Systematic Review of the Torsional Performance of Concrete-Filled Double Skin Steel Tube (CFDST) Members under Fire Conditions Following PRISMA Protocols

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Lateef" }, { "@type": "Person", "name": "Akram S. Mahmoud" } ], "publisher": { "@type": "Organization", "name": "F1000Research", "logo": { "@type": "ImageObject", "url": "https://f1000research.com/img/AMP/F1000Research_image.png", "height": 480, "width": 60 } }, "image": { "@type": "ImageObject", "url": "https://f1000research.com/img/AMP/F1000Research_image.png", "height": 1200, "width": 150 }, "description": "Concrete-Filled Double Skin Steel Tubes (CFDST) have emerged as a promising composite structural system that integrates the mechanical advantages of steel and concrete while achieving enhanced energy dissipation, reduced weight, and improved post-fire resilience. Over the past two decades, substantial experimental and numerical efforts have focused on understanding the torsional and thermal performance of CFDST and related CFST members. However, an integrated synthesis of these findings under a unified systematic framework has been lacking. This study conducts a comprehensive systematic review of 37 selected experimental and analytical studies addressing the torsional and fire behavior of CFDST and CFST members, following PRISMA guidelines “the PRISMA methodology, a standardized framework for conducting systematic reviews that ensures transparency, rigorous screening, and unbiased selection of relevant studies through a structured flowchart process”. The review identifies key influencing factors, including section geometry, wall thickness, concrete type, steel grade, axial load level, and fire exposure duration. Comparative analysis reveals that torsional resistance increases with lower hollow ratios, thicker outer tubes, and confined concrete cores, while elevated temperatures significantly reduce torsional stiffness and residual strength. Despite considerable research on CFST under fire and torsion separately, the coupling effect of post-fire torsional performance remains underexplored. Based on the identified research gaps, a new experimental program is proposed to investigate the pre- and post-fire torsional performance of CFDST columns with varying cross-sections and steel thicknesses. The study concludes with future research recommendations focused on developing constitutive models, hybrid materials, and fire-torsion interaction design equations for CFDST systems." } { "@context": "http://schema.org", "@type": "BreadcrumbList", "itemListElement": [ { "@type": "ListItem", "position": "1", "item": { "@id": "https://f1000research.com/", "name": "Home" } }, { "@type": "ListItem", "position": "2", "item": { "@id": "https://f1000research.com/browse/articles", "name": "Browse" } }, { "@type": "ListItem", "position": "3", "item": { "@id": "https://f1000research.com/articles/15-189", "name": "Systematic Review of the Torsional Performance of Concrete-Filled..." } } ] } Home Browse Systematic Review of the Torsional Performance of Concrete-Filled... ALL Metrics - Views Downloads Get PDF Get XML Cite How to cite this article Rajab OF, Lateef AM and Mahmoud AS. Systematic Review of the Torsional Performance of Concrete-Filled Double Skin Steel Tube (CFDST) Members under Fire Conditions Following PRISMA Protocols [version 1; peer review: 1 not approved] . F1000Research 2026, 15 :189 ( https://doi.org/10.12688/f1000research.176317.1 ) NOTE: If applicable, it is important to ensure the information in square brackets after the title is included in all citations of this article. Close Copy Citation Details Export Export Citation Sciwheel EndNote Ref. Manager Bibtex ProCite Sente EXPORT Select a format first Track Share ▬ ✚ Systematic Review Systematic Review of the Torsional Performance of Concrete-Filled Double Skin Steel Tube (CFDST) Members under Fire Conditions Following PRISMA Protocols [version 1; peer review: 1 not approved] Omar Fazaa Rajab https://orcid.org/0000-0003-1802-3238 1 , Assim M. Lateef 2 , Akram S. Mahmoud 3 Omar Fazaa Rajab https://orcid.org/0000-0003-1802-3238 1 , Assim M. Lateef 2 , Akram S. Mahmoud 3 PUBLISHED 05 Feb 2026 Author details Author details 1 Construction and projects department, University of Fallujah, Al-Fallujah, Al Anbar Governorate, Iraq 2 Civil Engineering, Tikrit University College of Engineering, Tikrit, Saladin Governorate, Iraq 3 Civil Engineering, University of Anbar College of Engineering, Ramadi, Anbar Governorate, Iraq Omar Fazaa Rajab Roles: Writing – Original Draft Preparation, Writing – Review & Editing Assim M. Lateef Roles: Supervision Akram S. Mahmoud Roles: Supervision OPEN PEER REVIEW DETAILS REVIEWER STATUS This article is included in the Fallujah Multidisciplinary Science and Innovation gateway. Abstract Concrete-Filled Double Skin Steel Tubes (CFDST) have emerged as a promising composite structural system that integrates the mechanical advantages of steel and concrete while achieving enhanced energy dissipation, reduced weight, and improved post-fire resilience. Over the past two decades, substantial experimental and numerical efforts have focused on understanding the torsional and thermal performance of CFDST and related CFST members. However, an integrated synthesis of these findings under a unified systematic framework has been lacking. This study conducts a comprehensive systematic review of 37 selected experimental and analytical studies addressing the torsional and fire behavior of CFDST and CFST members, following PRISMA guidelines “the PRISMA methodology, a standardized framework for conducting systematic reviews that ensures transparency, rigorous screening, and unbiased selection of relevant studies through a structured flowchart process”. The review identifies key influencing factors, including section geometry, wall thickness, concrete type, steel grade, axial load level, and fire exposure duration. Comparative analysis reveals that torsional resistance increases with lower hollow ratios, thicker outer tubes, and confined concrete cores, while elevated temperatures significantly reduce torsional stiffness and residual strength. Despite considerable research on CFST under fire and torsion separately, the coupling effect of post-fire torsional performance remains underexplored. Based on the identified research gaps, a new experimental program is proposed to investigate the pre- and post-fire torsional performance of CFDST columns with varying cross-sections and steel thicknesses. The study concludes with future research recommendations focused on developing constitutive models, hybrid materials, and fire-torsion interaction design equations for CFDST systems. READ ALL READ LESS Keywords CFDST, CFST, torsional performance, fire resistance, systematic review, PRISMA, composite columns, residual strength. Corresponding Author(s) Omar Fazaa Rajab ( [email protected] ) Close Corresponding author: Omar Fazaa Rajab Competing interests: No competing interests were disclosed. Grant information: The author(s) declared that no grants were involved in supporting this work. Copyright: © 2026 Rajab OF et al . This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. How to cite: Rajab OF, Lateef AM and Mahmoud AS. Systematic Review of the Torsional Performance of Concrete-Filled Double Skin Steel Tube (CFDST) Members under Fire Conditions Following PRISMA Protocols [version 1; peer review: 1 not approved] . F1000Research 2026, 15 :189 ( https://doi.org/10.12688/f1000research.176317.1 ) First published: 05 Feb 2026, 15 :189 ( https://doi.org/10.12688/f1000research.176317.1 ) Latest published: 05 Feb 2026, 15 :189 ( https://doi.org/10.12688/f1000research.176317.1 ) 1. Introduction Composite steel–concrete systems have become a cornerstone in modern structural design due to their high strength-to-weight ratio, superior ductility, and inherent fire resistance. Among them, Concrete-Filled Double Skin Steel Tubes (CFDST) represent an advanced evolution of conventional Concrete-Filled Steel Tubes (CFST), featuring an inner and outer steel tube separated by a concrete core. This configuration optimizes both structural efficiency and thermal stability, as the inner cavity mitigates weight and thermal stress while maintaining load-carrying integrity under fire and seismic conditions. 1 – 3 The torsional behavior of composite tubular columns is particularly significant in structures subjected to asymmetric loads, curved bridge girders, or combined lateral-torsional demands. Experimental research has shown that torsion-dominated loading can induce local buckling, concrete cracking, and degradation of stiffness. 4 , 5 In double-skin configurations, the confinement provided by both steel tubes enhances torsional ductility and prevents premature shear cracking. 6 Moreover, the hollow core modifies the stress flow, allowing for improved energy absorption and reduced stiffness degradation under cyclic torsion. 7 Simultaneously, the fire resistance of these composite systems remains a key determinant of structural safety. Under fire exposure, the outer steel tube experiences rapid temperature rise and loss of yield strength, while the concrete core and inner steel tube provide passive protection and structural continuity. 1 , 2 , 8 Studies on CFDST columns under ISO-834 and ASTM E-119 curves revealed that residual load-bearing capacity can retain up to 70–85% of its ambient value after moderate fire durations, depending on geometry and load ratio. 9 , 10 Despite the extensive work on either torsion or fire behavior separately, the coupled effect of fire exposure on torsional resistance of CFDST members has not yet been systematically investigated. This knowledge gap limits the development of design codes and predictive models addressing post-fire torsional stiffness and residual strength. Therefore, a systematic synthesis of existing findings is essential to identify parameters governing performance degradation, inter-material interaction, and potential synergies in CFDST systems. Accordingly, the present study aims to: 1. Conduct a systematic review of existing torsional and fire studies on CFST and CFDST members. 2. Compare and synthesize their performance trends in terms of torque capacity, ductility, stiffness, and fire resistance. 3. Identify key research gaps and propose a detailed experimental program for investigating post-fire torsional performance of CFDST columns. The review follows the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) methodology to ensure transparency, reproducibility, and comprehensive coverage of the relevant literature. 2. Research methodology (PRISMA framework) This systematic review adheres to the PRISMA 2020 guidelines, ensuring a structured, transparent, and replicable approach to literature identification, screening, eligibility, and inclusion. 11 2.1 Identification and data sources The literature search was conducted using databases including Scopus, ScienceDirect, Web of Science, and Google Scholar, covering publications from 2003 to 2024. Keywords used were: CFDST, CFST, torsion, fire resistance, post-fire behavior, double-skin tubular columns, composite torsion, and residual strength. Additionally, 37 experimental and analytical studies from previous systematic compilations were incorporated, as summarized in the provided data files. 2.2 Screening process Initial searches yielded approximately 108 articles. After removing duplicates, 103 records remained. Abstracts and titles were screened for relevance to torsional performance or fire response of CFST/CFDST systems. Numerical-only and purely theoretical papers without experimental or validated FE results were excluded unless directly linked to experimental datasets. This stage reduced the dataset to 78 studies. 2.3 Eligibility and inclusion criteria Final inclusion required that: • Specimens involve CFST or CFDST configurations, with or without inner tubes. • Studies present measured mechanical or thermal responses (torque, stiffness, axial capacity, temperature field, residual strength). • The methodology and results were reproducible and peer-reviewed. Following full-text analysis, 37 studies met the PRISMA inclusion criteria: • 19 torsional studies (2003–2024) focused on pure torsion, compression–torsion, and cyclic torsion of CFST/CFDST members. • 18 fire studies (2003–2022) examined ambient, elevated, and post-fire axial behavior. 2.4 PRISMA flow summary AS shown in ( Figure 1 ) summarizes the study selection process: • Records identified: 108 • Records screened after duplicates: 103 • Full-text assessed for eligibility: 87 • Full-text articles excluded: 78 • Studies included in synthesis: 37 Figure 1. PRISMA 2020 flow diagram illustrating the study identification, screening, eligibility, and inclusion process. The systematic flow ensured balanced coverage of both torsional and fire aspects, providing a foundation for cross-comparison and synthesis of performance characteristics under combined conditions. 3. Review of previous studies This section presents a comprehensive review and synthesis of prior experimental and analytical studies on the torsional and fire behavior of CFST and CFDST members. The discussion is structured into two main parts: • Section 3.1 — Studies on torsional performance under ambient and combined loading conditions. • Section 3.2 — Studies on behavior under elevated temperatures and fire exposure. The findings are critically analyzed to establish trends, identify controlling parameters, and assess the existing limitations that motivate the current experimental program. 3.1 Torsional behavior of CFST and CFDST members The systematic search identified 19 relevant studies (2003–2024) that addressed torsional resistance of CFST and CFDST members. The following table, ( Table 1 ), is a comparative summary of torsion studies. Table 1. Comparative summary of torsion studies on CFST and CFDST members. Reference No. Author/Year No. Specimens Member Type Section Shape Outer Dimensions (mm) Steel Thickness (mm) Inner Tube/Dimensions Concrete Type/Steel Strength MPa Test Type Ty/(kN m) θy/(deg.) Tp/(kN m) θu/(deg.) Failure Mode Key Findings 7 Mutlag, S. E., & Lafta, A. M (2024) 12 Beams Square SHS 100 × 100 1.5 & 2 With stiffening bars HSC fc = 65.36 Pure torsion HST2-5 = 25.55 0.187 rad/m HST2-5 =37.79 0.24 rad/m Local buckling delayed; higher ductility & energy absorption ↑ torsional strength vs unstiffened 5 Jia, Shi, Xian, Wang (2021) 6 CFST columns Circular Ø200 × 1000 4.2 — C50(fcu ≈ 54.), (fy = 353) Pure torsion, Compression–torsion CFST1-1 =64.5–72.0 CFST1-1 =2.4–3.3 CFST1-1 =74.8–84.6 CFST1-1 =7.8–9.2 Minor diagonal cracks (45°) in concrete, no buckling Compression before torsion ↑ 12% torque; low axial load enhances torsion, high axial load reduces it; FE model validated and design equation proposed. 17 Wang, Jia, Shi, Tan (2020) 18 SRCFST Columns//L = 1000–1500 mm Circular D = 200 mm, L = 1000–1500 mm 4.2 mm I-shaped (120 × 80 × 3), Cross (80 × 40 × 3), C.tube (Ø120 × 3) Concrete fc = 54.7 Steel fy = 353–378 Compression + Torsion CSFST1–1 = 71.5//CSFST1–2 = 69.3 CSFST1–1 = 3.8° //CSFST1–2 = 3.6° CSFST1–1 = 80//CSFST1–2 = 78.5 CSFST1–1 = 9° //CSFST1– 2 = 8.5° 45° diagonal cracks, no local buckling Embedded steel improved torsional strength & ductility; axial compression <0.4 enhances strength. 20 Wang, Wang, Yu, Zhou, Hu (2019) 72 (FE models) STRC Circular & Square D = 200–300, H = 1000 2–5 Reinforced concrete core fcu = 51.38; fy = 32–425; Pure torsion, Bending–torsion, Compression–bending–torsion ~10–28 ~1–2° ~15–45 ~5–10° Concrete crushing at end gaps, no steel buckling Verified FEM; axial load up to 0.3Nu ↑ torque, >0.4Nu ↓; parametric curves fitted (R 2 > 0.96); correlation equations proposed for design. 21 Xin, Wang, Li, Chen (2018) 8//4C+4S CFST Short Columns//L = 475 mm Circular (Ø200) & Square (200×200) ϕ200 × 6.23 & 200 × 200 × 5.82 6.23 (circular), 5.82 (square) Solid CFST fc = 40.97; Es200000 fy = 327.97–383.69 Pure Torsion, Bending-Shear, B-S-T ~60–130(varies specimen) //C-T = 105//R-T = 130 ~3–10°//C-T = 4°//R-T = 4° ~90–170//C-T = 130//R-T = 170 ~20–40°//C-T = 38°//R-T = 36° Buckling (square), ductile torsional deformation (circular) M/T ratio governs failure (B-S vs T); circular sections show higher ductility; simplified B-S-T interaction equations proposed. 19 Li, Han & Hou, 2018 20 (validation tests) Columns Circular & square (encased CFST) Ø120/SHS 120×120 ~3 Inner CFST Ø80–120 fc=30-80 fy=355 Compression + torsion//FEM matched ests (Torque ~20–30 kNm) - - Sc2-1=27 Sc2-1 = 18 Cracks in RC, buckling in steel Outer RC improved fire & ductility; formulas proposed 4 Wang, Lu & Zhou (2018) 6 CFDST column/length 475mm Circular & Square & Rectangular Ø 325/SHS 300×300/RHS 300×200 3.45 – 5.8.15 (varied) Ø 219 & 159/SHS 100×100 & 200×200/RHS 180×80 & 200×100 (fc ≈ 30 – 40) /Es≈ ((1.87-2.39)× 105) Cyclic loading (torsional/rotational) CT1= 333.45 – RT1=126.37 CT1=2.46 – RT1=0.87 CT2=519.74 – RT1=149.57 CT2 =11.87–RT1 = 5.34 Local buckling of steel tube + concrete cracking CFDST showed higher ductility and energy dissipation than CFST; increased steel thickness reduced stiffness degradation; larger hollow ratio reduced torsional resistance 22 Wang, Guo, Liu, Zhou, (2017) 8 L = 975 + 8 L = 475 CFST column Length 975mm & 475mm Circular & Square Ø 200/SHS 200×200 5.8 – 6.2 Solid (no inner hollow tube) fc ≈ (49–54.1) /fy ≈ (383.69) Combined torsion + eccentric compression R-T1 = 132//C-T1 = 101.4 R-T1 = 5.4//C-T1 = 6.7 R-T1 = 152.0//C-T1 = 127.2 R-T1 = 49.5//C-T1 = 50.5 Local buckling of steel + concrete cracking Higher eccentricity → lower torsional strength; larger steel ratio improved capacity; square > circular under eccentric compression 23 Ren, Han, Hou, Tao & Li (2017) 26 CE-CFST, RC hollow, CFST Square & Circular B&D = 200, H = 600 t = 2.98 di = 80, 100, 120 fci≈ 60, fco ≈ 40; fy ≈ 378 Es≈202 Combined axial load + torsion sc1-1= 22 sc4-1= 18 cc1-1= 20 cc4-1= 15 sc1-1= 0.09, sc4-1= 0.08 cc1-1= 0.08, cc4-1= 0.1rad sc1-1= 24.3 sc4-1= 18.5 cc1-1=21 cc4-1= 15.9 sc1-1= 0.3, sc4-1= 0.3rad cc1-1 = 0.3, cc4-1= 0.3rad Diagonal cracks in outer RC, local buckling in steel tube Inner CFST significantly improves torsional resistance; αcfst critical; axial load influence limited; superposition model predicts strength conservatively 24 Chen, Sheng, Fam, Wei (2017) 10 Dumbbell-shaped CFST member/L= 1200mm Two circular tubes + steel web Ø 100 & 108 &112 mm tubes, 4 & 6 Solid concrete fill between tubes fc ≈ (30–40) Pure torsion - - DCFST 25-40 =25.29//CCFST-6 = 22.20 Local buckling of tubes + concrete cracking Dumbbell-shaped CFST > circular CFST in torsional strength & stiffness; connecting web improved torsional transfer 25 Anumolu, Abdelkarim. ElGawady (2016) 6 HC-SCS Column length 625 mm Circular (Double Skin) D = 165 mm, H = 550 mm Outer: 3.0–4.6 Inner dia. = 42–75 and t= 3.0–5.0 fc= 50; fy=60–365 Pure torsion (cantilever) - -- CO111=24.6//CO312=54.3 CO1112.7=° //CO3123.5=° Steel rupture or concrete shell cracking Torsional capacity governed by outer steel tube and concrete shell thickness; FE model accurate (<5% error). 6 Huang, Han & Zhao (2013) CHS 7 & SHS 5 CFDST Length 550mm Circular & Square Ø 165/SHS 160×160 3–5 Ø 42 & 60 & 75 Normal (CHS fc=50) & (SHS fc=60) Pure torsion - - CO3I2 = 54.3//SO6I3= 48.8 CO3I2 = 5.8° //SO6I3 = 5° Ductile failure, local buckling + concrete cracking Wall thickness ↑ → torque ↑; hollow ratio ↑ → torque ↓; CFDST carried much higher torsion than hollow steel; proposed design equations matched tests 26 Wang, Nie, Fan (2013) 6//3C + 3R CFST columns Circular, Rectangular C(Ø220) & R (200×150) 6 mm None (solid CFST) fc= 49–58; fy = 336 Axial + bending + torsion ~15–20 ~1° 20–35 ~4-20° Local buckling of steel + concrete shear cracks Axial and bending loads reduce torsional resistance; CFST shows ductile behavior; concrete delays buckling 27 Wang, Nie, Fan (2013) Verification multiple past tests (≈74 specimens from L.R.) (9) CFST columns Circular Ø = 133, 114, 216.3//L =450, 387, 1620 t = 4.5 Solid CFST f’c ≈ 33.3, 27.4, 32.8, fy ≈ 324,280,362 Pure torsion & combined axial–torsion (numerical + experimental validation) TCB1-1 = 30, TB1-1 = 29, TCB1-1 = 21 TCB1-1 = 6°, TB1-1 = 8°, TCB1-1 = 9° TCB1-1 = 32, TB1-1 = 29, TCB1-1 = 22 TCB1-1 = 32°, TB1-1 = 18°, TCB1-1 = 33° Shear cracking in concrete, local buckling delayed New laminated tubes model accurately predicts torsional behavior; simplified equations for Tu proposed; axial load reduces torsional capacity 16 Nie, Wang, Fan (2013) 8//4C + 4R CFST Columns//Length 1090mm Circular & Rectangular CFST Circular (Ø220) & Rectangular (200×150) 6 mm None (solid CFST) Concrete fc= 49–58; Steel fy = 336 Compression + Bending + Torsion (cyclic) C-CT1 =113.8//R-CT1= 79.5 C-CT1 =2.1° //R-CT1= 2° ~C-CT1 =145.5//R-CT1= 94.8 C-CT1 =19.9° //R-CT1= 19.9° Local buckling (rectangular), diagonal cracking (circular)/or/Local buckling (steel tube), cracks along torsion axis Ductility & hysteretic energy dissipation high; M/T ratio critical; circular > rectangular in performance. /or/Good seismic performance; ductile hysteresis; bending reduces torsion capacity; M/T ratio governs failure type 15 Nie, Wang, Fan (2012) 8//4C + 4R CFST Circular, Rect. columns Circular Ø220, Rect. 200×150 H = 1100 4 & 6 Solid CFST fcu ≈ 55; fy = 285–336 Pure torsion, cyclic torsion, compression–torsion 47–114 1.7–2.5° 61–146 6–25° Cracks + buckling (at high compression) Cyclic torsion showed high energy dissipation; low axial load ↑ torque, high axial load ↓ torque; ductility excellent except at 0.6Nu. 14 Lee, Yun, Shim, Chang, G.C. Lee (2009) Compare 4 sp. (Xu 1991, Beck 2003, Han 2007) CFST (circular) Columns Circular D = 114–139.8, L = 1000 3.5–4.5 mm Solid CFST fc = 27–33; fy = 280–348 Pure torsion, Compression + Torsion ~15–25 ~2–4° ~35–42 >30° (حتى 10×θy) Empty steel tube → buckling; CFT → ductile, no torsional strength loss Steel resisted 65–75% of torque; confined concrete provided ductility; torsional strength ↑ with axial load up to 0.6Nu. 13 Han, Yao & Tao, 2007 12 (tests) + FEM L=450 – 2000mm CFST Columns Circular & square Ø114–1139.8/B114 3–4.5 Solid concrete core NC fc=20-36/fy=280-349 Pure torsion - - CH40 = 42 CH40 = 8 Local buckling prevented by concrete CFST much stronger than hollow steel; formula proposed 12 Beck & Kiyomiya, 2003 2 steel tubes, 3 CFST, 1 plain concrete column CFST (circular) + control steel & concrete Circular Ø139.8 × 1000 3.5, 4.0, 4.5 Solid concrete core fc ≈ 30, fy ≈ 340 Es=2.1× 10 5 Pure torsion (static) ≈ 31.9 ≈ 0.8–1.0 ≈ 40.1 >10 Steel: local buckling; CFST: concrete shear cracks CFST ~20% stronger than sum of steel + concrete; ductile post -yield; concrete prevented local buckling 3.1.1 Overview of experimental studies Research on the torsional performance of concrete-filled tubular columns has evolved over two decades, from early CFST investigations to recent explorations of double-skin composite (CFDST) and steel-reinforced variants. Pioneering work by Beck and Kiyomiya (2003) and Han et al. (2007) established the fundamental torque–rotation (T–θ) response of CFST members under pure torsion. 12 , 13 These studies revealed that the presence of infilled concrete prevents local buckling of steel tubes and enhances ductility by allowing stress redistribution after yielding. Subsequent works such as Lee et al. (2009) and Nie, Wang & Fan (2012, 2013) expanded testing to include combined compression-torsion and cyclic torsion, demonstrating that axial compression up to approximately 0.4 Nu enhances torsional strength, whereas higher compression ratios reduce it. Circular CFST columns consistently exhibited greater ductility and energy dissipation than square or rectangular ones due to uniform confinement. 14 – 16 3.1.2 Advancements with double-skin and reinforced configurations The development of Concrete-Filled Double Skin Steel Tubes (CFDST) introduced a new mechanism for controlling torsional stiffness and reducing overall weight. Huang, Han & Zhao (2013) performed one of the earliest experimental investigations on CFDST members under pure torsion, highlighting that increasing wall thickness substantially raised torsional capacity, while larger hollow ratios led to a reduction. Their results indicated that CFDST columns could resist 40–60% higher torque than equivalent hollow steel tubes due to confinement and composite action. 6 Wang, Lu & Zhou (2018) conducted cyclic torsion tests on circular, square, and rectangular CFDST specimens, showing that outer steel tube thickness significantly influenced stiffness degradation. Ductile failure was achieved through concrete cracking and steel yielding, confirming superior energy dissipation and rotational ductility in CFDST compared to CFST columns. 4 More recently, Mutlag & Lateef (2024) investigated high-strength CFST beams stiffened with internal cross-rods, achieving up to 40% improvement in torsional strength and delayed buckling onset. Internal steel stiffeners proved effective in enhancing confinement, showing potential for adoption in double-skin systems. 7 Complementary studies such as Jia et al. (2021) and Wang et al. (2020) explored coupled compression–torsion responses through both experiments and ABAQUS simulations. These works established that torque capacity increases under moderate pre-compression due to enhanced concrete confinement, but decreases at high axial ratios. The numerical models were validated within a 5–10% deviation from experimental results, providing reliable design-oriented equations. 5 , 17 3.1.3 Analytical and numerical insights Finite element (FE) models (for example, Wang et al., 2019; Li, et al., 2018) successfully reproduced the torque–rotation behavior and stress transfer mechanisms. 18 , 19 Analytical correlations between axial load ratio, hollow ratio, and torsional stiffness (GJ) were established with high accuracy (R 2 > 0.95). The consensus among these works indicates that torsional performance is governed by: • Concrete confinement efficiency, • Steel tube thickness and yield strength, • Hollow ratio and inner tube geometry, • Presence of axial compression, and • Concrete type (HSC > NSC). However, none of the studies incorporated post-fire residual torsional performance, leaving a significant gap in understanding the combined degradation of shear modulus (G) and torsional rigidity (GJ) after thermal exposure. 3.2 Fire behavior of CFST and CFDST members A total of 18 experimental and numerical studies (2003-2022) were identified that investigated the fire performance of CFST and CFDST members. The following table, ( Table 2 ), is a comparative summary Fire Studies. Table 2. Comparative summary of fire studies on CFST and CFDST members. Reference No. Author/Year No. Specimens Member Type Section Shape Outer Dimensions (mm) Steel Thickness (mm) Inner Tube/Dimensions Concrete Type/Steel Strength (MPa) Fire Code/Curve Heating Duration (min) Max Temp (°C) Test Type Ultimate Load/Capacity Failure Mode Key Findings 10 Chang et al., (2022) 24 (axial tests), 12 (post-fire tests) CFDST/CFSPT (UPVC inner tube) SHS outer CHS inner SHS (75–100 mm), CHS (31–37 mm) 1.2 Inner tube of steel or UPVC Normal concrete (fc’ ≈ 30) Elevated temp (post-fire residual) Residual capacity after heating - Axial compression 109–221 kN (average values per series) Local buckling of steel tube, concrete crushing Replacing inner steel with UPVC reduced cost and weight, while retaining good strength and ductility. Post-fire tests showed little effect of inner tube material on residual axial capacity. 35 Lopes & Rodrigues (2020) 12 Double-Skin & Double-Tube Square 220 outer/110 inner 8/6 Square inner tube PC, HSC, LWC/S355 outer, S275 inner ISO-834 up to collapse (~180 min) ~> 1000°C Experimental Measured Local buckling & concrete crushing Double-Tube with HSC inner gives highest ultimate collapse time 38 Wang, Huang, Yuan & Ye (2019) 12 CFST circular columns Slender CFST columns (L = 3470 mm, λ = 63.4) Circular (CHS) Ø 219 × 4.0 mm 4.0 mm Solid (no inner tube, only filled concrete) NSC fcu ≈ 27–33/fy ≈ 320 ISO-834 Standard Fire Curve Until failure (varied, 33–90+ minutes depending on load & preload) Furnace up to 1200°C (ISO-834 curve followed) Experimental (full-scale furnace tests) 395 – 923 kN (depending on load ratio) Overall buckling (dominant), with occasional local bulging - Preload ratio ↑ → Fire resistance time ↓ (up to 16.25%). - Thermal field not influenced by preload. - Structural deformation (axial & lateral) significantly larger with preload. - Neglecting preload in design may overestimate fire resistance. 39 Wang, He & Xiao/ (2019) Review (data from >30 years of studies) CFST columns Circular, square, rectangular, elliptical Various (150–1600 mm) Various (4–25 mm) Some studies CFDST NSC, HSC, SCC, fiber-reinforced ISO-834, ASTM E119, JIS A1304 Up to 300 min 1000+ (depending on furnace) Review of fire tests & numerical studies Summarized ranges from database Global buckling, local buckling, concrete crushing, debonding Larger cross-sections & lower load ratios improve FRR; circular best; Chinese & US codes most accurate; post-fire residual strength decreases with Tmax. 36 Tan et al. (2019) Numerical (validated with 19 prior tests) CFSST (stainless outer + carbon steel inner) Square e.g. 788 × 10 (model) 10 Inner carbon steel profile Concrete infill/Stainless outer + Carbon inner ISO-834 up to failure (~>180 min in simulations) Outer >1000°C/inner <125°C Finite Element Predicted Local & global buckling depending on slenderness Inner steel profile stays cool, sustaining load and enhancing fire resistance 33 Mohd et al. (2017) 54 stub CFDST columns Stub columns (L = 600 mm) Circular (CHS outer and inner) Ø101.6, Ø127, Ø152.4 with thickness 3 or 4 mm Outer: 3–4 mm; Inner: 3–4 mm Ø50.8, Ø76.2, Ø101.6 (t = 3–4 mm) NSC (fcu = 38–43) /Outer fy = 409–597; Inner fy = 449–762 ASTM E-119 Standard Fire Curve 60 min and 90 min (at 600°C) Furnace kept at 600°C Experimental (fire furnace + axial compression) Up to ~2000 kN (UTM capacity; actual failure loads varied 500–1600 kN depending on specimen) Outward local buckling of outer tube, crushing of concrete, inward/outward buckling of inner tube Longer exposure → more severe buckling & crushing. - RSI highest at 90 min (~22%). - Some specimens with t = 4 mm showed RSI negative (strength gain). - Secant stiffness dropped 11–64%. - DI increased in some cases, showing higher ductility after fire. - CFDST retains considerable residual strength after 90 min fire 40 Song, Tao, Han & Uy/2017 36 push-out CFST interface (bond study) Circular & square tubes Ø 150–200 (circular), 150×150 (square) 4–6 mm None NSC fcu ≈ 30, HSC fcu ≈ 70, SCC fcu ≈ 50 Stainless Carbon Elevated temperatures (20–800°C in furnace) Constant temperature (1–2 h) 800°C Experimental push-out test Bond strength reduced from ~2.5–3.5 MPa (20°C) → <0.5 MPa (800°C) Debonding at interface, concrete crushing near ends Bond strength decreases rapidly after 400°C; SCC moderate, HSC most sensitive; studs improve residual strength 20–40%. 9 Yao, Y., Li, H., Tan, K. (2016) 42 numerical models (FEA) + 6 experimental columns for validation Column Circular (CFDST) and Square (CFDST) Examples: 406×8, 219.1×5, 200×6, 350×8 Outer: 3–8 mm; Inner: 3–5 mm (depending on specimen) Examples: 165.1×3.0, 101.6×3.2, 89×3.5, 150×5 NSC fcu 30 and HSC fcu 60 fy(275+430+630) ISO-834 Standard Fire Curve Until failure (18–107 minutes depending on specimen) >1000°C (according to ISO-834 curve) Finite Element Analysis (ABAQUS) + Validation with experimental data Up to ~4420kN (specimen C4) Local buckling of outer steel tube, progressive load transfer to inner tube and concrete until collapse - Fire resistance decreases with higher slenderness ratio and load ratio. - Outer high-strength steel does not improve fire resistance. - Inner high-strength steel significantly improves performance. - Concrete strength has limited effect. - Larger inner steel area or concrete infill in inner tube enhances fire resistance. - Modified Rankine approach accurately predicts fire resistance compared with tests. 41 Ibañez, Romero & Hospitaler/ (2016) 360 (numerical parametric study) Concrete-Filled Tubular (CFT) Columns Circular D = 139.7, 193.7, 273, 323.9, 508 t = 3.2, 5, 6.3, 16 None (single-skin CFT) Normal strength concrete (fc ≈ 30) ISO-834 curve Simulated up to failure (R30–R120) Derived from ISO-834 exposure (not fixed, model-based) Numerical parametric study (fiber beam model) Reported as axial resistance ratio Nfi,Rd depending on λ, D/t, μ (no single value) Axial buckling after progressive loss of steel then concrete core Rotational restraint enhances FRR, axial restraint reduces capacity. Eurocode 4 (0.5L) unsafe for slender CFTs, UK NA (0.7L) more reliable. Authors recommend 0.7L in general, or 0.7L for stub & 0.5L for slender columns. 3 Romero, Espinos, Portolés, Hospitaler, Ibañez (2015) 12 columns (6 at room temperature + 6 under fire) Slender columns Circular double-tube (outer and inner CHS) Dext = 200 mm, thickness = 3 or 6 mm Outer: 3–6 mm; Inner: 3–8 mm (varied) Dint = 114.3 mm, thickness = 3–8 mm Normal-strength concrete (C30) and Ultra-high strength concrete (C150) /Fy 377-512 ISO-834 Standard Fire Curve Until failure (33–104 minutes depending on configuration) >1000°C (ISO-834 exposure) Experimental program (room temperature & fire tests) At room temperature: 1418 – 2076 kN (Nu); In fire tests: 283 – 415 kN (applied load ≈ 20% Nu) Overall buckling (no local buckling observed); load redistribution from outer tube → concrete → inner tube until collapse - At room temperature: thicker outer tube increased buckling capacity; filling inner tube with concrete slightly improved strength. - Fire tests: thick inner tube + thin outer tube gave best fire resistance (up to 104 min). - UHSC in the inner core had limited benefit (only ~9% increase in load capacity at room temperature, sometimes worse in fire). - Eurocode 4 (EC4) design methods were found unsafe for slender double-tube columns, especially with UHSC. - Suggested strategy: split steel into thin outer + thick inner tube, both filled with concrete, for improved fire resistance. 8 Zuki, Choong, Jayaprakash & Shahidan/ (2015) 9 (3 control, 3 exposed 60 min, 3 exposed 90 min) CFDST short columns Circular Ø 152.4 Outer 4 mm, Inner 2 mm Ø 101.6 × 2 mm Normal strength concrete fc’ ≈ 30 – 38 ASTM E-119 fire curve 60 and 90 min (at 600°C) Core 514–557°C, Inner steel 508–550°C Experimental fire test + monotonic concentric axial load Control: 1402 kN, 60 min: 1292 kN, 90 min: 1199 kN Local outward buckling (outer), inward buckling (inner), crushing of concrete Strength reduction only 7.8–14.5%; stiffness reduction more significant (11–36%); ductility nearly unchanged; concrete acted as effective thermal protection. 1 Han, Chen, Liao, Tao & Uy/ (2013) 5 (3 square, 2 circular) CFSST full-scale columns Square & circular 315×315×5, 630×630×10, Ø300×5 5–10 mm None SCC, fcu = 53–64/Es 2*10^5 ISO-834 Up to 240 min ~1000°C furnace, 500–600°C core Experimental fire test + FE modelling NF = 940–7870 kN depending on size/load ratio Local buckling, weld fracture (square), elephant’s foot bulge (circular), concrete crushing Fire resistance ranged 67–220 min; Larger size = better FRR; Stainless steel improved residual strength compared to CFST. 32 Lu, Han, Zhao (2010) 18 total (16 fire-tested, 2 ambient reference) Stub columns (800 mm length) Circular (CHS) and Square (SHS), inner and outer of same shape Circular: 406×8, 219.1×5; Square: 350×8, 200×6 Outer: 6–8 mm, Inner: 3–5 mm Circular: 165.1×3, 101.6×3.2; Square: 150×5, 89×3.5 (SCC) fcu =46.6 – 62.5, Steel fibre SCC, Steel + Polypropylene fibre SCC Fy =399-506 Standards Australia; 1997.AS 1530.4 Standard Fire Curve 18 – 138 minutes (until failure) Outer tube: 400-963°C, Inner tube: <200° 59-197C Experimental fire tests in gas furnace Up to 4420 kN (S1–S3 specimens) Compression failure with local bulging of steel tubes, crushing & cracking of concrete - SCC with steel fibres significantly increases fire resistance (esp. load ratio < 0.6). - Polypropylene fibres reduce spalling but limited effect on strength. - Inner tube remains cool (<200°C), ensuring residual strength. - CFDST has higher critical temperature than CFST/unfilled tubes. - Fire resistance strongly depends on load ratio and specimen size. 2 Lu, Han, Zhao (2010) 6 full-scale CFDST columns Slender CFDST columns (L = 3810 mm) Circular (CHS+CHS), Mixed (SHS+CHS), Square (SHS+SHS) CHS300×5, SHS280×5 5 mm (both outer and inner tubes) CHS125×5, CHS225×5, SHS140×5 Self-consolidating concrete (SCC), fcu ≈ 26–38/fy ≈ 320 ISO-834 Standard Fire Curve 40 – 240 minutes (depending on protection & load) Outer tube: up to 940°C; Inner tube: <500°C Experimental full-scale furnace tests 570 – 2050 kN (see specimen matrix) Overall buckling; local bulging in SHS; cracking in concrete; SCC spalling prevented by confinement - Fire resistance of unprotected CFDST: 40–115 min; protected: 165–240 min. - Limiting temperature of outer tube can reach 942°C, much higher than CFST. - Composite action (steel + concrete + inner tube) delays failure. - Larger outer perimeter & lower cavity ratio improve fire resistance. - Spray coating (10 mm) very effective in enhancing fire endurance. 31 Lu, Zhao, Han, (2009) 6 CFST stub columns Square SHS L=760 mm 150×150×5, 200×200×6, 5–6 None High-strength SCC fc ≈ (90–99) ISO 834/AS 1530.4 26–90 min, Tmax 920 °C - Axial compression fire test + FEA validation 2787–4702 kN ultimate Outward bulging of steel tube, crushing of core concrete SCC-filled CFST had similar fire behaviour to normal CFST. Main failure due to outward bulging and compressive crushing. Interaction between steel & SCC maintained integrity, giving good ductility under fire. 42 Yang & Han (2008) Theoretical CFDST Columns Circular & Square 200–1000 (parametric) 6–9 Circular inner tube Plain concrete/fy≈345 ISO-834 up to 180 min ~1200°C Numerical FEM Predicted Local buckling/thermal degradation Larger diameter & lower void ratio reduce inner tube temperature and increase fire resistance 28 Han, Zhao, Yang, & Feng. (2003) (8 without + 5 with) protective layers CFST columns CHS CHS D = 150-219-478 L3810 4.6-5-8 Solid (no inner tube) fc≈ 39.6 –68.8/fy≈ 259-293-381 ISO-834 up to 196 min C4-1=829°C/20min C4-2=434°C/177min Numerical and experimental - Global buckling Unprotected CFST columns do not provide sufficient resistance at high load ratios. Thermal protection is very effective in increasing FRR. Sectional diameter is the most important factor affecting FRR. 29 Han, Yang & Xu (2003) 11 Columns SHS (Square), RHS (Rectangular) 219×219×5.3,300×150×7.96, 300×200×7.96, 350×350×7.7 5.3–7.96 Solid (no inner tube) fcu =(18.7–49) Fy=341 Es=(1.87 + 2 + 1.83) * 10^5 ISO-834 60–169 min 500–786 Axial load (Concentric & Eccentric) 1795–4860 Compression, Buckling Fire protection reduced required coating by 25–70%; RSI and fire resistance formulas developed 3.2.1 Experimental studies under standard fire curves Research on fire performance has progressed extensively, focusing on axial compression and temperature-dependent degradation in both single-skin (CFST) and double-skin (CFDST) columns. Early foundational work by Han, et al. (2003) and Han et al. (2003) examined CFST columns under ISO-834 fire curves, demonstrating that larger cross-sections and lower load ratios yield longer fire resistance durations (up to 169 min). Fire protection layers were found to reduce required coating thickness by up to 70%. 28 – 30 The pioneering full-scale furnace tests by Lu et al. (2009, 2010) introduced self-consolidating concrete (SCC) and fiber-reinforced SCC for CFDST columns, showing that steel fibers enhanced fire resistance time by up to 60% and reduced spalling. The inner steel tube remained below 200°C even when the outer tube exceeded 900°C, maintaining post-fire load capacity and confirming CFDST’s superior thermal behavior compared to CFST. 31 , 32 Zuki et al. (2017) conducted axial fire tests on CFDST short columns under ASTM E-119, reporting only 7–15% strength reduction after 60–90 min exposure. The outer tube exhibited outward bulging, while the inner tube deformed inward—yet the residual capacity remained above 85% of the ambient value, underscoring effective concrete confinement and dual-tube protection. 33 , 34 3.2.2 Influence of material and geometry Yao et al. (2016) combined FE simulations and validation tests to analyze circular and square CFDST columns under ISO-834 exposure. 9 They found that inner high-strength steel significantly enhanced fire resistance, while outer high-strength steel had negligible effect due to early yielding. Concrete strength modestly affected fire endurance, but inner tube thickness and diameter were decisive. Similarly, Romero et al. (2015) studied slender double-tube columns under both ambient and fire conditions. Their results showed that using a thick inner tube and thin outer tube achieved the longest fire duration (up to 104 min), while ultra-high-strength concrete in the core yielded limited improvement. Lopes & Rodrigues (2020) extended this analysis to restrained square double-skin and double-tube columns, highlighting the influence of structural boundary conditions. 35 Axial restraint reduced fire resistance, while rotational restraint increased it. Columns with high-strength concrete cores achieved longer collapse times compared to normal concrete, validating the beneficial confinement effect even under thermal strain. 3.2.3 Innovations in inner tube materials A recent innovation by Chang et al. (2022) replaced the inner steel tube with UPVC pipes, introducing the Concrete-Filled Steel–Plastic Tubular (CFSPT) system. 10 Tests showed comparable axial strength to traditional CFDST columns with significantly reduced weight and cost. Importantly, post-fire residual capacities were nearly identical between steel and UPVC inner tubes, demonstrating that thermal degradation of UPVC did not critically compromise performance. Other hybrid studies 36 , 37 investigated stainless steel outer tubes or steel-reinforced concrete cores , achieving enhanced residual capacities after 120–180 min of fire. These configurations maintained structural integrity and ductility well beyond conventional CFSTs, marking a trend toward multi-material CFDST systems optimized for fire resilience. 3.2.4 Key observations and gaps A synthesis of all reviewed fire studies indicates that: • Fire resistance of CFDST members generally ranges between (60–180) min at (600-900) °C under ISO-834 conditions, depending on cross-section and load ratio. • The outer-to-inner steel thickness ratio and void ratio are the most influential geometric parameters. • The inner tube acts as a secondary load path, delaying collapse and ensuring residual stiffness. • Residual axial capacity often exceeds 70% of ambient strength, confirming the robustness of CFDST systems. However, despite extensive fire research, torsional or post-fire torsional testing has not been performed in any of these studies. The interaction between thermal degradation and torsional rigidity remains unexplored, particularly the potential changes in shear modulus (G) and biomaterial bonding at steel–concrete interfaces after heating. This gap forms the basis for the proposed experimental program in the present study. 4. Comparative discussion and synthesis This section integrates the findings from torsional and fire studies to establish the performance correlations, degradation mechanisms, and governing parameters influencing CFDST behavior under combined fire and torsional effects. The discussion also highlights the gaps that form the scientific rationale for the proposed experimental program. 4.1 Influence of section geometry The geometric configuration of CFDST members plays a critical role in determining both torsional stiffness and fire resistance. Across the torsional studies, 4 , 6 , 7 square and circular cross-sections exhibited distinct behaviors: • Circular CFDST columns achieved higher torsional ductility and smoother T–θ curves, with energy absorption capacity up to 30–40% greater than square sections. • Square and rectangular sections, however, provided improved torsional stiffness at small rotations but experienced earlier local buckling, particularly at flat steel faces. • Increasing wall thickness (t = 3–6 mm) consistently elevated both the elastic and ultimate torque capacities, delaying the onset of local shear buckling. In fire studies, 2 , 3 , 8 geometry influenced temperature distribution and failure sequence. Circular outer tubes provided more uniform confinement and lower inner-tube temperature gradients, leading to longer fire endurance. Square sections developed stress concentrations at corners, accelerating local failure. Hence, from a combined performance standpoint, circular CFDST members offer superior post-fire torsional resilience due to their balanced confinement, symmetric stress flow, and lower thermal strain differentials. 4.2 Effect of steel tube thickness and hollow ratio Both torsional and fire investigations emphasize the pivotal role of steel tube thickness and hollow ratio. In torsion, thicker steel tubes increase stiffness and delay local buckling, while smaller hollow ratios (i.e., smaller inner diameter) improve confinement efficiency. 4 , 6 Similarly, in fire conditions, a smaller cavity ratio reduces heat penetration and improves load retention. 1 , 9 For instance, CFDST columns with outer thickness ≥4 mm and inner-to-outer diameter ratio ≤0.6 maintained up to 80–90% residual strength after 90 min of exposure. 33 However, excessive steel thickness (t > 8 mm) provides diminishing returns due to increased thermal conduction. Therefore, optimization of steel distribution between outer and inner tubes—such as thick inner + thin outer configuration 3 - yields balanced fire and torsion resistance with improved ductility. 4.3 Concrete type and strength effects Concrete properties significantly affect torsional resistance and fire endurance. Under torsion, high-strength concrete (HSC) enhances stiffness and torque capacity but may reduce ductility due to brittle cracking. 7 In contrast, normal-strength concrete (NSC) or SCC promotes smoother post-yield rotation and higher energy absorption. During fire exposure, SCC with steel or polypropylene fibers effectively mitigates spalling and improves confinement performance. 2 Moreover, HSC cores in CFDST columns retain higher strength post-fire because of lower permeability and moisture diffusion, provided sufficient confinement exists. 35 Hence, hybrid systems employing fiber-reinforced SCC for fire resistance and HSC for pre-fire torsional stiffness may achieve optimal overall performance. 4.4 Influence of axial load and pre-stress Axial load interaction strongly affects both torsional and fire performance. Torsional tests 5 , 17 demonstrated that axial compression up to 0.3–0.4 Nu increases torsional strength by enhancing confinement. Beyond this limit, torsional capacity decreases due to premature concrete crushing. In fire conditions, 20 , 28 , 29 higher preload ratios significantly reduce fire resistance duration—by up to 16%—as thermal expansion amplifies internal stresses. When combined, these effects suggest that residual post-fire torsional stiffness will depend heavily on prior axial load levels during heating. Columns subjected to realistic service loads may exhibit pronounced degradation in both torque capacity and rotation ductility post-fire. 4.5 Failure modes and damage mechanisms Both sets of experiments revealed consistent failure patterns governed by the interaction between steel yielding and concrete cracking: • Torsional failure: shear cracking in the concrete core followed by local steel buckling along 45° planes. 4 , 13 • Fire failure: outward buckling of the outer tube, inward deformation of the inner tube, and crushing of the heated concrete core. 8 Post-fire torsional performance will likely be influenced by: 1. Reduction in steel yield strength (up to 60% at 600°C), 2. Degradation of concrete shear strength, 3. Loss of bond at steel–concrete interface, and 4. Residual geometric imperfections from thermal buckling. Thus, assessing post-fire torsional behavior requires capturing the residual material properties and interface conditions—none of which have been experimentally quantified to date. 4.6 Torsion–fire interaction: conceptual synthesis Integrating the torsion and fire literature suggests a coupled degradation model. Torsional stiffness GJ depends on both the shear modulus of steel and the integrity of the concrete core. Fire exposure simultaneously reduces steel’s shear modulus Gs and alters the concrete’s shear transfer capacity Gc. The combined reduction in effective torsional rigidity can be expressed conceptually as: ( GJ ) residual = T η · G s J s + C η · G c J c where T η and C η are temperature-dependent reduction factors derived from fire exposure history. The literature indicates that while axial and flexural residual strengths of CFDSTs are well-documented, the post-fire torsional reduction factor (T η ) remains unknown. Given the observed 50–60% reduction in axial capacity after severe fire, 1 , 38 it is reasonable to hypothesize a similar or greater decline in torsional rigidity due to the combined degradation of both materials. 4.7 Summary of comparative insights Aspect Torsional behavior Fire behavior Combined implications Cross-section shape Circular sections more ductile; square stiffer but prone to buckling. Circular sections distribute heat uniformly. Circular CFDST expected to retain higher post-fire torsional ductility. Steel thickness & hollow ratio Thicker tubes ↑ torque, smaller hollow ratio ↑ stiffness. Smaller cavity ratio ↑ fire resistance. Optimal range: t = 3–5 mm t = 3–5 mm t = 3–5 mm, inner/outer ratio ≤0.6. Concrete type HSC ↑ strength, SCC ↑ ductility. SCC with fibers ↑ fire resistance. Hybrid SCC–HSC mix ideal for torsion–fire resilience. Axial load Improves torque up to 0.4 Nu, then reduces. Higher load ratio ↓ fire resistance. Service load critical to residual torsional stiffness. Failure mode Shear cracking + steel buckling. Outward/inward buckling + crushing. Post-fire torsion governed by bond loss and steel softening. 4.8 Identified research gaps From the synthesis of 37 studies, the following gaps are identified: 1. No experimental program has yet examined post-fire torsional performance of CFDST members. 2. Lack of constitutive models linking temperature-dependent material degradation to torsional stiffness and ultimate torque. 3. Absence of validated finite element models incorporating both thermal damage and torsional loading. 4. Limited understanding of residual interface bond between steel tubes and concrete after heating. 5. No design equations currently account for fire-induced torsional reduction factors in composite columns. These gaps form the scientific foundation of the proposed experimental program, which aims to bridge the knowledge divide between isolated torsional and fire research. 5. Proposed experimental program (Present study) 5.1 Research objective To address the identified research gaps, the current experimental program is designed to investigate the torsional performance of CFDST members before and after fire exposure . The goal is to quantify the degradation in torsional stiffness (GJ), ultimate torque (Tu), and rotation capacity (θu) due to fire-induced thermal damage, while examining the effects of cross-sectional geometry, steel thickness, and inner tube configuration on both pre-fire and post-fire torsional behavior. This program represents the first systematic experimental attempt to couple fire exposure with torsional testing in CFDST columns, thereby linking two previously isolated research domains. 5.2 Experimental matrix A total of 36 full-scale CFDST beams specimens (L = 2000 mm) are planned, divided into two main groups: • Group A (Pre-fire): 18 specimens tested under torsion at ambient temperature. • Group B (Post-fire): 18 specimens first exposed to fire and then tested under torsion after cooling. Each group includes both square (100 × 100 mm) and rectangular (50 × 100 mm) sections, representing realistic cross-section geometries used in composite columns. 5.3 Variables and parameters (a) Outer Tube Thickness Three steel tube thicknesses will be investigated: • 1.2 mm, 1.7 mm, and 2.6 mm. • Each thickness level will include both square and rectangular specimens. • This variation allows examination of the effect of steel confinement and thermal degradation rate on torsional rigidity. (b) Concrete Core Configuration The concrete core will use ordinary normal-strength concrete (NSC) with a target strength of 30–35 MPa. Two filling conditions will be considered: 1. Fully filled concrete core (no inner tube). 2. Partially filled with an inner hollow tube (Type A or Type B), to simulate double-skin behavior and control hollow ratio. (c) Inner Tube Configurations • Type A : ○ Square outer : inner tube 50 × 50 × 1.2 mm (L = 1500 mm) ○ Rectangular outer : inner tube 25 × 50 × 1.2 mm (L = 1500 mm) • Type B : ○ Square outer : inner tube 25 × 25 × 1.2 mm (L = 1500 mm) ○ Rectangular outer : inner tube 10 × 30 × 1.2 mm (L = 1500 mm) This range covers three confinement levels : solid, wide cavity, and narrow cavity—corresponding to varying hollow ratios between 0.26 and 0.52 . 5.4 Fire exposure phase (Group B) For post-fire testing (Group B), specimens will be subjected to standard fire exposure following ISO-834 or ASTM E-119 temperature–time curves. The target temperature is expected to reach 500–700°C on the outer surface with corresponding inner temperatures of 150–250°C. The exposure duration will be selected to achieve realistic heating scenarios comparable to building fire durations (approximately 60–90 minutes). After heating, specimens will undergo natural air cooling to room temperature before torsion testing, representing realistic post-fire conditions. This phase aims to establish temperature-dependent reduction factors for steel and concrete and to correlate them with the post-fire torsional response. 5.5 Torsion testing phase After fire exposure (for Group B) or directly for Group A, specimens will be mounted in a pure torsion test rig, consisting of a fixed end and a rotating end. A controlled rotational loading will be applied at one end at a constant angular rate, while torque is measured using a calibrated torque transducer. Key parameters to be measured include: • Torque–rotation response (T–θ curve) • Ultimate torque capacity (Tu) • Secant torsional stiffness (GJ) • Rotation at yielding and at failure (θy, θu) • Energy dissipation capacity • Residual torsional strength (post-fire/pre-fire ratio) • Failure mode (local buckling, shear cracking, delamination) 5.6 Expected outcomes and hypotheses Based on trends from prior studies, the following hypotheses will be examined: 1. Torsional capacity (Tu) decreases after fire exposure by approximately 40–60%, proportional to the degradation in steel yield and concrete shear strengths. 2. Residual stiffness (GJ) strongly depends on outer tube thickness and hollow ratio; thicker outer tubes will retain higher stiffness after fire. 3. Square sections are expected to show more pronounced stiffness degradation than circular or rectangular ones due to corner heat accumulation. 4. The inner tube geometry will significantly affect residual torque capacity—narrower cavities (Type B) expected to maintain better confinement. 5. Failure modes will shift from ductile yielding (pre-fire) to brittle shear cracking and delamination (post-fire). 5.7 Significance and novelty This experimental program addresses the primary knowledge gap highlighted in the systematic review by providing the first direct experimental evidence of post-fire torsional degradation in CFDST columns. Its novelty lies in the integration of thermal and torsional loading regimes , which enables development of a temperature-dependent torsional stiffness reduction model applicable to design and assessment of fire-exposed composite structures. Furthermore, by including various geometries, hollow ratios, and thicknesses, the study will produce a comprehensive empirical dataset that supports: • Calibration of finite element simulations coupling heat transfer and torsion; • Formulation of design-oriented reduction factors for post-fire torsional rigidity; • Development of predictive empirical correlations between temperature exposure and torque retention. 6. Future research directions The synthesis of existing literature reveals that while significant progress has been achieved in understanding the isolated torsional and fire behavior of CFST and CFDST members, the combined fire–torsion domain remains largely unexplored. To advance the state of knowledge and develop reliable design provisions, several research directions are recommended. 6.1 Integrated fire–torsion testing framework Future research should prioritize the development of integrated experimental setups capable of simultaneously applying torsional and thermal loading. Unlike conventional axial–fire tests, these setups must allow controlled heating during torsion to simulate realistic loading sequences such as twisting of structural members during or immediately after a fire event. Key recommendations include: • Use of torsion–furnace systems with real-time thermal–mechanical coupling. • Incorporation of variable heating rates to assess transient and steady-state torsional degradation. • Testing of different fire exposure durations to establish residual torsional strength envelopes. Such hybrid setups will provide essential data for validating advanced fire–torsion constitutive models and finite element simulations. 6.2 Temperature-dependent material models Existing design codes (e.g., Eurocode 4, AISC 360) address temperature effects for axial and flexural performance but not for torsion. Future studies should therefore focus on developing temperature-dependent constitutive relationships for: • The shear modulus (G) of structural steel and its reduction with temperature, • The concrete shear strength (τc) degradation curve under thermal cycling, • The bond–slip characteristics at steel–concrete interfaces after heating and cooling. Experimental calibration of these parameters will enable reliable prediction of post-fire torsional stiffness (GJ) and residual energy dissipation capacity. 6.3 Multi-scale finite element and analytical modeling Validated finite element (FE) models integrating heat transfer , material degradation , and torsional loading are urgently needed. Future modeling efforts should: • Couple thermal–mechanical–damage formulations to simulate post-fire torque–rotation behavior; • Incorporate contact and interface degradation between concrete and steel; • Utilize multi-scale simulation frameworks linking material-level deterioration to global structural response. Such models can be used to derive simplified design equations for engineering applications, reducing the reliance on extensive experimental testing. 6.4 Development of post-fire design equations The absence of analytical or empirical expressions for torsional reduction factors after fire is a critical limitation in current design practice. Future research should aim to formulate: Tη = ( GJ ) residual ( GJ ) ambient Where T​ η represents the temperature-dependent torsional reduction factor. Parametric studies combining experimental data and FE simulations can be used to establish empirical correlations between T​ η , temperature, steel thickness, hollow ratio, and exposure duration. These correlations will form the foundation for next-generation fire design codes for composite tubular members. 6.5 Hybrid and sustainable materials To enhance fire and torsion performance while maintaining sustainability, future CFDST systems should incorporate advanced materials such as: • Stainless steel or aluminum alloys for outer tubes, offering superior oxidation resistance. • Fiber-Reinforced Concrete (FRC) or Geopolymer Concrete, providing reduced spalling and better post-fire recovery. • Recycled steel and lightweight concretes to minimize embodied carbon and improve constructability. Experimental investigations into CFDST–FRC and CFDST–geopolymer hybrids could reveal significant improvements in both torsional and thermal performance while supporting sustainable design objectives. 6.6 Long-term and cyclic post-fire behavior Real structures may experience torsional fatigue or cyclic twisting following fire events due to wind, seismic activity, or uneven thermal recovery. Future studies should examine: • Residual cyclic torsional stiffness and damping after fire exposure. • Creep and relaxation effects during prolonged thermal exposure. • Rehabilitation and strengthening techniques (e.g., external FRP wrapping or grouting of voids) for damaged CFDST columns. These investigations will bridge the gap between short-term post-fire tests and long-term structural serviceability assessments. 6.7 Data integration and machine learning applications Given the growing body of experimental and numerical data, machine learning (ML) models present a powerful tool for identifying nonlinear relationships among geometric, material, and thermal variables. Future research should: • Compile large databases of CFDST test results across torsion, fire, and combined scenarios. • Employ ML techniques (e.g., gradient boosting, neural networks) to predict post-fire torque capacity and stiffness retention. • Integrate ML-driven prediction models into probabilistic fire risk assessment frameworks for composite structures. This approach will enable data-driven optimization of CFDST design under uncertain loading and fire conditions. 6.8 Summary of future research priorities Table 3 shows summary of future research priorities. Table 3. Summary of future research priorities. Focus area Research need Expected contribution Hybrid Fire–Torsion Testing Experimental coupling of thermal and torsional loads Realistic performance data for CFDSTs Temperature-Dependent Models Constitutive laws for G and τ c Predictive post-fire stiffness models FE and Analytical Modeling Coupled heat–torsion simulations Mechanistic understanding and validation Design Equations Empirical torsional reduction factors (ηT) Codified fire–torsion design guidance Advanced Materials Use of FRC, geopolymer, stainless steel Enhanced ductility and sustainability Cyclic & Long-Term Behavior Post-fire torsional fatigue tests Improved durability assessment Machine Learning Integration Data-driven prediction tools Efficient and adaptive design strategies 7. Conclusions This systematic review comprehensively examined the torsional and fire performance of Concrete-Filled Double Skin Steel Tube (CFDST) members based on 37 selected studies (19 torsion-related and 18 fire-related), following the PRISMA 2020 protocol. The integration of findings provides the first unified perspective on how geometric, material, and thermal parameters jointly influence the mechanical and residual behavior of CFDST systems. The major conclusions are summarized as follows: 1. Distinct yet complementary behavior under torsion and fire:Torsional studies have demonstrated that CFDST columns exhibit superior energy dissipation, confinement efficiency, and rotational ductility compared to single-skin CFSTs. Conversely, fire studies confirm their exceptional thermal stability and residual load-bearing capacity due to the protective effect of the inner steel tube and concrete core. 2. Critical role of geometry and thickness:Both torsional resistance and fire endurance increase thicker outer steel tubes. Circular vs. Square/Rectangular Sections show more uniform confinement and reduced heat gradients, making them more resilient under post-fire torsional loading. 3. Material synergy and degradation:The dual steel–concrete system effectively delays local buckling and suppresses spalling under fire, but residual torsional stiffness may degrade by 40–60% depending on heating duration and section thickness. Concrete type (HSC, SCC, or fiber-reinforced) significantly influences the balance between strength and ductility. 4. Absence of post-fire torsional data:No prior study has experimentally assessed the post-fire torsional performance of CFDST members. This represents a critical research gap that limits current design code development and numerical model validation. 5. Proposed experimental program:The current study introduces a detailed testing matrix of 36 full-scale CFDST columns to quantify the degradation in torsional stiffness, torque capacity, and ductility before and after fire exposure. This program is expected to generate the first comprehensive database of post-fire torsional behavior for CFDST systems. 6. Future design and modeling implications:A multidisciplinary approach integrating thermal–torsional testing, temperature-dependent material laws, finite element modeling, and data-driven predictive tools is essential to develop reliable design-oriented torsional reduction factors (T η ​) for fire-exposed composite members. In summary, CFDST columns demonstrate exceptional promise as fire-resilient torsional members in modern composite construction. 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Publisher Full Text Comments on this article Comments (0) Version 1 VERSION 1 PUBLISHED 05 Feb 2026 ADD YOUR COMMENT Comment Author details Author details 1 Construction and projects department, University of Fallujah, Al-Fallujah, Al Anbar Governorate, Iraq 2 Civil Engineering, Tikrit University College of Engineering, Tikrit, Saladin Governorate, Iraq 3 Civil Engineering, University of Anbar College of Engineering, Ramadi, Anbar Governorate, Iraq Omar Fazaa Rajab Roles: Writing – Original Draft Preparation, Writing – Review & Editing Assim M. Lateef Roles: Supervision Akram S. Mahmoud Roles: Supervision Competing interests No competing interests were disclosed. Grant information The author(s) declared that no grants were involved in supporting this work. Article Versions (1) version 1 Published: 05 Feb 2026, 15:189 https://doi.org/10.12688/f1000research.176317.1 Copyright © 2026 Rajab OF et al . This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Download Export To Sciwheel Bibtex EndNote ProCite Ref. Manager (RIS) Sente metrics Views Downloads F1000Research - - PubMed Central info_outline Data from PMC are received and updated monthly. - - Citations open_in_new 0 open_in_new 0 open_in_new SEE MORE DETAILS CITE how to cite this article Rajab OF, Lateef AM and Mahmoud AS. Systematic Review of the Torsional Performance of Concrete-Filled Double Skin Steel Tube (CFDST) Members under Fire Conditions Following PRISMA Protocols [version 1; peer review: 1 not approved] . F1000Research 2026, 15 :189 ( https://doi.org/10.12688/f1000research.176317.1 ) NOTE: If applicable, it is important to ensure the information in square brackets after the title is included in all citations of this article. COPY CITATION DETAILS track receive updates on this article Track an article to receive email alerts on any updates to this article. TRACK THIS ARTICLE Share Open Peer Review Current Reviewer Status: ? Key to Reviewer Statuses VIEW HIDE Approved The paper is scientifically sound in its current form and only minor, if any, improvements are suggested Approved with reservations A number of small changes, sometimes more significant revisions are required to address specific details and improve the papers academic merit. Not approved Fundamental flaws in the paper seriously undermine the findings and conclusions Version 1 VERSION 1 PUBLISHED 05 Feb 2026 Views 0 Cite How to cite this report: Patra RK. Reviewer Report For: Systematic Review of the Torsional Performance of Concrete-Filled Double Skin Steel Tube (CFDST) Members under Fire Conditions Following PRISMA Protocols [version 1; peer review: 1 not approved] . F1000Research 2026, 15 :189 ( https://doi.org/10.5256/f1000research.194362.r477219 ) The direct URL for this report is: https://f1000research.com/articles/15-189/v1#referee-response-477219 NOTE: it is important to ensure the information in square brackets after the title is included in this citation. Close Copy Citation Details Reviewer Report 25 Apr 2026 Rakesh Kumar Patra , Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, India; C V Raman Global University, Bhubaneswar, Odisha, India Not Approved VIEWS 0 https://doi.org/10.5256/f1000research.194362.r477219 The manuscript claims adherence to PRISMA methodology, but the search strategy is not transparently reported. Key elements such as complete search strings, database-specific queries, and reproducibility steps are missing. This undermines the validity of the systematic review framework. ... Continue reading READ ALL The manuscript claims adherence to PRISMA methodology, but the search strategy is not transparently reported. Key elements such as complete search strings, database-specific queries, and reproducibility steps are missing. This undermines the validity of the systematic review framework. There are clear inconsistencies in the PRISMA flow data presented in the manuscript. The numbers reported for screened, excluded, and included studies do not logically reconcile. This raises concerns about the accuracy of the study selection process. The review lacks any formal critical appraisal of the included studies. There is no assessment of methodological quality, bias, or reliability of the experimental data. As a result, all studies are treated equally regardless of their rigor. Much of the manuscript is descriptive rather than analytical in nature. The authors summarize previous studies without synthesizing findings into deeper insights. This limits the scientific contribution of the work. No statistical or meta-analytical approach has been applied despite having a sizable dataset. The absence of quantitative synthesis reduces the strength of conclusions. A systematic review should ideally provide more than narrative summaries. The tables presented are excessively dense and difficult to interpret. Important findings are buried within large amounts of raw data. This significantly reduces readability and usability for readers. The manuscript contains multiple grammatical and language errors throughout. Sentence structure and phrasing are often unclear or awkward. Professional language editing is necessary before reconsideration. The claimed novelty through a proposed experimental program is weak. The proposal lacks sufficient technical detail and does not present actual experimental validation. This limits its contribution as original research. There is no development of a theoretical or predictive framework in the study. The review does not lead to new models, equations, or design formulations. Consequently, the outcomes remain largely qualitative. Repetition is evident across several sections of the manuscript. Similar observations regarding parameters like thickness and confinement are reiterated multiple times. This reduces conciseness and clarity. The integration between fire performance and torsional behavior is insufficient. These aspects are discussed separately with limited effort to combine them meaningfully. The central objective of coupled behavior is not fully achieved. Several conclusions are presented without strong supporting evidence. General statements are made without consistent comparison or validation across studies. This weakens the reliability of the findings. Figures and visual representations are limited and not effectively used. The PRISMA diagram is not clearly explained and no graphical synthesis of results is provided. Better visualization would improve comprehension. The literature review does not fully capture recent advancements in the field. Emerging approaches and modern analytical techniques appear to be missing. This questions the completeness of the review. The study does not clearly translate findings into practical engineering applications. There is little discussion on design implications or code development. This limits its usefulness for practitioners. Are the rationale for, and objectives of, the Systematic Review clearly stated? No Are sufficient details of the methods and analysis provided to allow replication by others? No Is the statistical analysis and its interpretation appropriate? Not applicable Are the conclusions drawn adequately supported by the results presented in the review? Partly If this is a Living Systematic Review, is the ‘living’ method appropriate and is the search schedule clearly defined and justified? (‘Living Systematic Review’ or a variation of this term should be included in the title.) Partly Competing Interests: No competing interests were disclosed. Reviewer Expertise: Concrete filled steel tube I confirm that I have read this submission and believe that I have an appropriate level of expertise to state that I do not consider it to be of an acceptable scientific standard, for reasons outlined above. Close READ LESS CITE CITE HOW TO CITE THIS REPORT Patra RK. Reviewer Report For: Systematic Review of the Torsional Performance of Concrete-Filled Double Skin Steel Tube (CFDST) Members under Fire Conditions Following PRISMA Protocols [version 1; peer review: 1 not approved] . F1000Research 2026, 15 :189 ( https://doi.org/10.5256/f1000research.194362.r477219 ) The direct URL for this report is: https://f1000research.com/articles/15-189/v1#referee-response-477219 NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article. COPY CITATION DETAILS Report a concern Author Response 28 Apr 2026 Omar Fazaa Rajab , Construction and projects department, University of Fallujah, Al-Fallujah, Iraq 28 Apr 2026 Author Response We sincerely thank the reviewer for the time and effort devoted to evaluating this manuscript. The comments provided are appreciated and have been carefully considered. PRISMA methodology and ... Continue reading We sincerely thank the reviewer for the time and effort devoted to evaluating this manuscript. The comments provided are appreciated and have been carefully considered. PRISMA methodology and search transparency Response: We acknowledge the comment regarding the level of detail in the search strategy. In the revised version, additional clarification will be included to improve transparency of the databases used, keyword combinations, and general search procedure. The study follows PRISMA principles, and this will be more clearly presented. PRISMA flow consistency Response: The PRISMA flow diagram and associated numbers will be rechecked and adjusted where necessary to ensure internal consistency and clarity of the selection process. Critical appraisal of included studies Response: The purpose of this review is to provide a structured synthesis of experimental findings. While a formal risk-of-bias tool was not originally applied due to the heterogeneous nature of the studies, an additional discussion will be included to better reflect differences in study reliability and experimental approaches. Descriptive versus analytical content Response: The manuscript primarily focuses on synthesizing findings from existing experimental studies. In the revision, further clarification and limited comparative interpretation will be added to strengthen the analytical aspect without altering the scope of the review. Quantitative synthesis Response: Due to variability in test configurations, materials, and reporting methods across the collected studies, a formal meta-analysis is not considered appropriate. However, selected comparative observations will be presented more clearly. Tables and readability Response: The tables will be reviewed and slightly reorganized to improve readability and presentation of key information. Language and clarity Response: The manuscript will undergo language revision to improve clarity and overall readability. Proposed experimental program Response: The proposed experimental program is intended as a conceptual extension based on the identified research gaps rather than a detailed experimental study. Minor clarifications will be added to better present its scope. Predictive framework Response: It is noted that the current literature does not provide an integrated predictive framework combining torsional behavior and fire performance. The manuscript reflects this condition and aims to highlight the need for such development rather than introduce a new model. Integration of fire and torsion behavior Response: The review indicates that existing studies generally treat fire performance and torsional behavior separately. This separation is acknowledged in the manuscript as a key gap, and the revised version will present this point more clearly. Repetition and structure Response: Some sections will be revised to reduce repetition and improve flow. Strength of conclusions Response: The conclusions will be refined to ensure closer alignment with the presented evidence. Figures and visualization Response: Minor improvements will be made to enhance figure clarity and presentation. Literature coverage Response: Selected recent studies will be considered for inclusion where appropriate. Practical implications Response: Additional brief remarks will be included to better relate the findings to engineering applications. We respectfully note that the current evaluation reflects the present version of the manuscript. The study is intended as a structured systematic review within a defined scope, and the ongoing revisions aim to enhance clarity, methodological presentation, and analytical depth without extending beyond its intended framework. We appreciate the reviewer’s comments and believe that the revisions will improve the clarity and presentation of the manuscript while maintaining its intended scope. Sincerely, The Authors We sincerely thank the reviewer for the time and effort devoted to evaluating this manuscript. The comments provided are appreciated and have been carefully considered. PRISMA methodology and search transparency Response: We acknowledge the comment regarding the level of detail in the search strategy. In the revised version, additional clarification will be included to improve transparency of the databases used, keyword combinations, and general search procedure. The study follows PRISMA principles, and this will be more clearly presented. PRISMA flow consistency Response: The PRISMA flow diagram and associated numbers will be rechecked and adjusted where necessary to ensure internal consistency and clarity of the selection process. Critical appraisal of included studies Response: The purpose of this review is to provide a structured synthesis of experimental findings. While a formal risk-of-bias tool was not originally applied due to the heterogeneous nature of the studies, an additional discussion will be included to better reflect differences in study reliability and experimental approaches. Descriptive versus analytical content Response: The manuscript primarily focuses on synthesizing findings from existing experimental studies. In the revision, further clarification and limited comparative interpretation will be added to strengthen the analytical aspect without altering the scope of the review. Quantitative synthesis Response: Due to variability in test configurations, materials, and reporting methods across the collected studies, a formal meta-analysis is not considered appropriate. However, selected comparative observations will be presented more clearly. Tables and readability Response: The tables will be reviewed and slightly reorganized to improve readability and presentation of key information. Language and clarity Response: The manuscript will undergo language revision to improve clarity and overall readability. Proposed experimental program Response: The proposed experimental program is intended as a conceptual extension based on the identified research gaps rather than a detailed experimental study. Minor clarifications will be added to better present its scope. Predictive framework Response: It is noted that the current literature does not provide an integrated predictive framework combining torsional behavior and fire performance. The manuscript reflects this condition and aims to highlight the need for such development rather than introduce a new model. Integration of fire and torsion behavior Response: The review indicates that existing studies generally treat fire performance and torsional behavior separately. This separation is acknowledged in the manuscript as a key gap, and the revised version will present this point more clearly. Repetition and structure Response: Some sections will be revised to reduce repetition and improve flow. Strength of conclusions Response: The conclusions will be refined to ensure closer alignment with the presented evidence. Figures and visualization Response: Minor improvements will be made to enhance figure clarity and presentation. Literature coverage Response: Selected recent studies will be considered for inclusion where appropriate. Practical implications Response: Additional brief remarks will be included to better relate the findings to engineering applications. We respectfully note that the current evaluation reflects the present version of the manuscript. The study is intended as a structured systematic review within a defined scope, and the ongoing revisions aim to enhance clarity, methodological presentation, and analytical depth without extending beyond its intended framework. We appreciate the reviewer’s comments and believe that the revisions will improve the clarity and presentation of the manuscript while maintaining its intended scope. Sincerely, The Authors Competing Interests: Competing Interests The authors declare that there are no competing interests. Close Report a concern Respond or Comment COMMENTS ON THIS REPORT Author Response 28 Apr 2026 Omar Fazaa Rajab , Construction and projects department, University of Fallujah, Al-Fallujah, Iraq 28 Apr 2026 Author Response We sincerely thank the reviewer for the time and effort devoted to evaluating this manuscript. The comments provided are appreciated and have been carefully considered. PRISMA methodology and ... Continue reading We sincerely thank the reviewer for the time and effort devoted to evaluating this manuscript. The comments provided are appreciated and have been carefully considered. PRISMA methodology and search transparency Response: We acknowledge the comment regarding the level of detail in the search strategy. In the revised version, additional clarification will be included to improve transparency of the databases used, keyword combinations, and general search procedure. The study follows PRISMA principles, and this will be more clearly presented. PRISMA flow consistency Response: The PRISMA flow diagram and associated numbers will be rechecked and adjusted where necessary to ensure internal consistency and clarity of the selection process. Critical appraisal of included studies Response: The purpose of this review is to provide a structured synthesis of experimental findings. While a formal risk-of-bias tool was not originally applied due to the heterogeneous nature of the studies, an additional discussion will be included to better reflect differences in study reliability and experimental approaches. Descriptive versus analytical content Response: The manuscript primarily focuses on synthesizing findings from existing experimental studies. In the revision, further clarification and limited comparative interpretation will be added to strengthen the analytical aspect without altering the scope of the review. Quantitative synthesis Response: Due to variability in test configurations, materials, and reporting methods across the collected studies, a formal meta-analysis is not considered appropriate. However, selected comparative observations will be presented more clearly. Tables and readability Response: The tables will be reviewed and slightly reorganized to improve readability and presentation of key information. Language and clarity Response: The manuscript will undergo language revision to improve clarity and overall readability. Proposed experimental program Response: The proposed experimental program is intended as a conceptual extension based on the identified research gaps rather than a detailed experimental study. Minor clarifications will be added to better present its scope. Predictive framework Response: It is noted that the current literature does not provide an integrated predictive framework combining torsional behavior and fire performance. The manuscript reflects this condition and aims to highlight the need for such development rather than introduce a new model. Integration of fire and torsion behavior Response: The review indicates that existing studies generally treat fire performance and torsional behavior separately. This separation is acknowledged in the manuscript as a key gap, and the revised version will present this point more clearly. Repetition and structure Response: Some sections will be revised to reduce repetition and improve flow. Strength of conclusions Response: The conclusions will be refined to ensure closer alignment with the presented evidence. Figures and visualization Response: Minor improvements will be made to enhance figure clarity and presentation. Literature coverage Response: Selected recent studies will be considered for inclusion where appropriate. Practical implications Response: Additional brief remarks will be included to better relate the findings to engineering applications. We respectfully note that the current evaluation reflects the present version of the manuscript. The study is intended as a structured systematic review within a defined scope, and the ongoing revisions aim to enhance clarity, methodological presentation, and analytical depth without extending beyond its intended framework. We appreciate the reviewer’s comments and believe that the revisions will improve the clarity and presentation of the manuscript while maintaining its intended scope. Sincerely, The Authors We sincerely thank the reviewer for the time and effort devoted to evaluating this manuscript. The comments provided are appreciated and have been carefully considered. PRISMA methodology and search transparency Response: We acknowledge the comment regarding the level of detail in the search strategy. In the revised version, additional clarification will be included to improve transparency of the databases used, keyword combinations, and general search procedure. The study follows PRISMA principles, and this will be more clearly presented. PRISMA flow consistency Response: The PRISMA flow diagram and associated numbers will be rechecked and adjusted where necessary to ensure internal consistency and clarity of the selection process. Critical appraisal of included studies Response: The purpose of this review is to provide a structured synthesis of experimental findings. While a formal risk-of-bias tool was not originally applied due to the heterogeneous nature of the studies, an additional discussion will be included to better reflect differences in study reliability and experimental approaches. Descriptive versus analytical content Response: The manuscript primarily focuses on synthesizing findings from existing experimental studies. In the revision, further clarification and limited comparative interpretation will be added to strengthen the analytical aspect without altering the scope of the review. Quantitative synthesis Response: Due to variability in test configurations, materials, and reporting methods across the collected studies, a formal meta-analysis is not considered appropriate. However, selected comparative observations will be presented more clearly. Tables and readability Response: The tables will be reviewed and slightly reorganized to improve readability and presentation of key information. Language and clarity Response: The manuscript will undergo language revision to improve clarity and overall readability. Proposed experimental program Response: The proposed experimental program is intended as a conceptual extension based on the identified research gaps rather than a detailed experimental study. Minor clarifications will be added to better present its scope. Predictive framework Response: It is noted that the current literature does not provide an integrated predictive framework combining torsional behavior and fire performance. The manuscript reflects this condition and aims to highlight the need for such development rather than introduce a new model. Integration of fire and torsion behavior Response: The review indicates that existing studies generally treat fire performance and torsional behavior separately. This separation is acknowledged in the manuscript as a key gap, and the revised version will present this point more clearly. Repetition and structure Response: Some sections will be revised to reduce repetition and improve flow. Strength of conclusions Response: The conclusions will be refined to ensure closer alignment with the presented evidence. Figures and visualization Response: Minor improvements will be made to enhance figure clarity and presentation. Literature coverage Response: Selected recent studies will be considered for inclusion where appropriate. Practical implications Response: Additional brief remarks will be included to better relate the findings to engineering applications. We respectfully note that the current evaluation reflects the present version of the manuscript. The study is intended as a structured systematic review within a defined scope, and the ongoing revisions aim to enhance clarity, methodological presentation, and analytical depth without extending beyond its intended framework. We appreciate the reviewer’s comments and believe that the revisions will improve the clarity and presentation of the manuscript while maintaining its intended scope. Sincerely, The Authors Competing Interests: Competing Interests The authors declare that there are no competing interests. Close Report a concern COMMENT ON THIS REPORT Comments on this article Comments (0) Version 1 VERSION 1 PUBLISHED 05 Feb 2026 ADD YOUR COMMENT Comment keyboard_arrow_left keyboard_arrow_right Open Peer Review Reviewer Status info_outline Alongside their report, reviewers assign a status to the article: Approved The paper is scientifically sound in its current form and only minor, if any, improvements are suggested Approved with reservations A number of small changes, sometimes more significant revisions are required to address specific details and improve the papers academic merit. Not approved Fundamental flaws in the paper seriously undermine the findings and conclusions Reviewer Reports Invited Reviewers 1 Version 1 05 Feb 26 read Rakesh Kumar Patra , Indian Institute of Technology Roorkee, Roorkee, India; C V Raman Global University, Bhubaneswar, India Comments on this article All Comments (0) Add a comment Sign up for content alerts Sign Up You are now signed up to receive this alert Browse by related subjects keyboard_arrow_left Back to all reports Reviewer Report 0 Views copyright © 2026 Patra R. This is an open access peer review report distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 25 Apr 2026 | for Version 1 Rakesh Kumar Patra , Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, India; C V Raman Global University, Bhubaneswar, Odisha, India 0 Views copyright © 2026 Patra R. This is an open access peer review report distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. format_quote Cite this report speaker_notes Responses (1) Not Approved info_outline Alongside their report, reviewers assign a status to the article: Approved The paper is scientifically sound in its current form and only minor, if any, improvements are suggested Approved with reservations A number of small changes, sometimes more significant revisions are required to address specific details and improve the papers academic merit. Not approved Fundamental flaws in the paper seriously undermine the findings and conclusions The manuscript claims adherence to PRISMA methodology, but the search strategy is not transparently reported. Key elements such as complete search strings, database-specific queries, and reproducibility steps are missing. This undermines the validity of the systematic review framework. There are clear inconsistencies in the PRISMA flow data presented in the manuscript. The numbers reported for screened, excluded, and included studies do not logically reconcile. This raises concerns about the accuracy of the study selection process. The review lacks any formal critical appraisal of the included studies. There is no assessment of methodological quality, bias, or reliability of the experimental data. As a result, all studies are treated equally regardless of their rigor. Much of the manuscript is descriptive rather than analytical in nature. The authors summarize previous studies without synthesizing findings into deeper insights. This limits the scientific contribution of the work. No statistical or meta-analytical approach has been applied despite having a sizable dataset. The absence of quantitative synthesis reduces the strength of conclusions. A systematic review should ideally provide more than narrative summaries. The tables presented are excessively dense and difficult to interpret. Important findings are buried within large amounts of raw data. This significantly reduces readability and usability for readers. The manuscript contains multiple grammatical and language errors throughout. Sentence structure and phrasing are often unclear or awkward. Professional language editing is necessary before reconsideration. The claimed novelty through a proposed experimental program is weak. The proposal lacks sufficient technical detail and does not present actual experimental validation. This limits its contribution as original research. There is no development of a theoretical or predictive framework in the study. The review does not lead to new models, equations, or design formulations. Consequently, the outcomes remain largely qualitative. Repetition is evident across several sections of the manuscript. Similar observations regarding parameters like thickness and confinement are reiterated multiple times. This reduces conciseness and clarity. The integration between fire performance and torsional behavior is insufficient. These aspects are discussed separately with limited effort to combine them meaningfully. The central objective of coupled behavior is not fully achieved. Several conclusions are presented without strong supporting evidence. General statements are made without consistent comparison or validation across studies. This weakens the reliability of the findings. Figures and visual representations are limited and not effectively used. The PRISMA diagram is not clearly explained and no graphical synthesis of results is provided. Better visualization would improve comprehension. The literature review does not fully capture recent advancements in the field. Emerging approaches and modern analytical techniques appear to be missing. This questions the completeness of the review. The study does not clearly translate findings into practical engineering applications. There is little discussion on design implications or code development. This limits its usefulness for practitioners. Are the rationale for, and objectives of, the Systematic Review clearly stated? No Are sufficient details of the methods and analysis provided to allow replication by others? No Is the statistical analysis and its interpretation appropriate? Not applicable Are the conclusions drawn adequately supported by the results presented in the review? Partly If this is a Living Systematic Review, is the ‘living’ method appropriate and is the search schedule clearly defined and justified? (‘Living Systematic Review’ or a variation of this term should be included in the title.) Partly Competing Interests No competing interests were disclosed. Reviewer Expertise Concrete filled steel tube I confirm that I have read this submission and believe that I have an appropriate level of expertise to state that I do not consider it to be of an acceptable scientific standard, for reasons outlined above. reply Respond to this report Responses (1) Author Response 28 Apr 2026 Omar Fazaa Rajab, Construction and projects department, University of Fallujah, Al-Fallujah, Iraq We sincerely thank the reviewer for the time and effort devoted to evaluating this manuscript. The comments provided are appreciated and have been carefully considered. PRISMA methodology and search transparency Response: We acknowledge the comment regarding the level of detail in the search strategy. In the revised version, additional clarification will be included to improve transparency of the databases used, keyword combinations, and general search procedure. The study follows PRISMA principles, and this will be more clearly presented. PRISMA flow consistency Response: The PRISMA flow diagram and associated numbers will be rechecked and adjusted where necessary to ensure internal consistency and clarity of the selection process. Critical appraisal of included studies Response: The purpose of this review is to provide a structured synthesis of experimental findings. While a formal risk-of-bias tool was not originally applied due to the heterogeneous nature of the studies, an additional discussion will be included to better reflect differences in study reliability and experimental approaches. Descriptive versus analytical content Response: The manuscript primarily focuses on synthesizing findings from existing experimental studies. In the revision, further clarification and limited comparative interpretation will be added to strengthen the analytical aspect without altering the scope of the review. Quantitative synthesis Response: Due to variability in test configurations, materials, and reporting methods across the collected studies, a formal meta-analysis is not considered appropriate. However, selected comparative observations will be presented more clearly. Tables and readability Response: The tables will be reviewed and slightly reorganized to improve readability and presentation of key information. Language and clarity Response: The manuscript will undergo language revision to improve clarity and overall readability. Proposed experimental program Response: The proposed experimental program is intended as a conceptual extension based on the identified research gaps rather than a detailed experimental study. Minor clarifications will be added to better present its scope. Predictive framework Response: It is noted that the current literature does not provide an integrated predictive framework combining torsional behavior and fire performance. The manuscript reflects this condition and aims to highlight the need for such development rather than introduce a new model. Integration of fire and torsion behavior Response: The review indicates that existing studies generally treat fire performance and torsional behavior separately. This separation is acknowledged in the manuscript as a key gap, and the revised version will present this point more clearly. Repetition and structure Response: Some sections will be revised to reduce repetition and improve flow. Strength of conclusions Response: The conclusions will be refined to ensure closer alignment with the presented evidence. Figures and visualization Response: Minor improvements will be made to enhance figure clarity and presentation. Literature coverage Response: Selected recent studies will be considered for inclusion where appropriate. Practical implications Response: Additional brief remarks will be included to better relate the findings to engineering applications. We respectfully note that the current evaluation reflects the present version of the manuscript. The study is intended as a structured systematic review within a defined scope, and the ongoing revisions aim to enhance clarity, methodological presentation, and analytical depth without extending beyond its intended framework. We appreciate the reviewer’s comments and believe that the revisions will improve the clarity and presentation of the manuscript while maintaining its intended scope. Sincerely, The Authors View more View less Competing Interests Competing Interests The authors declare that there are no competing interests. reply Respond Report a concern Patra RK. Peer Review Report For: Systematic Review of the Torsional Performance of Concrete-Filled Double Skin Steel Tube (CFDST) Members under Fire Conditions Following PRISMA Protocols [version 1; peer review: 1 not approved] . F1000Research 2026, 15 :189 ( https://doi.org/10.5256/f1000research.194362.r477219) NOTE: it is important to ensure the information in square brackets after the title is included in this citation. The direct URL for this report is: https://f1000research.com/articles/15-189/v1#referee-response-477219 Alongside their report, reviewers assign a status to the article: Approved - the paper is scientifically sound in its current form and only minor, if any, improvements are suggested Approved with reservations - A number of small changes, sometimes more significant revisions are required to address specific details and improve the papers academic merit. Not approved - fundamental flaws in the paper seriously undermine the findings and conclusions Adjust parameters to alter display View on desktop for interactive features Includes Interactive Elements View on desktop for interactive features Competing Interests Policy Provide sufficient details of any financial or non-financial competing interests to enable users to assess whether your comments might lead a reasonable person to question your impartiality. Consider the following examples, but note that this is not an exhaustive list: Examples of 'Non-Financial Competing Interests' Within the past 4 years, you have held joint grants, published or collaborated with any of the authors of the selected paper. You have a close personal relationship (e.g. parent, spouse, sibling, or domestic partner) with any of the authors. You are a close professional associate of any of the authors (e.g. scientific mentor, recent student). You work at the same institute as any of the authors. 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