Neither the National Building Code of Canada (NBCC) [1], nor any provincial code, such as the British Columbia Building Code (BCBC) [2], currently provide “acceptable solutions” to permit the construction of tall wood buildings, that is buildings of 7 stories and above. British Columbia, however, was the first province in Canada to allow mid-rise (5/6 storey) wood construction and other provinces have since followed. As more mid-rise wood buildings are erected, their benefits are becoming apparent to the industry, and therefore they are gaining popularity and becoming more desirable.
Forest product research has now begun to shift towards more substantial buildings, particularly in terms of height. High-rise buildings, typically taller than 6 storeys, are currently required to achieve 2 h fire resistance ratings (FRR) for floors and other structural elements, and need to be of non-combustible construction, as per the “acceptable solutions” of Division B of the NBCC [1]. In order for a tall wood building to be approved, it must follow an “alternative solution” approach, which requires demonstrating that the design provides an equivalent or greater level of safety as compared to an accepted solution using non-combustible construction. One method to achieve this level of safety is by ‘encapsulating’ the assembly to provide additional protection before wood elements become involved in the fire, as intended by the Code objectives and functional statements (i.e., prolong the time before the wood elements potentially start to char and their structural capacity is affected). It is also necessary to demonstrate that the assembly, in particular the interior finishes, conform to any necessary flame spread requirements.
The Technical Guide for the Design and Construction of Tall Wood Buildings in Canada [3] recommends designing a tall wood building so that it is code-conforming in all respects, except that it employs mass timber construction. The guide presents various encapsulation methods, from full encapsulation of all wood elements to partial protection of select elements. National Research Council Canada (NRC), FPInnovations, and the Canadian Wood Council (CWC) began specifically investigating encapsulation techniques during their Mid-Rise Wood Buildings Consortium research project, and demonstrated that direct applied gypsum board, cement board and gypsum-concrete can delay the effects of fire on a wood substrate [4].
There is extensive data on the use of gypsum board as a means of encapsulation for wood-frame assemblies and cold-formed steel assemblies. However, tall wood buildings are more likely to employ mass timber elements due to higher load conditions, requirements for longer fire resistance ratings, as well as other factors. There is little knowledge currently available related to using gypsum board directly applied to mass timber, or in other configurations, for fire protection. Testing performed to date has been limited to direct applied Type X gypsum board using standard screw spacing, and showed promising results [5, 6, 7]. This represents an opportunity for other configurations that might provide enhanced protection of wood elements to be investigated.
Being able to provide equivalent fire performance of assemblies between non-combustible and combustible construction will thus improve the competiveness of tall timber buildings by providing additional options for designers.
The objective of this research is to address a knowledge gap related to fire performance of midply shear walls. Testing has already been done to establish the structural performance of these assemblies. To ensure their safe implementation and their broad acceptance, this project will establish fire resistance ratings for midply shear walls. Fire tests will provide information for the development of design considerations for midply shear walls and confirm that they can achieve at least 1-hour fire-resistance ratings that are required for use in mid-rise buildings.
This research will support greater adoption of mid-rise residential and non-residential wood-frame construction and improve competition with similar buildings of noncombustible construction. This work will also support the development of the APA system report for midply walls, which will be a design guideline for using midply walls in North America.
Les panneaux en bois lamellé-croisé (CLT) peuvent offrir une excellente résistance au feu, souvent comparable à celle des constructions lourdes typiques non combustibles. En raison de la nature inhérente des pièces en gros bois d’oeuvre à carboniser lentement et à un taux prévisible, les systèmes en bois massif peuvent maintenir une capacité structurale significative pour des durées prolongées lorsque exposés au feu.
Afin de faciliter l’acceptation de futures dispositions du code pour la conception des panneaux de CLT résistant au feu, un projet de recherche d’un an a été lancé chez FPInnovations en avril 2010. L’objectif principal du projet était de développer et de valider une procédure générique pour calculer les caractéristiques de résistance au feu des assemblages de mur et de plancher de CLT. Une série d’essais de comportement au feu en vrai grandeur est en cours pour comparer le degré de résistance au feu mesuré lors de tests standards à celui calculé en utilisant la procédure proposée. Compte tenu du fait que le projet de recherche en est à ses débuts (lors de la rédaction de ce chapitre), une procédure de conception simple, mais conservatrice, est présentée dans ce chapitre, selon l’information de pointe provenant de l’Europe et de l’Amérique du Nord.
La norme canadienne sur les règles de calcul des charpentes en bois (CSA O86) peut être employée pour calculer le degré de résistance au feu des panneaux de CLT avec la même méthodologie qui est actuellement employée pour calculer les degrés de résistance au feu du bois de construction lamellé-collé et du gros bois d’oeuvre aux États-Unis, en Nouvelle-Zélande et en Europe. Cette méthode est connue sous le nom de « méthode de la section résiduelle ou efficace » et permet l’utilisation des valeurs de calcul que l’on retrouve dans la norme CSA O86.
On recommande de faire appel à un ingénieur qualifié en protection contre les incendies pour diriger ou surveiller la conception des assemblages de CLT afin d’obtenir le degré de résistance au feu désiré. L’ingénieur en protection contre les incendies devrait travailler étroitement avec l’ingénieur en structure afin de bien évaluer les effets de l’exposition au feu sur la structure.
Le bois lamellé-croisé (CLT), un système de construction relativement récent pour lequel l’intérêt ne cesse de croître dans le secteur de la construction d’Amérique du Nord, contribue à la définition d’une nouvelle classe de produits massifs en bois. Le CLT est un composant structural à base de bois très prometteur, qui présente un potentiel élevé pour fournir des solutions de construction rentables pour les bâtiments résidentiels, commerciaux et institutionnels ainsi que pour les grandes installations industrielles. L’acceptation de la construction en CLT dans l’environnement réglementaire canadien requiert la conformité aux dispositions relatives à la protection incendie du Code national du bâtiment du Canada (CNBC), entre autres choses.
Des essais au feu approfondis ont démontré la capacité du CLT à fournir un degré de résistance au feu pouvant atteindre près de 3 heures lorsque le matériau est mis à l’essai dans des conditions de plein chargement conformément à la norme CAN/ULC S101. De plus amples renseignements sont également fournis sur les propriétés de sécurité incendie connexes, y compris l’indice de propagation des flammes et les dispositifs coupe-feu.
Les éléments de CLT sont utilisés dans les systèmes de construction d’une façon similaire aux dalles de béton et aux éléments muraux massifs ainsi qu’aux éléments utilisés dans la construction en gros bois d’œuvre en limitant les vides de construction créés grâce à l’utilisation des éléments massifs en bois, ce qui contribue à réduire le risque d’incendies dans ces vides de construction. En outre, la construction en CLT utilise généralement des panneaux de CLT pour les planchers et les murs porteurs, ce qui permet d’obtenir une compartimentation ayant une résistance inhérente au feu et contribue une fois de plus à réduire le risque de la propagation d’un incendie au-delà de son point d’origine (compartiment d’origine).
Le présent document propose une méthodologie visant à déterminer la résistance au feu des éléments de CLT. En tant que modèle déterministe fondé sur les concept de calculs aux états limites, cette méthode permet de calculer la résistance des éléments de CLT soumis à une exposition au feu standard (soit la norme CAN/ULC S101) à l’aide des mécanismes techniques de base du bois pour les calculs de résistance au feu allant jusqu’à 3 heures, qui ne sont limités que par la disponibilité actuelle des données. La méthode utilise un ajustement linéaire échelonné de la vitesse de carbonisation, une épaisseur de la couche carbonisée de 7 mm dont la résistance et la rigidité sont supposées nulles, un coefficient de résistance égal à l’unité, un coefficient de durée d’application de la charge de courte durée, et des résistances spécifiées ajustées à leurs valeurs moyennes pour prédire les temps moyens de résistance au feu des éléments de murs et planchers de CLT qui suivent étroitement les temps réels de résistance au feu des éléments testés. Bien que certaines améliorations à cette méthode soient encore possibles, ces comparaisons suggèrent que la méthodologie prédit de façon prudente la résistance au feu des éléments de CLT.
Building regulations require that key building assemblies exhibit sufficient fire-resistance to allow time for occupants to escape and to minimize property losses. The intent is to compartmentalize the structure to prevent the spread of fire and smoke, and to ensure structural adequacy to prevent or delay collapse. The fire-resistance rating of a building assembly has traditionally been assessed by subjecting a replicate of the assembly to the standard fire-resistance test, (ULC S101 in Canada, ASTM E119 in the USA and ISO 834 in most other countries).
Massive wood elements such as solid sawn timbers, glued laminated timber (glulam) and structural composite lumber (SCL) can provide excellent fire-resistance. This is due to the inherent nature of thick timber members to char slowly when exposed to fire allowing massive wood systems to maintain significant structural resistance for extended durations when exposed to fire. Calculating the fire-resistance of massive wood elements can be relatively simple because of the essentially constant and predictable rate of charring during the standard fire exposure. Charred wood is assumed to no longer provide any strength and stiffness; therefore the remaining (or reduced) cross-section must be capable of carrying the load.
This report presents two (2) mechanics-based design procedures as alternative design methods to conducting fire-resistance tests in compliance with ULC S101 or to using Appendix D-2.11 of the NBCC, which is limited to glulam members stressed in bending or axial compression. The procedures are applicable to solid sawn timber, glulam or SCL structural members and aim at developing a suitable calculation method that would provide accurate fire-resistance predictions when compared to test data. The long-term objective is to provide recommendations for incorporating either method into CSA O86 and/or NBCC.
The comparisons between the proposed methodologies and the experimental data for beams, columns and tension members show good agreement. While further refinement of these methods is possible, these comparisons suggest that the use of the CSA O86 equations and a load combination for rare events adequately address fire-resistance design of massive wood members.
This report summarizes the results of a one-year project with the primary objective of documenting the results of six full-scale fire tests carried out on houses in Kemano BC in 2001. During the year, however, Forintek worked with a Ph.D. student at Carleton University to go beyond this objective and to compare the results of the experiments with the predictions of fire models.
These full-scale fire tests were conducted in order to assess the performance of wood-frame assemblies exposed to fire in furnished houses. The first item ignited in all tests was a waste-paper basket in contact with an item of upholstered furniture or a mattress. The fires were allowed to follow their natural course for a significant period of time without intervention by fire fighters so that the houses' wood-frame structures were challenged in a realistic fashion. Each experiment was instrumented to measure temperatures at up to 50 locations within rooms and building assemblies.
Observations taken onsite and a quick review of the raw data, allowed important conclusions to be drawn:
Fire spread quickly from the waste-paper basket to upholstered furniture or mattresses. Subsequent fire development was rapid with flashover occurring rather early. The temperature in the room of fire origin got much hotter than in standard fire-resistance tests.
Properly designed wood-frame walls and ceilings act as a significant barrier to fire spread.
The contents of a house (in particular, upholstered furniture and mattresses) are more of a fire-safety threat than the wood-frame structure. In all fires, untenable conditions developed before the structure was involved in fire.
In very large fires, a firewall provides a significant barrier to the spread of fire between two buildings of combustible construction.
A detailed analysis has also been undertaken whereby the fires were simulated using available fire models in order to assess whether the models give a good representation of real fires and of the performance of wood-frame assemblies. The results of this analysis, summarised below, were very encouraging.
The predictions of Forintek’s computer model WALL2D for the temperature between 15.9 mm fire-rated gypsum and wood studs in walls agreed very well with the measured values. Both experiment and theory demonstrated that fire-rated gypsum delays the involvement of studs in fire for a very long period of time.
The predictions of BREAK1, a commercially available computer model, were very close to the times at which window glass was observed to crack.
Using measured fire temperatures and a simple model, the rate of burning during the early stages of the fires, in which it was primarily a couch that was burning, was similar to that of upholstered furniture observed in a comprehensive European furniture study.
A simple model for predicting the maximum temperature rise in a fire-room with closed doors and windows was found to give predictions in good agreement with the experiments.
This report summarises the findings of four of the six tests conducted in Kemano. Each of these four tests has, however, been studied in more detail than originally planned. Forintek scientists will continue to study the data from the Kemano fires. In particular, simulation of these fires using available computer models will continue. If fires in wood-frame structures can be modeled accurately, one can begin to assess the advantages and disadvantages of various design options. In the end, the computer models will be used to evolve recommendations on how to improve the fire-safety performance of housing.
The objective of this work is to generate fire resistance data for NLT assemblies to address significant gaps in technical knowledge. This research will support designers and builders in the use of mass timber assemblies in larger and taller buildings, as well as provide scientific justification for Authorities Having Jurisdiction (AHJ) to review and accept this construction method. The intent is to demonstrate that NLT construction can meet or exceed NBCC fire safety requirements for use in buildings of mass timber construction.
The data could be used towards the inclusion of an NLT fire resistance calculation methodology into Annex B of CSA O86 – Engineering Design for Wood [4], which currently addresses only glue-laminated timber (GLT), structural composite lumber (SCL) and cross-laminated timber (CLT).
