Eccentric Compression Performance of Square Steel Tube Concrete Columns with Built-in High-Strength Angle Steel

Literature Overview and Research Motivation

The paper by Wu Bo, Wang Hui, and Liu Xiaozhu (2021), published in Journal of Architecture and Civil Engineering (Vol. 38, No. 2, pp. 16–25), investigates the eccentric compression behavior of square steel tube concrete (SRC) columns with built-in high-strength angle steel. Funded by the National Key R&D Program (2016YFC0701203) and the National Natural Science Foundation (51778240), this research addresses an innovative structural concept aimed at enhancing the performance of square SRC columns without increasing total steel consumption. The study combines experimental testing with analytical modeling to establish a practical design methodology.

Experimental Program and Key Parameters

The researchers conducted eccentric compression tests on eight built-in angle steel SRC columns and two conventional SRC columns for comparison. The experimental variables included:

Parameter	Description	Range / Levels
Steel ratio (取钢率)	Ratio of angle steel mass to total steel mass	Multiple levels tested
Net clearance	Gap between angle steel and inner steel tube wall	Multiple levels tested
Eccentricity ratio	Ratio of eccentric distance to column section dimension	Multiple levels tested
Total steel consumption	Total mass of steel per column	Kept approximately constant

The steel ratio is defined as the mass of angle steel divided by the total steel mass used in the column. This parameter allows researchers to systematically vary the contribution of the built-in angle steel while maintaining constant total steel consumption, enabling a fair comparison with conventional SRC columns.

Key Experimental Findings

The study produced several important conclusions:

Equivalence of performance: When the steel ratio and net clearance are appropriately designed, built-in angle steel SRC columns can achieve eccentric compression performance comparable to conventional SRC columns with the same total steel consumption.
Net clearance dominance: The net clearance between the angle steel and the inner steel tube wall has a more pronounced effect on eccentric compression capacity than the steel ratio itself.
Stiffness and ductility: When total steel consumption is held constant, the equivalent stiffness and ductility of built-in angle steel columns show limited variation compared to conventional columns.
Full strength utilization: At peak eccentric compression load, the high-strength angle steel in both compression and tension zones has fully yielded, indicating efficient material utilization.
Analytical model accuracy: The proposed practical calculation method for eccentric compression capacity demonstrated good accuracy for built-in angle steel SRC columns.

Technical Analysis and Engineering Practice

Mechanism of Improved Performance

The built-in angle steel concept leverages the high yield strength of structural angle sections (typically Q345 or Q390 grade) to enhance the load-carrying capacity of SRC columns. The angle steel, positioned at the corners or edges of the square steel tube, effectively increases the section modulus and moment of inertia without significantly increasing the overall steel consumption. The net clearance between the angle steel and the steel tube inner wall plays a critical role in facilitating concrete confinement and ensuring composite action.

Connection to Steel Tube Manufacturing

From a steel pipe manufacturing perspective, this research has several implications:

Surface quality requirements: The inner surface of the square steel tube must be smooth enough to allow proper concrete infill around the built-in angle steel. Surface roughness or weld bead protrusion on the inner wall can compromise the bond between concrete and steel tube.
Dimensional tolerances: The precision of the square steel tube's inner dimensions directly affects the fit of the built-in angle steel. Tolerances should be controlled to ensure uniform net clearance around the angle steel.
Welding considerations: If the angle steel is welded to the steel tube (rather than simply placed), the welding process must be carefully controlled to avoid distortion of the steel tube and to ensure proper fusion without excessive heat input that could degrade the base metal properties.

Comparison with Conventional SRC Columns

Aspect	Conventional SRC Column	Built-in Angle Steel SRC Column
Steel distribution	Uniform (tube wall only)	Concentrated at corners/edges
Moment of inertia	Lower	Higher (steel at extreme fibers)
Concrete confinement	Uniform	Potentially enhanced near corners
Fabrication complexity	Lower	Higher (angle steel placement)
Cost efficiency	Baseline	Comparable (same total steel)

Study Insights and Design Recommendations

This research presents a promising approach to optimizing SRC column design through strategic steel placement rather than simply increasing steel quantity. The finding that net clearance is more influential than steel ratio suggests that careful geometric design of the angle steel position is critical. Engineers should aim for a net clearance that is large enough to allow proper concrete flow and vibration but small enough to maintain composite action and prevent buckling of the angle steel.

The proposed analytical model provides a practical tool for design engineers, though its application should be validated against additional experimental data for different column geometries and loading conditions. The model's accuracy is particularly important for seismic design, where accurate prediction of ductile behavior and energy dissipation capacity is essential.

For steel tube manufacturers, this research suggests a potential market for high-precision square steel tubes with controlled inner surface quality, specifically designed for SRC applications with built-in reinforcement. The dimensional and surface quality requirements for such tubes may exceed those specified in general-purpose standards, necessitating dedicated manufacturing specifications and quality control procedures.