Compressive Performance And Seismic Performance Of Thin-walled Steeltube/Plybamboo Composite Column

Using bamboo plywood as the main material, the present study independently designed and developed a new thin-walled steel tube/multi-layer bamboo plywood composite hollow column with transverse binding bars(SBCCB). The core, which consisted of a thin-walled steel tube, increased the cross-sectional area of the composite column, decreased the slenderness ratio of the specimen, and could improve the stability of the pressure-bearing capacity and the ultimate failure load of the composite column.The use of binding bars effectively reduced the frequency of debonding failure and affected the ultimate failure mode of the SBCCB, thereby improving its pressure-bearing capacity. Different pressure-bearing requirements can be met through the flexible design of the dimensions and wall thickness of the core steel tube as well as the thickness of the bamboo plywood. SBCCBs can be used as vertical load-bearing components in multi-layer prefabricated bamboo buildings, where they can replace wood. SBCCBs have bright prospects for application. SBCCBs exhibit excellent compressive and seismic performance, have sections that are easy to assemble, and satisfy the current requirements for green building materials and industrial construction. The present study investigated the compressive and seismic failure modes, deformation, bearing capacity and debonding behaviour of the interface of an SBCCB using experimental and numerical methods and established a model for calculating the bearing capacity. The main tests conducted in the present study and their results are summarized as follows:(1) Two groups of square, short composite columns with and without binding bars(a total of 10 specimens) were subjected to compressive strength and elastic modulus tests. In addition, the bonded interface between the substrates was subjected to compressive and shears strength tests. The results showed that the bamboo plywood columns had a relatively high compressive strength parallel to the grain of the bamboo and a relatively high elastic modulus. The compressive strength of the bamboo plywood columns reached the level of that of medium-strength concrete.(2) By subjecting two groups of composite column specimens without binding bars(a total of 24 specimens) to axial and eccentric compression tests, the failure process,deformation characteristics, failure mode and characteristic load of the composite column were investigated, and the debonding, peeling and buckling failure modes were identified.The main compressive failure modes of the specimens without binding bars included debonding failure at the bonded interface between the end and body of the column andlocal peeling failure. The compressive and shear strength of the bonded interface affected debonding failure and the ultimate bearing capacity of the bonded interface. Based on the test results, the effects of the slenderness ratio, the net cross-sectional area, the eccentricity of the load and the hollow ratio on the compressive performance of a composite column were analysed. Based on a nonlinear regression analysis of the test data, a model for calculating a column’s ultimate pressure-bearing capacity was established.(3) By subjecting two groups of SBCCBs(a total of 18 specimens) to compressive strength tests, the failure process, characteristics of the development of the deformation,failure mode and characteristic load of an SBCCB were investigated. The debonding,peeling and local debonding and buckling failure modes were identified based on microscopic and macroscopic behaviours such as the cracking conditions during failure,the characteristics and locations of debonding and peeling failures, the strain relaxation at the interface between the substrates and the load-displacement relationship. Analyses of the effects of the slenderness ratio, the net cross-sectional area, the cross-section combination mode, the eccentricity of the load and the distance between the binding bars on the compressive performance of the SBCCBs contributed to a discussion of the relationship between the geometric design parameters and anti-cracking performance of a composite column. The axial compressive failure modes of the SBCCBs included flexural failure of the column body between the end of the column and the binding bars and debonding between the substrates; flexural failure of the column body was the main failure mode. The main eccentric compressive failure modes included debonding failure of the substrates at the column end, debonding and peeling failure of the compressed bonded interface between the binding bars and the column body and flexural failure of the bamboo plywood. The eccentric compressive failure mode was mainly controlled by the degree of lateral flexure. The greater the slenderness ratio was, the greater the deflection and the additional moment were and the more significant the decrease in the bearing capacity was.The ultimate pressure-bearing capacity of the composite column was not only related to the bonding performance, the cross-sectional area and the slenderness ratio but also affected by the cross-section combination mode. The results of the compression test of the SBCCBs showed that transverse binding bars effectively inhibit debonding failure and alter the ultimate failure mode of a specimen, thereby significantly improving its ultimate bearing capacity. Local debonding and flexural deformation decreased with the distance between the binding bars. A reasonable assemblage of binding bars and a thin-walled steel tube reduced the dependence on the strength of the bonded interface and thus retarded debonding failure. A finite-element-based numerical simulation was performed based onthe tests. The simulation and test results were complementary to each other. Based on the data from the test and numerical simulation, as well as the standards and methods for designing glulam structures, a model for the stability coefficient of the compressive strength of the SBCCB that considered the slenderness ratio and the material strength was established. The results obtained from the model were consistent with those of the test.Therefore, the model could provide a reference for engineering applications of SBCCB.(4) By subjecting nine SBCCB specimens to a low-cycle repeated quasi-static load test, the process and modes of the SBCCB’s failure were investigated, and the effects of the slenderness ratio, the net cross-sectional area, the cross-section combination mode and the binding bars on the mechanical and seismic performance of the SBCCB specimens were analysed. The results showed that the main failure modes of SBCCBs include cracking of the bonded interface at the base and transverse fracture of the bamboo plywood. The cross-section combination mode had a significant impact on the failure mode. Increasing the cross-sectional area and the slenderness ratio of a composite column could improve its seismic performance. The SBCCBs exhibited relatively good capacities for elastic deformation and seismic energy dissipation. The binding bars effectively ensured the integrity of the specimen and inhibited debonding of the interface between the substrates. The hysteretic curve of the SBCCB exhibited prominent slipping and pinching characteristics and overall bending or bending-shear failure characteristics. The cross-section combination mode had a significant impact on the seismic failure mode.Increasing the cross-sectional area and slenderness ratio of a composite column could improve its seismic performance. The composite columns exhibited relatively good capacities for lateral elastic deformation and energy dissipation. However, the SBCCBs exhibited no advantageous elastic-plastic deformation capacity or ductility.