Post-Fire Mechanical Behaviour of Eccentrically Loaded Hollow Sandwich High-Strength Steel Tube-Concrete Short Columns

Literature Overview

The paper by Li Yefeng, Mao Xiaoyong, Liu Xiao, and Liu Zhaohui, published in the Journal of Suzhou University of Science and Technology (Engineering Technology Edition), Volume 38, Issue 1, 2025, presents a comprehensive investigation into the full-process mechanical performance of hollow sandwich high-strength steel tube-confined concrete (HSTC) eccentrically loaded short columns subjected to fire exposure. The study is funded by the National Natural Science Foundation of China (Grant No. 52078306) and the Jiangsu Provincial Key Project of Natural Science Research (Grant No. 21KJA560002). The research addresses a critical gap in fire engineering design for composite structural members, particularly for columns with hollow sandwich configurations that are increasingly adopted in high-rise and industrial buildings for their lightweight and high-strength advantages.

Core Technical Content and Methodology

The authors first established the constitutive model for hollow sandwich steel tube-concrete members throughout the entire loading process, including the ascending branch, peak, and descending branch. A finite element model was constructed using ABAQUS software, and the full-process load-displacement curves were computed and validated against experimental results. The validation showed good agreement between numerical predictions and test data, confirming the reliability of the modelling approach.

The parametric study examined six key variables: heating duration, hollowness ratio, inner and outer steel tube yield strength, concrete compressive strength, slenderness ratio, and eccentricity. The following table summarises the principal findings:

The key finding is that material strength and hollowness ratio have limited influence on the residual bearing capacity coefficient, whereas eccentricity and slenderness ratio exert substantial effects. As eccentricity increases, the ultimate bearing capacity drops markedly, but the residual capacity coefficient gradually rises, indicating a relative improvement in post-fire performance under eccentric loading compared to axial loading. The slenderness ratio shows a pronounced adverse effect on residual bearing capacity, with longer and more slender columns experiencing greater strength loss. Heating duration has the most detrimental effect, causing a sharp decline in the residual capacity coefficient.

Engineering Practice Implications

From a structural engineering perspective, these findings carry significant implications for the fire design of hollow sandwich steel tube-concrete columns. The hollow sandwich configuration, which incorporates an inner steel tube, an outer steel tube, and a concrete core with a hollow space between, offers advantages in weight reduction and material economy. However, the fire performance of such members requires careful consideration.

The observation that eccentricity increases the residual capacity coefficient is counterintuitive but can be explained by the stress redistribution mechanism. Under eccentric loading, the fire-induced temperature gradient causes differential thermal expansion between the outer and inner steel tubes, leading to a more uniform stress distribution in the post-fire state. In contrast, axially loaded columns experience more concentrated stress concentrations that are more sensitive to fire damage.

For practical design, engineers should pay particular attention to the slenderness ratio and heating duration when assessing the post-fire safety of hollow sandwich columns. The slenderness ratio affects not only the buckling behaviour but also the thermal gradient distribution across the section, which is critical for fire resistance assessment. The heating duration parameter is directly related to the fire resistance rating, and the sharp decrease in residual capacity with prolonged heating underscores the importance of fire protection measures such as intumescent coatings or concrete encasement.

Key Questions and Reflections

Several questions arise from this study that warrant further investigation. First, the constitutive model developed for the full loading process under fire conditions assumes certain material degradation functions, and the accuracy of these functions at elevated temperatures needs validation with additional experimental data, particularly for high-strength steel grades above 460 MPa. Second, the hollowness ratio was found to have minimal influence on residual capacity, but this conclusion may not hold for very high hollowness ratios where the hollow space could affect heat transfer and concrete spalling behaviour. Third, the study focuses on short columns, and the transition to medium and long columns, where buckling becomes dominant, remains an open question.

The use of ABAQUS for finite element analysis is well-established, but the thermal-mechanical coupling effects, including the interaction between the hollow space and the surrounding materials, should be carefully modelled. The boundary conditions assumed in the numerical model may not fully capture the complex thermal boundary conditions encountered in real fire scenarios, such as the uneven temperature distribution from one-sided fire exposure.

Study Insights and Outlook

This paper provides valuable insights into the fire engineering design of hollow sandwich steel tube-concrete columns. The parametric analysis approach is systematic and the conclusions are practically relevant. The finding that slenderness ratio and heating duration are the dominant factors for residual capacity assessment should guide engineers in prioritising fire protection strategies. Future research should extend to full-scale member tests, consider cyclic loading after fire exposure, and explore the effectiveness of various fire protection methods on hollow sandwich configurations. The integration of these findings into design codes for composite structures subjected to fire is an important next step.