The LWF Blog
What is Fire Resistance | Part Two – BS 9999 vs Approved Document B & Fire EngineeringJanuary 9, 2014 7:07 pm
Various methods can be employed to assess and improve fire resistance in a building, i.e. the minimum time a building can withstand high fire temperatures and remain within certain safety boundaries.
In the past, LWF has designed buildings with various features and complexity in accordance with Approved Document B or BS 9999. BS9999, being the newer guidance, might be expected to make meeting fire resistance conditions easier for designers, however, the following examples show that this assumption is not always correct.
On a recent project, LWF was appointed as the fire-engineering consultant for a multi-storey department store. The height of the building is 15m and no sprinklers were fitted. In accordance with Approved Document B, the minimum level of fire resistance is 60 minutes. Following the guidance in BS 9999 with a risk profile B3, fast growth fire rate and ventilation conditions met, this would increase the structural fire resistance to 75 minutes.
Another example would be an office building with a façade height less than 5m and ground floor area less than 1000m2. In accordance with Approved Document B, the required period of fire resistance is 30 minutes, with or without sprinkler protection. On the contrary, for an identical building designed in accordance with BS 9999 with a risk profile A2, medium fire growth and not fitted with sprinklers the required fire resistance is 15 minutes. Potentially, fire resistance for the structural frame is not required because most of the hot rolled structural sections will achieve 15 minutes fire resistance without protection.
Another type of development, that is not unusual, is an open plan office building with a building height between 30m and 60m. Sprinklers are required because the building is over 30m in height and the minimum level of fire resistance for the structure would be 120 minutes in accordance with Approved Document B. For an identical building design, BS 9999 with a risk profile A2, medium growth fire rate and sprinklers provided would allow the structural fire resistance to be reduced to 75 minutes.
Based on the examples above, it can be concluded that the adoption of the newer standard BS 9999 shows that it can be used but may not always give a benefit to the designer in terms of reduced fire resistance requirements i.e. some building types have the potential to reduce the structural fire resistance and some will enhance the level of fire protection.
The Fire Safety Engineering Approach
It is collectively agreed in the fire industry that BS 9999 has advanced a level closer to fire engineered, performance base approach and it has the potential to provide invaluable benefits to the design team. However, for some social, technical and economic reasons, clients may have a desire to apply a fire engineered design/performance based approach to their buildings. LWF, who has a great deal of experience in fire engineered approaches has, in the past, applied the concept of time equivalence in accordance with BS EN 1991-1-28 or PD 6688-1-29 in order to reduce the required structural fire resistance in buildings.
The time equivalent method determines the required fire resistance period for a design based on compartment size and geometry. This method is a series of computations and compares the severity of a real fire to an equivalent period of exposure in a standard furnace test. The required input parameters for calculating the time equivalent analysis are fire load, compartment size (height and floor area), ventilation conditions and thermal properties of the compartment linings. In addition, assumptions are incorporated into the calculations and these are:
- The calculations are for post flashover fully developed fires
- Ventilation openings are provided by non-fire rated elements in the façade. The façade will not be intact under post flashover conditions.
- Internal partitions are constructed by non fire resisting construction
- The fire service won’t intervene on the fire growth
Once the input parameters are defined and the design fulfils the inherent assumptions, the assessment of time equivalent method and formulation can be verified as:
te,d = (qf,dkbwt)kc
te,d is the equivalent fire resistance (minutes)
qf,d is the design fire load (MJ/m2)
kb is a conversion factor dependent on thermal properties of linings
wt is the ventilation factor
kc is the correction factor dependent on material
The fire engineered approach is in contrast to the guidance given in Approved Document B in which each element of structure is assumed to be heated with no fire load, ventilation and compartment size considerations. By applying the fire engineered approach, the required fire resistance for compartments with high fire load and small ventilation openings will be greater than compartments with good ventilation conditions.
The fire engineered approach will be subject to approval from the approving authority and the local fire services. However, LWF have previously agreed similar approaches in UK and other projects abroad.
We hope to have provided an overview to the process when designing structural fire resistance and three methods that are available in the UK i.e. Approved Document B, BS 9999 and fire engineered approach (time equivalent method). It must be noted that all buildings will need to have adequate fire separation and in order to determine which approach is the most suitable one for a specific building, it is essential to consult with the fire engineer at an early stage of the project.
Considerable benefits can be given to a design through the use of either BS 9999 or fire engineered approach where the reduction of required structural fire resistance can often provide significant savings as well as design flexibility, and aid the design team to fulfil more creative aspirations.
If you would like to consult with Lawrence Webster Forest over the content of this article, or your project, please contact Peter Gyere on 020 8668 8663.
LWF are fire engineering and fire risk management consultants with over twenty-five years experience in the development of fire engineered technology and the application of fire safety standards including fire engineered techniques.