Axial strength of back-to-back cold-formed steel channels with edge-stiffened holes, un-stiffened holes and plain webs

doi:10.1016/j.jcsr.2020.106313

Journal of Constructional Steel Research

Volume 174, November 2020, 106313

https://doi.org/10.1016/j.jcsr.2020.106313 Get rights and content

Highlights

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A new generation of back-to-back CFS channels with edge-stiffened holes is examined.
•
Results from an experimental regime and elasto-plastic finite element are reported.
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A parametric study involving 135 FE models was conducted.
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Variables included column slenderness, hole size, screw spacing, stiffener lengths and stiffener fillet radius.
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Edge stiffened hole-channels can outperform plain channels in terms of axial strength.

Abstract

Cold-formed steel (CFS) back-to-back channels are becoming a popular choice for use in load-carrying members in building structures. These built-up channels often include web holes for installation of services. Traditional web holes are un-stiffened, which can restrict the size and spacing of holes. Recently, a new generation of CFS channels with edge-stiffened holes has been developed and used widely in New Zealand. No research has been reported in the literature to investigate the axial strength of back-to-back channels with edge-stiffened holes. Experiments are required on back-to-back channels with edge-stiffened holes so to understand the effects of composite actions between the back-to-back channels on the axial strength of such channels. This paper presents a total of 162 new results comprising 27 axial compression tests and 135 finite element analysis (FEA) results on the axial strength of back-to-back channels with edge-stiffened holes, un-stiffened holes and plain webs. Prior to compression tests, initial geometric imperfections were measured using a laser scanner. Tensile coupon tests were also conducted to determine the material properties of both the flat and corner portions of the channels. The test results show that for the case of back-to-back channels with edge-stiffened holes, the axial strength increased by 6.6% on average, compared to a back-to-back plain channel. For comparison, the same section with un-stiffened holes had a 12.4% reduction on average in axial strength, compared to a back-to-back plain channel. A nonlinear elasto-plastic finite element (FE) model was then developed, and the results showed good agreement with the test results. The validated FE model was used to conduct a parametric study involving 135 FE models to investigate the effects of column slenderness, diameter of hole, screw spacing, stiffener lengths and stiffener fillet radius on the axial strength of such channels. Finally, the tests and parametric study results were compared against the design strengths calculated in accordance with the American Iron and Steel Institute (AISI) (2016) and Australian and New Zealand Standards (AS/NZS) (2018) for back-to-back plain channels and against the design equations of Moen and Schafer (2011) for back-to-back channels with un-stiffened holes. It was found that the AISI (2016) and AS/NZS (2018) are only 3% conservative to the test results. The Moen and Schafer equations (2008, 2009, 2011) are conservative by 21% on average for back-to-back channels with un-stiffened holes.

Graphical abstract

Deformed shapes at failure from experiments and FEA.

Introduction

Back-to-back cold-formed steel (CFS) channels (Fig. 1) are used increasingly in wall studs, wall frames, and columns in portal frames due to their higher strengths. In built-up channels, intermediate fasteners at discrete points along the member length are used to prevent the individual channels from buckling independently. The channels often include un-stiffened holes (Fig. 1(a)) for the installation of services. Such holes, however, reduce the axial strength of the channels. Recently, a new generation of CFS channels with edge-stiffened holes (Fig. 1(b)) has been developed and used widely in New Zealand.

In the literature, limited research is available on the axial strength of such CFS single channels with edge-stiffened holes. Chen et al. [1,2] reported experimental and numerical studies on the axial strength of such single channels with edge-stiffened holes and found that the axial strength of single channels with edge-stiffened holes was greater than those of plain channels. This study extends the work of Chen et al. [1,2] from single channels to back-to-back channels with edge-stiffened holes, where the two channels are connected by intermediate fasteners, for which the axial strength can be expected to be higher than twice that of single channels with edge-stiffened holes. To quantify this, 27 new experiments were conducted, the results of which are all reported in this paper.

The American Iron and Steel Institute (AISI) [3] and the Australian and New Zealand Standards (AS/NZS) [4] describe a modified slenderness approach for back-to-back plain channels. Also, for determining the axial strength of CFS single channels with un-stiffened holes, design guidance is available in AISI [3] and AS/NZS [4]. However, no design guidelines are currently available for back-to-back channels with holes, whether they are edge-stiffened or un-stiffened.

In terms of back-to-back channels with un-stiffened holes, limited research is available in the literature. The effects of un-stiffened holes on the axial strength of back-to-back channels were analyzed by Stone and LaBoube [5]. This study [5] concluded that for thicker materials, the modified slenderness ratio was not necessary for the axial strength of back-to-back channels. The effect of holes on the axial strength back-to-back channels for thinner sections of higher grade was not considered by Stone and LaBoube [5].

For back-to-back plain channels, Roy et al. [[6], [7], [8], [9], [10], [11]] reported several studies where tests and finite element (FE) modelling of such back-to-back channels were conducted. The latter found that the effect of screws numbers on the strength of such specimens was demonstrated to be negligible for those stub columns. An experimental study was conducted by Fratamico et al. [[12], [13], [14]] to assess the composite action of back-to-back plain channels. For slender columns, it was found that the screws can increase the strength by as much as 21% for slender columns. For stub columns, the composite action of back-to-back plain channels was investigated numerically by Li et al. [15]. As expected, when the failure was through local buckling, the axial strength was equal to approximately two times that of the corresponding single channels. Dabaon et al. [16,17] experimentally and numerically investigated the behaviour of axially loaded back-to-back plain channels, but these were battened columns. They [16,17] found that the design strength determined from the AISI [3] was unconservative for the specimens which failed through local buckling. Lu et al. [18] conducted experiments on back-to-back CFS channels under compression and proposed a modification to the current direct strength method (DSM). Furthermore, Zhou et al. [19] presented an analytical approach to predict the axial strength of back-to-back CFS channels failing in flexural buckling. Zhang and Young [[20], [21], [22], [23]] experimentally and numerically studied the behaviour of CFS built-up open and closed section columns with web stiffeners, and modified DSM was proposed to closely predict the axial strength of such columns with web stiffeners.

As mentioned previously, this paper reports 27 new experiments on the axial strength of back-to-back channels with edge-stiffened holes, un-stiffened holes and plain webs. In addition, a total of 18 tests were conducted on corresponding single channels to study the effect of composite action. Prior to compression tests, initial geometric imperfections were measured using a laser scanner. Tensile coupon tests were also conducted to determine the material properties of both the flat and corner regions of the channels. A nonlinear elasto-plastic FE model was then developed and validated against the test results in terms of ultimate strength and deformed shapes. Using the validated FE model, a parametric study involving 135 models was conducted to investigate the effects of column slenderness, diameter of hole, screw spacing, stiffener lengths and stiffener fillet radius on the axial strength of such columns. The axial strength of back-to-back plain channels was determined from the AISI [3] and AS/NZS [4], while those specimens with un-stiffened holes and edge-stiffened holes were determined from two times the strength of corresponding single channels with un-stiffened holes. This is due to the unavailability of design guidelines for determining the axial strength of back-to-back channels with edge-stiffened and un-stiffened holes. Finally, the design strengths were compared against both the test and FEA results.

Section snippets

Test specimens

A total of 27 axial compression tests investigating the behaviour of back-to-back channels are reported in this study. This includes 9 tests on specimens with un-stiffened holes (Fig. 1(a)), 9 on specimens with edge-stiffened holes (Fig. 1(b)), and the remaining 9 on plain channels (Fig. 1(c)). The back-to-back channels were assembled from two single channels by the self-tapping screws at the web (Fig. 2). To study the effect of composite action, 18 tests were conducted on corresponding single

General

The non-linear FE analysis program ABAQUS was used to develop a numerical model for determining the axial strength of back-to-back channels with edge-stiffened holes, un-stiffened holes and plain webs. The measured dimensions of specimens, initial geometric imperfections and the material properties were incorporated in the FE model. Two different steps were used. The first step was to conduct a linear perturbation analysis (Eigen-value analysis) to obtain the buckling modes (Eigen-modes), while

General

The FE model developed in this study could closely predict the axial strength of back-to-back channels with edge-stiffened holes, un-stiffened holes and plain webs. Using the validated FE model, an extensive parametric study comprising 135 FE models was conducted to evaluate the influences of column slenderness, diameter of hole, screw spacing, stiffener lengths and stiffener fillet radius on the axial strength of such columns. The dimensions and the axial strength obtained from the parametric

Design for back-to-back plain channels [3,4]

Axial strength obtained from the numerical analysis and laboratory test were compared against the design strengths determined from the AISI [3] and AS/NZS [4] for back-to-back plain channels. The un-factored design strengths of back-to-back channels determined from the AISI [3] and AS/NZS [4] standards can be calculated using Eq. (3): $P_{AISI & AS / NZS} = A_{e} F_{n}$

The critical elastic buckling stress (F_n) was determined using Eqs. (4), (5) as follows:

For λ_c ≤ 1.5, $F_{n} = ({0.658}^{{λ^{2}}_{c}}) F_{y}$

For λ_c > 1.5, $F_{n} = (\frac{0.877}{{λ^{2}}_{c}}) F_{y}$

Where

Comparison of test and FEA strengths against design strengths

In terms of back-to-back plain channels, the comparison between the experimental results with the design strengths determined from the AISI [3] and AS/NZS [4] is shown in Table 5. The mean value of the P_EXP/ P_AISI&AS/ANS ratio is 1.03 with the COV of 0.02. As can be seen, the design strengths based on the modified slenderness approach [3,4] are conservative by only 3% on average, when determining the axial strength of back-to-back plain channels. Fig. 23 plotted the comparison of test and FEA

Conclusions

This paper presents a total of 27 experimental results on the axial strength of back-to-back channels with edge-stiffened holes, un-stiffened holes and with plain webs. In addition, 18 tests were conducted on corresponding single channels. Prior to compression tests, the magnitude of initial geometric imperfections was measured for all test specimens using a laser scanner. Tensile coupon tests were also carried out to obtain the material properties of both the flat and corner regions of the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Test specimens were provided by “Howick NZ. Ltd.”, which is greatly acknowledged by the authors. The compression tests were conducted in the structural lab of the University of Auckland. The contribution of Mark Byrami and Zhiyuan Fang in helping to set up the tests is greatly appreciated. The financial support provided by the China Council Scholarship (CSC) from the Chinese government is greatly acknowledged.

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Axial strength of back-to-back cold-formed steel channels with edge-stiffened holes, un-stiffened holes and plain webs

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Test specimens

General

General

Design for back-to-back plain channels [3,4]

Comparison of test and FEA strengths against design strengths

Conclusions

Declaration of Competing Interest

Acknowledgements

Thin-Walled Struct.

J. Constr. Steel Res.

Thin-Walled Struct.

Struct.

J. Constr. Steel Res.

J. Constr. Steel Res.

Thin-Walled Struct.

Eng. Struct.

Thin-Walled Struct.

Thin-Walled Struct.

J. Constr. Steel Res.

Thin-Walled Struct.

J. Constr. Steel Res.

J. Constr. Steel Res.

Thin-Walled Struct.

Thin-Walled Struct.

Thin-Walled Struct.