Analytical and experimental investigations on failure of bamboo culms in bending: Effects of shear-tension interaction and bimodulus material behavior

Full-culm bamboo is an eco-friendly construction material with remarkable mechanical properties. Due to the complex material properties and the tubular geometry, bamboo culms can fail under bending in a variety of mechanisms/modes. This study focuses on the effect of circumferential tension-shear interaction and the bimodulus elastic model (different compressive and tensile elastic moduli) on the failure of bamboo culms under flexure. Specifically, it compares the failure moment of these modes with the corresponding failure moment for longitudinal compression failure (disregarding the bimodulus behavior), splitting due to circumferential tension, shear parallel to the fibers, and Brazier instability. Findings indicate that bamboo culms under flexure are more likely either to fail under longitudinal compression (thick-walled culms) or to split at the side due to the interaction of shear and circumferential tension (thin-walled culms). Moreover, considering the bimodulus behavior results in a more precise longitudinal compression failure prediction compared to the standard bending theory. This study also validates the analytical approaches via conducting four-point flexural tests. It highlights the importance of mixed-mode failure and of the bimodulus elastic approach, contrary to the usual practice of disregarding these features in determining the failure moment of bamboo culms under flexure. Furthermore, the occurring strength values constitute a close approximation to the bending strength obtained according to ISO 22157:2019, effectively minimizing the need for bending tests and enabling accurate predictions based solely on culm geometric parameters and generic material properties.