Push Out Performance Of Novel Shear Connectors For Timber-concrete Composite Beams

Modern timber structure is developing towards multi-storey,high rise and large-span timber structures with various structural forms.China is rich in forestry resources,however,most of the timber species are fast-growing and the wood has relatively low strength,so that the structural timber is currently in short supply in China.Using timber combining with other traditional building materials(such as concrete,steel,etc.)is an effective way to solve the shortage of structural timber resources in China.As a novel type of composite structure,the mechanical mechanism of timber-concrete composite beam is that the lower timber beam is in tension and the upper concrete slab is in compression.The sliding and lifting action between the two members are resisted by the shear connectors,so that the timber beam and the concrete slab can work together.The performance of shear connectors is the key issue to the performance of timber-concrete composite beams.Previous studies have shown that the existing shear connectors of timber-concrete composite beams are generally flexible and have limited bearing capacity.In order to develop a shear connector with high bearing capacity,high stiffness and high ductility,three novel shear connectors are designed firstly,and their shear properties,including shear bearing capacity,stiffness and ductility,are studied by push-out tests,and their failure modes and failure mechanisms are verified by finite element simulation.Finally,the theoretical calculation formula of shear capacity of shear connectors for different failure modes is derived.The specific works are summarized as follows:(1)A total of 30 specimens grouped by 10 were designed and manufactured to experience push-out tests.The types of shear connectors are T-shaped steel,I-shaped steel and folded steel plate.The test results show that at the beginning of loading,the load shows approximately linear increase to displacement;with the load increased gradually,the curves begin to show nonlinear behavior;when the load increases to a certain level,the curves show obvious pseudo ductile behavior,indicating that the connector has good deformation ability.During the loading process,the obvious bending deformation of the anchors was observed,showing the "single hinge" and "double hinge" yield failure mode.At the same time,the timber around the anchors is obviously crushed,and the concrete slab cracks.In this test,the I-shaped steel shear connectors anchored by glued-in rods show the highest shear bearing capacity of 158.9k N.The folded steel plate shear connectors anchored by glued-in rods exhibit the highest initial shear stiffness up to 39.1k N/mm.The shear connectors anchored by screws show excellent ductility with a ductility coefficient is 34.3.Generally,the anchoring performance glued-in rods is better than that of the anchors with screws.(2)The finite element numerical simulation and parametric analysis were carried out by ABAQUS.The results show that the simulated load slip curves are in good agreement with the test curves with errors of both the shear capacity and the initial stiffness less than10%.Stress volume analysis verifies the failure mode of the tested specimens effectively,such as the yield of anchors,the crushing of timber and the deformation of shear connectors.The results of parameter analysis show that the diameter,depth,strength of anchors and friction coefficient between the anchors and timber have significant influence on the shear performance of the connectors.(3)Based on the Johansen yield theory,the theoretical calculation formula of the bearing capacity of the shear joints under single hinge / double hinge yield failure modes is deduced.The results show that the derived calculation formulas estimate the shear capacity of the novel shear connection for timber-concrete composite beams conservatively.The error between the theoretical value and the experimental value of shear capacity is less than 15%.The theoretical analysis of the initial stiffness shows that the existing initial stiffness formulas might not be able to precisely predict the initial stiffness of the shear connectors in this study.