Cold-formed Steel Built-up Columns Research Articles

Adoption of hooped-battens in cold-formed steel built-up columns for superior axial performance

Previous research on cold-formed steel (CFS) battened columns has identified the critical factors influencing their performance and accordingly, their limiting values for improved performance have been recommended. However, these studies involved connecting battens to the chords (channels) via their flanges, leaving the slenderest component disconnected from the web. This study introduces a novel hooped-batten (tubular-element) that links both webs and flanges of the chords together, thereby improving the structural integrity of the built-up system and curtailing the half-wave buckling length in the webs. As a result, axial strength and stability in these built-up columns may improve adequately. Firstly, a numerical model of a conventional CFS battened column was developed in ABAQUS and verified against test results on the same reported in literature. Afterward, the validated model was used to simulate the behaviour of CFS built-up columns with hooped-battens. Two key parameters i.e., unbraced chord slenderness and overall column slenderness were varied to explore their influence on the axial behaviour of built-up columns in terms of peak strengths, failure modes and load-displacement characteristics. The performance of the hooped-battened columns was compared with the identical conventional battened columns, which reflects that the former exhibits superior strength and stability characteristics over the latter.

Influence of Built-up Cold-formed Steel Columns Battened for Long-span Portal Frame

Influence of Built-up Cold-formed Steel Columns Battened for Long-span Portal Frame

Computational modelling of built-up cold-formed steel columns and parametric studies

The paper develops a detailed finite-element model for the reliable nonlinear collapse analysis of built-up cold-formed steel columns with discrete fastener connections. First, the main components of the computational model are discussed, including their available options and comparison between their results, followed by a rational justification for the options chosen in this study. Specifically, a detailed theoretical background is provided with conclusive comparisons of the options for enforcing end support conditions, applying contact conditions between the constituent plates, selecting connection elements for representing intermediate screw fasteners, and finally, the incremental load stepping and nonlinear solution schemes. The accuracy and performance of the proposed FE modelling strategy are verified against the experimental data, contributing to the calibration of the influencing parameters. The predictions of the proposed methodology are shown to be in excellent agreement with the test data in terms of the failure mode and the inelastic capacity curve up to ultimate capacity and beyond. Upon its successful calibration, the FE model is utilised to perform extensive parametric studies into the effects of various design parameters on the ultimate strength of built-up columns. This includes practical variations of cross-section dimensions, sectional slenderness, built-up section geometry, and fastener spacing. The parametric study results suggest a meaningful dependency of the ultimate capacity of built-up sections on the fastener spacing ratio, the critical buckling half-wavelength of the built-up section and the relative number of restrained components in the built-up configuration undergoing sectional buckling.

Numerical modelling of cold-formed steel built-up columns

Built-up cold-formed steel structural members can provide technically and economically advantageous solutions in cold-formed steel construction. However, there is a need for a deeper understanding of their stability behaviour, and this paper demonstrates that detailed, accurate and reliable finite element models provide an excellent means to achieve this. Apart from the typical issues to be considered in the high-fidelity modelling of thin-walled members, which include geometric and material nonlinearity, element type, mesh density, and the modelling of imperfections and boundary conditions, the paper focuses on two additional complications which arise in built-up members: the modelling of connectors, and contact between constituent parts. A number of alternative techniques to model connector behaviour are investigated and their effect on the predictions of the ultimate capacity quantified by benchmarking against experimental data. Various successful strategies to mitigate stubborn convergence problems, which almost inevitable originate when modelling complex contact problems, are also presented. It is demonstrated, however, that the inclusion of contact as a model feature does not have a significant effect on the predicted capacity in the majority of the considered cases, with its influence typically remaining limited below 10%. The validated models were further used in parametric studies which showed that once the connector spacing is short enough to prevent global instabilities of the component sections between connector points, its effect on the ultimate capacity of the built-up column is very small within the practical range of connector spacings. Modelling connector lines as continuous longitudinal constraints tying surfaces together, on the other hand, is bound to significantly overestimate the capacity.

Shear-slip constitutive model of screw-fastened connections and application in numerical analysis of built-up back-to-back cold-formed steel columns

In order to study shear-slip constitutive model of screw connections between cold-formed steel (CFS) profiles and its application in numerical analysis of CFS built-up columns, a total of 225 steel-to-steel screw-fastened connections were investigated under monotonic loads. The thicknesses of C-section steels and diameters of self-drilling screws were varied to study their effect on shear performance of connections. The experimental results show that the failure modes can be divided into three types, which include bearing failure of screw holes, shear failure of screws and pull-out of screws. Furthermore, the shear-slip constitutive model of screw-fastened connections was developed and then the methods for estimating stiffness parameters, strength parameters and displacement parameters were derived. In addition, the numerical models were put forward by applying the shear-slip constitutive model on basis of previous built-up back-to-back CFS columns tests. The results of numerical simulation are consistent with the previous test results by comparisons. The proposed shear-slip constitutive model can effectively simulate the shear-slip behavior of screws used in built-up back-to-back CFS columns.

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.

Experimental investigation of the sectional buckling of built-up cold-formed steel columns

This paper presents an experimental program investigating the local and/or distortional (sectional) buckling of built-up cold-formed steel sections subjected to pure compression. The experiments were designed to study the influence of key parameters, including buckling mode, cross-section geometry and fastener spacing, on the ultimate sectional capacity of built-up columns. Three singly- or doubly-symmetric built-up sections were considered with two sets of component lipped channel sections, one buckling predominantly in the distortional mode and one buckling in the local mode. The component sections were connected using self-drilling screws at three different spacings. Material properties, geometric imperfections and residual stresses of the specimens were measured prior to testing. A total of 36 specimens with a length of 1 m were tested under concentric compression between fixed ends, which was achieved through a special end plate connection. The specimens failed in either distortional or the interaction of local and distortional buckling modes. Compared to the sum of the capacity of component sections, the ultimate capacity in built-up columns with distortional buckling mode increased significantly with the reduction of screw spacing, whereas the increase was minor for specimens that failed through local-distortional interaction. The ultimate strength of the specimens was predicted using the current Direct Strength Method, where either conservative or overly unconservative predictions were obtained depending on the considered composite action level in the elastic buckling analysis.

Fire resistance of box-shape cold-formed steel built-up columns failing in global buckling: Test, simulation and design

Box-shape cold-formed steel built-up columns (CFS-QBC) are popular in multi-story CFS buildings. This paper conducted experimental research, numerical simulation, and parametric analyses to investigate its fire resistance. Twelve slender columns failing in global buckling were tested in a transient fire condition, where different initial load ratios and screw spacing were considered. The results showed that the specimen’s critical temperature decreased with the screw spacing, and the screw spacing could significantly affect the deformation modes; Besides, the end fastening group (EFG) had little effect on the fire resistance. When the screw spacing was less than 300 mm, the effectiveness of the composite action of the built-up section at elevated temperatures was demonstrated by the consistency of the deformation compatibility among the components of the built-up section. Numerical simulations were then performed, including heat transfer and structural analysis, calibrated and verified against the tests. Based on the numerical model, the load ratio and screw spacing were parametrically analyzed to study their effects on the fire resistance of the columns. Finally, a simplified design method was proposed to evaluate the load-bearing capacity of CFS-QBC at elevated temperatures.

Local-distortional interaction behaviour and design of cold-formed steel built-up columns

This research highlights the possibility of LD interaction in the built-up columns and provides a detailed design procedure to account for the strength erosion due to LD interaction. For this, a comprehensive numerical study using finite element (FE) method is performed where the FE models are validated using the test results in the literature. In this study, three types of built-up sections, i) back-to-back I section made of two lipped channels, ii) nested section made of two lipped channels, and iii) nested section made of one lipped and one unlipped channel, are selected. The effects of LD interaction, fastener spacing, and end fastener group (EFG) on the ultimate strength of built-up columns are investigated. The study reveals that the LD interaction behaviour of the built-up back-to-back I section in terms of failure mode and normalized ultimate strength result (Pu/Py) is similar to that of a lipped channel. For the built-up nested section, improved strength is observed when the lipped channel fails in distortional buckling. No improvement is observed in the LD strength of built-up sections with reduced fastener spacing or by using EFG. The study shows that the traditional direct strength method (DSM) based design procedure for LD interaction is conservative for built-up sections. Hence, a reliable design procedure is proposed based on the modified DSM equations obtained from the literature. The strength and failure mode predictions of the proposed method match well with those of experimental studies in the literature and the numerical study of this paper.

Strength-based requirements for end connections of built-up cold-formed steel columns

The requirements for end connections of built-up cold-formed steel columns in North American and Australian specifications were adapted from AISC provisions for built-up hot-rolled steel members, which were largely based upon judgment and experience. This paper presents a strength-based approach to establish end connection requirements. Examples are given to compare the proposed requirements to current specification provisions and stiffness-based requirements.

Buckling behavior of closed built-up cold-formed steel columns under compression

The versatility of cold-formed steel sections ensures the possibility of combining multiple individual sections forming built-up members with significantly higher load-bearing capacity. This paper reports detailed experimental, numerical and analytical research on closed built-up cold-formed steel columns with four different cross-section shapes under compression. Twelve experimental specimens were fabricated using individual C, U, and Σ-shaped sections and connected using fasteners. The material characterization and imperfection measurements were also performed. The interaction between the different components and the influence of spacing between connectors was investigated for a relevant range of slendernesses using the finite element method, providing recommendations concerning the effective use of fasteners to attain composite action between the individual shapes. The overlapping plates tend to buckle simultaneously when the spacing is smaller than the local buckle half-wavelength. The suitability of the design codes, namely, Eurocode and North American design predictions, according to Effective Width Method (EWM) and Direct Strength Method (DSM), respectively, were assessed for the built-up members. Reliability analysis was undertaken, and it was observed that generally, the DSM with the modified slenderness ratio is the most reliable methodology.

Numerical Studies on CFRP Strengthened Cold formed Steel built-up Columns

This paper reports the numerical investigation carried out on the Cold-formed steel (CFS) built up columns strengthened with a Carbon fiber-reinforced polymer (CFRP) by using Finite element software ABAQUS/CAE. The CFS sections used in the investigation are built-up Cold formed box sections connected together by screws. Totally 24 columns are considered for the analysis in that twelve columns used are plain CFS and other 12 columns used are CFRP strengthened CFS columns. The geometric properties of the materials considered as 0.6 mm, 0.75 mm and 1 mm thickness with 300 mm, 500 mm and 700 mm depth respectively. All the built-up columns were modeled and analysed using ABAQUS software. From the analysis results, the Ultimate load capacity, buckling behavior and Load-lateral displacement of the Plain and CFRP strengthened columns are obtained and presented in this paper. It is also noted that the numerical results obtained from the ABAQUS software is in good agreement with the experimental results.

Design of cold-formed steel battened built-up columns

Cold-formed steel (CFS) battened columns with adequately spaced chords have structurally outperformed the conventional CFS built-up columns with closely spaced chords, but have witnessed limited research previously. The relative slenderness of the unsupported chord (between the battens) to the built-up column, is a critical parameter that governs the behavior of battened columns, and has not been appropriately addressed in previous studies, and unlike hot-rolled steel battened columns, its limiting value for a wide range of column slenderness is missing in the literature. Therefore, the various design rules proposed by different researchers for the same may not be credible. The present study focuses on the axial performance of pin ended CFS battened columns composed of two adequately spaced plain channels to form closed built-up sections. The aspect ratio of the built-up sections was fixed as unity. Finite element (FE) models were developed using ABAQUS platform, and were validated against the relevant test results available in the literature. After rigorous validation, the robust FE model developed was used to conduct an extensive parametric study by varying the vital parameters like sectional compactness of the chords, relative slenderness of the unsupported chord and overall slenderness of the battened columns. The effect of each parameter on the behavior of battened columns has been discussed extensively. Furthermore, the strengths of CFS battened columns were predicted using the current North American and European codes. It was noted that both these codes over predict the strengths of CFS battened columns, particularly when the sectional compactness of the chords is low and the relative slenderness of the unsupported chord is high. Also, a range of relative slenderness of the unsupported chord has been identified for which the prediction of the current codes is satisfactory. Based on the wide pool of data points developed, new design rules were proposed for safe and reliable predictions for CFS battened columns. The proposed design rules were verified through a reliability analysis and also the test results on CFS battened columns available in the literature.

Numerical investigation of the strength and design of cold-formed steel built-up columns

The paper concerns built-up cold-formed steel (CFS) structural members composed of three or four component lipped channel sections. Finite element (FE) models are calibrated against recently conducted experimental results and used to perform a parametric study of the strength and behaviour of CFS built-up section columns, considering column the length, section thickness and screw spacing as independent variables. The strengths thus obtained combined with experimental ultimate strengths are compared with strength predictions obtained using the North American Specification for the Design of Cold-formed Steel Structural Members, which specifies the use of a modified slenderness ratio for built-up members composed of two sections connected back-to-back. The same data is used to assess a recently proposed design method for predicting the strengths of the studied 3C and 4C built-up sections, and potentially multi-cell built-up sections more generally, based on effective rigidities coupled with the Direct Strength Method. The proposal includes a design procedure for built-up sections experiencing flexural-torsional buckling, which is neither explicitly stipulated in current design standards nor discussed in-depth in previous studies, and generally provides accurate strength predictions. A reliability analysis is included in the paper to calculate resistance factors to accompany the proposed design provisions.

Experimental investigation and design considerations on cold-formed steel built-up​ I-section columns subjected to interactive buckling modes

An experimental investigation into the behavior of cold-formed steel (CFS) built-up column assembly compressed between fixed ends is presented in this paper. The built-up column assembly is designed to be doubly symmetric and locally slender to study the interactive buckling mode of failures. A total of forty-one columns were tested including different cross-section dimensions, lengths, intermediate connection spacing, and slendernesses. It is experimentally shown that the back-to-back connected doubly symmetric built-up cross-section assembly columns fail predominantly in interactive local and flexural–torsional buckling due to the shift in the line of action of the internal force caused by local buckling deformations. The influence of intermediate fastener spacing was also prevalent in the failure modes of the doubly symmetric CFS built-up columns. The appropriateness of the AISI’s maximum intermediate connection spacing limitation is verified to prevent global instability failures. The test and design results comparison indicated that the current AISI’s DSM design curve for interactive buckling is unconservative for the CFS built-up columns with predominant interactive local–global failure mode vulnerability. Therefore, a modified design curve for interactive buckling is suggested based on the test results with reliability analysis.

Structural optimization of closed built-up cold-formed steel columns

Cold-formed steel (CFS) members are commonly found in single sections, whereas built-up CFS members are usually composed of existing single sections. There is a lack of designed shapes for single sections targeting high structural performance built-up members, i.e., maximizing their resistance-to-weight ratio. Furthermore, traditionally, design optimization methods have a solid mathematical background (gradient-based methods for instance) and use design guidelines for the search of the optimal solution. The construction industry is responsible for a large share of the worldwide consumption of natural resources, and structural optimization plays an important role in improving the sustainability of the sector and reducing the impact of climate change. Non-traditional search optimization methods such as evolutionary algorithms are growing popularity in engineering optimization problems due to i) their nature-inspired technique, ii) their easy way (for the user) of solving complex real-world optimization problems, and iii) the current advanced computing machines, which provide sufficient computational speed to generate solutions to difficult problems in reasonable time. Therefore, optimized structural solutions for closed built-up CFS columns under compression are proposed by using the particle swarm optimization (PSO) algorithm and the finite element method (FEM). Design predictions for the optimum columns based on European Code and North American Specification are also given and compared with the numerical ones. Finally, the findings of this research work show how some parameters (including steel thickness, cross-section height and column length) can affect the optimum solution of the studied objective function (resistance-to-weight ratio), especially the steel thickness.

Design of locally buckling cold-formed steel built-up columns formed by unlipped channel sections

This study proposes a design procedure for the local buckling strength prediction of cold-formed steel (CFS) built-up I and box section columns based on the Effective Width Method (EWM) and the Direct Strength Method (DSM). For this, the ultimate strength of the built-up sections made of two plain unlipped channels is obtained using the finite element (FE) analysis. First, some compression tests on built-up sections are performed to validate the FE models. Then, a detailed parametric study using the validated FE models is conducted for sections with a wide range of slenderness (1≤λl≤3) where λl is the square root ratio of yield stress to the local buckling stress of the cross-section. The results show that the local buckling strength of built-up I sections is equal to the sum of its section’ strength, whereas for box sections, the strength can be 20%–25% higher as the nesting of sections will alter the local buckling characteristics. The EWM (with k factor approach) and authors’ DSM based design procedure can predict the strengths of the built-up I sections well but predict conservative strengths for the built-up box sections. This issue is resolved in this paper by proposing a simple cross-sectional model based on the constrained local buckling deformations of the built-up box section. When the buckling stress results of the proposed model were used with the EWM and authors’ DSM design procedures, the predicted local buckling strength results matched well with those of the present numerical study. In addition, the proposed design procedure provided more accurate strength predictions than the existing DSM and those presented in the literature for the built-up section 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.

Structural Fire Performance of Concrete-Filled Built-Up Cold-Formed Steel Columns.

Concrete-filled composite columns are widely used in the construction industry, exploiting the benefits of combining steel and concrete, providing, for instance, high load-bearing capacities and enhanced fire resistance. These solutions are extensively used in high-rise buildings and/or when high fire resistance performance requirements are imposed. In this exploratory research, a new type of concrete-filled composite column is investigated using fire resistance tests. Promoting the use of cold-formed steel products and developing innovative solutions for low-rise buildings with lower passive fire protection requirements led to the solutions presented in this research. Hence, a set of fire-resistance tests were undertaken on concrete-filled closed built-up cold-formed steel columns, where single cold-formed steel shapes are combined and fastened to create a box-shaped cross-section. The experimental results were then compared with the corresponding bare steel solutions to assess, in detail, the observed enhancements. Additionally, the effect of restraint on thermal elongation was assessed.

Axial compression behaviour of cruciform cold-formed steel built-up columns: Shape optimization and experimental study

In this paper, the cruciform CFS built-up cross sections are formed through four identical multi-roll lipped channels connected at their webs with high-strength bolts and filler plates. An optimization algorithm is primarily proposed that integrates sequential quadratic mathematical programming methods and finite strip analysis. It is desirable to effectively improve the buckling behaviour of cruciform CFS built-up columns without increasing the material volume. Eight different areas of built-up sections were optimized, each built-up open section was formed by fold-lines of the steel plate, and the width and angle of the constituent elements of the built-up section were selected as the main design variables. To simplify the optimization task and provide more practical structural components, usage constraints and practical manufacturing limits were imposed on the selected built-up sections during the optimization process. The results show that the adopted optimization process significantly increased the critical local buckling stress of the cruciform CFS built-up columns by 26.2%–58.9%. The design strength calculated according to the AISI standard was increased by 2.76%–18.15%. The axial compression bearing capacity tests with two different optimized cross sections were conducted, and the test loading capacities and failure modes of each specimen were obtained. The test results show that the failure modes were mainly driven from local and distortional buckling to torsional and flexural buckling with increasing slenderness, and the deformations were concentrated in the region near the end of column. Finally, the axial compression strengths of such cruciform CFS built-up columns obtained from the tests were used to assess the applicability of AISI standards. For columns failed by local and distortional buckling, AISI standard were un-conservative, whereas for torsional and flexural, AISI standard were conservative.