SFPE Discussions

Enclosing Rectangles and Subfloor Space

Consider a simple single level structure with suspended timber floor on piles and subfloor space. The floor being the lowest floor above ground is unrated as permitted under C/AS2 paragraph 4.14.1. When undertaking an enclosing rectangles analysis of an unrated wall under C/VM2 to determine distance separation requirement in terms of a relevant boundary, is there any requirement to include the subfloor space when measuring the height of the rectangle?

The enclosing rectangle method is a simplification of a radiation calculation where you are trying to determine if you have less the 30 kW/m² at the boundary and less than 16 kW/m² 1.0 m beyond the boundary. If you think that the above floor fire could easily spread to a subfloor space that has a fire load that would concurrently radiate with the above floor fire then yes I would include it in the enclosing rectangle method.
If your subfloor space has no fire load to speak ie is not going to generate subfloor fire compartment temperatures in the order of 800°C to1000°C then no I would not include the subfloor space in the enclosing rectangle method.
Robert Peart

Thanks for your response Robert - appreciate your common sense perspective.

While Robert’s comment is a common sense approach, where the radiating area is matched with the risk, the enclosing rectangle method refers to the bounds of the firecell. Since the subfloor space is part of the firecell above, then by the instructions for it, you would have to include the subfloor space.
To avoid this, you would have to be sure the subfloor space is fire sterile and going to stay that way, due to the size, height and access of the space since subfloor spaces are great places to keep old wood, lawnmowers and other junk.