© RISE Research Institutes of SwedenAbstractFire safety in cross laminated timber buildings; a reviewKey words: Cross laminated timber; CLT; fire safety; exposed CLT; auto-extinction; charring; delamination; detailingThis literature study presents recent research on fire safety in cross laminated timber (CLT) buildings. Results from large fire experiments and other studies in the period 2010 - 2018 are summarized, with focus on the following research questions:• How do constructions consisting of protected or exposed CLT contribute to the fire development in a room?• How can contribution to the fire development from detailing of CLT be avoided?There is an increasing desire to use wooden structures in tall buildings, as a substitute for more traditional construction materials. However, the use of combustible construc-tions in buildings in Norwegian Fire Class 3 (usually five floors or more) is not pre-accepted in the guideline to Regulations on technical requirements for construction works (TEK17), and fire safety must therefore be documented by analysis in such structures. When designing tall and complex timber buildings, it must be taken into account that a fire involving a timber construction may have more severe consequences than in buildings with constructions of steel or concrete, if the fire design of the construction and detail solutions is insufficient. Several studies show that fire exposed CLT, or CLT with insufficient protection, can cause a fire to develop faster, be more intense and last longer than a fire where the only fuel is the furniture and fixtures in the fire room. It is shown that the amount of fire exposed timber in a room may have impact on the extent and duration of a fire, but the knowledge has not yet been sufficient enough to be used in fire modeling, design and analysis.Research on charring rates, delamination and auto-extinction, all of which are factors that can have major impact on fire development and the fire resistance of the construction, takes place in Europe, Australia and North America. Although extensive research has been carried out, it is based on few large fire experiments, and the literature is still pointing to several knowledge gaps. However, the research projects have increased the knowledge of fire in timber buildings, and have contributed to the design of detail solutions, guidelines and development of models for function-based design. Revision of EN 1995-1-2 is under preparation and expected to apply from 2022. A knowledge base for the audit can be found in the network COST Action FP1404 Fire Safety Use of Bio-Based Building Products (COST FP1404) Working Group 2 (WG2). They have published several guidelines relevant for the fire design of CLT, including e.g. calculation methods for the prediction of charring rates and depths, determination of reduced CLT cross-section, design of CLT detailing and a suggested test method for evaluating adhesive performance.Based on the literature review, the following conclusions and recommendations are given for CLT constructions:• The design phase must sufficiently consider protection of the construction and con-tribution of the construction to the fire energy, and to a greater extent include the assessment of detailing and ventilation conditions. It should be considered whether analytic fire engineering design also should be required for buildings in the Norwegian Fire Classes 1 and 2 where more than one CLT wall is exposed.• By protecting all CLT surfaces of the structure with cladding, the construction may retain the stability and the load bearing capacity during the required time of fire resistance.• In buildings with only one exposed CLT wall in each fire cell, it may also be appropriate to use solutions that satisfy the pre-accepted performances, but one must consider whether a somewhat longer and more intense heat radiation and flame exposure on the facade outside window openings will require measures beyond the pre-accepted performances given in the guideline to TEK17.• Rooms where two or more CLT walls in addition to the ceiling are exposed, are configurations that should be avoided.• The risk of delamination can be reduced by using heat-resistant glue.• There is generally a need for relevant documentation for fire-resistant solutions for joints between CLT walls and floors and service penetrations in CLT constructions.• Test methods for testing of joints and penetrations in CLT constructions should be standardized. For example, there exists no standardized test for corner joints. Tests of penetration seals for CLT constructions are scarce, although they can be tested according to EN 1366-3. However, CLT is not a standard supporting construction according to EN 1366-3, and this must be taken into consideration when the test results are evaluated. Joints in glulam constructions should also be tested because they are often used in conjunction with CLT elements.

The goal of this project has been to establish new knowledge and methods for testing the penetration of hazardous soot and smoke particles into fire clothing. The aim has been to provide the basis for the development of new fire-fighter clothing with better protection against particle penetration. In cooperation with fire services, authorities and protection clothing producers, needs, requirements and recommendations have been investigated. For the documentation and relevant classification of protective clothing, test set-ups in small and larger scale have been developed. The aim has been to be able to achieve representative and repeatable fire- and smoke exposure for accurate measurement of the particle penetration into clothing and trough clothing layers for screening materials and design solutions. With regard to the performance of the clothing, the small-scale tests give indications of the textiles’ ability to block gases and particles from penetrating into the clothing. The large-scale tests give indications to how the design of the clothing as a whole is able to prevent intrusion of gases and particles.

Smoldering fires in stored or transported solid biofuels are very difficult to extinguish. The current study has explored heat extraction from the combustion zone as a method for extinguishing such flameless fires. Heat extraction from the sample was made feasible using water flowing through a metal pipe located inside the sample. The fuel container was a steel cylinder with insulated side walls, open at the top and heated from below. Wood pellets (1.25 kg, 1.8 l) was used as fuel. Results from small-scale experiments provide proof-of-concept of cooling as a new extinguishing method for smoldering fires. During self-sustained smoldering with heat production in the range 0 W to 60 W, the heat loss to the cooling unit was in the range 5 W to 20 W. There were only marginal differences between non-extinguished and extinguished cases. Up-scaling is discussed, cooling could be feasible for preventing smoldering fires in silos.

In industrial plants, such as oil platforms, refineries or onboard vessels carrying fuel, a rupture event of a pipeline could have dramatic consequences, as was demonstrated both in the Piper Alpha and Deepwater Horizon accidents. If surfaces are exposed to extreme conditions, both extreme cold (cryogenic spills) and extreme heat (jet fires), this can affect exposed surfaces, and can cause a domino effect of severe events.

Smoldering fires represent a severe, but often overlooked danger to society. Smoldering causes major economic losses for industrial facilities with production, transport and storage of biomass and biofuels worldwide. The smoke from post-flaming residual burning on the forest floor and in peatlands represents a major contributor to greenhouse gas emissions. [1]To prevent initiation of smoldering, and facilitate safe, efficient and complete extinguishment, a better fundamental understanding of smoldering is key. This is the aim of the research project EMRIS (Emerging Risks from Smoldering Fires). The consortium consists of 6 research institutes and universities in 5 countries, coordinated by Western Norway University of Applied Sciences in Haugesund, Norway. EMRIS started in 2015 and is now in its final stage. We will here present some points of interest from the project.Materials in the study include wood pellets, other biopellets, cotton, waste (wood chips), coal, wood fiber insulation and various pyrolysis products. Both experimental and modeling work has been done.Experimental work in small-scale has studied the sensitivity of smoldering ignition to a range of parameters [2], the impact of changes in air flow on the combustion [3], the effect of fire retardant content and fiber size [4], the transition from smoldering to flaming fire [5,6], early detection of smoldering [7]and heat extraction from the fuel bed with successfulextinguishment [8,9]. In medium scale experiments, initiationand propagation of reaction fronts have been studied [10]. TheEMRIS team also studies how particulate matter fromsmoldering fires can affect large scale phenomena, such ascloud formations, climate and public health.A cellular automaton model has been found to give a realistic representation of smoldering spread [11]. The method is based on a network of cells that mimic processes taking place in the material, which is easier to program and requires less computing power than traditional tools.The EMRIS project therefore represents progress within many different aspects of fire safety science. A continuation of the project is very much of interest, we welcome interested parties to discuss with us.

Smoldering fires represent domestic, environmental and industrial hazards. This flameless form of combustion is more easily initiated than flaming, and is also more persistent and difficult to extinguish. The growing demand for non-fossil fuels has increased the use of solid biofuels such as biomass. This represents a safety challenge, as biomass self-ignition can cause smoldering fires, flaming fires or explosions.

Smoldering and extinguishment in granular biomass was studied experimentally. The set-up consisted of a cylindrical fuel container of steel with thermally insulated side walls. The container was closed at the bottom, open at the top and heated from below by a hot surface. Two types of wood pellets were used as fuel, with 0.75-1.5 kg samples.

Logistic regression was used to determine the transition region between non-smoldering and self-sustained smoldering experiments, and to determine the influence of parameters. Duration of external heating was most important for initiation of smoldering. Sample height was also significant, while the type of wood pellet was near-significant and fuel container height was not.

The susceptibility of smoldering to changes in air supply was studied. With a small gap at the bottom of the fuel bed, the increased air flow in the same direction as the initial smoldering front (forward air flow) caused a significantly more intense combustion compared to the normal set-up with opposed air flow.

Heat extraction from the combustion was studied using a water-cooled copper pipe. Challenges with direct fuel-water contact (fuel swelling, water channeling and runoff) were thus avoided. Smoldering was extinguished in 7 of 15 cases where heat extraction was in the same range as the heat production from combustion. This is the first experimental proof-of-concept of cooling as an extinguishment method for smoldering fires.

Marginal differences in heating and cooling separated smoldering from extinguished cases; the fuel bed was at a heating-cooling balance point. Lower cooling levels did not lead to extinguishment, but cooling caused more predictable smoldering, possibly delaying the most intense combustion. Also observed at the balance point were pulsating temperatures; a form of long-lived (hours), macroscopic synchronization not previously observed in smoldering fires.

For practical applications, cooling could be feasible for prevention of temperature escalation from self-heating in industrial storage units. This study provides a first step towards improved fuel storage safety for biomass. 

Schwelbrände repräsentieren Brände die eine Gefahr im häuslichen, ökologischen und industriellen Umfeld darstellen. Diese flammenlose Form eines Brandes ist leichter zu starten, stabiler im Verlauf und schwieriger zu löschen als Flammenbrände.

Die wachsende Nachfrage nach nicht fossilen Brennstoffen hat die Verwendung von festen Bioenergieträgern, wie Biomasse, erhöht. Daraus resultieren neue Sicherheitsanforderungen, da die Selbstentzündung fester Biomasse einen Schwelbrand, einen Brand oder eine Explosion verursachen kann.

Schwelen und Löschen von granularer Biomasse wurde experimentell untersucht. Der experimentelle Aufbau bestand aus einem zylindrischen Brennstoffbehälter aus Stahl mit wärmeisolierten Seitenwänden. Der Behälter wurde unten geschlossen, oben offen gelassen und von unten durch eine heiße Oberfläche erhitzt. Als Brennstoff wurden zwei Arten von Holzpellets, in Proben von 0,75 kg bis 1,5 kg, verwendet.

Um den Übergang von nicht Schwelen zu selbständigem Schwelen und die beeinflussenden Parameter zu bestimmen, wurde logistische Regression verwendet. Um Schwelbrand zu erzeugen, war die Dauer der externen Erwärmung am wichtigsten. Die Probenhöhe war ebenfalls signifikant, während die Art der Holzpellets nahezu signifikant war, wobei die Höhe des Brennstoffbehälters nicht signifikant war.

Die Anfälligkeit des Schwelens gegenüber Änderungen der Luftzufuhr wurde untersucht. Mit einem kleinen Spalt am Boden des Behälters, ergab der erhöhte Luftstrom in der gleichen Richtung wie die anfängliche Schwelbrandfront (Vorwärtsluftstrom) eine signifikant intensivere Verbrennung im Vergleich zu der normalen Anordnung mit Rückwärtsluftstrom.

Die Wärmeentnahme aus der Verbrennung wurde mithilfe eines wassergekühlten Kupferrohres untersucht. Dadurch wurden Probleme in Verbindung mit direktem Brennstoff-Wasser-Kontakt (Aufquellen des Brennstoffes, Wasserkanalisierung und-abfluss) vermieden. Der Schwelbrand wurde in 7 von 15 Fällen gelöscht, wobei die Menge des Wärmeverlustes durch die Kühleinheit und die Wärmeerzeugung durch die Verbrennung im gleichen Größenbereich lagen. Dies ist der erste "Proof-of-concept" der sich mit Kühlen als Löschmethode bei Schwelbränden beschäftigt.

Geringfügige Unterschiede in der Erwärmung und Abkühlung führten zu Schwelbränden oder gelöschten Schwelbränden und zeigten das Brennstoffbett im Gleichgewicht zwischen Erhitzen und Kühlen. Niedrigere Abkühlungsniveaus führten nicht zum Erlöschen, aber das Kühlen führte zu einem vorhersagbareren Schwelen und verzögerte möglicherweise die intensivste Verbrennung. An diesem Gleichgewichtspunkt wurden pulsierende Temperaturen beobachtet; eine Form von langlebiger (Stunden), makroskopischer Synchronisation, die bisher bei Schwelbränden nicht beobachtet wurde.

Denkbar als praktische Anwendung wäre eine Kühlung in industriellen Lagereinheiten zur Verhinderung einer Temperaturerhöhung durch Selbsterwärmung. Diese Studie ist ein erster Schritt in Richtung Verbesserung der Lagersicherheit von Biomasse in industriellen Lagereinheiten.

Passive fire protection materials are used to protect critical structures against the heat from fires. In process plants with pressurized combustible substances there may be a risk of jet fires. Through risk analysis the severity of these jet fires is determined and these result in fire resistance requirements with different heat flux levels for different segments. The relevant test standard for fire resistance against jet fires does not include any measurements or definitions of the heat flux in the test flame which the tested object is exposed to. This paper presents methods for reaching different heat flux levels and how to measure them in a jet fire with limited deviations from the established jet fire test standard.

Jet fires are ignited leakages of pressurized liquid or gaseous fuel. In jet fire testing for the offshore industry, heat flux is the defining factor for the accidental loads. NORSOK S001 [1] defines two different heat flux levels of 250 kW/m2 and 350 kW/m2 depending on the leak rate of hydrocarbons. These heat flux levels are used in risk analysis and define what type of fire load bearing structures and critical equipment need to be able to resist in a given area. Examples of such ratings can be “250 kW/m2 jet fire for 60 minutes”, “350 kW/m2 jet fire for 15 minutes” or any other combination based on calculations in the risk assessment. Combined with critical temperatures this defines the performance criteria for the passive fire protection. Each configuration of the passive fire protection needs to be tested and verified. Manufacturers of passive fire protection request fire tests to document their performance against jet fires with these various heat flux levels. The challenge is that the standard for testing passive fire protection against jet fires [2] does not define any heat flux level or any method to define or measure it. We have developed a method for defining and measuring the heat flux levels in jet fires. This method can be used when faced with the challenge of testing passive fire protection against specific levels of heat flux. The method includes a custom test rig that allows jet fire testing with different heat flux levels. A large number of tests have been performed to verify the reproducibility and repeatability of the method. Heat flux is defined as the flow of energy through a surface. The heat flux from a fire to an engulfed surface of an object is dependent on both the engulfing flame and the properties of the surface. The properties of the surface may change during the exposure to the flame as it heats up and changes its surface properties. At some point the object inside the flame will reach a thermal equilibrium with the flame where the net flow of energy into the object is balanced by the energy emitted from the object. The heat flux for an object can be calculated as incident heat flux, emitted heat flux or net heat flux. A definition of heat flux needs to include parameters of the receiving object. These variations give a lot of degrees of freedom when calculating heat flux in a fire. Special water cooled gauges are designed to measure heat flux to a cooled surface, but these have proved to be very unreliable when placed inside a large fire. A more robust and easily defined method is to measure the equilibrium temperature inside an object placed inside the flame. This is the principle used in plate thermocouples used in fire resistance furnace testing [3]. In our experience, these plate thermocouples are often damaged during high heat flux jet fire tests. This raises questions to how long into the tests such measurements are reliable. Several other types of objects have been tested and the most convenient and reliable type was found to be simply a small 8 mm steel tube that is sealed in the end and has a thermocouple inside. One key difference between this small tube thermocouple and the plate thermocouple is that the plate thermocouple is directional and the tube is omnidirectional. Current works and tests will optimize the measuring objects in order to get the most relevant equilibrium temperature while still maintaining the robustness of the sensor during the test. The suggested heat flux calculation is to follow the Stefan-Boltzmann relation of temperature and heat flux. For a black body this gives 350 kW/m2 for 1303 °C and 250 kW/m2 for 1176 °C. A lower emissivity may be defined for the surface of the sensing object giving higher temperatures for the same flux levels. This method gives a simple, robust and reproducible correlation between heat flux levels and temperatures that can be measured during jet fire tests. The method does not differ between the varying convective and radiative heat transfer in the flame, but it is a representative measurement for the temperature that an object would reach when placed inside the flame.

A bench-scale experimental setup has been used to study the conditions necessary

for smouldering ignition in four types of loose-fill wood fibre thermal insulation, and

to study the development of the smouldering process. The products varied with regard to

wood species, grain size and fire retardant chemical additives. The test material was

placed in an insulated open top container and heated from below. Temperatures within

the sample and mass loss were measured during the tests. Both the fibre size and the level

of added fire retardant seem to influence the smouldering ignition. Two different types of

smouldering were identified in this study. Materials undergoing smouldering Type 1

obtained maximum temperatures in the range 380C to 440C and a total mass loss of

40 wt% to 50 wt%. Materials undergoing smouldering Type 2 obtained maximum temperatures

in the range 660C to 700C and a total mass loss of 80 wt% to 90 wt%. This

implies that Type 2 smouldering involves secondary char oxidation, which represents a

risk for transition to flaming combustion and thereby a considerable fire hazard. This has

been an exploratory project and the results must therefore be considered as indicative.

The findings may, however, have implications for fire safety in the practical use of loosefill

wood fibre insulation in buildings, and further experimental studies should be performed

with this in mind to obtain more knowledge about the topic.