OK, here's what I know,to get this chat started off ...
UL, FM, and others publish various directories of fire resistant construction assemblies. These are -generally- descriptions of assemblies that have actually been tested.
Strictly speaking, the ratings are only for the assembly AS TESTED. The directories leave you in the dark should there be any variation from the test sample. For example, what if the wall is 23 ft. tall, rather than the 8 ft.(IIRC) sample?
(My personal favorite: ALL the test samples have the drywall going 'up and down,' rather than laying on its' side. Yet, everyone lays it horizontally. Strictly speaking, this is no longer a rated assembly!)
To address this, manufacturers have their own design recommendations, and trades have their practices.
In theory, the construction details are spelled out in the plans, and the architect or engineer has used his "Professional judgement" to say that the rating is maintained if the plans are followed.
The "counterpoint" to the 'strict interpretation' is that, after looking over a couple thousand rated designs, you begin to see some patterns developing. For example: drywall is drywall, regardless of make, and only thickness counts. Or, that there is no difference in ratings whether you use wood studs or steel studs.
What DOES matter is whether the design assumes the fire will be on one particular side of the wall. The garage wall is one such example. On such a wall it does matter which side of the wall has the requisite drywall thickness. Building a 'box' around your inset panel can only work if the sides of the studs are also protected - both at the panel and where the wires penetrate them. This is also a concern if the wall is also used as a shear wall.
Another bugaboo regards those putty pads. Here the building code varies somewhat from the test data. The test data said 'no problem with 4-S's, yes problem with 4-11's, one per stud bay please.' The code folks re-wrote this to say '4-S's need the pads too - and the boxes need to be 2-ft. apart.' The code makes no distinctions between an opening with a metal box a plastic box, and no box at all.
Finally, it is common for local rules to specify construction methods, and these descriptions often have no relationship to any particular rated assembly.
Edited by renosteinke (12/13/10 02:38 PM)