Effect of a longitudinal crack on the flexural performance of bamboo culms

Abstract

Splitting parallel to the culm fibers is common in full-culm bamboo structural members, even early in a structure's lifespan. Currently, there is insufficient knowledge on the effect of splitting on member performance, which induces significant uncertainties in bamboo member engineering design. This is a potential threat to the safety of existing and future full-culm bamboo structures. This study investigates analytically the effect of a longitudinal crack on the stiffness of an originally intact bamboo culm in flexure. The study develops analytical expressions that describe stiffness loss in two flexure cases (a three-point bending and a four-point bending test), and verifies them with available experimental results and numerical simulations. Main cause of the stiffness loss are torsion-induced deflections, with secondary cause being shear deformations. Importantly, stiffness loss solely depends on two dimensionless parameters: shape factor (radius-to-thickness ratio), and a factor that is a function of material properties and ratio of shear span length to culm diameter. Additionally, the study proves analytically that friction at the load application points mitigates torsion-induced deflections. This has important implications for bamboo structure design and testing standards, indicating that the manner in which loads are transferred on beams affects the apparent beam stiffness when a crack appears.