Quasi-Static Finite Element Analysis of Bolted Square Steel Tube Concrete Column-Steel Beam Composite Frame

Literature Overview

This paper published in Progress in Steel Building Structures (Vol. 27, Issue 10, 2025, pp. 69–78) by Luo Jing, Yan Yuxiang, Lü Hui, and Shao Lian from Nanchang Hangkong University presents a comprehensive quasi-static finite element analysis of a bolted square steel tube concrete (STC) column-steel beam composite frame structure. The study employs ABAQUS software to establish a three-dimensional fine finite element model that captures the detailed behavior of all structural components including bolts, nuts, and T-stubs.

Finite Element Model Configuration

The model represents a one-bay, one-story composite frame with the following modeling characteristics:

The inclusion of solid modeling for bolts, nuts, and T-stubs—rather than simplified connection elements—represents a methodological advancement that enables accurate prediction of bolt slip, bearing deformation, and connection failure modes.

Key Analytical Results

Ductile Behavior and Hysteresis

The quasi-static analysis results demonstrate excellent agreement with existing experimental data regarding:

• Failure modes and damage patterns
• Load-displacement hysteresis curves
• Stiffness degradation versus displacement relationships

Bolt Slip and Axial Compression Ratio

The analysis reveals that bolt slip remains small throughout the loading cycles, indicating that the bolted connections maintain integrity under seismic-level deformations. The column axial compression ratio is also low, suggesting that the frame operates within the desired ductile range without premature column crushing.

Confining Action of Steel Tube

A critical finding is that the lateral deformation coefficient at the bottom of the steel tube exceeds 0.5 in all measurement points. This confirms that the steel tube exerts significant confining pressure on the core concrete, enhancing its compressive strength and ductility. The maximum compressive stress in the core concrete exceeds the uniaxial compressive strength, which is direct evidence of the confining effect.

Damage Mechanism and Energy Dissipation

The paper provides detailed insights into the damage distribution and energy dissipation characteristics:

• Column bottom concrete: More susceptible to cracking and crushing under cyclic seismic loading compared to concrete at beam-column joint regions.
• Plastic energy dissipation: The structure exhibits a dominant beam-based energy dissipation mechanism with column energy dissipation as secondary, confirming a typical "strong column-weak beam" design philosophy.
• Damage evolution: Progressive concrete crushing initiates at the column base and propagates upward, while beam plastic hinges develop at the ends away from the joints.

Engineering Practice Implications

The findings have direct relevance to the seismic design of composite steel-concrete structures:

1. Bolted connection design: The solid modeling approach validates that properly designed bolted connections can maintain integrity under large inelastic deformations, supporting their use in ductile frame systems.
2. Column protection: The higher damage concentration at column bases suggests that additional reinforcement or energy dissipation devices should be considered at column footings.
3. Confinement verification: The lateral deformation coefficient exceeding 0.5 validates the effectiveness of square steel tubes as confining elements for core concrete.
4. Design philosophy confirmation: The "strong column-weak beam" behavior observed in the analysis supports the fundamental seismic design principle of ensuring plastic hinging occurs in beams rather than columns.

Key Reflections

The detailed solid modeling of bolted connections represents a significant methodological contribution to the field of composite structural analysis. While computationally more demanding than simplified connection models, the approach provides irreplaceable insight into the actual behavior of bolted assemblies under cyclic loading. The validation against experimental data confirms the reliability of the modeling approach. For practicing engineers, the key takeaway is that bolted STC column-steel beam frames can achieve satisfactory seismic performance when designed with appropriate connection detailing and column protection measures. The study also highlights the importance of considering the confining action of steel tubes in the design of composite columns, as this effect significantly enhances the compressive strength and ductility of the core concrete beyond what would be predicted by conventional design codes.