Zhuojin Pipe Fitting Co., LtdNonlinear Analysis of CFST Frame-Central Brace Structures Against Progressive Collapse Using Fiber Element Models
Literature Overview
This paper, published in the Journal of Earthquake Engineering in 2023, authored by Wang Jingxuan, Zhang Yufeng, and Shen Yajun from the School of Civil Engineering, Lanzhou University of Technology, investigates the progressive collapse resistance of steel tube concrete (CFST) frame structures with central braces using nonlinear finite element analysis. The study is supported by the National Natural Science Foundation of China (Grants 52068047, 51708270) and the Gansu Provincial Youth Science and Technology Fund (Grant 20JR5RA437). The research employs fiber beam models to simulate five different CFST frame-central brace configurations under various column failure scenarios.
Core Technical Methodology
The study adopts a fiber beam element model, which discretizes the cross-section of each structural member into multiple material fibers, each governed by its own constitutive law. This approach allows for accurate representation of material nonlinearity, including steel yielding and concrete crushing, without the computational cost of a full three-dimensional solid element model. The model captures the interaction between the steel tube and concrete core through appropriate interface modeling and material constitutive relationships.
Constitutive Models Used
| Component | Constitutive Model | Key Parameters |
|---|---|---|
| Steel tube | Von Mises yield criterion with kinematic hardening | Yield strength fy, elastic modulus E, hardening ratio |
| Concrete core | Mander et al. confined concrete model | Compressive strength f'c, confinement ratio, ductility factor |
| Interface | Frictional contact with bond-slip relationship | Friction coefficient, bond strength |
| Braces | Same as steel tube with geometric nonlinearity | P-Δ effects included |
Analysis Results and Key Findings
The study evaluates five structural configurations under two failure scenarios: edge column removal and middle column removal. The results reveal several important engineering insights:
- Central braces improve overall structural performance: All configurations with central braces show enhanced overall stiffness and collapse resistance compared to bare frames. The braces provide additional load paths that redistribute forces away from the failed column region.
- Edge column failure is less critical than middle column failure: The improvement provided by central braces is more pronounced for edge column failure scenarios. This is because edge column failure creates a more localized damage pattern, while middle column failure affects a larger portion of the structural system.
- X-type braces provide the most consistent improvement: X-type central braces significantly enhance frame stiffness and load capacity across all failure scenarios and reduce vertical displacement at the failure node. The dual diagonal configuration provides bidirectional load transfer paths.
- Inverted diagonal braces offer superior ductility: The inverted diagonal brace configuration demonstrates better ductility and ultimate load capacity, making it more suitable for structures where energy dissipation is a priority.
Performance Comparison of Brace Configurations
| Brace Configuration | Stiffness Improvement (%) | Load Capacity Improvement (%) | Vertical Displacement Reduction (%) | Ductility Index |
|---|---|---|---|---|
| Bare frame (no brace) | Baseline | Baseline | Baseline | Baseline |
| Single diagonal (forward) | +15–25 | +20–35 | +20–30 | Moderate |
| Single diagonal (inverted) | +12–20 | +25–40 | +18–28 | High |
| X-type brace | +30–45 | +40–55 | +35–50 | Moderate-high |
| K-type brace | +20–30 | +28–42 | +25–38 | Moderate |
| V-type brace | +18–28 | +25–38 | +22–35 | Moderate |
Engineering Practice Implications
The findings have direct implications for the design of CFST frame structures in regions with progressive collapse risk, such as urban infrastructure, public buildings, and industrial facilities. The study provides quantitative data that can be used in performance-based design to select the most appropriate brace configuration for specific structural requirements.
For steel pipe selection in brace applications, the following considerations emerge:
- Brace pipe diameter: Larger diameters provide greater stiffness but may reduce the ductility of the system. A balance must be struck between stiffness enhancement and energy dissipation capacity.
- Brace pipe wall thickness: Thicker walls increase load capacity but also increase the risk of local buckling under compressive loading. The slenderness ratio of the brace member must be controlled to prevent premature buckling.
- CFST brace vs. hollow steel brace: CFST braces provide higher load capacity and better energy dissipation but at higher cost and weight. The choice depends on the specific performance requirements and economic constraints.
Design Recommendations Based on Study Findings
| Design Priority | Recommended Brace Configuration | Rationale |
|---|---|---|
| Maximum stiffness | X-type brace | Provides bidirectional load transfer with highest stiffness improvement |
| Maximum ductility | Inverted diagonal brace | Superior energy dissipation and ductility under cyclic loading |
| Balanced performance | X-type or inverted diagonal | Good combination of stiffness and ductility |
| Cost-sensitive design | Single diagonal brace | Lower cost with moderate performance improvement |
| Seismic design | X-type or inverted diagonal | Better energy dissipation under seismic loading |
Key Questions and Reflections
The study raises an important question about the applicability of the fiber element model to other types of composite structures, such as steel-encased concrete or steel-reinforced concrete columns. The fiber model approach has proven effective for CFST members, but its accuracy for other composite systems depends on the ability to accurately model the interface behavior between different materials.
Additionally, the study focuses on quasi-static progressive collapse scenarios, but the dynamic effects of sudden column failure are not fully captured. Future research should incorporate dynamic loading analysis to more accurately simulate real-world progressive collapse events, which involve high-rate loading and inertial effects.
Study Insights and Implications
This research provides valuable quantitative data for the progressive collapse resistance design of CFST frame structures with central braces. The use of fiber element models offers a computationally efficient yet accurate approach for nonlinear analysis, making it suitable for parametric studies and design optimization. The findings support the inclusion of central braces as an effective strategy for enhancing progressive collapse resistance, with X-type and inverted diagonal configurations offering the best performance. These results can be directly applied in performance-based design codes and standards for CFST structures, contributing to the development of safer and more resilient building systems.