Eccentric compressive behavior of square concrete-encased concrete-filled double-skin steel tubular stub columns

Concrete-encased concrete-filled double-skin steel tubular (CFDST) stub columns serve as structural members, encompassing an exterior reinforced concrete (RC) component alongside an internal CFDST component. In this paper, the eccentric compressive behavior of such stub columns was investigated. To this end, specimens with varying diameter-to-width ratios d_o/w and eccentricity ratios e/w were examined; their failure process, failure modes, strain distributions, and representative mechanical performance were recorded and analyzed. Based on the experimental results, the specimens' failure process transitions through four stages: elastic, elastic-plastic, descending, and stabilizing. With d_o/w escalating from 0.5 to 0.7, the eccentric compressive strength, flexural stiffness, and displacement ductility index improved by 16.1%, 15.2%, and 37.2%, respectively. Subsequently, a detailed finite element model (FEM) was established to analyze the eccentric performance of the concrete-encased CFDST stub columns. Following the FEM reliability validation, the working characteristics of such stub columns in the balanced failure mode were elucidated. The full-range analyses were carried out under failure modes controlled by compression and tension to unveil the loading distribution mechanisms. Moreover, the impacts of parameters like concrete compressive strength, diameter-to-width ratio, and steel tube yield strength on the N_u–M_u relation curves were quantified, ascertaining the key influencing factors. Finally, a formula was suggested to predict the corresponding eccentric compressive strength. Following the superposition principle and limit equilibrium theory, the simplified design equations put forth yielded satisfactory predictions based on the experimental and numerical results.