Bending Performance of Steel Tube Concrete Internal and External Flange Connection Joints

Literature Overview

This research investigates the bending performance of steel tube concrete (SRC) connection joints utilizing internal and external flange connections. Connection joints are critical structural elements that transfer forces between connected members while maintaining structural continuity. For SRC members, the connection design must accommodate the composite action between steel and concrete while providing adequate strength, stiffness, and ductility under bending loads. The comparison between internal flange and external flange connection configurations provides valuable guidance for selecting appropriate connection details in SRC structural systems.

Core Technical Analysis

Connection Configuration Comparison

Internal flange connections are positioned within the steel tube, while external flange connections are mounted on the outside surface of the tube. Each configuration has distinct advantages and disadvantages that influence the overall structural performance:

Feature	Internal Flange Connection	External Flange Connection
Location	Inside the steel tube	Outside the steel tube
Access for assembly	Requires tube opening	Full access from outside
Corrosion protection	Protected by steel tube	Requires coating
Moment transfer	Direct through tube wall	Requires additional stiffeners
Concrete interaction	Direct contact with concrete	Indirect through steel tube
Welding accessibility	Limited	Good
Cost	Lower material cost	Higher material cost

Bending Moment Transfer Mechanism

The bending moment transfer mechanism differs significantly between the two connection types:

Internal flange connections transfer bending moments through:

Direct bearing between the flange and the concrete core.
Shear transfer through the steel tube wall at the connection interface.
Friction between the flange surface and the concrete surface.

External flange connections transfer bending moments through:

Bolts in shear and tension across the flange interface.
Bearing between the flange plates and the steel tube surface.
Welded stiffeners that distribute forces to the steel tube.

The moment capacity of each connection type depends on the material properties, geometric parameters, and the interaction between steel and concrete components.

Experimental and Analytical Results

Load-Displacement Behavior

The bending performance is characterized by the load-displacement relationship, which typically exhibits three distinct stages:

Elastic stage: Linear relationship between applied moment and rotation, with the full composite section contributing to stiffness.
Yielding stage: Gradual transition from elastic to plastic behavior as yielding initiates in critical sections.
Plastic stage: Development of plastic hinges with reduced stiffness but maintained load-carrying capacity.

The ultimate bending moment capacity is determined by the formation of a plastic hinge in the connection zone, which may occur in the steel tube, the connection welds, or the bolts. The ductility ratio (ultimate rotation divided by yield rotation) is a critical measure of connection performance, with values above 3.0 generally considered acceptable for seismic applications.

Key Design Parameters

Parameter	Influence on Capacity	Influence on Ductility
Flange thickness	Directly proportional	Moderate positive effect
Bolt diameter	Increases shear capacity	Limited effect
Steel tube thickness	Increases confinement	Positive effect
Concrete strength	Increases bearing capacity	Slight negative effect
Connection length	Increases moment arm	Positive effect
Stiffener configuration	Redistributes stress	Improves energy dissipation

Failure Modes and Defect Analysis

Internal Flange Connection Failure Modes

Internal flange connections may fail through several mechanisms:

Concrete crushing: The concrete core beneath the flange bearing surface may crush under high bearing stresses, particularly if the concrete strength is inadequate or the bearing area is insufficient.
Steel tube local buckling: The steel tube wall near the internal flange connection may experience local buckling due to concentrated bearing stresses.
Weld failure: The welds connecting the internal flange to the steel tube may fail in shear or peel, particularly if the weld quality is inadequate.
Concrete-steel debonding: Loss of bond between the concrete and steel tube interior can reduce the composite action and connection efficiency.

External Flange Connection Failure Modes

External flange connections exhibit different failure characteristics:

Bolt shear failure: Bolts may fail in shear if the shear capacity is exceeded or if the bolts are not properly tightened.
Flange plate yielding: The flange plates may yield in bending or shear if they are not adequately sized.
Steel tube wall tear-out: The steel tube wall may tear around bolt holes or weld attachments if the local strength is insufficient.
Stiffener weld failure: Welds connecting stiffeners to the steel tube may fail under cyclic loading.

Engineering Practice Integration

Design Recommendations

Based on the comparative analysis of internal and external flange connections, the following design recommendations are provided:

Internal flange connections are preferred for applications where:

Corrosion protection is a primary concern.
Compact connection geometry is required.
The connection is not subject to frequent inspection or maintenance.
The concrete quality can be reliably controlled.

External flange connections are preferred for applications where:

Field assembly and inspection are critical.
High ductility is required for seismic applications.
The connection may need to be disassembled for maintenance or replacement.
The steel tube diameter is too large for practical internal connection fabrication.

Hybrid approaches combining elements of both types may provide optimal performance for specific applications, such as internal bearing surfaces with external bolted connections.

Quality Control Requirements

For both connection types, rigorous quality control is essential:

Weld inspection: Full radiographic or ultrasonic testing of all structural welds.
Bolt tightening verification: Torque or elongation verification for all bolts.
Dimensional inspection: Verification of connection geometry and alignment.
Load testing: Proof load testing to verify connection capacity before service.
Coating inspection: Verification of protective coating thickness and adhesion for external connections.

Study Insights and Implications

This research provides comprehensive comparative data on the bending performance of internal and external flange connections for SRC members, filling an important gap in the technical literature for this connection type. The findings demonstrate that both connection configurations can achieve adequate strength and ductility when properly designed, but with different performance characteristics and quality control requirements.

The key implication for engineering practice is that the selection between internal and external flange connections should be based on a holistic assessment of structural performance requirements, constructability, maintainability, and quality control feasibility. Internal flange connections offer advantages in terms of corrosion protection and compact geometry, while external flange connections provide better accessibility for assembly, inspection, and potential future maintenance. Engineers designing SRC structural systems should carefully evaluate both options for each specific application, considering the full lifecycle implications of the connection choice. Future research should explore advanced connection details that combine the advantages of both approaches, such as hybrid internal-external connections with improved ductility and maintainability characteristics.