Bending Capacity of Flange Connections in Steel Tube Structures

Literature Overview

This paper by Wang Yuanqing, Zong Liang, and Shi Yongjiu from Tsinghua University investigates the bending capacity of flange connection nodes in steel tube structures through four-point bending tests on specimens representing four basic flange connection configurations. The study is significant because flange connections are widely used in steel tube structures for their ease of assembly and field erection, but their bending behavior under moment loading is not fully understood. The research was supported by the National Natural Science Foundation (Grant No. 51038006) and published in the Journal of Hunan University (Natural Science Edition) in 2011.

Test Configuration and Instrumentation

Four basic forms of flange connections were tested, each representing a distinct configuration of tube-to-flange-to-bolt assembly. The test specimens were loaded in four-point bending to induce a constant moment region across the flange connection. A bolt strain measurement device was employed to monitor the strain in the bolts during loading, providing direct insight into the bolt force distribution and the development of prying action.

The four-point bending test configuration was selected because it provides a well-defined constant moment region, allowing direct measurement of the bending capacity without the complications of shear interaction that would arise in a three-point bending test.

Prying Action and Bolt Behavior

Prying Action Mechanism

The study confirms the existence of significant prying action in the tension zone of flange connections under bending. Prying action occurs when the flange plate deflects outward at the bolt location, creating an additional tensile force on the bolts beyond the direct bending tension. This prying force is self-reinforcing: as the flange deflects, the lever arm for the prying action increases, leading to higher bolt forces and further flange deflection.

Rigid vs. Flexible Flange Connections

The study finds that rigid flange connections exhibit significantly less prying action than flexible flange connections. This is because the thick flange plate in rigid connections has greater bending stiffness, which resists the outward deflection that drives the prying mechanism. In flexible connections, the thin flange plate deflects more readily, amplifying the prying effect. This finding has direct implications for connection design: rigid flange connections are more efficient in terms of bolt utilization, as a larger proportion of the bolt capacity is used for direct tension rather than prying.

Bolt Strain Distribution

The bolt strain measurements reveal that the bolt forces are not uniformly distributed across the connection. The bolts closest to the neutral axis experience lower strains, while the bolts in the extreme tension zone experience the highest strains. This non-uniform distribution is a result of the flange plate deflection and the prying action mechanism. Engineers must account for this distribution when checking bolt capacities, rather than assuming uniform bolt forces.

Yield and Ultimate Load Capacity

The experimental results provide yield loads and ultimate loads for each connection form. The yield load is defined as the load at which the first bolt or flange plate element reaches its yield stress. The ultimate load is defined as the maximum load the connection can sustain before failure. The ratio of ultimate to yield load, which represents the safety reserve, is approximately 1.2 for all connection forms. This safety reserve is relatively modest, indicating that flange connections have limited ductility and should be designed with appropriate safety factors.

Finite Element Analysis Validation

The experimental results were compared with finite element analysis results, and the agreement was good. This validates the finite element models and provides a basis for parametric studies and design optimization. The finite element models should incorporate geometric nonlinearity, material nonlinearity, and contact nonlinearity to accurately capture the prying action and flange plate deflection behavior.

Engineering Practice Implications

From a steel pipe manufacturing and welding perspective, the quality of the tube-to-flange weld is critical for the bending performance of flange connections. The weld must be designed to transfer both bending moments and shear forces from the tube to the flange plate. Full-penetration groove welds are typically required for high-strength connections, and the weld quality must be verified through non-destructive testing such as ultrasonic testing or radiographic testing.

The bolt holes in the flange plate should be drilled rather than punched to avoid work-hardening and micro-cracking at the hole edges, which can reduce the fatigue life of the connection. The bolt clearance should be controlled to allow for field assembly without compromising the connection stiffness. The bolt grade and tightening torque should be specified according to the design requirements, and the bolt shank should be checked for yield during loading.

Study Insights and Outlook

This research provides valuable experimental data on the bending behavior of flange connections in steel tube structures, with particular emphasis on the prying action mechanism and its dependence on flange stiffness. The finding that rigid flange connections exhibit less prying action than flexible connections has direct design implications: engineers should preferentially use thicker flange plates with closer bolt spacing to minimize prying action and maximize bolt utilization. The safety reserve of approximately 1.2 is relatively low, indicating that flange connections are not ductile and should be designed with appropriate safety factors. Future research should investigate the fatigue performance of flange connections under cyclic loading, the effects of bolt pretension on prying action, and the behavior of flange connections with different bolt grades and hole configurations. The finite element models validated in this study can be used for parametric studies and design optimization, but they should be verified against additional experimental data for different connection geometries and loading conditions.