Zhuojin Pipe Fitting Co., LtdSeismic Performance of Fire-Damaged Concrete Columns Strengthened by Thin-Walled Steel Tubes
Literature Overview
This 2017 study published in the "China Civil Engineering Journal" by researchers from Huaqiao University investigates the seismic performance of fire-damaged reinforced concrete columns strengthened with externally wrapped thin-walled steel tubes. The research, supported by the National Natural Science Foundation of China (Grant 51578255), conducted pseudo-static tests on 14 specimens including 2 unexposed control columns, 2 fire-exposed unstrengthened columns, and 10 fire-exposed columns strengthened with thin-walled steel tubes.
Test Program and Key Parameters
The test program systematically varied several important parameters: axial compression ratio, shear span ratio, steel tube thickness, and strengthening method. The fire exposure was conducted according to standard fire curves, simulating realistic fire damage conditions. The pseudo-static loading was applied to evaluate the hysteretic behavior, energy dissipation capacity, and ductility of the strengthened columns.
The results demonstrate that thin-walled steel tube strengthening can significantly improve the shear capacity, ultimate deformation, and cumulative hysteretic energy dissipation of fire-damaged concrete columns. After strengthening, the secant stiffness of the specimens was comparable to that of the unexposed control specimens, indicating that the strengthening effectively restores the structural stiffness.
Quantitative Performance Improvements
The study provides specific quantitative data on the improvement achieved through strengthening:
| Parameter | Shear Span Ratio 1.78 | Shear Span Ratio 3.0 |
|---|---|---|
| Capacity improvement | 41.8% to 47.0% | 38.5% to 74.4% |
| Primary failure mode | Shear-flexural | Flexural |
The axial compression ratio had a significant influence on the shear capacity, stiffness, energy dissipation, and ductility of the strengthened specimens, but had a relatively minor effect on ultimate deformation. The steel tube thickness had a notable impact on energy dissipation capacity but a smaller influence on shear capacity.
Strengthening Method Optimization
An important practical finding is that welding angle steel at the root of the thin-walled steel tube and anchoring it with bolts improves the strengthening effectiveness. This method addresses the potential failure mechanism of the steel tube being pulled away from the column under cyclic lateral loading.
The welding details at the connection between the angle steel and the steel tube are critical for the effectiveness of the strengthening system. The welds must be designed to withstand the combined effects of axial compression, bending moment, and shear force during seismic loading. Proper welding procedures, including preheating for thick sections, controlled heat input, and post-weld inspection, are essential to ensure the structural integrity of the strengthened system.
Engineering Practice Considerations
For post-fire structural assessment and strengthening, this study provides valuable guidance. The fire damage to concrete reduces its strength and stiffness, and the resulting loss of confinement to the reinforcing steel further compromises the column's seismic performance. The externally wrapped steel tube serves as a new confinement layer that compensates for the lost concrete confinement.
The variation in improvement with shear span ratio (38.5-74.4% for shear span ratio 3.0 versus 41.8-47.0% for shear span ratio 1.78) indicates that the strengthening is particularly effective for members with flexural-dominant failure modes. For shear-critical members, additional measures such as internal shear reinforcement may be necessary in conjunction with the external steel tube wrapping.