The Experimental Study On Fatigue Performance Of Notch-bolted Connector In Timber-Concrete Composite Beams

The timber-concrete composite(TCC)structure is a new construction technique in which a timber beam is connected to an upper concrete using different types of connectors.The best properties of both materials can be exploited because bending and tensile forces induced by external loads are resisted primarily by the timber and compression by the concrete topping.A connector with high composite action allows a significant reduction of the beam depth and an increase in the span when compared with a non-composite system.In recent decades,a wide range of connectors have been developed in different parts of the world.The connectors can be metal fasteners,notches cut in the timber and filled by concrete,or complex forms of combination.The structural efficiency of a TCC structure largely depends on the connector.In recent years,the research on TCC structures in China and abroad mainly focuses on the static properties of the connectors,and there are few researches on its fatigue performance.In the background of the 8-m-span timber-concrete composite beam bridge,five notch-bolted push-out specimens were manufactured,the corresponding finite element analysis model was established,and static and fatigue tests were carried out.By analyzing the phenomena,and comparing the experimental results with the calculated results,the fatigue performance and mechanism of the notch-bolted connector in timber-concrete composite beams are obtained.The main results are as follows:(1)At the initial stage of static loading,the load and the interface slip show a linear relationship,as the load increases,the load and deflection show a nonlinear relationship.The ultimate failure load of three notch-bolted connectors are 66.4kN,62.0kN and 70.6kN.The average is 66.3kN.(2)Under the first 500,000～600,000 fatigue cycles,the slope of load displacement curve as well as the stiffness increases.Then with the increase of fatigue load cycles,the slope of load displacement curve decreases,and the stiffness decreases gradually.(3)After 2 million times of fatigue cyclic load,the static failure load of notch-bolted push-out specimens in timber-concrete composite beam are 60.0kN and 58.3kN.The load capacity decreases by 9.5%and 12.1%respectively.It is shown that in the practical engineering background,the connectors have good fatigue performance under the load amplitude controlled by the stress level.(4)After 2 million times of fatigue cyclic load,the stiffness of notch-bolted connector is degraded on the whole process.The stiffness of Specimen-5 and Specimen-6 are reduced by 11.0%and 15.3%respectively.The degradation of the stiffness directly affects the degradation of the connectors’bearing capacity.(5)The numerical finite element models for rigorously analyzing push-out specimens were overviewed;the static calculation result and predictions of fatigue life were shown to compare favorably with the results of tests.The fatigue calculation formula is obtained by linear fitting with regression method,which can provide some references for the design of the TCC notch-bolted connectors in the future.