Thin-walled Cold-formed Steel Members Research Articles

Computational Analysis of Cold-formed Steel Columns with Initial Imperfections

Cold-formed steel (CFS) structures experience very complicated buckling and post-buckling performance. The presence of any kind of uncertainties complicates the calculation of such structures. Thin-walled CFS members are known to be particularly vulnerable to the influence of initial geometric imperfections. These imperfections may be the outcome of the manufacturing process, shipping and storage or the construction process. The article provides the results of nonlinear buckling analyses of CFS C-shaped compressed columns and evaluates the influence of imperfections on the load-bearing capacity of the tested members.

DISTORTIONAL BUCKLING STRENGTH OF COLD-FORMED THIN-WALLED STEEL MEMBERS WITH LIPPED CHANNEL SECTION

The half-wave length, the elastic buckling stress of the distortional-buckling of cold-formed thin-walled steel members with lipped channel sections and the corresponding stability coefficient of partially stiffened elements are developed, based on the energy method. With the comparison among the calculated results of elastic buckling stress using the proposed method and the finite strip method, the suitability and good precision of the developed method are illuminated. Then, a uniform formula for the stability coefficient of partially stiffened elements considering both local and distortional buckling effects is established. Finally, with examples’ analysis, it is shown that the proposed uniform formula has higher precision to calculate the stability coefficient of partially stiffened elements and the ultimate load-carrying capacity of cold-formed thin-walled steel members with lipped channel sections, comparing that of the current method provided by the technical code of cold-formed thin-walled steel structures (GB50018-2002).

Compressed Thin-Walled Cold-Formed Steel Members with Closed Cross-Sections

The paper presents fundamental information about experimental-theoretical research oriented to determinate the resistance of thin-walled compressed steel members. The investigated members had closed cross-sections made from homogeneous materials. The theoretical analysis in this research is oriented to determinate the resistance of mentioned members according to relevant standards, while the experimental investigation is to verify the theoretical results and to investigate the behavior of mentioned members during the loading process.

Analysis of the Resistance of Thin-Walled Cold-Formed Compressed Steel Members with Closed Cross-sections. Part 2

Part 1 of the paper presents fundamental information about the experimental and theoretical research oriented to determine the resistance of thin-walled cold-formed compressed steel members with closed cross-sections, (1). The investigated members were made from homogeneous material. The theoretical analysis was oriented to determine the resistance of mentioned members according to European and Slovak standards, while the experimental investigation was to verify the theoretical results and to investigate the behavior of the above-mentioned members during the loading process, (2). Part 2 is focused on the numerical analysis of the achieved results, as well as on 3D modeling and simulation of the experimental tests.

Analysis of the resistance of thin-walled cold-formed compressed steel members with closed cross-sections. Part 1

Analysis of the resistance of thin-walled cold-formed compressed steel members with closed cross-sections. Part 1

Theoretical and Experimental Stress-Strain Analysis of Compressed Steel Members with Closed Cross-Sections

Abstract The paper presents fundamental information about experimental and theoretic-numerical research to determinate the load-carrying capacity of thin-walled cold-formed compressed steel members. The investigated members have closed cross-sections made from homogeneous materials. The theoretic-numerical analysis in this paper is oriented on the modeling of the initial deviations effects, caused by production process, on the load-carrying capacities of mentioned members, while the experimental investigation is to verify the theoretical results and to investigate the behavior of mentioned members during the loading process.

Analysis of the Initial Imperfections Effect on the Thin-Walled Cold-Formed Compressed Steel Members

The paper deals with the influence of initial imperfections and presents fundamental information about experimental and theoretical-numerical research to determinate the load-carrying capacity of thin-walled cold-formed compressed steel members. The investigated members had closed cross-sections made of homogeneous material. The theoretical-numerical analysis in this paper is oriented on the investigation and modeling of initial imperfections effects, caused by production process, on the load-carrying capacities of mentioned members, while the experimental investigation is to verify the theoretical results and to investigate the behavior of mentioned members during the loading process.

Inelastic Bending Capacity of Cold-Formed Steel Members

AbstractThe objective of this paper is to provide and verify a general design method for prediction of inelastic bending capacity in cold-formed steel members potentially subject to local, distortional, and/or lateral-torsional buckling modes. An extensive experimental database of tested cold-formed steel beams is collected and indicates that inelastic reserve in the bending capacity of thin-walled cold-formed steel members is more common than typically assumed. Elementary mechanics for inelastic reserve are reviewed and simplified expressions provided for connecting the strain demand to the inelastic bending capacity in the range between the yield moment and the fully plastic moment. The strain capacity that can be sustained in inelastic local and inelastic distortional buckling is investigated through existing experiments coupled with nonlinear finite-element (FE) analysis. The nonlinear FE models provide a comprehensive means to investigate the relationship between cross-section slenderness, normalized...

Thin-Walled Cold-Formed Compressed Steel Members and the Problem of Initial Imperfections

The paper deals with the influence of initial imperfections caused by the production process and presents fundamental information about experimental and theoretic-numerical research to determine the load-carrying capacity of thin-walled cold-formed compressed steel members. The investigated members have closed cross-sections made from a homogeneous material. The theoretic-numerical analysis in this paper is oriented towards investigating and modelling initial imperfections effects on the load-carrying capacities of the mentioned members, while the experimental investigation verifies the theoretical results and investigates the behaviour of the members during the loading process.

Overall Stability of Open Cold-Formed Thin-Walled Steel Members with Hat Sections and Batten Plates under Axial Loads

Batten plates can play a significant role in reducing the bearing capacity of the entire component and preventing the upward warpage deformation in the opening section. The specific number of batten plates should be calculated for the open cold-formed thin-walled steel structure. By theoretical analysis, this study develops the flexural-torsional buckling formula for the open hat-section cold-formed thin-walled axially compressed members with batten plates. The calculating results show that, according to the configuration rule with 40 iy space between batten plates along the opening direction in the open thin-walled steel members, the warpage deformation will be effectively prevented in the opening direction. Besides, the bearing capacity of the entire member will be increased. The proposed calculation methods can actively complement the existing code.

Effects of Residual Stress on Bearing Capacity of Cold-Formed Thin-Walled Steel Members with “л” Section under Axial Compressive Force

In order to enhance the pertinence of agriculture building design, the experiment and numerical simulation on cold-formed thin-walled steel members with “л” section under axial compressive force were conducted to investigate the effects of residual stress on bearing capacity of them. For the members, there are inevitably initial defects, such as initial bending, initial eccentricity and residual stress due to the manufacturing error. In the numerical simulation, initial bending and initial eccentricity of the members were considered. The results show that the residual stress affects instability forms of the members. And as the member slenderness ratio increases from 69 to 200, the bearing capacity of member with residual stress decreases from 12.8% to 84.9% of that without residual stress.

Post-Buckling Shear Strength of Thin Steel Plates

Thin walled steel members, such as cold-formed steel (CFS) members, are susceptible for local buckling at low loads. However, initiation of local buckling of elements does not necessarily mean the ultimate limit state of the member, and thin-walled plate elements can exhibit substantial post-buckling strength. International cold-formed steel design specifications recognize the post-buckling strength of uniformly compressed stiffened and unstiffened elements, and the post-buckling strength of stiffened and unstiffened webs in flexure. However, potential post-buckling shear strength of webs is not considered in CFS design specifications, though post-buckling shear strength of stiffened webs is considered in structural steel plate girder design. The objective of this study is to quantify the post-buckling shear strength of thin-walled cold-formed steel members. This numerical investigation is based on finite element method.This paper presents the finite element modelling details associated with the thick/thin plates representing the web of a cold-formed steel member. The study considered simply-supported rectangular plates subjected to in-plane shear loadings experiencing buckling, post-buckling, and yielding until failure. The plate was modelled using geometrically non-linear quadrilateral shell elements, and non-linear steel stress-strain relationship derived from experiments. Total Langrangian with large displacement/small strain formulation was used for such analyses. The model also considered the initial geometric imperfections. The first part of the investigation established the ultimate shear strength of plates having different parametric dimensions. The next part compared such results with the strength values based on the current code provisions. For slender plates, the current code uses the shear buckling load as the strength, whereas the analyses indicated significant post-buckling strength. The third part of this paper establishes the shear design equations incorporating the post-buckling strength of plates.

Research on the Whole Stability of Axially Compression Member of Cold-Formed Thin-Walled Steel Sections in the Effect of Initial Imperfection

In calculating the reliable capacity of carrying load of cold-formed thin-walled steel members, there has not yet been a proper method which directly takes into account the impact of the residual stress. Through the research of residual stress, models close to the actual situation are established. Based on the ultimate load criterion, considering both cold-formed residual stress and initial out-of-straightness, the whole stability of axially loaded compression member of cold-formed thin-walled crimping angle steel and channel steel are analyzed using FEM. Based on a lot of FEA numerical results obtained, the φ-λ curves of different kinds of sections and size are obtained. Finally, φ-λ curve that reflects the fact can be obtained. By comparing the curve with the code and results of experience, it is proposed that the stability coefficient φ of small slenderness member of the code should be increased appropriately.

Plastic mechanisms database for thin-walled cold-formed steel members in compression and bending

Short members of thin-walled cold-formed (TWCF) steel sections, in compression and bending, fail by forming local plastic mechanisms. Taking into account the localised buckling pattern, the collapse of slender members, due to the interaction between local and overall buckling modes, is always characterised by local plastic mechanism failure mode. Based on these two observations, the ultimate strength in interactive buckling of these members can be regarded as an interaction between localised plastic mode and overall elastic one.The yield line mechanism method has been widely used to predict the sectional strength (e.g. local) of thin-walled cold-formed steel members that involve failure mode characterized by local collapse mechanisms. This method can be also used to study post-collapse behaviour and to evaluate the load-carrying capacity, ductility and energy absorption.This paper is based on previous studies and some latest investigations of authors, as well as the literature collected data. It represents an attempt to make an inventory, classify and range geometrical and analytical models for the local-plastic mechanisms aiming to characterize the ultimate capacity of some of the most used cold-formed steel sections in structural applications.

Residual stresses in press-braked stainless steel sections, I: Coiling and uncoiling of sheets

The manufacturing process of cold-formed thin-walled steel members induces cold work which can be characterized by the co-existent residual stresses and equivalent plastic strains and has a significant effect on their structural behaviour and strength. The present paper and the companion paper are concerned with the prediction of residual stresses and co-existent equivalent plastic strains in stainless steel sections formed by the press-braking method. This manufacturing process consists of the following two distinct stages: (i) coiling and uncoiling of the sheets, and (ii) press-braking operations. This paper presents an analytical solution for the residual stresses and co-existent equivalent plastic strains that arise from the first stage. In the analytical solution, the coiling–uncoiling stage is modelled as an inelastic plane strain pure bending problem; the stainless steel sheets are assumed to obey Hill’s anisotropic yield criterion with isotropic hardening to account for the effects of material anisotropy and nonlinear stress–strain behaviour. The accuracy of the solution is demonstrated by comparing its predictions with those obtained from a finite element analysis. The present analytical solution and the corresponding analytical solution for press-braking operations presented in the companion paper form an integrated analytical model for predicting residual stresses and equivalent plastic strains in press-braked stainless steel sections.

Residual stresses in press-braked stainless steel sections, II: Press-braking operations

The manufacturing process of cold-formed thin-walled steel members induces cold work which can be characterized by the co-existent residual stresses and equivalent plastic strains and has a significant effect on their structural behaviour and strength. The present paper and the companion paper are concerned with the prediction of residual stresses and co-existent equivalent plastic strains in stainless steel sections formed by the press-braking method. This manufacturing process consists of the following two distinct stages: (i) coiling and uncoiling of the sheets, and (ii) press-braking operations. This paper first presents an analytical solution for the residual stresses and the co-existent equivalent plastic strains that arise from the second stage while a corresponding analytical solution for the first stage is presented in the companion paper. In both solutions, plane strain pure bending is assumed and the effect of material anisotropy is taken into account. On the basis of these two analytical solutions, an analytical model is presented to predict residual stresses and equivalent plastic strains in press-braked stainless sections. The predictions of the analytical model are shown to be in close agreement with results from a finite element-based method, demonstrating the validity and accuracy of the analytical model. The analytical model provides a much simpler method for the accurate prediction of residual stresses and equivalent plastic strains in different parts of a press-braked stainless steel section than a finite element-based method.

Analysis and design of cold-formed steel channels subjected to combined bending and web crippling

The channel failures due to combined bending and web crippling may occur at the highly concentrated interior loading when there is no load stiffener in cold-formed thin-walled steel beams. This paper presents accurate finite element models to predict the behavior and ultimate strengths of cold-formed steel channels subjected to pure bending as well as combined bending and web crippling. Both geometric and material nonlinearities are considered in the finite element analysis. The nonlinear finite element models are verified against experimental results of cold-formed steel channels subjected to pure bending as well as combined bending and web crippling. The finite element analytical results show a good agreement with the experimental results in terms of the ultimate loads and moments, failure modes and web load-deformation curves thus validating the accuracy of the finite element models. The verified finite element models are then used for an extensive parametric study of different channel dimensions. The channel strengths predicted from the parametric study are compared with the design strengths calculated from the North American Specification for cold-formed steel structures. It is shown that the design rules in the North American Specification are generally conservative for channel sections with unstiffened flanges having the web slenderness ranged from 7.8 to 108.5 subjected to combined bending and web crippling. It is demonstrated that the nonlinear finite element analysis by using the verified finite element models against test results is an effective way to predict the ultimate strengths of cold-formed thin-walled steel members.

Complex Edge Stiffeners for Thin-Walled Members

The objective of this paper is to investigate the behavior, and provide recommendations on, the design of open cross-section thin-walled cold-formed steel members that employ complex stiffeners. Complex stiffeners are formed when the free edge of an open section is folded multiple times, as opposed to simple stiffeners which employ a single fold (a lip). Simple stiffeners become ineffective when long lips are required to stabilize the flange, as the lip itself initiates the instability. In the elastic buckling regime, complex stiffeners are shown to hold a distinct advantage in local buckling over simple stiffeners. Closed-formed solutions for both local and distortional buckling of members with complex stiffeners can provide conservative and reliable solutions, but remain cumbersome and somewhat limited in applicability, therefore numerical solutions are preferred. Nonlinear finite element analysis is used to examine the post-buckling and ultimate strength regime. Complex stiffeners are shown to provide improved ultimate strength performance over simple stiffeners, but with a slight increase in imperfection sensitivity. Further, for an equivalent amount of material, complex stiffeners only provide advantages for specific stiffener lengths, though global optimal designs (maximum strength while minimizing material) still favor complex stiffeners over simple stiffeners in the investigated examples. The cold-formed steel design specification in current use is shown to be a poor predictor for the ultimate strength of bending members with complex stiffeners. However, the direct strength method, recently adopted as an alternative design method in the North American Specification for cold-formed steel members is shown to be a reliable predictor of ultimate strength. The direct strength method is recommended for design and optimization of members with complex stiffeners.

高温下における冷間成形薄板軽量形鋼の材料ならびに座屈特性

Cold-formed thin-walled steel members with a thickness of around 1 mm have been used as structural members for residential houses since 1990s in Japan. The application of the members to the houses requiring fire resistance has been limited, however, due mainly to a lack of information on mechanical properties and buckling behavior of such a thin steel member under elevated temperatures. In this research, therefore, material tensile tests and compression column tests were conducted to clarify the characteristics and to examine if existing column design formulae for normal temperature can be applicable. The tests covered a thickness from 0.5mm to 1.6mm and a temperature up to 600℃. It was reported in this paper that much difference is not observed on yield strength and young's modulus degradations up to 400℃ from traditional thicker building members, and existing column design formulae for normal temperature may be applied to estimate the column strength of thinwalled steel members under high temperatures.

面材により部分拘束された曲げを受ける薄板軽量リップ溝形鋼のゆがみ座屈挙動

Cold-formed thin-walled steel members with thicknesses of around 1mm are relatively new structural members in Japan. It is because such a thin steel member had not been long allowed to use as a structural member under the Japanese Building Standard Law. This situation has been changed in recent years through the development of steel-framed houses. Although the use of such a thin steel member is currently possible, there are still some design issues to be clarified for realizing the broader applications, mainly related to its complex buckling behavior. The complex buckling is called distortional buckling and is often seen in the floor and roof panels in steel-framed houses. To study the behavior of distortional buckling in detail therefore, negative bending tests were conducted in this paper for panels consisting of two lipped-channels and structural plywood fastened together with screws. Twelve bending tests provided detailed behavioral information on distortional buckling and the possible strength evaluation methods for improving the design of thin-walled steel members.