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March 18, 2008

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External Fire Spread – The testing of building cladding systems

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Building designers now have a plethora of cladding materials and systems to choose from, but conventional small scale fire tests may not always be appropriate. Sarah Colwell sets out the latest developments in understanding how these systems behave in real fires.

Fires involving multi-storey buildings are fortunately rare, but they still entail a risk to life, property, as well as causing disruption to people’s lives and businesses. These concerns have been highlighted by events such as the Knowsley Heights fire in 1991, and the Irvine fire in 1999.

Fires within buildings have the potential to spread externally by breaking out through windows, travelling over or through the cladding system and then breaking back into the building at another level, so by-passing the internal compartmentation for the structure and thus potentially placing residents and building users at risk. Flames can extend over 2m above the window opening from which they break out, regardless of the type of cladding materials used. This can lead to a potential of secondary fires developing if the fire performance characteristics of the external cladding system are not fully assessed.

The mechanisms by which fire can spread externally include combustible materials and cavities – either as part of a system, or those created by delamination of the system or material loss during the fire. Once flames enter a cavity they have the potential to travel significant distances, giving rise to the risk of unseen fire spread within the cladding systems.

Designers of buildings have a wider range of options for cladding materials and insulation products than ever before. These include:

– timber

– plastics

– glass Reinforced Plastic

– glazing

– polymeric composites

– cement based products

– polymeric and mineral fibre based insulating products.

These systems often comprise a kit of parts and they undoubtedly provide designers with a host of creative cladding options. However, there also needs to be consideration of the implications associated with the design and use of these systems in practice, as effective facade design requires a wide range of inter-related technical provisions, such as maintaining ventilation to ensure that the durability of the system is not compromised whilst minimising the risk of fire spread through the cavity. So the needs of one provision can sometimes conflict with the needs of another and designers must be able to satisfy each one without contravening another.

Since the overall fire performance of the system is reliant on the interaction of all the elements within the cladding system, small scale component testing cannot always reflect the overall fire performance of the complete system when installed in a real building.

The project

Our research identified three different generic types of envelope design for consideration in this project, which can be summarised as follows:

– rendered insulation

– ventilated cavity cladding and

– built-up systems

A summary of the system details can be seen in Table 1. For each system type, the fire performance was assessed against four test methodologies:

– BS 476 Parts 6 and 7 (external finish and insulation only)

– European reaction to fire tests (EN 13501-1 – external finish and insulation only)

– ISO 9705 room test (the reference scenario for the European tests)

– The large scale test method specified in BR135 (BS 8414-1)

BS 476 part 6 and 7 are currently used in Approved Document B to determine Class 0 reaction to fire performance. These tests require the samples to be essentially flat and are primarily a material test, as they are not designed to assess the full product in an ‘end use’ scenario. The samples can be tested with a limited air gap or fitted directly to a suitable substrate.

Class 0 is not a classification defined by a British Standard, rather it is defined in Approved Document B as a sample that achieves Class 1 in a surface spread of flame test and achieves an index of performance (I) not exceeding 12 and a sub index (i1) not exceeding six in the fire propagation test.

European reaction to fire tests

The Construction Products Directive aims to remove barriers against trade between member states of the European Union, and part of this is the harmonisation of test methods. Reaction to fire testing results in a European classification A1-F.

Products are tested to simulate their end use application, so potentially products which have different end use applications can achieve different classifications in each case. This end use application includes the orientation of the product as it is to be used and its position in relation to other adjacent products such as the substrate and any fixings. For the purposes of this study the relevant test standards were:

– EN 13823: 2002 Reaction to fire tests on building products – single burning item test.

– EN ISO 11925-2: 2002 Reaction to fire tests on building products – Part 2: ignitability when subjected to direct impingement of flames.

The single burning item test (EN 13823) measures the spread of flame, heat release, smoke production and the extent of flaming debris/burning droplets. The small flame test EN ISO 11925-2 evaluates the ignitability of a product exposed to a small flame ignition source and measures flame distances, the burning time and any flaming droplets or particles generated. The data from these tests are then interpreted to provide a European classification for the product using the standard EN 13501-1: 2007.

The ISO 9705 room test

The systems were installed onto the walls of a block work room 2.4m x 2.4m x 3.6m adjoining a 3m calorimeter hood. The cladding systems were then subjected to a propane diffusion flame burner applied in the rear corner of the room at low level, and which provided 100kW for 10 minutes followed by 300kW for a further 10 minutes. The combustion products from the systems were collected and the data was then used to develop an indicative classification of fire performance for the systems using the methodology given in Annex A of

EN 13501-1: 2007. This annex explains the background to the development of the European reaction to fire classification system and uses this test as a reference scenario.

Large scale test

BS 8414 -1: 2002 is the test method for non- load bearing external cladding systems applied to the face of a building. The test determines the comparative burning characteristics of exterior wall assemblies by evaluating the following characteristics:

– Fire spread over the external surface

– Fire spread internally within the system being tested

– Mechanical response i.e. the degree of distortion and local or global collapse

The sample is installed on the main face and wing of the test rig. The system facility is a minimum of 8m in height from ground level and is subjected to a wooden crib ignition source with a peak heat output of 90kW/m2. The duration of the fire load is 30 minutes, though the test may run for up to 60 minutes if burning of the sample still occurs. After the test, damage from the following is recorded: flame spread on the surface; flame spread in the cavities or insulation; and the area of facade damaged or detached. Fire performance classification is addressed by the second edition of BR 135.

A second part of the BS 8414 standard – Test method for non-loadbearing external cladding systems fixed to and supported by a structural steel frame – has been published for use with panels and curtain walling systems where there is no masonry sub-structure. The fire load and thermal exposure test methods are the same as outlined in BS 8414 -1:2002 above. For complex systems, the BS 8414 test series provides a mechanism for understanding the fire break and cavity barrier details on a realistic scale. The need for a full scale test scenario has now also been accepted in Europe and a European test method is now being developed.

Results

The results from the test programme are summarised in the Table below. They indicate that material and individual component testing did not necessarily predict overall system performance, particularly in relation to the fire performance of overall system design details and fire break designs.

The primary conclusion from this work is that the use of test methods and assessments which more closely reflect the end use application on a building should provide novel designs and materials with a method of demonstrating their overall fire performance, as part of a system.

Sarah Colwell is principal consultant at BRE Global. This article is based on a presentation she gave at the BRE Trust conference on fire engineering projects in November. To view her presentation, visit http://www.bre.co.uk/presentations.jsp

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