Zhuojin Pipe Fitting Co., LtdEccentric Compression Performance of Hollow Sandwich Steel Tube Concrete Pipe Curtain Flange Bolt Connection Node
Overview and Research Context
This study investigates the eccentric compression behavior of a specialized connection node used in pipe curtain retaining structures. The connection involves a hollow sandwich steel tube concrete member joined to adjacent segments through flange bolt connections. Pipe curtain structures are increasingly used in deep foundation pit support, underground structure construction, and shield tunneling applications where a continuous, rigid retaining wall is required.
The connection node represents a critical structural element where load transfer occurs between pipe curtain segments. The eccentric compression loading simulates realistic service conditions where the pipe curtain experiences both axial loads from soil pressure and bending moments from differential settlement or lateral earth pressure variations.
Connection Configuration and Test Setup
The connection node under investigation consists of:
- A hollow sandwich steel tube concrete member with an outer steel tube, inner steel tube, and concrete infill between them.
- Flange plates welded to the ends of the steel tubes.
- High-strength bolts connecting the flange plates of adjacent segments.
- The eccentricity is introduced by offsetting the load application point from the centroid of the cross-section.
Test Parameters
| Parameter | Value | Description |
|---|---|---|
| Outer tube diameter | 600 mm | Main structural tube |
| Outer tube wall thickness | 10-12 mm | Q345 or Q355 steel |
| Inner tube diameter | 400 mm | Secondary tube |
| Inner tube wall thickness | 8-10 mm | Q345 or Q355 steel |
| Concrete strength | C40-C50 | Infill between tubes |
| Flange plate thickness | 20-30 mm | Q345 steel |
| Bolt grade | 10.9 | High-strength bolts |
| Bolt diameter | M24-M30 | Connection bolts |
| Eccentricity ratio | 0, 0.05, 0.1, 0.15, 0.2 | e/h where h is section depth |
| Number of bolts per flange | 12-16 | Arranged in circular pattern |
Failure Modes and Load-Displacement Behavior
The eccentric compression tests reveal several characteristic failure modes depending on the eccentricity ratio and connection configuration:
Identified Failure Modes
| Failure Mode | Eccentricity Range | Description | Critical Component |
|---|---|---|---|
| Bolt shear failure | Low eccentricity | Bolts fail in shear under high axial load | High-strength bolts |
| Flange yielding | Medium eccentricity | Flange plates yield in tension on the loaded side | Flange plate |
| Bolt tension failure | High eccentricity | Bolts fail in tension due to moment-induced prying | High-strength bolts |
| Concrete crushing | All eccentricities | Concrete crushes on the compression side | Concrete infill |
| Tube local buckling | High eccentricity | Steel tube walls buckle under combined compression and bending | Steel tube walls |
| Connection separation | Very high eccentricity | Flange plates separate at the interface | Bolt group |
Load-Displacement Characteristics
The load-displacement curves exhibit three distinct stages:
- Elastic stage: Linear relationship between load and displacement up to approximately 40-60% of ultimate load. All components remain elastic.
- Elastic-plastic stage: Non-linear behavior begins as the first yielding occurs, typically in the bolt group or flange plate on the tension side.
- Post-yield stage: Capacity continues to increase or remains stable as plastic redistribution occurs, until ultimate failure is reached.
Bolt Group Behavior Under Eccentric Compression
The bolt group behavior is the most critical aspect of the connection performance. Under eccentric compression, the bolt group experiences a combination of shear and tension forces:
Bolt Force Distribution
| Bolt Position | Eccentricity = 0 | Eccentricity = 0.1 | Eccentricity = 0.2 |
|---|---|---|---|
| Innermost bolts (compression side) | Shear only | Shear + slight tension | Shear + moderate tension |
| Middle bolts | Shear only | Shear dominant | Shear + tension |
| Outermost bolts (tension side) | Shear only | Shear + tension | High tension + shear |
The force distribution follows a modified prying force model where:
- The eccentricity induces a moment that creates tension in the outer bolts.
- The prying effect from the flange plate contact further increases bolt tension.
- The inner bolts may experience reduced shear or even tension under high eccentricity.
Design Recommendations and Code Comparison
Based on the test results, the following design recommendations are proposed:
Design Parameters
| Design Parameter | Recommended Value | Basis |
|---|---|---|
| Bolt utilization ratio | ≤ 0.75 of yield strength | Safety margin for fatigue |
| Flange plate thickness | ≥ 1.5 × bolt diameter | Prevent flange yielding |
| Eccentricity limit | ≤ 0.15 × section depth | Prevent bolt tension failure |
| Bolt spacing | 3-5 × bolt diameter | Uniform force distribution |
| Edge distance | ≥ 2 × bolt diameter | Prevent edge tearing |
| Concrete cover to bolts | ≥ 50 mm | Corrosion protection |
Comparison with Existing Codes
| Code | Bolt Design Method | Eccentricity Consideration | Applicable to This Connection |
|---|---|---|---|
| GB 50017-2017 | Shear + tension interaction formula | Limited eccentricity provisions | Partially applicable |
| AISC 360-16 | Bolt group analysis with moment | Detailed eccentric bolt group provisions | More applicable |
| Eurocode 3 (EN 1993-1-1) | Bolt group with moment | Semi-rigid connection provisions | Partially applicable |
| JTG/T 3365-2020 | Empirical formulas for pipe curtain | Specific to pipe curtain connections | Most applicable |
Engineering Practice Implications
The research findings have direct implications for pipe curtain design and construction:
- Bolt selection: High-strength bolts (grade 10.9 or higher) are essential, with careful attention to bolt hole tolerance and fit. Clearance holes with 1-2 mm clearance are recommended to prevent bearing stress concentration.
- Flange design: Flange plates should be designed to prevent yielding before bolt failure. The plate thickness should be verified against both bending and shear failure modes.
- Concrete quality: The concrete infill between the sandwich tubes must achieve full strength before loading. Early-age concrete with insufficient strength can lead to premature connection failure.
- Construction sequencing: The connection should be designed to allow sequential bolt tightening in a controlled sequence to ensure uniform load distribution.
- Inspection and maintenance: Regular inspection of bolt preload, flange condition, and concrete integrity is essential for long-term performance.
Key Questions and Study Insights
The research raises important questions about the long-term performance of the flange bolt connection under cyclic loading conditions, which may occur due to construction sequences, groundwater fluctuations, or seismic events. The prying force model used in the analysis should be validated against additional test data with varying bolt preload levels.
The hollow sandwich configuration offers advantages in terms of material efficiency and structural performance, but introduces complexity in the connection design. The interaction between the outer and inner tubes through the concrete infill creates a composite action that must be properly modeled. Future research should address the effect of corrosion on bolt performance, the influence of construction tolerances on connection behavior, and the development of simplified design methods suitable for routine engineering practice.
This study provides valuable experimental data and analytical insights that can directly inform the design of pipe curtain connection nodes, contributing to safer and more economical underground construction projects.