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

Literature Overview

This paper, published in Progress in Steel Building Structures (2025, Vol. 27, No. 10), authored by Luo Liang, Yan Yuxiang, Lü Hui, and Shao Lian from Nanchang Hangkong University, presents a comprehensive finite element analysis (FEA) of a bolted square steel tube concrete (CFST) column-steel beam hybrid frame structure using ABAQUS software. The study is supported by the National Natural Science Foundation of China, the Jiangxi Provincial Smart Construction Research Center Open Fund, and the Henan Provincial Science and Technology Project. The work addresses an increasingly important structural system in modern construction, where the combination of CFST columns and steel beams offers advantages in fire resistance, ductility, and construction efficiency.

Finite Element Model Development

The authors developed a three-dimensional refined finite element model of a single-story, single-bay bolted CFST column-steel beam hybrid frame. A key methodological contribution is the solid modeling of small components such as bolts, nuts, and washers, rather than simplifying them as rigid links or spring elements. This approach enables accurate simulation of bolt slip, preload relaxation, and friction contact behavior at connections.

Key Modeling Assumptions

The model incorporates several critical physical phenomena: the confinement effect of the steel tube on core concrete, the preload effect of bolts and nuts, and the frictional contact between bolts, T-stubs, steel tubes, and steel beams. The validation against existing quasi-static test results demonstrates good agreement in terms of failure mode, load-displacement hysteresis curves, and stiffness degradation-displacement curves.

Structural Behavior Analysis

The study provides detailed analysis of several structural response parameters:

• Bolt slip: The bolt slip displacement was found to be small throughout the loading process, indicating that the bolted connections maintain adequate clamping force even under cyclic loading. This is a positive finding for the seismic performance of bolted hybrid frames, as excessive bolt slip can lead to connection loosening and degradation of load-carrying capacity.
• Column axial compression ratio: The axial compression ratio of the column was found to be small, which is favorable for ductile behavior. High axial compression ratios can lead to brittle failure and reduced energy dissipation capacity, so maintaining a low axial compression ratio is essential for seismic design.
• Steel tube lateral deformation coefficient: The lateral deformation coefficient at the bottom measurement points of the steel tube all exceeded 0.5, indicating significant lateral expansion of the steel tube. This is consistent with the Poisson effect under axial compression and confirms the effectiveness of the steel tube in confining the core concrete.
• Core concrete stress-strain behavior: The maximum compressive stress in the core concrete exceeded the axial compressive strength, confirming the confining effect of the steel tube. The confinement increases the compressive strength and ductility of the concrete, which is a well-established principle in CFST column design.

Damage and Energy Dissipation Mechanisms

The study reveals that the hybrid frame structure exhibits a "strong column, weak beam" behavior pattern, with the beams dissipating the majority of energy and the columns dissipating secondary energy. This is the desired seismic design philosophy, as it ensures that plastic deformation concentrates in the beams (which are easier to repair) rather than in the columns (which are critical for gravity load support).

The study also found that under cyclic seismic loading, the concrete at the column base is more susceptible to cracking and crushing than the concrete at the beam-column joint. This is an important observation because it suggests that the column base region may require additional reinforcement or confinement to prevent premature concrete failure. In practice, this could be addressed by increasing the steel tube wall thickness at the column base, adding external confinement rings, or using higher-strength concrete in the base region.

Practical Implications for Bolted Hybrid Frame Design

The solid modeling approach for bolts and nuts, while computationally more demanding, provides significantly more accurate predictions of connection behavior compared to simplified models. This is particularly important for bolted connections in seismic applications, where the connection behavior under cyclic loading governs the overall structural performance. The study's finding that the solid modeling approach achieves both high computational accuracy and acceptable solution efficiency is encouraging for practical engineering applications.

From a design perspective, several recommendations emerge from this study:

• Bolt preload: Adequate bolt preload is essential to maintain connection integrity under cyclic loading. The study's finding of small bolt slip suggests that the preload was sufficient, but the actual preload value and its maintenance over time should be verified in practice.
• Column base design: The higher susceptibility of column base concrete to damage suggests that the column base region should be designed with additional attention to confinement and ductility.
• Beam-column joint: The joint region should be designed to ensure that plastic hinges form in the beams rather than at the joints, consistent with the "strong column, weak beam" philosophy.
• Steel tube thickness: The lateral deformation coefficient exceeding 0.5 at the column base suggests that the steel tube may require thicker walls or additional lateral support to prevent excessive deformation.

Study Insights and Recommendations

This study makes a valuable contribution to the understanding of bolted CFST column-steel beam hybrid frame structures through rigorous finite element analysis. The solid modeling approach for connection components is a methodological advancement that enables more accurate simulation of connection behavior, which is critical for seismic design of bolted hybrid frames.

The study's findings on damage distribution and energy dissipation mechanisms provide practical guidance for structural design. The "strong column, weak beam" behavior pattern is confirmed, and the higher susceptibility of column base concrete to damage is an important observation that should inform design practice.

For future work, I would recommend extending the study to include: (1) parametric studies on bolt diameter, bolt spacing, and T-stub thickness; (2) comparison with welded connection alternatives; (3) cyclic loading analysis with varying load amplitudes to simulate different seismic intensity levels; and (4) experimental validation of the FEA predictions for a full-scale or reduced-scale hybrid frame. Additionally, the study could be extended to consider the effects of fire on the bolted hybrid frame, as bolted connections may behave differently under elevated temperatures compared to welded connections.

In summary, this study provides a rigorous and insightful analysis of bolted CFST column-steel beam hybrid frame structures, offering both methodological advancements in finite element modeling and practical design guidance for the seismic performance of bolted hybrid frames.