Post-Earthquake Repair Performance of Externally Confined RC-CFST Pier Footings

Literature Overview

This study by Wang Xuanding, Gu Chao, Liao Yue, and Qiao Yunqiang, published in the China Civil Engineering Journal in 2024, addresses a critical practical issue in bridge engineering: the seismic repair and restoration of externally confined reinforced concrete (RC) concrete-filled steel tube (CFST) pier footings. The research was conducted at Chongqing University in collaboration with Lin Ting International Engineering Consulting (China) Co., Ltd. The study focuses on a specific damage pattern observed in externally confined RC-CFST piers, where plastic hinges form above the external confinement zone at the pier footing, and proposes a repair method involving embedded dowels and elevated external confinement layers.

Core Technical Findings

The study conducted pseudo-static testing on four repaired specimens to evaluate the effectiveness of the proposed repair method. The specimens were damaged to simulate post-earthquake conditions, and then repaired using the proposed technique before being re-tested to assess their restored seismic performance.

The proposed repair method effectively strengthens the damaged externally confined RC-CFST pier footings, reducing or preventing local buckling of the pier steel tube. The damage mechanism is successfully redirected from the CFST pier plastic hinge to the bottom of the footing confinement zone, which represents a more favorable failure mode for structural integrity. However, due to the presence of initial damage at the footing, the repaired specimens exhibit significantly lower lateral stiffness compared to the original specimens, and there exists a potential risk of fatigue fracture of damaged anchorage bars under low-cycle cyclic loading.

Interpretation of Technical Points

The research addresses a fundamental challenge in post-earthquake bridge engineering: how to effectively restore seismic capacity to damaged structures while acknowledging that complete restoration to pre-damage condition may not be achievable. The proposed repair method represents a pragmatic approach that prioritizes the most critical performance objectives: preventing steel tube local buckling (which can lead to sudden collapse) and ensuring adequate energy dissipation capacity.

The redistribution of damage from the pier body to the footing confinement zone is particularly significant. In the original design, the externally confined RC zone at the footing was intended to protect the pier base from damage. However, under strong seismic loading, the plastic hinge may form above this confined zone, leading to damage in the CFST pier body where repair is more difficult and the consequences more severe. The proposed repair method effectively extends the confined zone upward, creating a more favorable damage pattern where the footing zone absorbs the energy through controlled deformation.

The observation that repaired specimens have lower lateral stiffness but higher energy dissipation capacity reflects the fundamental trade-off between stiffness and ductility. The initial damage at the footing creates additional flexibility that reduces stiffness but allows for greater inelastic deformation before failure. This characteristic is actually beneficial for seismic performance, as structures with greater ductility can survive larger displacements without collapse.

Process and Standards Analysis

The repair method involving embedded dowels and elevated external confinement layers requires careful attention to several fabrication and welding aspects. The dowel bars must be properly embedded into the existing concrete and steel tube, requiring precise drilling and grouting operations. The welding of new external confinement reinforcement to the existing steel tube must ensure proper fusion and avoid thermal damage to the existing structure.

The welding of new reinforcement to the existing steel tube presents unique challenges due to potential differences in steel grade, heat treatment history, and residual stress state. Preheating may be required to prevent cold cracking, particularly if the existing steel tube has a high carbon equivalent. The welding procedure should be qualified through coupon testing, and welders should be certified for the specific process and position.

Relevant standards include GB 50011 for seismic design of buildings, JTG/T 2232 for bridge seismic design, and GB 50550 for fiber-reinforced polymer strengthening of concrete structures. The repair design should also comply with applicable codes for steel structure repair and rehabilitation.

Integration with Engineering Practice

This research has direct practical implications for bridge maintenance and post-earthquake emergency response. The proposed repair method provides a systematic approach to restoring seismic capacity to damaged CFST piers, which are increasingly used in modern bridge construction due to their superior strength and ductility characteristics.

In practical implementation, the repair process must be carefully sequenced to ensure structural stability during construction. Temporary shoring and support systems should be designed to maintain the pier's load-carrying capacity while repair operations are performed. The repair should be designed to accommodate the existing damage pattern and residual deformations, rather than attempting to restore the original geometry.

From a cost-benefit perspective, the repair method must be evaluated against the alternative of complete pier replacement. The decision should consider not only direct costs but also indirect costs such as traffic disruption, environmental impact, and long-term maintenance requirements. The proposed repair method offers a viable middle ground between complete replacement and simple patch repair.

Key Questions and Reflections

A significant concern raised by the study is the potential fatigue fracture of damaged anchorage bars under cyclic loading. This represents a long-term durability issue that may not manifest during the initial post-repair testing but could develop over the service life of the repaired structure. Engineers must consider this risk in the repair design and may need to incorporate additional monitoring or inspection requirements for the repaired pier.

Another important consideration is the applicability of the proposed repair method to different damage severities. The study focuses on a specific damage pattern, and the effectiveness of the repair method may vary for different types and extents of damage. Engineers should develop a damage assessment protocol to determine whether the proposed repair method is appropriate for a given damage condition.

The study also raises questions about the long-term performance of the repaired structure under repeated seismic events. While the repair method effectively restores seismic capacity for a single event, the cumulative effect of multiple earthquakes on the repaired structure requires further investigation. The repair design should ideally provide some reserve capacity for future seismic events.

Study Insights and Implications

This research makes a significant contribution to the field of post-earthquake structural repair and rehabilitation, particularly for CFST bridge piers. The proposed repair method demonstrates that effective restoration of seismic performance is achievable through targeted strengthening of the damaged zone, rather than requiring complete structural replacement.

For welding and fabrication professionals, this study highlights the importance of developing repair procedures that can be effectively implemented in field conditions. The welding and joining operations involved in the repair must be designed for practicality while maintaining the required quality standards. This may require the development of specialized welding procedures and inspection protocols tailored to repair applications.

The research underscores the principle that structural repair is not merely about restoring strength but about controlling the damage mechanism to ensure a predictable and survivable response under future seismic loading. The proposed method achieves this by redirecting damage to a more favorable location and preventing the catastrophic failure mode of steel tube local buckling.

Reference Value and Outlook

The research provides valuable guidance for the post-earthquake repair of CFST bridge piers, an area of increasing importance as more bridges are constructed using CFST technology. Future research should investigate the long-term performance of repaired piers under environmental exposure and repeated loading, develop simplified design procedures for practical repair applications, and extend the study to different pier configurations and damage scenarios. The development of performance-based repair criteria and inspection protocols would further facilitate the implementation of the proposed repair method in practice.