Axial Compression Performance of Steel Tubular Recycled Concrete Column After Fire Exposure

Literature Overview

This paper by Liu Wenchao and colleagues from Beijing University of Technology, published in the Journal of Natural Disasters (Volume 26, Issue 5, 2017, pages 45-50), investigates the axial compression performance of steel tubular recycled concrete (STRC) columns after fire exposure. The study was supported by the National Science and Technology Program (Project 2014BAL05B04). Six fire-exposed STRC column specimens were tested to evaluate the influence of concrete material type and steel tube wall thickness on the failure mode, load-displacement relationship, and load-strain relationship.

Background and Motivation

Recycled concrete, produced from crushed waste concrete and other construction demolition debris, is an environmentally sustainable alternative to conventional natural aggregate concrete. The use of recycled concrete in steel tubular concrete columns combines the advantages of composite action between the steel tube and concrete with the environmental benefits of recycled materials. However, the fire resistance performance of recycled concrete is generally lower than that of natural aggregate concrete due to the higher porosity and weaker interfacial transition zone. This study addresses the critical question of how fire exposure affects the structural performance of STRC columns.

Specimen Configuration and Test Program

The experimental program consisted of six STRC column specimens subjected to fire exposure followed by axial compression testing. The specimens were designed to investigate the influence of two primary variables:

The steel tube wall thickness is directly related to the steel ratio (steel content per unit volume of the composite column), which is a critical parameter in STC column design.

Key Findings

The test results revealed several important findings regarding the post-fire performance of STRC columns:

1. Failure mode - All fire-exposed STRC columns exhibited buckling failure characterized by lateral bowing of the column to one side, with severe deformation concentrated at the upper portion of the steel tube.
2. Steel tube wall thickness effect - When the steel tube wall thickness was small, the concrete material type had a significant influence on the column load capacity. As the steel tube wall thickness increased, the influence of the concrete material type rapidly decreased.
3. Steel ratio dominance - The steel tube wall thickness (steel ratio) was identified as the primary factor governing the load capacity of STRC columns, regardless of fire exposure.
4. Load-displacement behavior - The post-fire columns exhibited reduced stiffness and lower peak loads compared to unexposed columns, with the degree of degradation depending on the steel tube wall thickness.
5. Load-strain relationship - The strain capacity of the steel tube was significantly affected by fire exposure, with the high-temperature exposure reducing the yield strength and ductility of the steel.

Technical Analysis

From a welding and steel pipe fabrication perspective, the findings have important implications for the design and construction of STRC columns in fire-prone applications:

• The steel tube serves as both a structural member and a fire protection layer for the concrete core, and its wall thickness is critical for maintaining post-fire structural integrity
• The reduction in concrete strength after fire exposure is partially compensated by the steel tube, but this compensation is only effective when the steel tube is sufficiently thick
• The buckling failure mode indicates that the post-fire columns are susceptible to flexural instability, which must be considered in the design of fire-exposed structural systems
• The severe deformation at the upper portion of the steel tube suggests that the fire exposure temperature distribution along the column height is non-uniform, with the upper portion experiencing higher temperatures

Engineering Practice Considerations

For engineers designing STRC columns in fire-exposed applications, the following considerations are essential:

• The steel tube wall thickness should be selected to ensure adequate post-fire load capacity, particularly when using recycled concrete
• The steel ratio should be maintained above a minimum threshold to ensure that the steel tube dominates the load-carrying behavior after fire exposure
• Fire protection measures, such as intumescent coatings or concrete encasement, should be considered for thin-walled steel tubes
• The welding quality of the steel tube splices is critical for maintaining the structural continuity of the column, especially after fire exposure when the steel properties are degraded
• The recycled concrete mix design should be optimized for fire resistance, considering factors such as aggregate type, water-cement ratio, and admixture selection

Study Insights and Reflections

This study provides valuable experimental data on the post-fire performance of STRC columns, a relatively under-researched area in structural engineering. The finding that the steel tube wall thickness is the dominant factor in post-fire capacity is particularly significant for design practice, as it suggests that increasing the steel ratio is an effective strategy for improving fire resistance. From my perspective as a steel pipe and welding specialist, the study highlights the dual role of the steel tube as both a structural member and a fire protection element. The welding connections in the steel tube must be designed to maintain structural integrity after fire exposure, which may require additional consideration of weld quality and heat-affected zone properties. The use of recycled concrete in structural applications is an important sustainability initiative, and this study contributes to the understanding of how to safely incorporate recycled materials into structural systems.