## 2 Materials and Methods
### Sleeve Joint Test of the Samples  
The joint without fitting consists of reduced bamboo, sleeve, and adhesive, while the joint with fitting has reduced bamboo, adhesive, sleeve, wood fitting, and screws. Joint failure could take place either in the components, namely, bamboo, sleeve, fitting, and screw or in the glue lines between the bamboo-sleeve or bamboo-wood fitting. It is clear that a steel sleeve cannot fail among the components. Therefore, the failure of the joint without wood fittings took place in the bamboo and the adhesive. As the exodermis of bamboo is very hard to bond due to the wax and SiO\({}_{2}\), the glue layer between the bamboo and the steel sleeve is expected to be the first failure for joints without wood fittings [23]. Additionally, the relationship between strength properties and the density of bamboo is quite different from that of wood species. Overall, the strength characteristics of bamboo are directly related to their surface properties rather than density. The glue line between bamboo and a wood fitting did not fail because the glue layer was not loaded in the pullout test. Screws in the joint with wood fittings might fail or yield due to transverse shear load. The overall failure of the steel joint occurred in bamboo, glue line, and screw. Such findings were confirmed by the results of the pullout test of the steel joint. As screw failure always occurred right after adhesive failure, such failure was considered an adhesive failure [23]. Failure modes of the samples were simply classified as adhesive failure and bamboo failure, as shown in Figure 9.  
The number of failure modes for various joint types is depicted in Figure 10. As can be seen from this figure, forty adhesive failures and eight bamboo failures were determined for steel-sleeved joints. Bamboo failures took place only for the wood fitting joints and always occurred in the diaphragm where the pullout load was applied. Pullout test failures in bamboo parts referred to the glue layer, and, together, the screws were stronger than bamboo. The use of screws fixed in wood fittings significantly increased the joint strength. This implies that a steel-sleeved joint with wood fittings is stronger than bamboo by the use of more screws.  
The displacement of the crosshead of the testing machine was recorded as the deformation of the sleeve joint that took place during the test. Figure 11 illustrates the typical load-displacement curves for steel-sleeved Gui and Moso bamboo joints, with and without wood fittings, representing the load and deformation of the tested joints. As shown in Figure 11, the joints without wood fittings (blue square symbols) have abrupt drops of load after reaching the maximum pullout load, indicating that the joint failure is a brittle one. Drop of load represents the separation between bamboo and its sleeve. However, joints with wood fittings (red circular symbols) had ductile failure behavior. Initially, the magnitude of load did not drop to zero because even though the failure occurred between bamboo and sleeve, the screws still resisted a certain amount of pullout load until the screws either  
Figure 9: Bamboo failure (**a**) and adhesive failure (**b**) of sleeved Gui bamboo joints.  
yielded or failed. In the next step, the deformation of the joint with wood fittings was much larger than the joint without wood fittings. [PERSON] et al. (2018) [PERSON] et al. (2018) studied steel-reinforced bamboo square columns jointed by epoxy and screws. The steel-bamboo interface also showed similar load-displacement curves of brittle and ductile behavior, without and with screws, respectively.