Flexural capacity of gapped built-up cold-formed steel channel sections including web stiffeners

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Highlights

  • Eighteen laboratory tests are reported on the flexural capacity of back-to-back gapped built-up CFS channel sections under four-point bending.

  • Three different types of channels: plain channels, channels with one web stiffener and channels with two web stiffeners were used to for the gapped built-up beams.

  • A nonlinear FE model was then developed, which included initial imperfections and modeling of link-channels and fasteners.

  • An extensive parametric study was conducted to investigate the effects of web stiffeners and link-channel spacing on the flexural capacity of such gapped built-up beams.

  • It is shown that design in accordance with the AISI & AS/NZS can be conservative by as much as 27%.

Abstract

This paper describes an experimental and numerical investigation to study the flexural capacity of back-to-back gapped built-up cold-formed steel (CFS) channel sections under four-point bending. The gap between the back-to-back channels was formed through intermediate link-channels which were screwed to the webs of the back-to-back channels. A total of 90 results comprising 18 laboratory tests and 72 finite element (FE) analysis results are reported on the flexural capacity of back-to-back gapped built-up CFS channel sections under four-point bending. Three different types of channels were considered to form the built-up channels: plain channels, channels with one web stiffener and channels with two web stiffeners. Two different beam spans as 1000 mm and 2000 mm were tested. Initial geometric imperfections were also measured prior to bending tests for all test specimens. A nonlinear FE model was then developed, the results of which showed good agreement against the laboratory test results. Using the validated FE model, an extensive parametric study was conducted to investigate the effects of web stiffeners and link-channel spacing on the flexural capacity of such gapped built-up beams. It is shown that design in accordance with the American Iron and Steel Institute, AISI (2016) and Australia/New-Zealand standards, AS/NZS (2018) can be conservative by as much as 27%. The flexural capacity of back-to-back gapped built-up sections with two web stiffeners was increased by 10% on average, when compared to the capacity of gapped built-up beams with plain channel sections.

Introduction

In this paper, the results of 18 new laboratory tests on back-to-back gapped built-up cold-formed steel (CFS) channel sections, with the sections acting as beams, are presented. Fig. 1 shows the photograph of the built-up beam investigated in this paper. As can be seen from Fig. 1, the gap is formed through link-channels screwed between the webs of the back-to-back channel sections. Such gaps are commonly introduced in struts in steel trusses and beams in framed buildings (see Fig. 2), increasing the lateral stability and strength of such beams. The beneficial effects of the gap between the back-to-back channels can be summarized as follows:

  • (1)

    Improved ease of lifting and installation on-site, due to increased lateral stiffness.

  • (2)

    Fewer lateral braces required due to improved lateral-torsional buckling resistance.

  • (3)

    Ease of installation of electrical or plumbing services through the gap.

In Fig. 3, the 3D drawing and arrangement of the link-channels for back-to-back gapped built-up CFS channel sections are shown. The cross-sectional details of the three different types of back-to-back gapped built-up sections studied, is shown in Fig. 4. As can be seen from Fig. 4, the three different cross-sections were considered to form the gapped built-up beams, which include plain channel section, channel section with one web stiffener and channel section with two web stiffeners.

In the literature, no research work is reported which investigated the flexural capacity of back-to-back gapped built-up CFS channel sections. However, research is available on the same configuration of built-up section (back-to-back gapped built-up section) under axial compression. Rondal and Niazi [1] reported three experimental results in 1990 for such back-to-back gapped built-up channels under axial compression. Recently the authors of this paper [2] reported a detailed experimental investigation on the axial capacity of back-to-back gapped built-up CFS channel sections. In total, forty laboratory tests were reported by Roy et al. [2], covering a wide range of non-dimensional slenderness's from 0.23 to 1.42 for such gapped built-up columns. In addition, a finite element (FE) model was also developed which was validated against these test results [2]. The FE model was used for the purpose of a parametric study. From the parametric study results, a design modification was proposed for such back-to-back gapped built-up CFS channel sections under compression [2]. Furthermore, Anbarasu and Dar [3] reported a numerical study on the axial capacity of gapped built-up CFS channel columns. An extensive parametric study was conducted by varying the key parameters including plate slenderness of the lipped channels, unbraced chord slenderness, and global slenderness in order to investigate their variation on the axial capacity of these gapped built-up columns. From the results of the parametric study, new design equations were proposed for axial capacity of gaped built-up CFS channel section columns [3].

For the case of back-to-back sections without any gap, significant research is available in the literature. The flexural behaviour of the back-to-back built-up LiteSteel beams was investigated by Jeyaragan and Mahendran [4]. Wang and Young [5] conducted a series of beam tests on the CFS back-to-back built-up sections, but these were plain channels without any web stiffeners, and should also be noted that no gap was formed between these back-to-back channels. Laim et al. [6] conducted 12 four-point bending tests and 52 numerical simulations on CFS back-to-back and face-to-face built-up sections connected by screws, but again without any gap. Deepak and Shanthi [7] conducted a comprehensive experimental and numerical study on the distortional buckling–moment resistance capacity of built-up CFS hybrid double-I-box beams (HDIBBs) under four-point bending. On the other hand, Ye et al. [8] conducted an experimental study to investigate the local-distortional buckling interaction behaviour of back-to-back built-up CFS channel beams under four-point bending, It was found by Ye et al. [8] that the current design guidelines by the Direct Strength Method (DSM) were conservative, while predicting the ultimate moment capacities of such built-up beams failed by local-distortional buckling interaction. Ye et al. [9] also developed a FE model to study the strength and deflection behaviour of back-to-back built-up CFS channel section beams under four-point bending. The FE model was validated against the experimental results reported by Ye et al. [8].

In terms of CFS single channel section under bending, significant research is available in the literature, which investigated the flexural capacity and deflection behaviour of such single channel sections (Zhao et al. [10], Yu [11], and Moen et al. [12]). In recent years, some researchers investigated the structural behaviour of CFS single channel stiffened sections. Wang and Young [13] investigated the effect of intermediate web stiffeners on flexural behaviour of single channel sections subjected to bending. The study by Wang and Young [13] indicated that the nominal moment capacities predicted using the current DSM are quite conservative for the CFS channels with stiffened webs subjected to bending.

The design guidelines in accordance with the American Iron and Steel Institute (AISI) [14] and Australian and New Zealand Standards (AS/NZS) [15], do not include the effects of the gap between the back-to-back channels under bending. Therefore, current design practice rule to calculate the flexural capacity of gapped built-up beams is simply twice that of a single channel section. This is the case regardless of whether the Effective Width Method (EWM) or the DSM is used. The DSM was recommended by Wang and Young [13] for the design of single CFS channels with stiffened webs subjected to bending, however this [13] was for single channels and not for the built-up or even gapped beams.

As mentioned previously, this paper presents an experimental and numerical investigation on the flexural capacity of back-to-back gapped built-up CFS channel sections under four-point bending. Three different types of cross sections were considered to form the built-up gapped sections: plain channels, channel sections with one web stiffener and channel sections with two web stiffeners. In total, 18 laboratory tests were reported. The effects of web stiffener, link-channel spacing and gap between the back-to-back channels were investigated on the flexural capacity of the gapped built-up beams. The failure moment capacities, moment-displacement, and deformed shapes at failures are discussed for all test specimens.

A non-linear FE model was then developed and validated against these test results. The FE model included geometric imperfections and modeling of link-channels. Using the validated FE model, a parametric study was conducted to study the effects of web stiffener and gap between the back-to-back channels on the flexural capacity of such gapped built-up beams. Both the experimental and FE results were compared against the design strengths calculated in accordance with the AISI [14] and AS/NZS [15]. This paper has therefore presented the details of the experimental and numerical investigations and their results on the flexural capacity of back-to-back gapped built-up CFS channel sections under four-point bending.

Section snippets

Test specimens

In this study, a total of 18 simply supported back-to-back gapped built-up CFS channel sections acting as beam members were tested in the laboratory. The test specimens were firstly brake-pressed from G450 grade of CFS sheets to form the lipped channels, and then two of the same channel sections were connected together with the help of link-channels. The link-channels were connected through the webs of the back-to-back channels by intermediate screws to assembly the built-up gapped sections.

General

Finite element (FE) software ABAQUS [17] was used to develop a nonlinear elasto-plastic FE model for the back-to-back gapped built-up CFS channel sections under four-point bending. The FE model was based on the centre line dimensions of the cross-sections. In the FE model, the measured cross-section dimensions were used. The objective of numerical analysis in this study is to obtain the beam strengths rather than to investigate the failure of the screws, since the screws were not damaged before

Design approaches

The design of back-to-back gapped built-up CFS channel section beams is not covered in the current specifications [14,15]. The current direct strength method (DSM) implements rotational analysis method in thin-walled CFS through the direct use of member elastic buckling solutions. However, as mentioned previously the finite strip analysis program Thin-Walled-2 [24], based on the DSM, cannot be directly applied to the gapped built-up beams. Therefore, the moment capacity of the single channel

Parametric study

A parametric study comprising 54 models was conducted using the validated FE model. The same cross section (GBU155 × 45 × 20) as investigated in the experimental program, was used in the parametric study. A wide range of slenderness, covering length of beams from 300 mm to 2000 mm were considered in the parametric study (Table 5). As can be seen, three different numbers of link-channels were considered in the parametric study: 3 numbers, 5 numbers and 10 numbers.

In order to understand the

Conclusions

Experimental and numerical investigations of back-to-back gapped built-up CFS channel sections under four-point bending have been presented in this paper. A total of 18 laboratory tests were reported, 6 of which were for built-up gapped plain channel Sections, 6 were for built-up gapped channels with one web stiffener and the remaining 6 tests were on built-up gapped channels with two web stiffeners. The failure modes, moment capacity-displacement behaviour, and deformed shapes at failure are

Declaration of Competing Interest

None.

Acknowledgement

The authors gratefully acknowledge the support of Mr. Mei Chee Chiang from Ecosteel Sdn. Bhd., Kuching, Malaysia for kindly donating the test specimens. We also thank Mr. Tang Zing Wei for his assistance in the lab tests.

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