Tests on Short Outer Tube–Stiffened CFDST Columns: Effect of Stiffener Distribution on Axial Performance

Previous studies on concrete-filled double-skin tubular columns (CFDSTCs) have demonstrated a superior axial performance when outer steel tubes are reinforced with steel stiffeners. As a result, recommendations for stiffener design, specifically focusing on its cross-sectional area parameter, were proposed. However, as the steel tube diameter increases, increasing only the stiffener area (while maintaining a constant number of stiffeners) may satisfy the proposed recommendation, yet it could be ineffective in serving the intended purpose. Subsequently tests were conducted to address this issue through experimental investigation, as no tests or data are available for the optimal stiffener distribution in CFDSTCs, in continuation with the previous experimental research on outer tube–stiffened CFDSTs. The key focus of this study is to assess the impact of stiffener distribution in outer tubes of circular CFDSTs, by varying the stiffener width, stiffener number, outer tube diameter and stiffener spacing in terms of subpanel width. Failure characteristics in steel tubes, concrete core, peak loads, axial load–strain response, hoop-strain behavior, confined concrete strength, strength index and ductility index were the different performance indicators analyzed. Lastly, the peak test strengths were compared with those predicted by available design models, revealing inconsistencies in these approaches. The test findings highlight the significant impact of stiffener distribution on axial performance across all indicators, especially on the axial strength (up to 24%) and confined concrete strength (up to 28%). Consequently, results from a modified design model that integrated the improved confined concrete strength of short circular outer tube–stiffened CFDSTCs agreed well with the test results.