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.

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.

Final inclusion required that: •

Specimens involve CFST or CFDST configurations, with or without inner tubes.

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.

AS shown in ( Figure 1) summarizes the study selection process: •

Records identified: 108

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.

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.

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,

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.

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.

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.

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.

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.

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.

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.

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.

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.

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}

Post-fire torsional performance will likely be influenced by: 1.

Reduction in steel yield strength (up to 60% at 600°C),

Thus, assessing post-fire torsional behavior requires capturing the residual material properties and interface conditions—none of which have been experimentally quantified to date.

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.

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.

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.

These gaps form the scientific foundation of the proposed experimental program, which aims to bridge the knowledge divide between isolated torsional and fire research.

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.

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.

Each group includes both square (100 × 100 mm) and rectangular (50 × 100 mm) sections, representing realistic cross-section geometries used in composite columns.

(a)

Outer Tube Thickness

Three steel tube thicknesses will be investigated:

This range covers three confinement levels : solid, wide cavity, and narrow cavity—corresponding to varying hollow ratios between 0.26 and 0.52 .

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.

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)

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.

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;

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.

Such hybrid setups will provide essential data for validating advanced fire–torsion constitutive models and finite element simulations.

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,

Experimental calibration of these parameters will enable reliable prediction of post-fire torsional stiffness (GJ) and residual energy dissipation capacity.

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;

Such models can be used to derive simplified design equations for engineering applications, reducing the reliance on extensive experimental testing.

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.

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.

Experimental investigations into CFDST–FRC and CFDST–geopolymer hybrids could reveal significant improvements in both torsional and thermal performance while supporting sustainable design objectives.

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.

These investigations will bridge the gap between short-term post-fire tests and long-term structural serviceability assessments.

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.

This approach will enable data-driven optimization of CFDST design under uncertain loading and fire conditions.

Table 3 shows summary of future research priorities.