Experimental Study On Long-term Behaviors Of GluBam-concrete Composite Beams

GluBam-concrete composite beams fully utilize the compressive properties of concrete and the tensile properties of the Glubam,with a broad application prospect in the fields of green building and existing building reconstruction.Under the long-term action of load,the GluBam and concrete in the Glu Bam-concrete composite beams will creep and shrink at different rates.The difference between the two kinds of materials will lead to the redistribution of the stress on the section and cause a lot of additional deformation of the structure.The computation influenced by the factor of time has become a point which can not be ignored in the structural design.Based on the researches of shear and bending performance of GluBam-concrete composite system,this thesis selected four kinds of connector which are co ntinuous steel mesh,bolt,composite groove with bolt and preload force composite shear connectors,then carried out researches on the long-term behavior of the connectors and GluBam-concrete composite beams.The main research content of this thesis are as follows:Firstly,a long-term loading device for push-out tests on connectors of GluBam-concrete composite beams was designed and manufactured.The long-term loading device uses the lever principle to transform a small load into a higher load on the test specimen,so the loading efficiency is high.The loading frame is light in weight,small in space and easy to adjust the load.The load applied on the specimen can be adjusted by selecting the spring and changing the distance between the pin and the hinge support,which is convenient and practical.Then,the long-term push-out tests on connectors of GluBam-concrete composite beams were performed.Four groups of specimens were designed to sustaine a load by means of a spring load apparatus to simulate the qu asi-permanent load condition at serviceability limit state.The relative slip-time curve of every specimens was measured.The creep coefficients of connectors were obtained through the analysis of the test results.Based on the data of long-term push-out test,the connectors creep constitutive model was developed.On this basis,four GluBam-concrete composite beams with four kinds of connectors were tested under sustained load in sheltered indoor conditions.The midspan deflection was monitored in the serviceability load condition.There are many factors affecting the long-term properties of GluBam-concrete composite beams,such as the difference in creep properties of components materials,the variations of temperature and humidity and the level and distribution of stress.In order to account for the effects of creep in concrete,timber and connector,Effective Modulus Method uses the pure creep coefficients of different components,was proposed to fit the long-term deflection deformation of GluBam-concrete composite beams.This analytical approach significantly overestimated the experimental results.Therefore,The effective creep coefficients was proposed to modify the long-term deflection curve,and the modified long-term deflection curve is in good agreement with the test curve,then the method was used to extrapolate experimental deflections at the end of the service life.The results of this study provide important datas for long-term design of the GluBam-concrete composite beams.It is laid the foundation to the further study for this new composite structure.