the end fastening group (EFG) had little effect on the fire resistance. When the screw spacing was less than 300 mm, the effectiveness of the composite action of the built-up section at elevated temperatures was demonstrated by the consistency of the deformation compatibility among the components of the built-up section. Numerical simulations were then performed, including heat transfer and structural analysis, calibrated and verified against the tests. Based on the numerical model, the load ratio and screw spacing were parametrically analyzed to study their effects on the fire resistance of the columns. Finally, a simplified design method was proposed to evaluate the load-bearing capacity of CFS-QBC at elevated temperatures. • Fire-resistance tests of the built-up columns were conducted. • Different load ratios and screw arrangements were considered in the tests. • Numerical models were developed to reproduce the fire-resistance tests. • A simplified design method was proposed to evaluate the critical temperature of the built-up column. [ABSTRACT FROM AUTHOR]

• 144 tensile tests were conducted on post-fire thin-walled cold-formed steel, considering different steel grades, thicknesses, heating temperatures, and specimen locations. • The prediction formula of the post-fire elastic modulus, yield strength, and ultimate strength were developed based on the test results. • Numerical models of cold-formed steel columns were developed and verified against available tests, and parametric analyses were performed to investigate the post-fire behavior of the cold-formed steel columns by importing post-fire material properties. • The applicability of the direct strength method for the post-fire load-bearing capacity of cold-formed steel columns was assessed based on the parametric analysis results. This paper conducts a comprehensive investigation into the post-fire mechanical properties of thin-walled cold-formed steel (CFS) and evaluates the design method for the post-fire load-bearing capacity of CFS lipped channel section with web stiffener (Σ-shaped) columns. A total of 144 tensile tests were conducted to determine the mechanical properties of CFS after exposure to heating treatment. The study considered the effects of varied material strengths (Q235 and Q420), plate thicknesses (1.5 mm and 2.5 mm), extraction positions (flat plate, 90° corner, and 45° corner), as well as heating temperatures ranging from 20 °C to 800 °C. The research indicates that the heating temperature does not have a prominent influence on the elastic modulus of CFS, whereas its impact on yield strength is pronounced. After exposure to 800 °C, the yield strength of CFS can be reduced by 40 %, but the strain-hardening effect by cold-working still existed, which enhanced the yield strength by up to 20 %. This paper presents a predictive formula for the reduction factor of the mechanical properties. To assess the influence of post-fire mechanical properties on the load-bearing capacity of CFS columns, a finite element model of Σ-shaped columns was developed and calibrated. The simulation results indicate that the post-fire strain-hardening effect at corners would enhance the load-bearing capacity by less than 6 %. Finally, based on a parametric analysis, it was demonstrated that the Direct Strength Method (DSM) applied for distortional-local interaction buckling columns was still effective once the post-fire mechanical properties were incorporated when slenderness factor λ dl > 1.5. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the mechanical properties and thermal creep of Q355 cold-formed steel (CFS) through tests. The yield strength, ultimate strength, and elastic modulus of the steel at elevated temperatures were obtained through steady tensile tests. The thermal creep tests were conducted in the temperature range of 450 °C to 750 °C under four different stress ratios, and creep strain versus time curves were obtained. To assess the difference of creep behaviour between CFS and virgin steel, the test results were compared with that of the Q355 virgin steel sheet available in the literature. It was found that CFS was more susceptible to the thermal creep, and its creep strain rate was at least twice that of the virgin steel at the same condition. The test results were fitted into a creep model and incorporated into a numerical simulation to reproduce fire tests of the CFS columns conducted in transient-state temperature with different heating rates. The simulation results demonstrated that the models with considering of creep provided better predictions on fire behaviour of the column than those without accounting creep, and the creep could significantly reduce the fire-resistance of CFS columns. [Display omitted] • High-temperature steady-state tensile tests on Q355 cold-formed steel were carried out. • High temperature creep tests on Q355 cold-formed steel were conducted. • A creep model based on Fields & Fields creep model was proposed. • A finite element model was developed and calibrated by available test data. • Effect of creep strain on fire-resistance of cold-formed steel column was investigated. [ABSTRACT FROM AUTHOR]

Experimental and numerical studies on the fire resistance of cold-formed steel (CFS) columns with built-up box-shape sections were conducted in this paper. A series of coupon tensile tests were also performed before the fire resistance tests of the columns to obtain the stress–strain relationship of the steel plate from 20 °C to 800 °C. As a benchmark, the load-bearing capacity tests of the columns at ambient temperature were conducted first, and then the fire resistance tests were performed for eight specimens considering different load ratios and axial restraining stiffness. Afterward, numerical models, including heat transfer and structural analyses, were developed to reproduce the fire-resistance behavior of the columns and verified via test results. Finally, parametric analyses were performed to investigate the influence of load ratio, axial restraint, and rotational restraint on the fire-resistance behavior of the columns. Results indicated that the primary failure mode of the specimens was a combination of local and global flexural buckling, which was sensitive to the longitudinally non-uniform temperature field. Moreover, the increase in load ratio and axial restraining stiffness led to a significant decrease in the critical temperature and failure time. Moreover, even a low level of rotational restraint could enhance the fire-resistance behavior of the column. • Full-scale tests were carried out on restrained cold-formed steel built-up box columns at elevated temperatures. • Failure mode and critical temperature were obtained for axially restrained cold-formed steel built-up box columns. • Numerical models based on ABAQUS were developed and verified by the tests. • Factors influencing fire response of cold-formed steel built-up box columns were studied. [ABSTRACT FROM AUTHOR]

Cold-formed steel built-up box columns are commonly used in mid-rise cold-formed steel buildings to resist applied loads. Numerous studies have been conducted to evaluate the load-bearing capacity of the columns at ambient temperature but there is very limited research on the fire resistance of the built-up columns. In this paper, a full-scale experimental investigation on fire responses of sixteen axially loaded cold-formed steel built-up box columns was conducted. As a benchmark, the load-bearing capacity of the columns at ambient temperature was tested first. The effects of heating rate, load ratio, and temperature distribution pattern on the fire responses of the columns were investigated in the fire tests. The test results indicated that increasing the load ratio led to a significant decrease in the member critical temperatures, which consequently reduced the fire resistance of the columns considerably. For those columns with an applied load ratio lower than 0.6, they were expected to have member critical temperatures higher than 500 °C. Moreover, with thin-walled steel and no fire protection the column temperatures increased quickly, leading to failure in no more than 12 min in a fast fire even with a low load ratio. It was also found that creep deformation associated with the low heating rate was one of the important factors that could be detrimental to the fire resistance and member critical temperature. As for the effect of temperature distribution patterns in the columns on the member critical temperature, it was found that the member critical temperatures for specimens with non-uniform temperature distribution were slightly higher than those with uniform temperature distribution. • Full-scale tests on cold-formed steel built-up box columns were presented. • The tests contain two load-bearing capacity tests and sixteen fire-resistance tests. • Effects of Load ratio, heating rate and temperature distribution patterns were investigated in the tests. [ABSTRACT FROM AUTHOR]