Overview
ISO/TR 25439:2025 - "Design examples of concrete-filled steel tubular (CFST) hybrid structures in accordance with ISO 16521" - is a Technical Report that provides worked design examples and explanatory calculations to support the application of ISO 16521. It contains step‑by‑step design calculations for major CFST hybrid structural types (trussed and concrete‑encased), covers principal loading cases, and follows the design procedure given in ISO 16521:2024, Clause 6. The report also includes a global structural (fibre) analysis example of a concrete‑encased CFST hybrid arch and an informative annex with experimental verification data.
Key topics and technical requirements
- Design examples: Four trussed CFST hybrid examples and four concrete‑encased CFST hybrid examples that demonstrate sizing, indexing and verification procedures.
- Reference to ISO 16521: All calculations and verifications follow the methods, limit states and detailing rules given in ISO 16521.
- Structural indices and analysis: Calculation of cross‑section strength and stiffness indices (ISO 16521, Clause 10) and structural resistance checks for compression, bending, combined compression/bending and shear (Clauses 11 and 12).
- Fibre‑based global analysis: A detailed fibre model analysis, including mesh, stress–strain relationships and load–displacement/deformation studies for an arch example (Clause 6).
- Protective design: Procedures and examples for corrosion, impact and fire resistance design in accordance with the protective design clauses of ISO 16521 (Clause 14).
- Connections and detailing: Detailing requirements, joint resistance, fatigue checks and connection design examples (Clause 15).
- Core concrete void checks & long‑term effects: Verification of limiting core concrete void ratio and design for long‑term loads where applicable (Clause 16 and Clause 12.6).
- Experimental verification: Annex A presents published experimental data used to validate the design methods and examples.
Practical applications and who uses it
ISO/TR 25439:2025 is intended as a practical companion to ISO 16521 for:
- Structural engineers and designers working on CFST hybrid trusses, frames and arches.
- Design offices and consultants needing worked examples for specification, calculation checks and design documentation.
- Fabricators and contractors seeking clarity on detailing, joint design and protective measures.
- Researchers and educators using experimental verifications and fibre‑model examples for teaching and validation.
- Standards bodies and code writers referencing applied examples to harmonize national rules with ISO 16521.
Related standards
- ISO 16521 - Design of concrete‑filled steel tubular (CFST) hybrid structures (primary normative reference).
- ISO/TR 25439 complements ISO 16521 by providing worked examples, calculation guidance and experimental validation to aid practical implementation.
Keywords: ISO TR 25439:2025, CFST hybrid structures, concrete‑filled steel tubular, ISO 16521, design examples, trussed CFST, concrete‑encased CFST, fibre model, structural resistance, fire and corrosion design.
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