Seismic Performance of Prefabricated Square Steel Tube Concrete Column-Steel Beam Joints via Finite Element Analysis

Overview of the Study

This paper, authored by Wang Qi, Wang Lai, Zhou Shi-dong, Luo Tian-qi, and Zhang Hong-bin from Shandong University of Science and Technology, presents a finite element analysis of a novel prefabricated square steel tube concrete column-steel beam joint. Supported by the National Natural Science Foundation of China (51178295) and the Shandong Provincial Key R&D Program (2018GSF122006), the study was published in Earthquake Resistance and Retrofitting in 2022. The research addresses a critical challenge in earthquake-resistant construction: developing joint systems that combine the strength and stiffness of composite members with the constructability and quality assurance benefits of prefabrication.

Novel Joint Configuration

The proposed prefabricated joint is designed to facilitate off-site fabrication and on-site assembly while maintaining adequate seismic performance. The joint connects a square steel tube concrete (SRC) column with steel beams using end plates and vertical ribs. The key design features include:

• End plates: Bolted or welded to the column flanges, providing a moment-resisting connection.
• Vertical ribs: Reinforcing plates that increase the local stiffness and strength of the column walls at the beam connection zone.
• Prefabrication: The joint components are fabricated in the workshop and assembled on-site, reducing construction time and improving quality control.

The study established nine finite element models in ABAQUS, varying three key parameters:

Finite Element Modeling and Analysis

The finite element models were developed using ABAQUS, a general-purpose finite element analysis software. The models incorporated material nonlinearity, geometric nonlinearity, and contact interactions between the joint components. The concrete was modeled using the Concrete Damaged Plasticity (CDP) model, which captures cracking, crushing, and residual strength behavior. The steel components were modeled using the von Mises yield criterion with isotropic hardening.

The analysis included cyclic loading simulations to replicate earthquake loading conditions. The key performance indicators evaluated were:

• Hysteresis curves: Reflecting the energy dissipation capacity and stiffness degradation behavior.
• Skeleton curves: Showing the relationship between lateral force and displacement.
• Ductility: Measured by the ratio of ultimate displacement to yield displacement.
• Energy dissipation capacity: Calculated from the area enclosed by the hysteresis curves.
• Stiffness degradation: Tracking the reduction in joint stiffness as displacement increases.

Key Findings and Results

The analysis yielded several important conclusions:

1. Effect of axial compression ratio: Within the range of 0.3 to 0.6, increasing the axial compression ratio slightly reduces the ultimate bearing capacity and initial stiffness of the joint, but improves the energy dissipation capacity and ductility. This suggests that moderate axial compression can be beneficial for seismic performance.
2. Effect of end plate thickness: The presence of end plates significantly improves the joint performance, making the hysteresis curves more plump and increasing the ultimate bearing capacity and energy dissipation capacity. However, increasing the end plate thickness beyond a certain point leads to a reduction in ductility and energy dissipation capacity, as the stiffer end plate limits the plastic deformation of the joint.
3. Effect of vertical rib width: A vertical rib width of 90 mm provided the best combination of energy dissipation capacity and ductility. Wider ribs increased stiffness but reduced ductility, while narrower ribs did not provide sufficient reinforcement.

Engineering Practice Implications

The findings of this study have direct relevance to the design of prefabricated steel tube concrete structures for earthquake-prone regions. The proposed joint configuration offers a practical solution for achieving seismic performance while maintaining constructability. The finite element analysis provides a reliable tool for optimizing joint parameters before fabrication, reducing the need for costly full-scale testing.

From a fabrication and welding perspective, the joint components must be manufactured with high dimensional accuracy to ensure proper fit-up and bolt hole alignment. The welding of vertical ribs to the column walls must be performed with careful attention to weld quality, as these welds are subject to high cyclic stresses. The use of full-penetration butt welds or fillet welds with adequate throat thickness is recommended. Post-weld inspection using magnetic particle testing (MT) or ultrasonic testing (UT) should be conducted to ensure weld integrity.

The study also highlights the importance of considering the interaction between the axial load on the column and the lateral load on the joint. In seismic design, the column axial load is typically determined by gravity loads, while the lateral load is determined by seismic analysis. The interaction between these loads can significantly affect the joint behavior, and the design should account for the combined loading conditions.

Key Reflections and Study Insights

This paper demonstrates the value of finite element analysis in the design and optimization of seismic joints for composite structures. The parametric study approach is efficient and provides insights that would be difficult to obtain from physical testing alone. However, the accuracy of the finite element models depends on the validity of the material models and the boundary conditions. Validation against experimental data is essential, and the authors should be encouraged to conduct full-scale cyclic tests to confirm the numerical predictions.

One aspect that could be further explored is the effect of fabrication tolerances and bolt pre-tensioning on the joint performance. In practice, bolt holes may not align perfectly, and bolt pre-tensioning may vary, both of which can affect the joint stiffness and strength. Additionally, the long-term behavior of the joint under sustained loads and environmental exposure should be investigated, as bolt relaxation, concrete creep, and steel corrosion can degrade the joint performance over time.