Time-dependent modeling of concrete-encased concrete-filled double-skin steel tube stub columns considering humidity effects

Accurate prediction of time-dependent behavior is crucial for concrete-encased concrete-filled double-skin steel tube (CECFDST) columns used in high-rise load-bearing structures. However, existing models fail to predict the non-uniform distributions of shrinkage and creep influenced by humidity gradients. To address this issue, this study develops computational models for non-uniform shrinkage and creep in the outer concrete of CECFDST columns and proposes a simplified calculation method for predicting their time-dependent behavior. Firstly, a 210-day experiment was conducted to study moisture distribution in concrete and its time-dependent deformation. A relative humidity distribution model and a linear relationship between free shrinkage and humidity loss were developed using moisture field theory and micro-prestress solidification theory, followed by the development of non-uniform shrinkage and creep models for outer concrete through cross-sectional analysis. Subsequently, a numerical simulation of the time-dependent behavior of CECFDST columns was conducted, followed by a parametric study to quantify the influence of key parameters on long-term behavior. Results indicate that the concrete-filled double-skin steel tube ratio ( α CFDST ) is a crucial parameter, as the time-dependent effects of concrete decrease by 19 % when α CFDST increases from 0.5 to 0.7. The proposed non-uniform shrinkage and creep models effectively address the overestimation issue (over 10 % in specific cases) associated with current uniform models. Finally, a simplified calculation method was developed to facilitate practical engineering applications. • Humidity boundary variation yields outer concrete’s non-uniform shrinkage and creep. • Sandwiched concrete exhibits less shrinkage and creep than outer concrete. • Proposed models solve overestimation in CECFDST’s deformation due to uniform models. • RH -based design methods can be used for CECFDST serviceability design.