Sometimes, conventional fire tests on small scale samples are simply not sufficient to reproduce performance in real fires – particularly in some buildings which use modular building methods. Terry Day warns us to interpret such tests with caution.
Anyone involved in insuring or protecting buildings from fire needs to understand the difference between how structures act under test conditions compared to a real fire.
Generally, fire tests are carried out on a single manufacturer-supplied test specimen or set of specimens, so they are only a snapshot in time of the fire performance of a material or product. Additionally, fire tests do not simulate all aspects of a real fire – no single test result can reflect every aspect of each phase in a developing fire. Unfortunately, decisions are often made regarding the suitability of a building product without realising the limitations of a standard fire testing method.
Reaction-to-fire tests, which are used by regulators to control the use of materials and products in buildings, tend to be carried out on a small scale. A product’s fire performance may be influenced by its method of support, discrete fixings, the potential for delamination, the type of insulation, and the location of fire stops and joints. These features cannot always be recreated in small scale tests. Some tests limit the thickness of the tested sample, which may mean that it is no longer truly representative of what will be used in the building in practice.
Standard fire resistance tests are typically carried out on two-dimensional samples of elements around 3m x 3m in size. When forming part of a real building, many construction elements are much larger than this (for example a warehouse wall) and may not respond as the test sample. One particular limitation of standard fire resistance tests is that the interaction between a wall and a floor is not assessed, as each wall/floor system is tested in isolation. For some modern methods of construction this is a critical aspect. To overcome this limitation, BRE has developed LPS 1501 Fire test and performance requirements applicable to Pre-Assembled or Modular buildings. This standard for third party certification involves a three-dimensional construction representative of the building, and uses a fire exposure representative of the building use.
Third party certification by an independent certification body provides reassurance that the quality of the system should be reliable, and should achieve the same fire test performance as when originally tested – subject to correct installation. As such, third party approved products are now accepted by building control bodies as proof of compliance with the Building Regulations.
The risk of fire starting and the mechanism of fire spread are no different in modular buildings than in traditionally built ones. However, new modular construction techniques differ in a number of important ways from traditional techniques, and these differences can be significant in terms of the potential for fire spread throughout the whole structure. Fire spread mechanisms in traditional structures are usually well understood, being based on historical evidence of performance. It is important to determine whether modular construction techniques can result in the spread of fire through similar or different mechanisms, and to evaluate and quantify any new risks arising.
Plague on both houses
Both traditional and modular buildings are vulnerable to poor site supervision and inadequate levels of workmanship. While factory produced modules may be subjected to significantly better standards of quality control at the factory, they still require some final assembly on site where issues of poor workmanship are a potential problem.
Experience from actual fires sometimes shows the inadequacy of standard fire tests. In some cases only minor changes to tests may be required to address any shortcomings. In other cases, a large scale fire test, in conjunction with computational fire modelling, may be required (at least initially) to establish the fire risk. For certain types of construction products and systems commonly used in the UK, large scale fire tests of the systems are the most appropriate way of ensuring that a product will have satisfactory performance in a fire.
Experience from one actual fire led to the development of LPS 1581: Requirements and tests for LPCB Approval of non-loadbearing external thermal insulated cladding systems with rendered finishes (ETICS) or rain screen cladding systems (RSC) applied to the masonry face of a building. It showed that fire could spread through a cavity much quicker than had been assumed using standard reaction to fire tests, which are only able to test basic materials and not complete constructions.
Certainly, fire insurers’ experience from actual fires have indicated that some standard fire tests may not be truly representative of actual behaviour or the potential hazards present in real fires. It is for that reason that insurers strongly support the LPS 1181 series of standards, particularly for composite constructions that cannot readily be modelled in small scale fire tests.
Many tests and relevant guidance tends to assume that exposure to fire will occur from one side only. Additionally, test data may not necessarily be required for materials hidden in a cavity that are protected by linings offering required fire performance. However, during installation, refurbishment or maintenance, the exposure to fire may differ from that in a completed building. This may potentially lead to unforeseen fire risks and associated losses.
Fire tests are able to provide important information on one aspect of fire performance, but limitations of the test method(s) need to be fully understood for an accurate hazard assessment to be undertaken.
Terry Day is passive fire director of BRE Certification. For more details about a recent seminar on the subject of large scale fire testing, visit: www.bre.co.uk/eventdetails.jsp?id=1623
BRE Global (incorporating LPCB & BREEAM) is an independent third party approvals body offering certification of fire, security and sustainability products and services to an international market.
