Structural behaviour of prefabricated stressed-skin engineered timber composite flooring systems

Abstract

The primary focus of this study is understanding the behaviour and structural performance of prefabricated composite timber floor cassette systems with Oriented strand board (OSB) stressed-skins. These do not have a dedicated top flange, instead their plywood webs are integrally bonded to the OSB flooring skin through a chemical connection. Subsequently, local buckling of the stressed-skin was found to occur as the first failure mode. Specimens with 150 mm and 300 mm nail spacings were investigated for its effects on performance due to the criticality of the integrated web to floor skin connection in these systems. A total of 20 stressed-skin specimens were tested in three-point bending with recordings of the applied force, displacement, slip and failure modes. It was found that local buckling of the skin is prone to occur prior to reaching the designed SLS limit. A detailed Finite Element Analysis (FEA) which takes into consideration the full behaviour of the materials and the glue and nail connections along with failure modes has been validated and used to provide insight to potential design solutions. Key parameters investigated include the adhesive properties, the ratio of clear effective outstand width of the flange to the thickness of the stressed-skin (b_eff,o/t), ratio of the clear depth of the web to the thickness of the web (d_p/t_w), ratio of the clear span to the total depth of the composite beam (L_c/D) and finally the spacing of the nails. This has resulted in a broad level understanding of the effects of these design parameters to the behaviour of stressed-skin engineered timber flooring cassettes.

Introduction

Prefabrication is a growing method of modern construction that has vast opportunities in the fields of automation and advance manufacturing which are being refined and improved upon to better compete against traditional methods of construction. This is particularly in terms of cost competitiveness, quality of work, flexibility of design and sustainability [[1], [2], [3]]. Prefabrication is a suitable platform to adopt novel and efficient materials as well as to develop an optimum combination of several materials to form components of larger systems. One such material type and system which is focused upon in this research are engineered timber-based flooring systems which are suitable for Design for Manufacturing and Assembly (DfMA) and offsite construction. Engineered Wood Products (EWPs) are increasingly being developed and used for the prefabricated building industry as a more versatile and better performing solution compared to conventional solid sawn-cut wood and other construction materials [4]. Engineered timber I-beams have been relatively well studied in many areas such as flexural behaviour [5], bearing capacity [6], torsional rigidity [7], composite use of materials [8], shear behaviour [9], span and design guidelines [10] and behaviour with web openings [11]. Even there are standards such as ASTM - D5055 which have been developed to provide a ‘Standard Specification for Establishing and Monitoring Structural Capacities of Prefabricated Wood I-Joists’ [12]. EWPs have been well studied and their behaviour has been well understood. However, there is a lack of work in the effective combination of types of EWPs used together as a composite system and their further refinement. Engineered timber I-beams have a top and bottom flange with a web in-between. When adapted to prefabrication in the form of cassettes then they are also connected with rimboard along the outside edge to prevent overturning as shown in Fig. 1 [13].

This study forwards the use of reductive design to potentially further refine these systems. Reductive design as a philosophy builds on the principles of minimalism and removal of the non-essential through the minimisation of the number of components [[14], [15], [16]]. For this project this concept was applied to traditional engineering timber floor systems by the removal of the top flange since the web is now to be directly and integrally bonded to the skin/sheathing as shown in Fig. 2. By gluing the floor skin to the joist, a stressed-skin timber flooring solution where the flooring sheet carries load through composite action is achieved [17]. This was practically achieved due to prefabrication and controlled manufacturing methods which allowed both a glued and nailed connection to be repeatedly and reliably made rather than a nailed only connection.

Reductive design does not only reduce the number of components and offers ‘pure aesthetic’ [14] but it has vast practical advantages beyond direct cost savings. These are centred around prefabrication and thus are exploited in the proposed system. For example, manufacturing complexity decreases, supply chain and inventory is smaller and general management and quality assurance processes also become simpler. However, reducing complexity may limit technical functionality and performance. It has been discovered in early investigations of stressed-skin action that for joist spacings of 450 mm to 600 mm that between 62% and 83% of the floor panelling/sheathing acted in composite with the joists [18]. Therefore, full composite behaviour may not occur. Additionally, the spacing and the shear effects in the floor under compression results in less than full usage of the sheathing [19]. However, there is potential for floor skins to act in the same function as a top flange of a regular I-beam. Plywood (PLY) has been investigated as the skin material in the past [[19], [20], [21]], although since then Oriented Strand Board (OSB) has taken over many markets as it sources underutilised softwood species and is notably cheaper [22]. Some early work has been carried out in the investigation of creep in these OSB skinned systems [23], design equations for shear and flexural deflection under point load [24] and local buckling of OSB without perpendicular stiffening [25]. However, there is little to no research observing the use of OSB in stressed-skin flooring cassettes or on the structural analysis and improved design thereof.

In summary, the hypothesis is that if the top flange is removed and the flooring panel is adequately bonded through both adhesive and nails to the joists then the floor panel/sheathing can essentially act as the top flange of the joist. That is, it may take in-span compressive loads and hence become a stressed-skin. In this way, stressed-skin systems may be prone to local buckling prior to reaching serviceability and ultimate capacity limits. Thus, this investigation delves into the understanding of the force-deflection behaviour, failure modes and configuration adjustments in prefabricated stressed-skin floors which do not have a separate dedicated top flange in contrast to traditional flooring cassettes which do.