Fire Resistance Bearing Capacity of Axially Compressed Concrete Filled Steel Tube Members

Overview of the Literature

This paper by Li Guoqiang, He Junli, and Han Linhai (2001, Journal of Building Structure) addresses a critical engineering challenge: the calculation of fire resistance bearing capacity for axially compressed concrete-filled steel tube (CFST) members. The authors adopt the ECCS (European Convention for Constructional Steelwork) calculation model combined with an element analysis approach, leveraging computer programs to conduct extensive statistical analysis and derive regression formulas for the fire resistance bearing capacity of axially compressed CFST members. The results are compared with both domestic and international findings, showing good agreement.

Core Technical Content

The fire resistance of structural members is governed by the progressive degradation of material properties under elevated temperatures. For CFST members, two primary mechanisms govern the loss of load-bearing capacity: the weakening of the steel shell and the degradation of the concrete core. The ECCS model provides a well-established framework for evaluating these effects by defining temperature-dependent reduction factors for both materials.

Temperature-Dependent Material Degradation

The following table summarizes the typical temperature-dependent property reductions used in the ECCS framework:

The key insight is that concrete retains a relatively higher fraction of its compressive strength compared to steel at moderate temperatures (up to approximately 400°C), but degrades more rapidly at higher temperatures. Steel, while losing strength more gradually, also suffers from a significant reduction in elastic modulus, which affects the overall stability behavior of the member.

Regression Calculation Formulas

The authors derive regression formulas based on extensive parametric analysis. The fire resistance bearing capacity $N_{f,Rd}$ is expressed as a function of several key parameters:

• $A_s$: Cross-sectional area of the steel tube
• $A_c$: Cross-sectional area of the concrete core
• $f_{y,T}$: Yield strength of steel at temperature $T$
• $f_{c,T}$: Compressive strength of concrete at temperature $T$
• $D$: Outer diameter of the steel tube
• $t$: Wall thickness of the steel tube
• $\lambda$: Slenderness ratio of the member

The regression approach allows engineers to quickly estimate the fire resistance capacity without resorting to full finite element analysis, which is particularly valuable for preliminary design and code calibration.

Engineering Practice Implications

In practical fire protection design, CFST members offer inherent fire resistance advantages over unprotected steel members due to the thermal mass and passive cooling effect of the concrete core. However, several engineering considerations must be addressed:

1. Fire exposure duration: The design fire resistance rating (typically 60, 90, or 120 minutes) determines the maximum temperature profile, which in turn dictates the severity of material degradation.
2. Boundary conditions: The end conditions of the member significantly influence the slenderness ratio and thus the fire resistance capacity. Fixed-end members exhibit higher resistance than pinned-end members.
3. Imperfections: Initial geometric imperfections and residual stresses from cold forming of the steel tube become more critical at elevated temperatures due to the reduced elastic modulus.

Comparison with International Standards

The paper compares its regression formulas with results from ECCS and other international references. The comparison reveals that the regression formulas provide conservative estimates for most practical scenarios, which is desirable from a safety perspective. However, for members with very high slenderness ratios or very thin steel walls, the formulas may overestimate the capacity, suggesting the need for additional refinement in these regimes.

Key Questions and Reflections

A critical question arises regarding the applicability of these formulas to modern high-strength CFST members. The original ECCS model was calibrated primarily for structural steel grades up to S355, and the concrete strengths considered were typically below C60. With the increasing use of high-strength steel (S460, S690) and high-performance concrete (C80, C100), the temperature-dependent degradation factors may not be directly transferable.

Another important consideration is the interaction between fire and the surrounding structural system. In a real building, the fire resistance of a single CFST column is influenced by the continuity effects provided by connected beams and the constraint from adjacent structural elements. The isolated member approach used in this study, while conservative, may not capture these beneficial effects.

Study Insights and Implications

The work by Li Guoqiang et al. represents a valuable contribution to the fire engineering of CFST members. The regression formulas provide a practical tool for engineers to estimate fire resistance capacity during the preliminary design phase. The comparison with international results confirms the reliability of the approach. For engineering practice, I would recommend using these formulas as a first-pass estimation, followed by more detailed finite element analysis for critical members, particularly those with unconventional geometries or material combinations. The study also highlights the importance of considering material degradation at elevated temperatures in the design of composite structures, a consideration that should be integrated early in the design process rather than treated as an afterthought.