Eccentric Compression Behavior of Hollow Sandwich Steel-Concrete Tube Curtain Flange Bolted Connection Nodes

Literature Overview

This research investigates the structural performance of bolted flange connection nodes used in hollow sandwich steel-concrete tube curtain wall systems, specifically under eccentric compressive loading conditions. Steel-concrete composite structures have gained significant traction in modern high-rise and long-span construction due to their superior strength-to-weight ratio and enhanced fire resistance. The hollow sandwich configuration, which incorporates an air gap or secondary infill between the steel tube and concrete, offers additional advantages in thermal insulation and damping capacity. The flange bolted connection node serves as a critical junction where structural continuity is maintained, and its behavior under eccentric loading is fundamental to the overall stability and safety of the system.

Structural Configuration and Loading Analysis

The hollow sandwich steel-concrete tube curtain system typically comprises an outer steel pipe, an inner concrete core, and an intermediate cavity that may be filled with lightweight insulation material or left as an air gap. The flange bolted connection at each node transfers axial loads, bending moments, and shear forces between adjacent structural segments. Under eccentric compression, the connection experiences a combined state of axial force and bending moment, which induces a non-uniform stress distribution across the bolt group and the flange plate.

The eccentricity ratio, defined as the ratio of the eccentricity distance to the flange radius, is the primary geometric parameter governing the connection behavior. As the eccentricity increases, the stress concentration at the tension-side bolts increases significantly, while the compression-side bolts experience reduced load or even unloading. This asymmetric loading pattern can lead to bolt fatigue failure, flange plate yielding, and progressive joint degradation.

Key Geometric and Material Parameters

Experimental Findings and Failure Modes

The experimental program typically involves fabricating full-scale or scaled connection specimens and subjecting them to controlled eccentric compression loading using hydraulic actuators. Instrumentation includes strain gauges on flange plates and bolts, displacement transducers at the loading points, and crack detection equipment. The loading is applied in increments with hold periods to allow for stress redistribution and crack propagation observation.

Several distinct failure modes have been identified through such testing. The first mode involves yielding of the flange plate at the bolt holes, where localized tensile stresses exceed the yield strength of the steel. The second mode is bolt shear failure, typically occurring at the tension-side bolts where the combined shear and tension stresses are highest. The third mode is concrete crushing at the interface between the flange and the concrete core, which can lead to loss of composite action. The fourth mode involves progressive joint separation, where the bolts on the tension side elongate beyond their elastic limit, causing permanent joint opening.

Analytical Modeling and Design Implications

The analytical modeling of eccentric compression in these connection nodes requires consideration of the nonlinear interaction between the steel flange, the bolt group, and the concrete core. The moment-rotation relationship of the connection is typically nonlinear, with an initial elastic stiffness that degrades as yielding progresses. The stiffness degradation rate is influenced by the bolt pre-tension level, the flange thickness, and the concrete confinement effect.

For design purposes, the connection must satisfy strength requirements under both service and ultimate limit states. The bolt group analysis follows the standard approach of treating the bolt group as a rigid body rotating about a centroid, with individual bolt forces proportional to their distance from the centroid. However, the hollow sandwich configuration introduces additional complexity due to the potential for differential deformation between the outer steel tube and the inner concrete core.

Study Insights and Reflections

The eccentric compression behavior of flange bolted connection nodes in hollow sandwich steel-concrete tube systems represents a complex interaction of structural mechanics, material science, and connection engineering. The research findings emphasize that eccentricity is not merely a geometric parameter but a critical design variable that fundamentally alters the load path and failure mechanism of the connection. Engineers designing such systems should adopt a rigorous approach to eccentricity assessment, incorporating realistic load combinations and connection flexibility into their structural models. The hollow sandwich configuration, while offering thermal and acoustic advantages, introduces additional interface behaviors that must be carefully characterized through testing. Future research should focus on cyclic loading behavior, fatigue performance, and the long-term effects of environmental exposure on the bolted connection integrity.