Mechanical Performance Of Built-up Columns Assembled With Compressed Wood Dowels Under Axial Compression

Built-up column is a kind of wooden column with a large section which is made up of several dimension lumber with a small section connected by bolts or nails.It can be used in low-rise residential and commercial timber structures.Although its bearing capacity is lower than that of the glulam columns,this kind of adhesive-free laminated columns can be easily processed,which are also beneficial to reduce the impact of the adhesive on the environment.However,the metal connectors in the built-up column form a thermal bridge and the built-up columns with the metal connectors are not easy to cut and recycle,the wooden dowels may be more suitable connector for built-up columns.The common hardwood dowels have poor mechanical properties and obvious stress relaxation,which are not conducive to the tight connection between laminate and wood dowels.The mechanical properties of wood can be effectively improved and the stress relaxation can be obviously reduced by compression modification.Therefore,in this paper,compressed wood（CW）dowel was used as the connector,the axial compression behavior of built-up columnwith CWD was studied.The CW dowels with diameter of 12 mm were used to connect SPF,and to fabricate the built-up column.The axial compression test was carried out,and the comparison was made with the same size glulam columns and built-up columns assembled with bolts.The buckling capacity of CW dowelled built-up columns can reach 65% of that of the glulam columns.A three-dimensional finite element model with material nonlinearity and geometric nonlinearity was established by ABAQUS.The mechanical properties of the joints were simulated by axial.The mechanical property of axial was obtained by push-out shear test.By comparing the load-middle lateral deflection curves and the load-longitudinal strain curves obtained from the tests and numerical simulations,the validity of the finite element model was verified.The finite element model was used to analyze the geometrical parameters of CW dowelled built-up columns,and the influence of the diameter,spacing,row number of the dowel and the slenderness ratio of the columns on the buckling capacity of CW dowelled built-up columns were studied.The results show that the diameter,spacing and row number of dowel have no significant effect on the buckling capacity of CW dowelled built-up columns.In addition,the buckling capacity of CW dowelled built-up columns decreases with the increase of slenderness ratio.The paper summarizes the current design standards of timber structure and the calculation methods of the buckling capacity of built-up columns in the literature.The comparison between the theoretical calculation and numerical simulation shows that the existing methods can not accurately predict the buckling capacity of CW dowelled built-up columns.Therefore,the formula of the adjustment coefficient of the buckling capacity of the CW dowelled built-up columns is proposed considering the influence of the slenderness ratio of the members.Based on the calculation method of axial compression member buckling capacity in Chinese,American and Canadian standards for design of timber structures,the comparison between theoretical calculation and numerical simulation results shows that the buckling capacity the CW dowelled built-up columns can be accurately predict using Chinese and American standards for design of timber structures and the developed adjustment coefficient considering the influence of slenderness ratio of members.