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Limiting flame spread rates in large compartments with visible timber ceilings
RISE Research Institutes of Sweden, Safety and Transport, Fire and Safety.ORCID iD: 0000-0002-7663-1525
RISE Research Institutes of Sweden, Safety and Transport, Fire and Safety.ORCID iD: 0000-0001-8670-062x
RISE Research Institutes of Sweden, Safety and Transport, Fire and Safety.ORCID iD: 0000-0002-8001-401x
RISE Research Institutes of Sweden, Safety and Transport, Fire and Safety.ORCID iD: 0000-0002-4248-8396
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2023 (English)Report (Other academic)
Abstract [en]

The number of tall buildings combining both a visible mass timber structure and large open floor plans is growing rapidly introducing new fire safety challenges. One risk is that of very rapid flame spread in the ceiling, originating from a severe but localized fire, resulting in fires where the majority of large compartments burn simultaneously. Such phenomena have been observed in both tests and accidents, but knowledge of effective mitigation without the use of sprinklers is scarce. In Europe, this problem is commonly addressed in construction by complying to prescriptive rules of reaction-to-fire classification of linings. The reaction-to-fire classification, primarily based on the single burning item (SBI) test of EN13832, characterizes the material’s contribution to a fire in the very initial phase of the fire. Treatments can be used to improve the reaction-to-fire class of mass timber, which will reduce the risk of substantial fire development. Fires can, however, develop and grow large even without the contribution of lining materials. For this reason, and in light of the recent findings of research of large open floor plan compartments, it is of interest to assess the effectiveness of treatments to reduce the risk of rapid flame spread. Therefore, eight tests in 18.0 × 2.3 × 2.2 m3 compartments were performed. Six had exposed timber surface with a clear coating or impregnation in the ceiling, complying with a reaction-to-fire class B and two served as untreated timber and non-combustible reference tests. The fire source, representing a fire in moveable fuel, was severe enough (3 - 3.7 MW) for flame impingement on the ceiling. The rate of at which wood ignited from the heat in the ceiling, the temperature development at different heights, as well as external flaming were assessed and were used as indicators of performance. Additional indicators were the estimated tenability and ceiling char depths throughout the compartment. The untreated timber and the non-combustible ceiling represented the two extremes for most indicators with the class-B treated timber surfaces falling in between. Close to the fire source, the test indicators for treated timber surfaces performed similar to those of the untreated timber surface while the non-combustible ceiling performed significantly better. With increasing distance from the fire source, indicators from treated timber tests more resembled the non-combustible ceiling. This behavior was noticed for all types of indicators. With increasing distance from the fire source, the fire exposure is naturally less severe and thus, more similar to the small burner exposure used in SBI-testing which the treatments were developed against. Both final charring depth and temperature developments for ignition and tenability were clearly improved by the treatment, but the SBI test results (FIGRA and THR600s) did not correlate well to the compartment test indicators (Figure 92 andFigure 93). Nevertheless, using treatments assessed by SBI is a common strategy to mitigate fire spread in newly constructed mass timber buildings and practitioners should be aware that while the treatments have significant effects on the flame spread they are not to be treated as incombustible. We propose that addressing the ceiling spread problem requires an additional indicative test with more severe exposure than the SBI test setup. The impregnated timber experienced loss of integrity due to substantial shrinkage of the timber during the severe exposure. Such phenomena were not captured in the SBI testing. Comparisons of performance of the impregnated specimens indicates that it can be beneficial for the performance to implement more impregnation than needed for reaction-to-fire class B. Whether this holds for all treatments cannot be concluded.

Place, publisher, year, edition, pages
RISE Research Institutes of Sweden , 2023.
Series
RISE Rapport ; 2023:131
National Category
Civil Engineering
Identifiers
URN: urn:nbn:se:ri:diva-73100ISBN: 978-91-89896-18-5 (electronic)OAI: oai:DiVA.org:ri-73100DiVA, id: diva2:1856254
Available from: 2024-05-06 Created: 2024-05-06 Last updated: 2024-05-06Bibliographically approved

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Brandon, DanielSjöström, JohanJust, AlarLi, Tian

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