Jim,

What is the path of travel for the products of combustion to enter the attic from the bedroom?

What is the ceiling constructed of through its depth from bedroom to attic?

One could nominally calculate the energy or heat release rate based on surface area. From there, one can calculate the mass flow of the products of combustion through a plume into an upper layer and then through a flow path to the attic. In order for the smoke layer to descend in the attic to the level of the soffit, the area of the inflow must be much greater than the outflow area. More inflow than outflow (conservation of mass) creates an accumulation of mass within the attic and the how buoyant layer descends towards the level of the soffits. I am using the term "soffit" to be synonymous with eves or the lowest level of available ventilation in a traditional attic. Same holds true to soffits being located at the upper portion of the attic without the time delay of the smoke layer descending within the attic.

In general, these are "hand calcs" that can provide a bounding analysis that can be used to test the hypothesis of the smoke reaching the soffits at some prescribed time. If a flow path through a ceiling is present with flow from a ceiling layer, this complicates the physics and math to a point where an appropriate zone model, such as CFAST, may need to be used. Other specific complications may require the use of a more detailed CFD model or small-and large-scale testing.

Given that a specific time frame is provided as part of the technical basis for this opinion, I would say that "calculations" would be required to provide this level of both reliability and resolution with respect to the one minute and 15 second time frame.

Unless he/she has data of a scenario that is "substantially similar" to the conditions present in this fire, the use of testing may be prone some significant level of non-relevancy and unreliability. I doubt he/she has considerable experience with observing fires on beds with a small amount of gasoline that show smoke at soffits within one minute and 15 seconds. How do you assess the relevancy and reliability of his fire department experience in substitution of the reliable application of fire dynamics?

With that said, a basic understanding of the conditions necessary to thermally breach a gypsum wallboard ceiling, to say nothing of the time to reach those conditions in a fire, would far exceed the time frame of one minute and 15 seconds. This dopes not include any time associated with the growth and development of the fire on a mattress with "bed clothes" which can be significantly longer than a bare mattress. While this does not require putting pencil to paper for performing a series of "hand calculations," knowledge of fire dynamics can easily provide a reliable technical basis that either is consistent with this hypothesis or disproves the hypothesis. This knowledge can be based on the use and familiarity of the results of different calculations that can be applied.

For example, generally speaking, it would take nominally 1000 kW peak HRR to flashover the compartment to achieve the required thermal exposure to comprise the gypsum wallboard. The time to flashover in this room would be nominally 3 to 5 minutes. The time to breach the ceiling would nominally be about 10 minutes. Thus, a nominal time of 15 minutes would be required to just breach the ceiling, assuming one layer of 3/8 inch gypsum. Even with any consideration of uncertainty, this would disprove the hypothesis that it would take one minute and 15 seconds.


J L Mazerat Wrote:
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> Fire originated in a bedroom. The bedroom is
> 12X12. The wall covering is ⅜ gypsum board.
> The ceiling is fiber tiles. The investigator is
> saying the smoke produced from a fire on the bed
> spread into the attic and out of the soffits
> within a minute and fourteen seconds. The
> investigator is saying a small amount of gasoline
> was used as the first fuel ignited. He gives no
> idea as to then amount of gasoline. When asked if
> he tested his hypothesis, he states he did no
> calculations even though he knew they were
> available but used his own experience as a
> firefighter and observations of fire spread.

Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com