Lightening and hollow architecture are the common methods used for reducing the weight of structural members. This section introduces the effect of hollow architecture using the relationship of "bamboo structure and strength" as an example. <Reference: "Katachi to Tsuyosa no Himitsu (The Secret of Shape and Strength)" by Juuhachi Oda, published from Ohmsha, Ltd.>

= Toughness of bamboo =

・The external structure of bamboo trees can be described as a linear cylinder that becomes slightly thicker at the root. In addition, nodes are found at certain intervals along the inner cylinder.

・By ignoring the bamboo branches, bamboo trees have a cantilever structure anchored at the root area serving as a fixed support end.

・Aside from the theory, the bamboo structure can be described as follows:
(1)The bamboo's cylinder structure makes the object of the same cross-section three to four times stronger than the solid cylinder structure.
(2)The dimension structure of bamboo trunks that have thicker roots and thinner tips provides the maximum stiffness among all the other structures of the equivalent weight.

・This structure prevents bamboo from being bent even during the heavy snow season.

・Bamboo tree's material composition also contributes to its strength. The tissues in vertical direction are more densely distributed as it becomes closer to the epidermis. The functions of reinforced fiber similar to CFRP composite materials introduced in Vol.462 are working in this mechanism (see [Fig.1]).

・As seen from this bamboo example, we can prepare strong yet lightweight structural members by adopting hollow architecture to a structural member of the same cross-section and by designing the shape effectively (see [Fig.2]).