Zhuojin Pipe Fitting Co., LtdFire Resistance of Steel Tube Confined Recycled Concrete Columns Under Three-Sided Fire Exposure
Literature Overview
This study by Zhang Yuzhuo and colleagues (2021, Journal of Shenyang Jianzhu University) investigates the fire resistance limit of square steel tube confined reinforced recycled concrete columns subjected to three-sided ISO 834 standard fire exposure. The research employs ABAQUS finite element analysis to model the temperature field and fire resistance performance, examining the influence of recycled aggregate replacement rate, load ratio, concrete strength, slenderness ratio, steel ratio, and load eccentricity on sectional temperature distribution and ultimate fire resistance duration.
Core Technical Parameters and Findings
The study systematically varies multiple design parameters to quantify their effects on fire performance. The key quantitative findings are summarized below:
| Parameter | Variation Range | Effect on Fire Resistance |
|---|---|---|
| Load ratio | 0.2 – 0.8 | Significant negative correlation; higher load ratio reduces fire resistance |
| Steel ratio | 5.55% – 8.51% | Positive but diminishing returns; 14.77% improvement from 5.55% to 7.02%, only 3.09% from 7.02% to 8.51% |
| Concrete strength | C30 – C60 | Moderate influence; higher strength slightly reduces fire resistance due to lower thermal conductivity protection |
| Slenderness ratio | 8 – 20 | Higher slenderness reduces fire resistance through increased buckling susceptibility |
| Load eccentricity | 0 – 0.3h | Complex interaction; when load ratio < 0.5, effect depends on deflection direction induced by fire |
Technical Analysis from a Steel Tube and Welding Perspective
Steel Tube Selection and Welding Considerations
For square steel tube confined recycled concrete columns, the selection of steel tube grade and welding process is critical for maintaining structural integrity under fire exposure. The steel tube serves as both a formwork during construction and a passive fire protection layer. Key considerations include:
- Steel grade selection: Q345 or Q390 steel tubes are commonly specified per GB/T 1591. The yield strength reduction under ISO 834 fire follows the Eurocode 3 Part 1-2 (EN 1993-1-2) temperature-dependent curves, where fy(T) drops to approximately 60% at 600°C.
- Welding process: Longitudinal welds in the steel tube (typically SAW or HFW per GB/T 7115) must maintain adequate heat-affected zone (HAZ) toughness. The HAZ of low-carbon structural steel welds can experience temper embrittlement under prolonged elevated temperatures, potentially reducing fracture toughness by 20-30% at 400-600°C.
- Cross-weld quality: Transverse welds joining tube segments must comply with NB/T 47015 requirements for weld quality grade, as these locations are susceptible to premature failure under thermal gradients.
Recycled Aggregate and Fire Behavior
The use of recycled concrete aggregate (RCA) introduces additional complexity to fire performance. Recycled aggregate typically contains residual adhered mortar, which has lower thermal conductivity than natural aggregate but higher water content. During fire exposure:
- Free water in RCA evaporates at approximately 100°C, generating internal pore pressure.
- Bound water release occurs between 200-400°C, potentially causing explosive spalling in recycled concrete more readily than in natural concrete.
- The steel tube confinement mitigates spalling by maintaining lateral restraint, but the confinement effectiveness diminishes as the steel loses strength above 500°C.
Three-Sided Fire Exposure Challenge
Unlike uniform four-sided fire, three-sided exposure creates asymmetric thermal gradients that induce warping and additional bending moments. The steel tube on the unexposed side remains relatively cool while the exposed sides lose strength progressively. This differential heating generates:
- Thermal bowing: Lateral deflection toward the cooler side, compounding the applied eccentric moment.
- Differential expansion stresses: At weld joints and tube-to-end-plate connections, thermal expansion differences can reach 15-25 MPa, potentially exceeding the reduced yield strength of the HAZ at elevated temperatures.
Engineering Practice Implications
Design Recommendations
Based on the study findings and engineering experience, the following design guidelines are proposed:
| Design Parameter | Recommended Value | Rationale |
|---|---|---|
| Load ratio (N/Nu) | ≤ 0.6 | Balances structural efficiency with adequate fire resistance margin |
| Steel ratio | 6.0% – 7.5% | Optimal fire resistance improvement without excessive cost |
| Steel tube thickness | ≥ 6 mm | Adequate confinement and thermal mass; minimum per GB 50017 |
| RCA replacement rate | ≤ 40% | Limits spalling risk while maintaining sustainability benefits |
| Fire protection coating | Intumescent ≥ 2.5 mm | Extends steel tube temperature to below 500°C for 120 min |
Welding Quality Control Under Fire Design
For columns designed for fire resistance, welding quality control must incorporate fire-related considerations:
- Pre-weld heat treatment: For steel tubes thicker than 25 mm, pre-heating to 100-150°C is recommended to reduce HAZ cooling rates and minimize martensite formation. This also provides a baseline for evaluating fire-induced thermal cycling effects.
- Post-weld inspection: UT per GB/T 11345 Level II minimum, with particular attention to longitudinal welds where thermal cycling during fire could propagate existing defects.
- Residual stress management: Stress-relief annealing at 600°C for tubes thicker than 40 mm reduces residual stresses that could compound fire-induced thermal stresses.
Key Questions and Reflections
The study raises several important questions for further investigation:
- How does the welding process selection (SAW vs. HFW vs. ERW) affect the long-term fire performance of the steel tube confinement?
- What is the interaction between recycled aggregate porosity and fire-induced spalling pressure in confined conditions?
- Can advanced welding techniques such as friction stir welding (FSW) provide superior HAZ properties under thermal cycling?
From a practical standpoint, the diminishing returns observed in steel ratio enhancement (from 14.77% improvement to only 3.09% in the higher range) suggest that designers should focus on optimizing the load ratio and incorporating passive fire protection rather than simply increasing steel content. This aligns with the FMEA approach where the most effective failure mode mitigation often targets the root cause rather than adding redundant capacity.
Study Insights and Implications
This research contributes valuable quantitative data for the design of sustainable concrete-steel composite columns in fire-exposed environments. The finding that load ratio is the most influential parameter reinforces the importance of structural efficiency in fire design—over-designed columns that carry excessive load relative to their fire-resistant capacity will perform poorly. The steel tube confinement mechanism provides a natural fire protection layer, but its effectiveness is bounded by the steel's temperature-dependent strength loss. Engineers should integrate fire design considerations early in the structural design phase, selecting appropriate steel tube dimensions, welding specifications, and fire protection systems to ensure that the fire resistance limit meets or exceeds the required duration per GB 50016 and GB 51249. The use of recycled aggregate in confined columns is viable for fire-rated applications provided that the replacement rate is controlled and adequate fire protection measures are implemented at the steel tube level.