| With the progress of China’s transportation, Concrete-Filled Steel Tube(CFST) bridge as a relatively new type of bridge has been developed rapidly,more than200new such bridges have been built in recent20years, but it must be noted that the research works on CFST bridge design theory is relatively less compared with the engineering practices,the design codes are also imperfect,especially to the fatigue design of CFST.For CFST bridge, the tubular joint is not only the key part to loadcarring capacity, but also the weak link in the entire structure. Based on the existed research works on CFST bridge fatigue in China and abroad, the subjects as follows have been studied through theoretical analysis and experiments.1.The data of typical welded joints fatigue tests and extensive research results in China and abroad are collected.For actual project requirements and design issues to be resolved, the research subjects in this paper have been raised.2.By static load tests on the CFST welded joints, the nominal stress and hot spot stress are measured,the stress concentration factor of CFST welded joints are also obtained, and the refined three dimensional finite structure model to carry on the stress computation of specimen are established, and the calculated results are compared with the experimental results.3.Regarding stress concentration and stress concentrations factors, the differences in stress concentration between hollow joints and CFST welded joints are given, the concept of stress concentration mitigation factor q are raised.the equations to calculate the factor q are obtained by a finite element model utilizing the APDL method and NLSF(non-linear least-squares fitting) method by which the independent variables and the relative coefficients are analysed. Based on the hollow joint welded tube stress concentration factor formula, the stress concentration factor formula of CFST joint are given.4.By analyzing the fatigue test data of CFST joints,the fatigue failure criterion that surface crack lengths up to3times the wall thickness of CFST joints are raised.Through fatigue test and the crack extending survey, it’s proved that,due to the constraint of concrete, the criterion of fatigue failure for CFST joint is safe.5.By stress analysis to pipe joints based on fatigue tests, the pipe joints stress analysis mechanics model and the formula which can calculate the position of fatigue cracks in the hot spot region are proposed.This model can explain why the hot spot position of the hollow tube and the CFST may shift, and predict fatigue fracture initiation point of the pipe joints, and give analytical solution.Using the model’s formula to calculate the crack position of Cao’e Jiang bridge fatigue model, the result is in good agreement with experiment ones.6.The traditional S-N curve and fracture mechanics methods are used to study fatigue strength and the crack growth life of CFST.and fatigue strength correction coefficient of the CFST corresponding to the hollow tube(DT kind) are raised,and the CFST S-N curve modified equations are also obtained.CFST crack growth computational methods of the fracture mechanics are proposed, and the parameter value of crack growth formula through the fatigue data are determined.7.Taking the Cao’e Jiang bridge as the example, the entire bridge finite element model are established,and the S-N curve modified formula that raised in this paper and the fracture mechanic methods are used to calculate the fatigue life of the bridge. Two methods are mutually the confirmations, and the CFST joints fatigue life assessment process are established. |
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