Eccentric Compression Performance of Pin-Connected Nodes in Hollow Sandwich Steel Tube Concrete Pipe Curtain

Literature Overview and Technical Context

This study investigates the eccentric compression behavior of pin-connected nodes used in hollow sandwich steel tube concrete (HSTC) pipe curtain structures, which are increasingly employed in deep foundation pit support systems and underground engineering projects. The sandwich construction involves an outer steel tube, an inner steel tube, and a concrete layer sandwiched between them, creating a composite cross-section that combines the ductility of steel with the compressive strength of concrete. The pin connection node is a critical structural element that transfers loads between adjacent pipe segments, and its performance under eccentric loading conditions directly governs the overall stability and safety of the pipe curtain system.

The research methodology involves constructing scaled and full-scale specimens to simulate realistic loading conditions, with instrumentation including strain gauges, displacement transducers, and load cells to capture the complete load-deformation response. The eccentricity ratio, defined as the ratio of eccentricity distance to the cross-sectional dimension, is the primary variable that influences the stress distribution and failure mode of the connection.

Core Technical Points and Failure Mechanisms

The experimental results reveal that the pin-connected node exhibits a progressive failure mechanism under eccentric compression. At low eccentricity ratios (e/D ≤ 0.1), the node behaves predominantly in axial compression with relatively uniform stress distribution across the cross-section. As eccentricity increases beyond 0.2D, bending moments become significant, leading to localized yielding on the tension side of the connection while the compression side experiences increased compressive stresses. At higher eccentricities (e/D > 0.3), the node transitions into a flexural-compressive failure mode where plastic hinges form at the pin interface.

The sandwich concrete layer plays a critical role in enhancing the load-bearing capacity and ductility of the connection. The confinement effect of the outer and inner steel tubes on the sandwich concrete prevents premature concrete crushing and allows the concrete to develop higher compressive strengths compared to unconfined conditions. The composite action between the steel tubes and concrete ensures that the connection maintains substantial residual load-carrying capacity even after initial yielding.

Standards and Design Considerations

The design of pin-connected nodes in HSTC pipe curtains should reference GB 50011 (Seismic Design Code), GB 50017 (Steel Structure Design Standard), and JGJ 130 (Technical Code for Steel Pipe Pile Foundation). The eccentric compression design must account for both axial force and bending moment interactions, typically following the P-M interaction curve approach. The allowable eccentricity should be limited based on the ductility requirements of the specific application, with typical design eccentricity ratios not exceeding 0.25D for seismic zones and 0.35D for non-seismic applications.

Quality control during fabrication requires attention to the concentricity of the inner and outer tubes, the uniformity of the sandwich concrete placement, and the precision of pin hole drilling. Common fabrication defects include eccentricity between the inner and outer tubes exceeding 2 mm, incomplete concrete filling in the sandwich layer, and pin hole dimensional deviations beyond ±0.5 mm tolerance.

Engineering Practice Integration and Reflections

In practice, the pin-connected node design must balance structural performance with constructability. The use of larger pin diameters increases connection strength but complicates assembly and increases fabrication costs. A practical approach involves optimizing the pin diameter to satisfy shear strength requirements with a safety factor of 1.5 while keeping the pin diameter within the range of 0.3D to 0.5D for manageable assembly. The sandwich concrete should be placed using low-viscosity micro-concrete with a slump of 160-200 mm to ensure complete filling of the annular space.

The experimental findings suggest that the current design codes may be overly conservative for eccentric compression of sandwich steel tube concrete connections, as the composite action provides greater capacity than predicted by simple steel tube models. Engineers should consider using the experimentally validated capacity values in performance-based design, particularly for applications where cost optimization is critical.

Summary

The eccentric compression behavior of pin-connected nodes in hollow sandwich steel tube concrete pipe curtains is governed by the interaction between axial compression and bending moment, with the sandwich concrete layer providing significant confinement benefits that enhance both strength and ductility. The eccentricity ratio is the most influential parameter, and design practices should be refined to leverage the composite action more effectively while maintaining adequate safety margins for critical infrastructure applications.