Seismic Performance of Self-Compacting Self-Stress Rectangular Steel Tube Concrete Columns

Literature Overview

The experimental study by Jia Hongyu, Li Aivei, Li Fengge, and Han Chonggang (2018), published in Bulletin of the Chinese Ceramic Society, investigates the seismic performance of rectangular steel tube concrete (CFST) columns filled with self-compacting self-stress concrete (SCSSC). Funded by the Inner Mongolia University of Science and Technology College of Civil Engineering Research Center (Project TMY201702), the study fabricated three specimens with different aspect ratios and subjected them to cyclic loading to evaluate hysteretic behavior, ductility, energy dissipation, and stiffness degradation.

Specimen Configuration and Test Parameters

The three test specimens varied in aspect ratio (length-to-width of the rectangular section), which is a critical geometric parameter affecting both the confinement efficiency of the steel tube and the susceptibility to shear failure:

Specimen	Aspect Ratio	Key Characteristic
ZJGC-1	Lowest	Baseline reference
ZJGC-2	Medium	7% increase in max load over ZJGC-1
ZJGC-3	Highest	17% increase in max load over ZJGC-1

Self-compacting self-stress concrete combines two advanced concrete technologies: self-compacting concrete (SCC) eliminates the need for vibration, ensuring uniform filling within the confined steel tube, while self-stress concrete (SSC) generates internal compressive stress through expansive hydration of expansive aggregates. The combination is particularly beneficial for CFST columns because the expansive pressure from SSC enhances the bond between the steel tube and core concrete, improving composite action.

Critical Failure Mechanism: Corner Weld Failure

The most significant finding of this study is the identification of premature corner weld tearing as the root cause of sudden load capacity reduction. This finding has profound implications for welding quality control in rectangular CFST column fabrication:

Corner Weld Failure Analysis

The rectangular steel tube is fabricated by welding four flat plates together, creating four longitudinal corner welds. Under cyclic lateral loading, the corner regions experience complex multiaxial stress states:

Tensile stress from the opening moment that tends to separate the plates at the corners.
Shear stress from the diagonal tension field that develops during inelastic deformation.
Bending stress from the local flexure of the corner region under concentrated forces.

The combination of these stress components creates a highly unfavorable stress state at the weld toe, where stress concentrations are inherently high. When the corner weld tears prematurely, the confinement effect is immediately lost, leading to rapid concrete crushing and catastrophic load drop.

Welding Quality Control Requirements

Based on the failure observations, the following welding quality requirements should be implemented for rectangular CFST columns:

Requirement	Specification	Rationale
Weld type	Full-penetration groove weld with backing	Prevents incomplete fusion at root
Process	SMAW or FCAW with qualified WPS	Ensures consistent weld metal properties
Preheat	100–150°C for thicknesses >15 mm	Reduces HAZ hardness and crack susceptibility
Interpass temperature	≤250°C	Prevents grain coarsening in HAZ
NDE	100% UT of all corner welds	Critical structural element
Surface preparation	Grind weld toes to R ≥ 3 mm	Reduces stress concentration factor

Aspect Ratio Effects on Seismic Performance

The study reveals a clear trend: as the aspect ratio increases, the seismic performance degrades despite the higher ultimate load capacity. This apparent paradox can be explained through the following mechanism:

Higher aspect ratio columns have greater lateral stiffness, leading to higher elastic loads.
The increased stiffness concentrates deformation in the corner weld regions rather than distributing it uniformly.
The corner welds in higher aspect ratio columns fail at lower displacement levels due to the unfavorable stress concentration.
Post-weld failure, the ductility and energy dissipation capacity drop significantly because the confinement is lost.

This finding suggests that for seismic applications, the aspect ratio of rectangular CFST columns should be limited to a range of 1.0–2.0, beyond which the benefits of increased load capacity are offset by reduced ductility.

Engineering Practice Recommendations

For engineers specifying rectangular CFST columns in seismic zones, the following recommendations emerge:

Welding sequence optimization: Weld corners in a symmetric sequence to minimize residual stress asymmetry that could accelerate corner weld failure under cyclic loading.
Post-weld treatment: Consider peening or shot peening of corner weld toes to introduce compressive residual stress that counteracts the tensile stress concentration.
Concrete filling procedure: Use self-compacting self-stress concrete with controlled expansion rate (0.01–0.03% linear expansion) to ensure uniform confinement pressure without excessive early-age stress on welds.
Aspect ratio limitation: For seismic design, limit the aspect ratio to 2.0 or less, with detailed fracture mechanics assessment for ratios approaching 2.0.

Study Insights and Independent Thinking

This experimental study provides critical evidence that the welding quality of rectangular CFST columns is not merely a fabrication concern but a fundamental structural performance issue. The identification of corner weld tearing as the primary failure mode shifts the focus of seismic design from traditional concrete and steel strength considerations to weld integrity as the governing factor. In practice, this means that seismic design codes for CFST structures should mandate specific welding quality levels for corner joints that exceed the requirements for general structural steel connections. The self-compacting self-stress concrete technology offers a promising material solution, but its benefits can only be fully realized if the steel tube fabrication quality—particularly the corner welds—is brought to an equivalent level of excellence. Future research should investigate hybrid approaches combining rectangular CFST with internal reinforcing plates at corner regions to provide additional redundancy against weld failure.