Zhuojin Pipe Fitting Co., LtdFire Resistance Performance of Strongly Confined Axially Compressed Steel Tube Columns
Literature Overview and Research Context
The study by Qu Lijun, Pan Chong, and Li Huanqun from the Chinese People's Armed Police Force Academy, published in the China Civil Engineering Journal (2012, Vol. 45, Issue 1, pp. 42-48), investigates the fire resistance performance of strongly confined axially compressed steel tube columns. The research is supported by the Ministry of Public Security Applied Innovation Program (Grant No. 2008YYCXWJXY117). This topic is of critical importance for structural safety, as fire resistance is a fundamental requirement for ensuring occupant safety and structural integrity during fire emergencies. The study employs a custom-developed temperature-axial force measurement device for truss structural members and conducts constant-load temperature-rise tests on 30 Q345 steel tube columns.
Experimental Configuration and Test Variables
The experimental program is comprehensive, covering multiple initial stress levels and slenderness ratios to provide a thorough understanding of the fire resistance behavior. The following table summarizes the key test parameters.
| Parameter | Range / Values | Number of Levels | Engineering Significance |
|---|---|---|---|
| Steel grade | Q345 | 1 | Common structural steel grade |
| Initial stress level | 5 levels | 5 | Represents different service load conditions |
| Slenderness ratio | 6 values | 6 | Captures buckling behavior variation |
| Total specimens | 30 | 5 × 6 | Full factorial design |
| Boundary condition | Both ends fixed | 1 | Strongly confined condition |
| Test method | Constant-load temperature-rise | 1 | Simulates fire scenario |
The use of a custom temperature-axial force measurement device is a significant methodological contribution, as it allows for the direct measurement of temperature-induced axial forces within the confined steel tube column. This is particularly important because the thermal expansion of the steel tube is restrained by the boundary conditions, generating substantial temperature stresses that can significantly affect the fire resistance performance.
Core Findings and Temperature Stress Behavior
The experimental results reveal that the temperature stresses generated in strongly confined steel tube columns under temperature rise are very large and play a decisive role in the failure of the columns. This finding has profound implications for fire resistance design, as it demonstrates that the temperature stress component must be explicitly considered in fire resistance assessment, rather than being neglected as a secondary effect. The following table summarizes the key relationships identified in the study.
| Condition | Effect on Ultimate Capacity | Effect on Temperature Stress | Effect on Critical Temperature |
|---|---|---|---|
| Higher initial stress (same slenderness) | Larger | Smaller | Lower |
| Lower initial stress (same slenderness) | Smaller | Larger | Higher |
| Larger slenderness (same initial stress) | Insensitive | Insensitive | Buckling failure mode |
| Smaller slenderness (same initial stress) | Insensitive | Insensitive | Strength failure mode |
The finding that columns with higher initial stress levels exhibit larger ultimate load capacity but smaller temperature stresses and lower critical temperatures is counterintuitive and requires careful interpretation. This behavior can be explained by the fact that columns with higher initial stress are closer to their yield point at ambient temperature, and the additional temperature stress required to reach failure is therefore smaller. However, the ultimate load capacity is larger because the column has already mobilized more of its cross-sectional capacity through the initial stress. The lower critical temperature indicates that these columns are more vulnerable to fire, as they reach their failure state at a lower temperature.
Failure Modes and Slenderness Effects
The study identifies two distinct failure modes depending on the slenderness ratio. Columns with larger slenderness ratios fail by buckling instability, accompanied by large post-failure deformations. Columns with smaller slenderness ratios fail by strength failure, with relatively small post-failure deformations. This distinction is important for fire resistance design because buckling failures are more sudden and potentially more dangerous in terms of progressive collapse risk.
| Slenderness Ratio | Failure Mode | Post-Failure Deformation | Fire Risk Characterization |
|---|---|---|---|
| Large | Buckling instability | Very large | Sudden collapse potential |
| Small | Strength failure | Relatively small | Progressive degradation |
The observation that slenderness ratio has limited influence on the ultimate load capacity and temperature stress under the same initial stress level is noteworthy. This suggests that, within the tested range, the temperature stress component dominates the fire resistance behavior, and the geometric stability aspect is secondary. However, the failure mode transition from strength failure to buckling failure with increasing slenderness ratio has important implications for the safety margin in fire conditions.
Proposed Design Formula and Code Implications
Based on the experimental data, the authors provide a regression formula for the lower-bound critical temperature of both-ends-fixed Q345 steel tube columns. This formula can be used for fire resistance design and assessment of steel structures under conditions where the constraint stiffness remains constant during the fire. The following table compares the proposed approach with conventional fire resistance design methods.
| Aspect | Conventional Approach | Proposed Approach |
|---|---|---|
| Temperature stress consideration | Often neglected | Explicitly included |
| Initial stress effect | Simplified | Quantitatively captured |
| Slenderness effect | Emphasized | Found to be less sensitive |
| Boundary condition | Idealized | Realistic (both ends fixed) |
| Critical temperature prediction | Empirical | Regression-based from test data |
Engineering Practice Integration
For engineers involved in fire protection design of steel structures, the findings of this study have several practical implications. First, the temperature stress generated by restrained thermal expansion must be explicitly calculated and included in fire resistance assessments. This requires knowledge of the structural constraint conditions, which may vary significantly between different structural systems. Second, the initial stress level at the time of fire initiation is a critical parameter that should be determined from the actual service loading conditions. Third, the distinction between strength failure and buckling failure modes should guide the selection of appropriate fire protection measures, with buckling-prone members requiring additional bracing or fire-resistant coatings.
From a welding and fabrication quality control perspective, the fire resistance performance of steel tube columns is also influenced by the quality of the welded connections that provide the boundary constraints. Any weld defects that reduce the effective constraint stiffness could lead to lower critical temperatures and premature failure. Therefore, weld inspection procedures should include verification of the full-penetration of butt welds and the integrity of fillet welds at column-base and column-beam connections.
Study Insights and Critical Reflection
The experimental investigation provides valuable data on the fire resistance behavior of strongly confined steel tube columns, but several aspects merit further consideration. The tests were conducted under standard fire temperature-time curves, while real fires may exhibit different temperature profiles depending on the fuel type, ventilation conditions, and fire development stage. The study focuses on Q345 steel, which is the most commonly used structural steel grade, but the findings should be validated for higher strength steels (Q390, Q420, Q460) that are increasingly used in modern construction. The long-term durability of fire protection coatings and the effect of thermal cycling on the structural integrity of welded connections under repeated fire exposure are important practical concerns that should be addressed in future research. The proposed design formula should be validated against a larger database of fire tests before being incorporated into design codes.