Zhuojin Pipe Fitting Co., LtdPseudo-Dynamic Testing of Concrete-Filled Steel Tube Reinforced Concrete Composite Piers Under Mainshock-Aftershock Sequences
Literature Overview
The paper by Yuan Huihui, Cheng Jun, Wu Qingxiong, and She Zhimin from Fuzhou University, published in China Journal of Highway and Transport (2023, Vol. 36, No. 7, pp. 180-192), presents pseudo-dynamic test results for two 1:11.6 scale models of concrete-filled steel tube reinforced concrete (CFSTRC) composite piers subjected to mainshock-aftershock seismic sequences. The study was funded by multiple sources including the National Natural Science Foundation of China (51978169) and the National Key R&D Program of China (2017YFE0130300).
Structural Configuration and Test Setup
The CFSTRC composite pier combines a concrete-filled steel tube (CFST) column with reinforced concrete (RC) components, leveraging the confinement effect of the steel tube on concrete while benefiting from the composite action with surrounding RC. The pseudo-dynamic test method allows real-time feedback control of displacement based on measured structural response, enabling realistic simulation of seismic loading with the flexibility to apply multi-stage earthquake sequences.
Two specimens were tested: Specimen S1 and Specimen S2. The test program included elastic-stage loading with 10 strong-motion records (all scaled to PGA of 0.05g), followed by a mainshock at 9-degree rare event intensity (PGA approximately 0.7g), and then progressively increasing aftershocks.
Key Experimental Findings
| Loading Stage | Specimen S1 | Specimen S2 |
|---|---|---|
| Elastic stage (PGA 0.05g) | Elastic behavior | Elastic behavior |
| Mainshock (PGA ~0.7g) | Strength degradation stage | Elastic-plastic deformation stage |
| Aftershock sequence | Cumulative damage increase | Cumulative damage increase |
| Post-mainshock capacity loss | ~17% average reduction | ~17% average reduction |
| Hysteretic energy dissipation change | ~16% reduction | ~16% reduction |
| Maximum displacement increase | ~3% increase | ~3% increase |
Failure Modes and Damage Progression
The failure pattern consistently showed diagonal shear cracks along the loading direction on the web plates, horizontal cracks on the web plates perpendicular to the loading direction, and vertical cracks in the outer concrete caused by local buckling of the steel tube at the column base. This damage progression indicates that the steel tube local buckling is a critical failure mechanism that triggers concrete cracking and leads to significant stiffness degradation.
The Kobe earthquake JRT wave produced the largest displacement response among the 10 elastic-stage records, reaching 1.68 times the average value. This highlights the importance of ground motion selection in seismic testing and design verification.
Welding and Steel Tube Engineering Implications
From a steel pipe and welding engineering standpoint, the CFSTRC composite pier represents a hybrid construction where steel tubes are integrally welded to structural steel plates and connected to reinforced concrete through embedded connections. The local buckling of the steel tube at the column base, identified as a key failure mechanism, is directly related to the geometric proportions of the steel tube—specifically the width-to-thickness ratio. In accordance with standards such as GB 50017 and AISC 360, the width-to-thickness ratio limits for steel tubes in compression must be carefully controlled to prevent local buckling before the member reaches its plastic capacity.
The weld connections between the steel tube and the surrounding structural steel plates are subjected to complex multi-axial stress states during seismic loading. The test results showing 17% capacity reduction and 16% energy dissipation reduction under aftershock sequences imply that weld details must provide sufficient ductility to accommodate repeated inelastic cycling without premature fracture. This has direct implications for weld metal selection, welding procedure specification, and post-weld heat treatment requirements.
Damage Assessment Methodology
The authors propose a damage index based on a linear combination of maximum structural deformation and cumulative hysteretic energy dissipation. This approach provides a practical tool for post-earthquake structural assessment and repair prioritization. For steel tube members, the damage index can be correlated with measurable indicators such as steel tube local buckling amplitude, weld crack length, and residual deformation.
Study Insights and Reflections
The mainshock-aftershock testing protocol employed in this study is particularly relevant to real-world seismic events, where aftershocks frequently cause additional damage to already-compromised structures. The finding that the CFSTRC composite pier loses approximately 17% of its horizontal load capacity and 16% of its energy dissipation capacity under aftershock sequences underscores the need for ductile weld details and robust steel tube design. In engineering practice, this means that steel tube connections in bridge piers should be designed with generous weld reinforcement, adequate wall thickness to prevent local buckling, and possibly post-weld stress relief to minimize residual stress effects on fatigue and fracture resistance. The proposed damage index offers a quantitative framework for inspectors to assess post-earthquake condition of steel tube composite members, which is invaluable for rapid bridge reopening decisions.