ISO/DTR 25439

ISO/TC 71/SC 9/WG 1
Secretariat: SAC
Date: 2025-06-2007-29
Design examples of concrete-filled steel tubular (CFST) hybrid
structures in accordance with ISO 16521

DTR stage
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iii
ISO #####-#:####(X/DTR 25439:(en)
Contents
Foreword . v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Design examples of trussed CFST hybrid structures . 1
4.1 General requirements . 4
4.2 Calculation of indices for cross-sections . 5
4.3 Calculation of structural resistances . 7
4.4 Protective design . 21
4.5 Design of connections . 24
4.6 Calculation of core concrete void ratio (ISO 16521:2024, 16.3.10) . 26
5 Design examples of concrete-encased CFST hybrid structures . 26
5.1 General requirements . 29
5.2 Calculation of indices for cross-sections . 31
5.3 Calculation of structural resistances . 33
5.4 Protective design for fire resistance (ISO 16521:2024, 14.3) . 39
6 Structural analysis example of concrete-encased CFST hybrid arch . 40
6.1 Description of the arch structure . 40
6.2 Load conditions . 42
6.3 Establishment of the fibre model . 42
6.4 Analysis of load-displacement relationship . 43
6.5 Analysis of deformation . 44
6.6 Analysis of stress in steel and concrete . 44
Annex A (informative) Experimental verifications of design methods for concrete-filled steel
tubular (CFST) hybrid structures . 46
Bibliography . 72

© ISO #### 2025 – All rights reserved
iv
Foreword
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This document was prepared by Technical Committee ISO/TC 71, Concrete, reinforced concrete and pre-
stressed concrete, Subcommittee SC 9, Steel-concrete composite and hybrid structures.
A list of all parts in the ISO/TR 25439 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
v
ISO #####-#:####(X/DTR 25439:(en)
Introduction
This document provides informative design examples of concrete-filled steel tubular (CFST) hybrid structures
in accordance with ISO 16521, Design of concrete-filled steel tubular (CFST) hybrid structures. ISO 16521
stipulates the conceptional design, refined analysis method, simplified design formulae based on refined
analysis and experimental verifications, and detailing design of CFST hybrid structures. Detailed calculations
are provided in the design examples and the corresponding reference subclauses in ISO 16521 are indicated.
For trussed CFST hybrid structures, four design examples are provided, which include a three-chord structure
and two four-chord structures without concrete slab, and a four-chord structure with a concrete slab. General
requirements are verified and indices for cross-sections are calculated. The resistances of the hybrid
structures as well as the CFST chords in compression, bending, combined compression and bending, and
shear, are calculated in accordance with ISO 16521:2024, Clause 11. The protective design of corrosion
resistance and impact resistance is presented in accordance with ISO 16521:2024, Clause 14. The detailing
requirements and the resistances of the joints are verified in accordance with ISO 16521:2024, Clause 15.
Finally, the verification of the limiting value of the core concrete void in the steel tube is carried out in
accordance with ISO 16521:2024, Clause 16.
For concrete-encased CFST hybrid structures, four design examples are presented, i.e., a single-chord
structure, a six-chord structure, and a four-chord structure with circular CFST members and a four-chord
structure with rectangular CFST members. General requirements are verified and indices for cross-sections
are calculated. Resistances in compression, combined compression and bending, resistance considering long-
term load effects, shear resistance, are verified in accordance with ISO 16521:2024, Clause 12. Fire resistance
is calculated in accordance with ISO 16521:2024, Clause 14.
6Clause 6 of this document presents a global structural analysis example. A concrete-encased CFST hybrid
arch structure is analysed in accordance with ISO 16521:2024, Clause 10, which employs a fibre-based model.
It includes the determination of load, model establishment, load-displacement relationship analysis,
deformation analysis, and stress and strain analysis.
Annex AAnnex A is provided in this document to present some experimental data on CFST hybrid structures.
These published experimental data serve as an important basis for the development of ISO 16521.
Furthermore, they provide verifications of the design methods of CFST hybrid structures and examples for
design of the structures based on experimental data, as stipulated in the standard.
It is worth pointing out that the design examples are only provided to help readers in using ISO 16521 and
they are not based on any specific engineering projects.
© ISO #### 2025 – All rights reserved
vi
Design examples of concrete-filled steel tubular (CFST) hybrid
structures in accordance with ISO 16521
1 Scope
This document provides design examples of concrete-filled steel tubular (CFST) hybrid structures in
accordance with ISO 16521.
This document includes the design calculation of major structural types in ISO 16521, i.e., trussed CFST hybrid
structures, concrete-encased CFST hybrid structures. The design examples cover the major loading cases for
the structures and follow the design procedure presented in ISO 16521:2024, Clause 6.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 16521, Design of concrete-filled steel tubular (CFST) hybrid structures
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 16521 apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp
— IEC Electropedia: available at https://www.electropedia.org/
4 Design examples of trussed CFST hybrid structures
In this clause, the design of four examples of trussed concrete-filled steel tubular (CFST) hybrid structures
with and without a concrete slab are conducted in accordance with the corresponding clauses of ISO 16521.
The general design and construction procedure in ISO 16521:2024, 6.1, is followed, and the correspondence
between this clause and the detailed clauses of ISO 16521:2024 are shown in Table 1Table 1.
Table 1 — Design and construction procedure of trussed CFST hybrid structures
Procedure Clause in ISO 16521:2024 Subclause in this document
Preliminary structural design Clauses 5 to 8 4.14.1
Definition of actions (loads) Clause 9 /
Structural analysis Clause 10 4.24.2
Ultimate limit states design Clause 11 4.34.3
Serviceability limit states design Clause 13 /
Protective design Clause 14 4.44.4
Detailing design Clause 15 4.14.1 and 4.54.5
Construction and acceptance Clause 16 4.64.6
ISO #####-#:####(X/DTR 25439:(en)
Four examples are designed as shown in Table 2Table 2. Circular (Examples T1, T2, T4) and square (Example
T3) steel tubes are used for the CFST chords and circular steel tubes are used as the webs, and the interval
lengths of the chords l (Figure 13 in ISO 16521) are 3000 3 000 mm, 7750 7 750 mm, 7750mm7 750 mm, and
3000 3 000 mm for the four examples respectively.
Table 2 — Geometric and material properties of design examples
Web
Chords in
Chords in tension
Example compression D×t h (mm) b (mm) f (MPa) f (MPa)
d ×t i i y ck
w w
D×t or B×t (mm)
or B×t (mm)
(mm)
20002 00 20002 00
T1 D×t, 720×20 D×t, 720×20 392×18 355 41
0 0
D×t, D×t, 85008 50 85008 50
T2 850×18 420 60
14001 400×30 14001 400×30 0 0
B×t, B×t, 85008 50 85008 50
T3 850×18 420 60
14001 400×40 14001 400×40 0 0
18751 87 18751 87
T4 D×t, 400×10 D×t, 700×25 280×14 355 41
5 5
NOTE D is the outside diameter of the cir
...