AISI Specification Research Articles

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

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

Experimental investigation on cold-formed Z-sections under two-flange loadings

This article presents the updated web crippling coefficients of cold-formed Z-sections under two-flange loading conditions. AISI Specification defines end-two-flange (ETF) and interior-two-flange (ITF) load cases under two-flange loading conditions. The web crippling capacity can be calculated by the empirical equation provided by AISI, Eurocode, and IS Specifications. This empirical equation is associated with web crippling coefficients. This coefficient is not updated for the Z-section under two-flange loadings after the development of the AISI test standard for web crippling investigations. Hence, the authors conducted an experimental investigation of cold-formed Z-sections using 28 tests under ETF and ITF load cases. The experimental investigation is carried out according to AISI guidelines. The result of the experimental work is validated with finite element analysis (FEA) results. The web crippling capacity obtained from experimentation and FEA are compared with AISI, Eurocode, and IS Specifications. It is observed that web crippling strength prediction from cold-formed specifications is over-conservative under both ETF and ITF load cases. This research also highlighted the effect of higher web height to thickness and inside bend radius to thickness ratio. Hence, the authors suggested new web crippling coefficients that can better predict the web crippling capacity of Z-sections according to the AISI Specification.

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

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

Numerical simulation and specification provisions for cruciform cold-formed steel built-up columns

This paper addresses the nonlinear behavior of cruciform cold-formed steel (CFS) built-up columns formed by high-strength bolts and filler plates under axial compression. Nonlinear finite element modeling using ANSYS software is developed to simulate the behavior, including both geometric and material nonlinearities, and the numerical results are compared with the test results obtained by the authors and reported in another paper, showing good agreement. The finite element model in this paper predicts the ultimate loads and buckling modes of the specimens well. Following validation, the C89 series cross-section was selected for numerical and theoretical study using a varying slenderness ratio λ, plate thickness t and bolt spacing s. The results are compared with the nominal axial resistance calculated using the existing design standards, including the direct strength method (DSM) in the AISI Specifications and the effective width method (EWM) in the provisions of Chinese code GB50018-2002. The results show that the current AISI specifications have many overestimated resistance predictions for the cruciform CFS built-up columns, and the error decreases with the increase in the slenderness ratio, while the provisions in the Chinese code are overly conservative for predicting the resistance values of the cruciform CFS built-up columns. Therefore, a design procedure based on the effective width method is proposed for the resistance evaluation of axially compressed cruciform steel built-up columns.

Parametric study on cold-formed steel built-up columns using direct strength method

The use of built-up cold-formed channels in the metal lightweight industry increases for higher strengths offers in comparison with a single-channel section. However, their unavailability of design guidelines determining the axial strength had made most researchers seeking to formulate new designs rules for different built-up sections. This research aims to convey parametric study on built-up columns using a proposed Approach based on the direct strength method for further knowledge and to understand the behavior of cold-formed steel built-up columns. For that a nonlinear finite element was developed to perform a parametric study, a total of 93 finite elements models were analyzed to study the effects of the cross-section, the gap between the two channels, the spacing between battens, the addition of web stiffener, the angle variation in C-sections simple lip edge stiffener, different lengths of the profiles and the ratio (d/D), on the stability of built-up columns. An approach based on the direct strength method (DSM) was proposed, where elastic, local, distortional or global elastic buckling solutions are determined from finite element or finite strip procedures. The obtained results were compared with the current design standard the North American specifications and the Australia/New Zealand codes, the AISI specifications, and European code EC3. The results of the parametric study were compared with the values found by AISI, EC3 and proposed DSM approaches, it stated that, the design specification noted in the AISI are generally conservative for overall buckling mode and slightly conservative for local buckling mode columns. For EC3 specifications, there is a difference between open and closed sections. For open sections, the results are conservative for local buckling mode and slightly conservative for global buckling mode. For the proposed DSM approaches, the results are slightly conservative for global buckling columns and non-conservative for local buckling mode columns.

Design of cold-formed steel built-up open section members under combined compression and bending

The numerical investigation on cold-formed steel built-up open section members under combined compression and minor axis bending is described in this paper. Through validating against experimental results in the literature, a rational finite element model of built-up section beam–column members was developed. The verified numerical model was employed for a comprehensive parametric study to obtain the results of 280 eccentrically loaded built-up section members. Furthermore, the applicability of the design formulae provided in the AISI Specification, AISC Specification, AS/NZS Standard and European Code was examined by comparing nominal strengths with loading capacities of the 280 numerical data produced in this study and 33 existing test data. Overall, the strength predictions determined by the codified interaction formulae based on the nominal compressive and flexural resistances calculated by the current direct strength method (DSM) were found to be highly scattered and underestimated by average of 17% – 27% for the built-up open section components experiencing combined loading. Hence, the modified DSM was recommended to determine the nominal resistances of the members under isolated compression or bending for more accurate endpoints of the load–moment interaction curve. In addition, a bi-linear interaction expression was proposed in this study for the design of the cold-formed steel built-up section beam–columns, which offered better strength predictions in accuracy and consistency compared to the codified interaction formulae.

Experimental and numerical investigation of fixed-height cold-formed steel wall assemblies bearing on concrete slabs

Cold-formed steel bearing walls are frequently installed on concrete slabs, which do not provide perfectly rigid and uniform conditions. However, existing design guidance assumes bearing condition has no impact on axial capacity, though cold-formed steel compressive members are particularly susceptible to these end conditions. Additionally, studs are typically installed at the edge of the concrete slab, which is prone to spalling and geometric irregularities, and stud walls are occasionally installed inadvertently overhanging from them. In this paper, the impact of non-uniform and partial bearing conditions on the axial strength of cold-formed steel (CFS) wall assemblies is identified and characterized experimentally and numerically. Studs of various cross-sections and bearing conditions were considered. Bearing conditions include: full bearing (edge distance ≥ 20.32 cm (8 inches)), close to the edge, at the edge, and partially overhanging from the slab. In addition to the experiments, high-fidelity finite element modeling of all the systems was conducted to validate the experimental results and elucidate the impact of parameters not captured during the tests. The results provide a means of evaluating existing design guidelines presented in the North American Specification of the American Iron and Steel Institute (AISI S100-16). Twenty-seven tests were conducted on CFS wall assemblies. They reveal that there is a potential need for improvement of current AISI specifications, where all bearing conditions are assumed rigid and uniform. Design recommendations for improving the current specification are discussed.

Tests and design of built-up section columns

Built-up sections are increasingly used as structural elements in the cold-formed steel (CFS) industry. They are composed of two or more component sections connected by discrete fasteners, typically spaced evenly along the length and potentially with fastener groups at the ends. Conventionally, two singly symmetric C-sections are connected to form a doubly symmetric cross-section, and current design guidelines are limited to this particular application. As a means towards broadening the application of built-up CFS sections, the cross-sections in this study were composed of three or four lipped channel sections. The paper presents an experimental investigation of the strength and behaviour of built-up section columns of various lengths and cross-sectional shapes. As observed in the tests, singly-symmetric columns composed of three channel sections (3C) experienced either local, distortional and/or flexural-torsional buckling failure modes, while doubly-symmetric columns formed by four sections (4C) failed in local, distortional and/or flexural buckling modes. The current AISI Specification specifies the use of a modified slenderness ratio only for built-up sections composed of two sections connected back-to-back, whereas the design proposal in this paper recommends the use of an effective rigidity approach coupled with the Direct Strength Method for predicting the strengths of the test 3C and 4C built-up sections. This proposal also suggests a design procedure for the built-up sections experiencing flexural-torsional buckling, which is neither explicitly stipulated in current design standards nor in previous studies.

Parametric study and Improved design guidelines of CFS battened built-up columns

This paper presents the parametric study on the strength and behaviour of CFS closed built-up battened columns. The CFS closed built-up battened column consists of two lipped channels placed face-to-face with a uniform spacing of battens. Finite element models are validated with the results of the test specimens available in the thesis of the first author. The parametric study on 176 cold-formed steel built-up column sections is extended by using the validated finite element modeling covering a wide range of global column slenderness, plate slenderness and yield stress. The numerical results from the parametric study are compared with the design strengths calculated by the European specifications (EN1993-1-3:2006) and AISI Specifications (AISI S100:2016). The comparison of parametric results with the design strength predictions has indicated the design guidelines of the specifications need improvement. The design strength predictions of the specifications are also assessed by conducting the reliability analysis. Therefore, the exiting design rules are modified to improve the accuracy of the design strength predictions for the CFS closed built-up battened columns subjected to the axial compression. Furthermore, the reliability of the proposed methods is confirmed by means of reliability analysis.

Recommendations for design of sheathing bracing systems for slender cold-formed steel structural members

The use of sheathing material as a structural component in the Cold-formed steel (CFS) construction holds great potential for stability of the structure and savings in the construction cost. However, there is no robust design method to account the structural contribution of sheathing boards. This paper presents the experimental results of 107 (including unsheathed and sheathed) CFS wall frame studs subjected to out-of-plane loading to study the feasibility of using gypsum board as a structural bracing sheathing material. The test parameters include various shapes and slendernesses of the CFS frame stud, thickness of the sheathing board and spacing between the sheathing bracing connections. The out-of-plane loading is applied as it causes lateral torsional buckling in the CFS structural members thereby creates pull-through failure at the sheathing bracing connections. Moreover, the suitability of the current AISI and Eurocode specification for the design of sheathing braced CFS structural member is studied. The experimental results indicate that the lateral buckling of the symmetric shaped (against the loading axis) CFS wall frame studs can be inhibited by gypsum sheathing. Whereas most of the singly symmetric and point symmetric CFS studs exhibit lateral torsional buckling and biaxial bending, respectively, due to the inadequate bracing effect of gypsum sheathing resulting in pull-through failure at the sheathing bracing connections. Therefore, a set of limitations for the use of gypsum sheathing as a structural bracing is suggested in the form of a generalized design parameter. Finally, the experimental results indicates that the design strength of the CFS wall frame stud can be increased in the range of 39% to 595% based on the shape and slenderness.

Compressive Strength of Axially Loaded Built-up Sigma Cold Compressive Strength of Axially Loaded Built-up Sigma Cold Formed Sections Columns Formed Sections Columns

The use of cold formed steel sections in building constructions has lately seen rapid growth due to their light weight and easier erection. Generally, lipped channel sections are used more frequently .Sigma sections while requiring more effort in fabrication, they exhibit high strength to weight ratios compared with the lipped channel sections. This work presents experimental as well as numerical study of the strength of pinned-pinned axially loaded columns utilizing sigma sections. The column section consists of two back to back sigma cold formed sections. The two sections are connected through their webs with connecting fasteners. Eight specimens with variable plate element width to thickness ratio, and having different member slenderness ratios were tested. Residual stresses and geometrical imperfections were recorded. Moreover, the specimens were simulated by a nonlinear finite element model using four node isoparametric shell element that accounts for both geometric and material non-linearities. The measured geometric imperfections and residual stresses were included in the numerical model. The parameters studied include the properties of the cross-section such as the flange, web, and lip dimensions as well as the spacing of the web fasteners. Furthermore, several column heights are considered to study the different modes of failure. Finally, the results are compared with the ultimate strength predicted by the American AISI, the Eurocode-3 and DSM specifications. Results reflect that sectional (local/distortional) buckling governs the failure mode of short columns. Moreover, the AISI, the Eurocode-3 and DSM can reasonably predict the ultimate load capacity of such sections.

Behaviour of cold-formed steel built-up closed columns composed by angle profiles

This paper presents the experimental investigation of cold-formed built-up closed sections with intermediate web stiffeners under axial compression with hinged end conditions. The specimens were formed using two angle sections with edge and intermediate stiffeners, connected by self-tapping screws. Three series of tests were conducted by varying the width of the intermediate web element. Twelve columns were tested by varying the column length. The specimens were failed by local, flexural, and interaction of these buckling modes. Non-linear Finite-element Analysis (FEA) was carried out using ANSYS. FEA results were compared with the experimental results. It is observed that FEA results closely resemble to experimental results. An extensive parametric study was carried out using validated finite-element model by varying the cross-sectional geometries of cold-formed built-up closed section with intermediate stiffeners. The column strengths predicted from the FEA was compared with the design strengths calculated using the AISI specification of cold-formed steel structures. The reliability of the current design method was assessed by reliability analysis.

Cold-formed high strength steel SHS and RHS beams at elevated temperatures

The structural responses of cold-formed high strength steel (HSS) square and rectangular hollow section (SHS and RHS) beams at elevated temperatures were examined in this study. Stress-strain relationships of cold-formed HSS at elevated temperatures were proposed and verified against material test results. The proposed stress-strain relationships were then employed in a finite element (FE) analysis to study the behaviour of cold-formed HSS SHS/RHS beams at elevated temperatures up to 1000 °C. The developed FE model was verified with available test results of cold-formed HSS SHS/RHS beams; upon verification, a total of 252 numerical flexural capacities were gained from FE analyses. The numerical results were used to investigate the suitability of existing cross-section slenderness limits to the HSS tubular sections at elevated temperatures. The applicability of current flexural design provisions in the Eurocode 3, AISC and AISI specifications to the investigated HSS tubular beams at elevated temperatures was also examined. Overall, it is shown that the codified provisions can provide quite conservative predictions; an improved design rule is proposed by modifying the direct strength method (DSM) in the AISI specification. Furthermore, reliability analyses were carried out to assess reliability levels of codified and modified provisions. It has been demonstrated that the modified DSM can produce accurate and reliable design and therefore is recommended to be used for cold-formed HSS SHS/RHS beams at elevated temperatures.

Numerical study and design of thin walled cold formed steel built-up open and closed section columns

This paper, based on new proposed approaches, is a numerical investigation of the behavior of thin-walled cold-formed steel built-up columns under a uniform compression. Two types of cross sections have been studied, built-up closed and open sections formed by two channels connected by battens plates. First, a nonlinear finite element model has been developed and the results obtained show a good agreement against experimental results, such as the ultimate strength, the type of deformed shapes at failure and the lateral displacement. Second, a new proposed approach based on the DSM approach where the local, distortional or global buckling has been obtained numerically, using finite element method software, the thickness at the contact area in the web is taken as 1.0 times the thickness of the plate “t”, instead of 1.5 or 2.0 times “t”. In this case, the compound section is acting as one rigidly connected section. Comparisons with experimental data give a satisfactory degree of accuracy in the proposed design method (DSM-t) and the results are more conservative than those found by the DSM-1.5 and DSM-2. The results, for either closed or open sections, are also slightly conservative for columns failing in global modes than in local ones, if compared with the AISI and EC3 specifications. Some parameters, such as the spacing channels-web length ratio (B1/D) influence the stability and strength of the built-up columns.

Steady- and transient-state response of cold-formed steel-to-steel screwed connections at elevated temperatures

Cold-formed steel-to-steel screwed connections serve as the primary connectors in cold-formed steel framings. Most previous investigations of screwed connections focused on the failure limit states at ambient temperature. This paper presents details on single-shear experiments of cold-formed steel-to-steel screwed connections at ambient and elevated temperatures. A total of 189 steady-state tests and 36 transient-state tests are conducted. Four types of failure modes were identified, and the failure mode of the screwed connection at elevated temperatures is established to be quite similar to that of the same screwed connection under the steady-state condition of below 100 °C. The minimum loaded edge distance of three times the nominal diameter of the screw in AISI S240-15 is shown to be adequate for the shear design of cold-formed steel-to-steel screwed connections at elevated temperatures. The influence of design parameters, such as the screw diameter, steel type and thickness, is discussed, and the differences of the test results obtained from both transient and steady-state test methods are compared. In addition, the expressions from Laboube and Sokols' investigation are verified to be suitable for considering the group effect of two screws on the shear strength of screwed connections at elevated temperatures. The tested shear strength of cold-formed steel-to-steel screwed connections is also compared with the current AISI specification at ambient and elevated temperatures, and a conservative prediction is provided by using the reduced material properties of the corresponding steel.

Investigation on Flexural Behaviour of Cold Formed Steel Latticed Built-Up I Section with Drop Web

This paper deals with the study of flexural behaviour of cold formed latticed built-up I beam with drop web for various sections by varying depth, thickness and span length of the section. The numerical analysis was carried out by FEM software ABAQUS 6.13[11]. The experimental works carried out by Cheng Yu and Benjamin W. Schafer (2006) [1] were taken for validating the numerical analysis procedure. It provides a cost effective way to explore the performance of products in a virtual environment. The ultimate capacity is obtained by performing both linear and non-linear analysis. Theoretical investigation as per AISI specification was carried out for all the specimens to check the accuracy of the design proposal. Numerical finite element procedure is used to instruct the effect of various parameters on behavior of beams. The critical load and different modes of failure are studied by this numerical investigation. Comparison between deformed shape and load vs. displacement curves of I section with drop web were presented.

Cost Effectiveness of CFRP Repair Technique for Cold Formed Steel Beams

Objectives: Structural steel can be broadly classified into hot rolled steel and cold formed steel (CFS). CFS manufactured from roll forming or press braking operation. The main advantage of CFS sections are thin wall and light weight. These thin wall leads to buckling of structural member. This paper aims to study the capacity of Carbon Fibre Reinforced Polymer (CFRP) strengthened CFS channel beams, buckling behaviour, deflection and cost studies on CFRP strengthening. Methods/Statistical analysis: In this study three different length of CFS channel sections are selected for testing in which beams are tested for bending. All the beams were tested in plain section and CFRP strengthened series in universal testing machine. Findings: Experimental results shows the enhancement in capacity of beam due to CFRP strengthening. The local and distortional buckling modes were observed as failure modes. Experimental results were compared with maximum load resistance calculated by AISI specification which are in good agreement. Cost studies were compared on replacing a new member with CFRP strengthened channel member. Improvements/Applications: CFRP strengthening is suitable for enhancement of capacity of beams and repair. This CFRP strengthening technique can applied to reinforced concrete members, hot rolled steel, in filled steel tubes and bridges etc.

In-plane behavior of cold-formed thin-walled beam-columns with lipped channel section

This paper is mainly concerned with the in-plane behavior of cold-formed steel beam-column with lipped channel section. The tested members are classified into three series by loading types, including: axial compression and major axis bending (X), axial compression and minor axis bending (lips in tension, Y1), and axial compression and minor axis bending (lips in compression, Y2). A numerical model is developed and verified by the experimental results. The elastic local buckling loads and second-order effects are discussed based on results of test, numerical analysis and design methods. Comparing with test results in this paper, numerical analysis overestimates the elastic local buckling loads and the actual second-order effect is more severe than the prediction by current design methods. Finally, the comparison between test strength and predicted strength obtained by AISI specification indicates that the interaction equation can provide conservative strength prediction for beam-column.

Mechanical behaviours and design strengths of single shear bolted connections fabricated with aluminium alloy 7075-T6

In the study, experimental study has been performed to investigate the mechanical behaviours such as ultimate strength and fracture mode of single shear bolted connections with aluminium alloy 7075-T6. Main variables are bolt arrangement and end distance. As a result, the specimens failed by shear-out fracture and block shear fracture. Some specimens with a long end distance were accompanied by curling (out of plane deformation). The curling led to a sudden ultimate strength reduction. Moreover, the change of strain distribution was investigated according to curling occurrence. The reliability of design rules for single shear bolted connection in AAA, AISI and AIJ specification is evaluated through comparison between test results and predictions by each current design standard. Lastly, the strength equations considering curling effect and bolt arrangement were proposed and compared with test results.

Load capacity of cold-formed column members of lipped channel cross-section with perforations subjected to compression loading – Part I: FE simulation and test results

An investigation was carried out into the influence of perforations of various shapes on the buckling behaviour of cold-formed column members of lipped channel cross-section. A finite element analysis using ANSYS, an experimental investigation, and design code predictions using the AISI Specification, British Standards (BS) and EU Standards, are employed to determine the buckling load capacity. An experimental investigation of the buckling behaviour of flat and fixed ended columns is presented, and the findings from this are used to validate finite element results, and compare with design code predictions. The numerical results of load vs. displacement behaviour are shown to be in good agreement with those reported from the tests. It is shown that the ultimate failure load of the lipped channels under compression varies greatly with the presence of perforations.