Research On Shear Performance Of CLT With T&G Joints

Engineering wood products,such as cross-laminated timber(CLT),with the use of new structural systems and connectors,promote the development of middle-and high-rise timber structures.CLT is a kind of lightweight building material with strength and rigidity equal to concrete with equal thickness.Whether the structural effectiveness of CLT floor and wall system can be developed largely depends on the working efficiency of the connection system.In this paper,a new type of CLT connection,i.e.Tongue and Groove(T&G)connection,was designed.Through quasi-static monotonic loading tests and cyclic loading tests,the shear mechanical properties of CLT with T&G connection were studied,which created a new way for CLT connection and provided basic data for its practical application.Firstly,the tensile tests,bending yield tests and head pull through tests of self-tapping screws(STS)with different diameter(8mm or 6mm),head type(flange head or countersunk head)and thread type(fully-thread or partially-thread)made in China were carried out.The mechanical properties of STS were evaluated and the data for capacity calculation were provided.The test results showed that the tensile strength of the tested domestic STS met the requirements specified in the standard GB 27704-2011 and LY/T 2059-2012.Specimen(STS-C-6-F-E)had the highest tensile strength,1398MPa.The bending yield strength of the tested screw met the requirements of the standard LY/T 2059-2012.The bending strength of STS in STS-C-8-F-B group was the largest,with 1615MPa.The relative error between the predicted results using formula in EC5 and the test values was between 1.07%and 62.92%.The head pull-through strength of STS-F-8-P-A,STS-C-6-P-C and STS-C-8-P-D were13.54N/mm2,23.94N/mm2,18.07N/mm2,respectively.It was found that the relative error of head pull through strength between the test value and the calculation result of formula in EC5 is between 21.99%and 58.45%.Then,the shear tests of three-layer CLT with T&G connections were carried out as preliminary tests.The failure modes,load-carrying capacity,stiffness and ductility were evaluated.It was found that the yield,breakage head pull through,tip pull out of STS,and embedment failure of CLT were the main failure modes of T&G specimens.The load-carrying capacity,initial stiffness and ductility of the three kinds of T&G connections were 15%-31%,236%-338%and 95%-168%higher than those of the traditional half-lapped connections.The length of the groove will affect the capacity due to the influence of the edge distance of the screw.Among the three groove lengths in this test,TG-65 group had the highest stiffness and ductility,TG-50 group had the highest load-carrying capacity.The bonding length between each SPF and the main body of specimen should be considered to prevent shear failure of the adhesive layer.Finally,the shear tests of five-layer CLT with T&G connections were carried out.Five types of STS with different diameters(6mm and 8mm),head type(flange head or countersunk head)and thread type(fully-thread or partially-thread)were used to carry out quasi-static monotonic loading and cyclic loading tests and calculation was made using formula of European Yield Model in EC5.Some experimental results were obtained.In monotonic loading test the maximum load-carrying capacity,stiffness,ductility of T&G connections(TG-M-B)are higher than those of half-lapped connections with the same screw(HL-M-F).The ultimate load was increased by 81%,the initial stiffness was increased by 137%,and the ductility ratio was increased by 138%.The ultimate load of TG-M-A was the largest in the T&G connections with 16.38kN.The stiffness value of TG-M-D was the largest,with the initial stiffness of 2kN/mm.TG-M-E group was classified as middle ductile connection,while the rest of the T&G connections were classified as high ductile connection.In the cyclic loading tests,the strength degradation percentage of HL-C-f group was higher than that of TG-C-b group.The equivalent damping ratio of the T&G connection was basically in a rising state before400%?P_y,and the value of half-lapped connection decreases.The prediction value of the load-carrying capacity were 27%-55%less than the test values by using the embedment strength formula and head pulling-out capacity formula in EC5.The prediction value were24%-53%less than the test value by using the embedment strength formula and head pulling-out capacity formula proposed by Uibel&Bla?.The rope effect increases the prediction values,but the results are still relatively conservative.