Mechanical Properties of Steel Tube Concrete Columns under Combined Fire and Impact Loading

Literature Overview

The paper by Hu Wenwei, Wang Rui, Zhao Hui, and Zhang Li from Taiyuan University of Technology, published in 2022 in the journal Explosion and Shock Waves, Volume 42, Issue 2, pages 40-50, investigates the mechanical properties of steel tube concrete (SRC) columns under the combined action of fire and impact loading. The study was supported by the China Postdoctoral Science Foundation (2020M670656) and the Shanxi Provincial Overseas Returnees Science and Technology Activity Priority Funding Project (20210010). Using ABAQUS finite element analysis, the authors established a lateral impact model of SRC columns considering the effect of axial force at high temperatures, analyzed the failure modes and loading process, and investigated the influence of fire duration, material strength, steel ratio, and impact energy on the impact resistance of SRC columns.

Core Technical Content

The combined action of fire and impact on structural members is a critical concern for the safety of buildings and infrastructure, particularly for columns that may be exposed to fire followed by impact loading from falling debris or vehicle collisions. The study addresses this concern through a systematic finite element investigation of SRC columns under combined fire and impact loading.

The finite element model was developed in ABAQUS and included the following key components:

• Steel Tube: Modeled with a temperature-dependent material model that accounts for the degradation of steel strength and stiffness at elevated temperatures. The elastic modulus of steel decreases significantly above 400°C, and the yield strength decreases above 550°C.
• Concrete Core: Modeled with a temperature-dependent constitutive model that accounts for the loss of concrete strength, stiffness, and ductility at elevated temperatures. Concrete loses approximately 50% of its strength at 400°C and becomes essentially ineffective above 600°C.
• Steel-Concrete Interface: Modeled with a contact interaction that accounts for the bond, friction, and possible separation between the steel tube and concrete core at elevated temperatures.
• Axial Load: Applied as a constant axial force to simulate the effect of gravity loads on the column during impact.
• Impact Loading: Applied as a lateral impact using a rigid or deformable impactor with a specified impact velocity or energy.

The study identified several key findings regarding the behavior of SRC columns under combined fire and impact loading:

1. Failure Mode: The primary failure mode under combined fire and impact is bending failure, with the steel tube yielding and the concrete core crushing on the compression side.
2. Fire Duration Effect: After 15 minutes of fire exposure, the impact resistance of the column decreases significantly due to the degradation of both steel and concrete properties.
3. Axial Load Effect: The axial load ratio from 0 to 0.2 causes a 7.8% reduction in impact resistance after 60 minutes of fire exposure, indicating that axial compression has a detrimental effect on the lateral impact capacity.
4. Concrete Strength Effect: Increasing the concrete strength from 30 MPa to 50 MPa improves the impact resistance by approximately 85% after 90 minutes of fire exposure, demonstrating that higher concrete strength provides greater residual capacity at elevated temperatures.
5. Steel Tube Strength and Steel Ratio Effect: The strength of the outer steel tube and the steel ratio have relatively minor effects on the impact resistance at elevated temperatures.

Technical Analysis of Fire-Impact Interaction

The interaction between fire and impact loading creates a complex loading scenario that is significantly different from either fire or impact loading alone. The key aspects of this interaction include:

• Temperature-Dependent Material Degradation: Both steel and concrete lose strength and stiffness at elevated temperatures, reducing the structural capacity available to resist impact loading. The degradation is not linear and accelerates at higher temperatures.
• Thermal Expansion and Confinement: The differential thermal expansion of steel and concrete creates internal stresses that can affect the structural behavior. The steel tube confines the concrete, but at elevated temperatures, the confinement effect is reduced due to the loss of concrete strength.
• Axial Load Interaction: The axial load creates a biaxial stress state in the column cross-section, which affects the bending capacity. At elevated temperatures, the reduced material strength means that the same axial load represents a higher fraction of the remaining capacity, further reducing the lateral load resistance.
• Impact Duration and Rate Effects: The impact loading is typically applied over a short duration, and the rate of loading affects the material response. At elevated temperatures, the material may exhibit more ductile behavior, but the reduced strength means that the deformation capacity is also affected.

The finite element model employed in this study captures these interactions through appropriate material models and contact definitions. The temperature-dependent material models for steel and concrete are based on established research and standards, including EN 1993-1-2 for steel and EN 1994-1-2 for composite structures. The contact model accounts for the possible separation and friction between the steel tube and concrete core at elevated temperatures.

Engineering Practice Integration

The findings of this study have significant implications for the design and protection of SRC columns in fire-prone environments:

• Fire Protection Design: The results indicate that even short fire exposure times (15 minutes) significantly reduce the impact resistance of SRC columns. This suggests that fire protection measures, such as intumescent coatings, fire-resistant boards, or water spray systems, should be designed to provide adequate protection during the critical early phase of a fire.
• Concrete Strength Selection: The significant benefit of higher concrete strength on residual impact capacity suggests that higher-strength concrete should be specified for SRC columns in areas where combined fire and impact exposure is possible. However, the increased cost of higher-strength concrete must be balanced against the safety benefits.
• Axial Load Control: The detrimental effect of axial load on impact resistance suggests that the axial load ratio should be kept as low as possible in SRC columns that may be exposed to combined fire and impact loading. This may require structural design optimization to redistribute axial loads to other structural elements.
• Steel Tube Selection: The minor effect of steel tube strength and steel ratio on impact resistance at elevated temperatures suggests that the steel tube design should be optimized for other criteria, such as fire resistance, corrosion protection, and construction practicality, rather than for impact resistance alone.

From a fabrication and welding perspective, the study highlights the importance of ensuring high-quality welds in SRC columns that may be exposed to fire. Welds are potential weak points in the steel tube, and their quality directly affects the structural integrity at elevated temperatures. Welding procedures should be qualified for the specific steel grade and wall thickness, and non-destructive testing should be performed to verify weld quality.

Key Questions and Reflections

A critical question arising from this study is the applicability of the findings to different SRC column configurations. The study focuses on specific column dimensions and loading conditions, and the results may not be directly applicable to columns with significantly different aspect ratios, cross-sectional shapes, or loading configurations. Further research is needed to develop generalized design equations that account for a wider range of parameters.

Another important consideration is the effect of fire protection systems on the combined fire-impact behavior. The study does not explicitly consider fire protection systems, such as intumescent coatings or fire-resistant boards, which are commonly used in practice to protect steel structures from fire. The effectiveness of these protection systems in maintaining structural capacity under combined fire and impact loading requires further investigation.

The study also does not address the effect of repeated or cyclic impact loading, which may be relevant in certain scenarios, such as repeated vehicle collisions or seismic loading. The cumulative damage from multiple impacts, particularly at elevated temperatures, could significantly affect the structural integrity of SRC columns.

Study Insights and Implications

This study provides valuable insights into the behavior of SRC columns under combined fire and impact loading, which is a critical but under-researched topic in structural engineering. The findings demonstrate that fire exposure significantly reduces the impact resistance of SRC columns, and that the concrete strength is the most influential parameter in determining the residual capacity.

For pipe manufacturing and structural fabrication engineers, the implications are significant. The quality of the steel tube fabrication and welding directly affects the structural performance at elevated temperatures. High-quality welds with proper fusion and minimal defects are essential for ensuring the structural integrity of SRC columns under fire conditions. The study also highlights the importance of concrete quality control, as the concrete strength has a significant effect on the residual impact capacity.

The study contributes to the growing body of knowledge on the fire and impact resistance of composite structures and provides a basis for the development of design guidelines for SRC columns in fire-prone environments. The finite element methodology employed in the study can be adapted for specific project requirements, providing a practical tool for engineers to evaluate the fire and impact performance of SRC columns in different loading scenarios.