Finite Element Analysis of Square Steel Tube Concrete Column Dogbone Steel Beam Sleeve Connection Joint

Overview of the Study

The paper by Suo Nan, Qin Jinghan, Wang Liang, and Zhou Xiaolong investigates a novel connection node combining a square steel tube concrete column with a dogbone-type steel beam sleeve connection. The research is supported by the Shandong Provincial Natural Science Foundation (ZR2023ME044) and was published in the Journal of Qingdao University of Technology, Volume 46, Issue 4, 2025, pages 66–76. The primary objective is to evaluate the seismic performance of this innovative joint through ABAQUS finite element simulation, with the goal of transferring the plastic hinge away from the node zone into the dogbone-weakened beam section, thereby satisfying the well-established seismic design philosophy of "strong column, weak beam; strong joint, weak member."

Core Technical Content and Force Transmission Mechanism

The proposed connection node employs a sleeve-type interface where the steel beam is inserted into a sleeve attached to the square steel tube concrete column. The dogbone feature is a localized weakening segment on the beam flange, intentionally designed to concentrate plastic deformation at a predictable location. Through the finite element model, the authors validated their modeling approach by comparing simulated results against established benchmarks before proceeding to parametric studies.

The force transmission path is clearly delineated in the analysis. Axial tension and compression forces travel sequentially through the beam flange, the cover plate, and the sleeve into the core zone of the joint. Shear forces are transmitted through the beam web, shear connectors, and the sleeve into the same core zone. This dual-path mechanism ensures that the node zone itself remains essentially elastic during seismic loading, while the dogbone-weakened beam section absorbs the energy through controlled plastic deformation.

The following table summarizes the recommended geometric parameters derived from the parametric study:

Parametric Analysis and Design Recommendations

The parametric study systematically varied four key geometric variables: cover plate thickness, cover plate length, dogbone segment length, and dogbone weakening depth. Each parameter was found to influence the joint's seismic behavior in a predictable manner. Thicker cover plates increase the stiffness and load-carrying capacity of the sleeve interface but also increase the weight and material consumption. Longer cover plates improve the load distribution but may introduce additional weld length and fabrication complexity.

The dogbone segment length directly controls the location and extent of plastic hinge formation. A longer dogbone segment provides a more gradual transition from elastic to plastic behavior, reducing stress concentration at the weakened section. The dogbone weakening depth determines the ultimate rotation capacity of the beam; deeper weakening reduces the flexural strength but increases the ductility demand that can be accommodated.

From a welding and fabrication perspective, the sleeve connection requires full-penetration butt welds between the sleeve and the column tube, as well as fillet welds or plug welds for the cover plate attachment. The quality of these welds is critical because any lack of fusion or undercuts in the sleeve-to-column weld could create a brittle failure path that bypasses the intended dogbone plastic hinge. Welding procedures should follow applicable codes such as GB/T 19851 for submerged arc welding or GB/T 985 for GTAW/GMAW processes, with strict control of preheat temperature, interpass temperature, and post-weld heat treatment for materials with higher carbon equivalent.

Engineering Practice Implications and Reflections

This connection type offers a practical solution for steel tube concrete frames where field welding of the beam-to-column joint is difficult or undesirable. The sleeve connection allows the beam to be prefabricated and inserted into the column sleeve on site, reducing field welding volume and improving construction quality. However, the fit-up tolerance between the beam end and the sleeve must be carefully controlled; gaps exceeding 2 mm could lead to uneven stress distribution and premature local buckling of the sleeve wall.

The study's recommendation to keep the node zone elastic is consistent with the damage-limiting design philosophy adopted in modern seismic codes such as GB 50011 and ASCE 7. The key insight is that by providing a reliable ductile element (the dogbone beam) in series with a robust but potentially brittle connection (the sleeve), the overall system achieves both strength and ductility. This approach is analogous to the concept of replaceable fuses used in other structural systems.

One area that merits further investigation is the long-term performance under cyclic loading with varying amplitude and frequency, which better simulates real earthquake conditions. The current study appears to focus on quasi-static cyclic loading, which may not fully capture low-cycle fatigue effects at the sleeve weld interface. Future work should consider incorporating low-cycle fatigue criteria, such as those in IIW recommendations, to evaluate the durability of the sleeve welds under repeated seismic events.

In conclusion, the dogbone-type steel beam sleeve connection represents a well-conceived seismic design solution that effectively externalizes plasticity and protects the joint zone. The parametric recommendations provide practical guidance for engineers, while the force transmission analysis validates the structural logic of the system. Attention to weld quality, fit-up tolerances, and long-term cyclic fatigue behavior will be essential for successful implementation in full-scale projects.