AISI S100 Research Articles

Experimental and numerical investigations on cold-formed titanium-clad bimetallic steel angle and channel section stub columns

Titanium-clad bimetallic steel (TCBS) exhibits outstanding corrosion resistance, and can be advantageously applied in corrosive environments. This study investigated an innovative application of TCBS in cold-formed stub columns. During the cold-forming process, TCBS demonstrated no discernible separation at the bonding interface of the Q235 substrate layer and TA1 cladding layer, indicating the maintenance of synergistic deformation between the cladding and substrate layers. The compressive behavior and ultimate resistance of cold-formed TCBS angle and channel section stub columns were investigated using both experimental and numerical methods. These tests included material tensile flat and corner coupon tests, initial local geometric imperfection measurements, and stub column tests. The stub column test comprised four cold-formed TCBS angle section stub columns and three cold-formed TCBS channel section stub columns covering the non-slender and slender cross-sections. The experimental results were utilized to validate the finite-element (FE) model, which was then used in a parametric study to obtain further numerical data at different cross-sections. A comparative analysis of the ultimate resistance from the tests and numerical analyses with the predicted results from design approaches in EN 1993-1-1, AISI S100, and the direct strength method (DSM) was revealed. The evaluation findings generally showed that the EN 1993-1-1 design method forecasts for cold-formed TCBS channel section stub columns were dangerous. The non-slender cross-section classification coefficient, which was inapplicable to cold-formed TCBS channel steel stub columns, was the primary cause of the above unsafe prediction. As for the design approaches in AISI S100 and DSM for cold-formed TCBS angle and channel section stub columns, as well as those in EN 1993-1-1 for cold-formed TCBS angle section stub columns, they were judged safe but conservative. The above prediction error was mainly caused by ignoring the enhancement in strength properties of the TCBS after the cold-forming process. The considerations mentioned above led to the proposal and demonstration of modifying recommendations that offered safe, accurate, and consistent design approaches to ultimate resistance for the cold-formed TCBS angle and channel section stub columns.

Flexural behaviour and design of hybrid cold-formed steel built-up I-section beams

The current study investigates the flexural performance and design of hybrid cold-formed steel built-up I-section beams, which are increasingly popular in the construction industry. These beams combine the benefits of both an "I" section and a closed-box section in their cross-section profile. They are constructed from two identical plain channels placed back-to-back with space between them and top and bottom cover plates, fastened together by screws. A nonlinear finite element (FE) model was developed to consider initial geometric imperfections, material and geometric non-linearities. The model was validated against the test results of cold-formed steel (CFS) built-up beams from the companion paper and literature. Once validation was successful, a comprehensive parametric study using finite element modelling was conducted to generate data on hybrid cold-formed steel built-up I-section beams across a broader range of cross-sectional slenderness, hybrid ratio, aspect ratio of the cross section and thickness of the cover plates. The parametric results were used to analyse the influence of key parameters on the moment carrying capacity and buckling modes. The applicability of the current Direct Strength Method in the AISI S100–16 specifications was evaluated based on these results, revealing that it is unconservative. Additionally, modified design procedures and equations were suggested for hybrid CFS built-up I-section beams vulnerable to local buckling in major axis bending, which were then evaluated through reliability analysis.

High strength steel unlipped channel sections subjected to ETF loading: Laboratory testing, numerical simulations and web crippling design

This paper presents web crippling and structural design of high strength steel unlipped channel sections under end-two-flange (ETF) loading based on experimental and finite element investigations. The experimental programme was composed of six grade S690 and four grade S960 test specimens, and the corresponding test set-up, procedures and results were reported in detail. The web crippling test results were used in the numerical modelling programme to validate the developed FE models, which were then adopted to perform parametric studies to enlarge parameter ranges of the bearing length, web slenderness and inside bend radius. Given that codified design rules for S690 and S960 high strength steel unlipped channel sections undergoing web crippling are absent, the corresponding normal strength steel design provisions, as stipulated in the European code EN 1993-1-3 and American specification AISI S100, were evaluated for their suitability to high strength steel unlipped channel sections. It is shown that the ultimate strength of high strength steel unlipped channel sections nonlinearly increases with increasing bearing length but with decreasing web slenderness and inside bend radius. The design methods of current EN 1993-1-3 and AISI S100 provide inaccurate and scattered resistance predictions for high strength steel unlipped channel sections. Finally, a modified AISI S100 design method and a slenderness-based design method were proposed, which can outperform the codified design methods.

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.

Tests, simulations and web crippling design of S690 and S960 high strength steel unlipped channel sections under end-one-flange loading

This paper presents experimental and numerical investigations into the web crippling behaviour and structural design of high strength steel unlipped channel sections under end-one-flange loading. An experimental programme including six grade S690 and four grade S960 steel test specimens was firstly carried out, and the corresponding test set-up, procedures and results were reported. The web crippling test results were subsequently used in a finite element programme to validate developed finite element models, which were then adopted to perform parametric studies to cover wider parameter ranges of the bearing length, web slenderness and inside bend radius. It is found that the ultimate web crippling strength of high strength steel unlipped channel sections increases with increasing bearing length but decreasing web slenderness and inside bend radius. Considering that relevant codified design provisions are absent, the design rules of normal strength steel unlipped channel sections, as prescribed in EN 1993-1-3, EN 1993-1-5 and AISI S100, were assessed for their suitability to high strength steel unlipped channel sections. The codified design methods provide inaccurate and scattered resistance predictions for high strength steel unlipped channel sections under end-one-flange loading. Thus, an improved AISI S100 design method and a novel slenderness-based design method were proposed, which can outperform the codified design methods.

Experimental and numerical investigations of high strength steel unlipped channel sections under interior-two-flange loading

This paper presents laboratory tests, numerical simulations and web crippling design of high strength steel unlipped channel sections under interior-two-flange loading. An experimental programme comprising 10 test specimens (with six for grade S690 high strength steel and four for grade S960 high strength steel) was firstly implemented, and test results including failure modes, full load-deformation curves and ultimate loads were reported. Subsequently, finite element models were constructed and validated against the obtained test results, and then used to conduct supplementary parametric studies to widen the examined parameter ranges of high strength steel unlipped channel sections. Considering that relevant codified design rules are absent, the applicability of the design rules of EN 1993–1-3, EN 1993–1-5 and AISI S100 for normal strength steel unlipped channel sections to their high strength steel counterparts was evaluated against the obtained test and numerical results. It is shown that the resistance predictions from the codified design provisions were generally inaccurate and scattered. Hence, a modified AISI S100 design method and a slenderness-based design method were proposed and demonstrated to be accurate, consistent and reliable for the web crippling design of high strength steel unlipped channel sections under interior-two-flange loading.

Effect of solid blocking on lateral bracing requirements for cold-formed steel wall studs

The effectiveness of horizontal bracing in increasing the strength of cold-formed steel (CFS) load-bearing walls has been widely acknowledged. However, while significant research has been conducted regarding the bracing requirements for CFS bearing walls, the effect of solid blocking, which prevents columns (studs) within the wall from rotating, has not been fully explored. In this study, we propose an analytical method to quantitatively assess the effect of solid blocking on bracing requirements for CFS load-bearing walls. This method is comprehensive as it can be applied to systems with different bracing patterns, load patterns, or non-identical columns (studs), and it accounts for the columns’ initial curvature and semi-rigid end connections. It has been verified that considering the solid blocking always decrease the bracing requirements because the existence of solid blocking increases the system’s stiffness and subsequently decreases the additional displacement as well as the brace force, as expected. An example consisting of 23 CFS studs is presented to illustrate how the effect of solid blocking on bracing requirements is influenced by the location and interval of solid blocking, the stiffness of column end connections, and the characteristics of tie bracing. The results indicate that the effect of solid blocking on bracing requirements increases when the location of solid blocking is closer to the anchor and the solid blocking interval is smaller. Moreover, a modification to the equation in AISI S100 is proposed to account for the effect of solid blocking on the strength requirement of tie bracing. Overall, this research contributes to a better understanding of the role of solid blocking in CFS load-bearing walls and provides insights for optimizing bracing design in practice.

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.

Numerical investigation of cold formed steel sleeve connection for channel sections subjected to combined bending and shear

This study focuses on the behaviour of the sleeve connection to ensure proper distribution of moment at the location of discontinuity between two channel sections subjected to combined bending and shear using finite element (FE) modelling. The FE model was validated with the experimental data from previous authors studies and was found. A total of 180 simulations were carried out as part of the parametric study, by varying the depth, thickness, length of sleeve connector, length of the test specimen, and bolt configuration. Two modes of failure were identified based on sleeve length (Ls) and combination of sleeve length and channel thicknesses. The Ls/D ratios were suggested to achieve a similar strength and stiffness of sleeve specimen when compared with control specimen. The influence of sleeve length on the ultimate moment resistance was studied for combination of sleeve length and channel thickness. The increase in the channel thickness from 2 mm to 2.5 mm led to as significant increase in the ultimate moment resistance. An equation was proposed to calculate the nominal moment capacity of sleeved specimens and reliability analysis of this equation was carried out. The conservativeness of the proposed design procedure was checked by comparing it with the AISI S100 [1] interaction curve.

Experimental investigation on axial compression behaviour of cold-formed thick-walled steel tubes

This paper presents an experimental and numerical investigation on the stability behaviour of cold-formed thick-walled steel tubes. Axial compression tests were conducted on 15 steel tubes with square and rectangle cross-sections to investigate their structural behaviours. The primary test parameters were slenderness and width-to-thickness ratios. Material properties, failure modes, load-deflection curves, and load-strain curves of the specimens are discussed. Furthermore, a numerical simulation was conducted to further investigate the effects of the slenderness ratio, width-to-thickness ratio, and initial geometric imperfection on the axial compression behaviour of cold-formed thick-walled tubes. Based on the experimental and numerical results, the applicability of the design codes, GB50018–2002, AISI S100–16, and EN 1993-1-1 was assessed. Results revealed that all three codes predicted the yield strength well after considering the cold-formed effect. With an increase in the slenderness ratio, GB50018–2002 and AISI S100–16 yielded increasingly unsafe predictions for overall stability, whereas EN 1993-1-1 yielded safe predictions. Finally, a modified design method for the overall buckling resistance of cold-formed thick-walled rectangular tubes under axial compression was proposed based on EN 1993-1-1 and verified using test and FEM results.

Mechanical properties of corner material in cold-formed steel structure: From normal strength to high strength

Cold-forming effect varies from normal strength steel to high strength steel due to the different chemical compositions and mechanical properties of parent materials. To date, research studies on material characterisation of cold–formed steels have been mainly focusing on the normal strength structural steel, while systematic research on high strength structural steel remains scarce. In this paper, an experimental investigation into changes in mechanical properties of high strength steel undergoing various levels of plastic deformation is firstly presented. The test results have been combined with other available high strength steel data, and compared to the normal strength steel data. Of the analysis results indicated, unified predictive equations, modified from Karren’s model in 1967 can predict the strength enhancement behaviours of normal strength and high strength steel, utilising the ultimate-to-yield strength ratio of parent materials fu,f/fy,f and inner radius-to-thickness ratio after cold–forming ri/t. Moreover, the decreases in the Young’s modulus, ultimate strain and elongation at fracture of normal strength and high strength steel after cold-forming can be predicted with no further grouping required. Statistical evaluation results confirm the accuracy of the proposed predictive expressions. This paper extends the scope of the strength enhancement equation in current north American cold-formed steel design code AISI S100–16 to high strength steels with grade up to 960 MPa, and provides key predictive material equations that can be further used in the design or numerical modelling of cold-formed steel structures.

Buckling resistance of axially loaded cold-formed steel built-up stiffened box sections through experimental testing and finite element analysis

The use of cold-formed steel (CFS) built-up sections in engineering practice is widely gaining popularity due to their ability to provide sustainable solutions and optimised section design opportunities. This paper presents an experimental and numerical investigation conducted on CFS built-up stiffened box section columns under axial compression. A total of 13 new experimental tests were conducted with specimens having an overall web depth up to 550 mm. Detailed nonlinear elasto-plastic finite element (FE) models were then developed and verified against the test results. Upon verification, a parametric study involving 144 FE models was carried out to investigate the effects of section thickness, column length, and screw spacing on the buckling resistance of CFS built-up stiffened box sections. The test and finite element analysis (FEA) results were compared with the buckling resistances determined by the American Iron and Steel Institute (AISI S100) and Australian and New Zealand Standards (AS/NZS 4600). It was found that the buckling resistances determined by the AISI S100 and AS/NZS 4600 for CFS built-up stiffened box sections were conservative by 9.3% on average, compared to experimental and FEA results. Modified Direct Strength Method (DSM) design equations for predicting the buckling resistances of CFS built-up stiffened box sections were then proposed, and a reliability analysis was conducted to assess the feasibility of the proposed equations.

Reliability of Rack Columns Designed by the Direct Strength Method

Cold-formed rack profiles are produced mainly for use in industrial storage systems. Their complex geometry and the presence of hole patterns along their length make the strength prevision of these structural elements difficult. This article evaluates the safety presented in the design of rack columns by adaptations of the Direct Strength Method (DSM), the original formulation of which does not address perforations. The safety parameters used here are the reliability indexes, calculated by the First Order Second Moment (FOSM) method, used in the calibration of the AISI S100 current standard, the First Order Reliability Method (FORM) and Monte Carlo Simulation. Only methodologies that applied the reduced thickness method in cross-section modelling produce results close to the target reliability indexes suggested for cold-formed columns. The target indexes are met for all load combinations when only distortional buckling failure is considered. However, the DSM adaptations are still imprecise when considering failure with local buckling. New professional factors were calibrated for these cases, as the design using the current ϕ = 0.85 does not meet the safety requirements.

Experimental and numerical investigation on cold-formed steel zed section beams with complex edge stiffeners

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.

Structural behaviour of cold-formed steel elliptical hollow section stub columns after exposure to ISO-834 fire curve

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 hot-finished 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.

Experimental and numerical investigation of cold-formed steel telescopic studs under compression

This paper presents a thorough experimental and numerical investigation into the structural behaviour of a novel cold-formed steel (CFS) telescopic stud system under axial compression. A total of 40 specimens were tested, considering variations in screw configurations, steel grades, and section thicknesses. The test results revealed that tilting and bearing led to failure in all screw connections. In comparison to the single-screw connection with a gap at the middle of the web, the telescopic studs with 2 screws in the web, 2 screws in the flanges, and 4 screws in both the web and flanges showed approximately 35–37% higher strengths. A nonlinear explicit dynamic finite element model was then developed and validated against the test results in terms of its connection strengths and failure modes. The experimental results were compared with the design strengths determined by the current design standards (AISI S100 (2016), AS/NZS 4600 (2018), Eurocode 3 (2006), BS5950–5 (1998) and GB50018 (2002).). The comparison results revealed that the AISI S100 (2016) and AS/NZS 4600 (2018) are conservative, underestimating the connection strengths of telescopic studs with different screw layouts by 1–13%.

Design recommendation of cold-formed steel built-up sections under concentric and eccentric compression

A parametric investigation and design proposal for the back-to-back built-up cold-formed steel (CFS) channel sections with V-stiffeners and Σ-stiffeners under concentric and eccentric loading are presented in this paper. A nonlinear finite element model considering the initial geometric imperfection was developed and validated against experimental results. A total of 594 simulations were conducted involving a variety of slenderness ratios, eccentric axis directions, eccentricity, screw spacing and number of longitudinal holes. Simulation results including the load-carrying capacity, failure modes, ultimate bearing capacity versus eccentricity relationships, and load versus lateral displacement relationships were discussed. The influence of several member parameters on the load-carrying capacity and buckling modes was analyzed. Additionally, the applicability of the design strengths calculated from Chinese standard GB50018–2002 and North American specifications AISI S100–16 was evaluated based on the preliminary experimental results and simulation results. It was shown that both methods are conservative. Finally, a proposed DSM-based interaction equation considering the relationship between P/Pnd and M/Mnd was proposed and evaluated.

Proposal to Improve AISI S100 DSM Design Standards for Cold-Formed Steel Built-Up Closed Cross-Section Columns Subjected to Local–Global Interaction Buckling

Proposal to Improve AISI S100 DSM Design Standards for Cold-Formed Steel Built-Up Closed Cross-Section Columns Subjected to Local–Global Interaction Buckling

Reliability of Cold-Formed Steel Sections Subjected to Web Crippling

This study aims to evaluate the structural safety of cold-formed steel sections subjected to concentrated loads in regions without transverse stiffeners, susceptible to web crippling, and dimensioned according to the North American Specification (AISI S100) and the Norma Brasileira (NBR 14762 2010) standard. Experimental data from various sections reported in the literature were analysed, obtaining professional factor statistics. Target reliability indices commonly used in the calibration of cold-formed steel standards were considered. A reliability analysis according to the First-Order Reliability Method (FORM) was developed for several action combinations found in the AISI and NBR standards. In addition to the scenarios prescribed by the AISI (2016), a scenario for the NBR 14762 (2010) standard is proposed, which, although it does not yet exist in the standard, was adopted for research purposes. In the proposed scenario, it was discovered that the resistance factor stipulated by the NBR 14762 (2010) standard did not produce satisfactory outcomes regarding a target reliability index of 2.5 in 10 out of 25 examined cases.

Closed built‐up cold‐formed steel columns under compression

AbstractCold‐formed steel (CFS) structural profiles are extremely versatile enabling the combination of multiple individual cross‐section shapes to fabricate a built‐up member presenting enhanced structural performance (higher load‐bearing capacity and torsional stiffness). Built‐up members have the potential to contribute to further expanding the current use of CFS products in the construction industry. However, the existing design codes, such as the EN 1993‐1‐3 and AISI S100, do not present specific design methodologies for the design of such members, mainly due to a limited understanding of the complex behaviour of built‐up CFS members combining multiple components. The behaviour of built‐up CFS columns comprising 4 individual shapes was assessed through experimental tests and numerical modelling. The interaction between the different components and the influence of spacing between connectors was investigated using the finite element method, providing recommendations concerning the effective use of fasteners to attain composite action between the individual shapes. The suitability of the design codes, namely, Eurocode and North American design predictions, following to Effective Width Method (EWM) and Direct Strength Method (DSM), respectively, were assessed for the built‐up members.