# The LWF Blog

## Fire Safety Engineering – An introduction to Probability Risk Assessment – Part 1

May 22, 2014 9:26 am

Fire Safety for buildings designed and constructed in the UK is governed by legislation and supported by British Standards and approved documents. When a project is completed using the standards laid out in these guidances, it is called using the prescriptive approach. However, following only these guidances can lead to the design and its realisation becoming restricted or predictable.

Fortunately, architects and planners can utilise standards within the Building Regulations which allow for inventiveness and variety to be achieved, through performance-based design.

Simply put, performance-based design means that the intended design is one where fire engineering tools are put into practise to test the viability in terms of fire safety. The final approval is then given by the Fire Authority and Building control. One such fire engineering tool is Probability Risk Assessment.

The typical deterministic fire engineering study will still make certain assumptions about how fire scenarios will develop. This is where the Probabilistic Risk Assessment goes further. It is a fire safety engineering tool, considering that all assumptions made are always taken to be true. This approach not only calculates the consequences of the fire; it also takes into account the likelihood that a given set of consequences will occur. Using this method it is possible to work out a study for each individual building type, in considerable detail.

What is a Probability Risk Assessment?

The Probabilistic Risk Assessment is a method of hazard analysis that considers a range of fire scenarios likely to occur in a building. It represents a complete analysis of a building’s fire safety design. The assessment uses fire data gathered from previous fires – generally information taken from dedicated databases.

Probabilistic Risk Assessment or Analysis (PRA) is the generic term applied to risk-measurement studies. Risk is expressed as the likelihood that a set of consequences will occur; so the results of PRA studies produce numbers indicating the level of hazard posed to persons or property, but take into account how likely the event is. In practice this can mean, for example, that a common, yet low-consequence event can be looked on as a similar hazard to a rare yet high-consequence event.

Applying Probabilistic Risk Assessment

PRA can be applied to fire safety engineering in a number of ways. PRA can be used to compare the installation of two fire safety systems. For example, a designer may wish to determine whether a smoke control system or sprinkler system should be installed. An analysis of the building will be carried out and the method applied independently to the smoke control system and to the sprinkler system. Finally, it is possible to decide which system is best to fit in the same premises under different conditions. PRA is an excellent design tool for engineers and architects. Another application would be predicting how a building’s fire safety elements are likely to fail, and what damage would result.

With any Fire Engineering tool, it should be borne in mind that every tool has its limits. In the case of a Probabilistic Risk Assessment, the common limiting factor is the availability of relevant data. Mixing a deterministic approach with prescriptive fire engineering techniques has often bridged the gap between what data is readily available and what is absent, often by taking a conservative approach. The same kind of approach cannot be used in PRA studies. This is why complex mathematical techniques are used in finding unavailable data. However, the lack of basic data in complicated studies or the lack of data in simplistic studies can seriously hamper the use of PRA.

Next week’s blog will continue to look at the advantages and disadvantages of using PRA as a fire engineering tool for your design. In the meantime, if you have any questions about this subject, or wish to speak to one of our Fire Engineers, please contact Peter Gyere on 0208 668 8663.

Lawrence Webster Forrest is a fire engineering and fire risk management consultancy established in 1986, with experience in the development of fire engineered technology and the application of fire safety standards including fire engineered techniques.