Cold-formed Steel Research Articles

A Parametric Study on the Flexural Capacity of Cold-Formed Steel Stacked Built-up Sections

A Parametric Study on the Flexural Capacity of Cold-Formed Steel Stacked Built-up Sections

Numerical modelling and design of cold-formed steel battens subject to pull-out failures under bushfire conditions

Thin and high strength cold-formed steel (CFS) battens and claddings serve as non-combustible elements in roof and wall cladding systems, aligning with the recommendations of Australian bushfire standards. However, they are susceptible to significant damage from heat and strong winds experienced during bushfire events, particularly in the form of batten pull-out failures. Such failures may occur at the screw connections between cladding and battens, or battens and trusses/rafters. However, pull-out failures often occur in the thinner CFS battens, when the screw fastener pulls out of the batten’s top flange, leading to the loss of entire cladding and compromising the integrity of the building envelope. This study used finite element modelling of a range of CFS batten configurations to investigate their pull-out failures under combined wind action and bushfire conditions. Finite element models were developed to simulate the pull-out failures at elevated temperatures, validated using experimental results and used in a parametric study to investigate the effects of relevant parameters. Using the results, this study has proposed suitable design equations for predicting the pull-out capacities of CFS battens at elevated temperatures. Engineers can use them to design bushfire safe CFS cladding systems.

Simplified Direct Strength Method for the design of cold-formed steel channels with circular web openings under two-flange loadings

Previous studies have put forward reduction factor equations to determine the web crippling strength of cold-formed steel (CFS) channels with circular web openings. However, these equations overlook vital parameters such as material yield strength, flange width, and fillet radius of CFS channels. Consequently, this paper introduces simplified Direct Strength Method (DSM) equations, which account for plastic loads (Py) and elastic buckling loads (Pcr), integrating the aforementioned crucial parameters. Eight unique yield line models were devised, considering two-flange loadings: interior-two-flange (ITF) and end-two-flange (ETF), as well as the positioning of circular web openings, to compute the yield line length for Py. Similarly, new Pcr equations were also proposed. The reliability of the proposed equations was evaluated using a dataset of 236 test results sourced from the literature. This assessment aims to demonstrate the suitability of the equations for incorporation into future revisions of international design codes.

Practice-oriented numerical model for seismic response of cold-formed steel-framed mid-rise buildings

Cold-formed steel (CFS) framed buildings are becoming increasingly common, notably in the high seismic regions of the United States (US). However, research to understand the seismic performance of CFS-framed buildings specifically using large-scale (building-level) specimens has begun only recently. Therefore, efforts to validate and improve numerical modeling of whole building systems have been limited due to this lack of experimental data. A recently completed experimental program, conducted at the Large High-Performance Outdoor Shake Table at the University of California, San Diego, involved earthquake testing of a full-scale 6-story CFS-framed building. The test building adopted an assembly of continuous tie-rod and built-up compression stud pack at the ends of individual shear walls to resist seismic overturning moment and uplift forces within the building. The objective of this paper is to present a practice-oriented phenomenological finite element modeling approach intended for design engineers and compare the predictions from this model against the measured seismic response of the 6-story building. The model aims to provide a robust, yet computationally efficient approach, with minimal degrees-of-freedom, while also capturing the salient features of seismic response of mid-rise CFS-framed buildings suitable for nonlinear time history analyses-based design. The building model, developed within the OpenSeesPy finite element platform, includes the shear walls and gravity walls as well as the lateral strength and stiffness contribution from exterior and interior gypsum sheathing. The continuous tie-rod system is also modeled to capture the cumulative floor displacements caused by the axial elongation in the steel rods. The effect of built-up compression stud packs on the lateral behavior of a shear wall was estimated using available experimental data and a detailed numerical model of an isolated shear wall, then subsequently incorporated in the nonlinear hysteretic material model. The numerical model captured the natural frequencies of the translation modes of the building in its undamaged state with <3% error. It also predicted the peak roof drift ratio within an error of 0.03% drift ratio under the design earthquake event. These results provide confidence in the proposed numerical modeling approach as a tool for seismic response prediction of mid-rise CFS-framed buildings with similar structural detailing features.

Numerical Investigation of the Axial Load Capacity of Cold-Formed Steel Channel Sections: Effects of Eccentricity, Section Thickness, and Column Length

Cold-formed steel channel (CFSC) sections have gained widespread adoption in building construction due to their advantageous properties, including superior energy efficiency, expedited construction timelines, environmental sustainability, material efficiency, and ease of transportation. This study presents a numerical investigation into the axial compressive behavior of CFSC section columns. A rigorously developed finite element model for CFSC sections was validated against existing experimental data from the literature. Upon validation, the model was employed for an extensive parametric analysis encompassing a dataset of 208 CFSC members. Furthermore, the efficacy of the design methodologies outlined in the AISI Specification and AS/NZS Standard were evaluated by comparing the axial load capacities obtained from the numerically generated data with the results of four previously conducted experimental tests. The findings reveal that the codified design equations, based on nominal compressive resistances determined using the current direct strength method, exhibit a conservative bias. On average, these equations underestimate the actual load capacities of CFSC section columns by approximately 11.5%. Additionally, this investigation explores the influence of eccentricity, cross-sectional dimensions, and the point-of-load application on the axial load capacity of CFSC columns. The results demonstrate that a decrease in section thickness, an increase in column length, and a higher degree of eccentricity significantly reduce the axial capacity of CFSC columns.

Capacity Fragility of Screw Connections installed in Cold-Formed Steel Partition Walls

Capacity Fragility of Screw Connections installed in Cold-Formed Steel Partition Walls

Investigation of local buckling behavior of web perforated plain channel stub columns

As a sustainable material, cold-formed steel (CFS) is increasingly popular in structural components of buildings and industrial storage racks. The plain channel sections having a flange stiffened web element and unstiffened flange elements are prominently used as compression members in CFS systems. These elements are likely to undergo local buckling under compressive loading. The openings or closely spaced perforations along the longitudinal direction of the web for serviceability requirements on buildings and beam level adjustment requirements on storage racks lead to additional complications to the local buckling. Although the Effective Width Method and Direct Strength Method rationally cover the buckling behavior of plain channel sections, the influence of height-to-width ratio with perforations is not effectively accounted for in the design. Limited research details are available in the literature for the web perforations of lipped channels or rack sections. However, these sections do not have unstiffened flanges, where the unstiffened flanges can be more vulnerable to local buckling, e.g., plain channels. The web perforations also make the web more vulnerable to local buckling. This article examines the local buckling behavior of plain channel sections with the influence of web perforations through systematic experimental and comprehensive numerical studies. The influencing parameters of the cross-section geometry are assessed through the principal component analysis (PCA) to understand its correlation with local buckling. The PCA results shed light on mandatory parameters for the elastic critical local buckling load calculation and/or nominal local buckling strength prediction of the plain channel section.

Stability of cold-formed steel stud walls subjected to vertical compression

Cold-formed steel (CFS) stud walls are the primary vertical load-bearing units in CFS residential systems. These walls need to meet both strength requirements and good stability. In this study, a CFS stud wall was simplified into a mechanical model of an orthotropic plate rib system using the orthotropic plate (OP) method. A formula for calculating the equivalent stiffness of the stud wall was obtained, and the correctness of the simplified calculation model and equivalent stiffness calculation formula was verified by finite element analysis. Various factors that affected the equivalent stiffness of the wall were investigated. Additionally, the stability of the stud walls under a vertical load was derived by considering the stability theory of the plate. The results indicated that the stability of the stud walls was mainly related to the height-thickness ratio of the walls. Finally, the correction factor of the allowable height-thickness ratio of the walls was obtained by analyzing the parameters of the wall thickness, stud spacing, sheathing materials, and wall opening, and a case study was used to illustrate the procedure of this simplified method.

Fire performance of superabsorbent polymers protected full-scale cold-formed steel wall lined with plasterboards

Superabsorbent polymers (SAPs) have shown promise in enhancing the fire resistance of steel structures because of their exceptional absorbency and water retention capabilities. This study innovatively applies SAP insulation to cold-formed steel (CFS) walls with gypsum (GP) boards to improve fire performance. One full-scale CFS wall underwent axial compression testing at ambient temperature, and four full-scale fire experiments were conducted on CFS walls with load ratios of 0.27 and 0.85. All specimens were sheathed with double layers of GP boards, and rock wool served as the cavity insulation, with SAP insulation material applied to the cavity of wall studs. Additionally, a novel finite element model was developed to simulate the fluid-heat interaction within a furnace-CFS wall. The simulation results showed that SAP insulation material significantly changes the fire-induced failure mode in wall studs at a 0.27 load ratio, transitioning from hot flange (HF) local buckling to local buckling of the whole stud section. The inclusion of SAP insulation material absorbs heat, effectively delaying the rise in wall stud temperature. This property significantly enhances the structural performance of CFS walls during fire exposure. With SAP insulation material, the fire resistance time of the CFS walls increased by over 66 min compared to non-SAP insulation material counterparts. Moreover, increasing the stud web depth and reducing the HF width-to-thickness ratio can enhance the fire performance of CFS walls. The HF critical failure temperature of specimen S2 with an HF width-to-thickness ratio of 80 was 783 °C, 150 °C higher than that of specimen P2 with an HF width-to-thickness ratio of 67. Additionally, the adverse effect of screws on the fire performance of the CFS wall was localized. After 120 min of fire exposure, the temperature on the ambient side, which was severely influenced by the screws, was 360 to 370 °C.

The Evolution and Performance of Cold-Formed Steel Built-Up Battened Columns

The cold-formed steel (CFS) built-up battened column represents a transformative and innovative solution in the construction industry due to its versatility, cost-effectiveness, and superior strength-to-weight ratio. In design consideration, chord spacing and batten plates influence the structural performance and stability of the column. The emphasis lies in the design flexibility of built-up battened columns, in which the composite action of several chord members connected by batten plates can enhance axial compressive strength. The batten plate plays a crucial role in preventing buckling and increasing the stability of the column while distributing the load evenly. The batten plates need to be properly designed to ensure stability and reduce buckling failure in the built-up battened column. However, implementing CFS built-up battened columns presents several challenges, including design complexity, connection design, and limited standards and guidelines.

Experimental and finite element evaluations of single-T composite cold-formed steel beam with concrete slab

Steel-concrete composite constructions are of particular interest due to their properties such as lightweight, fast construction rate, and higher load carrying capacity, which may be enhanced by utilizing cold-formed steel rather than hot-rolled steel to get lightweight floor system. The structural behavior of hybrid composite beams has been studied through experimental testing and finite element analysis. The beams are made in the form of a single T-beam by joining a concrete slab and a mono-symmetric cold-formed section (CFS). The CFS has two flanges: the upper is shaped like a Y or a T, and the lower is hollow and rectangular in shape. By partially embedding the upper steel flange into the 1000 mm-wide concrete slab, the composite action was established. Sixteen finite element models were used in the finite element models' evaluation process, and eight composite beam specimens underwent a four-point loading flexure tests. Principal characteristics, like the CFS's upper (Y or T) flange with and without vertical web stiffeners, were investigated. The load-carrying capacity and failure mechanisms of each specimen were measured, recorded, and confirmed by the FEA results. The ultimate load-carrying capacity of the composite section was 120 % higher with the Y-beam CFS than with the T-beam CFS. Additionally, adding stiffeners increased the local buckling resistance of the CFS and increased the beam's load-carrying capacity by about 105 %. The FEA results showed that increasing the slab thickness increased the section moment capacity by about 123 % and using a flange thickness bigger than web thickness could increase the section moment capacity than using a flange thickness smaller than web thickness CFS beam thicknesses by about 109 %.

Experiment and design of cold-formed steel equal-leg lipped angle under axial compression

To study the buckling behavior and the design method of the load-carrying capacity of cold-formed thin-walled lipped angle, axial compression tests were carried out on 32 lipped angle columns with different sections and slenderness ratios which were manufactured with high strength zinc-coated grades G450 structural steel sheets. The experimental results showed that the long column with a small width-to-thickness ratio displayed global flexural-torsional buckling, local buckling was found for the short column with a large width-to-thickness ratio. Other specimens demonstrated interactive buckling with local buckling and flexural-torsional buckling. The specimens with torsional buckling all showed post-torsional buckling strength. Then ABAQUS finite element software was used to simulate the lipped angle specimens. The simulation results were in good agreement with the test results about buckling modes and ultimate strength which indicated that the established finite element analysis model was reasonable and feasible. Therefore, the influences of the slenderness ratio, width-thickness ratio, and width ratio of the leg to lip on the ultimate strength of cold-formed thin-walled lipped angle were analyzed using finite element software. The analysis showed that the ultimate capacity of the angle decreased greatly with the increase of the slenderness ratio. The ultimate capacity increased in the range of width-to-thickness ratios of 15–30 when the angles had the same lengths. The increasing of the width of the lips increased the ultimate capacity, but the increase of the ultimate capacity of the angles would lower when local buckling occurred at the lips. Finally, based on the test results, the modified effective width method and direct strength method were proposed to calculate the capacity of the cold-formed thin-walled lipped angle under axial compression. The comparison between the predicted results with the test and finite element analysis showed that the proposed methods were accurate and feasible.

Application of laced built-up columns with cold-formed lipped angle sections in wind turbine towers

Increasing the height of wind turbine towers effectively enhances their power output. Consequently, there is a growing demand for constructing wind turbine towers with higher hubs, especially in heavily populated and energy-intensive areas. Traditional approaches involve enlarging angled steel, posing production challenges and economic feasibility concerns. This study proposes replacing traditional angled steel with cold-formed lipped angles for improved structural performance. In this paper, a finite element model of the built-up column with cold-formed lipped angles is developed through ANSYS software. Finite element calculations are applied to various cross-sectional dimensions of cold-formed lipped angle sections to analyze column curves. Incorporating calculation methods from international standards, the paper presents a fitting formula and calculation method for the load-carrying capacity of the built-up column with cold-formed steel lipped angle sections. Finally, utilizing a wind turbine tower with a 120 m hub height as a case study, the paper conducts structural modeling and design analyses for both the built-up column with cold-formed lipped angles and the one with conventional angled steel. The research indicates that after adopting the built-up column with cold-formed lipped angle sections, the stress ratio of the tower columns is significantly reduced, with an approximate 16 % decrease in the stress ratio of the most critical section. The adoption of cold-formed lipped angle section effectively enhances the overall load-carrying capacity of the wind tower, providing a greater safety margin for the design. The calculation method provides valuable insights for future engineering applications.

Flexural Behavior of Galvanized Iron Based Cold-Formed Steel Back-to-Back Built-Up Beams at Elevated Temperatures

Cold-formed steel (CFS) sections have become popular in construction due to several advantages over structural steel. However, research on the performance of galvanized iron (GI)-based CFS under high temperatures, especially regarding its flexural behavior, has been limited. This study extensively investigates how GI-based CFS beams with varying spans behave under elevated temperatures and subsequent cooling using air and water. This study examines the impact of temperature loading and compares the effectiveness of air- and water-cooling methods. Experimental results are validated and analyzed alongside findings from finite element modeling (FEM) using ABAQUS (2019_09_13-23.19.31) and the Direct Strength Method (DSM). Additionally, this study conducts a parametric investigation to assess how beam span influences flexural capacity. Among beams heated to the same temperature, those cooled with water show slightly lower load capacities compared to those cooled with air. The highest load capacity observed is 64.3 kN for the reference specimen, while the lowest is 26.2 kN for the specimen heated for 90 min and cooled with water, a 59.27% difference between them. Stiffness decreases as heating duration increases, with the reference section exhibiting significantly higher stiffness compared to the section heated for 90 min and cooled with water, with a 92.76% difference in stiffness. As heating duration increases, ductility also increases. Various failure modes are observed based on different heating and cooling conditions across different beam spans. This study provides insights into how GI-based CFS beams perform under temperature stress and different cooling scenarios, contributing valuable data for structural design and safety considerations in construction.

A Comparative Analysis of Thin-Walled Sections Using Finite Element Modelling

Thin-walled channel sections, such as Folded Sections, C-Sections, and Z-Sections, are extensively used in structural applications, particularly in civil engineering. This research investigates the behavior of cold-formed steel sections, including folded flanges, C-sections, and Z-sections, with respect to their ultimate load-carrying capacities. A numerical model will be developed using the finite element method in Abaqus software to simulate the behavior of these sections under axial compressive loading, covering both the elastic and plastic ranges. Comparative analysis of the folded sections, C-sections, and Z-sections is conducted. Z-section stands out with its significantly higher ultimate load capacity compared to the folded and C-sections. This can be attributed to its Z-shaped design. The objective of this study is to establish more robust design guidelines for the safe and efficient use of these sections in building structures. The findings will contribute to enhancing the reliability and economy of design practices for these structural elements.

Research on post-fire mechanical properties of thin-walled cold-formed steel and its influence on the Σ-shaped columns after fire exposure

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 λdl > 1.5.

Buckling behavior and failure mechanism of cold-formed steel built-up special shape cross-section columns

This paper introduces a new type of cold-formed steel (CFS) built-up closed cross-section (CBC-C) column to connect multi-directional wall panels easily. Axial compression experiments and numerical analysis on 18 columns as well as theoretical calculations are carried out to evaluate the stability and bearing capacity of the proposed column. The effect of factors such as width-to-thickness, slenderness ratios, and screw spacing on the buckling behavior and failure mechanism was studied. As the slenderness ratio increased, the ultimate bearing capacity of specimens with the same section decreased. For specimens with a web width of 140 mm, the ultimate strength increased by 7.35 % when the slenderness ratio was reduced from 26.7 to 3.2. It was observed that the failure modes of short and intermediate long columns were generally local and distortion-related buckling, while the long columns experienced local, distortional, and finally global buckling when the slenderness ratio reached 36.73. The local buckling load and ultimate strength of specimens of the same length and section were reduced with the growth of the width-to-thickness ratio. On average, as the web width-to-thickness ratio increased by 10 %, the local buckling load decreased and the ultimate bearing capacity decreased by 22.37 % and 14.79 %, respectively. It was found that changing the section thickness had a greater effect on the stability and strength of the specimens compared to changing the web width of the CBC-C column. The mechanical properties of the CBC-C were not affected by the screw spacing and the differences among data were less than 5 %. Finally, the effective width method (EWM) and the direct strength method (DSM) in the AISI S100-16 wereassessed for CBC-C. The novel built-up column proposed in this paper has advantages in modular structures for their flexibility in configurations as well as high torsional rigidity and good stability.

Seismic design rules for lightweight steel shear walls with wood panel sheathing within the Eurocode format

Within the 2nd-generation of Eurocodes several new lateral force resisting systems (LFRS) typically used in lightweight cold-formed steel (CFS) buildings have been introduced, including shear walls with wood sheathing (SWWS). Therefore, extensive studies have been carried out to define the seismic design parameters according to the format of European code for seismic design, i.e. Eurocode 8. In current version of the 2nd-generation Eurocode 8 (prEN1998-1-2, CEN 2022), in case of SWWS the behaviour factor has been defined based on the FEMA P695 (FEMA, 2009) [3] methodology, whereas the other seismic design parameters have been set based on assumption validated with analyses on limited case studies. To overcome this lack, specific research devoted to defining the main seismic design parameters for SWWS belonging to Ductility class 3 (DC3) according to prEN1998-1-2 (CEN 2022) has been carried out at the University of Naples “Federico II”. The research shows that the Easley et al. (1982) method gives consistent in-plane horizontal wall resistance predictions if the shear resistance of wood panel-to-steel sheet screw connections (sheathing connections) is evaluated with reliable approaches and formulas for the evaluation of the shear resistance of sheathing connections have been calibrated. Based on the results obtained from this research, a partial safety factor for resistance evaluation of SWWS according to limit state design (LSD) equal to 1.3 is proposed and an overstrength (expected resistance) factor for the design of non-ductile elements according to capacity design equal to 1.7 is recommended.

DSM-Based failure load prediction of CFS pin-ended singly symmetric columns buckling in flexural-torsional modes

Recently, the authors investigated the post-buckling behaviour, strength and Direct Strength Method (DSM) design of cold-formed steel (CFS) fixed-ended singly symmetric columns buckling in major-axis flexural-torsional modes (FMT), some of which were shown to experience interaction with minor-axis flexural (Fm) buckling − FMT-Fm (global-global) interaction. The main fruit of this investigation was the development of an efficient DSM-based design approach capable of predicting the failure loads of such columns, regardless of their failure mode nature (pure FMT or FMT-Fm interactive). This work extends the scope of the above study to singly symmetric columns with three types of pin-ended support conditions, all fixed with respect to torsion and having warping fully prevented. Columns with seven cross-section shapes are considered, having their wall dimensions, lengths and yield stresses selected to ensure covering wide FMT slenderness ranges and various ratios between the Fm and FMT buckling loads. Following an investigation on the elastic and elastic-plastic post-buckling behaviours of the selected pin-ended columns, paying attention to the possible occurrence of FMT-Fm interaction, parametric studies are carried out to gather extensive pin-ended column failure load data, including some potentially associated with FMT-Fm interactive collapses. Then, the assembled numerical failure loads are used to show that (i) the available DSM-based strength curves are only able to predict adequately part of them and, thus, (ii) novel DSM-based design curves are needed to estimate the failure loads of the remaining pin-ended singly symmetric columns buckling in FMT modes. After developing such curves and assessing their merits (safety and reliability), it is concluded that different DSM-based design curves/approaches must be employed for columns with each type of pin-ended support conditions.

Cold-formed Q1200 ultra-high strength steel angle- and channel-section stub columns

This paper presents the experimental and numerical analysis of cold-formed Q1200 ultra-high-strength steel (UHSS) angle- and channel-section stub columns to elucidate their resistance performance. Initially, the mechanical property test including the flat and corner coupon specimens was conducted to quantify the effect of the cold-forming process on Q1200 UHSS. Then, the stub column test was conducted on 16 cold-formed Q1200 UHSS angle- and channel-section stub columns. Based on the experimental results, a finite element model was established to simulate the resistance performance of the cold-formed Q1200 UHSS angle- and channel-section stub columns. Subsequently, a parametric study involving 133 numerical models was performed. Then, the suitability of the design approaches in EN 1993-1-12, AISI S100, AS/NZS 4600, and the direct strength method (DSM) was evaluated. For angle-section stub columns, the design approach outlined in EN 1993-1-12 was deemed suitable and the effective cross-sectional area design approach and DSM in AISI S100 were found to be safe. However, these approaches were excessively conservative for slender cross-sections in directly predicting the ultimate resistance of angle sections. Because of unsafe predictions, the design approaches in EN 1993-1-12, AISI S100, and DSM were deemed inappropriate for channel-section stub columns. The modification suggestions were proposed for the current design approaches, where the beneficial effects from the strain hardening were included. Based on the comparisons, the revised design approaches in EN 1993-1-12, AISI S100, and DSM could be used to accurately predict the ultimate resistance of the cold-formed Q1200 UHSS angle- and channel-section stub columns.