Experimental Study On Flexural Behavior Of Reinforced Timber-Concrete Composite Beams With Novel Shear Connectors

With the development of timber processing technology,compared with traditional timber structures,modern timber structures have made a great breakthrough in strength and size of components.Modern timber structures have been widely used in western developed countries,and it also develops rapidly in China in recent years.However,China is in lack of structural timber currently.Therefore,the construction cost of pure timber buildings is relatively high currently in China.Using structural timber combining with traditional building materials can not only make full use of the material properties of the two components in the composite system,but also save timber resources and reduce costs to a certain extent.Timber-concrete composite（TCC）beam is one of the representatives.In TCC beams,the upper concrete slab is under compression,and the lower timber beam is under bending.The timber beam and the concrete slab are connected by shear connectors and work together,which effectively increases the bending capacity and stiffness of the structures.The existing research results show that the TCC beams usually failed by the tensile fracture at the bottom of the timber beam,while the upper concrete slab is basically elastic,which belongs to the brittle failure mode.The overall material utilization rate is generally low.In order to change the brittle failure mode of TCC beam and improve its material utilization rate,steel strips / CFRP strips are used to strengthen the bottom of the TCC beams to improve its structural performance.The flexural behavior of the reinforced TCC beams was studied by changing the type of strengthening materials,the thickness of strengthening materials and the strengthening methods.（1）Nine TCC beams including one unreinforced TCC beam was used as reference beam,and eight TCC beams reinforced with steel strips and CFRP strips were tested in four-point bending to investigate the bending capacity,the mid-span deflection,the relative slip between the concrete slab and the timber beam,the profile strain distribution and the effective bending stiffness of the TCC beams.The test results show that:compared with the unreinforced composite beam,the flexural capacity of the steel strips and the CFRP strips reinforced composite beams can be increased by 69.7%and 85.4%respectively,and the deformation capacity is also enhanced.The strain of the mid-section of the composite beam is basically linear along the cross-section height of the TCC beam.Because of the relative slip between the concrete slab and the timber beam,the strain at the interface between the concrete slab and the timber beam will be abrupt conforming plane-section assumption was not congruous.Compared with the unreinforced specimens,the maximum tensile strain of the reinforced composite beams can be increased by about 170%,and the maximum compressive strain of the concrete slab can be increased by 40%.Therefore,the timber-concrete composite beams and reinforced materials can fully use the compressive performance of concrete slab and the tensile performance of the timber beam,so that the flexural capacity of composite beams was improved.（2）The finite element results show that the simulation load-displacement curves and load-slip curves of the TCC beams are generally in good agreement with the experimental curves,which provides a reliable basis for the parameter analysis of the flexural behavior of the reinforced TCC beams with new shear connectors.The influence of the type,thickness and strengthening mode of reinforcement materials,the spacing of shear connectors and the spacing of screws at the bottom of timber beams on the mechanical behavior of composite beam were analyzed.（3）According to Eurocode 5,a calculation method for the cross-section of the TCC beams-"γ " method was proposed.The flexural capacity,mid-span deflection,relative slip of interface and flexural stiffness of the section of the composite beams were analyzed.The results show that the theoretical values are generally in good agreement with the experimental results,and some measuring points may have deviations,but they are still in a reasonable range.Therefore,the ultimate bending moment formula,effective bending stiffness formula and interface slip formula of composite beam basically meet the requirements.