The building industry has, in recent years, more frequently used different engineered wood products, like cross-laminated timber (CLT), glue-laminated timber and I-joists. The increased popularity is due to the many advantages of building with wood, like the possibility of prefabrication, the low carbon dioxide footprint, the easy handling and mounting of wood, and the aesthetic look. Conversely, when building with wood, additional combustible mass is introduced into the buildings. Given that these wooden products are exposed (i.e., visually present), they would produce pyrolysis gases when subjected to a fire and thus have an impact on the fire dynamics. For over a decade, different research groups have conducted compartment fire experiments with exposed CLT. Most of those experiments have been conducted in relatively small compartments with small ventilation openings. Thus, the role of the exposed CLT is better understood in small compartments than in large. Small compartments with exposed CLT could be relevant for certain buildings. However, CLT is used in a variety of different rooms, including open-plan offices, lobbies, canteens, dwellings, kindergartens, schools, etc. In other words, the great variation in the use of CLT, including room size, geometry, and orientation, challenges the current understanding of how exposed CLT affects a fire. The main focus of this thesis has been to increase the knowledge of the fire behaviour in large compartments with exposed CLT. The methodology consisted of conducting two large-scale (95 m2) compartment fire experiments. In the first experiment, named #FRIC-01, the ceiling was exposed, while in the second experiment, #FRIC-02, both the wall and the ceiling were exposed. The compartment had four open window openings along one wall, which caused a well-ventilated compartment. The fuel load density was representative of an office building and was represented by a continuous wood crib on the floor. These experiments aimed to better understand how two different configurations of CLT affect the fire dynamics, including fire spread inside the compartment, external flames, charring rate of CLT, decay phase, self-extinguishment of flames and delamination. In both experiments, the ignition of the CLT ceiling triggered a clear change in the fire dynamics, in which flames spread under the ceiling and caused a strong radiative heat flux to the wall and the wood crib. The increased radiative heat flux effectively preheated the wood crib (and the wall in #FRIC-02) and led to a significantly faster spread across the wood crib than before the CLT was ignited. In #FRIC-01, a new behaviour was observed, in which the flames in both the ceiling and of the wood crib travelled back and forth three times. As such cycles have not been reported earlier, we have named them flashing waves. Three such waves were observed before the fire was fully developed in the fourth wave. Despite retraction of the flames, the wood crib fire grew larger after each wave, contributing to a significantly faster fire spread rate than before the CLT ceiling ignited. In total, it took 13 minutes from ignition of the ceiling until the fire was fully developed. After a few minutes of intense burning, the flames in the ceiling started to extinguish, and over a period of 11 minutes, all flames in the ceiling were extinguished. This occurred while the wood crib was still burning. No reignition was observed within a total duration of four hours. In #FRIC-02, the contribution of having CLT exposed in both a wall and the ceiling was clearly seen. From ignition of the CLT ceiling, it took only 91 seconds before the entire compartment was burning. The fire spread was dominated by the rapid flame spread under the ceiling and upper part of the wall first. This caused a strong radiative heat flux to the wood crib and the other parts of the wall. The fire spread rate after ignition of the ceiling was 15 m/min across the ceiling and 11.7 m/min across the wood crib, corresponding to a fire growth rate faster than the ultrafast fire growth rate defined by Eurocode 1 (EN 1991-1-2). During the most intense burning phase, large external flames emerged mainly out of one window. For some period, the flames covered almost the entire window opening, reached above the facade wall (5.2 m), and extended about 3 m horizontally from the window. The flames effectively reduced the inflow of air through that window, resulting in more air being supplied through the other windows. This imbalance in air supply contributed to large temperature variations throughout the compartment. These non-symmetrical external flames are believed to be mainly due to the wind coming diagonally from behind the corner of the compartment, but also due to the very rapid fire spread. The characteristic behaviour of the external flames was successfully reproduced in a CFD simulation when similar wind conditions as in the experiment were considered. This strengthens the hypothesis that the non-symmetrical external flames were influenced by the wind conditions. After 10 minutes of intense burning, the gas temperatures started decaying, and also with this CLT configuration (wall and ceiling exposed), the flames of the CLT self-extinguished. About 50 minutes after the start of the decay phase, multiple small flames appeared at the surface of both the CLT wall and ceiling. Within 10 minutes, all combustible surfaces were burning again, corresponding to a second flashover. The temperatures were, for a short period, almost as high as after the first flashover. After that, the fire intensity varied strongly over the next 100 minutes but continued to burn until it was manually extinguished after almost 3 hours. This ongoing fire can be explained by the build-up of the CLT with thick (40 mm) outer layers and thin (20 mm) intermediate layers and the use of a regular PUR adhesive known to cause delamination. A side topic of this thesis has been to study the charring of I-joists in light timber framed assemblies with combustible insulation. In recent years, there has been increased interest in combining I-joists with new combustible insulation products, like wood fibre and cellulose insulation. However, as there is no available design model for calculating the load-bearing capacity of this combination when exposed to a fire, the outspread of this combination has been limited. This part of the research was aimed at producing experimental data to better understand the charring of I-joists and the recession rate of combustible insulation when these products are combined. This data could later be used to develop or validate design parameters for combustible insulation. Through five experiments, a combination of I-joists with different flange sizes and different combustible insulation types (wood fibre, cellulose and phenolic foam) were exposed to the standard fire curve (ISO 834) in a medium-scale furnace. Thermocouples were embedded into and outside of the flanges and used to determine the charring rate of the I-joist and the recession rate for the insulation. After exposure, the final char depth and the remaining cross-section were measured. The charring rates were compared against calculated values based on the design model for rectangular cross-sections in the current Eurocode 5 (EN 1995-1-2) and the new model for I-joists in the draft of the new Eurocode 5 (prEN 1995-1-2). Compared to those models, the charring rates were mainly on the conservative side. The charring rates decreased with increasing flange size and were comparable for the flanges of solid wood and laminated veneer lumber (LVL). Overall, the combustible insulation protected the I-joists well, and the recession rates were lower than values reported for glass wool insulation. The lowest values were obtained by cellulose- and wood fibre insulation. Due to few repetitions, the results must be considered as indicative. Still, the results strongly indicate that biobased, and thus more sustainable, insulation types deserve a great market share in the future. Altogether, the experimental work in this thesis has contributed to improved knowledge of both CLT and I-joists and could be considered a small but important step towards a more sustainable and fire-safe building sector.

The webinar looks more closely into the characteristic behaviour of the external flame observed in the large-scale CLT compartment fire experiment #FRIC-02 and show results where this experiment was simulated. The research has been carried out as part of WP3 in FRIC, as part of Bøe’s PhD work at NTNU, in collaboration with SINTEF, RISE Fire Research and other partners in FRIC. The research has been presented at the Fire Safety of Facades (FSF) conference in June 2024, and in the following publications:

#FRIC-02: https://doi.org/10.1016/j.firesaf.2023.103986 

#FRIC-01: https://doi.org/10.1016/j.firesaf.2023.103869

Thesis: https://ntnuopen.ntnu.no/ntnu-xmlui/handle/11250/3125216

Two fire experiments have been conducted to study sprinkler system extinguishing performance in a compartment (13 m2) with an adjacent corridor (12 m2), both with exposed cross-laminated timber (CLT). Four nozzles were installed in the corridor and two in the compartment. In Experiment 1, the sprinkler system was fully functional and successfully controlled a concealed fire. In Experiment 2, nozzles in the compartment were disconnected, while the corridor nozzles were operative, giving flashover after 5 min with large flames emerging into the corridor, rapidly worsening evacuation conditions. Despite four activated nozzles in the corridor, the temperatures remained high, and flames spread through the corridor along the CLT ceiling and the upper parts of the wall, an area that was not effectively protected by the nozzles. After flashover, the compartment temperatures remained stable at ∼1000 °C until experiment termination at 96 min. This continued fire in the compartment can be explained by water from the corridor sprinklers not reaching this area, extensive radiative feedback by the CLT surfaces and delamination of CLT elements of the 20 mm layers. The charring rate was ≥1.1 mm/min for large parts of the exposed CLT wall and ceiling in the compartment during the fire. 

Recent experiments have shown that exposed cross-laminated timber (CLT) can have a significant effect on the fire dynamics of large compartments. A simulation with the Fire Dynamic Simulator has been conducted to better understand the fire behaviour of open-plan compartments with exposed CLT. The simulation was set up to replicate a large-scale experiment, FRIC-02, with exposed CLT on the back wall and ceiling. The compartment was 95 m2 (18.8 m × 5.0 m × 2.5 m), with one long wall open (opening factor 0.18 m1/2). A continuous wood crib was used as the variable fuel load. The characteristic results of FRIC-02 with a rapid fire development and non-symmetrical external flames were successfully reproduced. With the wind coming diagonally from behind, as in FRIC-02, the external flames emerged mainly out of one window. The flames covered the entire window height, which effectively inhibited the inflow of air through that window. The imbalance in air supply also created large temperature differences throughout the compartment. With no implementation of wind, external flames and temperatures were more symmetrical. Despite a good match to FRIC-02, the method still has several limitations, including the adaption of the burning rate to the feedback from surroundings.

Denne rapporten oppsummerer resultater fra forskning utført i prosjekt 3.1 Novel construction products i forskningssenteret Fire Research and Innovation Centre (FRIC). Det overordnede målet med prosjektet er å undersøke hvordan nyere konstruksjonsmaterialer og produkter virker inn på brannsikkerheten i bygninger, å utvikle eksperimentelle og teoretiske metoder for å måle denne innvirkningen, samt å utarbeide brannsikre konstruksjonsoppbygninger

Fire safety in semi-automatic parking facilities

The main goal of this study is to contribute to increased safety in semi-automatic parking facilities. Semi-automatic parking facilities are parking facilities with a system for automatic stacking of cars, but in contrast to fully automatic parking facilities, these are not closed, compact, and unavailable for the public. The study is financed by The Norwegian Directorate for Civil Protection (DSB) and Norwegian Building Authority (DiBK). A fire simulation was conducted to compare fire spread in a semi-automatic parking facility to fire spread in an ordinary parking facility. The results indicate that the spread of fire from the car that was first ignited to another car happens approximately equally fast in the two scenarios. Thereafter, the fire spread faster in the semi-automatic parking facility compared to the ordinary parking facility. Although these results should only be considered as indicative, they do show that decreasing the distance between rows of cars can lead to a much faster fire spread. The simulation also shows that the size of a fire in a relatively closed-off parking facility is not necessarily controlled by the number of cars but by the access to air. Hence, the number of openings and properties of ventilation systems in such facilities are important factors to consider when assessing fire safety. A study of regulations and experiences with semi-automatic parking facilities in Norway and other countries as well as aspects that increase risks in semi-automatic parking facilities was conducted. No specific fire-related experiences were discovered, but this is not surprising when considering that fires in parking facilities are relatively rare and there are relatively few semiautomatic parking facilities. In addition, these types of facilities are relatively new. The study found regulations for fully automatic parking facilities in Norway, but semiautomatic parking facilities are not covered by the same regulations. The current regulations do not ensure that the authorities are informed when automatic car-stacking systems are installed in existing parking facilities that are open to the public. There are no regulations ensuring that a fire safety assessment is conducted when an automatic car-stacking system is installed in an existing building regulated for parking that is accessible to the public. It is our opinion that there is a need for a new assessment of fire safety when a system for car stacking is established in an existing parking facility.

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.