Bending Performance of Flange Connection Nodes in Steel Tube Structures

Literature Overview

This paper by Wang Yuanqing, Zong Liang, and Shi Yongjiu from Tsinghua University (2011), funded by the National Natural Science Foundation (Grant No. 51038006), investigates the bending resistance of flange-connected nodes in steel tube structures. Four-point bending tests were conducted on specimens representing four basic flange connection configurations, with bolt strain measurement devices installed to capture the force distribution characteristics. The study examines both rigid and flexible flange connection types and compares experimental results with finite element analysis.

Core Technical Findings

The research establishes that prying action in the tension zone is a critical factor governing flange connection behavior:

The finite element analysis results showed good agreement with experimental data, validating the analytical approach. Bolt strain measurements revealed non-uniform force distribution across the bolt group, with outer bolts experiencing higher forces than inner bolts, particularly during the later stages of loading.

Interpretation of Technical Points

The prying action phenomenon in flange connections represents a complex interaction between bolt tension, flange plate bending, and contact pressure distribution. When a flange connection is subjected to bending moment, the tension-side bolts are pulled outward, causing the flange plate to deflect and pry against the mating surface. This prying action effectively increases the tensile force in the bolts beyond what would be calculated from simple moment equilibrium.

The observation that rigid flange connections exhibit smaller prying action than flexible connections is counterintuitive but can be explained by the stiffness hierarchy. In rigid flange connections, the flange plate has sufficient bending stiffness to resist deformation, maintaining closer contact between flange surfaces and limiting the lever arm that generates prying force. In flexible flange connections, the thinner or less stiff flange plate deflects more readily, creating a larger separation gap and longer effective lever arm, which amplifies the prying force.

The safety reserve of approximately 1.2 between yield and ultimate loads indicates that flange connections have limited plastic reserve capacity. This is particularly important for seismic design applications where connections must sustain inelastic deformation without fracture. The relatively low safety factor suggests that flange connections should be designed to remain elastic during design-level seismic events, or that detailed fracture mechanics analysis is needed for connections expected to yield.

Welding and Manufacturing Quality Considerations

From a steel tube and welding perspective, the quality of flange connections is intimately tied to the manufacturing of the connected components:

• Flange plate material quality: Yield strength and toughness properties must be verified; low toughness materials are susceptible to fracture under prying action
• Weld quality at tube-flange junction: If flanges are welded to steel tubes, the weld HAZ must have adequate toughness to resist the high stress concentrations at the bolt holes
• Bolt hole fabrication: Hole geometry, dimensional accuracy, and surface finish affect stress concentration factors; drilled or reamed holes are preferred over punched holes
• Steel tube end preparation: Cut quality at tube ends where flanges attach must ensure full weld penetration and adequate fusion

For the steel tubes connected by these flanges, the tube end preparation is critical. The cut surface should be square and perpendicular to the tube axis, with burrs removed. If the flange is attached by fillet welds, the tube end should be prepared with a proper weld groove to ensure adequate weld throat thickness. If butt welds are used, full penetration with proper HAZ microstructure is essential. The residual stresses from tube manufacturing (cold bending for square tubes, welding for welded tubes) should be considered when determining the effective yield strength at the connection location.

Finite Element Analysis Methodology

The co-rotational coordinate method employed in this study is particularly suitable for large-deformation analysis of flange connections. The method separates rigid body rotation from deformation, allowing accurate tracking of contact conditions between flange surfaces as they separate and re-contact during loading cycles. The n+1 dimensional load-displacement space solution approach preserves the banded structure of the stiffness matrix, improving computational efficiency while maintaining accuracy.

Key modeling considerations for flange connection FEA include:

1. Contact elements between flange surfaces with appropriate friction coefficients
2. Bolt modeling with proper preload representation
3. Material nonlinearity capturing the yield behavior of both flange plates and bolts
4. Geometric nonlinearity to capture large deformations and prying action development
5. Stress concentration modeling at bolt holes with adequate mesh refinement

Engineering Practice Recommendations

Based on this research, the following recommendations are provided for flange connection design and construction:

• Prefer rigid flange connections where prying action is a concern, as they exhibit smaller prying forces and higher load capacity
• Design bolt groups with consideration for non-uniform force distribution; outer bolts should be checked for higher demand
• Apply the 1.2 safety reserve factor when checking connection capacity against design loads
• Ensure adequate flange plate thickness to maintain rigid behavior; minimum thickness should be verified against deflection criteria
• Conduct detailed FEA for critical connections, incorporating manufacturing imperfections and residual stresses
• Implement quality control procedures for bolt installation, including proper torque sequences and preload verification
• Specify material toughness requirements for flange plates in seismic applications to prevent brittle fracture

Study Insights and Reflections

This research provides valuable experimental data on flange connection behavior that complements existing design codes. The finding that prying action is significant even in nominally rigid connections suggests that current code provisions may be conservative for some connection types but potentially inadequate for others. The consistency between experimental and FEA results validates the analytical approach for detailed connection design. However, I note that the study focuses on monotonic loading conditions, while real structural connections experience cyclic loading during seismic events. The degradation of connection capacity under cyclic loading, particularly the progressive loosening of bolts and accumulation of plastic deformation, is not addressed. Future research should extend to cyclic loading tests and incorporate the effects of bolt relaxation and gasket compression to provide more comprehensive design guidance for seismic applications.