Study On Mechanical Behavior Of Timber-Concrete Composite Beams Under Different Boundary Conditions And Loading Rate

Timber-concrete composite structure is a new type of composite structure.Using timber combining with concrete,steel and other traditional building materials can not only give play to their respective advantages,but also alleviate the shortage of engineering wood resources in my country to a certain extent.Timber-concrete composite beam is a integrity consisting of the timber beam and the concrete slab connected with each other by shear connectors which resist the horizontal shear and slip between the timber beam and the concrete slab.At present,a large number of researches have been conducted on timber-concrete composite shear connectors by many domestic and abroad scholars.The push-out testes were usually conducted in double-shear single-shear configuration.However,there are some differences in the boundary conditions simulated by the two test devices.The influence of the boundary condition differences between the double-shear and single shear on the mechanical performance of the shear connectors has not yet been reported.In addition,the existing research results show that the loading rate of tests has a certain impact on the test results.At present,the push-out tests of timber-concrete shear connectors were carried out based on the loading protocol recommended by European standards.The influence of loading rates on the performance of shear connectors is worthy of further study.In view of this,a series of timber-concrete composite beam shear connector specimens were designed to do push out tests.Two kinds of connections namely,glued-in rod(GIR)and notch-GIR were considered as the shear connectors in the tests.Through experiments and theoretical analysis,different are studied.The mechanical properties of the shear connectors including shear capacity,stiffness,ductility under different were investigated under different boundary conditions and loading.The specific works are as follows:(1)A total of 54 specimens were designed for push-out tests.The test results show that the specimens with glued-in rod shear connectors mainly failed by double-hinge yield mode of GIR.The failure modes of the notched-GIR shear connectors push the specimens mainly include the double-hinge yielding and rigid failure of GIR.Under different boundary conditions and loading systems,the difference in bearing capacity,initial stiffness,and ductility between single-shear and double-shear is small.(2)Based on ABAQUS,numerical simulation and parametric analysis were carried out on the specimens of GIR and notch-GIR shear connectors.The results show that the error between the experimental value and the simulation value of both the shear capacity and the initial stiffness is less than 15%,and the simulated load-slip curves are generally in good agreement with the experimental curves.The diameter of GIR and the length of the notch have a significant impact on the mechanical properties of the shear connector,while the boundary conditions have a small effect on the mechanical performance of the shear connectors.(3)In view of different failure modes,the calculation formulas for the shear capacity of the timber-concrete shear connections with GIR and the notch-GIR were theoretically deduced,and the theoretical values of the shear capacity were compared with the experimental ones.The results show that the theoretical value is lower than the experimental value.The reason may be related to the lower value of the embedment strength of GIR in the concrete and the lower pull-out capacity of GIR.To a certain extent,conservatively estimate the planted bar shear connector and groove.The shearing capacity of the test piece is introduced from the mouth-planted bar composite beam.In addition,the calculated value of shear stiffness is obtained by referring to the existing calculation formula of shear stiffness and compared with the experimental value.The results show that there is a certain error between the calculated value of the existing shear stiffness formula and the experimental value.A calculation formula for the stiffness of planted reinforcement is proposed.The formula in this paper is in good agreement with the experimental results.