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Permalink Reply by Jack/dt on January 23, 2012 at 5:43pm

Devan,

I hope I didn't further confuse you, after re-reading I may not have been as clear as I had intended.

Legacy construction is dimensional lumber construction, the familiar 2x4, 2x6 et.al construction method.  Where I may have confused you is that there is no (as of yet) real distinction between legacy and lightweight construction. More over, some writers  define "legacy" as that construction where wooden members were solid, dimensional (a full 2"x4" etc) as opposed to the "nominal" (1 3/4"x3 3/4" or less) lumber used after WWII.  Other writers define legacy as the more modern, typical, "nominal" 2x4 lumber.  It can get confusing.  But I don't believe there is any significant difference in burn-through time between a dimensional 2x4 versus a nominal one.

It is very common to have a new construction (Type V, Wood Frame) incorporate lightweight materials/methods within a legacy-type stick framed building.  In my area ALL new construction (past 15 years at least) has incorporated lightweight materials such as roof truss systems, floor truss systems as well as now the lighter lightweight I-Beam joists.  Exterior and partition walls are STILL dimensional lumber, drywall is still the finish wall system of choice. (Although there are SIP wall and roof systems, yet another charming building method)

Where the hazard arises with the use of lightweight construction is, for example, where wooden I-Beam floor joists are used and are NOT covered with drywall (unfinished basement), fire/heat impingement on the exposed joists (typically 2x2 or 2x3 spruce chords with 1/2" OSB plywood web) WILL cause them to fail much more quickly than standard, dimensional 2v8/2x10 DF floor joists.  Compounding a faster fail time, floors made of wooden I-Beam joists are reported to exhibit little or no flex prior to collapse, whereas dimensional (legacy) joists, as they are weakened by fire will cause to the floor to bounce or sag.

Another issue with lightweight construction, specifically roof trusses is, because of the way they are constructed (essentially a series of wooden triangles joined with metal nailing plates -gussets, the wooden members typically 2x4), should flame or intense heat impinge on the exposed metal gusset plates, they will soften, the metal will also absorb and transmit heat into the wooden members, scorching the wood and further weakening and hastening the gussets to fail. 

Once ONE wooden member of a truss fails, that entire truss is compromised.  The strength of a truss depends upon the integrity of the entire unit.  This is NOT to say, should one or two trusses fail due to heat/flame impingement the entire truss roof system is compromised:  There have been incidents where individual trusses failed but the rest of the roof remained intact.  It's load bearing capacity may be greatly diminished but it can remain standing, tied together with the roof sheathing and roofing materials.  It may not be the safest place to be but it doesn't mean there is imminent collapse either.  It really depends upon the structure and the location, intensity and duration of exposure to fire.

A Type V Wooden Frame SFD, either wholly of legacy construction or incorporating lightweight materials/methods will not have the integrity and longer working time of heavy timber, timber framed, post and beam type structures.  This is due to the massive timbers used (typically, 4x6 or 6x6 posts, 4x8, 4x10, 6x8 or 6x10 beams and other equally large wooden members.  These larger timbers, simply due to their mass will retain their structural integrity for a much longer time (hours) rather than the an hour, or fraction of an hour that dimensional and lightweight members can withstand.

I'm pretty sure it takes the same temperature to produce ignitable vapors be it 1 3/4" stud or 6" post.  It's really just a matter of how quickly a wooden member will be consumed (or weakened to its failure point). On the other hand, modern furniture, fabrics and other petroleum based products may have lower ignition temperatures.  So along with MORE furniture and accessories comes not just a greater fire load, but a hotter one.  Flashover STILL occurs at roughly the same temperature regardless of contents, it's only that flashover will occur FASTER due to the high fire load and combustibility therein.  So having less time to work a room and contents, or a room and contents becoming a structure fire is more a result of the contents than the room. (building) construction.  But once the protective covering (drywall) has been impinged and failing then those lightweight structures WILL fail faster and generally, more catastrophically.

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Permalink Reply by Devan Mills on January 23, 2012 at 7:52pm

Hey Jack, i copy and pasted that, and saved it into my word documents i hope you do not mind ( not for reproducing as my own words, just something study on during a coffee break..lol) it was very informative, and no your previous post did not confuse me more, this just takes what you had said and broke it down more in depth, thanks for that. were covering building construction in my probie class last/this week i was kind of laid back, having already taken the class thought i knew more than our fundementals book would show ( typical young guy attitude.) thank you for the wake up call/correction/explanation. stay safe brother!

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