Zhuojin Pipe Fitting Co., LtdTorsional Performance of Steel Tube Concrete Columns under Eccentric Compression
Literature Overview
This paper by Wang Yuhang and colleagues from Chongqing University, published in 2017 in the China Civil Engineering Journal, investigates the torsional behavior of steel tube concrete (SRC) columns subjected to combined eccentric compression and torsion. The study was supported by the National Natural Science Foundation of China (project 5150852). The research addresses an important structural engineering challenge: the combined effects of eccentric compression and torsion on SRC columns, which is particularly relevant for curved bridge piers and arch bridge main arches.
The study conducted 8 SRC column specimens through monotonic pure torsion loading, cyclic pure torsion loading, and eccentric compression-cyclic torsion loading quasi-static tests. The specimens had different cross-sectional shapes, providing comparative data on the influence of section geometry on torsional performance.
Experimental Program and Loading Protocols
The test program comprised three loading protocols applied to different specimens:
- Monotonic pure torsion: A single loading cycle to ultimate failure to determine peak torsional capacity.
- Cyclic pure torsion: Repeated loading and unloading cycles to evaluate hysteretic behavior and energy dissipation capacity.
- Eccentric compression-cyclic torsion: Combined loading with a constant eccentric axial load and cyclic torsional moment to simulate realistic combined loading conditions.
Specimen Configuration
| Parameter | Variation | Purpose |
|---|---|---|
| Cross-sectional shape | Square, rectangular, circular | Study geometry effect on torsion |
| Aspect ratio (for rectangular) | Different values | Study slenderness effect |
| Steel tube wall thickness | Different values | Study confinement effect |
| Concrete strength | Different grades | Study material effect |
| Eccentric compression ratio | Different values | Study combined loading effect |
Hysteretic Behavior and Energy Dissipation
The cyclic pure torsion tests revealed that SRC columns exhibit relatively full hysteretic loops without the "pinching" phenomenon commonly observed in reinforced concrete members. This indicates that the steel tube provides effective confinement to the concrete core, preventing premature concrete crushing and maintaining bond integrity throughout the loading cycles. The strength and stiffness degradation of the specimens was relatively low, demonstrating good energy dissipation capacity.
Comparison of Short and Tall Column Behavior
| Performance Indicator | Short Column | Tall Column |
|---|---|---|
| Yield rotation angle | Smaller | Larger |
| Yield torque | Similar | Similar |
| Ultimate bearing capacity | Higher | Lower |
| Ultimate rotation angle | Smaller | Larger |
| Energy dissipation capacity | Better | Lower |
| Failure phenomenon | More pronounced | Less pronounced |
The short column specimens exhibited more pronounced failure phenomena compared to the tall column specimens. The short columns had smaller yield rotation angles but higher ultimate bearing capacities and better energy dissipation capacity. This behavior is attributed to the higher shear stress levels in short columns, which promote more ductile failure modes involving steel tube yielding and concrete crushing rather than brittle shear failure.
Effect of Eccentric Compression on Torsional Performance
The eccentric compression-cyclic torsion tests revealed that the eccentric compression load significantly affects the torsional behavior of SRC columns. For square-section specimens, the stiffness degradation was more pronounced under eccentric compression compared to pure torsion. The eccentric compression load reduced the energy dissipation capacity of the square SRC column specimens.
At the ultimate state, the eccentric compression load changed the buckling angle of the steel tube. This finding is significant because the steel tube buckling pattern directly affects the post-peak load-carrying capacity and the residual strength of the column. The change in buckling angle suggests that the interaction between eccentric compression and torsion creates complex stress states that alter the failure mode of the steel tube.
Stress State Interaction
| Loading Condition | Primary Stress Component | Failure Mode |
|---|---|---|
| Pure torsion | Shear stress | Steel tube shear buckling |
| Eccentric compression + torsion | Combined shear and compression | Modified buckling pattern |
| Cyclic torsion | Reversed shear stress | Progressive damage accumulation |
Engineering Practice Implications
For curved bridge piers and arch bridge main arches, the combined eccentric compression and torsion loading conditions are unavoidable. The design of SRC columns for these applications must account for the interaction effects between the two load types. The test results suggest that the torsional capacity of SRC columns under eccentric compression is lower than that under pure torsion, and the energy dissipation capacity is reduced.
From a steel pipe fabrication and welding perspective, the steel tubes used in these columns must maintain adequate mechanical properties under combined stress states. The longitudinal weld, if present, should be designed to resist the combined shear and compressive stresses without premature failure. The weld procedure should be qualified through full-scale weld tests that simulate the combined loading conditions expected in service.
The steel tube dimensions and wall thickness should be selected to provide adequate confinement to the concrete core while maintaining sufficient structural capacity under the combined loading. The diameter-to-wall thickness ratio should be limited to prevent local buckling of the steel tube under the complex stress state. For high-strength steels, post-weld heat treatment may be necessary to ensure adequate toughness in the heat-affected zone.
Study Insights and Reflections
This research provides valuable experimental data on the torsional behavior of SRC columns under combined loading conditions. The findings on the interaction between eccentric compression and torsion are particularly important for the design of curved bridge structures, where such combined loading is inherent. The observation that the eccentric compression load changes the steel tube buckling angle highlights the complexity of the failure mechanism and underscores the need for detailed design analysis.
The relatively full hysteretic loops and low strength degradation observed in the cyclic pure torsion tests are encouraging for seismic design applications. The SRC configuration provides inherent ductility and energy dissipation capacity that is superior to conventional reinforced concrete columns. However, the reduction in energy dissipation capacity under eccentric compression suggests that designers should apply appropriate reduction factors when evaluating the seismic performance of SRC columns subjected to combined loading.
Future research should extend the experimental database to include more specimens with different cross-sectional shapes and loading combinations. The development of simplified design formulas that account for the interaction effects between eccentric compression and torsion would be beneficial for practical engineering applications. Additionally, the long-term behavior of SRC columns under sustained combined loading should be investigated to provide guidance for the design of permanent structures.