Numerical studies into factors affecting structural behaviour of composite cold-formed steel and timber flooring systems

https://doi.org/10.1016/j.jobe.2021.102692Get rights and content

Highlights

  • A non-linear analysis of composite CFS and timber beam is presented and validated.

  • Parametric studies has been conducted to study the influence of various parameters.

  • A simplified design method is proposed based on numerical studies.

  • Design recommendation is made based on the FEA results observation.

Abstract

Lightweight flooring system made up of cold-formed steel joist, and timber floorboard is widespread but the benefits of composite action that arise due to the interaction of top flange of cold-formed steel joist and the bottom surface of timber floorboard as a result of mobilising the shear connection are not considered in their design. A three-dimensional (3D) finite element model was developed and validated against the experimental results for cold-formed steel and particle board flooring system. The validated numerical model was used for parametric studies to investigate the influence of various factors that affect the structural behaviour of the composite flooring system. The results from the parametric studies showed that higher strength and stiffness values of engineered timber product, as well as their increased thickness, enhances the moment capacity and stiffness of the flooring system. The reduction in the spacing of the cold-formed steel joist was found to increase the stiffness and hence the load-carrying capacity of the flooring system. The high strength to weight ratio of cold-formed steel flooring system is also demonstrated in this study. A simplified design method is proposed herein to predict flexural capacity of composite beams taking into account for the composite action. The finding in this study indicates that the design and construction of composite cold-formed steel and timber flooring system should be subjected to availability of the engineered timber product in the region,choice of timber floorboard thickness and joist spacing can be based on the ultimate strength and serviceability requirements of the flooring systems to make it cost-effective.

Introduction

The use of cold-formed steel as load-bearing and non-load bearing members in a variety of lightweight construction for residential, commercial and industrial buildings has significantly increased over the past decades [1,2]. The benefits of using cold-formed steel sections are high strength-to-weight ratio, ease of fabrication, and rapid installation [3,4]. Recent studies [5,6] have also highlighted the benefits of using cold-formed steel members in the building industry. The use of cold-formed steel joists with timber-based floorboards for the construction of lightweight flooring system in residential buildings is widespread. However, no design guidelines have been published yet for the flooring system to consider the beneficial effect of mobilising composite action on the flexural capacity [7,8].

The demand for lightweight flooring systems in the building construction industry is increasing over the recent years [9]. Initially, the application of cold-formed steel joist was limited to domestic floors, but the increasing urbanisation is causing a shift from domestic buildings to mid-rise apartments, making it as the best alternative to conventional timber floor joists [10]. Lightweight flooring systems which are made up of cold-formed steel joist and timber-based floor panels can be an economical and durable solution for the construction of flooring systems for the construction industry due to ease of mass production and rapid installation [11]. Composite cold-formed steel and timber flooring systems can be assembled off-site and be fixed on-site in a modular way which will reduce the construction time [12]. Such flooring system also offers an advantage of high strength to weight ratio, which eventually reduces the self-weight of floors and less imposed load on the foundation [13]. Most recently Karki and Far [14] has discussed about the trends and developments in composite cold-formed steel floors. Fig. 1 below shows a typical raised floor construction in a domestic building in Australia using cold-formed steel joists.

Several studies (e.g. Xu et al. [16], Xu and Tangorra [17], Parnell et al. [18]) evaluated the vibration performance of cold-formed steel lightweight floors. They investigated the key parameters that contribute to minimising floor vibrations without taking into account interaction between the timber floor-boards and cold-formed steel joist. Recently, researchers have started to explore the structural behaviour of this type of lightweight flooring system [8,19]. Li et al. [20] demonstrated the structural performance of cold-formed steel and bamboo composite floors to replace concrete or wooden slabs in residential buildings. The experimental and numerical investigation on composite flooring systems comprising cold-formed steel joist with cross-laminated timber [12], particleboard [11,19], and oriented strand board [8] demonstrated the potential for increased load carrying capacity and stiffness of composite flooring system when the shear connection is adequately provided taking into account the advantage of composite action. Far [21] also highlighted the importance of shear connection on composite flooring system comprising cold-formed steel and timber floorboards.

Experimental investigations alone can be time-consuming and costly to study all the associated factors that influence the strength and stiffness of the composite cold-formed steel flooring system [11,22]. Hence the need for finite element analysis that can cover the behaviour of the flooring system as captured in the laboratory is essential to study broadly about the topic and for further development. As a result, this study presents numerical studies to investigate the influence of type and thickness of timber floorboards, as well as spacing of joist that affect the structural behaviour of the composite flooring system considering the benefits of composite action that arises between the interface of cold-formed steel joist and timber floor-boards. The four-point bending tests carried out by Kyvelou et al. [19] have been used to validate the numerical models.

Section snippets

Development of finite element models

Several researchers [2,23,24] have demonstrated the importance of nonlinear finite element analysis to study the structural behaviour and performance of cold-formed steel. Few recent studies [8,11,25] have utilised nonlinear finite element analysis to simulate the behaviour of composite flooring system comprising cold-formed steel joists. The numerical results from those studies were found to be close enough to experimental measurements. ANSYS 19.1 [26] was used for the numerical investigation

Validation of finite element models

The accuracy of the numerical model developed in this study was confirmed after the numerical results were validated against the experimental test results performed by Kyvelou et al. [19]. The validated model was then used for parametric studies to study the influence of the different type of engineered timber products, the thickness of floorboard and joist spacing. The cross-sectional shape, height (250 mm), and thickness (3 mm) of the cold-formed steel joist were kept constant.

Kyvelou et al. [

Parametric studies and result discussions

The main objective of this study is to carry out parametric studies once the numerical model is validated. This section describes the influence of using different type of engineered timber products, varying the thickness of floorboard, and changing the spacing of the cold-formed steel joists on the flexural capacity of the studied composite cold-formed steel and timber flooring system. The ultimate moment capacity of the bare steel beam (without particleboard) was reported as 46.39 kN m from

Simplified calulation and analysis

The model validation and parametric studies results indicate that the ultimate moment capacity and stiffness of the composite cold-formed steel and timber flooring system could be significantly improved by the utilisation of shear interaction between top flange of CFS joist and bottom surface of timber floorboard. A cold-formed steel joist that is sheathed with timber floorboard acts as composite T-beam. While resisting bending in the composite member, timber floorboard acts as the compression

Conclusions

In this study, a finite element model has been developed and validated using four-point bending test results carried out by Kyvelou et al. [19]. The parametric numerical investigation was carried out to study the influence of utilisation of different types of engineered timber floorboards, variation in the floorboard thickness, and change in the joist spacing on the flexural behaviour of the flooring system. The investigation on the influence of different engineered timber floorboards indicates

Author statement

All persons who meet authorship criteria are listed as authors, and all authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in any other publication before its appearance in the Journal of Building

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.

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