Eccentric compression behavior of CFST columns strengthened with steel tube and sandwiched grout jackets

Concrete-filled steel tube (CFST) columns are widely used in civil engineering practice owing to its excellent mechanical properties and satisfying construction cost. However, the increasing application also increase the risk of unexpected damage for the CFST columns. To prolong the structure's life, the authors proposed to enhance the mechanical properties of the deficient CFST columns by setting an external confining steel tube outside the existing CFST columns and filling the gap in between with self-compacting grout. The preliminary study revealed the axial compressive behavior of the deficient CFST stub columns can be effectively strengthened by employing the configuration. To further promote the application of the new strengthening method, a total of 2 ordinary and 12 strengthened CFST slender columns were designed and tested under eccentrical compressive load. The main variables of the test include the steel ratio of the deficient CFST columns, slenderness ratio and load eccentricity, etc. Test results showed that due to the effective confinement by the additional outer steel tube, the significantly enhanced eccentric compressive strength can be obtained for the internal deficient CFST columns. Subsequently, the refined corresponding finite element (FE) model was developed and validated based on the experimental results, and the influence of key parameters including the slenderness ratio, load eccentricity, outer tube steel ratio, and so on were investigated and quantified systematically. Taking the slenderness ratio and load eccentricity as two basic parameters, a suitable calculating method was proposed to determine the eccentrical compressive strength of the strengthened CFST columns based on the prediction method of the axial compressive strength of the strengthened CFST stub columns.