Axial Compression Bearing Capacity Calculation for Rectangular CFST Short Columns with Through Bolts

Literature Overview

This paper, authored by Lu Songting, Kang Xiliang, and Gao Lei from the School of Civil Engineering at Lanzhou Jiaotong University, was published in Building Structure (Volume 54, Issue 8, 2024, pp. 11-19). The study investigates the axial compression behavior of rectangular steel tube concrete (CFST) short columns with through bolts and proposes a calculation method for their bearing capacity. The research was supported by the National Natural Science Foundation of China (Grant No. 52168070).

Technical Background and Motivation

Rectangular CFST columns are widely used in building construction due to their high strength, ductility, and architectural flexibility. However, the confinement effect provided by a rectangular steel tube on the enclosed concrete is inherently less effective than that of a circular tube, particularly at the corners and along the flat faces where the steel tube can buckle inward before the concrete reaches its full compressive potential. The introduction of through bolts—longitudinal steel bars passing through the core concrete and anchored in the steel tube walls—provides additional confinement and shear transfer, enhancing the overall structural performance.

The key advantages of through bolts in rectangular CFST columns include:

Enhanced confinement: The bolts act as internal reinforcement, restricting lateral expansion of the concrete and preventing premature outward bulging of the steel tube walls.
Shear transfer: The bolts provide direct shear transfer between the steel tube and the concrete core, improving composite action.
Crack control: The bolts help control the propagation of diagonal shear cracks in the concrete core, improving post-peak ductility.

Numerical Analysis Methodology

The authors used ANSYS finite element models calibrated against experimental test data as the baseline for parametric studies. This approach allows for efficient investigation of multiple parameters without the cost and time of physical testing. The key parameters studied include:

Parameter	Description	Effect on Performance
Bolt diameter	Diameter of through bolts	Larger diameter increases confinement and bearing capacity
Bolt longitudinal spacing	Spacing between adjacent bolts along column height	Smaller spacing improves confinement, ductility, and deformation capacity
Confinement ratio	Ratio of steel cross-sectional area to concrete area	Higher ratio increases bearing capacity
Width-to-thickness ratio	Steel tube width divided by wall thickness	Lower ratio (thicker walls) increases bearing capacity
Aspect ratio	Ratio of column width to depth	Influences failure mode and stress distribution
Material strength	Concrete compressive strength and steel yield strength	Higher strength increases bearing capacity

Key Findings and Results

Confinement Effect Enhancement

The study found that through bolts significantly improve the confinement effect of the rectangular steel tube on the core concrete. The stress distribution in the concrete core exhibits a distinctive pattern: the through bolts serve as boundaries dividing the core into two effective confinement zones, with the maximum stress values occurring at the four corners of the column cross-section. This observation is consistent with the theoretical understanding that rectangular CFST columns exhibit non-uniform confinement, with the corners receiving the highest constraint from the steel tube geometry.

Parameter Influence on Bearing Capacity

Bolt longitudinal spacing: Decreasing the spacing between bolts leads to improved bearing capacity, deformation capacity, and ductility. This is because closer bolt spacing provides more frequent lateral restraint against concrete dilation.
Bolt diameter: Increasing bolt diameter enhances the bearing capacity and ductility, as larger bolts provide greater tensile resistance against concrete lateral expansion.
Confinement ratio: Increasing the confinement ratio produces a moderate increase in bearing capacity. The effect is less pronounced than that of bolt spacing and diameter.
Width-to-thickness ratio: Reducing this ratio (i.e., using thicker steel tube walls) modestly increases bearing capacity by improving the local buckling resistance of the steel tube.
Material strength: Higher concrete and steel strengths contribute to increased bearing capacity, but the improvement is relatively modest compared to geometric parameters.

Proposed Calculation Method

The authors developed a calculation formula for the axial compression bearing capacity of rectangular CFST short columns with through bolts. The method was validated against experimental data and demonstrated good agreement between calculated and measured values. Importantly, the proposed method is also applicable to rectangular CFST short columns with confinement tie bars, extending its utility to a broader range of structural applications.

Engineering Practice Integration

From a practical engineering standpoint, the findings of this study have several important implications:

Design optimization: The parametric study results enable engineers to optimize through bolt configuration (diameter, spacing) for specific loading conditions and performance requirements. For example, in seismic design, where ductility is paramount, closer bolt spacing may be preferred even if it does not proportionally increase bearing capacity.
Cost-benefit analysis: The study provides a basis for cost-benefit analysis of through bolt reinforcement. Since bolt diameter and spacing have the most significant effects on performance, engineers can focus optimization efforts on these parameters to achieve the desired performance at minimum cost.
Code compliance: The proposed calculation method can serve as a basis for code provisions governing the design of through bolt-reinforced rectangular CFST columns. As these columns find increasing use in high-rise buildings and bridges, standardized design methods are essential for widespread adoption.
Construction considerations: Through bolts must be installed prior to concrete pouring, requiring careful coordination between steel fabrication, bolt placement, and concrete placement operations. The bolt spacing and diameter must be compatible with the available space within the steel tube and the practical limits of construction equipment.

Key Questions and Reflections

The study focuses on short columns, where flexural effects are minimal. The applicability of the proposed calculation method to slender columns, where bending and axial load interaction is significant, requires further investigation.
The cyclic loading behavior of through bolt-reinforced CFST columns, relevant to seismic design, is not addressed. The performance of the through bolts under repeated loading and unloading, including potential bolt fracture or pull-out, warrants experimental study.
The interaction between through bolts and the steel tube under local buckling conditions is not explicitly modeled. In practice, the bolts may provide some restraint against outward buckling of the steel tube walls, but this effect is not captured in the proposed calculation method.
The study does not address the effect of bolt corrosion on long-term performance. In aggressive environments, corrosion of the through bolts could reduce their confinement effectiveness over time.

Study Insights and Implications

This study makes a valuable contribution to the understanding and design of rectangular CFST columns with through bolt reinforcement. The identification of bolt longitudinal spacing and diameter as the most influential parameters provides clear guidance for design optimization, and the proposed calculation method offers a practical tool for engineers. The finding that the through bolts create two distinct confinement zones with maximum stress at the corners is particularly insightful, as it reveals the mechanism by which through bolts enhance the inherently non-uniform confinement of rectangular CFST sections. For the structural engineering community, this work advances the state of the art in CFST column design and provides a foundation for future research into the seismic performance, slender column behavior, and long-term durability of through bolt-reinforced rectangular CFST members.