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Justification de durée de résistance au feu de 90 et 120 minutes

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Conférence présentée dans le cadre du Congrès mondial WOODRISE à Bordeaux (12 au 15 septembre 2017). Workshop IMMEUBLES BOIS MOYENNE ET GRANDE HAUTEUR ET SÉCURITÉ INCENDIE.

Il s’agit, sur ce sujet sensible, de profiter des retours d’expérience des experts internationaux du domaine. Seront notamment abordés les thèmes concernant la mise en œuvre des sprinklers : à partir de combien d’étages faut-il mettre ces dispositifs en place, quels sont les risques de réhumidification des structures en bois ; quels sont les coûts d’entretien, etc.?

La problématique des balcons et leur prise en compte dans la propagation des feux en façade seront également des enjeux de cet atelier. Enfin la résistance au feu et notamment l’état de l’art des solutions REI 90, voire REI 120 au niveau des jonctions planchers / murs sera le troisième axe de travail.


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Titre : Justification de durée de résistance au feu de 90 et 120 minutes

Conférencier : 

Christian Dagenais, FPInnovations, Canada

Initialement présentée le : 14 septembre 2017

Description

The fire-resistance rating of building components has traditionally been assessed by subjecting a replicate of that component to a standard fire-resistance test, such as CAN/ULC S101 in Canada, ASTM E119 in the United States and ISO 834 in most other countries. Mass timber elements such as solid sawn timbers, glued laminated timber (glulam), structural composite lumber (SCL) and cross-laminated timber (CLT) can provide excellent fire-resistance due to the inherent nature of thick timber members to char slowly and at a predictable rate when exposed to fire. This allows mass timber systems to maintain significant structural resistance for extended durations under fire conditions. The “reduced cross-section method” is the most acknowledged analytical method around the world for evaluating the load-bearing function of mass timber elements exposed to fire (e.g. beams, columns, floor/roof slabs, decking, etc.). A linear charring rate, prescribed in relevant timber design standards, accompanied with a zero strength layer is typically used to calculate the reduced cross-section. Given it’s a mechanics-based method and that a number of tests exceeded 2 hours, there is theoretically no justification for limiting its application to a maximum structural fire-resistance rating (e.g. 2 hours). A large number of test data can be found in the public domain to verify and validate the application of the reduced cross-section method.

Furthermore, some building assemblies may require providing a separating function (insulation and integrity) to provide some level of compartmentation within a building (i.e. fire cells). The separating function of mass timber assemblies is however not as well-documented as their load-bearing function. Special considerations should therefore be given in attempt to ensure these assemblies also fulfill their separating functions, when needed. A number of fire tests on CLT assemblies demonstrated significant separating function, well in excess of 2 hours.

Lastly, connections between structural elements should provide sufficient fire-resistance so that they do not become the critical elements in a structural system. Currently, the Eurocode 5: Part 1-2 limits the design procedure for timber connections to 60 minutes when exposed to a standard fire. One can presume that any metallic fasteners located within the residual cross-section would not be severely impacted from thermo-mechanical degradation and would thus still fulfill its load-bearing function. However, very little test data for fire duration exceeding 1 hour can be found in the public domain. There is a need to rationalize the fire-resistance design of connections for tall wood building, which would most likely require 90 and 120 min fire-resistance.

This presentation will provide background information relating to the calculation of the load-bearing and separating functions of mass timber elements. It will also provide recommendations related to connections as well as information from recent research activities and test data conducted in support to tall wood buildings.

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