Insight into the behaviour of bamboo culms subjected to bending

Abstract

This study describes analytically the behaviour of bamboo culms subjected to bending, and predicts the failure load and stiffness loss after the linear-elastic stage. Basis of the failure load prediction is the identification of the critical failure mechanisms. The study examines analytically four distinct failure mechanisms: Brazier instability, longitudinal tension/compression, tension perpendicular to the fibres and shear parallel to the fibres. It concludes that, for the three bamboo species examined (Moso, Guadua and Kao Jue), critical failure mechanisms are tension perpendicular to the fibres (with potential tension–shear interaction) and longitudinal compression. Which of the two mechanisms occurs first depends on the case-specific material properties and culm radius-to-thickness ratio. Regarding stiffness loss, the main cause is longitudinal splitting. The extent of the stiffness loss depends on crack length, crack number and crack location along the culm circumference. Secondary causes are nonlinear geometric effects at the large deflection stage. Assuming a parabolic deformed shape, a single equation can describe the stiffness loss induced by nonlinear geometric effects, regardless of material properties and culm geometry. Comparing the analytical results with pertinent experimental data, the proposed equations are sufficiently accurate in their prediction of failure load and stiffness loss, although they tend to underestimate both.

Stress control mechanism by fiber distribution

“…Transverse forces ovalized the cross-section, and cracks or splitting developed at the four bending points [25]. Splitting failures commonly occur because of bending; more specifically, because of the tension being perpendicular to the fiber and longitudinal compression [26,28]. Circumferential stresses occur under bending, and transverse or circumferential tensile stresses cause the culm to split because the transverse tensile strength is low [28].…”

“…Splitting failures commonly occur because of bending; more specifically, because of the tension being perpendicular to the fiber and longitudinal compression [26,28]. Circumferential stresses occur under bending, and transverse or circumferential tensile stresses cause the culm to split because the transverse tensile strength is low [28]. Therefore, the cross-section is ovalized because of the transverse forces, longitudinal cracks and splits occur because of the circumferential tensile bending stress, and the cross-section buckles [22,25,29].…”

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

“…The complexity of bamboo structural member behavior partly stems from bamboo culm morphology, which comprises longitudinal fibers encased in a lignin matrix, much akin to a uni-directional fiber-reinforced composite [3,[7][8][9]. This leads to low tensile strength perpendicular to the fibers, and hence bamboo is prone to splitting parallel to the fibers [10][11][12][13]. Splitting can even occur during the process of bamboo culm drying, or because of ambient humidity variations [14].…”

“…Other studies have investigated bamboo fracture properties, which pertain to crack propagation [16][17][18][19]. Hence, there is little knowledge on the implications of splitting, and on member behavior after the appearance of cracks, and it mainly comes from very recent (2022) studies [13,20]. These studies indicate that, especially in full-culm bamboo flexural members, longitudinal cracks lead to significant stiffness loss, which starts notably earlier than eventual failure [12].…”

“…Additionally, the cross-section shear center shifts, inducing torsional effects, and hence additional torsion-induced deflections [21]. This study considers a single crack at the mid-height of the culm, as this is where cracks usually appear in bamboo culms subjected to bending, because of the high shear and circumferential tensile stresses in that location [12,13,15]. A crack at the mid-height of the culm is critical, because it maximizes the eccentricity of the applied transverse loads, with respect to the shear center, which in turn maximizes the applied torque on the culm.…”