Seismic Performance of Externally Steel-Jacketed Seismic-Damaged CFST Columns

Literature Overview

This study by Xu Chengxiang, Yang Bing, Zhao Bin, Zhang Jicheng, and Peng Wei (Yangtze University and HNA Industrial Group, 2015) investigates the feasibility and effectiveness of externally steel-jacketed reinforcement for seismic-damaged square concrete-filled steel tube (CFST) columns. Funded by the National Natural Science Foundation of China (Grant No. 51178057) and the Hubei Provincial University Excellent Young and Middle-aged Scientific and Technological Innovation Team Program (T201303), the research was published in the Journal of Guangxi University (Natural Science Edition), Vol. 40, No. 4, pp. 821-830.

Experimental Program and Specimen Design

The experimental program included four square CFST column models designed according to current design codes:

Specimen	Condition	Purpose
Specimen 1	Undamaged prototype	Baseline comparison
Specimen 2	Directly jacketed (no pre-damage)	Jacket performance without damage
Specimen 3	Moderately damaged + jacketed	Medium earthquake damage scenario
Specimen 4	Severely damaged + jacketed	Major earthquake damage scenario

All specimens underwent low-cycle reversed loading tests to evaluate the seismic performance of the externally steel-jacketed reinforcement approach.

Test Results and Key Findings

Performance Indicator	Prototype (Undamaged)	Jacketed (No Damage)	Jacketed (Moderate Damage)	Jacketed (Severe Damage)
Failure Mode	Flexural-shear	Flexural (strong shear, weak bending)	Flexural	Flexural
Ultimate Load Capacity	Baseline	Significantly increased	Increased	Increased
Ductility Factor	Baseline	Increased	Increased	Increased
Energy Dissipation	Baseline	Improved	Improved	Improved
Design Objective Met	Yes	Yes	Yes	Yes

Key findings include:

The externally steel-jacketed columns exhibit flexural-shear failure, satisfying the "strong shear, weak bending" seismic design objective.
Ultimate load capacity is significantly improved with external steel jacket reinforcement.
Ductility and energy dissipation capacity are enhanced.
Under moderate damage levels, the jacketed column can restore seismic performance to pre-damage levels.
The external steel jacket method is an effective seismic reinforcement approach for damaged CFST columns.

External Steel Jacket Design and Fabrication

The external steel jacket reinforcement method involves wrapping a steel jacket around the damaged CFST column, with appropriate connection details to ensure load transfer. Key design considerations include:

Design Parameter	Typical Value	Design Basis
Jacket steel grade	Q235 or Q345	Compatibility with existing structure
Jacket wall thickness	4-10 mm	Required confinement pressure
Jacket height	Column height or story height	Structural continuity
Connection method	Bolted or welded	Construction accessibility
Grout fill	High-strength grout	Load transfer between jacket and existing column

From a welding perspective, the external jacket introduces several welding considerations:

Welding to existing tube surface: The jacket welds connect to the outer surface of the existing steel tube, which may have surface imperfections, corrosion, or paint that must be removed before welding.
Preheating requirements: If the existing tube is Q345 or higher grade, preheating may be required to prevent cold cracking, particularly in cold weather conditions.
Weld sequence: The welding sequence should minimize distortion, with symmetric welding from the center outward or using back-step welding.
Weld inspection: All jacket welds should be inspected by UT and MT per applicable codes (GB 50661, AWS D1.1).

Damage Assessment and Repair Strategy

The study addresses an important practical question: when and how to repair seismic-damaged CFST columns. The damage assessment criteria used in this study are:

Damage Level	Description	Repair Strategy
Minor	Visible cracks, minor deformation	Monitoring, no immediate repair
Moderate	Significant deformation, local buckling	External steel jacket reinforcement
Severe	Major deformation, concrete crushing	External steel jacket + structural replacement
Collapse	Structural failure	Complete replacement

The external steel jacket method is most effective for moderate damage levels, where the existing structure retains sufficient integrity to serve as a core for the reinforced column. For severe damage, the jacket alone may be insufficient, and structural replacement may be necessary.

Engineering Practice and Retrofit Applications

The external steel jacket reinforcement method has several practical advantages for seismic retrofitting:

Minimal disruption: The method can be applied externally without requiring access to the concrete core or removal of existing finishes.
Speed of construction: Jacket fabrication and installation can be completed relatively quickly, minimizing building downtime.
Reversibility: In principle, the jacket can be removed if future modifications are required, unlike methods that involve drilling or cutting into the existing structure.
Cost-effectiveness: Compared to complete column replacement, external jacketing is significantly less expensive and less disruptive.

For steel pipe suppliers, the external jacket application creates demand for:

High-quality steel plates with consistent mechanical properties
Precision-cut and drilled jacket components
Appropriate coatings for corrosion protection in various environments
Welding consumables qualified for the specific steel grades involved

Key Reflections and Study Insights

This research provides valuable evidence supporting the external steel jacket method as an effective seismic reinforcement strategy for damaged CFST columns. The finding that moderate damage can be effectively addressed through jacket reinforcement has significant implications for post-earthquake recovery and retrofitting programs.

From a welding and fabrication standpoint, the study highlights the importance of weld quality in retrofit applications, where access may be limited and existing surface conditions may be less than ideal. Rigorous surface preparation, qualified welding procedures, and comprehensive NDT are essential to ensure the effectiveness of the reinforcement.

The study also raises important questions about long-term durability of the jacketed connection, particularly in environments where corrosion or freeze-thaw cycling may affect the grout interface or the jacket welds. Future research should address these durability concerns through accelerated corrosion testing and long-term monitoring of retrofitted structures.

The practical significance of this work extends beyond individual building retrofitting to broader disaster recovery planning, where rapid assessment and effective repair of damaged structures are critical for community resilience.