Eccentric Compression Bearing Capacity of Square CFST Columns with Constrained Tie Rods

Literature Overview

This paper by Cai Jian, Zhu Changhong, and Su Guangqun, published in the Journal of Guangxi University (Natural Science Edition) (2010, Vol. 35, No. 4, pp. 524–530), supported by the National Natural Science Foundation of China (Grant No. 50878087), investigates the eccentric compression bearing capacity characteristics of square concrete-filled steel tube (CFST) short columns incorporating constrained tie rods. The study employs a constitutive model specific to constrained tie rod CFST members and utilizes a fiber model-based nonlinear analysis program to numerically calculate the bearing capacity of the columns. The numerical results show good agreement with experimental data, validating the analytical approach.

The incorporation of constrained tie rods into CFST columns represents an innovative structural concept aimed at enhancing the confinement effect on the core concrete. Traditional CFST columns rely solely on the steel tube for lateral confinement, but the addition of internal tie rods can provide additional confinement, particularly in the interior regions of large-diameter columns where the steel tube confinement effect may be insufficient.

Core Technical Approach

Constitutive Model Development

The study develops a specific constitutive model for constrained tie rod CFST members that accounts for the enhanced confinement provided by the tie rods. The model considers:

• The confinement stress distribution within the concrete core
• The interaction between the steel tube and the tie rods
• The nonlinear stress-strain behavior of confined concrete under eccentric loading
• The contribution of the tie rods to the overall confinement effectiveness

Fiber Model-Based Nonlinear Analysis

The numerical analysis employs a fiber model approach, which is a well-established method for analyzing reinforced concrete and composite structural members. The key features of the analysis include:

• Discretization of the cross-section into discrete fibers, each representing a small area with a specific material type
• Assignment of appropriate stress-strain relationships to each fiber based on its material (steel tube, concrete, tie rod)
• Iterative equilibrium solution at each load step
• Tracking of the N-M (axial force-bending moment) interaction curve

Parameter Analysis Results

The study conducts a systematic parameter analysis examining the influence of various geometric and material parameters on the eccentric compression bearing capacity. The key findings are summarized below:

Influence of Tie Rod Configuration

The tie rod configuration parameters—spacing and diameter—have a significant and positive influence on the bearing capacity:

• Reducing tie rod spacing increases the number of confinement points, leading to more uniform confinement stress distribution and higher ultimate bearing capacity.
• Increasing tie rod diameter enhances the axial resistance of each tie rod, providing greater confinement force and improving the overall column capacity.

Influence of Steel Tube Wall Thickness

Interestingly, increasing the steel tube wall thickness causes the N/Nu-M/Mu interaction curve to contract inward. This counterintuitive result can be explained by the following mechanism:

• A thicker steel tube provides greater axial load capacity (Nu increases significantly)
• However, the bending moment capacity (Mu) does not increase proportionally because the concrete core confinement effect does not scale linearly with wall thickness
• The ratio Mu/Mu,ref may decrease, causing the interaction curve to contract inward relative to the normalized axes

Influence of Material Strength

Both concrete strength and steel strength have a positive influence on the bearing capacity, with the N/Nu-M/Mu curve becoming more convex as material strengths increase. This convexity indicates improved eccentric compression performance, as the column can sustain higher bending moments for a given axial load.

Directional Dependence of Bearing Capacity

The study reveals that the ultimate bearing capacity of bidirectionally eccentrically loaded short columns exhibits directional dependence:

• Maximum capacity occurs when the eccentricity is applied along the major principal axis of inertia
• Minimum capacity occurs when the eccentricity is applied along the minor principal axis of inertia

This is consistent with the general behavior of structural sections and confirms that the square CFST column with tie rods maintains the expected symmetry properties.

Similarity Between Uniaxial and Biaxial Eccentric Loading

The study finds that the influence of various parameters on the N/Nu-M/Mu interaction curves for uniaxial and biaxial eccentric loading is similar. This finding simplifies the design process, as parametric studies conducted for uniaxial eccentric loading can be reasonably extended to biaxial eccentric loading cases.

Engineering Practice Integration

Design Recommendations

Based on the parameter analysis results, the following design recommendations emerge for square CFST columns with constrained tie rods:

1. Tie rod spacing optimization: Reduce tie rod spacing to maximize confinement effectiveness, particularly for columns subjected to high eccentric loads. A spacing-to-diameter ratio of 3:1 to 4:1 is recommended for optimal confinement.
2. Tie rod diameter selection: Use larger diameter tie rods to enhance confinement, but consider the practical constraints of fabrication and installation. Tie rod diameters of 12–20 mm are typical for practical applications.
3. Steel tube wall thickness: Balance the steel tube thickness between axial load capacity and bending moment capacity. Excessive wall thickness may not proportionally improve eccentric compression performance.
4. Material selection: Higher strength concrete and steel grades improve bearing capacity but may reduce ductility. A balanced approach considering both strength and ductility requirements is recommended.
5. Eccentricity direction: Design for the critical eccentricity direction (minor principal axis) to ensure adequate safety margins.

Comparison with Conventional CFST Columns

Study Insights and Reflections

This research contributes a valuable analytical tool for the design of square CFST columns with constrained tie rods. The fiber model-based nonlinear analysis, validated against experimental data, provides engineers with a reliable method for predicting the eccentric compression bearing capacity of these innovative structural members.

The parameter analysis results are particularly useful for design optimization, providing clear guidance on the relative importance of various geometric and material parameters. The finding that tie rod spacing and diameter have the most significant influence on bearing capacity highlights the importance of optimizing the tie rod configuration in design.

The counterintuitive finding that increasing steel tube wall thickness causes the N/Nu-M/Mu interaction curve to contract inward is an important insight that should be considered in design. Engineers should not assume that increasing steel tube thickness always improves eccentric compression performance and should instead optimize the thickness based on the specific loading conditions.

One limitation of the study is the focus on short columns. The behavior of slender CFST columns with constrained tie rods may differ significantly due to the influence of second-order effects (P-Δ effects) and potential buckling of the tie rods. Future research should extend the investigation to slender columns and consider the stability of the tie rod system under high axial loads.

The study also does not address the long-term performance of the tie rod system, including potential corrosion of the tie rods, relaxation of the tie rod pre-stress, and the effect of sustained loading on the confinement effectiveness. These factors are important for the design of durable and reliable structural systems and should be considered in future research and practical applications.