THE SKY’S THE LIMIT?

Over my career, logistics buildings have been growing in footprint and in height. The days of the 10,000m² warehouse with a height approaching 12m are a distant memory along with the vision of a fleet of tall slender automated warehouses exceeding 20m in height. However, as the scale of warehouses has grown, the understanding of fire protection challenges and the relevant regulatory guidance has often lagged behind.

Take regulatory guidance in England as an example.  This week I heard a discussion about new projects where the steel portal frames had a ‘haunch’ height of 18m. The haunch typically signals the usable free height of the building, so what surprised me was how many people were unaware the regulatory guidance sets an 18m ceiling height for a storage building without sprinklers. The buildings being discussed would all exceed that limit. So, if used as a warehouse, the guidance would trigger a sprinkler requirement, with significant consequences for structural loadings and fit out. Given the limited demand for factory buildings at this height, it is clear the main market for these buildings is warehouse use.

I’ll concede that Table 8.1 in Volume 2 of Approved Document B (ADB) takes a bit of studying but it has been in place for nearly 20 years. The table is confusing because it uses two different height-based metrics, each defined differently, in the same table. But read carefully and the limits are clear. My prior assumption was that many buildings stayed below the 15m haunch height to ensure they did not breach this 18m ceiling limit and trigger the sprinkler requirement. 

It is important to note the trigger is not related to a volume or compartment size. Compartmentation does not deal with the height issue and it is also technically demanding to deliver in steel portal frame structures.

There are limits to fire protection

In the same conversation, I found myself explaining that current standards for active fire protection did not include ‘ceiling-only’ solutions for warehouses at 15m let alone such a higher building height. To be clear: those operating storage buildings would prefer ceiling-only sprinklers as a solution as it gives them more freedom in how they configure their storage. Traditionally, protecting open-rack storage structures has required a combination of ceiling sprinklers and in-rack sprinklers, quite literally sprinklers running within the racking.  Obviously, this can limit the flexibility of storage arrangements.

From a fire behaviour perspective, a modern storage array is an ideal configuration for rapid fire growth. Fires in such storage arrays quickly become impossible to fight with manual means alone and often the fire will be too large before a fire service can intervene. 

If the outcome is to protect the array then active fire protection is needed. To design such a scheme takes a careful consideration of what commodities are stored, how they are stored, any obstructions within the racking systems or indeed near to the sprinkler heads themselves.  To appreciate the challenge, all of that then has to be coupled to a hydraulic design capable of delivering the right quantity of water quickly enough to suppress or control the fire.

Large-scale fire testing and research shows that it can be achieved using larger orifice sprinklers delivering substantial amounts of water. However, the research into ceiling-only sprinkler system designs mean that there are height limits. For certain types of storage, there are no designs at a ceiling height above 15m and for others the limit is closer to 12.5m. These heights are well below the ceiling heights proposed for a number of speculative builds.

Similarly low levels of storage in a tall building are equally challenging to active fire protection. It takes time for the fire to cause the sprinklers to operate and when they do, more are needed. The options to deal with this are to significantly restrict the levels of storage and spread them out. If the stated aim of the project is a warehouse it is a bit of challenge.

The consequences of higher buildings is that they will require specific automatic sprinkler designs to protect them.  Occupiers will be faced with decisions over layouts and perhaps compromises over their storage arrangements. If the structural design loads for the fire protection are not considered there may well be a clash with the ability to place active fire protection in the building in the first place. The industry may feel that the sky’s the limit, but our regulatory guidance and fire-protection capability needs to be considered in these buildings. We can’t afford to allow ignorance to define a fleet of buildings that cannot be properly protected, leaving them as stranded assets or even worse compromising the safety of both occupants and firefighters.

Image by THAM YUAN YUAN from Pixabay