With mass timber structural systems being increasingly used in tall and large buildings, the Fire Protection Research Foundation (FPRF) initiated a series of research projects to help address fire safety challenges of tall mass timber buildings. Previously completed projects include a review of mass timber building fire performance, identification and prioritization of research needs, and experimental and modelling works to quantify the contribution of cross laminated timber (CLT) building elements to compartment fires. These projects focused on fire safety and protection of occupants, fire fighters, and property during a fire. A topic that has not been well addressed is the repair of mass timber after a fire event. Tall mass timber buildings are expected to resist significant structural failure or collapse during and after a fire. However, technical information and guidance on how to repair and recertify a mass timber structure after a fire are lacking. Therefore, the Fire Protection Research Foundation initiated this research program is to develop guidance on repair and recertification of mass timber structures after a fire. The objective of this phase of the project was to develop a research plan for future research projects to help bridge the gap in knowledge needed to enable repair and recertification of a mass timber structure after a fire.

Sample tests were conducted to obtain thermal and kinematic parameters for wood and high-density polyethylene (HDPE) that were used in a series of intermediate scale tunnel fire tests with and without water-based fire suppressions systems. The thermal properties were measured using Transient Plane Source (TPS) and Transient Line Source (TLS). The pyrolysis kinetics parameters were tested based on Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). Different methods were used to obtain pyrolysis kinetics parameters. Different oxygen concentrations exposed to samples were tested and the results showed its significant influence in the charring process.

Post-flashover fires inherently lead to external fire plumes, constituting a hazard for rapid fire spread over façades. As multi-storey mass timber buildings with internal visible timber surfaces become more common, there are concerns that such buildings would produce larger external plumes and hazards (assuming all other parameters equal). The literature reveals only indications of this, and how the actual exposure relates to different test methods for assessment is unknown. Here we utilise a series of full-scale mass timber compartment tests to quantify the exposure to the external façade. An incombustible external façade is instrumented with gauges at positions corresponding to reference data from several different assessment methods. The results show that there is an increase in plume duration, height, and temperatures when increasing the areas of exposed timber, but that this increase is less for normal- to large-opening compartments, than was previously seen in small-opening compartments. Also, normal variations in external wind speed have a larger influence on plume heights than the effect of doubling exposed timber surfaces. Test methods used for regulatory compliance differ significantly not only in exposure but also in pass/fail criteria. The proposed European large exposure method and the BS8414 method exhibit exposures on par with the severe end of what could be expected from mass timber compartments, whereas methods like SP Fire 105 and Lepir II produce significantly less severe plumes. However, the safety level is always a combination of exposure and assessment criteria. This data can help justify assessment criteria from a performance perspective. © 2023 The Authors. 

Exposing cross-laminated timber (CLT) structures in buildings is increasingly popular in modern buildings. However, large timber surfaces, window facades, and different geometries can change the fire dynamics in a compartment. The effect of those parameters, therefore, needs to be studied. Two large-scale CLT compartment fire experiments (95 m2) have consequently been performed. The experiments were designed to represent a modern office building with an open-plan space and large window openings. In this experiment, #FRIC-01, the ceiling was exposed. The wood crib fire developed slowly and travelled approximately 1.5 m before the ceiling ignited at 32.5 min. Thereafter the fire spread rapidly across the ceiling and wood crib before it shortly after retracted. Three such cycles of rapid spread followed by a retraction occurred within 13 min, whereby the wood crib fire grew larger for each cycle. After the flames extended through the compartment for the fourth time, the fire remained fully developed. After a short period of intense burning, the CLT self-extinguished while the wood crib fire was still burning. The compartment withstood full burnout, and no reignition occurred despite some delamination and using an adhesive that lacks a demonstrated resistance against glue-line integrity failure. © 2023 The Authors

Cross-laminated timber (CLT) is becoming increasingly popular due to its many advantages. However, it has been shown that exposed CLT can have a significant effect on fire dynamics and spread rates. Further studies are therefore needed to better understand the impact of CLT to fire safety. Two large-scale CLT compartment fire experiments (95 m2) representing a modern office building have been performed, #FRIC-01 and #FRIC-02. This paper presents the second experiment, #FRIC-02, with exposed CLT on the back wall and the ceiling. The fire developed fast and spread across the room in less than 3.5 min from ignition of the wood crib on the floor and in 1.5 min after the ignition of the ceiling. Large external flames were observed, despite the compartment being well-ventilated. The 5-layer CLT, which comprised a 40 mm thick exposed outer layer and was face-bonded using a common European polyurethane adhesive, exhibited glue-line integrity failure and led to a second flashover after a significant period of decay. Subsequent layers of 20 mm also delaminated before the fire was manually extinguished after 3 h. Compared to #FRIC-01, the fire spread rate was faster, and temperatures, charring rates, heat release rates and external flames were higher. 

With the worldwide construction sector being responsible for one third of carbon dioxide emissions, as well as forty percent of the world’s energy use and waste production, a shift to sustainable and renewable construction techniques is crucial. Engineered timber, a champion of sustainable construction materials, has evolved to a stage that enables the construction of not only family housing but also taller buildings so far commonly built from concrete or steel. Designing taller timber buildings made is more demanding than their concrete and steel counterparts. Whereas different design aspects (architectural, structural, fire safety, acoustics, etc.) of concrete buildings can work almost independently, the design of taller timber buildings should be performed with intensive collaboration among the design teams. It is therefore crucial to address taller multi-storey timber buildings from a collaborative and interdisciplinary perspective, considering static, dynamic, fire, acoustic, human health, and other aspects in parallel and not in isolation. Only through interdisciplinary analysis and interaction can a set of holistic design guidelines be developed that will enable the safe construction of taller timber buildings, as well as respect human wellbeing demands. In this paper, the COST Action CA20139 will be presented and the main aims will be discussed.

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.

In opdracht van het Nederlands Normalisatie Instituut (NEN) en in overleg met werkgroep 351 007 00 07 ´Brandveiligheid en bouwen met hout, heeft RISE met medewerking van Arup en TNO een literatuurstudie uitgevoerd. Deze literatuurstudie is stap 1 van meerdere te nemen stappen om antwoord te kunnen geven op motie nr. 28325-220 d.d. 20 april 2021 van de Tweede Kamer waarin wordt geconstateerd dat het Bouwbesluit / Besluit Bouwwerken Leefomgeving nog niet is toegerust op de toenemende toepassing van nieuw of hernieuwd bouwmateriaal, zoals hout, voor nieuwe hoogbouw. Het doel van deze literatuurstudie is specifieke punten te identificeren die aandacht behoeven in de regelgeving om te zorgen voor brandveilige gebouwen waarin veel hout is toegepast. Het onderzoek geeft op basis van literatuur inzicht in nut, achtergronden, noodzaak en relevantie van gebruik van bestaande regelgeving, beoordelingsmethoden en de uitgangspunten daarin. De vervolgstappen op dit rapport omvatten in de literatuurstudie geïdentificeerde punten die aandacht behoeven, waaronder mogelijke aanpassing van de Nederlandse bouwregelgeving en bestaande normen, zoals NEN 6068, NEN 6069 en de Eurocodes 1995-1-2, 1991-1-2 en de in de Eurocode 1995-1-2 aangeduide bepalingsmethode, NEN-EN 13381-7. Uit deze literatuurstudie blijkt onder andere dat: • de huidige brandveiligheidseisen uit het Bouwbesluit / Besluit Bouwwerken Leefomgeving niet zonder meer adequaat zijn voor alle massieve houtconstructies, omdat de huidige prestatie-eisen geen directe relatie kennen met de eventueel verhoogde permanente vuurbelasting; • wanneer de huidige prestatie-eisen uit het Bouwbesluit en BBL worden toegepast voor gebouwen met de nieuwe houten bouwsystemen dan levert dit voor die gebouwen een mogelijke onderschatting van het bereikte veiligheidsniveau, en wordt er mogelijk aan de functionele eisen van het Bouwbesluit en BBL onvoldoende voldaan; • de huidige prestatie-eisen uit het Bouwbesluit / Besluit Brandveiligheid Leefomgeving en bijbehorende bepalingsmethodes (NEN-normen) zijn niet altijd voldoende toegesneden op het beoordelen van nieuwe typen houten bouwsystemen, zoals ‘engineered wood’-producten waaronder CLT (Cross Laminated Timber) en NLT (Nailed laminated timber) en LVL (Laminated veneer lumber). Dit is gerelateerd aan de grotere hoeveelheid brandstof in constructies met deze materialen, waardoor de vuurlast, de brandrisico’s en gevolgen voor de omgeving mogelijk zijn verhoogd. Nader moet worden onderzocht welk deel van de constructie, rekening houdend met repressieve inzet, een bijdrage levert aan de vuurbelasting, intensiteit en de duur van de brand; • het blussen van een brand en de aanvullende brandrisico’s vergen extra inzet, middelen en aandacht van de brandweer voor wat betreft het blussen en volledig doven van smeulende resten in het gebouw (en dus ook in de bouwconstructie); Op basis van de resultaten van het literatuuronderzoek komt de werkgroep tot de conclusie dat de toename van de vuurlast bij houtbouw de brandveiligheid in potentie negatief beïnvloedt. De werkgroep heeft de indruk dat voor gebouwen met een beperkt brandrisico en een beperkte hoeveelheid brandbare bouwmaterialen en hout, en ook met een laag gevolg-risico, de invloed beperkt is en de huidige eisen en bepalingsmethoden mogelijk gehandhaafd kunnen blijven. Voor de overige situaties zullen aanvullende maatregelen of aangepaste bepalingsmethoden moeten worden ontwikkeld, om aanvullende brandrisico’s te beheersen, en/of zullen de eisen en de bepalingsmethodes moeten worden herzien. Om op dit moment aantoonbaar aan de functionele eisen van het Bouwbesluit / Besluit Bouwwerken Leefomgeving te voldoen is een nadere studie op basis van een integrale aanpak, en gebruikmakend van fire safety engineering, nodig. Zowel de op fire safety engineering gebaseerde aanpak als een mogelijke aanscherping van de prestatie-eisen moeten nader worden uitgewerkt.

This chapter provides an overview of the application of performance-based approaches to the fire safety design of timber buildings. Performancebased design methods are relevant for the design of tall timber buildings and other timber buildings that vary from accepted prescriptive solutions. Performance-based design approaches are commonly categorised as deterministic or probabilistic methods and should be applied in accordance with the applicable regulations, building codes and standards. This chapter provides references to detailed information that should be consulted when undertaking performance-based designs.

Different countries world-wide have different legislation concerning the performance of facades exposed to fire and often significantly different ways to assess this performance. Although it is recognized that standard façade fire testing aims to distinguish façade systems that limit fire spread to an acceptable level from systems that do not, it has historically been considered important that the fire exposure of such tests is representative for real fires.

In this study five real scale compartment fire tests, constructed of Cross Laminated Timber and Glued laminated timber were performed with instrumentation on a façade extension above the ventilation openings, providing a means to compare façade performance tests against the exposure generated by realistic compartment fires. The fuel load and openings of four of these tests were determined from a statistical analysis to represent severe fire exposure within a realistic range. Of these tests the surface areas of exposed Cross Laminated Timber and Glued Laminated Timber were varied, allowing an assessment of the influence having internal areas of exposed timber surfaces on the façade fire exposure.

For these tests, an increase of roughly 40 m2 exposed surface area (from ~54 to ~94 m2 or from 113 % to 196 % of the floor area) resulted in a temperature increase of roughly 100 to 130 °C at the façade at all heights up to 3.5 m above the opening. Additionally, an increased fire plume height of 0 to 1 m was observed. The most significant effect of increased exposed areas was a prolonged duration of the flashover phase.

The British BS 8414 standard façade fire tests and the recently proposed European façade fire test have been identified to be the most representative for the tested residential fire scenarios in terms of façade fire exposure. Temperature measurements of the North American methods (NFPA 285 and CAN/ULC-S134) are towards the end of the tests also close to the those of the compartment tests. The Swedish SP Fire 105 test imposes the lowest exposure for a relatively short duration to the façade. It should, however, be noted that a lower exposure in the standard test method does not with necessity mean lower threshold for regulatory compliance as the test criteria also differ between different countries.

One of the tests were characteristic of open plan office buildings and it was shown that the fire exposure is both shorter and lower compared to typical residential compartment tests. All standard tests that were used for comparison here exhibited both longer and higher exposure than the office building compartment test.