The Experimental Study On Fatigue Performance Of Bolted Sheer Connector In Timber-Concrete Composite Beams

Timber-concrete composite beam is a new construction technique in which a timber beam or deck is connected to an upper concrete flange using different types of connectors.The best properties of both materials can be exploited because bending and tensile forces induced by gravity loads are resisted primarily by the timber and compression by the concrete topping.As a renewable resource,timber has the characteristics of high strength to weight ratio,good flexibility,impact resistance,low energy consumption and easy processing and decomposition.These features of timber can reduce energy consumption and pollution,compared with concrete and steel.They also can promote the sustainability of the environment.In recent years,the research on timber-concrete composite structures in China and abroad mainly focuses on the static properties of the composite beams and the specimens.Because of the lack of theoretical and experimental research,The fatigue property analysis of timber-concrete composite structure is based on experience.It is found that the static performance,long-term behavior and fatigue property of composite structure mainly depend on the performance of shear connectors.The most shear connectors are the bolt shear connectors,but the shear connectors fatigue research in the domestic is few.Consulting an actual timber-concrete composite beam bridge in foreign countries,7 specimens are made and the corresponding nonlinear finite element analysis model is established.Finally,the static and fatigue tests are carried out.The fatigue performance and failure mechanism of bolted shear connectors are obtained by analyzing the experimental phenomena and comparing the experimental data with calculated results.The main results are as follows:(1)At the initial stage of static loading,the load has a linear relationship with the vertical displacement.With the load further increased,the load has a nonlinear relationship with the slip.Finally,the ultimate failure load is 38.9kn.(2)During the cyclic loading process,the bolts and the timber adjacent to bolt were partially damaged.Besides,the timber were extruded and deformed.However,the structure was not damaged after the fatigue cyclic loading for 2 million times.(3)With the number of fatigue load cycles increasing,the stiffness of the specimens has a significant degradation.Finally,the stiffness of the specimen F1 and F2 decreases by 17.74%and 22.9%after experiencing the fatigue load.(4)By comparing the finite element model with the test results,the ultimate bearing capacity of the specimen differs from the model by 2.57%.The fatigue load-slip curve and load-strain curve obtained in the fatigue test are consistent with the fatigue cloud diagram of the model.The calculation formula of bolt fatigue is obtained as followslg Nf=0.982-0.3523(RI/Psh).(5)According to the fatigue tests of timber-concrete composite beams in other literature,the shear connector at the end of the support is the first part damaged.Besides,the fatigue failure form of the shear connector is also consistent with the experimental phenomenon in this paper.