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Fire Safety in Timber Buildings

Created on Sunday, April 17, 2016 and posted in Industry News
Fire Safety in Timber Buildings
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A new and completely revised third version of the Fire Safe Timber Buildings handbook has been published as part of a Nordic-Baltic project with the support of NICe, the Nordic Innovation Centre.  Leading experts and scientists in all countries have contributed to the work, which guarantees its quality and relevance.  The book has been first published in Swedish, but translations to other languages, including Estonian, are planned.

‘Fire Safe Timber Buildings 3’ provides information and guidance on all aspects of importance for the outline design, detailed design and detailing of fire-resistant timber buildings.  It also contains the latest information on European and national regulations.

The handbook has been written for architects and designers.  It can also be used by public authorities, fire and rescue services, in training applications and for product developers working in the construction and timber industries.

Timber Buildings

Image 1: An office building in timber-structure in Tapiola, Esbo, Finland.

The handbook

The aim of the handbook is to provide practical guidance and advice concerning the design of timber structures and products in order to meet fire safety requirements in the Nordic and Baltic countries through application of European classes and design methods in accordance with Eurocode 5, and with new design methods that are not yet included in Eurocode 5.  Detailing is also very important, and particular attention is paid to this.

Although the handbook is concerned primarily with the design and construction of multi-storey buildings, with greater use of timber elements, it is naturally also suitable for low-rise buildings.  The instructions are concerned mainly with residential buildings, although many design solutions are suitable for general application.

The handbook covers the use of materials and requirements that are directly related to timber.  More general technical guidance can be found in more widely drawn handbooks on fire safety design.

Although methods of fire testing and classification systems have been harmonised in Europe, regulations are still national.  The European systems exist on a technical level, but fire safety falls under national legislation, which means that it lies on a politicallevel.  National fire safety regulations will therefore remain, although the European harmonisation provides a good basis for more consistent national regulations.  A European guide to fire safety in timber buildings has recently been published, describing the application of the European systems on the use of timber in construction.  The Nordic-Baltic handbook contains more information on fire regulations, design, construction, materials and several practical examples.

The main new contents of Fire Safe Timber Buildings 3 are:

•    New methods of calculating the fire resistance of timber structures, both the fire separation and of load-carrying capacity, which are not yet included in the Eurocodes.
•    Detailed information on fire classes for wood as a visible surface material in accordance with European systems.
•    Wood products with fire protection ability, known as European K-classes.
•    New methodology of evaluating design alternatives through the installation of sprinklers.
•    New systems for assessing fire retardant wood products, their long term durability and workmanship in practice.
•    Fire design documentation for high rise timber buildings.
•    Fire protection on building sites.

Contents

The handbook starts information on regulations, European systems for fire safety in buildings, and fire protection requirements in the Nordic and Baltic states.  This is followed by a chapter on fire safety design of buildings using function-based design and presenting examples of fire safety design.  The chapter also includes examples of timber building projects from the Nordic and Baltic states.  The following chapters deal with specific timber-related subjects.

Fire resistance of timber structures

Two chapters describe both fire separating and load bearing timber structures, designed in accordance with the calculation methods in the fire part of Eurocode 5.  These chapters also describe new methods of calculation deriving from the latest research, which are seen as potential input for future revisions of Eurocode 5.  In both cases, many examples of timber structures and their calculated fire resistance are described.

The fire separating performances of timber structures are presented using a new and improved addition method.  Based on extensive experimental results and calculations, the method is suitable for design of timber structures having an unlimited number of sheet material and insulation material layers.

The  chapter on load bearing structures presents methods of calculation for new types of timber structures that are not yet included in Eurocode 5.  This applies primarily for solid cross laminated timber sheets used as wall and floor/ceiling elements as well as for I beams, of which the flanges need to be protected from fire in order to maintain their load carrying capacity.

Another new feature is that the fire resistance properties of gypsum board must be assumed, as their thermal and mechanical properties when exposed to fire are insufficiently described in European standards, such as EN 520.  The time to collapse of Type F gypsum boards must be determined by measurement, with data being supplied by the producers, but there is at present no common European system for this.

Wood products with fire protection ability

Wood-based panels, solid wood panelling and cladding can meet the new European K classes for fire protection ability.  These classes are based on testing of fire resistance, with the main parameter being the temperature behind the panel.  Three classes have been defined:  10, 30 and 60 minutes.  The criteria for classification of wood products in K classes is based mainly on the thickness of sheets.

K210 is used in the Nordic states together with surface cladding class B-s1,d0 for designation of what was previously called ‘ignition-preventing cladding’ in the Swedish Building Regulations.  Gypsum boards have traditionally fulfilled this fire class.  Other products are now also available, such as fire retardant wood products with Class K210/B-s1,d0.

 

Detailing and special structures

Design details are important for the fire performance of timber buildings, and present a major challenge area.  Although timber structures have a predictable behaviour in fires, structural details must be designed and incorporated carefully in order to ensure the fire safety of the building as a whole.  Many examples are given in the handbook.

Fire stops must be installed in cavities in order to prevent hidden fires from spreading between fire cells.  The preferred arrangement is to ensure that structures do not have cavities (e.g. are fully insulated).  Penetrations such as ventilation ducts, pipes and cables must not reduce fire resistance.  Fire stops in ventilated structures, such as external cladding or roof spaces, must permit ventilation.

Eaves must not reduce fire resistance.  Eaves and ventilation openings must be so designed that they withstand the effect of external fires.

Fire classes for wood surfaces

Wood products have a stable fire behaviour, and can therefore be classified as a group in accordance with European systems.  The handbook includes detailed fire classification of five different wood products:  wood-based panels, structural timber, glulam, solid wood panelling and wooden flooring.  Wood products thicker than about 10 mm normally fulfil at least class D-s2,d0, but classification can be affected by density, the presence of joints, underlying materials, air gaps behind the wood product and surface profiles.

Wooden facades

There is no European system for façades, and no common Nordic view.  However, this chapter includes material on fire scenarios, requirements and risk evaluation of timber façades.

Fire requirements for façade materials are determined by the risks of spread of room fires or some other external fire risk.  The most severe fire exposure comes from a flashover fire, with flames billowing out through a broken window.  The façade cladding in such cases must not contribute to the spread of fire along the façade.

The flames from windows in a flashover room are normally so severe that any window immediately above is exposed to very severe fire conditions regardless of the cladding material.  This is regarded as an accepted risk level in the Building Regulations.


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