Experimental Study On Nail-holding Performance Of Bamboo Structures

With global warming becoming more and more serious,people’s awareness of environmental protection is gradually strengthened and low-carbon life has gradually become a trend.Bamboo is widely welcomed by the public for its green and environmental protection characteristics.Compared with traditional building materials,it has superior mechanical properties and seismic performance which makes it becoming a rare ideal building engineering material.The strength,durability and reliability of the structure are directly affected by the joints,which has the function of transferring loads and ensures the overall stability of the structure and the joints are a key Lingk in the structural design.The connection methods of modern bamboo-wood structures,such as tenon and mortise connection,step joint,pin connection and so on,is various.Among them,the nail joint has simple connection method,good connection performance,safety and good bearing capacity,which is suitable for on-site construction and has been widely used in modern bamboo-wood structure buildings.Laminated bamboo lumber,as a new type of material,stands out among many engineering bamboo materials with excellent mechanical properties,and it has become the object of study by many experts and scholars.However,the current research on laminated bamboo lumber is still relatively limited,and the design specifications are mainly based on domestic and international design standards for wood structures,which seriously restricts the popularization and application of laminated bamboo lumber in the engineering field.In order to break through the limitations of laminated bamboo lumber in the field of civil engineering and expand its application range,this paper used laminated bamboo lumber as the material,the material properties of laminated bamboo lumber and the bending resistance of nails were measured.The experiment and theoretical analysis of the mechanical properties of the nail holding force of laminated bamboo lumber nail joint were carried out,the constitutive model suitable for the nail holding force of laminated bamboo lumber nail joint was determined,and the finite element simulation was carried out,which provided a reliable technical and theoretical basis for the design of laminated bamboo lumber nail joint.The main research results were summarized as follows:(1)The bending yield strength of six different kinds of nails and the basic mechanical properties such as tensile strength and compressive strength of laminated bamboo lumber were measured by nail bending test and laminated bamboo lumber material property test.(2)The nail pull-out test of laminated bamboo lumber nail joints was carried out.The effects of different sections of laminated bamboo lumber,nail diameter,density,moisture content,nail penetration depth and nail types on the nail holding force of nail joints were studied.The results showed that the pull-out grip strength of the diameter section of laminated bamboo lumber was greater than that of the chord section of laminated bamboo lumber and the end section of laminated bamboo lumber.The nail holding force of laminated bamboo lumber increased with the increase of nail diameter,which can be increased by 3 times at most.Density was positively correlated with the strength of nail-holding force.In addition,the non-Lingear regression analysis of test piece density and grip nail force yielded constant values of the index for all six nail types that were higher than American wood structure design formulas.The strength of nail-holding force was significantly affected by moisture content,but the change of moisture content had little effect on the nail-holding force of floor nails and self-tapping screws on all surfaces of laminated bamboo lumber.The deeper the nai Lingg depth,the greater the nail holding strength of laminated bamboo lumber,which can be increased by 2.42 times at most.The nail-holding strength of laminated bamboo lumber on self-tapping screws was the highest,followed by flooring nails,stainless steel nails,galvanized twist nails,cement steel nails and bare round rod nails.(3)Based on the bond slip model,Cosenza model,Malvar model,Ling model,Yuan model and Gao model were used for non Lingear fitting with the test data.It was found that Ling model was the most suitable bond-slip model for predicting the grip strength of laminated bamboo lumber,which basically matched the experimental curves perfectly.(4)Laminated bamboo lumber was considered to be an orthotropic material.considering the influence of material damage,the finite element numerical simulation of the pull-out test of laminated bamboo lumber nails was carried out by using ABAQUS software.The results showed that the established finite element model could realistically simulate the force deformation of the nail joints during the pullout process,and the load-slip curve of the model is basically consistent with the experimental results.