Size matters: understanding size and scale effects for the safe design of very large engineered timber structures

This proposal aims to ensure that the safe design of large, engineered timber structures is not compromised by inadequate understanding of the influence of increasing member size on structural capacity. In the face of the pressing need to address the infrastructure and building needs of rapidly growing urban populations whilst simultaneously and drastically reducing the environmental impacts of the built environment, there is a widespread resurgence in interest in the use of timber in construction. The development of advanced engineered timber materials formed of adhesively or mechanically bonded lamellae in a parallel or sub-parallel arrangement has prompted rapid translation of this technology into larger scale structures. In contrast to the rapid pace of implementation, little research has been conducted into the behaviour of axially loaded engineered timber members at very large sizes. A particular danger with allowing research to lag practice in this context is that the relatively high safety factors used to ensure suitably low probability of failure in civil structures mean that unrecognised or inadequately understood effects may reduce reliability by orders of magnitude before this reduction begins to become evident in practice. Since it is inevitably the case that the largest structural members required in design cannot be tested to failure, extrapolation based on properly corroborated theory is inevitable. It is therefore necessary to conduct specially designed test series across the range of sizes that can be tested to failure, in order to provide a suitable evidentiary basis for any theory or modelling used to predict behaviour at larger sizes. The research hypothesis is that the strength of large, engineered timber members is significantly affected by at least two confounding and competing phenomena: an adverse size effect relating to the quasi-brittle nature of the constituent materials; and a beneficial system effect relating to the lay-up of the engineered material. Safe design of large, engineered timber members therefore requires the influence of both phenomena to be understood and quantified. This research proposal addresses this research hypothesis in relation to large glued laminated timber members subject to axial loads. Specifically, the research objectives are to: Determine experimentally whether a size effect is present in glued laminated timber members subject to axial loads; the degree and nature of this size effect, and whether the nature of this size effect changes with size. Determine experimentally whether a system effect is present, the degree and nature of this system effect, and whether this changes with size. Determine the nature of the interaction of the two effects and the implications for the behaviour of large glued laminated timber members in compression and tension. Develop numerical models that adequately capture structural behaviour including these size and system effects and their interaction at lab scale and beyond. Evaluate the implications of the findings for the reliability of current large, engineered timber structures and produce guidance for the safe design of future structures. This research will enable structural designers to exploit the potential of glued laminated timber for the design of more sustainable large structures and infrastructure with a proper understanding of the influence of size and system effects on behaviour. This research will also open new avenues of research into size and system effects in other engineered timbers and may shed light on opportunities to further 'engineer' such materials to improve performance.