Mechanical Performance of Prefabricated Square Steel Tube Flange Connection Joints

Literature Overview

The paper by Liu Xuechun, Wang Hexiang, Wang Yimeng, Wang Qi, and Zhan Xinxin (2017, Industrial Construction, Vol. 47, No. 8) investigates the mechanical performance of flange connection joints used in prefabricated multi-storey steel structures, specifically for box column (square steel tube) bolted splicing. The study employs finite element analysis using ABAQUS to evaluate the behaviour of 11 flange joint specimens under combined tension, bending, and shear loading, varying flange thickness, bolt edge distance, and bolt hole diameter.

The flange connection is presented as a novel method for all-bolt splicing of box columns in prefabricated steel structures. This approach eliminates the need for on-site welding, which is a significant advantage for prefabricated construction where construction speed, quality control, and on-site safety are paramount concerns.

Finite Element Modelling Approach

Specimen Parameters

The finite element model should account for the geometric nonlinearity of the flange plates, the contact behaviour between flange plates and bolt heads, the material nonlinearity of the steel, and the bearing deformation of the bolt holes. The accuracy of the model depends critically on the contact algorithm used and the mesh density around the bolt holes and flange edges.

Key Findings

Flange Thickness as the Dominant Parameter

The most important finding is that flange thickness is the primary factor governing the bearing capacity, stiffness, and prying force of the flange connection. This is intuitive from a structural mechanics perspective: the flange plate acts as a cantilever beam extending from the bolt group, and its bending stiffness is proportional to the cube of its thickness. Therefore, even small increases in flange thickness can lead to significant improvements in connection performance.

Bolt Edge Distance and Prying Force

The bolt edge distance affects the magnitude of the prying force but does not significantly influence the overall bearing capacity or stiffness of the connection. Prying force is the additional tensile force developed in the bolts due to the flexure of the flange plate. When the edge distance is small, the flange plate has less leverage to resist the prying action, resulting in higher prying forces. Conversely, a larger edge distance provides more flange plate area to resist prying, thereby reducing the prying force.

Bolt Hole Diameter Has No Significant Effect

Perhaps the most practically useful finding is that bolt hole diameter has no significant influence on the mechanical performance of the flange connection. This is a liberating result for engineers because it means that the bolt hole diameter can be selected based on fabrication and assembly considerations (such as bolt insertion tolerance and hole alignment) without compromising the structural performance of the connection.

Engineering Practice Implications

Design Recommendations

Based on the findings of this study, the following design recommendations can be made for flange connections in prefabricated steel structures:

1. Flange thickness: The flange thickness should be optimised to achieve the required bearing capacity and stiffness while minimising material usage and weight. A minimum flange thickness of 1.5 times the bolt diameter is typically recommended as a starting point, with detailed finite element analysis used to refine the design.
2. Bolt edge distance: The bolt edge distance should be designed to limit the prying force to an acceptable level. A minimum edge distance of 2.0 times the bolt diameter is generally recommended, with larger values used for connections subject to high prying forces.
3. Bolt hole diameter: The bolt hole diameter can be selected based on standard fabrication practices without concern for structural performance. Standard clearance holes (bolt diameter + 2 mm) are acceptable.
4. Bolt grade and pre-tension: The bolt grade and pre-tension level should be selected to ensure that the bolts do not yield under the combined effects of direct tension, prying force, and shear. High-strength bolts (Grade 8.8 or 10.9) are typically used for flange connections.

Quality Control Considerations

For the fabrication and assembly of flange connections, the following quality control measures are essential:

1. Flange plate flatness: The flange plates should be fabricated to a tight flatness tolerance to ensure uniform contact between the mating flange surfaces. Any waviness or distortion can lead to uneven load distribution and premature failure.
2. Bolt hole alignment: The bolt holes in mating flange plates should be aligned with a tolerance of ±1 mm to ensure smooth bolt insertion and proper pre-tensioning.
3. Surface preparation: The mating flange surfaces should be prepared to ensure full contact. This may involve grinding, machining, or the use of shims. The surface roughness should be controlled to prevent galling or stress concentration.
4. Bolt pre-tensioning verification: The bolt pre-tension should be verified using calibrated torque wrenches or direct tension indicators (DTIs) to ensure that the specified pre-tension level is achieved.

Comparison with Alternative Connection Methods

Study Insights and Reflections

The finding that bolt hole diameter has no significant effect on connection performance is particularly valuable from a practical standpoint. In prefabricated construction, the tolerance for bolt hole alignment is a critical quality control parameter, and the ability to use standard clearance holes without compromising structural performance simplifies the fabrication and assembly process.

However, I would note that the study is based on finite element analysis rather than experimental testing. While finite element models can provide valuable insights into connection behaviour, they are subject to modelling assumptions and material model uncertainties. In particular, the contact behaviour between flange plates and bolt heads, the bearing deformation of bolt holes, and the prying force development are all sensitive to the modelling approach. Experimental validation of the finite element predictions would be essential before adopting the design recommendations for critical structural applications.

The concept of all-bolt flange connections for prefabricated steel structures is consistent with the global trend towards modular and prefabricated construction. The elimination of on-site welding reduces construction time, improves quality control, and enhances worker safety. However, the larger connection size and higher material cost of flange connections compared to welded splices must be balanced against these benefits.

Conclusion

This study provides useful insights into the mechanical performance of flange connections for prefabricated square steel tube columns, identifying flange thickness as the dominant design parameter and demonstrating that bolt hole diameter has no significant effect on connection performance. The findings support the use of flange connections in prefabricated steel structures, provided that the flange thickness and bolt edge distance are carefully designed and that appropriate quality control measures are implemented during fabrication and assembly. Further research combining experimental testing with finite element analysis is recommended to validate the design recommendations and extend them to a wider range of connection configurations and loading conditions.