Zhuojin Pipe Fitting Co., LtdNumerical Analysis of Square-in-Square Hollow Interlayer Steel Tube Concrete Column with Steel-Concrete Beam Single-Side Bolt End-Plate Connection
Literature Overview
This paper by Wang Jingfeng, Duan Mengjun, Guo Lei, and Zhang Meng from Hefei University of Technology (published in Progress in Steel Building Structures, Vol. 22, No. 6, 2020, pp. 46-55) presents a numerical investigation of a composite connection between a square-in-square hollow interlayer steel tube concrete (SC) column and a steel-concrete composite beam using single-side high-strength bolt end-plate connections. The research was supported by the National Natural Science Foundation of China (Grant 51178158) and the Ministry of Education New Century Excellent Talents Support Program (NCET-12-0838).
Numerical Modeling Approach
The authors developed a finite element model using ABAQUS software that accounts for:
- Geometric nonlinearity (large deformation effects)
- Material nonlinearity (elasto-plastic behavior of steel and concrete)
- Complex contact interactions between multiple structural components
- Material constitutive relationships for steel, concrete, and composite materials
The numerical model was validated against low-cycle reversed loading test data, with satisfactory agreement in failure mode and hysteresis curve behavior, confirming the model's reliability for parametric analysis.
Parametric Study Results
Influence of Geometric Parameters
| Geometric Parameter | Effect on Ultimate Load | Effect on Initial Stiffness |
|---|---|---|
| End-plate thickness | Positive correlation | Strong positive correlation |
| Bolt diameter | Moderate positive effect | Moderate positive effect |
| Column outer dimension | Positive correlation | Positive correlation |
| Inner tube dimension | Moderate effect | Moderate effect |
| Beam depth | Positive correlation | Positive correlation |
Influence of Material Parameters
The material properties of the steel tube, concrete, and composite beam significantly affect the connection behavior. Higher yield strength of the steel tube and higher concrete compressive strength both increase the ultimate bearing capacity and initial stiffness of the connection. The constitutive model for concrete under confined conditions (accounting for the square-in-square interlayer confinement) is critical for accurate prediction of the column's contribution to connection behavior.
Influence of Load Parameters
The loading pattern and magnitude affect the connection's response. The study examined the effect of different loading conditions on the load-displacement relationship, providing insight into the connection's behavior under various seismic scenarios.
Technical Analysis and Engineering Relevance
Square-in-Square Hollow Interlayer SC Column
The square-in-square hollow interlayer SC column is an innovative structural system that combines the benefits of steel tube confinement with the additional stiffness provided by the interlayer configuration. From a fabrication perspective, this column type requires:
- Outer square tube fabrication: Typically formed from steel plates using submerged arc welding (SAW) or flash butt welding processes, with precise dimensional control to ensure proper fit with the inner tube.
- Inner square tube fabrication: Smaller dimension tubes, also fabricated from steel plates, requiring precise dimensional accuracy for the interlayer gap.
- Assembly and concrete placement: The inner tube is positioned within the outer tube with controlled interlayer spacing, after which concrete is placed in both the inner tube and the interlayer space. The welding of any internal bracing or connection plates must be performed before concrete placement.
- Weld quality requirements: All welds connecting the inner and outer tubes (if welded together) must achieve full penetration and pass UT inspection, as these welds transfer forces between the two tube systems.
Single-Side Bolt End-Plate Connection
The single-side high-strength bolt end-plate connection is a practical and economical connection type for prefabricated construction. Key technical considerations include:
| Connection Component | Design Consideration | Welding/Assembly Requirement |
|---|---|---|
| End plate | Thickness and bearing area | Welded to beam end with full-penetration groove welds |
| High-strength bolts | Grade 8.8 or 10.9 | Pre-tension control and installation sequence |
| Column-side connection plate | Welded to column | Fillet or groove welds with full inspection |
| Contact surfaces | Surface preparation | Grinding to SA2.5 or better finish |
The single-side configuration means all bolts are on one side of the connection, which simplifies field assembly but requires careful design to ensure adequate connection strength and stiffness.
Failure Mechanism
The numerical analysis reveals that the connection failure involves progressive yielding of the end plate, bolt elongation, and plastic deformation in the column and beam regions adjacent to the connection. The square-in-square column provides enhanced confinement and bending resistance compared to conventional SC columns, contributing to the connection's overall seismic performance.
Integration with Prefabricated Construction
This connection type is specifically designed for prefabricated composite frames in high seismic intensity zones. The single-side bolt configuration allows for:
- Easy field assembly without requiring access to both sides of the connection
- Modular construction approach where column and beam components are fabricated separately
- Quality control in factory conditions for welded components
- Reduced on-site welding requirements, minimizing the risk of field weld defects
From a quality assurance perspective, the factory-fabricated welded components (end plates welded to beams, connection plates welded to columns) should undergo comprehensive NDT including UT for groove welds and MT/PT for fillet welds, with documented traceability for all materials and welds.
Key Questions and Reflections
The numerical model, while validated against test data, relies on idealized material constitutive relationships that may not fully capture the complex behavior of the square-in-square interlayer system under extreme cyclic loading. Questions remain about the long-term performance of the bolted connections under repeated seismic events, including bolt relaxation, bearing surface wear, and potential for progressive loosening. Additionally, the interaction between the interlayer concrete and the inner tube under high strain rates (as in near-fault earthquakes) may differ from quasi-static test conditions.
Study Insights and Implications
This research demonstrates that the square-in-square hollow interlayer SC column with single-side bolt end-plate connections offers excellent seismic and structural performance suitable for prefabricated composite frames in high seismic intensity zones. The proposed numerical analysis methodology provides a practical tool for design engineers to evaluate connection behavior under various parameter combinations. For steel pipe fabrication and welding practitioners, the key insight is that the quality of factory-welded connection components is paramount, as field assembly relies on the integrity of these pre-fabricated welds to achieve the predicted seismic performance. The single-side bolt configuration represents a practical solution that balances fabrication complexity, field assembly convenience, and structural performance.