Building control: fire engineering analysis
A new approach
27 September 2013
Richard Rankin explains how fire engineering analysis can limit external fire spread
Those who believe that installing a sprinkler system or introducing fire-resisting cladding to the façade is the only way to limit external fire spread can think again. On a recent warehouse project, WSP was able to use fire engineering analysis to demonstrate that the risk of external fire spread to an adjacent building was adequately mitigated without resorting to such measures. A specific radiation analysis was carried out using in-house software to determine that the level of radiation received at the boundary was within the limits recommended in Approved Document (AD) B.
The warehouse is used for the storage of car parts and comprises a single-storey unit measuring 84m x 84m x 6.5m with some office and mezzanine storage facilities. It is constructed of a steel-frame metal-cladding system with brick facing around the office area, which is segregated from the main warehouse and represents a separate fire compartment, has a total area of around 5,600m2 and includes low- and high-level rack storage.
To satisfy functional requirement B4 of the Building Regulations, it must be demonstrated that the external walls of a building will adequately resist the spread of fire
To satisfy functional requirement B4 of the Building Regulations, it must be demonstrated that the external walls of a building will adequately resist the spread of fire, and the design must ensure that the fire will not extend to the adjacent property. Where buildings on the same site are operated and managed by the same organisation they usually do not need to be assessed for this risk, but they do when under different management or ownership. Standard guidance documents such as AD B or BS 9999 recommend using the methodology in BR 187 to determine whether the distance to the boundary is acceptable or what percentage of the façade needs to be fire resisting. An initial assessment using this methodology suggested that either 50% of the façade needed to be fire resisting or a sprinkler system would need to be installed to allow the boundary distance to be doubled.
However, it is important to consider some of the assumptions in the guidance methodology that, particularly in large-volume buildings such as warehouses, can lead to onerous requirements in terms of minimum distance to the boundary or limitations on unprotected openings. One key assumption is that flashover has occurred and the entire façade of the fire compartment is radiating at a defined intensity based on the purpose group of the building. For a warehouse, the radiating element is assumed to be at an intensity of 168kW/m2, equating to 1,031°C. To put this in perspective for this building, the standard guidance methodology assumes an 84m x 84m x 6.5m rectangle radiating at a uniform temperature of 1,031°C.
For flashover to occur, a build up of heat and gases needs to develop within an enclosure. Due to the physical size of the warehouse, a fire size of approximately 150MW would be required (estimated using Thomas’ Flashover criteria, which estimates the heat-release rate needed based on the geometry and ventilation conditions of the compartment). On this calculation, the scale of fire would consist of five articulated truck and trailer units fully involved in a fire. This is an unrealistic size for a warehouse that stores non-combustible metal car parts in a limited amount of packaging and palleting.
For flashover to occur, a build up of heat and gases needs to develop within an enclosure
Likely failure modes of the non-fire resisting structure were considered. The type of roof construction consists of an unprotected metal-cladding system incorporating unprotected lighting, which also has an impact when considering flashover. A large fire in the order of 10MW-20MW would cause the roof to suffer local failure and collapse (given that the racking extended to within 0.5m of the roof). This would:
- provide ventilation to the warehouse to remove the hot smoke, reduce the build-up of heat and gases therefore the likelihood of flashover
- increase the fire size necessary for flashover to occur. This would subsequently cause more of the roof to fail, providing more ventilation.
On that basis, it is considered that the worst probable fire scenario is a localised fire involving a portion of the façade and not a fully involved post-flashover fire. Once we determined that flashover was unlikely to occur within this building, the next step was to challenge the assumption that the temperature of the radiating element is 1,031°C, which assumes post-flashover conditions within the compartment. Babrauskas (Ref. 1) found that fires in warehouses with rack storage had an average solidflame temperature of 870°C. The average temperature at the visible flame tips was 450°C, but the range was between 300°C-600°C. To give a level of conservatism, the solid-flame temperature of 870°C was used.
WSP carried out specific fire analysis using in-house software, which calculated the level of radiation received at the boundary for a number of fire scenarios. This demonstrated that in all cases the radiation intensity received was less than 12.6kW/m2 (as recommended in AD B). Therefore, by challenging the assumptions in the guidance documents and carrying out specific fire engineering analysis, WSP could demonstrate that the building complied with the Building Regulations. This novel approach allowed the client to omit the sprinkler system without the need to provide fire resistance to the façade, leading to a significant cost saving and more sustainable building.
This approach is not limited to warehouses. A fire engineer can assess the fire-loading and geometry of a variety of building types to determine whether flashover is likely to occur and hence whether the assumptions in standard guidance are valid. If not, then a fire-engineered solution could allow a greater percentage on unprotected openings than a standard guidance approach, reducing cost and increasing design freedoms.
Richard Rankin is an Associate at WSP Group