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

Literature Overview

This 2001 paper by Li Guoqiang, He Junli, and Han Linhai from Tongji University and Harbin University of Architecture and Engineering addresses a critical but historically underdeveloped area in structural engineering: the fire resistance design of concrete-filled steel tube (CFST) axially compressed members. The authors adopt the ECCS (European Committee for Constructional Steelwork) calculation model combined with a unit analysis computational pattern, and through extensive statistical analysis and computer programming, derive regression formulas for predicting the fire resistance bearing capacity of CFST columns. The results are benchmarked against both domestic and international data, demonstrating good agreement.

Core Technical Approach

The fundamental challenge in fire resistance design of CFST members lies in the coupled degradation of steel and concrete properties at elevated temperatures. As temperature rises, the yield strength of structural steel drops significantly—typically to approximately 60% of its ambient value at 600°C—while the compressive strength of concrete begins to degrade above 400°C, with progressive loss of aggregate bonding and internal cracking. The authors' approach follows the ECCS methodology, which treats the steel tube and concrete core as interacting components whose combined resistance is calculated at each time-temperature step.

The unit analysis computational pattern divides the member into discrete elements along its length, allowing the incorporation of second-order effects (P-Δ) that become increasingly significant as material properties degrade during a fire exposure. This is particularly important for CFST members, which often have relatively high slenderness ratios and where the loss of stiffness in the steel tube at elevated temperatures can trigger premature buckling failure.

Key Technical Parameters and Regression Formulas

The paper's primary contribution is the set of regression formulas obtained from large-scale statistical analysis. While the specific coefficients are detailed in the original publication, the general form follows the established pattern:

The regression formulas express the residual bearing capacity as a function of the above parameters and the fire exposure time, incorporating the temperature-dependent reduction factors for both steel and concrete.

Standards Comparison and Context

The ECCS model referenced in this paper is part of the European fire design framework that was subsequently incorporated into Eurocode 4 (EN 1994-1-2) for composite structures. At the time of publication in 2001, China's GB 50017 and GB 50010 did not yet contain specific provisions for the fire design of CFST members, making this work particularly significant for Chinese engineering practice. The comparison with international results validated the applicability of the ECCS-based approach to Chinese material grades and construction practices.

The paper also implicitly addresses the limitation of the earlier Chinese code approach, which treated CFST members under fire conditions using simplified methods that did not adequately account for the interaction between the steel tube and concrete core at elevated temperatures.

Engineering Practice Implications

In practical engineering applications, the regression formulas derived in this paper enable structural engineers to perform fire resistance verification of CFST columns without resorting to full numerical simulation for every design case. This is particularly valuable for large-scale projects such as high-rise buildings, industrial plants, and bridge piers where CFST columns are commonly employed for their superior strength-to-weight ratio and ductility.

However, several practical considerations merit attention. First, the regression formulas assume a standard fire temperature-time curve (ISO 834 or equivalent), and deviations from this standard exposure—such as those encountered in hydrocarbon fires or pool fires in industrial settings—require separate analysis. Second, the formulas are calibrated for axially compressed members; members subjected to combined axial load and bending, which are common in frame structures, require additional consideration of the biaxial or uniaxial bending interaction.

From a materials and fabrication perspective, the fire performance of CFST members is also influenced by the quality of the concrete filling process. Incomplete concrete filling, voids, or weak interfaces between the steel tube and concrete can significantly reduce the actual fire resistance below the predicted values. Engineers should ensure that the concrete placement process achieves full compaction and that the steel tube interior surface is clean and free of rust or mill scale prior to pouring.

Key Questions and Reflections

A notable limitation of the 2001 paper is that it does not address the effects of fire on the weld joints or bolted connections in CFST frames. In practice, the connection details often govern the overall fire performance of the structural system, and the loss of connection stiffness at elevated temperatures can lead to mechanism formation even when the individual members retain adequate strength.

Furthermore, the paper predates the widespread adoption of high-performance concrete (HPC) and ultra-high-performance concrete (UHPC) in CFST applications. The behavior of these advanced concretes under fire exposure differs from ordinary concrete, particularly regarding the risk of explosive spalling due to steam pressure buildup in low-permeability concrete matrices. Modern fire design of CFST members should incorporate provisions for spalling protection, such as the use of polypropylene fibers or appropriate fire-resistant coatings.

Study Insights and Implications

The fundamental contribution of this paper lies in establishing a systematic, code-compatible methodology for fire design of CFST columns in the Chinese context. The regression formulas provide a practical tool for engineers, bridging the gap between fundamental research and design practice. The approach of combining the ECCS framework with statistical regression based on extensive computational data represents a sound engineering methodology that balances accuracy with practicality.

For contemporary practice, the formulas should be viewed as a baseline tool that can be supplemented with more refined numerical methods when the structural importance or fire risk warrants. The paper's emphasis on the interaction between steel and concrete degradation mechanisms under fire exposure remains highly relevant, and its conclusions continue to inform modern fire design codes and standards for composite structures.