The strength of plants: theory and experimental methods to measure the mechanical properties of stems

From the stems of agricultural crops to the structural trunks of trees, studying the mechanical behaviour of plant stems is critical for both commerce and science. Plant scientists are also increasingly relying on mechanical test data for plant phenotyping. Yet there are neither standardized methods nor systematic reviews of current methods for the testing of herbaceous stems. We discuss the architecture of plant stems and highlight important micro- and macrostructural parameters that need to be controlled and accounted for when designing test methodologies, or that need to be understood in order to explain observed mechanical behaviour. Then, we critically evaluate various methods to test structural properties of stems, including flexural bending (two-, three-, and four-point bending) and axial loading (tensile, compressive, and buckling) tests. Recommendations are made on best practices. This review is relevant to fundamental studies exploring plant biomechanics, mechanical phenotyping of plants, and the determinants of mechanical properties in cell walls, as well as to application-focused studies, such as in agro-breeding and forest management projects, aiming to understand deformation processes of stem structures. The methods explored here can also be extended to other elongated, rod-shaped organs (e.g. petioles, midribs, and even roots).This work is part of a project funded by the Leverhulme Trust (Project title: ‘Natural material innovation’). The project forms a collaboration with the Department of Applied Mathematics and Theoretical Physics, Department of Biochemistry, Department of Chemistry, and Department of Plant Sciences at the University of Cambridge

Proposal for Defining a Tall Timber Building

In order to talk about ‘tall’ buildings and more particularly ‘tall, timber’ buildings, it is useful to clarify what is meant by a ‘tall’ and a ‘tall, timber’ building. This clarification facilitates both productive discussion of the subject and secures the basis on which meaningful comparisons can be made between buildings using different structural systems and materials. The historic duopoly of steel and concrete as the structural materials of tall building construction is in the process of being broken by the introduction of engineered timber and it is appropriate that this change is reflected in the language of tall building design. The approach adopted in this paper has three parts. Existing definitions and terminology for ‘tall’ buildings are explored. A study is made of the structural systems and materials of existing buildings that use structural timber and have some claim to ‘tallness’. A proposal is made for the clarification and expansion of existing criteria for tall building terminology and definitions to accommodate the use of structural timber in tall building.This is the author accepted manuscript. The final version is available from the American Society of Civil Engineers via http://dx.doi.org/10.1061/(ASCE)ST.1943-541X.000161

Thermal relaxation of laminated bamboo for folded shells

Laminated bamboo is emerging as a novel material in design and construction. As a natural fibre composite, it has unique mechanical properties that allow for innovations that are not possible in other materials. Here, we discuss one new application of those properties: the development of a novel bending technique using high temperature, and we explore its implications for design. We have explored the fundamental properties of laminated bamboo and its thermal relaxation asit passes the glass transition temperatures of its constituent polymers.By mechanically thinning engineered bamboo material, score lines allow precise, controlled and localised heating that promotes limited but essential elasto-plastic behaviour. Concentrated heating above the glass transition temperature induces property evolution and structural morphology changes, which results in thermal relaxation with minimal recovery and full set upon cooling.This original technology is then deployed in the design and construction of a folded plate helical shell composed of thin laminated bamboo sheets.The presented work is supported by a Leverhulme Trust Programme Grant, and EPSRC Grant EP/K023403/1

What is tall timber? Towards the formal classification of timber as a material of tall building design

The emergence of taller buildings using engineered timber as a structural material raises important questions about the language that is used to describe tall buildings. In the absence of formal definitions it is difficult to make meaningful comparisons between buildings using different materials, structural systems and building forms. Claims to the title of 'tallest timber building' are frequently made and may be subject to dispute. This paper discusses the role of the CTBUH Criteria for Defining and Measuring Tall Buildings in the classification of tall buildings and the challenges to the existing criteria raised by the emergence of engineered timber as a contemporary structural material. The paper highlights the authors' proposal for updating the existing terminology to accommodate the use of timber in the design of tall buildings and details the progress that has been made in moving towards a revision of the CTBUH Criteria to include timber. This progress is significant as it represents a critical step forward in bringing timber engineering into the mainstream discourse of tall building construction and places timber on a level playing field with steel and concrete.Leverhulme Trust, EPSR

Cell geometry across the ring structure of Sitka spruce.

For wood to be used to its full potential as an engineering material, it is necessary to quantify links between its cell geometry and the properties it exhibits at bulk scale. Doing so will make it possible to predict timber properties crucial to engineering, such as mechanical strength and stiffness, and the resistance to fluid flow, and to inform strategies to improve those properties as required, as well as to measure the effects of interventions such as genetic manipulation and chemical modification. Strength, stiffness and permeability of timber all derive from the geometry of its cells, and yet current practice is to predict them based on properties, such as bulk density, that do not directly describe the cell structure. This work explores links between micro-computed tomography data for structural-size pieces of wood, which show the variation of porosity across the wood's ring structure, and high-resolution tomography showing the geometry of the cells, from which we measure cell length, lumen area, porosity, cell wall thickness and the number density of cells. High-resolution scans, while informative, are time-consuming and expensive to run on a large number of samples at the scale of building components. By scanning the same volume of timber at both low and high resolutions (high-resolution scans over a near-continuous volume of timber of approx. 20 mm3 at 15 μm3 per voxel), we are able to demonstrate correlations between the measurements at the two different resolutions, reveal the physical basis for these correlations, and demonstrate that the data from the low-resolution scan can be used to estimate the variation in (small-scale) cell geometry throughout a structural-size piece of wood.This work was funded in major part by a Leverhulme Trust Programme Grant. The X-ray imaging work was supported by the Advanced Imaging of Materials (AIM) facility (EPSRC Grant No. EP/M028267/1), the European Social Fund (ESF) through the European Union’s Convergence programme administered by the Welsh Government

Stiffness and slip in multi-dowel flitch-plate timber connections

Large multi-dowel connections can provide the strong and ductile connections required for large, highly-loaded timber structures, but their slip under load is not well understood. This is important because accumulated local displacements at connections can have significant implications for overall building serviceability. Empirical relationships for the slip of a single-dowel connection do not capture the dowel interaction effects of the multi-dowel connections used in larger structures. We present the results of an experimental test series and probabilistic numerical analysis investigating the development of stiffness in multi-dowel timber flitch plate connections. The influence of the diameter and number of dowels on the stiffness of the connection are investigated, including the influence of off-centring of dowels due to manufacturing tolerances. The test series is used to validate a probabilistic model for the stiffness of such a connection. The model incorporates the nonlinear stiffness and hole opening observed in single-dowel connections to predict the behaviour of the group. The study shows that the random off-centring of dowels in multi-dowel connections reduces the range of displacements over which the connection displays zero stiffness, but that this zone is not eliminated as a result of irreversible hole opening under load

Correction to: Identifying Efficient Transport Pathways in Early‑Wood Timber: Insights from 3D X‑ray CT Imaging of Softwood in the Presence of Flow (Transport in Porous Media, (2021), 136, 3, (813-830), 10.1007/s11242-020-01540-8)

A correction to this paper has been published: https://doi.org/10.1007/s11242-021-01569-3The contributions of TR, GW, DUS, OAS, MHR and PFL were funded by a Leverhulme Trust Programme Gran

Lightweighting with Timber: An Opportunity for More Sustainable Urban Densification

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal

Proposal for Defining a Tall Timber Building

© 2016 American Society of Civil Engineers.Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal

Duelling timber floats of Japan's Fushiki Hikiyama festival

Each spring the town of Fushiki in Japan's Toyama prefecture plays host to a unique cultural event - the Fushiki Hikiyama festival. Timber engineers will find interest in the culmination of the festival: a series of duels in which predominantly wooden festival floats armed with what can only be described as battering-rams, are brought together at speed in a series of spectacular collisions. The authors were brought together by the Civic Cultural Heritage Network Tottori, a Japanese cultural organisation, to carry out a preliminary investigation of the festival floats; both for engineering and cultural interest, and as a possible exemplar of the behaviour of traditionally carpentered structures subject to high dynamic loads. The second, third and fourth authors travelled to Fushiki to observe the May 2016 festival and to carry out an after-the-battle inspection of one of these unique duelling floats. High-speed and high-resolution video was used for digital image correlation measurement of the collisions, to quantify the magnitude of the impact