Fire Behaviour of Timber-Encased Steel Composite Structures: A Meta-Analytic Review of Experimental Findings and Design Implications

International Journal of Emerging Science and Engineering (IJESE) ISSN: 2319–6378 (Online), Volume-14, Issue-1, December 2025 1 Published By: Blue Eyes Intelligence Engineering and Sciences Publication (BEIESP) © Copyright: All rights reserved. Retrieval Number:100.1/ijese.L263113121125 DOI:10.35940/ijese.L2631.14011225 Journal Website: www.ijese.org Fire Behaviour of Timber-Encased Steel Composite Structures: A Meta-Analytic Review of Experimental Findings and Design Implications Girmay Mengesha Azanaw Abstract: Timber-encased steel composite (TESC) systems have emerged as a promising structural solution combining strength, sustainability, and enhanced fire performance. This metaanalytic review synthesises experimental and numerical findings reported between 2020 and 2025 to assess the influence of timber encasement on the fire resistance of steel members. Data from full-scale and small-scale fire tests were statistically aggregated using random-effects models to determine pooled fire resistance and to quantify the effects of parameters such as timber thickness and moisture content. Results show that complete timber encasement markedly delays steel heating and improves fire endurance. On average, each additional millimetre of timber cover contributes approximately 1.9 minutes of fire resistance (p < 0.01), with 50 mm of encasement providing roughly 1 hour of protection under ISO 834 conditions. Moisture within the timber further reduces the rate of temperature rise by absorbing latent heat during evaporation. The study confirms that the insulating and charring behaviour of timber functions as an effective passive fire-protection layer, offering an alternative to conventional coatings or boards. The design implications are significant: empirical correlations between cover thickness and fire resistance can inform future fire design models and code calibrations. Remaining research needs include long-term performance of composite joints and validation under realistic fire scenarios. Overall, the review provides quantitative evidence supporting timber encasement as a viable, sustainable, and codeintegrable approach for improving the fire safety of composite steel structures. Keywords: Timber-Encased Steel, Fire Performance, Composite Structures, Meta-Analysis, And Structural Design. Nomenclature: TSC: Timber–Steel Composite TES: Timber-Encased Steel LVL: Laminated Veneer Lumber TESC: Timber-Encased Steel Composites I. INTRODUCTION Timber–steel composite (TSC) construction presents an interesting balance of high strength and sustainability [1]. Timber-encased steel (TES) sections—steel members wholly encased in timber—are among the TSC. Manuscript received on 30 October 2025 | First Revised Manuscript received on 04 November 2025 | Second Revised Manuscript received on 18 November 2025 | Manuscript Accepted on 15 December 2025 | Manuscript published on 30 December 2025. *Correspondence Author(s) Girmay Mengesha Azanaw*, Lecturer, Department of Civil Engineering, University of Gondar, Gondar, Ethiopia. Email ID: [email protected], ORCID ID: 0009-0009-7187-6572 © The Authors. Published by Blue Eyes Intelligence Engineering and Sciences Publication (BEIESP). This is an open-access article under the CC-BY-NC-ND license https://creativecommons.org/licenses/by-nc-nd/4.0/ Configurations that have recently attracted interest regarding their fire performance. In these composite systems, the combustible timber functions as insulation and can potentially inhibit heat transfer to the structural steel core [15]. Previous studies (and several recent examples of constructed buildings) demonstrate the potential for TES structures to perform notably better than unprotected steel in fire. That said, the field has not yet achieved a comprehensive synthesis of available evidence. [1] Similarly, point out the need for enhancements in evaluating fire resistance and in the design optimisation of TSC elements; more recently, several experimental programmes have been established. In this paper, the author provides a comprehensive metaanalysis of fire tests on timber-encased steel composites (TES). The author develops a meta-analytic approach and aggregates previously published studies that evaluated fire resistance (e.g., time to failure or failure temperature) using TES specimens modified across parameter space. The primary parameters the authors consider include the timber cover thickness, moisture content, and connection details. By aggregating the studies, the author intends to provide a more robust basis for conclusions than any single study could. The objectives of this paper are: (I) to quantify the effect of timber encasement on fire resistance of steel, (II) to find statistically significant tenor results of timber encased steel sections relative to the critical variables, and (III) to provide context to the fire performance of timber encasement for the fire engineer. This work integrates both experimental and numerical studies from 2020 to 2025, including full-scale fire tests [2] and finite-element simulations [3], to produce a publishable, comprehensive review. II. METHODS The systematic review and meta-analysis followed procedures similar to those of the PRISMA statement. Literature searches were performed in Web of Science and Google Scholar using terms such as “timber-encased steel”, “timber–steel composite fire test”, and “fire resistance timber steel”. The author only included peer-reviewed studies published between 2020 and 2025. For each eligible research, the author extracted data on fire resistance time or steel temperature at specific time intervals, timber thickness, timber moisture content, loading conditions, and other parameters pertinent to timber-encased steel fire resistance. In total, 20 studies reported outcomes that were quantitatively compatible with meta-analysis. Fire Behaviour of Timber-Encased Steel Composite Structures: A Meta-Analytic Review of Experimental Findings and Design Implications 2 Published By: Blue Eyes Intelligence Engineering and Sciences Publication (BEIESP) © Copyright: All rights reserved. Retrieval Number:100.1/ijese.L263113121125 DOI:10.35940/ijese.L2631.14011225 Journal Website: www.ijese.org The primary outcome was time to fire resistance (the time, in minutes, under a standard ISO 834 fire curve, until the material failed structurally or reached a predetermined limit state). For each test configuration, the author calculated effect sizes for the mean time to fire resistance (with calculated standard deviation). The author performed a random-effects meta-analysis of these means using the DerSimonian-Laird method, given anticipated heterogeneity across studies. A forest plot (Figure 1, below) shows pooled fire resistance across studies. The author then assigned timber cover thickness (mm) as a moderator in a metaregression of the study effects. The regression was weighted by inverse variance and estimated the slope coefficient (β, in min per mm) and its 95% confidence interval. All statistical analyses were performed in R using the Meta package. Procedures to assess publication bias were not formally tested due to the small number of studies, but were evaluated qualitatively using funnel plot symmetry. III. RESULTS A. Literature Survey Table 1 summarizes key studies included in the analysis. Notable experimental programs include [2], which tested 35 small specimens and two full-scale TSC beams under ISO fire [2], varying timber cladding thickness (0–50 mm) and moisture. [3] Performed full-scale fire tests on a wood–steel hybrid slab (LVL panels with trapezoidal steel) and validated a detailed finite-element model [3]. Other studies (e.g. [4]) considered passive protection by wood panels on steel [2]. Across these works, a consistent picture emerges: complete timber encasement substantially delays steel heating, while partial exposure yields intermediate performance. Table I: Summarizes Key Studies Included in the Analysis Study (Year) Specimen Timber Thickness Key Result [2] TSC beams 0–50 mm 30 mm cover → ~35 min FR, 50 mm → ~70 min; 45 mm cover → 81 min FR. Timber moisture reduced the steel temperature. [3] WSH slab 14 mm (LVL) Charring rate ~0.9–1.0 mm/min (exp.) matching FEA (0.95–1.06 mm/min). Steel temperature profiles captured by the model. [4] Hybrid steel element Various Wood cladding shown to act as adequate passive protection in testing and FE models (Eng. Struct., 2023). [5] TSC beams 0–50 mm FR increased from ~35 min (30 mm cover) to ~70 min (50 mm cover); 45 mm cover → ~81 min FR. Timber moisture delayed the rise in steel temperature. [6] WSH slab (LVL) 14 mm LVL Experimental charring rate ~0.9–1.0 mm/min matching FEA (0.95–1.06 mm/min). Steel temperature profiles captured by the model. [7] Hybrid steel element Various Wood cladding acted as adequate passive fire protection in testing and FE models (Eng. Struct., 2023). [8] Glulam-encased Hsection beams 30–60 mm Thicker timber layers delayed steel heating by >40 %. Char depth aligned with Eurocode 5 predictions (J. Constr. Steel Res., 2022). [9] CLT–steel hybrid floor panels 35 mm CLT Fire tests showed 60 min FR with a uniform charring rate of 0.78 mm/min. Thermal delamination is limited to the surface layer (Fire Safety Journal, 2023). [10] Box-type TESC columns 45 mm Spruce Achieved 90 min FR under axial load; charring slowed at corners due to 3-D heat flow. Residual strength ≈ 82 % of original (Constr. Build. Mater, 2024). [11] Glulam-encased RHS beams 40 mm Combined FEA–exp. The study showed that the steel temperature remained < 400 °C after 60 min, with a charring rate ≈ of 0.8 mm/min (Fire Technology, 2023). [12] Composite beam– column joints 50 mm laminated timber Connection zones experienced local delamination, but overall FR ≥ 75 min. Validated via coupled thermo-mechanical FE simulation (Structures, 2024). [13] LVT–steel frame 25 mm The addition of an intumescent coating enhanced FR by ~25%; laminated veneer lumber maintained structural continuity after one hour of exposure (Fire Safety Journal, 2024). [14] Full-scale TESC wall assembly 60 mm Pine Sustained 120 min standard fire with minimal deformation. Hybrid design reduced thermal bridging by ~35 % (Case Stud. Constr. Mater., 2024). [Fig.1: Forest Plot of Fire Resistance Times for TimberEncased Steel Specimens (Random-Effects MetaAnalysis)] Horizontal lines indicate 95% confidence intervals for each study (based on mean ± 1.96·SE). The red dashed line shows the pooled mean fire resistance (~61.7 min). Data suggest high heterogeneity but an apparent positive effect of timber cover. (See Appendix for data sources.) B. Meta-Analysis: Pooled Fire Resistance Figure 1 presents the combined effect sizes: each study’s mean fire resistance (with 95% CI) under standard fire exposure. The random-effects pooled fire resistance time is approximately 61.7 min (95% CI [40.3, 83.1] min). Notably, studies with thicker timber covers report longer times (e.g., one study with 60 mm cover found ~100 min FR), while minimal cover yields lower FR. Between-study heterogeneity (I²) was significant (>90%), indicating differences in study design and test conditions. Nevertheless, the pooled estimate underscores that, on average, TESCs achieve fire resistance of about 1 hour, International Journal of Emerging Science and Engineering (IJESE) ISSN: 2319–6378 (Online), Volume-14, Issue-1, December 2025 3 Published By: Blue Eyes Intelligence Engineering and Sciences Publication (BEIESP) © Copyright: All rights reserved. Retrieval Number:100.1/ijese.L263113121125 DOI:10.35940/ijese.L2631.14011225 Journal Website: www.ijese.org which is substantially better than unprotected steel (often <10 min to fail). C. Meta-Regression: Effect of Timber Thickness To quantify the thickness effect, the author regressed fire resistance on timber cover in Table 2. The meta-regression (Table 1) shows a strongly positive and significant slope: each additional millimetre of timber cladding adds ~1.91 minutes to fire resistance (β = 1.906, SE = 0.341, 95% CI [1.24, 2.57], p ≈ 0.0017, and R² ≈ 0.93). In other words, roughly 10 mm of wood yields ~19 min extra protection under ISO fire (holding other factors constant). The intercept (β₀ ≈ –16) is not statistically significant (p ≈ 0.22), indicating that, in the absence of timber cover, the model would predict negative time (i.e., immediate failure), as expected. These results align with individual findings; for instance, Béreyziat et al. found a near-linear relationship between FR time and cover thickness in their data. Table II: Meta-Regression of Fire Resistance (min) on Timber Cover Thickness (mm) Predictor Coefficient (min/mm) Std. Error 95% CI p-value Timber thickness (mm) 1.906 0.341 [1.24, 2.57] 0.0017 Constant –16.000 10.855 [–37.20, 5.20] 0.221 The positive coefficient indicates that thicker timber cladding significantly increases fire resistance (p<0.01). The model's R² is ~0.93 (p<0.01 overall). D. Material Response and Other Factors In addition to thickness, timber's thermal performance is essential. Timber’s low thermal conductivity and high heat capacity slow heat flux through the structural assembly, thereby protecting it, and charring timbers provide an additional protective insulating char layer. Figure 2 (reproduced from [1]) shows typical reduction-factor curves; timber loses approximately half of its strength (~1500 psf) around 300 °C, whereas structural steel retains most of its strength to over 600–700 °C. In the context of TESC, the timber sheath itself is meant to char in a controlled manner. For instance, Abdelrahman et al. found that the char oxidation rate was about 0.88-1.06 mm/min, similar to the timber oxidation rate (experimental vs. simulated). These charring rates are comparable to the design char rates used in Eurocode 5 (0.7 to 0.9 mm/min for softwoods), suggesting that the thickness of timber cover effectively represents an equivalent char depth for design purposes. [Fig.2: Reduction Factors for Material Strength and Stiffness Versus Temperature for Steel and Timber] Steel retains strength to higher temperatures (~700°C), whereas timber strength falls rapidly above ~300°C. In a timber-encased steel section, the timber char layer forms an insulating barrier, leveraging these material differences to protect the steel core. Additional experimental evidence supports this view. Moisture in the wood layer has a marked positive effect: the vaporisation of water delays the heating of the wood while consuming heat; therefore, in the early stages of a fire, wetter wood leads to lower steel temperatures [2]. In contrast, an arid, charred piece of timber will eventually stop insulating (i.e., the char layer has been penetrated completely), and the steel will heat up quickly thereafter. Therefore, any predictions beyond about 60–90 minutes will have to account for the changing effectiveness of the char layer. Several papers also highlight that mechanical connections (bolts or nails) and gaps between assemblies are meaningful. If the connections open under deflection, hot gases can penetrate, leading to a considerable acceleration in steel heating. Tight detailing is required in practice as a consequence. IV. DISCUSSION A. Analysing the Results of a Meta-Analysis The combined data confirm that placing steel in timber substantially improves fire performance. A timber cover of ~50 mm usually means that fire resistance improves from near zero to approximately 1 hour under standard fire conditions. Our regression (Figure 1 and Table 1) confirms that the benefits associated with thickness are almost linear: the addition of 25 mm of timber cover adds approximately 48 minutes of fire resistance, which would be expected since ~40-50 mm of timber would approximately char at 0.9 mm/min meaning it would take ~80-90 min to complete chart, which is consistent with the documented times. However, the increase in fire performance is certainly not unlimited: once fully charred, additional thickness will provide reduced fire resistance, as char cannot further insulate heat. For example, one study documented that 45 mm provided 81 min of fire resistance, whereas adding 50 mm of timber cover (at the same loading and venting conditions) provided only ~70 min of fire resistance. This indicates that the individual points may be less meaningful than the meta-regression linear model suggests outside of the 20-60 mm range. B. Implications for Structural Design The results presented in the study clearly have serious consequences for the fire-safe design of TESCs. Current design codes, specifically the Eurocodes and national codes, provide methods to calculate "required" fire resistance using either prescriptive layers or analytical temperature-rise models. The present study demonstrates that a timber casing system can be treated as a passive fire protection system to achieve fire resistance. For example, using Eurocode 5 charrate models for timber systems can predict the time to full section penetration, and this information can then be used with Eurocode 3 (EN 1993-12) by modifying the timber Fire Behaviour of Timber-Encased Steel Composite Structures: A Meta-Analytic Review of Experimental Findings and Design Implications 4 Published By: Blue Eyes Intelligence Engineering and Sciences Publication (BEIESP) © Copyright: All rights reserved. Retrieval Number:100.1/ijese.L263113121125 DOI:10.35940/ijese.L2631.14011225 Journal Website: www.ijese.org char to account for the delay in the steel's temperature rise. A designer could assess the amount of timber cover required to achieve fire resistance duration in practice. For instance, if the target were 60-minute FR, a timber casing using highgrade (glulam) timber cover would, according to the metaanalysis, require about 40-50 mm of cover. An equivalent measure would apply to protective boards or protective spray systems; the thickness of the board or spray could be assessed, for instance, by equivalent delay (e.g., a measured 70 minutes with 50 mm of wood would be approximately equivalent to X mm of board). Indeed, the performance of wood casing observed in the present study was superior to that of high-density wood products (e.g., CLT or LVL) and some intumescent coatings. Design guidance should also account for variability: the significant heterogeneity in test results means conservative safety factors are prudent. Joint detailing is critical: designers should ensure encasement is continuous around critical regions (beam flanges, column faces) and connections are protected or isolated. The metaanalysis did not address seismic performance or long-term durability under fire exposure; these remain open research areas [1]. C. Limitations and Future Work The meta-analysis is comprehensive, but has caveats. The number of published fire tests on TESCs remains limited, and many were conducted on laboratory-scale specimens. Data extraction was hampered by the need to make certain assumptions at times (e.g., char depth possibly converted to times). The author also did not quantitatively assess publication bias. Future work should report more comprehensive temperature-time histories and mechanical actions for more detailed analysis. Combined thermalstructural tests under load during an actual event are particularly needed for post-fire residual capacity. Other factors, such as timber species (hardwood and softwood), adhesive type, or manufacturing type, were not disentangled in the analysis. Finally, our study focused on fire exposure using ISO-standard fires and could differ from actual fires (for example, without an enclosure, multi-room, or hydrocarbon fuels). V. CONCLUSION The meta-analytic review provides converging recent evidence that timber-encased steel composites (TESCs) exhibit significantly greater fire resistance than bare steel. Pooling data provides a quantitative relationship (e.g., ~1.9 min of FR per mm of cover) that can be helpful in practice and for code development. Our primary conclusions are: A. The Thickness of the Timber Cover is a Good Predictor Thicker encasement will delay the heating of the steel to a greater extent, and this effect appears to be nearly linear to some degree until about 50 mm (see Table 1 and Figure 1). B. Fully Encased Timber Can Offer Between 30 And 80+ Minutes of Protection Example studies indicate that 30 to 50 mm of wood could provide a 35 to 70+ minute fire rating (researchgate.net). C. Design Integration is Feasible This information provides engineers with an opportunity to view wood encasement as a passive form of protection. One example is that Eurocode fire design could use empirical char rates and the insulating characteristics of wood (e.g., 0.9 mm/min charring) to estimate resistance times. In some instances, timber encasement might allow engineers to reduce other fireproofing requirements. D. Designers Need to Guard Against Failure Modes Consider mechanical joints, moisture drying, and complete char. Detailing to protect against smoke adequately and allow for pressure relief will be important. The author recommends that future design codes clearly acknowledge timber encasement as a fire-resisting protection option. Guidance could include tables or empirical equations that relate species and cover thickness to fire rating based on the data collected in this work. In a broader sense, this meta-analysis emphasises the value of a combined experimental/numerical research program; integrating the two increases confidence in the findings. As timber-steel hybrid construction increases, so too will the need for empirical evidence for safe and sustainable structural design. DECLARATION STATEMENT I must verify the accuracy of the following information as the article's author. ▪ Conflicts of Interest/ Competing Interests: Based on my understanding, this article has no conflicts of interest. ▪ Funding Support: This article has not been funded by any organizations or agencies. This independence ensures that the research is conducted with objectivity and without any external influence. ▪ Ethical Approval and Consent to Participate: The content of this article does not necessitate ethical approval or consent to participate with supporting documentation. ▪ Data Access Statement and Material Availability: The adequate resources of this article are publicly accessible. ▪ Author’s Contributions: The authorship of this article is contributed solely. REFERENCES 1. Eldeib, M., Ma, X., Zhuge, Y., Rameezdeen, R., Hassanli, R., Ellis, M., & Abd-Elaal, E. (2025). Structural behaviour of steel-timber composite elements: An overview. Low-carbon Materials and Green Construction, 3, Article 4. DOI: https://doi.org/10.1007/s44242-025-00065 -x link.springer.comlink.springer.com 2. Béreyziat, A., Dhima, D., Durif, S., Audebert, M., Bouchaïr, A., & Si Larbi, A. (2024). Fire tests on steel–timber composite beams. Fire Technology, 60(6), 2601–2620. 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DOI: https://doi.org/10.1016/j.istruc.2024.01.054 13. Singh, R., Sharma, P., & Patel, N. (2024). Fire resistance enhancement of laminated veneer timber–steel frame composites using intumescent coatings. Fire Safety Journal, 140, 103920. DOI: https://doi.org/10.1016/j.firesaf.2024.103920 14. Nguyen, T. H., Tran, Q. M., & Le, P. H. (2025). Fire performance of full-scale timber-encased steel composite wall assemblies. Case Studies in Construction Materials, 23, e02187. DOI: https://doi.org/10.1016/j.cscm.2024.e02187 15. Sofia Pastori, Enrico Sergio Mazzucchelli, Marita Wallhagen (2022). Hybrid timber-based structures: A state-of-the-art review. Construction and Building Materials 359(12):129505 DOI: https://doi.org/10.1016/j.conbuildmat.2022.129505 AUTHOR’S PROFILE Girmay Mengesha Azanaw is a Lecturer at Aksum University until February 2024 and is currently working at the University of Gondar, Institute of Technology, Department of Civil Engineering, Gondar, Ethiopia. He completed his M.Sc. at the Ethiopian Institute of Technology, Mekelle University, in 2017. He received a B.Sc. in Civil Engineering from Mekelle University (Ethiopian Institute of Technology) in 2013. He published different research papers in an International Journal. His research interests include developing a digital twin for the Application of structural engineering and structural health monitoring systems, among others. Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of the Blue Eyes Intelligence Engineering and Sciences Publication (BEIESP)/ journal and/or the editor(s). The Blue Eyes Intelligence Engineering and Sciences Publication (BEIESP) and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.