Seismic Energy Response Analysis of CFST Diagonal Grid Shell Structures

Literature Overview and Research Context

The 2023 paper by Cai Wenzhe, Wang Bin, Shi Qingxuan, and Ge Minglan, published in "Journal of Vibration and Shock," presents a comprehensive seismic energy analysis of concrete-filled steel tube (CFST) diagonal grid shell structures. This research was supported by multiple funding sources including the Shaanxi Provincial Natural Science Foundation (2022JQ-392), National Natural Science Foundation (52278215), and the Western Green Building National Key Laboratory Open Research Fund (LSKF202212). The study employs a novel axial restoring force model for CFST columns and performs elastoplastic time-history analysis to investigate the energy distribution patterns within the structure at three levels: structural, component, and floor levels.

Core Technical Framework and Methodology

The research framework is built upon the development of an axial restoring force model for CFST columns that captures the nonlinear behavior under cyclic loading. This model is then incorporated into a finite element analysis framework to perform elastoplastic time-history analysis of the diagonal grid shell structure. The energy analysis is conducted from three perspectives: (1) the overall structural energy balance, (2) the energy dissipation by individual components (diagonal columns, connecting beams, and other elements), and (3) the energy distribution along the height of the structure.

The diagonal grid shell structure is a spatial structure characterized by its diagonal members forming a grid pattern, which provides excellent load-bearing capacity and stiffness efficiency. The use of CFST columns in this structural system combines the high strength and ductility of CFST members with the geometric efficiency of the diagonal grid configuration. The following table summarizes the key energy parameters analyzed in the study:

Key Findings and Energy Distribution Patterns

The research revealed that the structure primarily relies on damping energy and hysteretic energy to balance the seismic input energy. The damping energy accounts for the elastic energy dissipation through material damping and viscous damping, while the hysteretic energy represents the inelastic energy dissipation through plastic deformation of structural members. This finding is consistent with the fundamental principle of seismic energy balance and confirms that the diagonal grid shell structure has adequate energy dissipation capacity.

The study identified that the diagonal columns and connecting beams are the primary energy-dissipating components of the diagonal grid shell structure. This is expected because these members undergo the largest displacements and deformations under seismic loading, leading to significant plastic deformation and energy dissipation. The energy dissipation by the diagonal columns was found to decrease with increasing floor height, which is consistent with the typical cantilever bending behavior of the structure. However, a notable finding was the presence of abrupt changes in the hysteretic energy distribution at module junctions, where the structural configuration changes and stiffness discontinuities occur.

The dynamic characteristics of the structure, including natural frequencies and mode shapes, were found to influence the energy distribution pattern. The fundamental frequency of the structure determines the primary period of energy input, while the higher-order modes contribute to the distribution of energy among different components. The seismic ground motion parameters, including amplitude, duration, and frequency content, also significantly affect the energy distribution. Stronger ground motions with longer duration lead to greater cumulative hysteretic energy dissipation and more extensive plastic deformation.

Connection to Steel Pipe Manufacturing and Welding Quality

From the steel pipe manufacturing perspective, the CFST columns used in diagonal grid shell structures are typically fabricated from circular or square hollow structural sections. The quality of these steel pipes directly affects the seismic performance of the structure. For circular CFST columns, the manufacturing process should ensure uniform wall thickness and minimal ovality, as geometric imperfections can lead to premature local buckling under compressive loading. The diameter-to-thickness ratio should be controlled to ensure adequate local buckling resistance; a D/t ratio below 60 is generally recommended for seismic applications.

The welding quality of the connections between CFST columns and connecting beams is critical for the seismic performance of the structure. These connections must be designed to provide adequate strength, stiffness, and ductility under cyclic loading. Full-penetration groove welds or high-strength bolted connections with slip-critical design are commonly used. The welding procedure specifications should include appropriate heat input control, preheat requirements, and post-weld heat treatment to minimize residual stresses and ensure adequate weld toughness.

The material grade of the steel pipes should be selected to ensure adequate strength and ductility. Q345 or Q390 grade structural steel is commonly specified, with minimum yield strength of 345 MPa and elongation of at least 20%. For seismic applications, the elongation at fracture should be verified to ensure sufficient ductility for the steel pipes to undergo large plastic deformations before failure. The chemical composition should be controlled to limit carbon equivalent (CEV ≤ 0.45%) for good weldability.

Key Questions and Engineering Reflections

The research raises several important questions for further investigation. First, the effect of the axial restoring force model parameters on the energy distribution pattern deserves more systematic study, as the model parameters are derived from experimental data and may not accurately represent the behavior of all CFST columns. Second, the influence of the concrete strength and steel tube material properties on the energy dissipation capacity should be investigated more thoroughly. Third, the long-term behavior of the structure under repeated seismic events, including the accumulation of damage and the degradation of energy dissipation capacity, is not addressed in this study.

In terms of practical design considerations, the research highlights the importance of avoiding large stiffness discontinuities at module junctions in diagonal grid shell structures. The abrupt changes in hysteretic energy distribution at these locations indicate that the stiffness transition should be gradual to ensure uniform energy distribution and prevent localized damage. This can be achieved by varying the section size or material properties of the members near the module junctions, or by introducing transition elements that provide a smooth stiffness transition.

Study Insights and Practical Implications

This research provides valuable insights into the seismic energy behavior of CFST diagonal grid shell structures and offers a framework for evaluating the seismic damage and energy dissipation capacity of such structures. The key takeaway is that the energy analysis approach provides a more comprehensive understanding of the seismic behavior than traditional force-based analysis, as it captures the cumulative damage and energy dissipation patterns that are not directly visible from force-based results. For steel pipe manufacturers, this research underscores the importance of producing high-quality steel pipes with consistent mechanical properties and minimal geometric imperfections, as these factors directly affect the energy dissipation capacity and seismic performance of the structure.