Zhuojin Pipe Fitting Co., LtdPressure Distribution on Steel Tube Concrete Column Surfaces Under Blast Loading - Study Note
Literature Overview
The paper by Sun Shanshan, Zhao Junhai, and Zhang Changguang (2018), published in the journal Blasting, investigates the pressure distribution on the surface of steel tube concrete (CFT) columns subjected to blast loading. The research was supported by multiple funding sources including the National Natural Science Foundation of China (Grant No. 51708035), the Postdoctoral Science Foundation of China, and central university basic research funding. The study conducted large-scale static blast tests using TNT charges of 3 kg and 50 kg on CFT columns, along with a 50 kg free-field static blast test, to obtain empirical data on blast pressure distribution.
Core Technical Content and Experimental Setup
The experimental program was designed to address a critical gap in the understanding of how blast waves interact with CFT column surfaces. Unlike free-field blast waves, the presence of a structural member modifies the pressure distribution through reflection, diffraction, and shielding effects. The study compared measured pressure distributions against several empirical formulas to evaluate their predictive accuracy and applicability.
| Test Condition | TNT Charge | Purpose |
|---|---|---|
| CFT column static blast test 1 | 3 kg | Near-field pressure distribution |
| CFT column static blast test 2 | 50 kg | Far-field pressure distribution |
| Free-field static blast test | 50 kg | Baseline reference for incident pressure |
The scaled distance (z), defined as the distance from the charge center to the measurement point divided by the cube root of the charge mass (m/kg^(1/3)), is the fundamental parameter governing blast wave characteristics. The study identified a critical transition at z = 3 m/kg^(1/3), below which near-field effects become dominant and the pressure distribution deviates significantly from far-field predictions.
Key Findings and Technical Interpretation
Pressure Distribution Patterns on CFT Column Surfaces
The study revealed several important characteristics of blast pressure distribution on CFT columns:
- Windward surface negative pressure zone: The region on the windward face adjacent to the column edge experienced relatively small negative pressure values. This is attributed to the diffraction of the blast wave around the column edge, where the wave front separates from the surface, creating a low-pressure wake region.
- Leeward surface pressure: The pressure on the leeward (shadow) side of the column was found to be small and could be neglected in design calculations. This finding is significant because it simplifies the load model for blast-resistant design of CFT columns.
- Reflection pressure effect: The CFT column surface influenced the reflected pressure on the windward face, but the effect on the positive-phase impulse was relatively minor. This suggests that the column's surface roughness and geometry affect the instantaneous pressure peak more than the integrated impulse.
Scaled Distance Dependence
The relationship between reflected pressure and incident pressure showed strong dependence on scaled distance:
| Scaled Distance Range | Reflected/Incident Pressure Ratio | Behavior |
|---|---|---|
| z > 3 m/kg^(1/3) | Approximately 2.5 (average) | Spherical and hemispherical wave predictions converge |
| z < 3 m/kg^(1/3) | Increases as z decreases | Near-field effects dominate, ratio changes rapidly |
Empirical Formula Comparison
The study compared the measured data against several empirical formulas for incident pressure, reflected pressure, and positive-phase duration:
| Formula | Incident Pressure | Reflected Pressure | Applicability |
|---|---|---|---|
| TM 5-1300 (U.S. Army) | Good agreement | Reasonable agreement | Most reliable overall |
| Henrych formula | Acceptable | Underestimates (biased low) | Less accurate for reflected pressure |
| Chengqing Wu formula | Acceptable | Underestimates (biased low) | Less accurate for reflected pressure |
| Other formulas | Variable | Variable | High prediction scatter |
The study concluded that TM 5-1300 provided the most reasonable predictions for the blast load characteristics acting on CFT columns, although the reflected pressure prediction still exhibited some scatter.
Engineering Practice Implications
For engineers designing blast-resistant structures incorporating CFT columns, this study provides several important design considerations:
- Load model simplification: The finding that leeward surface pressures are negligible simplifies the load model, allowing designers to focus on the windward surface pressure distribution.
- Scaled distance classification: The critical threshold at z = 3 m/kg^(1/3) provides a practical criterion for distinguishing between near-field and far-field blast loading regimes, which have different pressure distribution characteristics.
- Formula selection: TM 5-1300 should be the preferred empirical formula for predicting blast load parameters on CFT columns, particularly for reflected pressure calculations where other formulas tend to underestimate.
- Surface pressure measurement: The study demonstrates that the pressure distribution on a CFT column is non-uniform, with significant variation around the column circumference. Design calculations that assume uniform pressure over the windward face may be inadequate for detailed design.
Key Questions and Reflections
The study raises important questions about the structural response of CFT columns to the non-uniform pressure distributions identified. The local pressure concentrations near the column edges, where diffraction effects are strongest, may induce local buckling or fracture of the steel tube, particularly in thin-walled configurations. The study does not address the structural response of the CFT column to these pressure distributions, which is essential for establishing design criteria. Additionally, the effect of column geometry (diameter, length, and end conditions) on the pressure distribution is not systematically investigated, limiting the generalizability of the findings. The study also does not discuss the effect of soil confinement on columns embedded in ground, which would significantly alter the pressure distribution compared to free-standing columns.
Study Insights and Implications
This study provides valuable empirical data on blast pressure distribution on CFT column surfaces, filling an important gap in the blast engineering literature. The identification of the critical scaled distance threshold and the comparative evaluation of empirical formulas offer practical guidance for blast-resistant design. For steel pipe engineers, the study highlights the importance of understanding how the steel tube surface interacts with blast waves, as the tube's geometry and surface condition directly influence the reflected pressure distribution. The findings also underscore the need for detailed structural analysis of CFT columns under non-uniform blast loading, particularly at locations where pressure concentrations are expected. The recommendation to use TM 5-1300 as the primary predictive tool provides a standardized basis for design calculations, although engineers should remain aware of the inherent scatter in empirical blast pressure predictions and incorporate appropriate safety factors.