Analytical Formula for Load-Carrying Capacity of Eccentrically Loaded CFDT Columns and Performance Comparison with CFST Columns Under Seismic Loads

Concrete-filled double-tube (CFDT) columns are a new idea in designing and building structures. In this manuscript, the effect of various parameters (yield stress of the steel tube, the characteristic strength of concrete, the arrangement of steel tubes, and the ratio of the diameter to the thickness of the steel tubes) on the bearing capacity of eccentrically loaded CFDT columns is studied for the first time. Using ABAQUS software, the finite element model is validated based on available laboratory specimens. The bearing capacity of eccentrically loaded CFDT columns with different material and mechanical specifications is compared in the form of P-M diagrams. The results show that by increasing the yield stress of the outer tube, increasing the diameter of the outer tube and increasing the strength of the outer concrete, the bearing capacity of the CFDT column is increased. Based on the failure modes of the CFDT column under the eccentric load, the outer concrete has the most damage in the tension and compression mode. In addition, the ductility and energy absorption capacity are compared between the CFDT and the concrete-filled steel tube (CFST) columns. Based on this comparison, the performance of the CFDT column is better under eccentric loading. Finally, a formula for designing and determining the bearing capacity of eccentrically loaded CFDT columns is proposed. In the last part of the work, the seismic performance of multi-story buildings equipped with the CFDT and CFST columns under a real earthquake is examined. In terms of seismic performance, the frames equipped with the CFDT columns have more energy absorption and base shear than the frames equipped with the CFST columns. Also, the displacement of the roof related to the bending frame with CFDT columns is less than the frame with CFST columns. The results indicate that CFDT columns improve the seismic performance of tall buildings.