Long-term Behaviour Of Prefabricated Timber-concrete Composite Beams

The timber-concrete composite beam(TCC)is composed by the bottom timber beam and upper concrete slab through by shear connectors.This type of structures shows the advantages in bearing capacity and bending stiffness compared with pure timber bending systems in structural performances,as well as better acoustic behaviour,fire resistance and vibration control performance.However,compared with the steel-concrete composite structures which showed‘full composite action',the shear connection of TCC structures showed relatively low slip stiffness coupled with obvious interface slip which was restricted by material performances,and the composite action of TCC structures was considered as ‘partial composite action'.These problems lead to that researches for the shear connections and the long-term behaviour have been the hot topics in recent years.Accordingly,the short-and long-term behaviours of both TCC beams and corresponding shear connectors were researched comprehensively in this thesis.The main research contents and conclusions are summarised as following:(1)The steel plate-screw connectors can realise that the timber beam and concrete slab were constructed separately,and assembled together on the construction site.The effects of screw quantity,angle steel reinforcement,and timber notches for steel plates on the shear performance of this type of shear connectors were investigated through by push-out tests.Testing results showed that although two rows of screws can improve the bearing capacity,the slip moduli declined due to the rotation of steel-plate.In addition,the application of timber notches for steel-plates can obviously improve the ultimate strength and slip moduli of the steel plate-screw type connector.(2)Notch screw connectors are more suitable for integral prefabrication of TCC structures.The effects of loading modes,the shape of the notch,the width of the notch,and self-tapping screw reinforcement for timber shear planes on the bearing capacity,slip moduli,and ductility of the notch-screw connectors were investigated through by push-out tests.Testing results showed that the notch-screw connections showed superior bearing capacity and slip moduli,but relatively low ductility.While the timber shear planes were reinforced by self-tapping screws or adopting slant notch,the ductility of the notch-screw connection can be improved obviously.(3)Trough by bending tests of full scale TCC beams,the failure modes,bearing capacity,bending stiffness,as well as composite efficiency of TCC beams with steel plate-screw connectors or notch-screw connectors were obtained.It was found that the main failure mode of TCC beams was the bending failure of timber beams.For the TCC specimens with notchscrew connectors,the brittle failure of the shear connector at beam ends was also observed.In addition,the TCC beams with notch-screw connectors showed superior composite action,and the composite efficiency reached to 70%.(4)The long-term tests of push-out specimens with steel plate-screw connectors and notchscrew ones were conducted under constant temperature and humidity conditions.Two loading levels including the 10% and 30% of the short-term ultimate bearing capacity of the push-out specimens were considered.The creep coefficients of the steel plate-screw connection were3.85 and 1.72,respectively,for the 15% and 30% loading levels.Correspondingly,the creep coefficients of the notch-screw connections were 1.66 and 1.45,respectively.(5)The Kelvin,Burger,five-element,and Kelvin-Log models were adopted to simulate the creep coefficient of shear connectors.Based on the prediction tendency of each creep models for long-term period,it was found that multi-body Kelvin model,five-element model,and Kelvin-Log model showed reasonable prediction tendency.Based on the three fitting results,the creep coefficients of the steel plate-screw connector and the notch-screw connector at the end of service life were predicted.In addition,the long-term stiffness and the stiffness reduction were also analysed.(6)The long-term deformation of the timber and the creep of concrete were analysed in this thesis.The long-term slip mechanism of TCC connectors is investigated by combining the timber creep with the concrete creep.The calculation method for long-term slip was proposed.The analytical results of interface slip during the whole service life are consistent with the fitting one based on the long-term tests.The influences of environment humidity and concrete curing time on the long-term behaviour were analysed.(7)One glulam beams and four TCC beams were tested to evaluate their long-term behaviour under indoor uncontrolled conditions.The long-term deflection,timber strain,and interface slip correspond to time was recorded.The long-term deflections of the timber beam specimen and TCC ones were simulated through by creep models,the final deflection and long-term effective stiffness were also predicted(8)The long-term bending stiffness of TCC structures was obtained by introducing the creep coefficient of timber,concrete,and shear connectors to the calculation equation of the effective bending stiffness of ‘?-method',which agree well with the fitting results.Based on the theoretical analysis,the effects of initial stiffness and creep behaviour of shear connector,the curing time of concrete,as well as the environment humidity on the long-term performance of TCC beams were investigated.Through by the parametric analysis,the effective measures and design suggestions for improving the long-term behaviours of TCC beams were proposed.