Testing times for tall buildings?
In the wake of The Address Downtown Dubai hotel fire and other high profile fires in tall buildings, Niall Rowan outlines the Association for Specialist Fire Protection’s initiatives to provide an affordable fire test for manufacturers of cavity barriers used in façades and rain screens.
Another week, another big façade type fire in a tall building, another YouTube video and the collective wringing of hands of interested parties. What is going on? When we have been constructing buildings for a number of years in a controlled regulatory framework where we can mitigate the risks to an acceptable level by the use of codes and statutory guidance; how on earth has a new type of fire hazard sprung out of nowhere? The problems lie in a mixture of inadequate design, unsuitable materials used where they should not be used (either by design or lack of effective policing) and an inadequate understanding of the hazard.
While many building designers understand fires in buildings and use a number of tools from compartmentation to suppressions systems, smoke extraction and other measures; when it comes to fire outside the building there is not the same clarity of thought or approach. This often leads to an inadequate evaluation of a proposed design with the result that something potentially unsafe is built. Fortunately, most of the fires, apart from that in Baku in May 2015 which killed 15 people, have had few or no fatalities, mainly because the fires have, for the most part stayed outside the building.
Conventional fire testing
When it comes to façade fires, there is agreement that the use of conventional reaction to fire tests (small scale tests which measure how materials ‘react’ when subjected to heat source) are unsuitable. Fire resistance tests (large scale tests using furnaces run to a standard temperature time curve to model a post flashover cellulosic fire) are also often not suitable, because while they are ‘severe’ enough, they don’t model the fire scenario of a façade fire which can comprise a convective plume and a complicated geometry of façade panels, glazing, supports and so on. So conventional standard fire tests, which are good at mitigating fires inside buildings, don’t really work when the fire is outside.
Consequently, a new breed of fire tests has been developed including NFPA 285, BS 8414, SP 105 etc. All these are large scale and most use large wooden cribs (piles of finely stacked timber) to give a large high intensity fire with a large convective component (more so than in a furnace test). One example of these tests is BS 8414.
BS 8414 is typical of a number of large scale tests used to evaluate the fire performance of façades. It is a ‘full scale’ test with two ‘wings’ at right angles to each other (see fig 1). Each wing is sized so that there is a minimum of 6m height above the 2m x 2m combustion chamber; the narrower wing being >1.5m wide and the wide wing >2.8m. A wooden crib producing 45-90kW/m2 (see photo 1) is designed to burn for 30 minutes and the test is run for 60 minutes to ensure all components have completed any combustion by the end of the test. Damage is observed in terms of flame spread on the surface and in the cavities and the temperature is measured at a number of positions. The results are classified according to BRE report BR135 which contains the pass/fail requirements.
BS 8414 and tests of its kind are being used to evaluate façades in many countries. However, some countries have either not taken up this type of test, or if it is used, the façades installed have not been evaluated to this type of test. Most of the façades in UAE which are causing such a problem at the moment are composed of aluminium-faced combustible cored panels. A façade made from such panels would not normally pass a large scale fire test such as BS 8414. You can see a video of BS 8414 testing here https://www.youtube.com/watch?v=V4KA8S4yLoI
Large v small/medium scale tests
The merits of large scale tests is that they:
- model at full scale
- model the complete system and the interaction between various components
- can be directly related to a given fire scenario.
However, they are very expensive – especially for manufacturers of small components that go to make up a façade. One example of this is the situation with rain screen cladding which is commonly used in buildings to add waterproofing and insulation to the outside of a building for thermal efficiency reasons. Such rain screens need cavity barriers installed horizontally to prevent vertical flame propagation up through the cavity. However, they must be ventilated i.e. open to allow for free air circulation under normal conditions. Manufacturers of these small components will baulk at the tens of thousands of dollars for a single large scale test – especially if they have many different products to evaluate, or wish to do some comparison testing for R & D purposes. Medium and small scale tests can be used effectively and have been used to successfully evaluate penetration seals, linear joint seals and air transfer grilles for a number of years.
Medium scale tests
In Europe, there is a fire resistance test standard for components of curtain walling EN 1364-4. This is used by manufacturers to be able to test components with the eventual aim of obtaining CE marking.
For fire barriers used at slab edge seals, the relevant test scenario is Type A (non-fire resisting) glazing and configuration number 5 (see fig 2 and photos 2 and 3). The ability of the seal to prevent flame spreading upwards between the slab edge and the spandrel panel is evaluated in terms of integrity (resistance to passage of heat and flames) and insulation (ability to restrict temperature rise of the unexposed (upper) face of the seal to below certain levels).
Some manufacturers of fire barriers have tested their products to EN 1364-4 and obtained CE marking. However, even these tests are quite expensive and so there is some pressure for an even smaller/simpler test using the same principles as EN 1364-4. Another driver for a smaller scale test is that manufacturers of such seals want third party product conformity certification for them and this means that the certification bodies will need a test that can be used to evaluate the seal itself.
Small scale tests
Parallel activities by the Association for Specialist Fire Protection (ASFP) and in the European standards body CEN have resulted in two very similar furnace based tests for cavity barriers. In both cases, the tests model the gap between the façade or rain screen and the building by means of aerated concrete upstands mounted above a fire resistance furnace.
The upstands are spaced apart so that when the seal is installed there is still a gap of anything between 25mm to 75mm which is filled by the seal as it expands on heating. (See fig 3 and photo 4). Thermocouples are mounted above the seal to measure insulation and also determine when the seal closes. Measurements of integrity are made by cotton pad and the observation of sustained flaming as per the EN 1364-4 fire resistance test.
Differences between the ASFP and European test are highlighted in the table above. However for most seals, one would expect the same result in both cases. A number of manufacturers have tested to the ASFP method. The method is published as ASFP Technical Guidance Document 19. You can down load it here: http://is.gd/FCjaV1 . At the time of writing the draft European standard is undergoing CEN enquiry which will end in July 2016.
Small scale tests have been successfully used in lieu of large scale tests in a number of areas and for some time. However, it would be desirable to be able to correlate the results of the small scale tests to those obtained in the large scale tests. This is the intention of the CEN committee drafting the small scale test once a decision has been made on which test will be used by the European Commission for façades.
The Commission is currently evaluating the options and is minded to have one large (BS 8414 type) test for all Member States with a transitional period for those Member States who still want the intermediate scale test such as DIN 4102-20. After the transitional period, the intermediate scale test can only be used in those Member States that still want it, but the large test will be valid everywhere. So in time everyone will probably move to the large test.
For more information, go to www.asfp.org.uk