I've got to disagree with your use of 8.1 as a basis to say the standard does not address the issue. Allow me to start with saying that although your logic is reasonable, your outcome is flawed. One piece that was neglected is the part that momentum plays in the ceiling jet. Look at 8.6.4.1.1.3 where a 3 ft ceiling change can be ignored except for obstructions concerns. That's not based on a fire being under the lower ceiling and traveling upward. You are not intended to take 8.1 and attempt to analyze these issue especially from the perspective of comparison to just a flat ceiling. So, 13 does address the described situation by defining the type of construction for dictating the distance below ceiling and then accounting for obstructions in placing the sprinklers. A beam is not a wall and is not to be treated as such. Does it impact activation patterns? Certainly does. You should see the activation pattern in a room with exposed wood joist. The first to open is often not the one closest to the fire but that doesn't change the spacing of the sprinklers.
AS a side note, panel construction is still in the newer editions. We moved it out of the Annex because without something in the body of 13, it technically violated the old definitions. Roland As a member of the NFPA 13 committee, I need to say that the above is just my opinion and is not to be considered an official position of the NFPA (although if you contact the NFPA 13 liaison, I'm confident he will concur that you are over-cooking the stew and that the 13 criteria does indeed apply) On Feb 10, 2010, at 6:52 PM, Michael J. Ricks wrote: > Not to be the contrarian here, this being my first post to this > forum, but, > I believe the AHJ has a point. > > > > Given your beam spacing of over 7'-6" on center, you are dealing with > unobstructed construction here. NFPA 13 8.6.4.1.2, regarding > placing the > deflector above the bottom of the structural member, relates to > obstructed > construction. In obstructed construction, we are able to place > deflectors > up to 22" below roof decks since the obstructions help collect heat to > assure prompt activation of the sprinkler head. (If you remember > your older > versions of NFPA 13, we used to have to deal with the "panelized roof > limitations of 300 square feet to get the added deflector > distance.) NFPA > 13 doesn't specifically address the issue of placing the deflector > above the > beam in unobstructed construction and although you are meeting the > obstruction requirements of 8.6.5.1.2, the issue here is response > time. > 8.1.1 (2) states "Sprinklers shall be positioned and located so as to > provide satisfactory performance with respect to activation time and > distribution." A8.1 also says "The installation requirements are > specific > for the normal arrangement of structural members. There will be > arrangements of structural members not specifically detailed by the > requirements." Just because your head is not obstructed and you > have met > the deflector distance requirements for unobstructed construction > doesn't > mean it will work as intended. > > > > The theory behind automatic sprinkler performance requires that the > head be > in the jet stream or in a pocket that will collect heat in a timely > manner. > With limited obstructions to collect heat, that distance will be 5 > to 12% of > the height of the ceiling above the fire source. This is the > theory behind > the 12" deflector distance rule for unobstructed construction. > Given the > language of NFPA A8.1 and 8.1.1 the AHJ is not off base in being > concerned > with response times. Unless you have a 30'+ ceiling, I would have > concerns > that the jet stream would be cut off by the beams and that the heat > would > not collect fast enough to assure prompt activation of the head and/or > skipping would occur causing head outside of the fire area to > activate prior > to heads in and around the fire area due to the jet stream being > directed > away from those heads by the beams. > > > > Michael Ricks > > Katco Fire Sprinkler Design > > P.O. Box 607 > > 555 South 100 East > > Salem, UT 84653 > > 801-423-3096 Cell: 801-380-4651 > > [email protected] > > > > > > _______________________________________________ > Sprinklerforum mailing list > [email protected] > http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum > > For Technical Assistance, send an email to: [email protected] > > To Unsubscribe, send an email to:[email protected] > (Put the word unsubscribe in the subject field) > _______________________________________________ Sprinklerforum mailing list [email protected] http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum For Technical Assistance, send an email to: [email protected] To Unsubscribe, send an email to:[email protected] (Put the word unsubscribe in the subject field)
