Keywords: bamboo stem, orthotropic, strength, stability, failure, numeric iteration.

Natural building materials play an important role in modern construction and architecture. Further to timber, in other parts of the world giant bamboos are widely used in various applications. From the simple household usage to large engineering structures the possibilities of applications are innumerable.

Over a thousand different bamboo species are classified; many of them are middle size or large timber bamboos, which are most significant for engineering purposes. These species are highly specialized in their structure and exhibit an outstanding mechanical behaviour in strength. Bending tests show that a bamboo stem typically fails by developing longitudinal splits leading to a sudden and immediate collapse of the hollow stem. Scientific interest in bamboo concentrates on its practical utilization for engineering purposes, mainly on the determination of strength failure due to various loading or on developing structural elements and connections made of bamboo [1,2].

In this study we were interested in the mechanical behaviour of the living timber bamboos in their natural environment. They are typically slender and tall columns divided into segments by diaphragms. Our aim is to investigate the possible failure modes. The bamboo stem may fail either by the exhaustion of strength or by the loss of stability. We have raised the question whether the appearance of splits is initiated by stresses at stable equilibrium or alternatively, by local buckling at a relatively low stress level. Our hypothesis assumed a balance between the two failure modes.

We set up a material model of the complex composition of the bamboo stem. The mechanical model was composed of an internode segment with two nodes under a bending load. We applied anisotropic plate theory [3] and second order theory of displacements. A numerical analysis was carried out on various geometric configurations to determine the collapse load. We developed our own computer program, which enabled us to set any parameter easily. We conclude that the calculations support our hypothesis. A group of species, typically the ones with long and thin internodes are bound to fail by the loss of stability, on the other hand other group of species, typically the ones with short and wide internodes fail by the stresses exceeding of the strength of the material. Between these extremes, many species can be regarded as an optimal middle ground where the two failure modes occur at similar load levels.

1

J.J.A. Janssen, "Bamboo in building structures", Doctorate dissertation, University of Eindhoven, The Netherlands, 1981.

2

O.A. Arce-Villalobos, "Fundamentals of the design of bamboo structures", Doctorate dissertation, University of Eindhoven, The Netherlands, 1993.

3

J.M. Whitney, "Structural analysis of laminated anisotropic plates", Technomic Publishing Company, Lancaster, Pennsylvania, USA, 1987.

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