Zhuojin Pipe Fitting Co., LtdNonlinear Analysis of Steel Tubular Concrete Mega-Diagrid Cylindrical Structure Under Seismic Loading
Literature Overview
This paper by Han Xiaolei and colleagues from South China University of Technology, published in Earthquake Engineering and Engineering Dynamics (Volume 29, Issue 4, 2009, pages 77-84), presents a nonlinear analysis of a steel tubular concrete (STC) mega-diagrid cylindrical structure used as the outer shell of a tube-in-tube super-tall building. The study employs the PERFORM-3D software with fiber elements to conduct nonlinear analysis under seismic excitations corresponding to seismic intensity levels 7, 8, and 9. The research was supported by the Open Fund of the State Key Laboratory of Subtropical Building Science, South China University of Technology (Grant 2008KA01).
Structural System Description
The mega-diagrid cylindrical structure is a structural system where diagonal members arranged in a grid pattern form a cylindrical outer shell. When combined with STC columns and beams, this system provides exceptional lateral stiffness and strength for super-tall buildings. The key features of this structural system include:
- High lateral stiffness due to the triangulated diagrid pattern
- Efficient load path from the lateral loads to the foundation through the diagonal members
- Reduced need for internal cores compared to conventional tube structures
- Aesthetic appeal of the exposed diagrid pattern on the building facade
The tube-in-tube configuration combines the mega-diagrid outer shell with an internal core tube, providing redundant lateral load-resisting systems and enhanced overall structural stability.
Analytical Methodology
The nonlinear analysis was conducted using the following approach:
| Analytical Parameter | Description |
|---|---|
| Software | PERFORM-3D |
| Element type | Fiber elements (fiber beam-column elements) |
| Material models | Concrete and steel fiber models with nonlinear stress-strain relationships |
| Seismic intensities | 7, 8, and 9 degrees (Chinese seismic intensity scale) |
| Analysis type | Nonlinear static or dynamic analysis |
| Performance criteria | Based on performance-based seismic design methodology |
The fiber element approach is particularly suitable for STC members, as it allows for the accurate modeling of the composite action between the steel tube and the concrete core, including the nonlinear behavior of both materials under seismic loading.
Key Findings
The nonlinear analysis results demonstrated the following:
- The STC mega-diagrid cylindrical structure exhibits excellent seismic performance under all investigated seismic intensity levels.
- The structure demonstrates progressive damage rather than sudden failure, providing warning before collapse.
- The inter-story drift ratios remain within acceptable limits even under seismic intensity 9.
- The plastic deformation is distributed throughout the structure rather than concentrated at specific locations, indicating good structural redundancy.
- The structure is suitable for high seismic fortification regions, providing a basis for its application in super-tall building design.
Technical Interpretation from a Steel Fabrication and Welding Perspective
The mega-diagrid structural system presents unique challenges for steel pipe fabrication and welding:
- The diagonal members are typically fabricated as steel tubes or steel box sections at various angles, requiring precise cutting and welding
- The intersections of the diagonal members with the horizontal ring beams and vertical columns create complex multi-directional connections
- The welding of diagonal members to the STC columns requires careful consideration of the connection details to ensure adequate moment transfer
- The large number of welded connections in the diagrid system requires systematic quality control procedures
The following table summarizes the key welding considerations for the mega-diagrid system:
| Connection Type | Welding Challenge | Quality Control Requirement |
|---|---|---|
| Diagonal to column | Multi-directional moment transfer | Full penetration welds, NDT inspection |
| Diagonal to ring beam | Complex geometry, angle variation | Weld sequence control, distortion management |
| Column to column (STC) | Axial load continuity | Weld quality verification, HAZ inspection |
| Ring beam to column | Shear and moment transfer | Bolted or welded connection design verification |
Engineering Practice Implications
The mega-diagrid cylindrical structure is particularly suitable for:
- Super-tall buildings in high seismic regions
- Buildings where architectural aesthetics require an exposed structural system
- Applications where the tube-in-tube configuration provides redundancy and robustness
- Seismic regions where performance-based design is required
The nonlinear analysis results provide confidence that the STC mega-diagrid system can be safely applied in high seismic fortification regions. The performance-based seismic design approach allows engineers to evaluate the structural response under specific seismic scenarios and ensure that the structure meets the desired performance objectives.
Study Insights and Reflections
This study contributes valuable analytical data for the seismic design of STC mega-diagrid structures, a relatively new structural system gaining popularity in super-tall building design. The use of fiber elements in the nonlinear analysis provides accurate modeling of the composite behavior of STC members, which is essential for predicting the structural response under severe seismic loading. From my perspective as a steel pipe and welding specialist, the mega-diagrid system represents both an opportunity and a challenge. The opportunity lies in the potential for innovative structural design that combines structural efficiency with architectural expression. The challenge lies in the fabrication and welding of the complex diagonal member connections, which require precise engineering and quality control. The study's conclusion that the structure is suitable for high seismic fortification regions is encouraging, and with proper attention to fabrication and welding quality, the STC mega-diagrid system represents a promising solution for super-tall building design in seismic regions.