Zhuojin Pipe Fitting Co., LtdSeismic Performance of Fire-Damaged Concrete Columns Strengthened with Thin-Walled Steel Tubes
Literature Overview
This study, published in the Journal of Civil Engineering in 2017 (Vol. 50, No. 7, pp. 39–49) by Xu Yuye, Ren Yakuan, Dong Yuli, and Luo Yi from Huaqiao University, presents a comprehensive pseudo-static test program on 14 specimens: two undamaged reinforced concrete (RC) columns, two fire-damaged RC columns without strengthening, and ten fire-damaged RC columns strengthened with externally bonded thin-walled steel tubes. The research was funded by the National Natural Science Foundation of China (Grant 51578255), the Fujian Provincial Key Science and Technology Program (2014Y0033), and the Huaqiao University Young and Middle-aged Faculty Research Enhancement Program (ZQN-YX204). The primary objective was to investigate how axial compression ratio, shear span ratio, steel tube wall thickness, and strengthening configuration affect the seismic performance of fire-damaged RC columns after external steel tube confinement.
Core Experimental Design and Parameters
The test matrix was carefully designed to isolate the influence of each parameter. The specimens were subjected to controlled fire exposure to simulate post-fire degradation of concrete and reinforcement, followed by strengthening with thin-walled steel tubes of varying configurations. The key parameters studied included:
| Parameter | Levels Tested | Purpose |
|---|---|---|
| Axial compression ratio | Multiple levels | Evaluate load level effect on seismic response |
| Shear span ratio | 1.78 and 3.0 | Distinguish shear-dominated vs. flexure-dominated behavior |
| Steel tube wall thickness | Multiple thicknesses | Assess confinement effectiveness |
| Strengthening configuration | Tube only vs. tube with angle steel and bolt anchorage | Compare connection methods |
The pseudo-static loading protocol simulated cyclic lateral displacement to replicate seismic loading conditions, allowing measurement of hysteretic behavior, energy dissipation, stiffness degradation, and ductility.
Key Technical Findings
The experimental results demonstrated several important conclusions regarding the effectiveness of thin-walled steel tube strengthening on fire-damaged RC columns:
- Shear capacity improvement: For fire-damaged columns with a shear span ratio of 1.78, the strengthened specimens showed a 41.8% to 47.0% increase in shear capacity compared to unstrengthened counterparts. For shear span ratio 3.0, the improvement ranged from 38.5% to 74.4%, indicating that the strengthening effect is more pronounced in flexure-dominated members where confinement benefits are more fully realized.
- Stiffness recovery: The secant stiffness of strengthened specimens approached that of undamaged specimens, suggesting that the steel tube confinement effectively compensates for the stiffness loss caused by fire damage to the concrete and reinforcement.
- Energy dissipation and ductility: The cumulative hysteretic energy dissipation and ultimate deformation were significantly enhanced by the steel tube strengthening, with the wall thickness of the tube having a notable influence on energy dissipation capacity.
- Axial compression ratio effect: The axial compression ratio had a significant influence on shear capacity, stiffness, energy dissipation, and ductility of strengthened specimens, but had a relatively minor effect on ultimate deformation. This suggests that while higher axial loads reduce the overall seismic performance, the ductility margin provided by the steel tube remains relatively stable.
- Wall thickness effect: Thicker steel tube walls improved energy dissipation capacity more noticeably than shear capacity, indicating that the tube thickness primarily contributes to sustained cyclic performance rather than peak strength.
- Anchorage configuration: The configuration of welding angle steel at the tube base and anchoring with bolts significantly improved the overall strengthening effectiveness, likely by preventing local buckling and slippage at the tube-column interface.
Proposed Shear Capacity Formula
The authors proposed a practical calculation formula for the shear capacity of fire-damaged RC columns strengthened with externally bonded thin-walled steel tubes. The formula accounts for the residual shear capacity of the fire-damaged concrete and reinforcement, plus the additional shear contribution from the confined concrete within the steel tube. This formula represents a significant advancement for the practical design of post-fire rehabilitation of RC columns, providing engineers with a quantifiable tool for evaluating strengthening schemes.
Engineering Practice Implications
From a practical engineering standpoint, this research has several important implications:
- Post-fire rehabilitation strategy: The findings confirm that external steel tube confinement is an effective rehabilitation method for fire-damaged RC columns, particularly when combined with proper anchorage details at the tube base.
- Shear span ratio consideration: Engineers should note that the strengthening benefit is more pronounced in flexure-dominated regions (higher shear span ratio), which aligns with the fundamental mechanics of confinement effectiveness.
- Tube thickness selection: While thicker tubes improve energy dissipation, the marginal benefit on shear capacity diminishes, suggesting that thickness selection should balance seismic performance requirements against material cost and constructability.
- Anchorage detail criticality: The welding of angle steel and bolt anchorage at the tube base is not merely a construction detail but a critical factor in achieving the full strengthening potential.
Study Insights and Reflections
This study addresses a critical gap in post-earthquake and post-fire structural rehabilitation research. The combination of fire damage and seismic vulnerability is a well-recognized risk scenario in modern infrastructure, and the proposed strengthening method offers a practical solution. The relatively small difference in secant stiffness between strengthened and undamaged specimens is particularly encouraging, as it suggests that the original seismic design assumptions may be partially recoverable through this strengthening approach. However, engineers should note that the fire exposure conditions in the laboratory may not fully replicate the complexity of real fire scenarios, including uneven heating, spalling, and reinforcement exposure. The formula proposed should be validated against additional test data before widespread application in design codes. The use of thin-walled tubes rather than thick-walled ones also raises questions about the long-term corrosion protection of the steel tube in exposed environments, which warrants further investigation.