Seismic Performance Testing of Steel Tube Concrete Composite Tubular Structure

Literature Overview

This paper published in Sichuan Building Science (2009, Vol. 35, No. 6, pp. 201-204) by Chen Bowa, Wang Haibo, and Cao Guohui from Hunan City College presents the results of a shake table test on a 1:10 scale model of a 13-story steel tube concrete composite tubular structure. The model features an external steel tube concrete frame-supported frame-tube structure and an internal reinforced concrete core tube. The test was conducted under four seismic intensity levels with peak ground accelerations of 0.22g, 0.40g, 0.62g, and 1.0g. The study provides experimental validation of the seismic performance of composite tubular structures, which are increasingly used in high-rise building construction.

Structural Configuration and Test Setup

The tested model represents a typical composite tubular structural system used in modern high-rise construction. The key structural features are:

The external frame-supported frame-tube structure provides lateral stiffness and strength through the interaction of the steel tube concrete columns and the frame beams. The internal core tube provides additional lateral stiffness and serves as a shear wall for seismic load resistance. The combination of these two systems creates a composite tubular structure with enhanced seismic performance compared to either system alone.

Two-Degree-of-Freedom Equivalent Model

The authors equivalent the composite tubular structure to a two-degree-of-freedom system, which captures the essential dynamic characteristics of the structure:

• First degree of freedom: Represents the global bending response of the external frame-tube, characterized by the fundamental lateral displacement of the structure
• Second degree of freedom: Represents the relative displacement between the external frame-tube and the internal core tube, capturing the interaction between the two structural systems

This equivalent model is appropriate for the dynamic analysis of composite tubular structures because it accounts for the coupling between the external and internal structural systems. The relative displacement between the two systems is a critical parameter that influences the internal forces in the connecting elements and the overall seismic response.

Seismic Performance Results

The shake table tests were conducted under four increasing seismic intensity levels. The results demonstrate the following key findings:

Response at 0.22g (Design Earthquake)

At the design earthquake level, the structure exhibited elastic or near-elastic behavior. The displacements were within the elastic range for both the external frame-tube and the internal core tube. The interstory drift ratios were well below the damage threshold, indicating that the structure can withstand the design earthquake without significant damage.

Response at 0.40g (Major Earthquake)

At the major earthquake level, the structure began to exhibit inelastic behavior. Plastic deformation developed in the steel tube concrete columns and the reinforced concrete core tube. However, the deformation was distributed across multiple stories, preventing localized damage concentration. The interstory drift ratios remained within acceptable limits for life safety performance.

Response at 0.62g (Rare Earthquake)

At the rare earthquake level, the structure experienced significant inelastic deformation. Plastic hinges formed at the column bases and beam-column joints. The steel tube concrete columns demonstrated excellent energy dissipation through the yielding of the steel tube and the confinement of the concrete core. The structure maintained its overall stability and did not experience progressive collapse.

Response at 1.0g (Extreme Earthquake)

At the extreme earthquake level, the structure experienced severe inelastic deformation. The steel tube concrete columns underwent significant plastic deformation, but the steel tube provided effective confinement to the concrete core, preventing concrete spalling and maintaining the axial load capacity. The structure survived the extreme earthquake without collapse, demonstrating excellent collapse prevention performance.

Key Performance Characteristics

The experimental results reveal several important performance characteristics of the steel tube concrete composite tubular structure:

1. Excellent energy dissipation capacity. The combination of the steel tube concrete external frame and the reinforced concrete core tube provides multiple mechanisms for energy dissipation. The steel tube yields and dissipates energy through plastic deformation, while the concrete core provides additional damping through cracking and crushing.
2. Good ductility. The steel tube concrete columns exhibit high ductility due to the confinement effect of the steel tube on the concrete core. The steel tube prevents concrete spalling and maintains the integrity of the column under large deformations.
3. Effective load redistribution. The composite tubular structure allows for load redistribution between the external and internal structural systems. When one system approaches its capacity, the other system can take on additional load, preventing premature failure.
4. Robust performance under extreme loading. Even at the 1.0g extreme earthquake level, the structure maintained its overall stability. This robustness is attributed to the redundancy provided by the dual structural system and the ductility of the steel tube concrete components.

Engineering Practice Implications

For engineers designing steel tube concrete composite tubular structures, the following recommendations emerge from this study:

1. Validate analytical models with experimental data. The two-degree-of-freedom equivalent model provides a useful analytical framework, but the model parameters should be calibrated against shake table test results for structures with similar configurations.
2. Design for multiple performance levels. The structure should be designed to meet specific performance objectives at each seismic intensity level: immediate occupancy at the design earthquake, life safety at the major earthquake, and collapse prevention at the rare earthquake.
3. Ensure adequate ductility in steel tube concrete columns. The steel tube wall thickness should be designed to provide sufficient confinement to the concrete core while also allowing for plastic deformation of the steel tube. The ductility of the column is a critical factor in the overall seismic performance.
4. Design connecting elements for relative displacement. The connections between the external frame-tube and the internal core tube must be designed to accommodate the relative displacement between the two systems. Inadequate connection design can lead to premature connection failure and loss of composite action.
5. Consider the effect of concrete-steel tube interface. The bond between the concrete core and the steel tube is a critical factor in the seismic performance of the column. The interface should be designed to maintain composite action under cyclic loading, preventing relative slip that would reduce the effective stiffness and strength.

Key Reflections

This shake table test provides valuable experimental validation of the seismic performance of steel tube concrete composite tubular structures. The results confirm that this structural system offers excellent seismic performance, with the ability to withstand extreme earthquake loading without collapse. The combination of the steel tube concrete external frame and the reinforced concrete core tube creates a highly redundant and ductile structural system.

The study also highlights the importance of experimental validation in structural engineering. Analytical models, while useful for design, may not capture all the complex behaviors that occur during seismic loading. Shake table tests provide direct evidence of structural performance under realistic seismic conditions, including the effects of higher-mode responses, nonlinear material behavior, and structural interaction.

For the steel tube concrete construction industry, this study provides confidence in the use of composite tubular structures for high-rise buildings in seismic regions. The demonstrated performance under extreme loading conditions supports the adoption of this structural system for critical infrastructure and occupied buildings where seismic safety is paramount.

The paper also contributes to the understanding of the interaction between the external and internal structural systems in composite tubular structures. The two-degree-of-freedom equivalent model captures the essential dynamics of this interaction, providing a practical analytical tool for design engineers. However, the study also demonstrates that the actual behavior is more complex than the simplified model, with additional energy dissipation mechanisms and load redistribution pathways that contribute to the overall seismic performance.

This research represents an important contribution to the seismic design of composite structures. As the construction industry continues to seek more efficient and resilient structural systems, the steel tube concrete composite tubular structure emerges as a promising solution for high-rise buildings in seismic regions. The experimental validation provided by this study supports the continued development and application of this structural system in practice.