Reference
Fire Ratings
Understanding Fire Ratings of Assemblies - Resistance is Futile
By Mark Hughes, P. Eng.
© 2003 Sintra Engineering Inc.
Most fire investigators and those involved in the construction industry are familiar with the fire resistance ratings of different types of assemblies, but few understand how these ratings are developed and their true significance.
In general, the fire resistance rating of an assembly refers to the relative rating of a wall, door or roof structure when exposed to a fire. For example, the Alberta Building Code may require a particular wall to be constructed with a 45 minute fire resistance rating. On the surface it would appear that this rating gives you 45 minutes before the fire goes through the wall but in practice this is not the case. What the fire resistance rating gives you is the ability to compare different assemblies (e.g. walls) to see their relative ability to withstand a particular fire (e.g. A 3/4 hour rated assembly will last longer than a 1/2 hour rated assembly).
Fire resistance ratings were initially developed in the 1920's as a method of assessing the vulnerability of different construction methods to fire. To compare different assemblies, a standard fire exposure was developed. In a test furnace, fire is imposed on the assembly and the temperature is measured at a location six (6) inches away from the assembly. This temperature is controlled to match a mathematical curve that is the standard fire curve. All assemblies are exposed to this heat/temperature profile and the resistance ratings are based upon the performance under these fire conditions. These curves form the basis of American Society of Testing and Materials (ASTM) Standard E119 for testing and rating construction assemblies.
The problem with the Standard ASTM E119 fire curve is that no fire in nature ever looked like this temperature profile (or ever will). The standard curve increases in temperature until the assembly fails. In real life, a fire will increase in intensity and eventually die out as the fuel being burned is consumed. As there has never been an instance where the ratings for assemblies rated this way have failed before the rated values, the technique for testing has stuck even though the real life performance is different.
As the assembly rating only gives a relative understanding of how an assembly breaches, it may or may not provide an understanding of what happens when the assembly is breached. For example, a 1" plywood covered wall stud assembly may have a fire rating of 20 minutes and a 1/2" type C drywall assembly may have a similar rating, however, when the plywood is breached, it is a fire load, when drywall is breached, the drywall is not a fire load, only the objects behind the wall are.
As an investigator, when considering fire spread through wall assemblies,
it is important that the investigator not rely on the rating as a sole measure
of how long a fire takes to spread through a building or home. The ratings
only provide a relative indicato.
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