Study On Axial Compression Behavior Of Pre-stressed Thin-walled Steel Confined Short Circular Timber Column

Timber,one of the environmental friendly and traditional materials,has a lot of merits about lightness,high ration of strength to weight,ductility,superior seismic performance,aesthetic,easy to produce and plasticizing ability.However,as a natural material,timber exists some natural defects(knots,shrinkage,crack and creep,etc.)and easily deteriorates due to changes in temperature and moisture.Meanwhile,timber has complex anisotropic mechanical properties that show high mechanical properties along the grain direction,but low strength and stiffness perpendicularly the grain direction.In recent years,scholars take a lot of measures to improve the properties of timber columns,for example,timber columns with different cross-section shapes were placed in thin-walled steel cylinders to form steel-timber composite columns,and FRP was used to wrap the timber column or steel hoop was used to reinforce the timber column.Otherwise,there are still exist a few defects that the mechanical properties of the timber column were enhanced within limits and the combination efficiency is not high by the above reinforcement methods.Hence,a kind of pre-stressed thin-walled steel confined short circular timber column is presented in the article.The thin-walled steel cylinder is used to apply transverse initial pre-stress to the core timber column in order to take the core timber column in the mechanical state of tri-axial compression when contact to load,so that the deformation of timber is effectively limited and the stress hysteresis of steel is avoided.In order to comprehensively study the behavior of pre-stressed thin-walled steel confined short circular timber column under axial compression,the following task is done in the article:(1)Mechanical properties test of timber and steel basic materialsThrough the material property test of timber,the moisture content,density p,Poisson’s ratio,uij of three main directions(longitudinal,radial and tangential),elastic modulus Ei,yield compressive strength fc,y and ultimate compressive strength fc of timber were obtained.According to the standard,the shear modulus Gij of three shear planes of timber was calculated.Besides,the yield tensile strength and ultimate tensile strength of steel without weld ft,y and steel with weld ft were tested by material property test.(2)Axial compression performance test of pre-stressed thin-walled steel confined short circular timber columnOne timber column,one thin-walled circular steel cylinder,three unpre-stressed thin-walled steel confined short circular columns and four pre-stressed thin-walled steel confined short circular columns were designed.The failure process,failure modes,ultimate loads and ductility indexes of each specimen were acquired by axial compression tests,and load-displacement curves and load-strain curves were drawn.This article explores the improvement effect of pre-stressed composite column on mechanical properties compared with original specimens,test results show that the ultimate load of pre-stressed composite column is greater than the sum of pure timber column and thin-walled steel cylinder on the same size of core timber column.Even if the size and design conditions are worse,the pre-stressed composite columns without extension length have better ultimate load than the unpre-stressed composite columns with extension length,because of the confined of the active confining pressure on the core timber columns.What’s more,the influence of the extension length and different initial transverse pre-stress k on the mechanical properties under axial compressive of the pre-stressed composite column was investigated as well.The test results show that for unpre-stressed column with different extension length,the unilateral extension length 15 mm of which possesses the highest ultimate load.In the case of the same extension length,when initial transverse pre-stress degree k=0～0.20,the mechanical properties of the composite column are enhanced obviously.(3)Finite element simulation analysis of axial compression performance of pre-stressed thin-walled steel confined short circular timber columnIn order to put forward the calculation and analysis method of pre-stressed thin-walled steel confined short circular timber column,and to further investigate the axial compression performance of the composite column,the numerical analysis model of the pre-stressed thin-walled steel confined short circular wooden column was established by the finite element analysis software ABAQUS,and the axial compression performance of the specimen was analyzed.The numerical results are in good agreement with the experimental results.According to the model,the influence of higher initial transverse pre-stress degree k on the mechanical properties of composite column was further investigated.When k=0.20～0.30,the mechanical properties of composite columns continue to improve,but the increment is limited.When k=0.30～0.40,the mechanical properties gradually decrease.(4)Theoretical study of axial compression performance of pre-stressed thin-walled steel confined short circular timber columnBasing on existing research,the peak stress fco of pure timber column was calculated according to different standards and literatures.Referring to the calculation method of peak stress of concrete-filled steel tube to analysis the effective confinement stress fl of core timber column,then combining with the peak stress fcc got from the test and the finite element method,and the formula for calculating the peak stress of pre-stressed thin-walled steel confined short circular timber column is given.Trilinear constitutive models of composite timber columns under axial compression were proposed.