Analytical and numerical investigation of ultra-high-performance circular concrete-filled double-tubes under fire conditions

This study presents an analytical and numerical investigation of ultra-high-performance concrete-filled double-skin tubular (UHPC-CFDST) columns with circular cross-sections under fire conditions. An automated algorithm was employed to develop and verify a finite element (FE) model capable of accurately simulating CFDST columns incorporating different concrete types (normal concrete, ultra-high-performance concrete (UHPC), and lightweight concrete) in both core and ring regions. The validated model was used to examine the influence of key parameters, offering deeper insight into the behavior of such columns under fire. Three temperature-dependent material models were developed to represent UHPC, lightweight concrete, and high-strength steel at elevated temperatures. Additionally, a finite difference-based thermal model was proposed to simulate the temperature distribution across the column cross-section and to predict fire resistance (FR) time. To the best of the authors’ knowledge, existing fire design standards do not provide specific models for CFDST columns under fire, and available research in this area is limited. This study addresses this gap by evaluating the applicability of Eurocode 4 (EC4) and proposing a simplified modification that improves the prediction accuracy of fire resistance for UHPC-CFDST columns.