Investigations on global buckling behaviour of concrete-filled double-skinned steel tubular columns

[EN] Concrete-Filled Double-skinned Steel Tubular columns (CFDST) are proved to possess exceptional structural resistance in case of fire and multi-hazard situations. This superior quality of CFDST makes it preferable in long column applications. However, studies on the long column behaviour of CFDST is very few, and their results are not in line with the behaviour of CFST long columns. Whereas, several researches on stub column CFDST shows that, the axial compression behaviour of CFDST is similar to CFST. In this paper, selected results (4 numbers of circular CFDST specimens) from a large test data is presented. Axial compression behaviour of long column CFDST specimens is studied, with non-dimensional slenderness λ around 1.0, and hollowness ratio as the governing parameter for study. Test results namely, axial load carrying capacity, axial deformation and lateral deflection are presented in this paper. Numerical models are also developed and validated with the experimental results, to carry out more parametric studies. Further, the experimental axial capacity values are compared with modified capacity equations from EC4 and AISC. Results show that extended EC4 and AISC equations gives conservative predictions for CFDST column even in the long column range. Moreover, the initial imperfections in the specimen and their corresponding boundary conditions for load application, are found to be governing parameters in long column buckling study.Sulthana, UM.; Jayachandran, SA. (2018). Investigations on global buckling behaviour of concrete-filled double-skinned steel tubular columns. En Proceedings of the 12th International Conference on Advances in Steel-Concrete Composite Structures. ASCCS 2018. Editorial Universitat Politècnica de València. 419-425. https://doi.org/10.4995/ASCCS2018.2018.7144OCS41942

Direct Strength Method Based Design of Cold-Formed Steel Built-Up Columns: Challenges and Solutions

The buckling behaviour and strength of cold-formed steel (CFS) built-up columns have been thoroughly explored in the literature. Additionally, different design procedures have been proposed for strength estimation as the current codes of practice suggest limited guidelines. In this paper, the proposed design procedures are compiled and evaluated against the ultimate strength results of experimental studies presented in the literature. The challenges and applicability of each design procedure are highlighted. As per the direct strength method (DSM), the nominal design strength is a function of yield stress and elastic buckling stress of the section. A wide discrepancy is observed while evaluating the strength of CFS built-up sections, as the proposed design procedures are based on the approximate elastic buckling stress calculation methods. Hence, the present study highlights the need for a unique design procedure and suggests design steps based on the existing solutions to obtain the elastic buckling stress of built-up section columns for better strength predictions.The first author thanks Rowena Rachel, Research Associate, Indian Institute of Technology Madras and Rangaraj Baskar, Research Intern, PSG College of Technology, who helped collect test data. The authors thank the Indian Institute of Technology Madras for providing infrastructure facilities for the research work

Investigations on buckling behavior of intermittently fastened cold-formed steel built-up columns using spline finite strip method

In load-bearing cold-formed steel (CFS) framed wall systems, built-up columns are preferred over single CFS sections for their improved structural performance. In this study, a numerical formulation is presented to compute the elastic stability of isolated and compound wall studs, braced intermittently to the sheathing. These bracings can alter the buckling behavior of the compound columns altogether, especially in the distortional and global buckling modes. The present study is towards improving the design provisions of built-up columns presented in AISIS100 (2016) which suggests adopting a modified global slenderness ratio. Although, this is intended to accommodate the loss of shear rigidity due to discrete fastener spacing, no guidance is provided to accommodate this effect on other buckling modes. In the literature, approximatemethodologies have been reported, but comprehension of the composite behavior of built-up sections is still missing. In this paper, a numerical methodology using compound spline finite strip method is developed to compute the elastic buckling stress of built-up steel columns braced withand without sheathing. A compound model is generated by adding the stiffness matrix of fasteners into system global stiffness matrix where stiffness matrix of the fasteners is computed by adopting a three-dimensional beam model with adjustable geometrical properties and stiffness of wall system is added to the model with the help of translational and rotational springs. All the resultsare compared with FE based software ABAQUS and results are found to be in good agreement. A clarity is brought out in this paper between the effects of restraints provided by the presence of wall sheathing on the overall performance of built-up wall studs in comparison to the unsheathed ones

A Computational Study on Buckling Behavior of Cold-Formed Steel Built-Up Columns Using Compound Spline Finite Strip Method

This paper presents a computational methodology to compute the critical buckling stress of built-up cold-formed steel columns joined with discrete fasteners. The fasteners are modeled as three-dimensional beam elements, and their effect is integrated into the spline finite strip framework, evolving the compound strip methodology. Although this technique has been presented in the literature, this paper presents yet another robust framework for the buckling load evaluation of compound cold-formed steel columns with arbitrarily located fasteners. The proposed framework is applied to study the effect of fasteners on the formation of local, distortional, and global buckling modes of built-up section and a comparison is drawn with the buckling behavior of a single section. In this study, the proposed formulations are also used to get insights into the stability behavior of single-span and multi-span compound cold-formed steel columns in the presence of (i) fasteners with varied spacings with respect to span and (ii) the presence of the additional restraining system such as wall panels. For different buckling modes, a significant increment in buckling stress for a built-up section from a single section is observed when the fastener spacing is kept less than the critical buckling half-wavelength of the respective buckling modes. The study on the effect of wall panels shows that in comparison to unsheathed wall studs, the sheathed wall studs that produce additional constraints lead to the elimination of the global buckling deformations. The proposed formulations are simple, yet rigorous and have been validated using finite element-based numerical results

Global Buckling Behaviour of Intermittently Fastened Cold-Formed Steel Built-Up Columns

In the load-bearing wall frame system, built-up sections are utilized over single sections to enhance the performance of cold-formed steel (CFS) structural systems. Built-up CFS sections can be formed in different shapes by the use of intermittently connected fasteners in the longitudinal direction. The spacings of these intermittent fasteners can alter the global buckling behavior of built-up CFS columns, and hence, they are investigated in the present study. The change of global buckling load from a single section CFS column to a built-up section CFS column is studied with different fastener spacing. In this paper, a numerical methodology using a compound spline finite strip method is developed to compute the elastic critical buckling load of CFS built-up columns. The results of buckling analysis are compared with FE based software ABAQUS and results are found to be in good agreement. Parametric studies on back-to-back connected I section with different web to flange ratios have been carried out, and it is found that global buckling behavior of open built-up sections will move towards fully composite section buckling behavior with the reduction in the fastener spacing

Explicit incremental matrices for the postbuckling analysis of thin plates with small initial curvature

The postbuckling behaviour of thin plates is an important phenomenon in the design of thin plated structures. In reality plates possess small imperfections and the behaviour of such imperfect plates is of great interest. To numerically study the postbuckling behaviour of imperfect plates explicit incremental or secant matrices have been presented in this paper. These matrices can be used in combination with any thin plate element. The secant matrices are shown to be very accurate in tracing the postbuckling behaviour of thin plates

Improved secant matrices for the postbuckling analysis of thin composite plates

To study the postbuckling behavior of imperfect laminated composite plates, improved incremental or secant matrices are presented in this paper using what is called additional displacement formulation (ADF). These secant matrices are derived using the Marguerre's shell theory and they can be used in combination with any thin plate finite element. The advantage of the present formulation is that it involves no numerical approximation in forming the initial imperfection matrices as opposed to earlier secant matrices published in the literature using total displacement formulation. With the addition of shear stiffness matrix and little modification, the present incremental matrices could be extended to model postbuckling behavior of plates using the first-order shear deformation theory. The secant matrices presented in this study are shown to be very accurate in tracing the postbuckling behavior of thin isotropic and laminated composite plates with general initial imperfections

Global buckling strength of discretely fastened back-to-back built-up cold-formed steel columns

This study proposes a design equation for built-up cold-formed steel (CFS) columns to eliminate the inconsistencies present in Section I1.2 of AISI S100 specification. The proposed equation was developed by using the results from the authors' compound spline finite strip-based numerical framework and 228 experimental and numerical results from literature on built-up CFS columns, failing by minor axis buckling. Investigations were conducted using numerical models with mode-specific deformations incorporated in compound spline finite strip framework. The shear slip behaviour between the individual columns, characterised by (i) fastener spacing and (ii) slenderness ratio of fully-composite cross-section, was investigated using parametric variations. The study shows that the modified slenderness ratio (MSR) in Section I1.2 of AISI S100 yields conservative strength predictions with increased fastener spacing. This is possibly because the MSR considers the effect of fastener spacing and overall slenderness ratio as disjoint and treat them as additive components. In reality, they interact, and hence a compound slenderness ratio (CSR) is proposed with a term that captures the interaction of these two entities. The prediction of strength by the proposed CSR and direct strength method (DSM) is in good agreement with the results of experimental and numerical studies, with the mean close to one. When incorporated in the DSM design equations, the proposed CSR yields matching reliability with AISI S100.</p

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.</p

Design of Cold-Formed Steel Built-Up Back-to-Back Columns Subject to Local-Flexural Interactive Buckling

The North American Standard (NAS) and the European Standard (EC3) for cold-formed steel design provide limited guidelines for the strength predictions of built-up columns. The existing design equations are limited to flexural buckling and local-flexural interactive buckling. The present study evaluates the local-flexural interactive design equations for cold-formed steel built-up back-to-back I-section columns against the results of experimental and numerical studies. The results show that the existing design guidelines for local-flexural interactive buckling are unconservative by about 17%, 5%, and 15% for DSM-NAS, EWM-NAS, and EWM-EC3, respectively. Furthermore, a maximum reduction in the built-up column strength by local-flexural interaction is observed when its nominal local and flexural buckling loads are close. Based on the results and observations, new design equations are proposed by including the product of nominal local and flexural buckling loads for the North American and European standards, which capture the local-flexural interactive buckling in built-up I-section columns. The effect of fastener spacing on the ultimate load for local-flexural interactive buckling is also evaluated, and its effect is found to be small, unlike for global buckling. The minimum fastener spacing criteria of NAS are assessed, and suitable recommendations are provided for fixed-end columns. Overall, the present study provides insights into the local-flexural interactive buckling behavior of built-up I-section columns and proposes reliable design equations to predict the ultimate load.</p