Mark you ignorant slut: I have been out of my office all day, so this is the first opportunity I've had to sit thoughtfully at a keyboard. I sincerely apologize that yesterday's responses didn't meet your standard of care. Nonetheless, I'm glad you feel sated or satiated or salivated or however it is that you feel. Feelings are good. However (and Ken, please feel free to join if you feel compelled) I am not sure that Tom's answer is complete, and as such I respectfully demur on a couple of points.
The concept at work is the different "harmonics" of movement in the building and the relativity of anchorage. If mains are braced, lines restrained and ceilings also rigidly braced to the SAME structural assembly above, then the likelihood of them moving harmoniously in an earthquake is high, which reduces and can even eliminate the counter directional movements such as caused by branch line whiplash. But rigidly bracing a ceiling is very expensive and very rarely undertaken, so in nearly every new building where ceilings are simply laterally braced and furnished with compression struts, the overhead piping is going to be moving with the roof or floor above, while the ceiling is expected to move differentially and/or float between structural assemblies all potentially moving differentially from one another. The ASCE prescriptives are based on the concept of building flexibility into the interface between overhead piping and ceiling, that interface being ... arm-overs and drops. ASCE 7 requires at non-rigidly braced ceilings that either the oversized annular space, or a swing joint or a flexible hose connection be provided. So laterally restraining such piping is counter-intuitive. And, as far as "lines in the sand" or the issue of super long arm-overs, I am not sure that we're done. Sprinklers have lagged behind other disciplinary designs for the past 20 years for seismic protection. Since harmonizing 13 with ASCE 7, we've joined the 21st Century for sure but as the 2010 and 2013 editions are adopted, and as more states and AHJ's begin to implement requirements for seismic protection of building systems, these practices are going to come as something of a shock for sprinkler contractors and designers. I never thought we'd have to be structural engineers, but there are elements of SE in what we're doing here in CA right now for certain agencies. If you look at Ken's slides from the convention (link was posted yesterday), you'll see at the end an exercise for calculating what is called "deflection" of an exemplar riser nipple. It's not called that in the slides, but that's the force being applied. There are forces - deflection and compression - and there is displacement. ASCE 7 requires that building systems be proven adequately protected from force and displacement and there are formulas for calculating this stuff. The ASCE 7 method (simplified but not short-cut in NFPA 13) for calculating braces is based on worst-case scenarios; if you were to do site specific calculations it's even more complicated as you have to multiply by factors for different kinds of structure, even different weights of concrete in some cases, and differing seismic force factors from site to site. So Ken's exercise is a taste of what may be to come, as it's sound engineering principal to also take deflection into account for hangers and braces (that's what the references to net vertical reaction forces allude to in the standard now). ASCE 7 has a formula for calculating the longest allowable distance of hangers, based on piping weight and the seismic force, and it takes the potential deflection and displacement of the piping into account in that formula. I guess the point is that is a systematic, rather than a specifically elemental approach. The flexibility of drops, combined with the rigidity of mains and lines, is what makes it work. The establishment of adequate resistance to force in the piping and hangers and sway brace arms are all important factors in assessing the overall design. I hope that answers your question(s) ... My opinion only but I'm not on the committee so the disclaimer is just to make me sound important, Steve Leyton -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Mark A. Sornsin, P.E. Sent: Tuesday, October 22, 2013 9:40 AM To: [email protected] Subject: RE: NFPA 13 '10 9.3.6 BL Restraint Tom, Thank-you for your thoughtful and insightful reply. It addressed our question and, therefore, should be a model for all of the Forum - especially Steve and Ken. Bottom line is, yes, there are 8 ft. arm-overs that would not require restraint to meet seismic requirements; and there are 8 ft. branch lines feeding two sprinklers that would require restraint. It seems to be a bit of a cross-over in the rules, but ultimately doesn't happen much and thus isn't considered to be a real problem. I can't speak for Chris (despite having done so before), but I feel satiated. Mark A. Sornsin, P.E. | Karges-Faulconbridge, Inc. | Fire Protection Engineer | Fargo, ND | direct: 701.552.9905 | mobile: 701.371.5759 | http://www.kfiengineers.com -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Tom Wellen Sent: Tuesday, October 22, 2013 10:00 AM To: [email protected] Subject: Re: NFPA 13 '10 9.3.6 BL Restraint Mark and Chris, I'll take a stab at this. The west coast is still grinding the coffee beans. The length of an armorer is not expected to be very long. The adjacent branch lines cannot exceed 20 feet apart for extended coverage sprinklers and 15 ft for standard spray sprinklers. If the armorer is over 10 ft long coming off a cross main or branch line, it should be considered to be piped from the adjacent branch line. The main objective is to use less pipe, correct? Why waste pipe length? So, if that is the case, the committee is not worried about a line with a single sprinkler and provided the definition of an armover. A line with two or more sprinklers is defined as a branch line where they can be of considerable length. The longer the line, the more it will have a whipping action. Thus, Table 9.6.3.4(a) specifies the spacing needed for restraints based on the Cp in that area and the pipe size ranging from 26 ft to 53 ft. An armorer shouldn't whip around so much due to its short length so restraint is not required. Can there be instances where there are long armorers? Sure, especially if there is a drop from a 40 ft ceiling to a ceiling at an 8 ft level. 9.3.6.7 Drops and armorers shall not require restraint. What a mess that would be to restrain a drop located 32 ft from the ceiling. Steve commented that ceiling grids should be rigidly braced. If not, then the oversized hole for the sprinkler is necessary in the ceiling to keep the ceiling from bre aking the fusible element. Will fusible elements still still break during an earthquake from the ceiling? Probably. The cost benefit of providing restraints on every drop does not warrant them to be installed. NFPA 13 is full "lines in the sand" that we don't know the true history behind them but it seems mostly just to give a warm fuzzy (i.e. 4 ft obstruction rule, area limitations of 52,000 sf, etc....) and armorers and branch lines is just one of them. It doesn't fit all applications, but the baseline requirement is established. Tom Wellen AFSA _______________________________________________ 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
