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ISO/TR 25439:2025 — Design Examples of Concrete-Filled Steel Tubular (CFST) Hybrid Structures
Practical design calculations for trussed and concrete-encased CFST structures per ISO 16521
1. Introduction to CFST Hybrid Structures and ISO/TR 25439
Concrete-filled steel tubular (CFST) hybrid structures combine the complementary strengths of steel and concrete: the steel tube provides tensile strength, confinement to the concrete core, and serves as permanent formwork, while the concrete core enhances compressive capacity and stiffens the steel tube against local buckling. CFST hybrid structures have become increasingly popular in large-span buildings, bridge piers, and high-rise construction due to their excellent seismic performance, high load-bearing capacity, and favorable constructability.
ISO/TR 25439:2025 provides detailed design examples that demonstrate the practical application of ISO 16521 (Design of concrete-filled steel tubular (CFST) hybrid structures). The document covers two major structural types — trussed CFST hybrid structures and concrete-encased CFST hybrid structures — with step-by-step calculations that follow the design procedure specified in ISO 16521:2024, Clause 6. Each example includes explicit cross-references to the relevant subclauses in ISO 16521, making it an effective training and reference tool.
Developed by ISO/TC 71/SC 9 (Steel-concrete composite and hybrid structures), ISO/TR 25439 bridges the gap between theoretical design provisions and practical engineering application. The design examples are not based on specific engineering projects but are carefully constructed to illustrate the full range of design considerations.
2. Design Examples for Trussed CFST Hybrid Structures
2.1 Structure Configurations
Four design examples are presented for trussed CFST hybrid structures: a three-chord structure without concrete slab, two four-chord structures without concrete slab (featuring different configurations), and a four-chord structure with a concrete slab. Each example addresses material compatibility (ISO 16521:2024, 5.2.6), requirements for CFST chords (Clause 7.1), and requirements for webs (Clause 15.2.4). The variety of configurations ensures that engineers can find a reference case closely matching their specific design scenario.
| Example | Configuration | Chords | Concrete Slab | Key Design Checks |
|---|---|---|---|---|
| 1 | Trussed | 3 chords | No | Cross-section indices, axial compression, bending |
| 2 | Trussed | 4 chords | No | Compression-bending interaction, shear resistance |
| 3 | Trussed | 4 chords | No | Connection detailing, joint resistance |
| 4 | Trussed | 4 chords | Yes | Composite action, slab interaction |
2.2 Structural Resistance Calculations
The design procedure progresses through calculation of cross-section indices (strength and stiffness per ISO 16521:2024, 10.3), followed by structural resistance verification including axial compression resistance (Clause 11.2.1), bending resistance (Clause 11.2.2), combined compression and bending (Clause 11.2.3), and shear resistance (Clause 11.3). Protective design considerations cover corrosion resistance (Clause 14.2.2) and impact resistance (Clause 14.4). Each calculation step is demonstrated with numerical values, showing how the theoretical formulae from ISO 16521 are applied with realistic material properties and geometric parameters.
A critical engineering consideration in CFST design is the void ratio of core concrete within the steel tube. The standard requires verification of the limiting void ratio (Clause 16.3.10) because excessive voids reduce the confinement effect and compromise the composite action that gives CFST structures their superior performance. Void ratio verification must be performed using appropriate quality control methods during both design and construction phases.
3. Concrete-Encased CFST and Global Structural Analysis
3.1 Concrete-Encased CFST Examples
Four additional examples cover concrete-encased CFST hybrid structures: a single-chord structure, a six-chord structure, a four-chord structure with circular CFST members, and a four-chord structure with rectangular CFST members. These examples verify resistances in compression, combined compression and bending, long-term load effects (including creep and shrinkage), and shear in accordance with ISO 16521:2024, Clause 12. Fire resistance calculations follow Clause 14, addressing a critical safety consideration for building applications.
3.2 Global Structural Analysis Example
Clause 6 of the standard presents a comprehensive global structural analysis of a concrete-encased CFST hybrid arch structure using a fibre-based model per ISO 16521:2024, Clause 10. The analysis includes load determination, model establishment, load-displacement relationship analysis, deformation analysis, and stress-strain analysis. This example demonstrates how refined analysis methods can capture the nonlinear behavior of CFST structures under extreme loading conditions, accounting for material nonlinearity, geometric nonlinearity, and the confinement interaction between steel tube and concrete core.
3.3 Experimental Verification
Annex A provides experimental data that served as the basis for developing ISO 16521. These published experimental results verify the design methods and provide engineers with benchmark examples for validating their own analysis models against physical test data. The experimental comparisons cover a wide range of member geometries, concrete strengths, steel yield strengths, and loading configurations.
One of the most valuable contributions of ISO/TR 25439 is its demonstration of how simplified design formulae (derived from refined analysis and experimental verification) can be applied in routine practice. The detailed cross-referencing between design steps and ISO 16521 clauses makes it an excellent training tool for engineers new to composite structure design.
4. Frequently Asked Questions
Q1: What are the main advantages of CFST hybrid structures over traditional steel or reinforced concrete structures?
A: CFST structures offer higher strength-to-weight ratios, excellent seismic energy dissipation, reduced construction time (the steel tube acts as permanent formwork), and superior fire resistance when concrete encasement is provided. The steel-concrete interaction creates a confinement effect that significantly enhances the compressive capacity of the concrete core.
A: CFST structures offer higher strength-to-weight ratios, excellent seismic energy dissipation, reduced construction time (the steel tube acts as permanent formwork), and superior fire resistance when concrete encasement is provided. The steel-concrete interaction creates a confinement effect that significantly enhances the compressive capacity of the concrete core.
Q2: Can ISO/TR 25439 be used independently of ISO 16521?
A: No. The technical report is intended as a companion to ISO 16521 and provides worked examples that reference specific clauses of the parent standard. Users need access to ISO 16521 to understand the underlying design provisions and to apply the methodology to their own projects.
A: No. The technical report is intended as a companion to ISO 16521 and provides worked examples that reference specific clauses of the parent standard. Users need access to ISO 16521 to understand the underlying design provisions and to apply the methodology to their own projects.
Q3: Are the design examples applicable to seismic design?
A: While the examples primarily cover gravity and basic load combinations, the design methods in ISO 16521 address seismic considerations through ductility requirements and capacity design principles. Engineers should consult local seismic codes and supplementary provisions for earthquake-specific design.
A: While the examples primarily cover gravity and basic load combinations, the design methods in ISO 16521 address seismic considerations through ductility requirements and capacity design principles. Engineers should consult local seismic codes and supplementary provisions for earthquake-specific design.
Q4: How does the void ratio of core concrete affect structural performance?
A: The void ratio directly influences the confinement effect provided by the steel tube. Higher void ratios reduce the effective composite section and can lead to premature local buckling of the steel tube. ISO 16521 specifies limiting void ratios that must be verified during design to ensure intended composite action.
A: The void ratio directly influences the confinement effect provided by the steel tube. Higher void ratios reduce the effective composite section and can lead to premature local buckling of the steel tube. ISO 16521 specifies limiting void ratios that must be verified during design to ensure intended composite action.
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