Post-Fire Lateral Impact Performance of Concrete-Filled Steel Tube Members

Literature Overview

This study by Ji Sunhang, Shi Yanli, and Wang Wenda from Lanzhou University of Technology, published in Vibration and Shock (2021, Vol. 40, No. 4, pp. 179-187), investigates the lateral impact resistance of concrete-filled steel tube (CFST) members after exposure to fire. The research is supported by the National Natural Science Foundation of China (Grant No. 51778274) and related provincial funding. The authors established a validated finite element model to simulate the post-fire impact behavior of CFST members under lateral drop-weight loading, systematically analyzing how fire exposure duration affects deflection, impact force, and sectional bending moment.

Core Technical Content and Methodology

The study adopts a numerical simulation approach using validated finite element analysis (FEA) to model CFST members subjected to varying fire exposure durations followed by lateral impact loading. The key variables examined include:

Fire exposure time (varying duration)
Mid-span deflection under impact
Impact force time-history curves
Sectional bending moment distribution
Energy absorption coefficient
Post-fire dynamic bending moment enhancement factor

The FEA model was verified against experimental data to ensure accuracy before parametric studies were conducted. The drop-weight impact simulation captures inertial effects, particularly during the peak force stage, where the distribution morphology of bending moment and shear force changes significantly compared to quasi-static loading conditions.

Key Technical Findings

Effect of Fire Exposure Duration on Impact Response

Parameter	Trend with Increasing Fire Time	Engineering Implication
Mid-span deflection	Significant increase	Loss of structural stiffness
Impact duration	Lengthens	Reduced dynamic stiffness
Ratio of steel tube to concrete dynamic ultimate moment	Increases	Steel tube bears proportionally more load
Impact force plateau value	Decreases	Reduced structural resistance
Sectional dynamic ultimate bending moment	Decreases	Degraded flexural capacity
Energy absorption coefficient	Decreases	Reduced impact tolerance
Post-fire dynamic moment enhancement factor	Decreases	Loss of strain-rate hardening benefit

Failure Mode and Energy Dissipation

The post-fire failure mode is characterized as overall bending deformation, with the member dissipating the kinetic energy of the drop hammer primarily through global deformation rather than local crushing or buckling. This is significant from a pipe/steel tube engineering perspective because it indicates that the steel tube maintains its geometric integrity even after fire exposure, but with reduced load-bearing capacity.

Inertial Force Effects

The study highlights that inertial forces significantly influence the distribution of bending moment and shear force, particularly during the peak stage of impact. During this phase, the distribution morphology changes markedly, which has implications for understanding how steel tube members behave under combined thermal and dynamic loading scenarios such as fire followed by structural collapse or impact events.

Engineering Practice Implications

From a Steel Tube and Pipe Engineering Perspective

For engineers involved in steel tube fabrication and structural applications, several critical observations emerge:

Material degradation under fire: The steel tube material properties (yield strength, elastic modulus) degrade with temperature, directly affecting the post-fire impact capacity. This is analogous to the heat-affected zone (HAZ) degradation observed in welding processes, where microstructural changes reduce mechanical properties locally.
Composite action degradation: The confining effect between steel tube and concrete, which is fundamental to CFST design philosophy, is weakened after fire exposure. The ratio of dynamic ultimate moments carried by the steel tube versus concrete increases, suggesting that the concrete's contribution diminishes more rapidly than the steel tube's contribution under thermal degradation.
Residual stress interaction: Fire exposure introduces residual thermal stresses in the steel tube wall, which interact with impact-induced dynamic stresses. From a welding quality control perspective, this is comparable to how residual stresses from welding processes affect the fatigue and fracture behavior of welded joints.

Design Recommendations

For critical infrastructure (bridges, industrial facilities) where fire followed by impact scenarios are possible (e.g., vehicle impact on bridge piers after a fire event), the post-fire impact capacity should be explicitly evaluated rather than relying solely on post-fire static strength.
The proposed maximum deflection calculation formula provides a practical tool for preliminary assessment, with good agreement to simulation results.
Engineers should consider that the dynamic strengthening effect (strain-rate sensitivity) is reduced after fire exposure, meaning the member cannot rely on the same level of dynamic enhancement that exists in unheated conditions.

Key Questions and Reflections

The study raises important questions about the interaction between thermal and dynamic damage mechanisms in steel tube structures:

How does the microstructural evolution of the steel tube material (similar to grain coarsening observed in HAZ regions) correlate with the observed reductions in dynamic moment capacity?
What are the implications for steel tube specifications (e.g., ASTM A500, EN 10216-1) when considering combined fire-impact scenarios?
Can protective coatings or material upgrades to the steel tube (such as high-strength low-alloy grades) mitigate the post-fire impact capacity loss?

Study Insights and Reference Value

This research provides valuable quantitative data for engineers designing CFST structures in scenarios where fire followed by impact is a credible hazard. The energy absorption coefficient and post-fire dynamic moment enhancement factor serve as useful metrics for comparing different steel tube configurations under post-fire impact conditions. The finding that the member primarily dissipates energy through overall bending deformation rather than local failure is encouraging from a ductility standpoint, suggesting that even degraded steel tube members retain some post-fire impact tolerance.

The work is particularly relevant for bridge engineering and industrial facility design, where steel tube piers and columns may be exposed to fire events and subsequently subjected to impact loads. The proposed deflection formula offers a practical design tool that bridges the gap between detailed numerical simulation and preliminary design calculations. Future work should extend these findings to include experimental validation under combined thermal-dynamic loading conditions and consider the effects of different steel grades and tube geometries on post-fire impact performance.