Nonlinear Stability And Realiability Of Concrete Filled Tube

Because of the good mechanical performance, the concrete-filled tube (CFT) is widely applied in the structure. CFT usually is demonstrated as a suitable structural element for arch ribs through bridge engineering practices, for the CFT element exhibits excellent compressive resistance capacity. Rigid framework with CFT was in service as super high pier in Ya Lu High Way. The stability of CFT piers are studied, relying on the LaBaJin and HeiShiGou continuous rigid frame bridge in this thesis, of which is the highest pier in Asia (182m). The dissertation includes three parts mainly:In the first part, based on a large number of literatures, the overviews of CFT structures and reliability are undertaken. The research project to study the ultimate resistance and reliability of CFT structures is put forward.In the second part, the nonlinear analysis of ultimate stability resistance is solved. First, a modified stress-strain relation for confined concrete which involves the influence of load eccentricity on confining pressure is proposed. The nonlinear finite element software of CFT is presented, adopting in formulation of degenerated beam element. To study the whole-process behavior of the CFT structure, the arc-length method is introduced in the nonlinear analysis program. The nonlinear behaviors of experimental CFT short columns are investigated, and the accuracy of the proposed constitutive model for confined concrete is mainly concerned. The results demonstrate that because of the crack of confining concrete with increment of eccentricity, the volume expansion of core concrete may decrease gradually, which leads the steel tube to provide weaker confining pressure on core concrete. It can be found that the core concrete is under whole-section compression, for the small eccentrically loaded column. On the contrary, as large eccentrically loaded column, a large portion of core concrete has been tensile cracked, and trivial confining pressure can be predicted. The case can be viewed as an intermediate state between small and large eccentrically loaded column, and a reduction of confining pressure may be an appropriate way to reflect the actual behavior. To solve the ultimate stability resistance of CFT, the geometrical nonlinearity should be considered as well as material nonlinearity. Second, geometrical stiffness matrix induced by large deformation is derived by updated Lagrange formulation. The behaviors of CFT slender test columns and test arch are simulated by the program developed in this part. The numerical data are suitable for the test results well. The effect of geometrical nonlinearity is studied by parametric analysis. In additional, the effects of concrete strength, steel strength and eccentricity to the ultimate resistance of high pier in LaBaJin Bridge are investigated. A model experiment of CFT high pier is accomplished. The accuracy of the nonlinear analysis program is proved by the experimental results.In the third part, the main propose is to present a reliability analysis method of nonlinear ultimate stability resistance of CFT. Probability distributions of ultimate resistance capacity of axial loaded short CFT column are investigated by Monte Carlo method. Means and mean-square deviations of those are presented by regression analysis. The distributions don't refuse normal distributions through K-S method. The reliability index can be calculated easily with the suggested probability distribution model. Next, consistent mode imperfection method and random field imperfection method are introduced to deal with the initial imperfection of CFT structures. The upper limit of random variables of exponential decay model and square exponential decay model are induced, considering the range of variables in person computer. Based on the interpolating technology, a new method is suggested to mesh random field, verified by exiting literature. Finally, a general and versatile reliability analysis program of nonlinear ultimate bearing capacity of CFT is developed by response surface method. The difficult of searching designing point induced by nonlinear is solved through Sequential Unconstrained Minimization Technique (SUMT). The stability coefficient and influences of geometrical, initial imperfection, concrete strength, steel strength and D/t to reliability index are discussed in the numerical examples.