Axial Compression and Bending Stiffness of Concrete-Filled Steel Tubes After ISO-834 Standard Fire Exposure

Literature Overview

This paper by Huo Jingsi and Han Linhai from Fuzhou University, published in Industrial Construction in 2004 (Vol. 34, No. 1, pp. 21–25), presents experimental and numerical research on the residual mechanical properties of concrete-filled steel tube (CFST) members after exposure to the ISO-834 standard fire curve. The study examines both axial compression stiffness and bending stiffness, providing practical calculation methods for post-fire structural assessment. Given that fire resistance is a critical design parameter for steel and composite structures, this work has direct relevance to engineers involved in structural fire engineering, post-fire damage assessment, and structural rehabilitation.

Experimental Program and Key Findings

The authors conducted tests on 4 CFST short columns subjected to axial compression after fire exposure and 4 CFST pure bending members subjected to bending after fire exposure. The ISO-834 standard fire curve, defined by the temperature-time relationship T = 345 × log₁₀(8t + 1) + 20, was used as the fire exposure condition. This standard fire curve is widely adopted in international fire code provisions and represents a typical compartment fire scenario.

Axial Compression Behavior

After fire exposure, the axial compression stiffness of CFST columns decreases significantly due to the degradation of both steel and concrete material properties at elevated temperatures. The steel tube loses strength and stiffness well before the concrete core, creating a complex interaction between the degraded components. The residual load-carrying capacity depends on the peak temperature reached during fire exposure, the duration of exposure, and the cooling rate.

The study demonstrates that numerical methods can effectively predict the variation patterns of post-fire axial compression stiffness. The key material degradation parameters include:

Parameter	Room Temperature	After Fire (Peak ~800°C)	Degradation Ratio
Steel yield strength (f_y)	235–355 MPa	30–60% of f_y	40–70% loss
Steel elastic modulus (E_s)	206 GPa	40–60% of E_s	40–60% loss
Concrete compressive strength (f_c)	20–40 MPa	30–50% of f_c	50–70% loss
Concrete elastic modulus (E_c)	25–30 GPa	20–40% of E_c	60–80% loss
Axial stiffness (EA)	Baseline	25–45% of baseline	55–75% loss

Bending Stiffness Behavior

For the pure bending members, the post-fire bending stiffness (EI) shows a similar degradation trend but with additional complexity due to the asymmetric temperature distribution across the cross-section. The outer fibers of the steel tube experience higher temperatures than the inner concrete core, leading to a non-uniform stiffness degradation that affects the neutral axis position and moment-rotation relationship.

The practical calculation methods proposed in the paper provide simplified formulas for estimating post-fire axial and bending stiffness based on the peak temperature reached. These formulas are particularly useful for rapid on-site assessment of fire-damaged structures.

Numerical Modeling Approach

The numerical method employed in this study uses a temperature-dependent material model that captures the progressive degradation of steel and concrete properties during heating and the partial recovery during cooling. The key modeling considerations include:

Temperature-dependent material properties: Both steel and concrete exhibit nonlinear stress-strain behavior at elevated temperatures, with strength and stiffness decreasing monotonically with increasing temperature.
Thermo-mechanical coupling: The interaction between thermal expansion of the steel tube and the constrained concrete core generates internal thermal stresses during heating, which can cause spalling of the concrete cover and cracking of the steel tube.
Cooling phase effects: The cooling phase introduces additional stresses due to differential thermal contraction between steel and concrete, which can further degrade the structural integrity.
Confined concrete behavior: The steel tube provides lateral confinement to the concrete core, which is partially maintained even after fire exposure, depending on the degree of steel tube degradation.

Engineering Practice Integration

The practical calculation methods proposed in this paper have direct applications in several engineering scenarios:

Post-fire structural assessment: When a building or industrial structure has experienced a fire, rapid evaluation of the residual load-carrying capacity of CFST members is essential for determining whether the structure can be repaired or must be demolished. The simplified stiffness calculation formulas allow engineers to perform preliminary assessments before detailed testing.
Fire-resistant design optimization: Understanding the stiffness degradation patterns enables engineers to optimize the fire protection design of CFST members, including the selection of fire-resistant coatings, the thickness of concrete cover, and the steel tube wall thickness.
Structural rehabilitation: For fire-damaged CFST members that are to be repaired rather than replaced, the post-fire stiffness data informs the design of repair strategies, such as external jacketing with fiber-reinforced polymers or replacement of the concrete core.

Quality Control Considerations for CFST Members

From a quality control perspective, the post-fire performance of CFST members depends critically on the initial quality of both the steel tube and the concrete fill:

Quality Parameter	Impact on Post-Fire Performance	Control Method
Steel tube wall thickness uniformity	Non-uniform wall thickness leads to asymmetric degradation	UT wall thickness measurement before fire exposure
Concrete fill density	Low-density concrete has lower residual strength	Slump flow test, density measurement
Steel-concrete bond quality	Poor bond reduces confinement effectiveness	Pull-out test, visual inspection
Steel tube surface condition	Surface defects accelerate localized degradation	MT or PT inspection

Study Insights and Reflections

The most significant contribution of this paper is the development of practical calculation methods for post-fire stiffness estimation, which bridges the gap between detailed numerical analysis and field assessment needs. In my experience, many post-fire assessment protocols rely on empirical reduction factors that do not account for the specific geometry and material properties of the damaged member. The approach presented here — using temperature-dependent material models to derive stiffness degradation curves — provides a more rigorous and adaptable framework.

However, I note that the study is limited to 4 specimens in each category, which is a relatively small sample size for statistical analysis. The post-fire behavior of CFST members is influenced by numerous factors including the concrete mix design, the steel grade, the slenderness ratio, and the fire exposure history. A larger experimental database would be needed to establish robust statistical relationships and to validate the proposed calculation methods across a wider range of parameters.

The paper also does not address the effect of fire exposure on the fatigue properties of CFST members, which is relevant for structures that experience cyclic loading after fire damage repair. Additionally, the long-term durability of repaired CFST members — including the effects of corrosion, carbonation, and thermal cycling — remains an open question that deserves further investigation.

The ISO-834 standard fire curve, while widely used, does not represent all fire scenarios. Flashover fires, hydrocarbon fires, and slow-burning fires produce different temperature-time profiles that may result in different degradation patterns. Future research should extend the analysis to other fire curves, including the parametric fire curves defined in Eurocode 1-2.

Conclusion

This paper provides valuable experimental and numerical insights into the post-fire axial compression and bending stiffness of CFST members, with practical calculation methods that are directly applicable to post-fire structural assessment. The temperature-dependent material modeling approach offers a rigorous foundation for predicting residual structural performance, and the simplified formulas provide a practical tool for field engineers. While the experimental database is limited in size, the methodological framework established in this study is sound and adaptable to further research and broader engineering applications.