| Bamboo/glubam beam is a basic element of modern bamboo structure,and it has important influence on the span and the interior net storey heightof the bamboo structure.Relatively lower flexural stiffness leads to relatively large deflection of glubam beam.It significantly limits the span of structures and becomes a bottleneck problem for developing modern bamboo structure.Based upon the existing technologies of the timber-concrete composite(TCC)system,developing glubamconcrete composite or bamboo-concrete composite(BCC)beam or floor may be a feasible solution for increasing strength and stiffness properties of glubam f lexural members.However,bamboo and wood materials exhibit obvious creep phenomenon,and the concrete slabs and connections also have creep under the long-term loading,which cause the deflection of composite beams increases.Therefore,the long-term behavior of BCC on serviceability limit state(SLS)needs to be investigated.Predicting the long-term deformation of TCC/BCC is more complex than that of general beams,considering the stress redistribution between timber beam and concrete slab.In this paper,long-term behavior of BCC beams was studied through carrying out the long-term load tests on typical connectors and the large-scale BCC beams.A new calculation method for the long-term deformation of BCC was proposed and the residual performance of BCC after long-term loading was also studied and disscussed.Based on the results obtained from tests and analysis,the following conclusions can be drawn:Firstly,the steel plate mesh connector(SM),screw connector(SC),notch connector(NC),pre-tightening notched connector(PNC)and RPC-steel composite connector were selected to make symmetrical shear specimens.The long-term load test was carried out under indoor conditions.Holding time was not less than 9 months.The slip-time curves of the interface were obtained and used to evaluate the long-term performance of connectors.The test results show that the slip increases rapidly in the first 30 days followed by a slowly increasing trend.Finally,it alomost exhibits a stable state after about 200 days.The environmental temperature and humidity have influence on the slip development,corresponding a certain fluctuation of curves.On the other hand,the fluctuation is relatively small under the indoor conditions.On the point of slip increment,the long-term performance of SC connector is better than RPC-steel composite connector.Secondly,several large-scale BCC beams with SM connector,SC connector,NC connector,PNC connector and RPC-steel composite connector were performed the long-term bending tests under the indoor environment.The test results show that the midspan deflection increases rapidly in the beginning 30 days.Then,the increasing rate significantly decreases.After about 550 days,the deflection basically reaches steady state.The environmental temperature and humidity had obvious effect on the deflection development of BCC,correspondint that the obvious fluctuation of the midspan deflection-time curve can be observed as the temperature and humidity changed.Besides,the fluctuation of deflection in autumn and winter is relatively significant compared with spring and summer.Under the test conditions,the final midspan deflection of BCC beams after 700 days is about 2.03～2.73 times of the initial deflection.Thirdly,Burgers model and double-chain Kelvin mechanical model are used to construct the creep constitutive model of BCC connections.The test results show that the predicting accuracy of double-chain Kelvin model is relatively higher.A creep constitutive model is proposed by modifying the stiffness reduction method.The model is introduced into predicting the long-term deflections of BCC beams.The results indicate that new method can well predict the final deflection and deflection growth trend of BCC beam under long-term load,and prediction accuracy of the proposed method is much better than that of the original method.Based on the new method,the long-term deformation of BCC beam under the quasi-permanent design load is predicted and discussed.The results reveal that the final deflection of typical BCC beam atfer loading 50 years meets the requirement of controlling deflection of SLS according to the design code.This paper suggests that the upper limit of long-term load of BCC beam can be determined by 10% of its ultimate bearing capa city under shortterm load.Finally,the push-out tests and bending tests are carried out for the connections and composite beams after the end of the long-term test.The test results show that the long-term loading history causes mechanical performance degradation of connections.The degree of degradationis related to the type of connectors.It also found the degradation degree of cast-in-place connections was relatively large compared with the assembled connector.The residual bending capacity and bending stiffness of BCC beams after long-term loading slightly increase,while its ductility reduce.The current study preliminarily reveals the deformation development characteristics of BCC system under long-term loading,and the residual mechanical performance after long-term load.It provides the foundation for the design and application of BCC beam. |
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