Thank you, Allan, very informative and well put... Jeff Garrison
On Thu, Jun 6, 2013 at 11:30 PM, AKS-Gmail-IMAP <[email protected]> wrote: > No. The 15,000 psi is a maximum allowed stress. It is not yield stress > psi. 30,000 psi yield stress would be for a minimum grade steel and is what > you might use for steel yield stress for some unknown steel grade. You take > that number and apply a factor to arrive at the maximum allowed stress that > you are trying not to exceed. The factor number is proscribed by whatever > steel design standard you are using and that number is different for the > situation in question - as per that standard. The factor is not to be > viewed as a factor of safety as you might be thinking. There are many > things that come into play that you might not realize. Load cycling is a > big one for example. As a general rule steel loaded to no more than 50% of > its yield strength can be repeatedly loaded and unloaded for practically an > infinite number of times before failing. That is why the 0.5 factor has > been used for a long while and may be the origin for the 15,000 number when > steel grades were typically bottoming out as 30,000 yield. > > The next time you are driving your car down the highway think about how > many times you have stressed and unstressed in torsion the steel rod > connecting the steering wheel to the steering gear box and whether or not > someone used a yield or allowed stress number when sizing that bar. > > Allan Seidel > St. Louis, MO > > > > On Jun 6, 2013, at 7:27 AM, Jeff Garrison <[email protected]> > wrote: > > > Something else to remember / revise in your calculations is that the > minimum > > > > Yield Strength of almost all steel pipe today is 30,000 psi, some going > to > > 33 or 35k. > > > > The 15,000 used in the "red book" (how long since it's been red?) is > quite > > out of date. > > > > Jeff Garrison > > Project Manager > > C 586-871-3705 > > Detroit Automatic Sprinkler Co. > > 28501 Ryan Rd., Suite C > > Warren, MI 48092 > > P 586-574-0100 > > F 586-574-0101 > > [email protected] > > > > > > > > > > -----Original Message----- > > From: [email protected] > > [mailto:[email protected]] On Behalf Of > Brad > > Casterline > > Sent: Wednesday, June 05, 2013 5:44 PM > > To: [email protected] > > Subject: RE: required section modulus (it don't get no easier) > > > > & Ken I wish your PL/4 would have occurred to me sooner (I was re-reading > > your post last night). Since the trapeze member has two supports I see > each > > one handling half the force time half the distance, or, .5P x .5L, which > is > > equivalent to your PL/4. I would have simply asked you to give that some > > thought... > > > > > > -----Original Message----- > > From: John Denhardt [mailto:[email protected]] > > Sent: Wednesday, June 05, 2013 4:18 PM > > To: [email protected] > > Subject: RE: required section modulus (it don't get no easier) > > > > 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]. > > org> > >>>> http://lists.firesprinkler.**org/listinfo.cgi/** > >>>> > > sprinklerforum-firesprinkler.**org< > http://lists.firesprinkler.org/listinfo.c > > gi/sprinklerforum-firesprinkler.org> > >>>> > >>>> > >>> > >>> ______________________________**_________________ > >>> Sprinklerforum mailing list > >>> > > Sprinklerforum@lists.**firesprinkler.org > <[email protected]. > > org> > >>> http://lists.firesprinkler.**org/listinfo.cgi/** > >>> > > sprinklerforum-firesprinkler.**org< > http://lists.firesprinkler.org/listinfo.c > > gi/sprinklerforum-firesprinkler.org> > >>> > >>> > >> > >> ______________________________**_________________ > >> Sprinklerforum mailing list > >> > > Sprinklerforum@lists.**firesprinkler.org > <[email protected]. > > org> > >> http://lists.firesprinkler.**org/listinfo.cgi/** > >> > > sprinklerforum-firesprinkler.**org< > http://lists.firesprinkler.org/listinfo.c > > gi/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 > > > > _______________________________________________ > > 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 > > _______________________________________________ > 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
