Zhuojin Pipe Fitting Co., LtdBearing Capacity Calculation for Square Steel Tube Concrete Columns Under Biaxial Compression-Bending After High Temperature Exposure
Literature Overview
This paper by Jiang Shaofei, Yu Qinghai, and Li Yuewu, published in the Journal of Shenyang Jianzhu University (Natural Science Edition) in 2009, presents a simplified calculation method for evaluating the residual bearing capacity of square steel tube concrete (STC) columns subjected to biaxial eccentric compression after exposure to high temperatures. The research is based on experimental investigation of 16 specimens, numerical simulation verification, and regression analysis. Funded by the Ministry of Education Key Project (208064), Shenyang Science and Technology Bureau (1022040-1-04), and Fujian Provincial Youth Science and Technology Talent Program (2007F3054), the work addresses a critical need in post-disaster structural assessment.
Problem Statement and Methodology
Post-fire assessment of steel tube concrete structures presents unique challenges due to the complex interaction between steel and concrete at elevated temperatures. The steel tube provides confining pressure to the concrete core, but this confining effect diminishes as both materials degrade at high temperatures. Under biaxial eccentric loading — a common condition in multi-story buildings — the residual capacity is further complicated by asymmetric stress distributions.
The research methodology follows a systematic approach:
- Experimental program: 16 square STC column specimens subjected to controlled high-temperature exposure followed by biaxial eccentric compression testing.
- Numerical verification: Finite element analysis validated against experimental results to ensure simulation accuracy.
- Parametric study: Identification of key factors influencing the bearing capacity damage coefficient kr.
- Regression analysis: Development of a simplified kr expression correlating damage with exposure conditions.
- Method integration: Combination of kr with ambient-temperature calculation formulas for residual capacity prediction.
Key Parameters and Damage Coefficient
| Parameter | Range/Type | Effect on kr |
|---|---|---|
| Exposure temperature | 300–1000°C | Decreases kr with increasing temperature |
| Eccentricity ratio | Variable | Higher eccentricity amplifies damage effects |
| Steel tube wall thickness ratio | D/t ratio | Thinner walls show greater capacity loss |
| Concrete strength grade | C20–C50 | Higher strength concrete shows greater sensitivity |
| Load combination angle | Biaxial direction | Affects stress redistribution pattern |
Technical Analysis of the Damage Coefficient
The bearing capacity damage coefficient kr represents the ratio of post-fire residual capacity to the ambient-temperature capacity. The regression analysis reveals that kr is primarily governed by the exposure temperature, with secondary dependence on geometric parameters and load eccentricity.
At temperatures below 300°C, the residual capacity retains approximately 95–100% of the ambient-temperature value. Between 300°C and 600°C, kr decreases progressively as concrete loses strength and the steel tube enters the strain-hardening-to-softening transition. Above 600°C, the rate of capacity loss accelerates significantly, particularly for specimens with thin steel tubes where the confining effect is critically compromised.
The simplified calculation method integrates kr with the conventional biaxial eccentric compression formula for STC columns. This approach preserves the established design framework while incorporating fire damage effects through a single multiplicative factor — an elegant solution from a practical standpoint.
Comparison with Existing Methods
| Method | Approach | Advantages | Limitations |
|---|---|---|---|
| Full finite element analysis | Explicit material degradation models | High accuracy | Computationally intensive, requires expertise |
| Equivalent stress method | Temperature-dependent constitutive models | Good accuracy | Complex implementation |
| Proposed kr method | Damage coefficient multiplication | Simple, practical | Assumes linear degradation behavior |
| Empirical design codes | Prescriptive strength reduction | Quick assessment | Conservative, lacks physical basis |
Engineering Practice Integration
In post-disaster structural assessment, the proposed method offers significant practical advantages:
- Rapid field assessment: Engineers can estimate residual capacity using exposure temperature (measured with thermocouples or inferred from fire duration) and existing structural drawings.
- Repair decision support: The method enables quantitative comparison of residual capacity against design loads, informing decisions on repair, strengthening, or replacement.
- Code compatibility: The integration with existing ambient-temperature formulas maintains consistency with established design codes.
For strengthening design following fire damage, the method provides the baseline residual capacity against which strengthening effectiveness must be evaluated. Typical strengthening approaches include steel jacketing, external FRP wrapping, and internal concrete replacement — each requiring knowledge of the post-fire condition.
Critical Reflections
The linear regression approach for kr, while practical, may underestimate capacity loss at intermediate temperatures (500–700°C) where material degradation exhibits nonlinear behavior. The assumption of uniform temperature distribution across the cross-section may not hold for large sections with significant thermal gradients. Additionally, the method does not account for potential buckling of the steel tube under residual stresses after cooling, which can further reduce capacity.
The experimental program of 16 specimens provides a reasonable statistical basis, but the parameter ranges covered may not encompass all practical scenarios. Future research should extend the investigation to include: cyclic loading after fire exposure, long-term creep effects on residual capacity, and the influence of fire exposure duration (not just peak temperature) on material degradation.
Study Insights and Implications
This work makes a valuable contribution to the post-disaster assessment methodology for composite structural systems. The development of a practical, code-compatible calculation method addresses a genuine need in engineering practice, where rapid and reliable assessment is essential for public safety decisions. The approach demonstrates that complex material degradation phenomena can be captured with relative simplicity when the primary variable (exposure temperature) is well-characterized.
For practicing engineers, the key takeaway is that residual capacity assessment should not be treated as a binary pass/fail determination but as a quantitative evaluation that informs a spectrum of response options. The proposed method provides the quantitative foundation for this spectrum, enabling rational decisions that balance safety, economy, and practicality in post-fire structural management.