Interesting analysis and welcome to the jungle. However, if you read the handbook commentary it would seem that your concern about ceiling jets is addressed in the definitions of obstructed and unobstructed construction. I am far from an expert on the behavior of ceiling jets, but I would think in this situation as described, that the sprinklers as positioned, especially in a 17 ft span, would be within that jet.
At 09:52 PM 2/10/2010, you 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) Todd G. Williams, PE Fire Protection Design/Consulting Stonington, Connecticut www.fpdc.com 860.535.2080 _______________________________________________ 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)
