Thanks Roland, So following your train of thought, an attic truss with 2x4 top cord is unobstructed and a different truss with a 2x6 (or larger) cord is obstructed. Now for the fun part, how do I (we) justify this to the AHJ who says that's not how it's in the book? The AHJ on this project will accept this, but there are others who will want to fight this tooth and nail.
Thanks again, Bob Knight -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Roland Huggins Sent: Thursday, June 21, 2007 12:32 PM To: [email protected] Subject: Re: Wood Truss Construction Your ending question hit the nail on the head. It does NOT matter what you call the structural member but as the definition says, whether it impedes heat flow. If the top chord is deep enough to be readily identified as obstructed when it is a joist then it IS obstructed construction. The root cause of our problem is that there is nothing that quantifies how deep is too deep for unobstructed. I attempted to get this straightened out in either 99 or 02 edition and the TC response was that it is defined in the Annex. When I pointed out with a comment what the Annex quantities and it isn't depth DOWN from the ceiling the TC still failed to comprehend. You can lead a horse to water but you can't make it drink (DROWNING it is an option though). The annex identifies 4 inches deep as part of what differentiates a joist from a beam (rightly called thickness and width in this section) and a bar joist from a truss (called depth in this section) but it is the distance ACROSS the structural member. Nothing discusses distance down from the ceiling. Few would call a 2 x 6 joist unobstructed but what about a 2 x 4 stringer? AS a REASONABLE point of demarcation, I use the 2x4. Looking at the figures in the annex of a Truss, it looks like a 2x4. We know some amount of protrusion is acceptable and a 2x6 joist is generally accepted as obstruction so WALLAH, a 2x4 is unobstructed and anything beyond it REGARDLESS of the name of the member is obstructed (assuming the spacing is as defined of course). Being a member of 13 and discussing such an ambiguous issue, I have to state the NFPA caveat that this is not to be considered a formal interpretation of the NFPA or its technical committees . Roland On Jun 21, 2007, at 9:59 AM, Bob Knight wrote: > I have a situation in an attic space that is using a standard wood > truss w/ > 2x6 top cords, but in addition to this there is another 2x6 > sistered to the > top cord so that the top cord is now 11" deep. Per 13, truss > construction > is considered unobstructed, but I'm having a difficult time > considering this > unobstructed. This is a dry system. If this is unobstructed > construction, > then the deflector is to be 1" to 12" down which leaves only 1" of > play for > sloping the branch lines. At a 1/2" per 10' there is not enough > play to > provide proper slopeage. Can this be considered obstructed > construction due > to the depth of the double top cord? > > As a side question, I've never understood why a truss with 2x4 or > 2x6 top > cord is considered unobstructed when a joist that is a 2x4 or 2x6 is > considered obstructed. What's the difference? > > Thank you, > > Bob Knight, CET > (208) 495-2057 > [EMAIL PROTECTED] > www.firebyknight.com > > > No virus found in this outgoing message. > Checked by AVG Free Edition. > Version: 7.5.472 / Virus Database: 269.9.1/854 - Release Date: > 6/19/2007 > 1:12 PM > > > _______________________________________________ > Sprinklerforum mailing list > [email protected] > http://lists.firesprinkler.org/mailman/listinfo/sprinklerforum > > To Unsubscribe, send an email to:Sprinklerforum- > [EMAIL PROTECTED] > (Put the word unsubscribe in the subject field) > _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/mailman/listinfo/sprinklerforum To Unsubscribe, send an email to:[EMAIL PROTECTED] (Put the word unsubscribe in the subject field) No virus found in this incoming message. Checked by AVG Free Edition. Version: 7.5.472 / Virus Database: 269.9.1/854 - Release Date: 6/19/2007 1:12 PM No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.472 / Virus Database: 269.9.1/854 - Release Date: 6/19/2007 1:12 PM _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/mailman/listinfo/sprinklerforum To Unsubscribe, send an email to:[EMAIL PROTECTED] (Put the word unsubscribe in the subject field)
