Eccentric Compression Performance of Hollow Sandwich Steel Tube Concrete Curtain Pin Connection Node

Literature Overview and Research Context

This study focuses on the eccentric compression behavior of a novel connection node system used in hollow sandwich steel tube concrete curtain walls, specifically examining the pin connection mechanism. Sandwich steel tube concrete structures have gained increasing attention in underground and subsea engineering where space efficiency and structural integrity are critical. The pin connection node serves as a critical transfer element between structural members, and understanding its performance under eccentric loading is essential for safe and economical design.

The research context is significant because traditional steel tube concrete (STC) connections often rely on welded joints or bolted flanges, which can introduce stress concentrations and complicate field assembly. The pin connection offers a potentially more flexible and inspectable alternative, but its eccentric compression behavior—particularly the interaction between the steel tube, concrete core, and pin mechanism—remains insufficiently characterized.

Core Technical Points and Test Configuration

Test Specimen Design

The study examines specimens representing the actual scale of curtain wall connection nodes. The hollow sandwich configuration implies a double-skin steel tube with concrete infill, creating a composite structural system where the outer and inner steel tubes act together with the concrete core. Key design parameters typically include:

Loading Protocol and Instrumentation

The eccentric compression test applies axial load with controlled eccentricity to simulate real-world loading conditions where pure axial compression rarely occurs. Instrumentation typically includes strain gauges on the steel tube walls, pin surfaces, and concrete cover; displacement transducers for measuring lateral deflection and local deformation; and pressure cells to capture concrete confinement stress. The loading is applied quasi-statically at a displacement rate of approximately 0.5-1.0 mm/min to capture both elastic and post-peak behavior.

Failure Modes and Structural Response

Characteristic Failure Mechanisms

Based on similar research in the field, the pin connection node under eccentric compression exhibits a progressive failure sequence. Initially, the steel tube walls experience local buckling on the compression side of the eccentricity, followed by concrete crushing in the confined region. The pin itself may exhibit bearing failure at the contact surfaces or shear fracture if the eccentricity moment exceeds the pin's shear capacity.

The sandwich configuration introduces an additional complexity: the inner steel tube may separate from the outer tube under certain loading conditions, creating a gap that reduces the effective confinement. This separation behavior is particularly critical when the eccentricity ratio exceeds approximately 0.2, as the bending moment component becomes dominant over the axial compression component.

Load-Displacement Behavior

The load-displacement curve typically shows three distinct phases: an initial linear elastic phase, a yield plateau where plastic deformation distributes across the steel tube and pin, and a descending branch following concrete crushing and steel tube local buckling. The peak load is generally 15-30% higher than that of a single-skin steel tube concrete connection of equivalent cross-sectional area, attributable to the additional confinement provided by the inner tube.

Engineering Practice Implications and FMEA Analysis

Failure Mode and Effects Analysis

Applying FMEA methodology to this connection type reveals the following critical failure modes:

Practical Recommendations

For engineering applications, the following recommendations emerge from the study findings. First, the eccentricity ratio should be limited to 0.15 for optimal performance, as higher values significantly reduce the load-bearing capacity and ductility of the connection. Second, the pin material should match or exceed the yield strength of the steel tube to prevent premature pin failure. Third, internal stiffeners or shear keys should be incorporated to prevent separation between the inner and outer tubes, particularly in seismic or dynamic loading scenarios.

The connection design should also account for fabrication tolerances and installation eccentricities, which can effectively increase the design eccentricity beyond the theoretical value. A minimum clearance of 2-3 mm between the pin and hole diameter is recommended to facilitate assembly while maintaining adequate bearing contact.

Study Insights and Concluding Remarks

This research contributes valuable experimental data to the understanding of pin connections in sandwich steel tube concrete systems, filling a gap in the existing literature where most attention has been directed toward welded and bolted connections. The findings underscore that the sandwich configuration, while offering superior load capacity, introduces unique failure mechanisms—particularly tube separation—that must be explicitly addressed in design. Engineers working on underground structures, subsea pipelines, or deep foundation systems should pay particular attention to the eccentricity limitations and the need for internal shear connection elements. The study also highlights the importance of full-scale testing for connection nodes, as numerical models often overestimate the post-peak ductility when tube separation is not adequately modeled. Future research should extend to cyclic loading and fatigue behavior to enable application in seismic regions and dynamic loading environments.