Theroretical And Experimental Research On Creep And Shrinkage Effect Of Steel And Concrete Composite Beams

Steel-concrete composite beam has significant technical and economic benefits and broad application prospects. Over time, the concrete members have properties of creep and shrinkage, which constraints by steel beam. The internal stress redistribu-tion occurs in statically determinate structures, even internal forces redistribution in statically indeterminate structures, changing the final stress state of composite beams. The value is several times than elastic deformations, which is positively bad for the structure strict in deflection. However, how to accurate stress redistribution and the resulting in a series of structure effects accurately and conveniently caused by creep and shrinkage, still not been well solved. The completed work and conclusions of this thesis through methods of theoretical calculation, numerical analysis and experi-mental study are listed as follows.1. Analytic formulas of stress redistribution caused by internal constraints in statically determinate composite beams are deduced by Internal Force Distribution Method. The concrete creep constitutive relation is set up first with Dischinger dif-ferential equation and Trost-Bazant algebraic equation respectively, and the accurate solutions of internal forces distribution are obtained in the two equations respectively. By ignoring the influence of concrete axial strain by bending moment redistribution of concrete, the approximate solutions are obtained in some applicable conditions. The results show that the results of approximate solution are in good accord with that in exact solution, which can be used to practical calculation in engineering applica-tion.2. Analytic formulas of stress redistribution caused by internal and external coupling constraints in statically indeterminate composite beams are deduced also by Internal Force Distribution Method. The secondary moment under creep restraint is solved in three basic conditions: â‘  Structural system transformation dur-ing construction; â‘¡Composite beam structure subjected to uneven settleme nt; â‘¢ Different creep (shrinkage) characteristics in each component of composite beam structure. The results show that the negative bending moment in simply support-ed-continuous composite beam is smaller than the overall cast-in-place continuous beam. The middle bearing negative bending moment increases with time by creep, and the greater the creep, the closer to a negative moment of the cast-in-place con-tinuous beam. Creep plays a favorable role on the settlement of bearing. There is no creep secondary moment in intermediate bearing with same creep characteristics of two cross in composite beams or same span. Both two cases of different across span and creep characteristics, or different loads and creep characteristics, lead to creep secondary moment of middle branch. If considering shrinkage effect, all cases bring out secondary moment. If considering shrinkage effect, all cases bring out secondary moment.3. Putting forward "Direct Method", the stress formula of composite beam is obtained, with conversion factor of material, creep adjustment coefficient and con-traction coefficient. Adopting this method, the section stress in composite beam is calculated according to single material rule directly without solving differential equation or algebraic equation, with the corresponding coefficient, which covers the characteristics relationship of concrete and steel beam, aging characteristics of creep and shrinkage. The results show that the section stress results of composite beams using the Direct Method is good accord with that using the Internal Force Distribu-tion Method. It is very convenient and effective to determine the accuracy of calcu-lation result through creep stress calculation by found in "zero point of creep stress".4. Concrete constitutive relation of creep (shrinkage) is simulated accurately in ANSYS with APDL procedures, through solving creep (shrinkage) coefficient of constitutive equation. Theoretical method and ANSYS is used to contrast the results of axial force and bending moment of concrete and steel beam, deflection and strain in mid-span of the example. The restraint degree of girder to concrete is key influence factors of creep (shrinkage) characteristic in composite beam, and using coefficient "j" to distinguish. The results show that the results of internal force and deformation of composite beams using ANSYS is very good with theoretical calculation results, which validate the accuracy of theoretical calculation method and confirmed that the software is applicable to analyze the long-term mechanical performance of composite beams. It is also found that the greater the constraint degree of girder to concrete, the more significant the distribution.5. Four simply supported composite beams are designed for experiment de-pending on constraint degree of girder to concrete of composite beam, through me-chanical observation of experimental beams at 300 days. The accuracy of theoretical method is verified and the long-term mechanical laws obtained by compared with theoretical calculation, numerical analysis and the experimental data. The strain and deformation values using the three methods are accorded well with overall trend. Strain and deformation of composite beam increase with time, which is about three times higher at 300 days old than that at the early loading. Creep and shrinkage ef-fects of composite beams should be cause enough attention.