Direct Strength Method Research Articles

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.

Local buckling behaviour of Q690 high strength steel press-braked elliptical hollow section stub columns: Testing, numerical modelling, and design

This paper presents comprehensive investigations on the local bucking behaviour of Q690 high strength steel (HSS) press-braked elliptical hollow section (EHS) stub columns subject to concentric compression experimentally. In comparison with conventional hot-finished EHS or cold-rolled EHS, the press-braked EHS provides high precision, great construction productivity and the fabricated geometries are not limited to production assembly instrumentation. HSS press-braked EHS fabricated through press-braking process was expected to exhibit distinct behaviour compared with their counterparts constructed from the conventional fabrication routes. Well-understanding of their performance at the levels of material, and cross-section level is essential. Material properties were obtained through tensile coupon tests. Residual stresses in both membrane and bending types were measured and recorded and their impact on structural performance was also assessed. Initial local geometric imperfection measurements were conducted for all the specimens. In conjunction with experimental studies, the finite element (FE) models of Q690 HSS press-braked EHS stub columns were developed. The accuracy of the FE model for HSS press-braked EHS stub column was asserted by comparing the numerical predictions with the tested results. An extensive parametric study was conducted to generate structural performance data with a wider range of cross-section slenderness to complement the test data bank. The cross-section classification for conventional CHS codified in existing standards and the proposed slenderness limits by various researchers were evaluated. The existing slenderness limits cannot be extended to HSS press-braked EHS investigated in this study, a new slenderness limit is proposed. Moreover, the advanced design approaches of Direct Strength Method (DSM) and Continuous Strength Method (CSM) were assessed for their applicability to the cross-section resistance predictions of the examined EHS stub columns.

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.

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.

Local buckling behaviour of Q690 high strength steel cold-formed round-ended oval hollow section stub columns under axial compression: Experimental investigation, numerical modelling and design

This study presents a comprehensive investigation of Q690 high strength steel (HSS) cold-formed round-ended oval hollow section (ROHS) stub columns subjected to concentric compression, utilising a combination of experimental and numerical analyses. Material properties were determined through tensile coupon tests. Residual stresses in both membrane and bending types were measured and recorded and their impact on structural behavior was evaluated. Initial local geometric imperfection measurements were conducted for all specimens. In addition to experimental studies, the numerical investigation was conducted to develop a finite element (FE) model of Q690 HSS cold-formed ROHS stub column. The accuracy of the FE model for HSS cold-formed ROHS stub column was validated through a comparison of the numerical predictions with the experimental test results. An extensive parametric study was conducted to produce structural performance data for HSS cold-formed ROHS with an expanded range of cross-sectional slenderness, aiming to enhance the existing data repository. The cross-sectional resistance of stub columns obtained from experimental and numerical investigations were compared with the design strengths predicted by the advanced design methods of Direct Strength Method (DSM) and Continuous Strength Method (CSM). The comparison indicate that the existing design methods provide over-conservative and unreliable cross-sectional resistance predictions. Modifications on DSM and CSM are proposed, based upon which notable improvements in cross-sectional resistance in predictions are obtained in a reliable manner.

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.

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.

DSM design of cold-formed steel fixed-ended lipped channel columns undergoing distortional-global interaction

This work reports a numerical investigation dealing with the post-buckling behaviour, strength, and Direct Strength Method (DSM) design of cold-formed steel fixed-ended lipped channel columns experiencing coupling between distortional and global (flexural-torsional) buckling − distortional-global (D–G) interaction. Various cross-section dimensions and lengths are considered, to ensure that the columns selected undergo different levels and types (“true”, “secondary-bifurcation distortional” or “secondary-bifurcation global”) of D–G interaction. The results presented and discussed, determined by means of Abaqus shell finite element geometrically and materially non-linear analyses, consist of elastic and elastic-plastic post-buckling equilibrium paths, failure loads and collapse modes. The steel material behaviour is deemed elastic-perfectly plastic and several yield stresses are considered, thus making it possible to cover a wide column D–G slenderness range. Particular attention is devoted to identifying the most detrimental initial geometrical imperfection shapes, in the sense that they lead to the lowest column failure loads. The numerical failure load data gathered are subsequently used to assess the merits of the available DSM-based design approaches developed to handle cold-formed steel columns undergoing D–G interaction. Since these design approaches are shown to be either inefficient and/or improvable, the above failure load data are also used to propose modifications/improvements aimed at achieving an efficient failure load prediction in the specific context of fixed-ended lipped channel columns. The success of this endeavour provides encouragement to the authors in their search for a safe, accurate and reliable DSM-based design approach capable of handling cold-formed steel columns with arbitrary cross-section shapes and/or end support conditions that fail in D–G interactive modes.

Moment capacity of perforated cold-formed aluminium channels–Tests, analysis, and design

This paper investigates the moment capacity of perforated cold-formed aluminium alloy (CFAA) channel sections through a combination of experimental and numerical studies. In total, 12 new experimental tests were conducted on CFAA with different cross-section sizes, web hole spacing, and numbers of web holes, under pure bending, followed by a numerical study comprising 601 validated finite element (FE) models. In the numerical study, the effects of various parameters were examined, namely web hole size, the number of web holes, section thickness, web depth, and the grade of aluminium alloy. The moment capacities obtained from both the experimental tests and finite element analysis (FEA) were utilized to evaluate the design guidance provided in the Aluminium Design Manual (ADM), Eurocode (EC), Australian and New Zealand Standard (AS/NZ), and the American Iron and Steel Institute (AISI) standards. Along with the Direct Strength Method (DSM), the Continuous Strength Method (CSM) was also evaluated. The results from both the experimental tests and their FEA indicate that DSM and EC9 are slightly unconservative, by 1% and 9%, respectively. In contrast, ADM is found to be conservative by 10%, whereas CSM is found to be unconservative by 50%.

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.

Eccentric compressive performance and design of asymmetric steel columns with complex edges

Box-type houses often use asymmetric cold-formed thin-walled steel columns with complex edges to meet structural requirements. However, there is a need to clarify the mechanical properties of eccentric loading in this section of the steel column. This study conducted eccentric compression tests on 24 asymmetric cold-formed thin-walled steel columns with complex edges, clarifying the specimens' ultimate load-bearing capacity and buckling mode. The results showed that the resistance to pressure provided by the corner was higher than that provided by the two crimped edges. Subsequently, a finite element model was established, and its accuracy was verified by comparing it with experimental results. Finally, based on the formula for calculating the nominal axial strength in AISI, the direct strength method formula for the stable bearing capacity of asymmetric cold-formed thin-walled steel columns with complex edges under in-plane eccentric compression was proposed. This method takes into account the local-global interactive buckling and distortional-global interactive buckling. The comparison between the theoretical calculation results and numerical simulation results demonstrated that the design method proposed in this study is effective and conservative. It can provide a reference for future engineering applications.

Interactive buckling behaviour of Q420–Q960 steel welded thin-walled H-section long column

Considering the broad application prospect of high-strength steel (HSS) in engineering structure, and the limitations present in existing studies on interactive buckling of HSS welded thin-walled box-section long column, both test and finite element analysis were employed to investigate the interactive buckling behavior exhibited by 12 specimens fabricated from Q420–Q960 steels. The detailed analysis considered various aspects, including the failure mode, axial deformation, interaction of local and overall buckling deformation, and the ultimate bearing capacity. Observations revealed that the utilization rate of steel strength diminished with an escalation in both the plate width–thickness ratio and steel strength. An increase in the plate width–thickness ratio correlated with an earlier onset of local buckling, and the influence of steel strength was found to be negligible. More importantly, the limit of the plate width–thickness ratio for columns undergoing interactive buckling increased with a rise in column slenderness and decreased with an increase in steel strength. Taking into account the influence of column slenderness and steel strength, a novel calculation formula for determining the limit of the plate width–thickness ratio was derived. It is noteworthy that Eurocode 3 and JGJ/T 483–2020 were found too conservative for calculating the ultimate bearing capacity, while ANSI/AISC 360–22 was found suitable. Additionally, a calculation method for the ultimate bearing capacity of Q420–Q960 steel welded thin-walled H-section long column was proposed based on the direct strength method.

Numerical investigation on post–fire resistance of cold–formed Q960 ultra high strength steel channel section stub columns

Numerical analyses on the residual resistant performance of cold–formed Q960 ultra high strength steel (UHSS) channel section stub columns after fire exposure were reported. The proposed numerical modelling method was validated by the existing experimental results of the cold–formed EN 1.4420 austenitic stainless steel, S690 high strength steel (HSS) and S960 UHSS channel section stub columns at room temperature, and the hot-rolled EN 1.4301 austenitic stainless steel channel section stub columns after exposure to fire, where the ultimate resistance, load–end shortening curve and failure mode were considered. The satisfactory comparisons demonstrate the appropriateness of the proposed numerical modelling methodology for the Q960 UHSS channel section stub columns after fire exposure. A parametric study considering different geometric dimensions and exposure temperatures was conducted, where a total of 410 numerical models were included. The geometric dimensions of cold–formed Q960 UHSS channel section stub columns changed the effects of exposure temperature on the load–end shortening curve. For the conditions after exposure temperature less than 600 °C, the slenderness limit of Class 3 in European code could be used for cold–formed Q960 UHSS channel section stub columns. When the exposure temperatures were at 700, 800 and 900 °C, the slenderness limit of Class 3 became unsuitable. The accuracy of design approaches in EN 1993–1-12, AISI S100, AS/NZS 4600 and direct strength method (DSM) was assessed. When the exposure temperatures were at 800 and 900 °C, there were noticeable differences between the numerical results and results obtained from the abovementioned design approaches. The modification methods considering effects of exposure temperature and geometric dimensions were proposed for the EN 1993–1-12 design approach to predict ultimate resistance of the cold–formed Q960 UHSS channel section stub columns after different exposure temperatures.

Optimization of cold-formed steel beam cross-sections for pallet rack storage systems

This paper aims to obtain optimized cold-formed beam cross-sections for industrial pallet rack storage systems. The optimal sections were obtained by applying fitness functions aimed at reducing cross-sectional area and/or displacement and solving them using the Genetic Algorithm. These functions imposed a behavioral constraint so that all optimized solutions had the same resistant bending moment. The calculation of the characteristic resistant bending moment was carried out according to the direct strength method, with load factors defined by the elastic stability analysis with CUFSM, in an analysis routine implemented on the MATLAB platform. Four parameterized sections were studied, verifying the optimum set of height, width and stiffening elements that met the objectives defined by functions, assuming reference values of a rectangular hollow section. Manufacturing constraints were also taken into account. The results show that it is possible to manufacture cold-formed beams that consume less material and/or have a better moment of inertia in relation to the reference section, without compromising the strength of the structural element.

Assessment of design models for aluminium alloy CHS beam-columns with stocky cross-sections

There is little information on the prediction accuracy of the international design standards for aluminium alloy circular hollow section (CHS) beam-columns. To bridge this gap, this paper presents a comprehensive numerical investigation on the performance of aluminium alloy CHS beam-columns with stocky cross-sections. A non-linear finite element (FE) model was developed and validated against experimental results. The validated FE model was employed to conduct an extensive parametric study of aluminium CHS members subjected to combined axial compression and bending over a range of lengths, cross-sections and applied eccentricities. The effect of these parameters was investigated for two material grades, 6061-T6 and 6082-T6, resulting in a total of 168 FE models. The numerically obtained capacities were used to assess the accuracy of the strength predictions of Eurocode 9 (EC9), the Aluminium Design Manual (AA) and the Direct Strength Method (DSM). On the basis of the results, EC9, with the exponent of the interaction formula, ψc, calculated as a function of the buckling reduction factor χ, was found to provide the most accurate strength predictions with a mean value equal to 1.01 for both examined material grades. The most conservative predictions were given by the DSM, whilst the most scattered predictions by the AA, with a Coefficient of Variation (COV) equal to 0.17. Finally, based on the DSM buckling strength and the plastic cross-sectional moment resistance, a new interaction curve is proposed for CHS beam-columns with stocky cross-sections.

Behaviour and design of cold-formed high-strength steel built-up beams

The adoption of high-strength steel in the construction sector can provide potential advantages, such as supporting large-scale structures with small cross-sectional sizes leading to decreased self-weight and improved space availability. In this paper, detailed numerical investigations are presented to investigate the nonlinear behaviour of cold-formed high-strength steel (HSS) built-up beams subjected to in-plane flexure along with their bending moment capacities, deformed shapes, and moment-curvature curves. Finite element (FE) models incorporating the initial geometric imperfections of cold-formed sections and material nonlinearities were established and validated against the experiments presented in the literature by comparing the moment-curvature curves, deformed shape, and ultimate bending moment capacities. Following validation, the calibrated FE models were utilized to perform extensive parametric studies to generate further numerical data by varying three different parameters: cross-sectional yield strengths (S700, S900 and S1100), geometries (such that the local slenderness, λl values varied from 1.07 to 4.65) and cold-formed steel (CFS) sheet thicknesses (1.0, 1.5, 2.0, and 2.5 mm). The specimens considered in the present study were restrained to inhibit lateral-torsional buckling and demonstrated failure owing to local buckling. The applicability of the design principles for predicting bending capabilities was assessed using numerical data in accordance with direct strength method (DSM) of AISI codal provisions and continuous strength method (CSM). The obtained numerical results demonstrated that the existing standards lack precise design procedures for cold-formed HSS built-up members and additional research is still required to provide accurate design procedures. Therefore, an improved rational extension of DSM of AISI codal provisions and CSM is recommended to predict the ultimate bending moment resistance.

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.

Improving the local and distortional resistance of CFS trapezoidal self-supporting roof members

This paper reports the development, numerical implementation and application of a methodology to perform the multi-objective optimisation, with respect to local and distortional failures, of cold-formed steel (CFS) trapezoidal cross-section shapes used in members belonging to self-supporting roof systems and manufactured by bending the steel sheets with roll-forming machines. Initially, a brute-force approach is adopted to search for optimal unstiffened cross-section shapes of members subjected to positive (sagging) and negative (hogging) bending moments. Next, an optimisation procedure based on a Genetic Algorithm (GA) is employed to find stiffened cross-sections that (i) outperform the original (plain/unstiffened) ones, as far as the resistance against local and distortional failures is concerned, and (ii) can be fabricated with minimal additional cost, through the sole addition of small intermediate stiffeners, whose possible locations are pre-defined in accordance with a map of executable fold-line positions – the original cross-section typology, overall dimensions and wall angles are retained. The design approach adopted to estimate the member ultimate strengths is based on the Direct Strength Method (DSM), and Generalised Beam Theory (GBT) is used to calculate the elastic buckling stresses due to bending. The application and capabilities of the proposed methodology are illustrated, by means of numerical results presented in terms of cross-section families belonging to optimal Pareto Fronts – an excellent performance of the procedure was found in all cases.