Zhuojin Pipe Fitting Co., LtdFire Resistance Performance of Square Steel Tube Confined Recycled Concrete Columns Under Three-Sided Fire
Literature Overview
This paper by Zhang Yuzhuo and colleagues from Shenyang Jianzhu University investigates the fire resistance limit of square steel tube confined reinforced recycled concrete columns subjected to three-sided fire exposure under ISO 834 standard fire conditions. The research employs ABAQUS finite element analysis to establish temperature field and fire resistance limit models, examining the effects of recycled aggregate replacement ratio, load ratio, concrete strength, slenderness ratio, steel ratio, and load eccentricity on section temperature distribution and fire resistance duration.
Core Technical Findings
The study reveals several critical relationships governing fire performance:
| Parameter | Effect on Fire Resistance | Key Quantitative Result |
|---|---|---|
| Load ratio | Significant negative correlation | Higher load ratio reduces fire resistance limit substantially |
| Steel ratio (5.55% to 7.02%) | Positive improvement | 14.77% increase in fire resistance at load ratio 0.6 |
| Steel ratio (7.02% to 8.51%) | Diminishing returns | Only 3.09% additional improvement |
| Load eccentricity | Complex, direction-dependent | Below load ratio 0.5, bending direction under fire governs |
| Recycled aggregate replacement ratio | Affects thermal conductivity | Higher replacement increases thermal diffusivity |
Technical Analysis of Three-Sided Fire Exposure
Three-sided fire represents a realistic fire scenario where one face of the column remains shielded, typically by adjacent structural elements or fire protection. This creates asymmetric temperature gradients across the section, inducing additional bending moments due to differential thermal expansion. The steel tube confinement plays a dual role: it provides lateral restraint to the concrete core during heating, while simultaneously experiencing significant strength degradation at elevated temperatures.
The temperature-dependent material properties are critical to model accuracy. Steel strength reduction follows the Eurocode 3 formula where yield strength at temperature T is expressed as:
f_y(T)/f_y(20) = (1 - 0.0032(T - 600)) for T between 20°C and 600°C, and further degrades to 0.38 at 1000°C. Concrete loses compressive strength rapidly above 400°C, with the recycled aggregate potentially accelerating this degradation due to higher porosity and thermal conductivity of the recycled material.
Engineering Practice Implications
From a manufacturing and construction perspective, this research has direct relevance to steel pipe selection for composite columns in fire-prone environments:
- Steel tube wall thickness selection: The optimal steel ratio appears to be in the 5.5% to 7.0% range for typical load conditions. Beyond this threshold, additional steel provides diminishing fire resistance returns while increasing material cost and weight.
- Load ratio management: Designers should target load ratios below 0.6 to maximize fire resistance margins. This has implications for column sizing during the structural design phase.
- Recycled concrete considerations: The use of recycled aggregate introduces higher thermal conductivity, which accelerates heat penetration to the steel tube. Compensatory measures such as increased fire protection thickness or enhanced steel tube wall thickness may be required.
Key Reflections
The diminishing returns observed at higher steel ratios suggest an economic optimization point for steel tube specification. In engineering practice, specifying a square steel tube with wall thickness yielding approximately 6-7% steel ratio represents a balanced design choice for three-sided fire scenarios. The complex interaction between load eccentricity and fire-induced bending direction warrants careful consideration in seismic zones where columns may experience combined fire and residual seismic loading. The research underscores that recycled concrete, while environmentally beneficial, demands more conservative fire design approaches compared to conventional concrete, particularly regarding thermal protection specifications for the steel tube component.