Comparative Experimental Study on Strengthening Methods for Earthquake-Damaged Square Steel Tube Concrete Columns

Literature Overview

This paper, published in the Journal of Guangxi University (Natural Science Edition, 2016, Vol. 41, No. 1, pp. 53-62) by Xu Chengxiang, Yang Bing, Lu Mengxiao, and Zha Xinfeng from Yangtze University and Wuhan University of Science and Technology, presents a comprehensive experimental investigation into two post-earthquake strengthening methods for square steel tube concrete (CFST) columns. The research was funded by the National Natural Science Foundation of China (Grant No. 51178057) and the Hubei Provincial University Excellent Young and Middle-aged Scientific Innovation Team Program (Grant No. T201303).

Experimental Program Design

The experimental program is notably rigorous, involving seven column specimens subjected to a three-stage testing protocol:

1. Initial pseudo-dynamic loading to simulate earthquake damage at different severity levels.
2. Repair and strengthening using either external steel jacketing or carbon fiber reinforced polymer (CFRP) wrapping.
3. Post-strengthening low-cycle reversed loading to evaluate the restored seismic performance.

This sequential testing approach is superior to many studies that only test either undamaged or pre-damaged specimens, because it captures the actual behavior of repaired structures under continued seismic demand.

Strengthening Methods Compared

The two strengthening methods evaluated are:

External Steel Jacketing

The steel jacketing method involves welding steel plates onto the exposed faces of the square CFST column. This method effectively increases the cross-sectional area and moment of inertia of the column, thereby enhancing both its axial load capacity and flexural stiffness. The welded connection between the jacket plates and the existing steel tube creates a composite action that is critical to the effectiveness of the strengthening.

From a welding quality perspective, several critical aspects must be controlled:

• The weld between the jacket plate and the existing steel tube must achieve full penetration to ensure composite action.
• Welding-induced residual stresses must be managed to avoid reducing the strength of the existing steel tube in the heat-affected zone.
• Preheating and interpass temperature control are essential when welding to thick steel sections, particularly if the existing tube contains high-strength steel grades.

CFRP Wrapping

The CFRP wrapping method involves bonding carbon fiber sheets to the external surface of the column using structural epoxy adhesive. This method provides confinement to the concrete core and arrests crack propagation, but does not significantly increase the axial load capacity of the column.

The effectiveness of CFRP wrapping depends critically on:

• Surface preparation of the steel tube (grinding to bare metal, cleaning)
• Quality of the epoxy adhesive bond
• Wrapping pattern and fiber orientation
• Overlap length and termination details

Test Results and Comparative Analysis

The experimental results reveal distinct performance characteristics for each method:

The steel jacketing method proved superior in restoring ultimate load capacity and stiffness, while the CFRP wrapping method excelled in improving ductility and energy dissipation capacity. Importantly, the study demonstrated that under certain damage levels, the strengthened columns could recover or even exceed their pre-damage seismic performance.

Damage Degree Classification

The study implicitly addresses the concept of damage classification, which is essential for determining whether a damaged column can be repaired and strengthened or must be replaced. Based on the experimental observations, damage levels can be categorized as:

1. Minor damage: Visible cracking in concrete, minor yielding of steel tube, no significant geometric deformation. Both strengthening methods are effective.
2. Moderate damage: Extensive cracking, significant yielding of steel tube, moderate local buckling. Steel jacketing is more reliable.
3. Severe damage: Severe local buckling, concrete spalling, significant geometric deformation. Replacement may be necessary rather than strengthening.

Engineering Practice Integration

The findings have direct implications for post-earthquake structural assessment and repair procedures. In practice, the decision between steel jacketing and CFRP wrapping should consider:

• Accessibility: CFRP wrapping requires less equipment and can be applied in confined spaces, while steel jacketing requires welding equipment and access to all column faces.
• Durability: Steel jackets are susceptible to corrosion and require maintenance, while CFRP composites are corrosion-resistant but may degrade under UV exposure or moisture ingress.
• Cost: CFRP materials are generally more expensive per unit area, but the labor cost of welding steel plates can be comparable.
• Seismic performance requirements: If the priority is restoring load capacity, steel jacketing is preferred. If the priority is improving ductility and energy dissipation, CFRP wrapping may be more appropriate.

Key Questions and Reflections

A significant question raised by this study is the long-term durability of the strengthened columns under subsequent seismic events. The tests evaluate performance under one additional seismic event after strengthening, but real structures may face multiple seismic events over their service life. The cumulative effect of repeated cyclic loading on the welded connections and adhesive bonds requires further investigation.

Another important consideration is the interaction between the strengthening method and the existing damage. The study tests columns damaged to different degrees, but the interface between the strengthening material and the damaged substrate may vary significantly. For instance, CFRP bonding to a heavily deformed steel tube surface may not achieve the same bond quality as bonding to an undamaged surface.

Study Insights and Implications

The most important practical insight from this research is that both strengthening methods are viable for post-earthquake repair of CFST columns, but they serve different purposes. Steel jacketing is the preferred method when load capacity restoration is the primary concern, while CFRP wrapping is more appropriate when ductility improvement is the goal. In many practical scenarios, a combination of both methods may provide the most balanced restoration of seismic performance.

For welding engineers involved in structural repair, this study highlights the importance of weld quality in achieving the full strengthening potential of steel jacketing. The welded connection must be designed and executed to ensure full composite action between the jacket and the existing column, which requires careful attention to weld procedure qualification, welder certification, and non-destructive testing of completed welds.