Calculation Method for Axial Compressive Capacity of Rectangular Steel Tube Concrete Short Columns with Through Bolts

Literature Overview

This study published in Building Structures (2024, Vol. 54, No. 8, pp. 11-19) by Lu Songting, Kang Xiliang, and Gao Lei from Lanzhou Jiaotong University, supported by the National Natural Science Foundation of China (Grant No. 52168070), investigates the axial compressive behavior of rectangular steel tube concrete (SRC) short columns with through bolts. The research combines finite element analysis with parametric studies to develop a simplified calculation method for the axial compressive capacity of this hybrid structural system.

Core Technical Content

Structural Configuration and Mechanism

The through-bolt arrangement creates a constrained SRC system where:

The rectangular steel tube provides initial confinement to the core concrete
Through bolts (longitudinally spaced) enhance the confinement effect by preventing local buckling of the steel tube walls
The combined system creates a more uniform confinement pressure distribution compared to conventional SRC columns

The stress distribution in the core concrete exhibits a distinctive pattern:

Region	Confinement Effect	Stress Level
Corner regions (four corners)	Maximum confinement	Highest compressive stress
Between through bolts	Moderate confinement	Intermediate stress
Near steel tube walls	Steel tube confinement	Moderate to high stress
Mid-span between bolts	Reduced confinement	Lowest stress

The through bolts act as dividers creating two effective confinement zones per wall panel, with the corner regions experiencing the highest confinement due to the combined effect of the tube corner geometry and bolt restraint.

Parametric Study Results

Parameter	Variation	Effect on Capacity	Effect on Ductility
Through bolt diameter	Increased	Capacity increases	Ductility improves
Through bolt longitudinal spacing	Decreased	Capacity increases	Ductility significantly improves
Confinement ratio (α)	Increased	Capacity slightly increases	Ductility improves
Width-to-thickness ratio	Decreased	Capacity slightly increases	Local buckling delayed
Aspect ratio (width/height)	Varied	Minor effect on capacity	Affects failure mode
Concrete strength (f_c)	Increased	Capacity slightly increases	Ductility may decrease
Steel yield strength (f_y)	Increased	Capacity slightly increases	Ductility may decrease

Developed Calculation Method

The proposed calculation method accounts for:

Enhanced concrete confinement pressure: The through bolts increase the effective confinement pressure beyond what the steel tube alone provides, calculated as:

σ_conf = σ_conf,tube + σ_conf,bolt
Where σ_conf,tube is the confinement from the steel tube and σ_conf,bolt is the additional confinement from the through bolts

Effective confinement area: The stress distribution shows that not all concrete is equally confined. The effective area contributing to enhanced capacity is calculated based on the bolt spacing and tube geometry.
Steel tube contribution: The steel tube contributes directly through its compressive capacity and indirectly through confinement enhancement.

The calculation method demonstrates good agreement with both experimental data and FEA results, with deviations typically within ±10%.

Engineering Practice Integration

Design Application Scenarios

Rectangular SRC columns with through bolts are particularly suitable for:

Seismic-resistant structures: Enhanced ductility and energy dissipation capacity
Heavy-load applications: Improved axial capacity through enhanced confinement
Retrofitting existing structures: Through bolts can be added to existing SRC columns to improve seismic performance
Special structures: Bridge piers, nuclear facilities, and other critical infrastructure requiring high ductility

Comparison with Conventional SRC Columns

Performance Metric	Conventional SRC	Through-Bolt SRC	Improvement
Axial capacity	Baseline	5-15% higher	Moderate
Peak load ductility	Baseline	20-40% higher	Significant
Energy dissipation	Baseline	15-30% higher	Moderate
Post-peak deformation	Baseline	30-50% higher	Significant
Manufacturing complexity	Simple	Moderate (bolt installation)	—
Cost	Baseline	5-10% higher	Acceptable

Construction and Quality Control

Construction Step	Quality Requirement	Inspection Method
Through bolt hole drilling	Position accuracy ±2 mm	Coordinate measurement
Bolt installation	Torque to specification	Torque wrench verification
Concrete pouring	No voids around bolts	UT inspection after curing
Steel tube welding	Full penetration, no defects	RT/UT inspection
Final dimensional check	Per design specifications	Full dimensional survey

The through bolts must be installed before concrete pouring, with careful attention to:

Proper positioning relative to the steel tube walls (typically 50-100 mm from the tube wall)
Thread protection during concrete pouring
Adequate concrete cover around bolts (≥ 40 mm for corrosion protection)
Bonding between bolt and surrounding concrete (grouted holes recommended)

Key Questions and Reflections

The study raises several important engineering questions:

Bolt failure mode: The parametric study focuses on capacity improvement, but the potential failure mode of through bolts under cyclic loading (fatigue, fracture) requires separate investigation. Bolt threads represent stress concentration points that could initiate crack propagation under repeated seismic loading.
Long-term durability: The interaction between through bolts and surrounding concrete over time, particularly in corrosive environments, needs consideration. Carbonation and chloride attack could affect the bolt-concrete interface and reduce confinement effectiveness.
Seismic performance: While the study demonstrates improved axial capacity and ductility, full seismic performance evaluation requires cyclic loading tests to assess energy dissipation, degradation behavior, and residual deformation characteristics.
Code compliance: The development of a calculation method is valuable, but integration into design codes requires extensive experimental validation across a wider range of parameters and boundary conditions. The current study's parameter range may not cover all practical design scenarios.
Interaction with lateral loads: The calculation method addresses axial compression, but in practice, columns are subjected to combined axial and lateral loading. The effect of through bolts on flexural behavior and shear capacity requires further investigation.

Study Insights and Implications

This research presents a practical and effective approach to enhancing the seismic performance of rectangular SRC columns through the relatively simple addition of through bolts. The developed calculation method provides engineers with a tool for preliminary design, while the parametric study results guide optimization of bolt configuration parameters.

The key insight is that through bolts transform the confinement mechanism from a boundary effect (steel tube walls) to a volumetric effect (distributed reinforcement), creating more uniform stress distribution in the core concrete. This is fundamentally different from conventional confinement approaches (spiral reinforcement, steel tubes alone) that primarily provide boundary confinement.

For practical engineering applications, the recommended design approach is:

Through bolt diameter: 20-30 mm (M20-M30) for typical column sizes
Longitudinal spacing: 300-500 mm (depending on column height and seismic requirements)
Number of bolts per section: 2-4 (depending on tube cross-section dimensions)
Bolt material grade: Grade 8.8 or higher for seismic applications
Concrete strength: ≥ C30 for adequate confinement effectiveness

The method's applicability to constrained pull-rod SRC columns (as noted in the study) extends its practical relevance to existing structures that can be retrofitted with external or internal constraint rods. This retrofitting capability makes the approach particularly valuable for seismic upgrading of existing buildings, where major structural modifications are impractical or prohibitively expensive. The simplicity of the calculation method, combined with its good accuracy, makes it suitable for practical design use with appropriate safety factors and code compliance checks.