Axial Compression Mechanism Analysis of Hollow Sandwich High-Strength Steel Tube Concrete Columns After Fire

Literature Overview

This paper by Liu Xiao and colleagues from Shenyang University, published in the journal Industrial Construction in 2019, investigates the post-fire axial compression behavior of hollow sandwich high-strength steel tube concrete columns. These columns, referred to as high-strength CFDST columns, feature a hollow sandwich configuration with an outer steel tube, an inner steel tube, and a concrete core between them. The study was supported by the National Natural Science Foundation of China (Grant 51308347) and the Shenyang Science and Technology Plan Project (18-013-0-16). The authors used ABAQUS finite element analysis software to establish computational models, validated them against experimental data, and conducted parametric studies to understand the influence of various factors on post-fire structural performance.

Core Technical Content and Model Validation

The hollow sandwich steel tube concrete column is an advanced structural system that combines the advantages of high-strength steel and concrete in a compact, efficient cross-section. The hollow core between the inner and outer tubes can be used for utility routing or to reduce self-weight, while the sandwich configuration provides enhanced confinement of the concrete core.

The finite element model was developed in ABAQUS using appropriate constitutive models for the concrete core. The model was validated against experimental test data, and the validation results showed good agreement, confirming the reliability of the computational approach.

The parametric study investigated the effects of four key variables:

A particularly important finding was the shift in load-bearing proportions when the outer tube yield strength was increased from Q345 to Q460. The core concrete contribution to total capacity decreased from 60% to 30%, while the outer tube contribution increased from 25% to 50%. This shift indicates that higher-strength outer tubes significantly change the load-sharing mechanism within the composite column.

Post-Fire Mechanism Analysis

The study revealed several important mechanisms governing the post-fire behavior of hollow sandwich high-strength steel tube concrete columns:

• With increasing fire exposure time, the ultimate load capacity of the CFDST columns decreases due to the degradation of both steel and concrete material properties at elevated temperatures.
• The interaction between the inner and outer steel tubes and the concrete core is a critical factor in post-fire performance. The confinement provided by the outer tube helps maintain the integrity of the concrete core even after fire exposure.
• The load-sharing mechanism changes significantly with the outer tube strength. Higher-strength outer tubes bear a larger proportion of the load, which can be beneficial for post-fire performance because steel retains a higher fraction of its strength at elevated temperatures compared to concrete.
• The hollow ratio affects the post-fire capacity by changing the amount of concrete in the core and the interaction between the inner and outer tubes.

Engineering Practice Implications

The findings have important implications for the design and assessment of hollow sandwich steel tube concrete columns in fire-prone environments:

1. Post-fire structural assessment should account for the significant degradation of both steel and concrete properties, with the load-sharing mechanism potentially changing after fire exposure.
2. The use of higher-strength outer tubes (Q460 or higher) can improve post-fire performance by increasing the proportion of load carried by the steel component, which degrades less severely than concrete at high temperatures.
3. The hollow ratio should be optimized to balance structural efficiency with post-fire performance, as higher hollow ratios reduce the concrete volume and the confinement effect.
4. Fire protection measures should be designed to limit the temperature exposure of the concrete core, as concrete strength degradation is a major contributor to post-fire capacity loss.
5. The validated finite element model can be used for post-fire structural assessment and for evaluating repair and strengthening strategies.

Study Insights and Reflections

This paper provides valuable insights into the post-fire behavior of advanced composite structural systems. The hollow sandwich configuration represents an innovative approach to steel tube concrete columns that combines structural efficiency with functional advantages such as utility routing through the hollow core. The finding that higher-strength outer tubes shift the load-bearing proportion from concrete to steel is particularly significant for fire engineering, as it suggests a design strategy for improving post-fire resilience. The parametric approach used in the study is systematic and provides clear guidance for designers. However, engineers should be aware that the validated model uses specific constitutive relationships for concrete, and these should be verified for different concrete grades and fire exposure conditions. The study also highlights the importance of considering the interaction between components in composite columns, as the post-fire behavior is not simply the sum of the individual component responses. Overall, this research contributes to the understanding of advanced composite structural systems and provides a foundation for developing design guidelines for hollow sandwich steel tube concrete columns in fire-prone applications.