Long-term Behavior Of Axially Loaded Concrete Filled Steel Tubular Stub Columns Under High Stress

Concrete filled steel tubular (CSFT) members have gained their popularity inboth high-rise buildings and large-span arch bridges applications due to their highcompressive strength and excellent ductility. These mechanical advantages makeCFST members do not suffer strict limits on the ratio of the sustained load over thecross-sectional capacity. Especially for members with circular cross-sections, somedesign code even cancels such specification, considerably reduces the cost of thestructures, but increases the stress level in the concrete cores of CFST members inservice condition which may lead to nonlinear creep.Extensive experimental researches have been conducted to investigate thetime-dependent behavior of concrete under high stress levels. Test results showedthat creep strains were much greater than those predicted by linear creep assumptionfor medium stress levels (ranging from40%to80%of short-term compressivestrength fc). The concrete members may even fail after a while when subjected torather high stress levels (more than80%of fc). Using linear creep theory to predictthe long-term responses of CFST members under high stress levels may leads topotential unsafety for structures. Up to date, no theoretical investigation wasconducted to well predict the nonlinear creep of CFST members. Though someexperiments have been done to investigate the long-term response of CFST memberssubjected to sustained high stress level. The conclusions are not consistant or evencontradictory to each other. Arguments are still carrying on as to the limitation.In this context, this thesis presents experimental and numerical analysis on thelong-term response of CFST members under high stess levels, which mainly focuson the following works.Existing concrete nonlinear creep models was benchmarked against availablenonlinear creep tests to evaluate the reliability of those models in predicting thetime-dependent behavior of concrete under high stress levels. Based on the results,the concrete model with a reasonable description of long-term behavior measured inthe reported experiments was recommended.The long-term experiments were carried out to investigate the nonlinear creepbehavior of axially loaded CFST members under high initial concrete compressivestress levels. For this purpose,6CFST stub columns were subjected to differentlevels of initial stress in concrete core at the same concrete age. The role ofconfinement for the specimens was then evaluated based on the experimentalmeasurements of both longitudinal and circumferential strains. An accurate method was developed to evaluate the nonlinear creep behavior ofaxially loaded CFST members. This method was validated using available testresults. An extensive parametric study was then performed to evaluate the influenceof time effects on the static response of CFST members under high initial concretecompressive stress levels. Considered parameters included the duration of thesustained load, the concrete compressive strength, the ratio of steel area overconcrete area and the concrete age at first loading.