Hysteretic Performance of Hollow Sandwich Steel Tube Concrete Columns After High Temperature Exposure

Literature Overview

This 2016 study published in the Journal of Railways and Road Engineering by Liu Xiao, Bao Juntao, and Wang Bing from Shenyang University investigates the post-fire hysteretic behavior of hollow sandwich steel tube concrete (HSSC) columns subjected to combined axial compression and bending. Supported by the National Natural Science Foundation of China (Grant 51308347), the research employed a rational constitutive model to establish finite element models and analyzed the effects of axial compression ratio, fire temperature, and inner steel tube yield strength on post-fire seismic performance. The findings provide critical data for the seismic assessment of steel tube concrete structures exposed to fire events.

Core Technical Findings

The study examined HSSC columns that experienced fire temperatures up to 900°C and then were subjected to cyclic lateral loading to evaluate their residual seismic capacity. The finite element models were validated against experimental results with good agreement. Key findings include systematic degradation of energy dissipation capacity, displacement ductility, lateral load-bearing capacity, and stiffness with increasing fire exposure temperature and axial compression ratio.

Technical Interpretation of Post-Fire Deterioration Mechanisms

The degradation of structural performance after fire exposure in HSSC columns is governed by multiple interacting mechanisms:

Material property degradation: Steel loses strength progressively with increasing temperature. At 600°C, structural steel retains approximately 50% of its room-temperature yield strength. Concrete experiences progressive strength loss due to dehydration of cement paste, cracking of aggregates, and spalling. At 900°C, concrete strength may be reduced to 20-30% of its original value.

Confinement effect reduction: The hollow sandwich configuration provides confinement through both inner and outer steel tubes. After fire exposure, the reduced steel yield strength diminishes the confinement pressure available to restrain concrete dilation, leading to accelerated concrete degradation and loss of composite action.

Stiffness degradation pattern: The study observed that stiffness degrades rapidly in the early loading cycles and then stabilizes. This pattern is characteristic of progressive damage accumulation in composite structures, where initial cycles cause additional cracking and interface debonding, while subsequent cycles encounter a more stable but degraded structural configuration.

Load-bearing capacity at 900°C: The more than 50% reduction in lateral load capacity after 900°C exposure reflects the combined loss of steel strength, concrete strength, and composite interaction efficiency. This represents a severe degradation that would necessitate structural replacement or major rehabilitation for most building applications.

Parametric Analysis and Design Implications

The finding that inner steel tube yield strength has no significant influence on post-fire performance is particularly noteworthy from a design perspective. This suggests that the post-fire behavior is governed more by the overall geometric configuration, fire exposure history, and axial load level than by the specific grade of the inner steel tube. Designers should therefore prioritize:

1. Fire protection strategies — External fire-resistant coatings or encasements that limit peak temperature exposure to below 600°C can substantially preserve post-fire seismic capacity.
2. Axial compression ratio control — Limiting the axial compression ratio to lower values (below 0.3-0.4) provides greater post-fire ductility margin and energy dissipation capacity.
3. Section configuration optimization — The hollow sandwich geometry offers advantages in fire resistance compared to conventional filled STC due to the thermal insulation provided by the hollow space, but this benefit diminishes at extreme temperatures.
4. Residual strength assessment — Post-fire structural assessment should incorporate temperature-dependent constitutive models for both steel and concrete to accurately predict residual capacity.

Key Reflections and Engineering Practice

This research addresses a critical gap in structural engineering practice: the seismic performance of steel tube concrete members after fire exposure. In real scenarios, buildings may experience fire events followed by seismic activity, or require post-fire seismic evaluation for occupancy decisions. The systematic parametric study approach, combining validated FE models with clear quantitative results, provides actionable guidance for engineers involved in post-fire structural assessment. The finding that 900°C exposure causes more than 50% capacity loss underscores the importance of fire protection design for steel tube concrete structures in seismic regions. Engineers should integrate fire resistance design and seismic design considerations from the outset, recognizing that these two hazard scenarios are not independent and may interact in ways that compound structural vulnerability. The rational constitutive model development for post-fire conditions represents a methodological advancement that can be extended to other composite structural systems requiring multi-hazard performance evaluation.