Parametric Analysis of Axial Compression Performance of High-Strength CFST Columns After Fire Exposure

Research Background and Fire Safety Concerns

The paper by Wang Xiaochu and colleagues, published in the "Journal of Shenyang University (Natural Science Edition)" in 2024, investigates the axial compression performance of high-strength concrete-filled steel tube (CFST) columns after fire exposure. Fire safety is a critical concern in structural engineering, and understanding the residual structural capacity of CFST columns after fire exposure is essential for post-fire assessment, structural repair, and design of fire-resistant systems. The research employs ABAQUS finite element software to simulate the post-fire axial compression behavior of high-strength CFST columns, with steel tube yield strength, fire exposure duration, and concrete strength as the primary parameters.

Finite Element Modeling and Simulation Approach

The finite element model was developed to capture the complex interactions between the steel tube and concrete core under elevated temperature conditions. The model incorporates temperature-dependent material properties for both steel and concrete, accounting for the reduction in yield strength, elastic modulus, and ultimate strength of steel at elevated temperatures, as well as the loss of compressive strength and tensile strength of concrete. The fire exposure was simulated using a standard fire curve, and the post-fire cooling phase was also modeled to capture the residual stresses and damage accumulation.

The load-displacement curves obtained from the simulations were analyzed to determine the ultimate load-bearing capacity, initial stiffness, and ductility of the post-fire columns. The typical load-displacement curve characteristics were also examined to understand the failure mechanisms at different fire exposure durations.

Key Findings on Post-Fire Structural Performance

The parametric analysis revealed several important findings regarding the post-fire axial compression behavior of high-strength CFST columns:

1. As the steel tube yield strength increases, the ultimate load-bearing capacity of the post-fire column increases proportionally. This confirms that higher-grade steel tubes provide better fire resistance performance due to their higher initial strength margin.
2. As the concrete strength increases, the ultimate load-bearing capacity also increases, but the improvement becomes less pronounced at higher concrete grades. This is because the concrete strength reduction at elevated temperatures is more significant for higher-strength concrete grades.
3. As the fire exposure time increases, the ultimate load-bearing capacity shows a decreasing trend. The rate of capacity loss is more significant at longer exposure times, reflecting the progressive degradation of both steel and concrete material properties.
4. For fire exposure times of 60, 90, and 120 minutes, the concrete in the column bears the primary load, indicating that the steel tube has undergone significant strength degradation at these exposure durations.

The load-displacement curves of post-fire columns exhibit a characteristic nonlinear behavior with reduced initial stiffness and lower ultimate capacity compared to unexposed columns. The failure mode shifts from steel tube local buckling in unexposed columns to concrete crushing in post-fire columns, particularly at longer exposure times.

Temperature-Dependent Material Degradation and Structural Implications

The degradation of material properties at elevated temperatures is a critical factor in post-fire structural performance. For structural steel, the yield strength begins to decrease significantly at temperatures above 400°C, with a reduction of approximately 50% at 600°C. For concrete, the compressive strength starts to decrease at temperatures above 300°C, with rapid degradation above 500°C due to the decomposition of calcium hydroxide and the loss of internal bonding.

The differential degradation rates between steel and concrete create a complex interaction in the post-fire CFST column. At lower temperatures, the steel tube retains most of its strength and continues to provide effective confinement to the concrete. At higher temperatures, the steel tube weakens significantly, reducing its confinement effectiveness and allowing the concrete to expand and potentially spall. This transition in the load-sharing mechanism between steel and concrete is a critical factor in determining the post-fire structural capacity.

Engineering Practice and Post-Fire Assessment Procedures

For practical engineering applications, the findings of this research have important implications for post-fire structural assessment and repair. After a fire event, structural engineers must evaluate the residual capacity of CFST columns to determine whether the structure can be safely occupied or requires repair or replacement. The following assessment procedures are recommended:

1. Visual inspection of the steel tube surface for discoloration, oxidation, and deformation.
2. Measurement of the maximum temperature reached at critical locations using thermocouple records or thermal imaging analysis.
3. Assessment of concrete spalling and cover loss, which indicates the severity of fire exposure.
4. Mechanical testing of coupon specimens cut from the steel tube to determine residual yield strength.
5. Core drilling and compressive testing of the concrete core to determine residual compressive strength.
6. Finite element analysis using the measured residual material properties to estimate the post-fire load-bearing capacity.

The results of this parametric study can be used to develop simplified design formulas for the post-fire load-bearing capacity of high-strength CFST columns, which would facilitate rapid post-fire assessment in emergency situations.

Study Insights and Conclusions

This research provides valuable insights into the post-fire structural behavior of high-strength CFST columns and highlights the importance of considering fire exposure in the design and assessment of structural systems. The parametric analysis demonstrates that the steel tube yield strength and fire exposure time are the most influential parameters on post-fire load-bearing capacity, with the concrete strength playing a secondary but still significant role. Engineers should incorporate fire resistance considerations into the design of CFST columns from the outset, selecting appropriate steel grades and concrete strengths that maintain adequate residual capacity after fire exposure. The findings also underscore the need for standardized post-fire assessment procedures and repair guidelines for CFST structures, which remain an area requiring further standardization and codification.