I like Brad's point here. I think we all tend to try to visualize what is happening in a fire and fluid mechanics are far too complicated (both the reality and the mathematics) to just "picture" in our heads. We also seem to have a tendency to simplify this picture into several erroneous categories: That the head bis supposed to extinguish a fire rather than to contain it to 1500, 2000, etc. sqft, that heat spread is uniform like an upside down version of pouring a viscous liquid out of a spout onto a flat and level surface, that heads go off when the air temperature reaches the heads temperature listing and that that heat is uniform throughout the design area, that there actually is .01 gallons per minute hitting each square foot of the head's area of coverage evenly throughout each minute of discharge, that 'overflowing," whatever that is, is a bad thing, that the head somehow meters the water and distributes it appropriately exactly per the calc in a perfect rectangular, no matter what rhombic we use, pattern, that all shadowing will result in disaster, I can go on.... I would suggest that when using prescription one should assume that it works, regardless of whether it exists because of observed long-term success, testing, a manufacturer's lobbying to force the sale of a product or a bit of someone's bad potato, and just try to apply the rule rather than try to mind picture the event because you'll invariably bring only a partial picture into play even if you're one of those "grand old men" of the industry. Interpret the rule rather than your vision and ignore intent unless you have a dispute, either with yourself or with an outside authority. For a good example of how ridiculous we can look when trying to apply four decimal places to variable event I suggest attending a lecture on the precision of water availability data gathering and calculating by Steve Scandialato. Mind changing. Particularly for theoretical types with lots of engineering education, or high school mathematicians for that matter. (Sent without editing--a big mistake but I have to convince fresh minds that 2.2 x 2.2 cannot equal 4.84 in real world applications.)
On Thu, Feb 11, 2010 at 6:31 AM, Brad <[email protected]> wrote: > Response Time Index is based on the temperature AND velocity > of the heat- the faster any head is receiving heat the faster > it will activate, Quick or Standard Response. If a head is in > an area where it has to "wait" for the heat to collect and bank > down for it to heat up enough, the emergency is not in that area. > > -----Original Message----- > From: Michael J. Ricks [mailto:[email protected]] > Sent: Thursday, February 11, 2010 8:04 AM > To: [email protected] > Subject: Beam obstruction rules > > It really depends on the location of the fire plume in relationship to the > heads and where the jet will be directed by the roof/ceiling elements. When > I said the AHJ "may" have a point. I was responding in the hypothetical. > It the original post the second head would have been 10' away from the first > beam and 19' away from the wall. Since fire plumes go straight up and then > outward, if the fire were 11' feet away from the wall, two feet away from > the first beam, chances are the first head located on the other side of the > beam which is covering that part of the floor would not fuse. I assumed > that there were more heads spaced along in the same beam pockets all spaced > the same distance away from the wall. My guess is the row of heads 19 feet > away from the wall and 8' away from the fire plume in the second beam pocket > would all fuse before the head that is only six feet away. We also don't > know what's past the second beam-since he said beam and not wall, what's > happening past that point. I would need to see the plans to know what I > really felt. My point is that just because you meet deflector distance (I > do realize that his deflectors are less than 12" per the unobstructed rules) > and obstruction rules, based on 8.1.1 and the fact that NFPA 13 doesn't > address heads above the bottom of beams in unobstructed construction, your > design may still be deficient when you look at the head layout in relation > to fire sprinkler performance theory. > > As sprinkler designers, we often fall into the trap of thinking covering the > floor area is what is most important. It is very important to look at our > designs with regard to response time as well. Where will the heat go when > there is a fire? Unfortunately, fires don't always start directly below our > sprinkler heads. Unless there is a breeze in the building, the heat will go > straight up and then outward from there. If there is nothing to cause it to > collect it will follow the path of least resistance. It doesn't care if > there is a sprinkler head 6 feet away on the other side of a beam. It might > as well be a wall. > > Personally, I would ere on the side of caution and space to the beams in > this case. > > Mike Ricks > Katco Fire Sprinkler Design > > 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 >> >>mike at fire-sprinkler-design.com > <http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum> >> >> >> >> >> >>_______________________________________________ >>Sprinklerforum mailing list >>Sprinklerforum at firesprinkler.org > <http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum> >>http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum >> >>For Technical Assistance, send an email to: support at firesprinkler.org > <http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum> >> >>To Unsubscribe, send an email to:Sprinklerforum-request at > firesprinkler.org > <http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum> >>(Put the word unsubscribe in the subject field) > > Todd G. Williams, PE > Fire Protection Design/Consulting > Stonington, Connecticut > www.fpdc.com > 860.535.2080 > > > > > > 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) > -- Ron Greenman InstrucRon Greenman, CET, FPE Instructor Fire Protection Engineering Bates Technical College Tacoma, WA Member: SFPE, ASCET, NFPA, AFSA, NFSA AFAA, NIBS, WSAFM, WFC _______________________________________________ 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)
