Ultimate Bearing Capacity of Centrifugal Steel Tube-Concrete Short Columns Under and After Fire

Literature Overview

The paper by Jin Weiliang and Yuan Weibin, published in the Journal of Zhejiang University (Engineering Science) in 2006, presents a study on the ultimate bearing capacity of centrifugal steel tube-confined concrete (CCFST) short columns under fire and after fire. The research addresses fire resistance design, damage assessment during fire, and post-fire structural evaluation and repair. The authors analyze the high-temperature and post-fire stress-strain relationships of steel and concrete, establish a finite element analysis model for four-sided uniform fire exposure, and propose simplified formulas for the ultimate bearing capacity under the ISO-384 standard temperature rise curve and after fire exposure.

Core Technical Framework

The research is structured around three key aspects: the constitutive behavior of materials at elevated temperatures, the thermal and structural response of CCFST columns under fire, and the development of simplified design formulas.

High-Temperature Material Behavior

The stress-strain relationships of steel and concrete at elevated temperatures are fundamental to the fire resistance analysis of CCFST columns. For structural steel, the yield strength and elastic modulus decrease progressively with temperature. The reduction factors for steel strength at elevated temperatures follow well-established empirical relationships, with significant strength loss occurring above 500 degrees Celsius.

For concrete, the behavior is more complex. The compressive strength decreases with temperature, but the rate of decrease depends on the concrete grade, aggregate type, and curing conditions. The confined concrete within the steel tube may exhibit different behavior compared to unconfined concrete, as the lateral restraint from the tube may mitigate some of the temperature-induced strength loss.

The centrifugal concrete used in CCFST columns has a denser microstructure compared to conventionally placed concrete, which may influence its high-temperature behavior. The denser packing of aggregates and reduced porosity could provide better resistance to thermal degradation, but this requires experimental validation.

Finite Element Analysis Model

The finite element model for the CCFST short column under four-sided uniform fire exposure couples thermal and structural analyses. The thermal analysis determines the temperature distribution within the column cross-section, accounting for the heat transfer through the steel tube, the concrete core, and the air gap between them if present.

The structural analysis uses the temperature-dependent material properties to calculate the load-deformation response. The model must account for:

• Thermal expansion of both steel and concrete
• Differential thermal expansion between steel and concrete
• The development of thermal stresses due to temperature gradients
• The progressive degradation of material properties with temperature
• The interaction between the steel tube and concrete core under thermal loading

The load-deformation curves obtained from the finite element analysis provide the complete response history of the column under fire. These curves show the initial elastic response, the progressive strength degradation, and the ultimate failure under sustained loading at elevated temperature.

Simplified Design Formulas

The simplified formulas proposed by the authors are based on the finite element analysis results and are intended for practical design use. The formulas express the ultimate bearing capacity as a function of the temperature rise time under the ISO-384 standard temperature curve.

The key finding is that the ultimate bearing capacity decreases dramatically with increasing temperature rise time, and the relationship between bearing capacity and temperature field distribution is essentially linear. This linearity simplifies the design calculation and allows for straightforward interpolation between discrete temperature points.

Post-Fire Structural Assessment

The post-fire analysis addresses the critical need for structural evaluation after a fire event. The authors consider the residual strength of the column after cooling, which is typically lower than the strength at ambient temperature due to irreversible material degradation.

The post-fire assessment involves determining the residual bearing capacity of the column based on the maximum temperature reached during the fire. This information is essential for deciding whether the column can remain in service, requires repair, or must be replaced.

The repair considerations include:

• The feasibility of restoring the concrete core if the steel tube remains intact
• The assessment of the steel tube's residual strength and any fire-induced deformation
• The evaluation of the bond between the steel tube and concrete core after thermal cycling
• The determination of the appropriate load reduction factor for continued service

Engineering Practice Integration

The research provides practical tools for the fire resistance design of CCFST structures. The simplified formulas allow engineers to estimate the fire resistance duration of CCFST columns without performing detailed finite element analysis for each design case. This is particularly valuable for routine design applications where computational efficiency is important.

For post-fire assessment, the research provides a framework for evaluating the structural integrity of CCFST columns after fire exposure. Engineers can use the proposed formulas to estimate the residual capacity based on the maximum temperature reached, which can be determined from fire investigation data or fire exposure calculations.

The following table summarizes the key design considerations for CCFST columns in fire:

Key Technical Insights

The finding that the bearing capacity decreases dramatically with temperature rise time highlights the vulnerability of CCFST columns to prolonged fire exposure. Even though the composite action between steel and concrete provides inherent fire resistance, the progressive degradation of material properties eventually leads to significant capacity loss.

The linear relationship between bearing capacity and temperature field distribution is a valuable simplification that enables practical design. This linearity suggests that the temperature distribution within the column cross-section is the primary governing factor for the fire resistance performance, which is consistent with the fundamental principles of heat transfer in composite members.

The centrifugal concrete used in CCFST columns may offer advantages over conventionally placed concrete in terms of fire resistance. The denser microstructure could provide better resistance to thermal degradation, but this requires further experimental investigation to quantify the improvement.

Study Insights and Implications

This research makes a significant contribution to the fire resistance design of CCFST structures. The development of simplified design formulas based on rigorous finite element analysis provides engineers with practical tools for fire resistance evaluation. The post-fire assessment framework addresses an important aspect of structural safety that is often overlooked in practice. For engineers involved in the design and assessment of CCFST structures, this work provides a comprehensive understanding of the fire behavior of these composite members and practical methods for evaluating their performance under and after fire exposure. The research also highlights the need for further experimental studies on the high-temperature behavior of centrifugal concrete and the long-term effects of fire exposure on the structural integrity of CCFST members.