Behaviour of plasterboard-lined steel-framed ceiling diaphragms

Highlights

• Performance of steel-framed ceiling diaphragms is mainly governed by the behavior of the sheathing-to-framing connections.

• Ceiling diaphragm test showed a failure of the plasterboard connections at the corners of the specimen.

• Presence of end-wall top plate increased the load carrying capacity and displacement capacity of ceiling diaphragm.

• Finite element model results showed good agreement with the experimental results.

• Design engineer can prepare design charts to provide maximum spacing of bracing walls for houses constructed in Australia.

Abstract

Behaviour of low rise cold-formed steel-framed residential structures when subject to lateral loading (e.g. wind and earthquake loads) is significantly influenced by both structural and non-structural elements. The ceiling is normally considered as a horizontal diaphragm for the distribution of such lateral loads to the bracing walls. In Australia, the ceiling diaphragm, in single and two storey cold-formed steel-framed houses, is made of plasterboard lining fixed to ceiling battens which in turn are screwed to the roof trusses. Determination of the stiffness and strength of the ceiling diaphragm is crucial for accurate distribution of the lateral load to the lateral resisting elements (e.g. bracing walls). This paper presents experimental results from tests of typical ceiling diaphragms. Further, a detailed finite element model is described which was validated against the experimental results. The model is capable of reproducing the initial stiffness, non-linear deformation and ultimate strength. The finding from this work would assist engineers in designing the lateral load resisting system for houses.

Introduction

A diaphragm is a structural system (usually horizontal) that acts to transmit lateral loads to the vertical lateral resisting system (such as bracing walls) as illustrated in Fig. 1. Stiffness characteristics of diaphragm are essential for accurate assessment of the behaviour of structures under lateral loading. Knowledge of diaphragm stiffness is crucial for proper distribution of the lateral load to the bracing walls. In wood light-framed buildings, the diaphragm is designated as either flexible or rigid for the purpose of lateral load distribution [2]. The International Building Code (IBC) [3] has considered either a “flexible” or “rigid” diaphragm based on the relative stiffness of the diaphragm to the bracing walls. However, in Australian design standards, there is no reference to the rigidity of the ceiling/roof diaphragms. Reardon [4] conducted a full scale testing of a steel-framed house and specified that the ceiling diaphragm may be considered as rigid. Paevere et al. [5] stated that the roof/ceiling diaphragm behaved as a rigid diaphragm, while Breyer et al. [2] mentioned that ceiling/roof diaphragms can be considered as flexible. If the load distribution is not anticipated appropriately, an engineered structure can produce a false sense of confidence of safety and performance compared to the one that has not been ‘engineered’ [6]. Therefore, the under- or over-simplification of the structural behaviour of light-frame structures may result in inefficient designs or potential failures [7].

In Australia, plasterboard is typically used as interior lining on light-framed residential structures. As an interior lining it can be used to provide bracing resistance for walls as per AS1684 and NASH Standard Part 2 [8]. This has been confirmed by testing of walls systems and full houses under simulated wind, cyclonic and earthquake loads [9,10]. Based on the observation of failure modes of plasterboard lined frames when subjected to lateral load, Liew [11] found that board density and mechanical properties of the paper lining are key parameter influencing the bracing performance of plasterboard. Accordingly, Liew et al. [12] developed a simple test method to assess the quality of plasterboard as a composite material in resisting in-plane shear loads when it is used as a bracing material.

Light sheet cladding has been used for some time on steel and timber frames as part of roof diaphragms. One of the most common applications is the use of light gauge corrugated roof sheeting fastened to cold formed steel trusses or frames which are typically design as stressed skins [[13], [14], [15], [16], [17]]. However, there is very limited tests and verified models for plasterboard lined ceilings in steel-framed houses. Indeed, for timber-framed houses there is very limited information to allow a designer to determine the capacity of ceiling diaphragm to transfer lateral loads. For instance, Walker and Gonano [18]; Walker and Gonano [19]; Walker et al. [20] investigated the ceiling diaphragms action in timber-framed structures and developed empirical design data which are contained in the Australian Standard, AS1684.2 [21]. However, if the geometric configurations (e.g., spacing between bracing walls) fall outside the specific limits in AS1684.2 [21] there are no guidance whatsoever. Therefore, there is a need to evaluate the strength and stiffness of ceiling diaphragm to correctly design the lateral load-resisting system. The development of a rational design method would allow Australian designers and manufacturers to develop optimised systems rather than relying on extrapolation of historical empirical data.

This paper is part of a larger investigation undertaken to develop a rational method for design of plasterboard lined steel framed ceiling diaphragms [22]. This paper in particular presents the experimental tests of plasterboard-lined steel-framed ceiling diaphragms and utilises the test results to develop finite element models for evaluating the stiffness and strength of various diaphragm geometries and boundary conditions. Knowledge of the stiffness and the strength of diaphragm would assist designers to develop the appropriate lateral load distribution to the bracing walls through the ceiling diaphragm.