Experimental Study on Single-Shear Joint of Self-Tapping Screw of Steel-Timber Composite Structure

The steel-timber composite system combines the advantages of both materials and has prospects for application in village houses,long-span structures and other building systems.Pin connections are common connection forms in steel-timber systems,and their mechanical properties significantly affect the performance of the structural system.Among them,bolts and screws are the common forms of pins connections.In order to accurately evaluate the shear resistance of thin-walled steel-timber self-tapping screw connections,connection shear tests and theoretical derivation were investigated in the paper.The mechanical properties of the potential structural material,fast-growing wood,were also calibrated.
To investigate the shear performance of the connections between timber and thin-walled section steel (2 mm) with small diameter self-tapping screws (diameter ＜ 6 mm),11 groups of single shear connections under monotonic loading were designed and tested.Timber type,dimension of connections,and connection direction are the main parameters explored.The mechanical properties such as damage mode,bearing capacity and stiffness,ductility coefficient,and cut line stiffness of the self-tapping screw connections were obtained.On the basis of Standard for Design of Timber Structure(GB 50005-2017),the bearing capacity of typical damage modes of thin-walled steel-timber self-tapping screw connections was analyzed.
The results include:the strength of fast-growing timber can meet the connection requirements,the change of timber species has a certain effect on the shear performance of self-tapping screw connections,but the change of screw size,type and embedment length significantly affects the damage mode and peak load capacity of the connections.The performance of Chinese fir,poplar and pinus radiata is lower than that of common building materials such as douglas fir and pinus sylvestris,but they still have sufficient connection strength.The self-tapping screw connections in the paper mainly occur in three damage modes:self-tapping screw rod shearing,screw pull-out and timber damage.Among them,self-tapping screw shearing is the most common damage.When the embedded length of the screw is small,the connection is prone to damage from self-tapping screw pull-out,in addition,the timber cross-grain connection specimen is prone to brittle fracture due to cross-grain tension,the actual situation should be avoided.Self-tapping screw connection has a good peak bearing capacity (about 3 kN) in steel-timber composite structures and members,but the ductility is poor due to the brittle fracture of the screw,the average ductility coefficient is 2,reducing the embedment length can increase the ductility of the connection.Through the theoretical derivation and mechanism analysis of the shear bearing capacity of self-tapping screw connections,as well as the analysis and calibration based on GB 50005-2017 and other domestic codes,it shows that the existing standards do not fully consider the rope effect,the bearing capacity are less than the actual measured value of the test.Different from the steel or wood pin-type connections,the thin-walled steel-timber composite self-tapping screw connections are dominated by bending and shear damage,and the design values derived from GB 50005-2017 account for about 30%-45% of the actual value of the bearing capacity.