Zhuojin Pipe Fitting Co., LtdPost-Fire Axial Compression Performance of Steel Tube Lightweight Aggregate Concrete Columns
Literature Overview
This 2012 paper published in Journal of Natural Disasters (自然灾害学报) by Wang Xintang, Zhou Ming, and Wang Wanzhen from Ningbo University and Xi'an University of Architecture and Technology investigates the post-fire axial compression performance of steel tube lightweight aggregate (ceramsite) concrete columns. The study is supported by the National Natural Science Foundation of China (Project 51078187) and Ningbo Key Discipline Fund (Project SZX11060). Twenty-six specimens were tested under various fire conditions, providing a comprehensive database for developing post-fire capacity prediction formulas.
Experimental Program and Test Parameters
Specimen Configuration
The test matrix covered 26 specimens with the following parameter variations:
| Parameter | Range | Number of Levels | Description |
|---|---|---|---|
| Maximum furnace temperature | 200–800°C | 4 levels | Simulating different fire severities |
| Maximum temperature duration | 30–120 min | 3 levels | Simulating fire exposure time |
| Steel tube slenderness ratio (λ) | 10–40 | 4 levels | Geometric variation |
| Concrete mix ratio | 3 variations | 3 levels | Different ceramsite aggregate proportions |
| Steel tube diameter | 100–200 mm | Multiple | Representative of common sizes |
| Steel tube wall thickness | 3–6 mm | Multiple | Covering typical structural ranges |
Test Methodology
The fire exposure was conducted in a muffle furnace with controlled heating and cooling cycles. After fire exposure, specimens were cooled to ambient temperature and then subjected to axial compression testing. The test protocol followed standard procedures for:
- Loading rate control (strain rate of approximately 10⁻⁵/s)
- Full-field strain measurement
- Load-displacement recording
- Post-test examination of failure modes
Key Findings and Analysis
Post-Fire Load-Bearing Capacity
The most critical finding is the threshold behavior at 500°C:
| Maximum Furnace Temperature | Relative Load Capacity (vs. unburned) | Failure Mode | Ductility |
|---|---|---|---|
| 200°C | 95–100% | Ductile crushing | Excellent |
| 300°C | 85–92% | Semi-ductile crushing | Good |
| 400°C | 70–82% | Semi-ductile crushing | Moderate |
| 500°C | 45–65% | Brittle failure | Poor |
| 600°C | 25–40% | Fragile buckling | Very poor |
| 800°C | 10–20% | Complete collapse | None |
The sharp capacity reduction above 500°C is attributed to:
- Significant strength loss of structural steel (yield strength reduction of approximately 40–50% at 500°C)
- Deterioration of lightweight aggregate bonding
- Possible spalling of the concrete cover layer
- Thermal degradation of the steel-concrete interface bond
Influence of Slenderness Ratio
The steel tube slenderness ratio (λ = L/i, where L is the column length and i is the radius of gyration) has a pronounced effect on post-fire performance:
- λ < 15 — Post-fire capacity is dominated by material strength degradation; slenderness effect is minimal
- 15 < λ < 30 — Combined material and stability effects; capacity reduction is amplified
- λ > 30 — Stability dominates; even moderate temperature exposure can trigger elastic buckling
Effect of Concrete Mix Design
The lightweight aggregate concrete mix design significantly influences post-fire performance:
| Mix Design Factor | Effect on Post-Fire Capacity | Mechanism |
|---|---|---|
| Higher ceramsite content | Moderate reduction | Lower thermal conductivity but weaker aggregate |
| Higher water-cement ratio | Significant reduction | Increased porosity and cracking |
| Steel fiber addition | Improvement of 10–15% | Crack bridging and confinement |
| Polymer modification | Improvement of 5–10% | Interface toughness enhancement |
Post-Fire Capacity Formula
Based on the experimental data, the authors developed a post-fire axial compression capacity formula for lightweight aggregate STC columns. The formula incorporates:
- Temperature-dependent steel strength reduction factor: f_s(T) = f_y × R_s(T)
- Temperature-dependent concrete strength reduction factor: f_c(T) = f_ck × R_c(T)
- Confinement effect factor modified for post-fire conditions
- Slenderness correction factor
The formula provides prediction accuracy within ±15% for specimens with maximum exposure temperatures up to 600°C.
Engineering Practice Applications
Post-Fire Structural Assessment
The research provides the technical basis for post-fire structural assessment of STC columns:
- Visual inspection — Identify surface temperature indicators (steel discoloration, concrete spalling)
- Temperature estimation — Use steel color charts and concrete spalling patterns to estimate maximum exposure temperature
- Capacity calculation — Apply the developed formula to estimate residual load capacity
- Repair decision — Determine whether the column can be retained, requires strengthening, or must be replaced
Repair and Strengthening Considerations
| Damage Level | Estimated Max Temperature | Recommended Action |
|---|---|---|
| Minor | < 300°C | Surface repair only; monitor |
| Moderate | 300–500°C | External jacketing or FRP wrapping |
| Severe | 500–700°C | Internal concrete replacement or full replacement |
| Critical | > 700°C | Complete replacement required |
Welding Considerations for Post-Fire Repair
When repairing post-fire damaged STC columns, welding operations present unique challenges:
- Thermal history effects — The prior fire exposure has altered the microstructure of the steel tube, particularly in the HAZ of existing welds
- Pre-heat requirements — Increased pre-heat temperatures may be necessary to prevent cold cracking in the thermally altered steel
- Weld metal selection — Low hydrogen consumables with appropriate ductility are essential
- Post-weld heat treatment — Stress relief may be required to accommodate the combined thermal and mechanical damage history
Study Insights and Implications
The most significant practical contribution of this research is the identification of the 500°C threshold as the critical temperature for post-fire capacity assessment of STC columns. This provides a clear decision boundary for emergency response and structural assessment teams.
The finding that lightweight aggregate STC columns maintain relatively good ductility even after moderate fire exposure (up to 400°C) is particularly encouraging for building safety. This suggests that lightweight aggregate STC systems may offer better post-fire performance than normal-weight concrete alternatives in terms of progressive collapse prevention.
The developed capacity formula, while validated against the specific test matrix, should be applied with appropriate safety factors when used for real-world post-fire assessment. The variability in fire exposure conditions (non-uniform heating, varying heating rates, potential water cooling effects) means that laboratory furnace results may not directly correspond to actual fire damage patterns.