Good point John. I don't recall anything in the prescription that says you can use less than 1/2" rod on a trapeze supporting 6" pipe. Although I am hanging from more than one rod and I could calculate the distributed loads between them wouldn't that need to be engineered. Or have I missed a provision that allows me to do such a thing while remaining prescriptive?
On Wed, Jun 5, 2013 at 2:18 PM, John Denhardt <[email protected]>wrote: > Rahe - I am so confused. This is a requirement of the standard already. > Are you requiring that they comply with the standard? That would be a > great starting point! > > John > > John August Denhardt, P.E., FSFPE > Strickland Fire Protection Incorporated > 5113 Berwyn Road > College Park, Maryland 20740 > Office Telephone Number: 301-474-1136 > Mobile Telephone Number: 301-343-1457 > FIRE SPRINKLERS SAVE LIVES - Can you live without them? > > > -----Original Message----- > From: [email protected] [mailto: > [email protected]] On Behalf Of Rahe Loftin > - 7PCE > Sent: Wednesday, June 05, 2013 5:04 PM > To: SprinklerFORUM > Subject: Re: required section modulus (it don't get no easier) > > Ken, > You might mention that if one is trying to support a 6 inch pipe, requiring > 1/2 inch all thread rod with a trapeze using 3/8 inch all thread rod, as > the 6 inch pipe is moved from the middle of the span, one of the 3/8 ATRs > will begin to be supporting more of the 6 inch pipe load. At some point, > the 3/8 inch rod will no longer share the load properly leaving one of the > rods trying to support too much of the load. Even if the section modulus > is correct, I always have the contractor use 1/2 inch ATR to support the > trapeze if they are trying to support 6 inch pipe. > > * > * > *Thanks* > * > * > *Rahe Loftin, PE * > *Fire Protection Engineer (AHJ)* > *General Services Administration* > *Region 7 - TX, OK, NM, AR, LA* > *Design & Construction 7PCE* > * > * > *c - 817-371-3102* > *o - 817-978-7299* > > > On Mon, Jun 3, 2013 at 9:59 AM, ParsleyConsulting < > [email protected] > > wrote: > > > Whoops, forgot that to get to 1.09 for the required section modulus, you > > first have to divide PL by 4, then by 15,000. See how easy it is for > even > > a smart aleck like me to make a mistake? The answer was correct, but the > > explanation was flawed, that's why my teachers always asked to show the > > work. > > > > 65,696.4divided by 4= 16,421 and that divided by 15,000 = 1.094 > > > > Forgetting a step like that got me much lower grades. > > * > > > > PARSLEY CONSULTING > > > > * > > > > Ken Wagoner > > > > 760.745.6181 voice > > > > 760.745.0537 fax > > > > website: www.ParsleyConsulting.com <http://www.ParsleyConsulting.**com< > http://www.ParsleyConsulting.com> > > > > > > > On 6/3/2013 7:51 AM, ParsleyConsulting wrote: > > > >> Brad, > >> Part 1 - Gibberish. Pure and simple. The tables in NFPA 13 didn't > >> reduce the "force" and "distance" by half in their calculations. It > simply > >> used the formula to calculate the maximum moment at the point of the > load > >> for a simple beam, based on the maximum length of various pipe sizes > >> between hangers. That's why the figures for 1" and 1-1/4" are based on > 12' > >> between hangers. > >> The maximum moment is the "worst case", and was what the committee saw > as > >> providing system designers with enough information to allow them to not > use > >> trapeze members which would have excessive loads placed on them. > >> The formula to determine maximum moment is PL/4, with P=weight of > >> pipe being supported in pounds (including 250#), and L=the length of the > >> span of the trapeze in inches. The moment is then divided by the maximum > >> allowable bending stress, which for steel is 15,000. > >> The resulting equation looks like this: (PL/4)/15,000 = required > >> section modulus. So, for 15' of 1-1/2" sch 40 pipe, the value of P would > >> equal 15' x 3.61 = 54.15 + 250 = 304.15#. For an 18' span, the value > of L > >> = 216". Multiply PxL = 65,696.4, and then divide that value by 15,000, > and > >> the result is a required section modulus for the worst case location of > the > >> load (mid span) of 1.09 in³. That is just slightly less than doubling > the > >> value for a 9'-0" span, however using that method would result in a > larger > >> required section modulus, so it would work out in the end. > >> The 4ab/a+b you reference is a method to calculate the equivalent > >> length of the trapeze, in order to reduce the value of "L" in the > original > >> equation. Going back to your 18' span, if the load were 3' from one end, > >> and 15' from the opposite end, that results in the equation > >> 4x180x36/180+36, which equals 120", or 10'. The value of L in the above > >> equation is changed from 216"to 120", reducing the required section > >> modulus. PL/4 now becomes 304.15 x 120 = 36,498which is divided by 4, > >> resulting in 9,124.5, which is further divided by the allowable bending > >> stress of 15,000 for steel, with a final required section modulus of > 0.6083 > >> in³. Round that up to 0.61, and it matches the value in NFPA 13, Table > >> 9.1.1.7.1(a) (2013). > >> No reduction by half anywhere in those equations. > >> Part 2 - Relatively correct regarding doubling the require section > >> modulus for a 9' span, however as the structural engineers in the room > when > >> this table was expanded to include spans up to 16'-0" notedyou can keep > >> increasing the span to the point where the weight of the trapeze itself > now > >> becomes an issue in the overall capacity to resist bending, and that > sort > >> of calculation was one they were reluctant to provide without a > significant > >> amount of additional information. > >> Please note that this is my opinion only, and does not reflect that of > >> the NFPA 13 Hanging and Bracing Committee. > >> * > >> > >> PARSLEY CONSULTING > >> > >> * > >> > >> Ken Wagoner > >> > >> 760.745.6181 voice > >> > >> 760.745.0537 fax > >> > >> website: www.ParsleyConsulting.com <http://www.ParsleyConsulting.**com< > http://www.ParsleyConsulting.com> > >> > > >> > >> On 5/31/2013 7:36 PM, Brad Casterline wrote: > >> > >>> saw an e-mail the other day about 18 foot trapeze spans. > >>> > >>> wished someone had asked me. > >>> > >>> i would have said---either: > >>> > >>> section modulus is moment divided by stress. moment is half the > force(lb) > >>> times half the distance(in). > >>> > >>> the force is the weight of the water-filled pipe plus 250. the distance > >>> is > >>> 4ab/a+b. > >>> > >>> the stress is 15,000. > >>> > >>> or: double the required section modulus for a nine foot span. > >>> > >>> > >>> Brad Casterline, NICET IV > >>> > >>> Fire Protection Division > >>> > >>> > >>> FSC, Inc. > >>> > >>> P: 913-722-3473 > >>> > >>> [email protected] > >>> > >>> www.fsc-inc.com > >>> > >>> > >>> Engineering Solutions for the Built Environment > >>> > >>> > >>> ______________________________**_________________ > >>> Sprinklerforum mailing list > >>> Sprinklerforum@lists.**firesprinkler.org< > [email protected]> > >>> http://lists.firesprinkler.**org/listinfo.cgi/** > >>> sprinklerforum-firesprinkler.**org< > http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org > > > >>> > >>> > >> > >> ______________________________**_________________ > >> Sprinklerforum mailing list > >> Sprinklerforum@lists.**firesprinkler.org< > [email protected]> > >> http://lists.firesprinkler.**org/listinfo.cgi/** > >> sprinklerforum-firesprinkler.**org< > http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org > > > >> > >> > > > > ______________________________**_________________ > > Sprinklerforum mailing list > > Sprinklerforum@lists.**firesprinkler.org< > [email protected]> > > http://lists.firesprinkler.**org/listinfo.cgi/** > > sprinklerforum-firesprinkler.**org< > http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org > > > > > _______________________________________________ > Sprinklerforum mailing list > [email protected] > > http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org > _______________________________________________ > Sprinklerforum mailing list > [email protected] > > http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org > -- Ron Greenman (Пушечное ядро Ивана 6 дюймов) Instructor Fire Protection Engineering Technology Bates Technical College 1101 So. Yakima Ave. Tacoma, WA 98405 [email protected] http://www.bates.ctc.edu/fireprotection/ 253.680.7346 253.576.9700 (cell) Member: ASEE, SFPE, ASCET, NFPA, AFSA, NFSA, AFAA, NIBS, WSAFM, WFC, WFSC They are happy men whose natures sort with their vocations. -Francis Bacon, essayist, philosopher, and statesman (1561-1626) _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org
