Ultimate Bearing Capacity of Centrifugally Cast Steel Tube Concrete Short Columns Under Fire and Post-Fire Conditions

Literature Overview and Research Context

The paper by Jin Weiliang and Yuan Weibin, published in the Journal of Zhejiang University (Engineering Science) in 2006, addresses a critical yet underexplored aspect of steel tube concrete (STC) structural engineering: the fire resistance and post-fire residual capacity of centrifugally cast steel tube concrete (CSTC) short columns. Centrifugal casting of concrete inside steel tubes offers superior density and uniformity compared to conventional pouring methods, making CSTC columns increasingly attractive for high-rise and infrastructure applications. However, fire scenarios remain a dominant design challenge, particularly for composite columns where the interaction between steel and concrete under elevated temperatures introduces complex thermal-mechanical coupling effects. This study establishes finite element models for CSTC columns subjected to uniform four-side fire loading following the ISO-834 standard temperature curve, and subsequently evaluates the residual load-bearing capacity after fire exposure. The research bridges a significant gap between fire engineering theory and practical structural assessment, offering simplified calculation formulas that engineers can apply during both design and post-incident evaluation phases.

Core Technical Content and Methodology

The authors begin by analyzing the stress-strain relationships of both steel and concrete at elevated temperatures and after cooling from fire exposure. For the steel component, the yield strength and elastic modulus degrade progressively with temperature, with critical thresholds typically occurring above 550°C where steel loses approximately 50% of its room-temperature strength. For the concrete component, the centrifugally cast matrix exhibits different thermal behavior compared to normal-weight concrete due to its denser microstructure and reduced porosity. The authors model the thermal degradation of concrete strength using temperature-dependent constitutive laws that account for microcracking, dehydration of calcium silicate hydrates, and spalling phenomena at temperatures exceeding 600°C.

The finite element model incorporates several key assumptions: the steel tube and concrete core are treated as perfectly bonded with no slip at the interface, the fire loading follows the ISO-834 standard curve (T = 345 × log₁₀(8t + 1) + 20°C, where t is time in minutes), and the column is subjected to uniform heating on all four sides. The model captures the full load-deformation response, including the initial elastic phase, the progressive degradation phase, and the ultimate failure phase. Post-fire analysis involves simulating the cooling process and evaluating the residual load capacity under continued axial compression.

Key Findings and Engineering Implications

The study reveals that the ultimate bearing capacity of CSTC short columns decreases dramatically with increasing fire exposure time. This reduction is not linear with time but rather follows a curve that steepens significantly beyond 60 minutes of fire exposure. The authors note that the bearing capacity and the temperature field distribution are essentially linearly related, which simplifies the formulation of simplified calculation expressions. The simplified formulas proposed in the paper incorporate the room-temperature ultimate bearing capacity expression as a baseline, with correction factors applied for thermal degradation of both steel and concrete components.

From a practical engineering standpoint, this research has profound implications for three distinct scenarios. First, during the design phase, engineers can use the simplified formulas to estimate the fire resistance rating of CSTC columns without resorting to full-scale fire testing. Second, during post-fire structural assessment, the residual capacity formulas allow for rapid evaluation of whether a column can remain in service or requires replacement. Third, for repair and rehabilitation purposes, the analysis provides a basis for determining the extent of damage and the feasibility of strengthening interventions.

The centrifugal casting process itself introduces additional considerations for fire performance. The denser concrete core provides better fire resistance compared to conventionally cast STC columns because the reduced porosity limits the penetration of water vapor pressure buildup, which is a primary mechanism for concrete spalling. However, the steel tube in a CSTC column is exposed to direct fire attack on the exterior surface, leading to rapid temperature escalation that can compromise the steel's load-bearing capacity before the concrete core reaches critical temperatures. This differential heating creates complex stress redistribution patterns within the composite section.

Integration with Standards and Practice

The research aligns with several international standards governing fire design of steel and composite structures. Eurocode 4 (EN 1994-1-2) provides the general framework for fire design of composite steel-concrete structures, while GB 50017 in China covers the design of steel structures including fire resistance provisions. The simplified formulas proposed by the authors can be viewed as complementary to these standards, offering a more specific tool for CSTC columns that may not be fully covered by existing code provisions.

In practice, the findings underscore the importance of fire protection measures for steel tube components in CSTC columns. Intumescent coatings, concrete encasement, or fire-resistant spray applications on the exterior of the steel tube can significantly extend the fire resistance rating. The study also highlights the need for post-fire inspection protocols that include visual examination for concrete spalling, steel tube distortion, and overall column alignment, followed by non-destructive testing to assess residual structural integrity.

Reflections and Study Insights

This paper exemplifies the rigorous analytical approach necessary for fire engineering of composite structures. The integration of material-level constitutive modeling with structural-level finite element analysis provides a comprehensive framework that can be adapted to other composite column configurations. The simplified formulas derived from the detailed analysis represent a practical contribution that reduces computational burden while maintaining acceptable accuracy for engineering applications.

One notable limitation is that the study assumes uniform four-side heating, which represents a conservative simplification. Real fire scenarios may involve compartment fires with non-uniform temperature distributions, direct flame impingement on specific faces, or water spray cooling effects. Future research could extend the analysis to consider these more realistic fire scenarios, as well as the effects of axial load eccentricity and lateral restraint conditions on fire performance.

The work also raises important questions about the long-term durability of CSTC columns after fire exposure. Even if the residual capacity is sufficient for continued service, the thermal cycling during fire and subsequent cooling may introduce microcracks in the concrete and residual stresses in the steel tube that could affect long-term behavior under sustained loading, fatigue, or seismic events. Engineers should therefore consider both immediate structural safety and long-term performance degradation when making post-fire assessment decisions.