Seismic Performance of Multi-Chamber CFST Column Mega-Frame Systems

Literature Overview

The study by Peng Bin, Cao Wanlin, Wang Zhihui, Dong Hongying, and Zhang Jianwei (Beijing University of Technology, 2012), published in the Journal of Structural Engineers (Vol. 28, No. 3, pp. 128-134), presents shaking table test results for a 1/25 scale model of a mega-frame structure with multi-chamber concrete-filled steel tube (CFST) columns. Funded by the National Natural Science Foundation of China (51178010) and the Beijing Science and Technology Program (D09050603720000), this research directly addresses the seismic design of super-tall building mega-frame systems, which are increasingly used in high-rise construction in China.

Core Technical Content

The mega-frame system tested consists of: (1) multi-chamber CFST mega-columns with irregular cross-sections; (2) steel truss mega-beams; (3) V-shaped box-section steel braces in the bottom mega-story; and (4) X-shaped box-section steel braces in the second and third mega-stories. The 1/25 scale model represents three mega-stories at the base of a super-tall building, which is the most critical region for seismic performance.

Structural Configuration

The multi-chamber CFST column design is a significant innovation. Unlike conventional single-chamber CFST columns, the multi-chamber design divides the cross-section into multiple concrete-filled cells using internal steel partitions. This design offers several advantages: (1) improved buckling resistance of individual chambers; (2) enhanced confinement effect due to the partition walls; (3) increased cross-sectional efficiency for irregular column shapes; and (4) potential for improved seismic energy dissipation through the interaction of multiple chambers.

Interpretation of Technical Points

Hysteretic Behavior and Energy Dissipation

The shaking table test results reveal the following hysteretic characteristics:

1. Initial stiffness: The initial lateral stiffness of the mega-frame is dominated by the axial stiffness of the mega-columns and the bending stiffness of the mega-beams. The braces contribute minimally to the initial stiffness due to their relatively small cross-section compared to the mega-columns.
2. Yield sequence: The test demonstrates a well-defined yield sequence: (a) braces yield first as the first line of defense; (b) mega-beam top and bottom chords yield next; (c) mega-column bases develop plastic hinges last. This sequence is ideal for seismic design because it ensures that damage is concentrated in replaceable elements (braces) before the primary gravity-load-resisting elements (columns) are compromised.
3. Stiffness degradation: The stiffness degradation follows a typical pattern for steel-braced frames, with the most significant degradation occurring after the braces have yielded and the mega-beams begin to yield. The multi-chamber CFST columns maintain relatively high stiffness throughout the test, indicating effective confinement of the concrete core.

Ductility and Yield Mechanism

The test achieved a displacement ductility ratio of approximately 3.5-4.0, which is adequate for seismic design in high seismic zones (seismic intensity 8 per GB 50011-2010). The yield mechanism is characterized by:

• Brace yielding: Inelastic deformation in the braces provides the primary energy dissipation mechanism. The box-section braces undergo flexural yielding followed by local buckling of the box walls.
• Beam chord yielding: After brace yielding, the mega-beam top and bottom chords develop plastic hinges at the mega-column connections. The steel truss configuration provides redundancy, as yielding of one chord does not immediately compromise the load-carrying capacity of the beam.
• Column plastic hinge: The last stage of yielding occurs at the base of the mega-columns, where the multi-chamber CFST section develops a plastic hinge through the yielding of the steel tube and the crushing of the concrete core. The multi-chamber design delays this stage by providing additional confinement and partition wall stability.

Comparison with Conventional CFST Mega-Frames

Welding and Fabrication Considerations

The fabrication of multi-chamber CFST columns presents significant welding challenges:

1. Internal partition welding: The internal steel partitions must be welded to the outer tube, creating complex multi-wall weld joints. The weld access is limited, and the heat input must be carefully controlled to avoid excessive distortion of the thin partition plates.
2. Weld sequence optimization: A critical welding sequence is essential to minimize residual stresses and distortion. The recommended sequence is: (a) weld the internal partitions to the tube in a symmetric pattern; (b) weld the tube segments together at the longitudinal seams; (c) weld the diaphragms to the tube. This sequence minimizes the interaction between welding-induced residual stresses and the structural stresses.
3. Non-destructive testing (NDT): The internal welds are difficult to inspect using conventional NDT methods. Ultrasonic testing (UT) from the outside of the tube is feasible for the longitudinal partition welds, but the weld access holes may require additional inspection. The recommended NDT coverage is: 100% UT for all internal partition welds, 100% MT for all external welds, and 10% UT for all longitudinal tube welds.
4. Heat-affected zone (HAZ) control: For Q345 steel with wall thicknesses of 16-30 mm, the HAZ width is typically 3-6 mm per side. In the multi-chamber configuration, the HAZs from adjacent welds may overlap, creating a broader zone of altered microstructure. This overlap zone may have reduced toughness and increased susceptibility to cracking. The welding procedure should include preheating to 100-150°C and interpass temperature control to 200-250°C for wall thicknesses exceeding 25 mm.

Standards Compliance

The design and fabrication of the mega-frame system must comply with multiple standards:

• GB 50011-2010 (Code for Seismic Design of Buildings): Governs the seismic design requirements, including ductility requirements, yield mechanism requirements, and performance-based design criteria.
• GB 50017-2017 (Standard for Design of Steel Structures): Governs the design of steel members, including the mega-columns, mega-beams, and braces.
• GB 50010-2010 (Code for Design of Concrete Structures): Governs the design of the concrete core in the CFST columns.
• JGJ 4-2008 (Technical Specification for Concrete-Filled Steel Tubular Structures): Provides specific design provisions for CFST members, including interaction curves and confinement models.
• GB 50661-2011 (Code for Welding of Steel Structures): Governs the welding quality requirements, including weld classification, NDT requirements, and acceptance criteria.

Key Questions and Reflections

The shaking table test provides valuable insights into the seismic performance of multi-chamber CFST mega-frames, but several questions remain open:

1. Cyclic loading beyond shaking table capacity: Shaking table tests are limited by the maximum displacement and acceleration of the test facility. The test may not capture the behavior at very large displacements (> 5% inter-story drift) where the multi-chamber CFST columns would be expected to exhibit significant inelastic deformation. Supplemental quasi-static cyclic tests on subassemblies are recommended to extend the understanding of the behavior at large displacements.
2. Concrete-steel interface behavior under cyclic loading: The bond-slip behavior at the steel-concrete interface in multi-chamber CFST columns is not explicitly addressed in the study. Under cyclic loading, the bond may degrade progressively, leading to reduced confinement effectiveness and accelerated damage accumulation. This is particularly relevant for the internal partition walls, where the concrete is confined on all sides and the bond behavior may differ from that in single-chamber CFST columns.
3. Long-term performance and fatigue: The study focuses on seismic performance under dynamic loading but does not address the fatigue behavior under wind loading, which is also significant for super-tall buildings. The interaction between fatigue damage from wind loading and seismic damage from earthquake loading is an important consideration for the design life of the structure.

Study Insights and Implications

This study demonstrates that multi-chamber CFST mega-columns are a viable and effective solution for the seismic design of super-tall building mega-frames. The well-defined yield sequence — braces first, beams second, columns last — is a hathe writing systemark of good seismic design and ensures that the structure can undergo significant inelastic deformation without collapse. The multi-chamber design provides additional confinement and redundancy compared to conventional single-chamber CFST columns, leading to improved ductility and energy dissipation capacity.

The practical implications are significant. As China continues to build taller and taller buildings, the demand for high-performance structural systems that can resist both gravity and lateral loads is increasing. The multi-chamber CFST mega-column system offers a promising solution that combines the advantages of steel (strength, ductility, constructability) and concrete (compressive strength, fire resistance, stiffness) in an efficient composite configuration. Future research should extend this work to include full-scale testing, parametric studies of the multi-chamber geometry, and the development of simplified design methods for routine engineering applications.