Hysteretic modelling and strength-envelope estimation models of concrete filled steel tubular members

The concrete filled steel tubular (CFT) columns have been widely used in steel structures due to many merits, including large strengths, large stiffness and high construction convenience. Many previous studies of CFTs have verified that the confinement of the infilled-concrete was significantly affected by the dimensions, deformation levels and shapes of the steel tubes. Various estimations of the confined strengths of the concrete were previously developed upon the experimental results; however, apparent deviations were shown. This study presents a comprehensive study on the strengths of CFTs subject to monotonic loadings, especially the confined strengths of the concrete, through parametric analyses using finite element modelling (FEM) method. The estimations of the average confined strengths of the infilled-concrete for the CFTs were established and showed improved accuracy compared to the existing others. To achieve the modelling of the hysteretic behavior of the CFTs, the general fiber element modelling (FBM) method of CFTs was then developed. The general estimations of stressstrain models of the infilled-concrete of the CFTs were proposed and accurately captured the hysteretic behavior of CFT columns. Finally, the developed FBM was adopted to establish the general estimations of hysteretic-strength-envelop models of CFTs.