Seismic Performance of Hollow Sandwich Steel Tube Concrete Column with Combined Beam Single-Side Bolt Connection

Literature Overview

This paper by Wang Jingfeng and colleagues from Hefei University of Technology, published in 2019 in the Journal of Hefei University of Technology (Volume 42, Issue 5, pages 655-663), investigates the seismic performance of prefabricated frame connections between round hollow sandwich steel tube concrete (CST) columns and steel-concrete composite beams using single-side high-strength bolt connections. The research was supported by the National Natural Science Foundation of China (Grants 51478158 and 51178156) and the Ministry of Education New Century Outstanding Talent Support Program (NCET-12-0838). This study is particularly relevant to the growing trend of prefabricated construction in seismic regions, where connection design and welding quality play critical roles in seismic performance.

Test Configuration and Specimen Design

Four interior column joint specimens were tested under horizontal low-cycle reversed loading. Each specimen consisted of a round hollow sandwich CST column connected to a steel-concrete composite beam through single-side high-strength bolts with either flush or extended end plates. The hollow sandwich CST column features an inner and outer steel tube with a concrete layer sandwiched between them, creating a novel structural configuration that combines the advantages of composite action with the flexibility of prefabricated construction.

From a welding and fabrication perspective, the hollow sandwich CST column requires precise welding of the inner and outer tubes to the sandwich concrete layer. The welding of the inner tube to the outer tube (if connected) or the welding of connection plates to the outer tube must be executed with high quality to ensure the integrity of the composite action. The single-side bolt connection introduces a semi-rigid behavior that is fundamentally different from fully rigid welded connections, which has implications for the seismic force distribution and energy dissipation mechanism.

Key Findings and Parametric Analysis

The study examines the influence of two primary parameters on the seismic performance of the connection:

The study classifies the connection as a semi-rigid connection and a partial strength connection, evaluated according to European relevant codes. This classification is important for the seismic design of prefabricated frames, as semi-rigid connections introduce additional complexities in the force distribution and deformation patterns compared to fully rigid or fully pinned connections.

Welding Quality and Prefabricated Connection Implications

The prefabricated nature of this connection system places significant demands on the welding quality of the column and beam components. The hollow sandwich CST column, with its concentric tube configuration, presents unique welding challenges:

1. The welding of the outer tube to the connection end plate must ensure full moment transfer capacity, typically requiring full-penetration welds with backing.
2. The inner tube, if welded to the outer tube through the sandwich concrete, requires specialized welding techniques such as submerged arc welding (SAW) or electroslag welding (ESW) for the internal joints.
3. The bolt holes in the end plates must be precisely located and drilled to ensure proper bolt fit-up, as misalignment can introduce unintended eccentricities that compromise the connection behavior.

The single-side bolt arrangement means that the bolt group is subjected to both shear and tension under seismic loading. The bolts must be high-strength grade (typically 10.9 or 12.9 per GB/T 1228 or ISO 898-1) and installed with proper tightening torque to achieve the designed pre-tension. The welding of the end plate to the column tube must be designed to accommodate the bolt group loading without premature failure.

Study Insights and Engineering Practice

This study provides valuable experimental data for the seismic design of prefabricated CST frame connections. The finding that the extended end plate form outperforms the flush end plate in terms of bearing capacity and energy dissipation is intuitive but important to quantify. The extended end plate provides a longer lever arm for the bolt group, reducing the tensile stress on the bolts and the bearing stress on the end plate.

From a quality control standpoint, I would emphasize the importance of weld inspection for the hollow sandwich CST columns. The internal welds between the inner and outer tubes are difficult to inspect using conventional NDT methods. I recommend the use of phased array ultrasonic testing (PAUT) or time-of-flight diffraction (TOFD) for internal weld inspection, as these techniques can detect internal defects with high sensitivity. Additionally, the bolt installation quality must be verified through torque wrench calibration and bolt elongation measurement.

The classification of the connection as semi-rigid and partial strength has important implications for the seismic design methodology. Engineers must use appropriate analysis methods that account for the semi-rigid behavior, such as the component method or the moment-rotation curve approach. The partial strength classification means that the connection may yield before the plastic hinge forms in the column or beam, which can be advantageous for seismic energy dissipation but requires careful detailing to ensure ductile connection behavior.

In conclusion, this study contributes to the advancement of prefabricated CST frame technology and provides practical guidance for the design and quality control of semi-rigid bolted connections in seismic applications.