The LWF Blog

Fire Engineering Design and Risk Assessment – Hazard Classifications – Part 14

September 28, 2017 11:36 am

In LWF’s fire engineering and risk assessment blog series for architects and others in the building design business, we have been looking at fire suppression in terms of sprinkler protection and, recently, hazard classification. In Part 13, we ascertained there are three main divisions based on fuel load – light, ordinary and high hazard, with further subdivisions of the second two. In Part 14, we’ll continue discussing hazard classifications and light hazard in particular.


Within the hazard classification is dictated not only the minimum amount of water provided in the form of water spray, but also the expected maximum area of the sprinkler system that will be activated by the fire. This is known as the assumed maximum area of operation (AMAO) and is expressed in square metres.


Particular information on hazard classes can be found in PD 7974 Part 4 Application of fire safety engineering principles to the design of buildings. Detection of fire and activation of fire protection systems. (Sub-system 4), which was last updated in 2003, and is current as of 2017.


When considering the information given regarding design densities and AMAO of sprinkler operation, consideration should be given to the equivalent number of operating sprinkler heads, particularly in cases of high hazard classification. The equivalent number is the amount of sprinkler heads expected to operate before control of a fire is achieved.


A light hazard classification will always be a non-industrial scenario where the amount of contents is low and the combustibility of those contents is also low. Typical light-hazard environments include schools, hospitals, hotels etc. The maximum fire load for this classification is 400 MJ.m-2.


An ordinary hazard classification will consist of commercial and industrial environments, including the storage and processing of a stock of materials, all of which would fall into the category of ‘ordinary combustibles’, i.e. they will not develop quickly into an intensely burning fire. The maximum fire load would be 1000 MJ.m-2.


If you’re noting that there is quite a gap between the light hazard classification’s fire load and that of the ordinary hazard classification, you would be correct and this is why the ordinary hazard classification is broken into further sections to allow for more accurate calculations.


Even when the correct classification is ascertained, the manner of storage can still have an effect. For instance, while the storage of goods is permissible under ordinary hazard classification, the tendency for an upwards velocity to a fire plume is such that a sprinkler may be unable to cope adequately with the amount of stored consumables or the height at which they are stored.


When classifying the hazard of an occupancy and ascertaining the correct amount of fire protection for an area, it is always wise to seek informed advice from a suitably qualified fire engineer.


In Part 15 of this series, we will continue talking about ordinary hazard classifications before moving on to talk about high hazard risks. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.


Lawrence Webster Forrest has been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.


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