Nonlinear Seismic Analysis of CFST Mega Diagonal Grid Tube-in-Tube Structures

Literature Overview

The 2009 paper by Han Xiaolei and colleagues, published in Earthquake Engineering and Engineering Dynamics, presents a nonlinear seismic analysis of a mega diagonal grid tube-in-tube structure using steel tube concrete (CFST) columns. The research employs the PERFORM-3D software with fiber element modeling to evaluate the seismic performance of a super-high-rise tube-in-tube structure under strong earthquake loading at seismic intensity levels of 7, 8, and 9 degrees. The study is motivated by the need to assess the seismic performance of this advanced structural system, which combines the high lateral stiffness of the mega diagonal grid outer tube with the strength and ductility of CFST columns.

Core Technical Approach

The mega diagonal grid structural system is characterized by large diagonal members that form a rigid outer tube, providing exceptional lateral stiffness and strength. When used as the outer tube of a tube-in-tube configuration, the system offers a highly efficient structural solution for super-high-rise buildings. The CFST columns within this system benefit from the confinement effect of the steel tube, which enhances the compressive strength and ductility of the concrete infill. The nonlinear analysis employs fiber element modeling, which discretizes the cross-section into multiple material fibers, each with its own constitutive behavior. This approach accurately captures the nonlinear material behavior, including concrete crushing, steel yielding, and the interaction between steel and concrete.

The analysis is conducted under performance-based seismic design methodology, which focuses on the structural performance under specific seismic scenarios rather than meeting prescriptive code requirements. The study compares the structural response under three seismic intensity levels, providing insight into the structure's behavior across a range of earthquake intensities.

Test Results and Key Findings

The nonlinear analysis results demonstrate that the CFST mega diagonal grid tube-in-tube structure exhibits excellent seismic performance under strong earthquake loading. The structure maintains acceptable performance under seismic intensity levels of 7, 8, and 9 degrees, with the degree of damage increasing with seismic intensity but remaining within acceptable limits for each level. The fiber element modeling captures the complex nonlinear behavior of the CFST columns, including the progressive crushing of concrete, the yielding of steel fibers, and the interaction between the diagonal grid members and the core tube.

The analysis reveals that the mega diagonal grid outer tube provides significant lateral stiffness, which reduces the inter-story drift and accelerations within the structure. The CFST columns contribute to the overall strength and energy dissipation capacity of the system. The nonlinear interaction between the diagonal grid members and the core tube is an important aspect of the structural behavior, as it affects the load distribution and the formation of plastic hinges.

Engineering Practice Implications

For engineers designing super-high-rise buildings in high-seismic regions, the CFST mega diagonal grid tube-in-tube system offers a promising structural solution. The combination of the mega diagonal grid's lateral stiffness and the CFST columns' strength and ductility creates a structural system that can effectively resist strong earthquake loading. The performance-based design approach used in this study is particularly appropriate for such complex structural systems, as it provides a clear understanding of the expected structural performance under specific seismic scenarios.

From a fabrication and quality control perspective, the mega diagonal grid members require precise fabrication and welding to ensure the required geometric accuracy and structural integrity. The diagonal members are typically long, large-diameter steel tubes or built-up box sections, and their welding quality is critical to the overall structural performance. The CFST columns require careful concrete placement to ensure full compaction and adequate bond with the steel tube. The use of fiber element modeling in the analysis provides engineers with a powerful tool for predicting the nonlinear behavior of the structure, but the accuracy of the model depends on the quality of the input material properties and the constitutive model parameters.

Study Insights and Reflections

This research is valuable for the structural engineering community because it demonstrates the seismic performance of an advanced structural system through rigorous nonlinear analysis. The use of fiber element modeling is a significant methodological contribution, as it provides a more accurate representation of the nonlinear material behavior than conventional beam element models. The performance-based design approach is particularly relevant for super-high-rise buildings, where the consequences of structural failure are severe and the seismic design requirements are stringent. The study provides a basis for the application of the CFST mega diagonal grid tube-in-tube system in high-seismic regions, and it highlights the importance of nonlinear analysis in the design of complex structural systems. For engineers working on super-high-rise projects, this paper serves as a reference for the selection of structural systems and the application of performance-based seismic design methodology. The findings reinforce the principle that structural efficiency and seismic performance can be achieved through the rational combination of structural systems and advanced analysis methods, and that the CFST mega diagonal grid tube-in-tube system is a viable option for high-seismic-intensity regions.