Axial compressive behavior and capacity reduction of circular CFST stub columns with internal discrete voids

To investigate the influence of internal discrete voids on the axial compressive behavior of circular concrete‐filled steel tubular (CFST) stub columns, 24 specimens were tested under axial compression. Regularly distributed spherical voids were introduced using thin‐walled hollow stainless steel spheres. The effects of nominal void volume ratio χ , tube wall thickness t , and column height L on failure mode, axial capacity, secant stiffness at 0.4 N u , and steel‐tube strain response were examined. Internal voids promoted local buckling near the hollow spheres and between adjacent voids, accompanied by localized crushing and circumferential cracking of the core concrete. Increasing χ generally reduced axial capacity and secant stiffness at 0.4 N u , whereas increasing t improved structural resistance and delayed local deformation. For the tested specimen dimensions and nominal void‐ratio range ( χ = 0%–12%), a linear capacity reduction factor kr = 1 − 0.0221 χ was established based on the normalized capacity ratio and combined with the GB 50936‐2014 reference capacity. The full‐dataset calibration and leave‐one‐out cross‐validation gave MAPEs of 4.45% and 4.66%, respectively.