Substructure Pseudodynamic Testing and Finite Element Analysis of Steel Tube Concrete Columns under Variable Axial Load

Literature Overview

This paper published in the journal "Steel Structures" in 2017 (Volume 32, Issue 6, pages 31-38) presents a comprehensive investigation into the seismic performance of circular steel tube concrete (CFT) columns under variable axial load conditions. The authors, Zhang Guowei, Chen Boshan, Zhan Yuan, Guo Yurong, and Gao Yuchun from Beijing University of Civil Engineering and Architecture and Hunan University, conducted substructure pseudodynamic tests on six CFT column specimens under four levels of seismic excitation. The study is supported by the National Natural Science Foundation of China (Grant No. 51308026) and the Youth Talent Program.

The significance of this research lies in addressing a critical gap in CFT column seismic design. Conventional seismic tests on CFT columns typically apply a constant axial load throughout the cyclic loading history, which does not reflect the actual behavior of columns in multistory frames where the axial load varies significantly during seismic events due to P-delta effects, gravity load redistribution, and inertial effects. Understanding the influence of variable axial load on the hysteretic behavior, stiffness degradation, and energy dissipation capacity of CFT columns is essential for developing more accurate seismic design provisions.

Experimental Program and Test Configuration

Specimen Design

Six circular steel tube concrete column specimens were tested, varying in parameters that influence seismic performance:

The specimens were designed to represent typical CFT columns found in multistory buildings and bridges. The steel tube provides lateral confinement to the concrete core, significantly enhancing the ductility and energy dissipation capacity compared to reinforced concrete columns. The diameter-to-thickness ratio is a critical parameter that governs the local buckling behavior of the steel tube under cyclic loading.

Pseudodynamic Testing Methodology

The substructure pseudodynamic test method was employed, which combines experimental testing of critical structural components with numerical simulation of the remaining structure. This approach offers several advantages over full-scale shake table testing:

1. Cost efficiency: Only the column specimen requires physical construction and testing.
2. Controlled conditions: The test environment can be precisely controlled, eliminating environmental noise and interference.
3. Real-time feedback: The test system provides real-time force-displacement feedback, enabling accurate simulation of the dynamic interaction between the column and the rest of the structure.
4. Multiple earthquake scenarios: The same specimen can be tested under multiple earthquake records without requiring repeated full-scale testing.

The OpenSEES software was used for the finite element analysis of the global structure, with the column substructure represented by the experimentally measured hysteretic behavior. The test system synchronizes the numerical and experimental components through a real-time control algorithm that updates the boundary conditions at each time step based on the measured column response.

Key Experimental Results

Hysteretic Behavior and Pinching Effect

The most significant finding of the study is the pronounced pinching effect observed in the hysteretic curves of variable axial load columns compared to constant axial load columns under rare earthquake conditions. The pinching effect manifests as a reduction in the slope of the hysteretic loop during unloading and reloading phases, indicating stiffness degradation and possible separation between the steel tube and concrete core.

The pinching effect is attributed to the cyclic variation in axial load, which causes repeated compression and decompression of the concrete core. When the axial load decreases significantly, the confinement pressure on the concrete is reduced, leading to microcracking and potential debonding between the steel tube and concrete interface. Upon reloading, the damaged interface requires additional displacement to re-establish contact, resulting in the characteristic pinching behavior.

Stiffness Degradation

The stiffness degradation pattern differs between constant and variable axial load conditions. Under constant axial load, the stiffness degradation follows a relatively smooth curve as the displacement amplitude increases. Under variable axial load, the stiffness degradation is more erratic, with sudden drops corresponding to moments of low axial load when the confinement effectiveness is temporarily reduced.

Failure Mode

The primary failure mode observed in all specimens was local buckling of the steel tube in the plastic hinge region. The location and extent of local buckling were influenced by the axial load history. Variable axial load columns exhibited more extensive local buckling at lower displacement levels because the repeated variation in confinement pressure accelerates the initiation and propagation of local buckling.

Finite Element Analysis Validation

The OpenSEES finite element model was calibrated against the experimental results to verify its predictive capability. The model represents the steel tube using shell elements with appropriate constitutive models that capture the local buckling behavior, and the concrete core using solid elements with a confinement-enhanced concrete model.

The comparison between analytical and experimental results demonstrates good agreement in terms of:

• Peak load capacity
• Displacement at peak load
• Overall hysteretic loop shape
• Stiffness degradation trend
• Pinching effect magnitude

The finite element model captures the essential features of the variable axial load behavior, including the pinching effect and the reduced energy dissipation capacity. This validation confirms that the OpenSEES software with appropriate constitutive models can accurately simulate the seismic behavior of CFT columns under variable axial load conditions, providing a reliable tool for performance-based seismic assessment.

Engineering Practice Implications

Design Code Considerations

The study results have direct implications for seismic design codes that currently assume constant axial load for CFT column verification. The reduced lateral displacement capacity and energy dissipation under variable axial load suggest that existing design provisions may be non-conservative for columns subjected to significant axial load variation during seismic events.

Key design considerations include:

1. Axial load combination: The design axial load should account for the maximum expected variation during seismic events, not just the gravity load combination.
2. Ductility demand: The reduced displacement capacity under variable axial load increases the ductility demand on the column, requiring more ductile detailing.
3. Confinement effectiveness: The diameter-to-thickness ratio should be selected to maintain adequate confinement even under minimum axial load conditions.
4. Energy dissipation: The reduced hysteretic loop area under variable axial load must be considered in seismic energy dissipation calculations for the overall structure.

Practical Recommendations

Based on the study findings, the following recommendations are proposed for engineering practice:

1. Variable axial load analysis: Seismic analysis of CFT columns should incorporate the variation in axial load due to P-delta effects and gravity load redistribution.
2. Enhanced confinement: Columns subjected to significant axial load variation should have enhanced confinement through reduced D/t ratios or additional external confinement.
3. Performance-based assessment: The OpenSEES-based finite element methodology validated in this study should be employed for performance-based seismic assessment of CFT structures.
4. Testing standards: Future testing standards should include provisions for variable axial load testing to better represent actual seismic behavior.

Study Insights and Critical Reflection

This study makes a significant contribution to the understanding of CFT column seismic behavior by addressing the often-neglected influence of variable axial load. The pronounced pinching effect observed under rare earthquake conditions is particularly concerning because it indicates that the column's ability to sustain large inelastic deformations is significantly compromised when the axial load varies. This finding challenges the conventional assumption that CFT columns inherently possess superior ductility and energy dissipation capacity compared to reinforced concrete columns.

The use of pseudodynamic testing is commendable because it bridges the gap between controlled laboratory testing and full-scale seismic testing. The method provides realistic dynamic loading conditions while maintaining the precision and repeatability of laboratory testing. The validation of the OpenSEES finite element model against experimental results further extends the applicability of the findings to practical engineering design through numerical simulation.

However, the study is limited to six specimens and four earthquake levels, which may not fully capture the range of possible variable axial load histories. Future research should investigate the influence of earthquake duration, frequency content, and multi-directional loading on the variable axial load behavior of CFT columns. Additionally, the long-term durability implications of the cyclic loading-induced damage observed in this study warrant further investigation.

Summary

This literature provides valuable experimental and analytical insights into the seismic behavior of circular steel tube concrete columns under variable axial load conditions. The key findings include the pronounced pinching effect in hysteretic curves, reduced lateral displacement capacity and energy dissipation, and accelerated local buckling of the steel tube. The validated OpenSEES finite element model offers a reliable tool for performance-based seismic assessment of CFT structures. For practicing engineers, the study underscores the importance of considering variable axial load effects in the seismic design and assessment of CFT columns, particularly for structures subjected to rare earthquake events where the axial load variation can significantly compromise the column's ductility and energy dissipation capacity.