Lipped Channel Sections Research Articles

Moment capacity of cold-formed steel trusses with Howick Rivet Connectors: Tests, modelling and design

Cold-formed steel (CFS) trusses are increasingly used in lightweight building construction. A new type of pin-jointed connector, known as the Howick Rivet Connectors (HRCs), has been developed for CFS trusses. Previous studies focused on testing T-stub connections and shear strengths of CFS trusses with HRCs, providing insights into their structural performance and shear capacity. This paper extends the research by testing ten new CFS trusses with HRCs to determine their moment capacity. The study involved two different cross-sections: lipped channel sections and hat sections, used as truss chords. Variations in the number and locations of lateral supports were introduced to simulate different boundary conditions. A parametric study explored the effects of span-height ratio and variability in lateral support locations on truss strength and failure modes. Finally, the trusses with lipped section chords were designed according to the American Iron and Steel Institute (AISI 2016) and Australia/New Zealand standards (AS/NZS 2018), achieving an average test-to-design strength ratio of 0.86.

High-Strength Cold-Formed Steel Stiffened Channel Section: Axial Compressive Strength and Initial Geometric Imperfections

This paper presents a summary of experimental findings from axial compression tests on columns featuring a cold-formed lipped channel section with intermediate stiffeners and return lips, roll-formed from high-strength low-allow steel with a nominal yield strength of 690 MPa (100 ksi). Additionally, the paper provides an analysis of the elastic stability of the studied section, a complete description of the initial geometric imperfections of the tested columns, results of tensile coupon tests, and comparison of the observed strengths of the columns with design predictions. The results provide important additional benchmarks for the wider adoption of high-strength cold-formed steel sections and indicate conditions where existing design methods may be reliably extended.

An experimental study on web-bearing resistance of cold-formed steel sigma-shaped sections with web holes under interior-two-flange loading case

Cold-formed steel (CFS) sigma-shaped sections used as flooring joists and bearers are commonly fabricated with web holes to accommodate building services. These sections featuring web stiffeners and curved lips exhibit lower web-bearing resistance than traditional lipped channel sections. However, the web-bearing resistance of CFS sigma-shaped sections with web holes has not been thoroughly investigated. To address this gap, a detailed experimental investigation was conducted, testing 29 CFS sigma-shaped sections with web holes under an interior-two-flange (ITF) loading case. For comparison, specimens without web holes were also tested. Variables such as hole diameter ratio, hole location, bearing length, and flange condition were examined. A finite element (FE) model was developed and validated against the test results. The results indicated that the web-bearing resistance for specimens with web holes was reduced by 24.0% on average. To assess the accuracy of existing design specifications, the test results were compared against the design strengths predicted by the American Iron and Steel Institute (AISI) (2016), Australian and New Zealand Standards (AS/NZS) (2018), European Standard (EC3) (2006), and Uzzaman et al. (2012). The comparison revealed that the design strength predicted by Uzzaman et al. (2012) agreed well with the test results.

Local buckling behaviour of web perforated cold-formed steel lipped channel columns

To connect beams and bracings with storage rack uprights, closely spaced perforations are provided along the web, flanges, and rear flanges of uprights. These perforations can significantly lower the ultimate capacity of such compression members with the possible influence of its natural buckling modes. This capacity reduction can depend on various parameters such as (a) geometrical shape, proportioning of cross-section, and stiffeners; (b) perforation shape, size, spacing, and location; (b) slenderness of member, cross-section, and elements of cross-section; and (d) material properties. A thorough understanding of the influence of the above-mentioned factors is necessary for the accurate strength prediction of perforated Cold-formed steel (CFS) compression members. Even though the current design standards are updated for the accurate strength prediction of unperforated CFS compression members, they do not collectively account for the influence of all the aforementioned factors on the load-carrying capacity of the perforated CFS members, particularly for the local buckling capacity. Though the Direct Strength Method (DSM) of design is the most accepted method for accurate strength prediction of CFS members even for complex cross-sectional shapes, recent research on the strength evaluation of perforated CFS members using DSM has emphasized the need for refinement in DSM. The Modified Direct Strength Method (MDSM), which accounts for the simultaneous buckling of flanges and web, includes the cross-section aspect ratio and cross-section slenderness to predict more accurately the local buckling design strength. However, it was developed only for unperforated specimens. Hence, a systematic experimental and numerical investigation was done to understand the influence of the perforation in the local buckling behavior of the lipped channel section. In total, 14 specimens, including 2 unperforated and 12 web perforated CFS lipped channel stub columns were physically tested with fixed support conditions. The Finite Element Analysis using ABAQUS software was used to conduct an extensive parametric numerical study. The results were used to compare the strength curves of DSM and MDSM and the modification in the design curves has been proposed by considering the erosion in strength due to the presence of perforation.

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.

Web crippling design of cold‐formed ultra‐high strength steel lipped channels under ITF loading: A numerical parametric investigation

AbstractIn recent years, there has been a compelling need to adopt cold‐formed ultra‐high‐strength steel (CFUSS) in the construction industry owing to its numerous advantages, such as a higher strength‐to‐weight ratio, flexibility in achieving desired shapes, and adaptability over longer spans. Among the various applications, CFUSS lipped channel sections are commonly used as purlins and joists in steel structural systems. However, these sections are susceptible to different failure modes, particularly web crippling, which presents significant challenges. Currently, the current design rules lack specific guidelines for estimating the web crippling capacity of CFUSS sections. To address this crucial gap, the present study focuses on a comprehensive numerical investigation of the web crippling response of CFUSS lipped channel sections under interior‐two‐flange (ITF) loading conditions. Finite element (FE) models were developed using the ABAQUS package, verified against published test data, and subsequently used in an extensive parametric study. The ultimate web crippling capacity obtained from the parametric study was used to evaluate the accuracy of the current design equations in various design standards. The findings revealed that the existing design equations inadequately predicted the ultimate web crippling capacity of CFUSS lipped channel sections subjected to the ITF loading condition. Consequently, a modified design equation is proposed, utilizing the same approach as the current design standards, and a new direct strength method (DSM) approach is developed and verified through reliability analysis. The proposed modified design equations offer promising solutions to ensure safer and more reliable design practices for CFUSS structures in the construction industry.

Predicting the load-bearing capacity of lipped channel section cold formed steel profiles under combined effects using machine learning

The application areas of cold formed steel (CFS) profiles are progressively expanding. Due to the thin cross-sections of these profiles, buckling problem occurs. Experimental and numerical studies to investigate this behavior are very difficult and time consuming. Therefore, this study aims to predict the load bearing capacity of CFS profiles with different slenderness under combined effects using advanced machine learning (ML) models. In this scope, a detailed finite element model including material nonlinearity and geometrical imperfections was created and 2240 finite element analyses (FEAs) were performed using the model validated by experimental studies. A comprehensive data set consisting of 2240 data points and 9 different features was created from the FEAs. Then, five ML models such as support vector regressor (SVR), random forest (RF), multilayer perceptron (MLP), gradient boosting regressor (GBR) and extreme gradient boosting (XGB) were applied to the dataset to predict the load bearing capacity of CFS profiles. In order to obtain effective results during the implementation phase, cross-validation and hyperparameter fine-tuning were performed to obtain optimum parameters. At the conclusion of the study, all five ML models were considered to be highly effective in predicting the load bearing capacity of CFS profiles. MLP model made better predictions compared to the other four models. In the load bearing capacity prediction, the accuracy of the MLP model is ± 4.26 kN, which is less than 4 % of the average load bearing capacity. It was also found that flange length and section thickness have more effect on the load bearing capacity compared to other properties.

Test, numerical investigation and design of perforated advanced high-strength steel I-shaped built-up compression members

This study investigates the effects of perforations on the buckling behavior and load-bearing capacity of advanced high-strength steel I-shaped built-up compression members. The built-up section was fabricated by connecting two identical lipped channel sections made of complex phrase steel HC700CP980 using pull rivets. The perforations were classified into two groups: holes in web and holes in both web and flanges. A comprehensive parametric study, combining both experimental results and finite element analysis, demonstrated the decrease in load-bearing capacity caused by the presence of perforations, particularly noticeable in specimens featuring holes in both web and flanges. The extend of the reduction is closely linked to the quantity and dimensions of the holes, with a higher hole-element width ratio resulting in a more substantial loss of load-bearing capacity. Additionally, the obtained test and FE results were used to evaluate the accuracy of direct strength method (DSM) for the examined advanced high-strength steel I-shaped built-up compression members. The evaluation findings highlight the limitations of the DSM in the design predictions for built-up columns featuring multiple web holes and holes on both web and flanges. Therefore, a modified calculation method was proposed, which integrated strength reduction factors to consider the influence of the position of holes relative to the flexural buckling axis of built-up sections. The modified method has shown notably improved design accuracy and consistency for the perforated advanced high-strength steel I-shaped built-up sections.

Numerical and Experimental Analysis of Buckling and Post-Buckling Behaviour of TWCFS Lipped Channel Section Members Subjected to Eccentric Compression.

The paper presents a static analysis of the buckling and post-buckling state of thin-walled cold-formed steel (TWCFS) lipped channel section beam-columns subjected to eccentric compression. Eccentricity is taken into consideration relative to both major and minor principal axes. An analytical-numerical solution to the buckling and post-buckling problems is described. The solution is based on the theory of thin plates. Equations of equilibrium of section walls are derived from the principle of stationary energy. Then, to solve the problem, the finite difference (FDM) and Newton-Raphson methods are applied. Linear (buckling) and nonlinear (post-buckling) analyses are performed. As a result, pre- and post-buckling equilibrium paths are determined. Comparisons of the obtained numerical results, FE simulation results, and experimental test results are carried out and presented in comparative load-shortening diagrams. Additionally, a comparison of the buckling forces and buckling modes obtained from theoretical analysis and experiments is presented.

Experimental and numerical investigations of high-strength cold-formed steel multi-limb built-up section columns

This paper investigates the buckling behaviour and load-carrying capacity of G550 high-strength cold-formed steel multi-limb built-up section columns under axial compression. Each built-up section consists of multiple lipped and unlipped channel sections connected by self-tapping screws. The C3U1 section is formed by three C-sections (i.e. lipped channel section) and one U-section (i.e. unlipped channel section), while the C2U2 section is formed by two C-sections and two U-sections. The experimental programme involved tensile coupon tests, measurements of initial geometrical imperfection, and column tests on ten C3U1 and ten C2U2 multi-limb built-up section columns. The numerical modelling programme was then performed, with the nonlinear finite element models created and validated against the experimental results. The applicability of the codified Effective Width Method and Direct Strength Method for predicting the strengths of G550 high-strength cold-formed steel multi-limb built-up section columns, as specified in the American Specification and Australian Standard, was then evaluated. The evaluation results showed that the codified Effective Width Method led to rather conservative and less accurate failure load predictions for G550 high-strength cold-formed steel multi-limb built-up section columns, while the codified Direct Strength Method provided accurate and consistent failure load predictions and thus applicable to the design of G550 high-strength cold-formed steel multi-limb built-up section columns. Concerning the lack of design guidelines specifically for multi-limb built-up sections, a modified Direct Strength Method was thus proposed as an alternative design method and shown to provide accurate failure load predictions.

Experimental axial-compressive behaviour of bare cold-formed-steel studs with semirigid-track and ideal-hinged boundary-conditions

Studs are the primary load-bearing components in cold-formed steel (CFS) wall panels, connected to tracks at both ends with self-tapping screws, forming a semirigid boundary condition (BCT). Most existing tests on the axial compressive behaviour of bare CFS studs are based on either theoretically-hinged (BCH) or fully-fixed boundary conditions. Previous researchers have employed BCT only on sheathed stud-wall panels. However, practicing engineers and current design codes, e.g., Eurocode 3, follow an all-steel design. Therefore, this research experimentally investigated bare-CFS-studs' axial compressive behaviour with BCT, considering, for the first time, the combined effect of the tracks' warping rigidity, stud-to-track gap, non-linear connection stiffness, and bare studs' various cross-sectional slenderness. Forty-two industry-standard lipped channel sections (studs) of five thicknesses (1.2-3 mm), three depths (75–125 mm), and two heights (1.2 & 1.5 m) were tested under static-concentric axial compressive loading with BCT. Another fourteen studs were tested with BCH, a comparator to BCT. Results demonstrated that the studs' global failure mechanisms were flexural-torsional in BCT instead of flexural in BCH. Studs' axial stiffness was two-phased in BCT due to the stud-to-track gap, compared to single-phased stiffness in BCH. >1.8 mm stud-to-track gap caused stud-to-track connections' failure and studs' sudden capacity reduction during gap closure. Studs achieved 1.22 times higher axial-compressive strength, 2.3 times more axial-shortening, 0.7 times lower axial stiffness, and 58% lower axial-compressive strain at the web-midheight under BCT-PhaseII than BCH. Tested strengths were compared with EC3 design strength, and an effective-length-factor of 0.65 was suggested for efficient design of studs with BCT.

Buckling behavior of cold-formed steel sigma and lipped channel section beam-columns: Experimental and numerical investigation

In this study, the buckling behavior of cold-formed beam-column members with sigma and lipped channel sections under the combined effect of axial load and biaxial bending moment (P-Mx-My) was compared. A total of 30 specimens (15 with sigma sections and 15 with lipped channel sections), each measuring 1500 mm in length, were tested under monotonic loading. The performances of the specimens under eccentric loading such as strength and energy absorption capacity were investigated experimentally and numerically. The local, distortional and global buckling modes that determine the bearing capacity of the section and the length of the sigma and lipped channel member where all these buckling modes can be seen were determined. Finite element analysis (FEA) was performed with Abaqus software, and the numerical results were verified against experimental results. Using the validated finite element model, a comprehensive parametric study was carried out by performing 2894 FEA on sigma and lipped channel section members with various cross-sectional dimensions and lengths. Experimental results show that there are significant differences in load bearing capacity, energy absorption capacity and buckling modes of sigma and lipped channel section members. In the FEA/ AISI-S100 code comparison made at the end of the parametric study, it was determined that differences occurred according to the lengths, cross-sectional properties and loading points of the members. It was determined that the error rate was safer when the lips were subjected to tensile stresses than when they were subjected to compressive stresses. Finally, it was determined that the FEA/AISI-S100 error rate decreased as the length of the sigma-section members increased.

Elastic critical local buckling load and behavior of local buckling for cold‐formed lipped channel steel member under compression

AbstractThe objective of this study was to analyze the elastic critical local buckling of cold‐formed lipped channel sections using the energy method. A mechanical model and unique displacement functions simulating the local buckling behavior were proposed to consider the plate element interaction. Using the proposed mechanical mode, an analytical formula for local buckling that directly reflects the parameters of the channel member was proposed. Based on the proposed formula, the relationship between the cross‐sectional geometry and local buckling behavior was investigated. In addition, a simplified design equation was proposed. The reliability and effectiveness of the proposed equations were presented by comparing with the results of eigenvalue analysis based on the finite element method.

Axial compressive behavior of cold-formed steel built-up box-shape columns with longitudinal stiffeners

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 λg and λl 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.

Tests, numerical simulations and design of G550 high strength cold-formed steel lipped channel section columns failing by interactive buckling

This paper presents the experimental and numerical investigations into the structural performance and strengths of G550 high strength cold-formed steel (CFS) lipped channel section columns failing by interactive buckling. A testing program was first conducted, comprising material testing, measurements of initial local, distortional and global geometric imperfections and ten pin-ended column tests. The interaction between different buckling modes was observed and discussed for all the examined specimens upon testing. A numerical modeling program was subsequently performed, where numerical models were created and verified with reference to the test results and afterwards utilized to conduct parametric analysis to produce more numerical data over an extensive spectrum of cross-section geometries and effective member lengths. The effective width method (EWM) codified in the European code and direct strength method (DSM) specified in American and Australian standards were evaluated, based on the combined set of numerical and test data, for their suitability to G550 high strength CFS lipped channel section columns susceptible to interactive buckling. The evaluation results indicated that (i) the EWM yielded scattered and conservative ultimate strengths for columns with small and intermediate slenderness ratios, but relatively accurate results for those of large slenderness ratios and (ii) the DSM led to more consistent and accurate strength predictions for G550 high strength CFS lipped channel section columns failing by interactive buckling, though a number of strength predictions were on the unsafe side.

Test and design of cold-formed steel closed built-up beams with double sigma sections and triple lipped channel sections

Built-up section beams fabricated from several single sections are finding increasing use in the cold-formed steel (CFS) industry due to various advantages such as higher bending capacities, innovative new shapes, and elimination of eccentricity. However, a built-up beam is connected at discrete points along the longitudinal direction, which weakens the shear and flexural rigidities of the built-up section. The structural behaviour of built-up CFS beams is therefore more complicated than that of single section beams. This paper presents an experimental investigation on the moment capacity of screw fastened built-up CFS beams. To investigate the local, distortional, and flexural-torsional buckling of built-up CFS beams, four-point bending tests of 8 braced and 24 unbraced built-up C2C (fabricated from 2 sigma sections) section beams and C3C (fabricated from 3 lipped channels) section beams are conducted. Unbraced bending test specimens are tested at three lengths covering inelastic failure at short lengths to essentially elastic failure at long lengths. The strength predictions for the tested built-up beams are calculated using the current Direct Strength Method (DSM) in the AISI-S100 Specification. The comparison between the experimental results and DSM predictions indicates that the current DSM in AISI specification is overly conservative and unreliable for the types of built-up beam sections examined in this research, due to the incorrect failure mode prediction and underestimation of the critical elastic buckling stresses which is a key input to the DSM.

Instability-driven family optimization of cold-formed steel lipped-channel cross-sections with strength and stiffness constraints

The objective of this study is to advance a solution scheme that can produce a family of optimized thin-walled cold-formed steel lipped-channel sections under combined compressive and flexural loading subjected to both strength and stiffness constraints. In this new study, the solution scheme is extended from previous work considering strength constraints only to include stiffness constraints to address the serviceability requirement as well for a more practical reflection of behavior whereby stiffness is reduced due to member instabilities. The stiffness constraint is driven by a novel method for estimating effective moment of inertia due to cross-section instabilities such as local and distortional buckling modes. A two-level optimization framework is utilized to produce a family of optimized sections. The first level focuses on individual member optimization of the P−M demand space as derived from current commercially available lipped-channel sections in the United States; the second level focuses on a new family of optimized sections capable of covering the same design space at a minimal family size. The results of the study show that a family of only 8 unique optimized sections can replace 186 commercially available sections and still achieve the same or improved performance in terms of both strength and stiffness compared to strength constraints only. The developed family of sections demonstrates that optimization techniques, employing complicated design constraints, have a great potential for improving cold-formed steel production on currently widely-used cross-section shapes such as lipped channels.

Effects of section slenderness and concrete strength on the cross‐sectional behaviour of lightweight concrete‐filled cold‐formed built‐up box sections

AbstractThis paper concerns axial compressive behaviour of lightweight concrete‐filled cold‐formed built‐up box sections (CFBBS) at cross‐sectional level. A total of twenty‐four lightweight concrete‐filled CFBBS stub columns and their hollow counterparts—formed by nesting two asymmetric lipped channel sections front‐to‐front—were tested. The varying parameters in the present experimental study included three cross‐section slenderness (h/t) ranging from 75 to 125 and three concrete grades (fc) with measured cylinder strengths varying from 33.5 to 55.6 MPa. The typical mechanical behaviour of the stub column tests was discussed in terms of load‐end shortening response. The parametric influences on the load‐end shortening response and ultimate compressive load were scrutinised.

Flexural behaviour of CFS stacked built‐up beams: experiments, finite element modelling and direct strength method

AbstractThis paper presents the initial results and findings from the experimental and numerical study of recently developed CFS stacked built‐up beams. The specimens are constructed by joining either three or four 75×150 lipped channel sections using screws. A cap of LC 89×41.3 with a nominal thickness of 1.2 mm screwed to the open flange to provide a closed section with a greater flexural capacity. A six‐point loading system was considered to study the flexural behavior of these built‐up beams. The load‐displacement curves, strength and failure modes are reported. A nonlinear elastic‐plastic finite element (FE) model is developed. The bending strengths are also calculated using the direct strength method (DSM) in the North American Specification (AISI) and Australian/New Zealand Standard (AS/NZs) for cold‐formed steel structures. Both results obtained from the FE and the direct strength methods are compared against the experimental.

Experimental Research on the Cold-Forming Effect of Cold-Formed Thick-Walled Steel

To study the cold-forming effect on the yield strength distribution of cold-formed thick-walled steel, material tensile tests and axial compression tests were conducted on steel-lipped channels. To undertake the material tensile tests, five types of cold-formed lipped channel sections with varied thicknesses and nominal yield strengths were selected. The material test samples were taken from the web, flange, curled edge, and corner of the lipped channel sections, respectively. Next, a corresponding distribution model for the yield strength of cold-formed thick-walled lipped channel sections was developed. Following this, axial compression tests of five short columns with the same section as the coupon tests were conducted to further investigate the cold-forming effect. Finally, the results from the short column tests, the results based on the proposed yield strength distribution model, and the results calculated for related codes in various countries were compared. These revealed that the yield strength distribution, bearing capacity, and ductility were considerably influenced by the cold-forming effect and mainly depended on the width-to-thickness ratio (h/t) of the plate involved. When h/t was less than 34, the curve slowly decreased after reaching the peak load and had good ductility. Conversely, when h/t was greater than 71, local buckling had an obvious influence on the bearing capacity of the specimen, and the column ductility was poor. Overall, the results based on the Chinese code GB 50018-2002 were closest to the experimental values with the smallest variation coefficient, and therefore this code has the best applicability.