Experimental and Numerical Study of Single-Side Bolted End-Plate Connection Nodes for Circular Hollow Sandwich Steel Tube Concrete Columns

Literature Overview

The paper by Wang Jingfeng, Zhang Meng, and Zhang Na (2018), published in Progress in Steel Building Structures (Vol. 20, No. 2, pp. 36-43), presents an experimental and numerical study of single-side bolted end-plate connection nodes for circular hollow sandwich steel tube concrete (CHSSC) columns. The authors are affiliated with Hefei University of Technology, supported by the National Natural Science Foundation of China (Grant Nos. 51478158 and 51178156) and the Ministry of Education New Century Excellent Talent Support Program (Grant No. NCET-12-0838). This research addresses an important structural engineering challenge: the design of reliable and ductile connections for the emerging CHSSC column system, which combines the advantages of steel tube concrete columns with enhanced energy dissipation through a sandwiched steel plate layer.

Core Technical Content

Test Program

The authors conducted low-cycle reverse loading tests on four CHSSC column connection specimens. The test matrix was designed to investigate the effects of two key parameters:

The low-cycle reverse loading simulated seismic action, with displacement-controlled loading at multiple amplitude levels to capture the full hysteretic behavior from elastic to post-peak conditions.

Numerical Model Development

Finite element models were developed using commercial finite element software, incorporating:

1. Material constitutive models: Nonlinear material models for steel and concrete, including plasticity and damage evolution.
2. Contact modeling: Complex contact conditions between the end plate, bolts, column, and beam were defined using appropriate contact algorithms.
3. Bolt modeling: Detailed bolt modeling including pre-tension, thread engagement, and potential slip conditions.
4. Mesh refinement: Appropriate mesh density in critical regions such as bolt holes and weld connections.

Key Findings and Technical Analysis

Failure Modes

The experimental observations revealed several distinct failure modes:

• End-plate bending failure: Local yielding and buckling of the end plate around bolt holes.
• Bolt shear failure: Shear fracture of bolts under cyclic loading.
• Column wall local buckling: Inward or outward buckling of the CHSSC column wall near the connection.
• Weld fracture: Failure of welds connecting the end plate to the column or beam.

Hysteretic Performance

The connection nodes exhibited good hysteretic performance characterized by:

• Full and stable hysteresis loops indicating good energy dissipation capacity.
• Adequate ductility with displacement ductility ratios exceeding code requirements.
• Moderate stiffness degradation with increasing displacement amplitude.
• Strength degradation that remained within acceptable limits for seismic design.

Classification According to Eurocode 3 (EC 3)

Based on the European standard EC 3 classification criteria, the connection nodes were classified as:

• Semi-rigid: The connections exhibited measurable rotation under load, not fully rigid.
• Full-strength: The connections maintained their strength capacity without premature failure, allowing the connected members to reach their full plastic moment capacity.

Effect of End-Plate Type

The two end-plate types investigated showed different performance characteristics:

• Type A (single-side bolted) provided adequate performance with simpler fabrication.
• Type B (modified configuration) showed improved rotational capacity but with more complex detailing.

Effect of Column Hollow Ratio

The hollow ratio of the CHSSC column influenced connection performance:

• Higher hollow ratios reduced the column's local stiffness near the connection.
• This could lead to earlier local buckling of the column wall.
• The sandwiched steel plate layer helped maintain connection integrity even at higher hollow ratios.

Integration with Engineering Practice

Design Recommendations

Based on the research findings, the following design recommendations are proposed:

1. End-plate thickness: Should be designed to prevent excessive bending deformation while allowing adequate rotation for ductile behavior.
2. Bolt specification: High-strength bolts should be used with appropriate pre-tension to prevent slip under cyclic loading.
3. Column wall reinforcement: Local reinforcement (e.g., internal stiffeners or external reinforcement plates) should be considered to prevent column wall buckling.
4. Weld detailing: Welds should be designed for full penetration and inspected for quality to prevent premature weld fracture.
5. Hollow ratio limitation: The column hollow ratio should be limited to ensure adequate connection performance.

Fabrication and Quality Control

Key quality control measures for CHSSC column connections include:

• Inspection of the sandwiched steel plate layer for proper bonding and thickness.
• Verification of bolt hole alignment and fit-up before bolt installation.
• NDT of welds including UT or MT for critical connections.
• Verification of bolt pre-tension using calibrated torque wrenches or direct tension indicators.

Study Insights and Outlook

This research makes a valuable contribution to the understanding of connection behavior for CHSSC columns, which represent an emerging structural system with significant potential for high-rise and long-span applications. The combination of experimental and numerical approaches provides a comprehensive understanding of the connection behavior under cyclic loading.

The classification of the connections as semi-rigid and full-strength according to EC 3 is particularly important for design practice, as it provides a basis for analytical modeling and design verification. Future research should investigate the effect of fire exposure on connection performance, the behavior of connections under combined axial, shear, and moment loading, and the development of simplified design equations for practical engineering use. The findings of this research will support the continued development and adoption of CHSSC column systems in seismic-resistant structural design.