Seismic Performance of Square Steel Tube Concrete Columns Strengthened by Steel Jacketing

Literature Overview

This 2015 paper by Yang Bing and colleagues from Yangtze University and Wuhan University of Science and Technology investigates the seismic performance of square steel tube concrete columns strengthened using two different steel jacketing methods. The study involves low-cycle reversed loading tests on six column specimens and examines the influence of jacketing method, steel jacket wall thickness, welding, and strengthening height on the seismic performance. The research is directly relevant to my expertise in steel pipe and pipe fitting technology because it deals with the mechanical behavior of square steel tubes under complex combined loading conditions involving axial compression and cyclic lateral loading.

Test Configuration and Key Parameters

The experimental program included six square steel tube concrete column specimens, which were subjected to low-cycle reversed loading to simulate seismic action. The two strengthening methods studied were the outer steel jacketing method, where the steel jacket is installed on the exterior of the existing column, and the internal steel jacketing method, where the steel jacket is installed inside the existing column. The study systematically varied the steel jacket wall thickness, the presence or absence of welding between the jacket and the existing structure, and the height of the strengthened zone.

Influence of Strengthening Parameters

The study found that the internal steel jacketing method significantly improves the bearing capacity and stiffness of the column while also improving ductility to some extent. In contrast, the outer steel jacketing method provides limited improvement in bearing capacity but substantially enhances the ductility of the column, significantly improving its seismic performance. This distinction is important for engineering practice because the choice of strengthening method should be guided by the specific performance requirements of the structure.

Welding Quality and Its Role in Strengthening Performance

The study explicitly examines the effect of welding between the steel jacket and the existing column on the strengthening effectiveness. The finding that welded connections produce better performance is entirely consistent with my experience in pipe fitting welding and quality control. The weld joint provides a continuous load transfer path between the jacket and the existing structure, preventing relative slip and ensuring that the jacket and the column act as a composite member.

From a welding metallurgy perspective, the quality of the weld joint is critical to the overall performance of the strengthened column. The weld must achieve full fusion between the steel jacket and the existing structure, without defects such as incomplete fusion, porosity, or cracks. The heat-affected zone must have adequate toughness to resist crack initiation under cyclic loading. The residual stresses introduced by welding must be managed to prevent stress concentrations that could initiate fatigue cracks during seismic loading.

Welding Procedure Considerations

The welding of steel jackets to existing concrete columns presents several challenges that require careful procedural control:

1. Surface preparation of the existing steel tube surface to remove rust, paint, and contaminants to ensure proper weld fusion
2. Selection of appropriate welding process, such as SMAW or GTAW, depending on accessibility and joint configuration
3. Control of heat input to prevent excessive distortion of the steel jacket and avoid damaging the existing concrete
4. Implementation of post-weld inspection, including visual testing, magnetic particle testing, and ultrasonic testing, to verify weld quality
5. Consideration of post-weld heat treatment if required to relieve residual stresses and improve weld toughness

The welding quality requirements for seismic strengthening applications are particularly stringent because the weld joints are subjected to cyclic loading during earthquakes. Fatigue resistance of the weld is essential, and the weld geometry should avoid sharp notches or stress concentrators. This is directly analogous to the welding quality requirements for offshore platforms and marine structures, where weld fatigue performance is a primary design consideration.

Seismic Design Philosophy and Strengthening Mechanism

The study emphasizes that the steel jacketing method effectively ensures the "strong shear, weak bending" seismic ductility design objective for reinforced concrete columns. This is a fundamental principle in seismic design, which requires that the plastic hinge mechanism form in the bending zones rather than in the shear zones. The steel jacketing provides confinement to the concrete core, preventing premature shear failure and promoting ductile bending failure.

The confinement mechanism of the steel jacket is similar to the confinement effect provided by spiral reinforcement in reinforced concrete columns or by the steel tube in concrete-filled steel tube columns. The jacket restrains the lateral expansion of the concrete under axial compression, which increases the compressive strength and ductility of the concrete core. This triaxial stress state in the confined concrete is well understood from materials science and concrete mechanics perspectives.

The hysteresis curves and skeleton curves presented in the study demonstrate the typical behavior of ductile structural members under cyclic loading. The area enclosed by the hysteresis curves represents the energy dissipation capacity, which is a key measure of seismic performance. The strength and stiffness degradation coefficients quantify the progressive damage accumulation under cyclic loading, which is essential for performance-based seismic design.

Engineering Practice Implications

For engineers involved in seismic retrofitting of existing structures, this study provides clear guidance on the selection of strengthening methods. When the primary objective is to increase bearing capacity, the internal steel jacketing method is more effective. When the primary objective is to improve ductility and energy dissipation, the outer steel jacketing method is preferred. In many practical situations, a combination of both methods may be the optimal solution.

From a materials and fabrication standpoint, the steel jacket must be manufactured with appropriate steel grade and wall thickness to achieve the desired confinement pressure. The steel grade should have adequate yield strength and ductility to prevent brittle fracture under seismic loading. The fabrication process, including cutting, bending, and welding, must be controlled to ensure dimensional accuracy and weld quality.

The study also highlights the importance of strengthening height. A taller strengthening zone provides better performance because it ensures that the plastic hinge forms within the strengthened zone rather than at the boundary between the strengthened and unstrengthened regions. This is analogous to the importance of proper heat-affected zone control in welding, where the transition zone between the weld and the base metal must have adequate properties to prevent crack initiation.

In conclusion, this paper provides valuable experimental data and practical guidance for the seismic strengthening of square steel tube concrete columns using steel jacketing methods. The findings on welding quality, jacket configuration, and strengthening parameters are directly applicable to engineering practice and reinforce the importance of composite action and quality-controlled fabrication in structural strengthening applications.