This study investigated the material properties of cold-formed ferritic stainless steel grade EN 1.4003 rectangular hollow sections (RHS) at elevated temperatures. A test program consists of 55 tensile coupon tests was carried out. Both steady state and transient state test methods were employed. The steady state tests covered temperatures ranged from 22 to 900 °C for 13 different temperatures, and a total of 11 stress levels were selected in the transient state tests. Tensile coupons were extracted from cold-formed ferritic stainless steel RHS 80×60×4 and RHS 100×40×2 with nominal plate thickness of 4mm and 2mm, respectively. Key material properties obtained from the experimental study were compared with predictions from the current European code and design recommendations by previous researchers for ferritic stainless steel. The comparisons revealed that the existing design rules could not accurately predict the material properties of the cold-formed ferritic stainless steel RHS at elevated temperatures. A new set of predictive equations is proposed, which shows good agreement with the test results and is recommended for determining the material properties of the studied ferritic stainless steel at elevated temperatures.

Cold-formed stainless steel structural members, characterised by exceptional durability and high reusability, offer a promising pathway toward a low-carbon and sustainable construction sector. This study experimentally and numerically investigated the structural performance of cold-formed stainless steel built-up open section beams with circular web holes. Firstly, the built-up section specimens were fabricated by assembling edge-stiffened channel components back-to-back using self-drilling screws, which were brake-pressed from EN 1.4301 austenitic stainless steel plates with nominal thicknesses of 1.0, 1.5, and 2.0 mm. Circular web holes were introduced at the midspan of the built-up section specimens, with hole diameter-to-web height ratios ranging from 0.25 to 0.75. Four-point bending tests were conducted to examine the effect of circular web perforations on the behaviour of cold-formed stainless steel built-up open section beams. The experimental results were obtained in terms of moment-curvature response, failure mode, and moment capacity. Subsequently, by calibrating against the four-point bending test results, a nonlinear finite element model was established and used to perform extensive parametric study. An additional 108 numerical data were generated for austenitic stainless steel built-up open section beams with circular web holes, covering broader ranges of geometric dimensions, cross-sectional slenderness, and hole diameter-to-web height ratios. Furthermore, the suitability of design provisions specified in ASCE/SEI 8-22 was evaluated for EN 1.4301 austenitic stainless steel web-perforated built-up section members subjected to bending. It was revealed that the codified direct strength method generally underestimated the flexural strengths of cold-formed austenitic stainless steel built-up open section beams with circular web holes.

This paper proposes a novel high strength steel square hollow section with semi-circular stiffeners, aiming to delay or eliminate premature local buckling failure of the steel tube and lead to improved compressive performance. A total of 9 stiffened and unstiffened stub columns were tested to investigate the failure modes and compressive responses. The test results showed that the stiffened sections achieved significantly higher compressive strength with delayed local buckling failure compared to the unstiffened counterparts. Tensile coupons were extracted from flat, corner and curve portions of the steel tubes in order to determine the corresponding material properties. Finite element (FE) models were established to numerically simulate the tested stub columns, in which both failure modes and load-end shortening curves could well replicate the test results. Parametric studies were then implemented using the validated FE model to extend the dataset. The obtained test and numerical results were used to assess the applicability of existing design standards and methods. It was found that EC3, ANSI/AISC 360-22, AS 4100 and Direct Strength Method (DSM) could be safely adopted in cross-section classifications and cross-section capacity predictions.

An experimental investigation to examine the buckling behaviour of cold-formed stainless steel built-up section columns is presented in this paper. Stainless steel sheets of high-strength austenitic grade S35657 and duplex grade S22053 (equivalent to AISI 2205) were adopted to brake-press unlipped and lipped channel components, which were employed to assemble built-up closed section members using self-plugging rivets. A total of 26 fixedended column tests were conducted to examine two built-up closed sectional configurations consisting of two unlipped channels or one lipped and one unlipped channels, two nominal plate thicknesses of 1.5 and 2.0 mm, three nominal member lengths of 600, 1200 and 1800 mm, as well as fastener spacing varying from 100 to 400 mm and two different stainless steel types. Based upon the experimental results obtained from this study, the effects of stainless steel type, sectional configuration, member slenderness and fastener spacing on failure modes and loading capacities of the cold-formed stainless steel built-up closed section members experiencing axial compression were discussed. By evaluating against the ultimate loads obtained from the column tests, it was found that the design rules provided in the Chinese Standards and American Specification resulted in conservative strength predictions by averages of 14% and 19% on grade S35657 as well as 29% and 17% on grade S22053, respectively, for the fixed-ended cold-formed stainless steel built-up closed section columns.

This study investigates the plastic seismic design of moment-resisting frames using concrete-filled steel tube columns (MRF-CFST) by utilizing the theory of plastic mechanism control (TPMC). Fundamentally, within the rigid-plastic analysis framework, the TPMC relies on a kinematic approach and an equilibrium curve for mechanisms. Employing this theory in the design of steel frames ensures the formation of a global collapse mechanism while deterring undesirable occurrences like a soft story. Using the TPMC, the kinematically admissible multiplier for seismic horizontal forces linked to the global mechanism is the minimum among all the kinematically admissible multipliers for other generic mechanisms. In this study, three methods were used to address the TPMC conditions: (a) Story-based method in which only the sum of the required plastic moments for columns in each story are unknown-identified as Design Method 1 (DM1); (b) Story-based method in which the sum of required plastic moments for both beams and columns in each story are unknown-denoted as Design Method 2 (DM2); and (c) Member-based method in which the demands for all members are unknown-denoted as Design Method 3 (DM3). A linear programming (LP) problem has been formulated for all design methods that incorporate the TPMC conditions as constraints. The DM1 used in previous studies serves as the conventional method for solving the TPMC conditions. A comparison between the three design methods has been conducted. The results show that DM3 stands out as the superior method, effectively minimizing weight and maximizing the strength of structures. Meanwhile, DM2, despite having fewer unknown parameters compared to DM3, yields the results that are comparable to DM3. To show the practical application of the proposed design methods, seismic design of three MRF-CFST with 5, 10, and 15 stories has been conducted. In order to verify the seismic design goal of achieving a global mechanism, both pushover analysis and incremental dynamic analyses (IDA) were utilized. The results illustrate the successful establishment of a global mechanism and the outstanding seismic performance of the designed structures.

This paper aims to investigate the structural behavior of cold-formed stainless steel (CFSS) elliptical hollow section (EHS) members under combined axial load plus biaxial bending by using numerical method. By using a validated finite element (FE) model for cold-formed (CF) tubular beam-column members, an extensive parametric study was conducted in this study. In order to cover various stainless steel alloys, a wide range of crosssection geometries, aspect ratios, cross-section slenderness values, member slenderness values as well as loading angles and eccentricities of the CFSS EHS beam-column members were examined. The FE predicted ultimate loads were compared with the design strengths predicted from the current interaction design expressions for CFSS structures specified in prEN 1993-1-4:2023, AS/NZS 4673 and ASCE/SEI 8-22 together with their codified slenderness limits for CFSS circular hollow sections (CHS). The Direct Strength Method (DSM) was recently proposed in the literature to predict the compression and bending resistances of CF normal grade steel EHS subjected to combined actions. The design expressions in AS/NZS 4673 and ASCE/SEI 8-22 associated with the DSM end points were also evaluated for CFSS EHS beam-column members. As the existing versions of Continuous Strength Method (CSM) were only suitable for CFSS square (SHS) and rectangular (RHS) hollow section as well as CHS members subjected to axial load plus uniaxial bending. Therefore, in this study, a modified CSM was proposed for the design of CFSS EHS beam-column members, by developing new CSM end points for CFSS EHS as well as the modified CSM interaction factors and modified CSM design formulae for CFSS EHS subjected to axial load plus biaxial bending. The comparisons revealed that the existing codified interaction design curves provided scattered and conservative predictions. By adopting the Chen and Young's modified DSM to predict the end points, the accuracy of the design formulae in AS/NZS 4673 and ASCE/SEI 8-22 can be highly improved. However, the proposed modified CSM can achieve more accurate and reliable design predictions for CFSS EHS beam-column members than the existing codified methods and DSM.

Steel modular construction is gaining momentum for constructing high-rise buildings with demonstrated advantages including accelerated construction, enhanced quality, and reduced waste. However, limited intermodule connection systems have been developed for high-rise steel modular buildings and their structural behaviour has not been fully understood, which imposes a risk to the wide adoption of steel modular construction. This study aims to develop an innovative inter-module connection system for high-rise steel modular buildings and examine its structural performance under compression loads. This connection system was developed by considering both structural and constructional requirements, and its fundamental compressive behaviour was examined through laboratory tests and finite element analysis. Accurate finite element models were then established, and numerical simulations were followed for a parametric analysis with full factorial design that helped reveal the effect of various strengthening measures on the compressive behaviour. The results illustrated that the failure of the connection should be controlled by the load-bearing capacity of the regions near the openings. Furthermore, the adoption of high-strength steel and the involvement of stiffening ribs were found to significantly improve the compressive strength by approximately 100 % and 50 %, respectively. This study provides an innovative and practical solution to inter-module connections for high-rise steel modular buildings and contributes to a better understanding of the compressive behaviour of the connection system for inspiring connection designs.

Abstract Experimental investigation of cold‐formed austenitic stainless steel (CFASS) tubular T‐joints are presented in this paper. The CFASS tubular joints were fabricated by braces and chords with circular hollow sections (CHS). The CFASS tubes had five different cross sections (d×t in millimetre) of 60×2.8, 76×3.0, 114×3.0, 140×3.0 and 165×3.0, where d and t are the nominal outer diameter and thickness, respectively. Each of these sections was used as the chord member of the T‐joints. The corresponding brace member was paired by selecting the five nominal section sizes that had outer diameter smaller than or equal to that of the chord member. Hence, the geometric parameters of the CHS T‐joint specimens were varied, including the ratios of brace diameter‐to‐chord diameter ( β ), chord diameter‐to‐chord thickness (2 γ ), and brace thickness‐to‐chord thickness (τ). A total of sixteen CFASS CHS T‐joints were tested by applying axial compression through the brace without preloading in the chord. The structural behaviour of the CFASS CHS T‐joints in terms of load‐deformation curve, strength and failure mode were obtained. The test strengths were compared with the predictions by using design equations for carbon steel CHS T‐joints provided in CIDECT and EN‐1993‐1‐8. It is shown that these predictions are generally unconservative.

Lunar regolith has been demonstrated as the most easily available in-suit resource for lunar construction. Aluminium alloy, which is possible to be produced from the rich aluminium oxide in lunar regolith, can be used together with lunar regolith geopolymer to form composite members with promising structural performance. In this study, a lunar regolith simulant (LRS) CQU-1 was developed, followed by the preparation of LRS geopolymer with two strengths (G70 and G85) through alkali activation under the curing temperatures of 70 degrees C and 85 degrees C. The G70 and G85 could achieve the cylinder compressive strengths of 37.6 MPa and 57.2 MPa, respectively. The elastic moduli of G70 and G85 are 7.6 GPa and 8.4 GPa, respectively, which are obviously lower than that of normal concrete. A total of 23 hollow and LRS geopolymer-filled aluminium alloy circular hollow section (CHS) and square hollow section (SHS) stub columns was experimentally investigated. Parameters including cross section slenderness and infilled LRS geopolymer strength were examined. Test results revealed that both the strength and ductility of the composite section were significantly enhanced compared to those of the aluminium alloy tube or LRS geopolymer individual components, indicating the effectiveness of LRS geopolymer-filled aluminium alloy CHS and SHS columns. The applicability of existing design equations for steel-concrete composite structures as well as aluminium structures was evaluated with the test results obtained from this study. It was found that design methods using EN 1994-1-1 combined with Aluminum Design Manual, EN 1999-1-1 and GB 50429-2007, respectively, provided close predictions for the cross section resistance of the LRS geopolymeraluminium alloy composite columns.

This paper presents an experimental investigation on web crippling behaviour of cold-formed stainless steel square and rectangular hollow sections (SHS and RHS) at elevated temperatures. A total of 21 web crippling tests were conducted under the Interior Two-Flange (ITF) loading condition as codified in ASCE/SEI 8–22 Specification at various temperatures up to 800 °C. Tensile flat and corner coupon tests were conducted to obtain the material properties of the cold-formed stainless steel SHS and RHS at various temperatures corresponding to those pre-set in the web crippling tests. Details of the test specimens, setups and procedures are comprehensively documented in this paper. Furthermore, the specimen temperatures together with the test results, including failure modes, web crippling strengths and load-deformation curves are fully reported. The obtained test results were used to evaluate the suitability of codified web crippling design provision as per ASCE/SEI 8–22, where reduced material properties were used in calculating the web crippling strengths at elevated temperatures. In addition, the test results were also compared with the predictions obtained from existing design rules in literatures for cold-formed stainless steel SHS and RHS at elevated temperatures. Moreover, reliability analyses were conducted to assess the reliability levels of these design provisions. It is demonstrated that the available web crippling design provisions can provide generally conservative and reliable strength predictions, and therefore are deemed suitable for predicting the web crippling strengths of cold-formed stainless steel SHS and RHS at elevated temperatures.

Steel structures are often used in buildings due to their advantage in weight-to-strength ratio. However, their structural capacity deteriorates in fire as the temperature of the structures rises. Investigation of cold-formed stainless steel (CFSS) structures at elevated temperatures is still limited, especially for rectangular hollow section (RHS) beams having a single web hole in the mid-span (perforated web). Therefore, a numerical investigation was conducted to evaluate the current design provisions to calculate the strength of such beams at elevated temperatures ranging from 22 - 900 degrees C. A total of 400 specimens of stainless steel grades austenitic (EN 1.4301) and lean duplex (EN 1.4162) were considered. The investigation used finite element analysis (FEA) to simulate the behaviour of RHS beams with perforated web under pure bending. The finite element (FE) model was validated against a series of experimental results available in literature. The comparison between flexural strengths obtained from FEA with design values calculated from the current design rules showed that the design rules are conservative. However, they are not always reliable and safe for RHS beams without and with a perforated web for the two material grades at elevated temperatures. In this study, only the design rules specified by Eurocode 3 are shown to be reliable and safe.

This paper presents an experimental investigation on the performance of cold-formed stainless steel built-up closed section stub columns. The built-up sections were composed by assembling two channel components at the flanges, which were brake-pressed from high-strength austenitic and duplex stainless steel thin sheets. Three types of built-up sectional profiles were formed by connecting two unstiffened channels (i.e., unlipped channels), two edge-stiffened channels (i.e., lipped channels) as well as one unstiffened channel and one edge-stiffened channel, respectively, using self-plugging rivets. Four sectional series were devised to cover nominal overall web height-to-plate thickness ratio values of 50, 60, 67 and 80. In order to obtain the actual material properties considering the cold-work effect, tensile coupon tests were carried out on both flat and corner longitudinal coupon specimens. In addition, a total of 29 fixed-ended stub columns including 19 specimens without holes and 10 specimens with circular web holes were tested under axial compression. The experimental results involving failure modes, ultimate capacities and responses of load versus axial shortening were obtained and fully documented. The effects of sectional profile types, overall web height-to-plate thickness ratio and hole diameter-to-overall web height ratio on the performance of cold-formed stainless steel built-up closed section stub columns were examined. Moreover, the appropriateness of design rules as prescribed in the recently updated American Specification ASCE/SEI 8–22 was evaluated. It was revealed that the codified design provisions generally provided unconservative strength predictions for the high-strength austenitic and duplex stainless steel built-up closed section stub columns both without and with circular web holes. Reliability analyses were performed in this study, and the compressive strength predictions of the built-up closed sections according to the existing design rules were found to be unreliable for the high-strength austenitic stainless steel stub columns without holes as well as the duplex stainless steel stub columns without and with circular web holes.

Concrete-filled steel tubular members have been widely used in construction industry in recent years, and highstrength steel (HSS) is gaining its popularity as a construction material. This paper presents a numerical investigation of concrete-filled high-strength steel tubular members subjected to bending. An extensive parametric study with 144 specimens covering a wide range of geometric parameters was performed. The specimens consisted of square hollow sections (SHS) and rectangular hollow sections (RHS) with high-strength steel grades of 700 MPa and 900 MPa, and filled with concrete of grades C40, C70 and C110. The numerical results, together with test results reported in literature, were compared with design strengths calculated by existing design rules. Reliability analysis was conducted to evaluate the suitability of different design rules for concrete-filled highstrength steel tubular beams. The American Specification was shown to provide very conservative predictions of moment capacities for slender sections. On the other hand, theoretical plastic moment and European Code 4 with plastic moment assumption provide better predictions. Finally, a design method that incorporated confinement effect between the two materials was proposed. It is shown that the proposed design method provides more accurate and less scattered predictions compared to the aforementioned design rules.

With advantages including high productivity and sustainability, modular construction has attracted increasing interest in building project developments. As modular buildings are stepping higher, the modular systems should be better designed to ensure structural safety. In such systems, different types of connections, such as module-to-module (M2M) connections, play an important role in the loading transfer. However, the investigation into the rotational stiffness of the M2M connection was still insufficient in the literature. This paper aims to examine the effects of M2M connection rotational stiffness on the structural performance of high-rise steel modular buildings. A theoretical analysis was conducted on a hypothetic three-storey three-span steel frame to simplistically demonstrate the significant effect of connection rotational stiffness on the internal force distribution of modular buildings. To assess the applicability of preliminary findings in real-life high-rise steel modular buildings, validated numerical models were employed to determine the rotational stiffness of a newly proposed innovative and practical M2M connection. Subsequently, structural models of a 40-storey steel modular building with different M2M connection rotational stiffness values were established to conduct comprehensive investigations. The analysis results indicated that the rotational stiffness of M2M connections plays a critical role in determining the behaviours of critical structural members. The findings provide important references for the design of M2M connection stiffness and structural performance analysis of high-rise steel modular buildings.

This paper describes the experimental and numerical investigation on the flexural performance of cold-formed steel (CFS) zed section members bent about the neutral axis parallel to the flanges. In the test program, twelve pairs of zed section specimens fabricated from steel sheets of grades G450, G500 and G550 were loaded under four-point bending. Three series of sectional shapes, namely the zed sections with plain flanges as well as complex edge stiffeners consisting of double-fold inward and outward return lips, were devised for the test specimens. In the numerical investigation, the finite element model of four-point bending members, which was developed using ABAQUS and calibrated against the experimental results, was adopted to predict the behaviour of CFS unstiffened and edge-stiffened zed section beams over wide ranges of flange-to-web width ratio, lip-to-flange width ratio, return lip-to-lip width ratio and cross-sectional compactness. Furthermore, underpinned by the bending capacities acquired from the 12 experiments and 222 FE analyses in this study as well as 22 tests available in the literature, it was demonstrated that the current direct strength method (DSM) codified in the AISI S100 generally provided conservative flexural strength predictions for the unstiffened zed section members, while led to overall slightly unconservative design for the edge-stiffened zed section beams. In addition, based upon the DSM-based approaches that were addressed in the previous studies to account for the effect of local-distortional interaction, the nominal flexural strengths of the CFS zed section members with simple and complex edge stiffeners were found to be underestimated by 17% to 21% on average. Accordingly, the modified DSM formulae were recommended in this study for the CFS unstiffened and edge-stiffened zed section beams bent about the neutral axis parallel to the flanges.

The present study undertakes the post -fire stub column behaviour of cold -formed steel elliptical hollow sections (CFS-EHS) by experimental and numerical analyses. A total of 18 CFS-EHS stub column specimens made up of four cross-section series was firstly prepared, heated in a gas furnace as per the ISO -834 standard fire and then naturally cooled down to ambient temperature. Four fire exposure temperatures of 300 C, 550 C, 750 C and 900 C were adopted. Details of the experimental campaign, e.g., specimen preparation, test procedures and results, are presented. Accurate finite element (FE) model was established to mimic the experimental responses from various perspectives. Exhaustive parametric studies were subsequently conducted on 224 post -fire CFS-EHS stub columns to generate substantial data. The test and FE results were compared with the strength calculations using the equivalent diameter method and equivalent rectangular hollow section method developed for hotfinished steel elliptical hollow sections without fire exposure, the Direct Strength Method (DSM) stipulated in the American Specification AISI S100, as well as the modified DSM previously calibrated for CFS-EHS without fire exposure. By replacing the material properties obtained at ambient temperature with the post -fire material properties in the aforementioned design methods, it is shown that the equivalent section methods and the existing DSM overly underestimate the column strengths, whilst the modified DSM yields precise and reliable strength estimations. It is recommended to adopt the modified DSM for calculating the residual compressive strengths of CFS-EHS stub columns after exposure to the ISO -834 standard fire.

This study investigates the elastic and inelastic major-axis flexural buckling of cellular steel columns. Based on the stationary principle of potential energy, the existing elastic buckling load equation is refined to incorporate the local bending deformations at web posts. The column strength curve for determining the critical loads is then derived. For the elastic buckling, the refinement provides a significant improvement. For the inelastic buckling, the effects of residual stresses and initial geometric imperfections on the column strength are examined by the validated nonlinear finite element (FE) models. The initial geometric imperfection amplitudes of L/300 and L/500, where L is the column length, are recommended for global buckling of cellular steel columns fabricated from the parent steel shapes with h/b <= 1.2 and h/b > 1.2, respectively, where h/b is the depth-to-width ratio of the parent steel shapes. The developed column strength curve conservatively predicts the major-axis flexural buckling strength of practical cellular steel shapes, with an average prediction-to-FE buckling stress ratio of 0.97. A comparison also shows that the current EC3 and AISC360 equations are applicable for predicting the strength of practical cellular columns, provided that the refined elastic buckling load is adopted.

The structural behaviour of octagonal hollow section (OctHS) tubes under lateral tensile loads applied through blind-bolted T-stubs were experimentally and numerically investigated in this study. Two series of T-stub connections – single-side and double-side bolted T-stub connections, were tested to investigate the influence of the tube geometry and boundary conditions on the failure modes, deformation, and strength. Finite element (FE) models were developed and validated against the test results. Based on the validated FE models, parametric studies were conducted to further assess the influence of width-to-thickness ratios, tube length-to-width ratios and boundary conditions on the tube deformation and strength. Furthermore, the applicability of the tube deformation of 3% of a tube width to be the ultimate state criterion for OctHS tubes was evaluated using tube-face component shell FE models. A simplified two-dimensional analytical model was developed, and the calculation equations were proposed based on either small deformation assumption or large deformation assumption. The comparison of the strengths obtained from the proposed equations, numerical simulations and tests demonstrated that the proposed equations are able to give reasonable predictions on the yield and ultimate strengths of blind-bolted OctHS tube connections under tensile loading.

This paper presents the numerical investigation and design of cold-formed steel (CFS) built-up section beam -column members experiencing non-uniform minor axis bending. Detailed considerations of material nonline-arity and initial imperfections were involved in developing finite element models of the thin-walled built-up section members undergoing eccentric compression. The finite element models calibrated against the existing experimental results were then employed for a parametric study. Numerical predictions of 279 built-up section beam-columns over a broad spectrum of cross-sectional geometries and moment distributions along the member length were obtained in this investigation. For the built-up open and closed section members included in the numerical analyses, non-uniformly distributed moments resulted in the strength enhancements by average of 39% and 37%, respectively. Furthermore, based on the 279 numerical predictions obtained in this investigation and 91 data collected from the literature, the nominal strengths predicted by the design rules codified in the current standards and proposed in the previous studies were examined for the CFS built-up section beam-column members with end moment ratios ranging from-1.0 to 1.0. Underpinned by the assessment results, design recommendations and modified design rules were provided for the CFS built-up open and closed section members undergoing synchronous axial load and non-uniform minor axis moment, respectively.

The detailed numerical investigation and design of cold-formed S960 steel grade square bird-beak (SBB) T- and X-joints have been presented in this paper. The SBB joint is one of the novel bird-beak tubular joint configurations and obtained by rotating the chord member of a conventional square hollow section (SHS) joint along its centroidal axis by 45°. In this investigation, accurate finite element (FE) models were developed for SBB T- and X-joints using the tests carried out by the authors. The developed FE models successfully replicated the static strengths, load vs deformation curves and failure modes of test specimens. In order to gain an in-depth understanding on the static behaviour of SBB joints, a comprehensive FE parametric study was performed using the verified FE models. The joint failure strengths and joint ultimate capacities of a total of 220 SBB T- and X-joints specimens, including 200 FE specimens investigated in this study, were evaluated against the nominal strengths predicted from the literature and European code. All SBB T- and X-joints test and FE specimens were failed by the chord crown failure (C) mode. It has been shown that the design provisions given in the literature and European code are unsuitable and uneconomical for the design of cold-formed S960 steel grade SBB T- and X-joints investigated in this study. Therefore, accurate and reliable design equations are proposed in this study for predicting the static strengths of the investigated SBB T- and X-joints.