Mechanical Properties of Novel Square Steel Pipe Column Inner Sleeve Assembly Connection

Literature Overview

This paper by Liu Xiuli, Sun Fengbin, Lu Yang, and Cao Yuanzheng, published in Science Technology and Engineering (Vol. 21, Issue 3, 2021, pp. 1123-1130), presents research on a novel assembly connection for square steel pipe columns using an inner sleeve. The study was funded by the National Natural Science Foundation of China (Youth Science Fund, Grant No. 51508290). The authors propose a new connection configuration that relocates the column splicing position above the beam-column node core zone, thereby providing space for high-strength bolt installation at the beam-column joint while maintaining node zone continuity. Parametric finite element analysis using ANSYS was conducted to investigate the influence of inner sleeve thickness, inner sleeve length, tie bolt spacing, and end plate thickness on the hysteretic performance, ductility, and energy dissipation capacity of the connection.

Technical Background

Square steel pipe columns are widely used in modern steel structures due to their efficient cross-sectional properties, ease of fabrication, and compatibility with bolted connections. However, traditional splicing methods for square steel pipe columns often place the splice at the beam-column node location, which creates several challenges:

1. Limited access for bolt installation in the confined node zone
2. Potential interference between splice components and beam flanges
3. Reduced continuity of the node zone, which can compromise seismic performance

The proposed novel connection addresses these challenges by relocating the column splice above the beam-column node, creating a cleaner and more accessible connection geometry.

Core Technical Findings

Connection Configuration

The novel connection consists of an inner sleeve inserted into the upper column segment, which overlaps with the lower column segment. High-strength bolts connect the inner sleeve to the lower column through end plates, and tie bolts (through-bolts) pass through both column segments to provide lateral restraint and moment resistance. The key innovation is the placement of the splice above the beam-column node, which allows the beam-column connection to be designed as a conventional moment-resisting joint without interference from splice components.

Parametric Study Results

Inner Sleeve Thickness

The inner sleeve thickness is identified as the most influential parameter on the connection's mechanical performance. Increasing the sleeve thickness from the base case significantly improves the ultimate bearing capacity, stiffness, ductility, and energy dissipation. However, the study notes a threshold effect: when the sleeve thickness exceeds the column wall thickness by more than 2 mm, the improvement in mechanical performance becomes more pronounced. This suggests that the sleeve thickness should be at least equal to the column wall thickness, with a recommended minimum of column wall thickness plus 2 mm for optimal performance.

Inner Sleeve Length

Counterintuitively, increasing the inner sleeve length leads to decreased stiffness, ductility, and energy dissipation capacity. This is because a longer sleeve introduces more flexibility into the connection, reducing the overall stiffness. The authors recommend that the sleeve length should be minimized while still meeting construction and geometric requirements. A practical guideline would be to use the minimum sleeve length that provides adequate lap length for bolt engagement and allows for proper fabrication and erection.

Tie Bolt Spacing

The tie bolt spacing has a moderate but positive effect on connection performance. Increasing the spacing between tie bolts improves the stress distribution in the connection and reduces stress concentrations at the bolt holes. However, the spacing should not be increased excessively, as this would reduce the number of load transfer points and potentially lead to localized deformation. The optimal spacing balances the need for uniform load distribution with the need for adequate load transfer points.

End Plate Thickness

The end plate thickness has a positive effect on connection performance, with thicker plates providing higher capacity, stiffness, ductility, and energy dissipation. However, the authors note that excessively thick end plates are not economical, as they add material cost and weight without providing proportional performance benefits. The optimal end plate thickness should be determined through a cost-benefit analysis that considers both the structural performance requirements and the economic constraints of the project.

Engineering Practice Implications

The study provides several design recommendations for the novel inner sleeve assembly connection:

1. Sleeve thickness: The inner sleeve thickness should be at least equal to the column wall thickness, with a recommended minimum of column wall thickness plus 2 mm. Thicker sleeves provide better performance but at increased cost.
2. Sleeve length: The sleeve length should be minimized to meet geometric and construction requirements. Excessive length reduces stiffness and energy dissipation capacity.
3. Tie bolt spacing: Tie bolts should be spaced to provide uniform load distribution without excessive stress concentrations. Typical spacing of 3-5 times the bolt diameter is recommended.
4. End plate thickness: The end plate thickness should be sufficient to prevent yielding under the design loads, but not excessively thick. A thickness of 1.5-2 times the column wall thickness is a reasonable starting point for design.
5. Bolt grade: High-strength bolts (Grade 8.8 or 10.9) should be used for all connections to ensure adequate load transfer capacity.

Study Insights and Reflections

This research addresses an important practical challenge in steel structure design: how to achieve efficient column splicing without compromising the beam-column connection. The proposed novel connection is a significant improvement over traditional splicing methods, as it separates the splice from the beam-column node, allowing each connection to be designed optimally for its specific function.

From a structural engineering perspective, the hysteretic performance of the connection is critical for seismic design. The study demonstrates that the novel connection exhibits good ductility and energy dissipation capacity, which are essential for seismic-resistant design. The parametric study provides valuable design guidance, identifying the key parameters that influence connection performance and the optimal ranges for each parameter.

The finding that increasing the inner sleeve length reduces performance is particularly interesting and has practical implications. In traditional bolted connections, longer overlap lengths are generally associated with better performance. However, in this inner sleeve connection, the sleeve itself acts as a flexible element, and increasing its length introduces additional flexibility that reduces the overall connection stiffness. This insight is important for designers who might otherwise assume that longer sleeves are always better.

The economic consideration of end plate thickness is also noteworthy. In structural design, there is often a trade-off between performance and cost, and the identification of the optimal thickness range is valuable for practical application. The study's recommendation to balance performance and economy is consistent with sound engineering practice.

The novel connection concept has the potential to be widely adopted in steel structures, particularly in seismic regions where ductile connections are essential. The finite element analysis approach used in this study provides a reliable and efficient method for evaluating connection performance under various loading conditions, and the parametric study framework can be extended to other connection configurations and structural applications.

The study contributes to the ongoing development of steel structure design methods, providing practical design guidance that can be directly applied to engineering projects. The combination of experimental validation and finite element analysis is a robust approach that provides both physical insight and computational efficiency, making it suitable for use in design practice.