Seismic Performance Testing of Steel Tube Concrete Composite Tubular Structures

Overview of the Study

The paper by Chen Bowan, Wang Haibo, and Cao Guohui, published in Sichuan Building Science (2009, Vol. 35, No. 6, pp. 201-204), presents the results of a shake table test on a 1:10 scale model of a 13-story steel tube concrete (SRC) composite tubular structure. The model featured an outer tube composed of an SRC frame-supported frame-tube system and an inner tube composed of a reinforced concrete core wall. The research was funded by the Hunan Provincial Natural Science Foundation and the Hunan Provincial Department of Education. The study was conducted at Hunan City University and represents a significant contribution to the understanding of the seismic behavior of composite tubular structures, which are increasingly used in high-rise buildings in seismic regions.

Model Description and Test Configuration

Structural Configuration

The tested model represents a dual-tube composite structure, a configuration that combines the high stiffness and strength of the outer SRC frame-tube with the lateral load resistance of the inner reinforced concrete core wall. The key structural parameters are:

Test Loading Protocol

The model was subjected to four loading conditions with progressively increasing peak ground acceleration (PGA):

The model was idealized as a two-degree-of-freedom (2DOF) system for analysis purposes, with the outer tube and inner tube treated as two coupled oscillators. This simplification allows for a more tractable analysis of the inter-tube interaction and energy dissipation mechanisms.

Test Results and Seismic Performance

Overall Seismic Performance

The test results demonstrated that the SRC composite tubular structure exhibits excellent seismic performance across all loading conditions. The key observations are:

1. No catastrophic failure — Even at the highest loading level (1.0 g PGA), the model did not experience collapse or severe structural damage. This indicates that the dual-tube system provides sufficient redundancy and energy dissipation capacity to withstand extreme seismic events.
2. Progressive damage pattern — Damage developed progressively with increasing loading intensity, following a predictable pattern that is consistent with the design intent of a ductile structural system.
3. Effective energy dissipation — The composite action between the steel tubes and the concrete cores provided significant energy dissipation through inelastic deformation, reducing the overall seismic demand on the structure.

Damage Mechanisms

The damage observed during testing followed a predictable pattern:

• At lower loading levels (0.22 g and 0.40 g), the structure remained essentially elastic, with only minor cracking in the reinforced concrete core wall.
• At moderate loading levels (0.62 g), the outer SRC columns began to yield, with visible plastic hinging at the column bases and at the top and bottom of the frame-supported columns. The steel tubes showed localized buckling of the walls between stiffeners.
• At the highest loading level (1.0 g), significant inelastic deformation occurred, with extensive plastic hinging in the outer tube columns, cracking and spalling of the concrete in the core wall, and localized buckling of the steel tube walls. However, the structure maintained its load-bearing capacity and did not collapse.

Inter-Tube Interaction

One of the most important findings of this study is the effective interaction between the outer and inner tubes. The dual-tube system behaves as a coupled system in which the outer tube and inner tube share the lateral load and deform together. This coupling provides several advantages:

• Load sharing — The lateral load is distributed between the two tubes, reducing the demand on each individual tube.
• Stiffness contribution — Both tubes contribute to the overall lateral stiffness, reducing interstory drift and improving the seismic response.
• Energy dissipation — The inelastic deformation of both tubes contributes to energy dissipation, reducing the overall seismic demand.

Technical Analysis

Comparison with Single-Tube Systems

The dual-tube composite system offers several advantages over a single-tube system:

The redundancy provided by the dual-tube system is particularly valuable in seismic design. If one tube sustains significant damage, the other tube can continue to carry the load, preventing collapse. This is a fundamental principle of seismic design: the structure should have multiple lines of defense against failure.

Energy Dissipation Mechanisms

The energy dissipation in the SRC composite tubular structure occurs through several mechanisms:

1. Plastic hinging in SRC columns — The steel tubes and confined concrete in the columns undergo inelastic deformation, dissipating energy through hysteresis.
2. Steel tube wall buckling — Local buckling of the steel tube walls between stiffeners dissipates energy through plastic deformation.
3. Concrete cracking and crushing — The reinforced concrete core wall dissipates energy through cracking and crushing, although this mechanism is less ductile than the steel-based mechanisms.
4. Slip at the tube interface — If the outer and inner tubes are not rigidly connected, relative slip at the interface can dissipate energy through friction.

The relative contribution of each mechanism depends on the loading level and the structural configuration. At lower loading levels, the energy dissipation is primarily through elastic deformation and minor cracking. At higher loading levels, the steel tube mechanisms dominate.

Key Questions and Reflections

Several questions and observations arise from this study:

1. Scale effects — The test was conducted on a 1:10 scale model. Scale effects in shake table testing can influence the results, particularly for brittle failure modes such as concrete crushing. The results should be interpreted with caution when extrapolating to full-scale structures.
2. Material characterization — The paper does not provide detailed information on the material properties of the model, including the steel grade, concrete strength, and reinforcement ratio. These parameters are critical for interpreting the test results and for comparing them with analytical predictions.
3. Long-term durability — The test evaluates the seismic performance of the structure under a single loading event. However, the long-term durability of the structure after seismic damage is also important. Can the structure be repaired and returned to service after a major earthquake? What is the residual capacity of the damaged structure?
4. Comparison with analytical models — The paper does not appear to include a detailed comparison between the test results and analytical predictions. Such a comparison would be valuable for validating analytical models and for understanding the accuracy of simplified analysis methods.

Study Insights and Implications

This study provides valuable experimental evidence for the seismic performance of SRC composite tubular structures. The results confirm that the dual-tube system is an effective structural configuration for high-rise buildings in seismic regions, offering high stiffness, significant energy dissipation capacity, and excellent damage tolerance.

The progressive damage pattern observed in the test is consistent with the design philosophy of a ductile structural system. The structure degrades gracefully under increasing seismic demand, with damage developing in a predictable sequence that allows for early warning and potential evacuation. This is a critical requirement for life-safety design in seismic regions.

The effective inter-tube interaction is a particularly important finding. The coupling between the outer and inner tubes provides a level of redundancy and load-sharing that is not achievable with a single-tube system. This makes the dual-tube configuration particularly attractive for critical infrastructure, such as hospitals, emergency command centers, and nuclear facilities, where the consequences of structural failure are unacceptable.

The study also highlights the importance of experimental validation in structural engineering. Analytical models, while useful for design, can be inaccurate for complex structural systems with nonlinear behavior. Shake table testing provides direct evidence of structural performance and can reveal failure modes that are not predicted by analytical models. The results of this study should be used to validate and refine analytical models for SRC composite tubular structures, improving the accuracy of future designs.

In conclusion, the shake table test results presented in this paper provide strong evidence for the seismic performance of SRC composite tubular structures. The dual-tube system offers a robust and reliable structural configuration for high-rise buildings in seismic regions, and the experimental results should be incorporated into the development of design codes and guidelines for these structures.