Experimental Investigation on the Neutral Axis of Wulung Bamboo (Gigantochloa Atroviolacea) BEAM

Previous studies have demonstrated that bamboo exhibits distinct mechanical properties under tensile and compressive loading, characterized by differing moduli of elasticity. This asymmetry implies that the neutral axis of a bamboo cross-section under bending should not lie at mid-height. However, experimental validation of the neutral axis position remains limited. This study aims to comprehensively investigate the bending behavior of Wulung bamboo by considering the tensile and compressive elastic moduli parallel to the grain. Tensile, compressive, and flexural tests were performed according to ISO 22157:2019. The results revealed that the average tensile modulus of elasticity was 15,789 MPa for internodes and 13,716 MPa for nodes, while the average compressive modulus of elasticity was 21,270 MPa and 15,264 MPa, respectively. The neutral axis was consistently located below the geometric center of the cross-section, ranging from approximately �0.03D to �0.09D, where D is the average outer diameter of the bamboo culm. A modification factor for the tensile and compressive elasticity moduli was introduced to achieve internal force equilibrium in flexural analysis. A finite element analysis is recommended to further simulate the interaction between the position of the neutral axis, the modified tensile and compressive elastic moduli parallel to the grain, and the overall flexural behavior of the bamboo. ISO 22156:2021 remains applicable at the design stage, provided material behavior remains within the linear-elastic domain. However, a more detailed analysis of the stress-strain distribution along the fiber direction would help support and refine the structural design process. © 2025 The Korean Society of Wood Science & Technology.