Seismic Performance of Prestressed Steel Strip Confined Concrete-Filled Steel Tube Columns

Overview and Research Context

This study investigates the seismic behavior of concrete-filled steel tube (CFST) columns enhanced by prestressed steel strips, a hybrid confinement system designed to address the well-known ductility deficiency of conventional CFST columns under cyclic lateral loading. As an engineer who has spent decades working with steel tube structures and their weld integrity, I find the concept of adding an external prestressed steel strip constraint particularly intriguing because it introduces a fundamentally different load-path mechanism — one that leverages active confinement rather than relying solely on the passive interaction between the steel tube and the infill concrete. The research addresses a critical gap in the seismic design of high-rise and bridge structures where large-diameter CFST columns are routinely employed, yet their post-peak behavior often becomes brittle once the steel tube yields and local buckling initiates.

Core Technical Findings

The prestressed steel strip system functions as an additional active confining ring that maintains compressive stress on the concrete core even after the steel tube has undergone significant plastic deformation. Key findings from the research include:

• The prestressed strip effectively delays the onset of concrete core crushing by maintaining lateral confining pressure throughout the loading cycle
• Peak load capacity increases by approximately 15-25% compared to conventional CFST columns of identical geometry
• Ductility improvement is substantial, with displacement ductility ratios reaching 6-8 times the elastic displacement, compared to 3-4 for unconfined CFST
• The energy dissipation capacity improves markedly due to the sustained confinement preventing premature concrete spalling and steel tube local buckling

From a welding and fabrication standpoint, the attachment of prestressed steel strips introduces additional weld details at the strip-to-tube junctions and at the strip splices. These welds must be designed to accommodate the cyclic tensile loading imposed by the prestress force and the seismic response. The weld configuration typically involves fillet welds or groove welds at the anchor plates, and the fatigue life of these connections under low-cycle fatigue becomes a governing design consideration.

Welding and Fabrication Implications

The integration of prestressed steel strips into the CFST column assembly raises several quality control concerns that warrant careful attention during fabrication:

1. The anchor plate welds connecting the prestressed strip to the column must achieve full penetration and be inspected by ultrasonic testing (UT) per GB/T 11345 or equivalent standards
2. Residual stresses from the welding of anchor plates may interact adversely with the prestress forces, potentially reducing the effective prestress level by 5-10% if not properly accounted for
3. The thermal distortion from welding must be controlled to prevent geometric imperfections that could initiate local buckling at lower loads than predicted
4. Material compatibility between the steel strip grade (typically Q345 or Q460) and the steel tube grade (Q235-Q460) must be verified to prevent galvanic corrosion in humid environments

The heat-affected zone (HAZ) at the anchor welds is particularly critical because these regions experience combined tensile and bending stresses during seismic loading. Microstructural examination of the HAZ should confirm that no excessive grain growth or martensitic transformation has occurred, which would compromise toughness. Charpy V-notch testing at the HAZ, particularly at the weld toe, is essential to verify that impact energy values remain above the minimum specified in the applicable design standard.

Engineering Practice and Quality Control

In practical implementation, the prestressing sequence must be carefully coordinated with the construction schedule. The prestress should be applied after the concrete core has achieved sufficient strength (typically 75% of design strength) to ensure that the confinement effect is fully mobilized. The prestress level is typically calibrated to 60-70% of the yield strength of the steel strip to provide adequate margin against fatigue failure while still delivering meaningful confinement.

Non-destructive testing (NDT) protocols for these hybrid columns should include:

• Magnetic particle testing (MT) on all anchor welds to detect surface and near-surface cracks
• Ultrasonic testing (UT) for volumetric defects in groove welds
• Visual inspection of the steel tube outer surface for geometric deviations exceeding 1/1000 of the member length
• Hydrostatic testing of the column assembly if the design requires leak-tightness in the confined concrete

The research provides valuable insight into the synergistic interaction between active confinement and the inherent composite action of CFST members. For engineers involved in the design and construction of seismic-resistant steel structures, this work demonstrates that modest additions in fabrication complexity — specifically the prestressed strip system — can yield disproportionately large improvements in seismic performance. The key engineering challenge lies in ensuring that the additional weld details and connections maintain their integrity over the full range of seismic demand, which requires rigorous weld design, material selection, and quality control throughout the fabrication and erection process.