Section Modulus for Trap Members 10'-6" On Center

[ShawnO1]

NFPA 13 gives a table of Section Modulus for varying spans of trapeze members based on 15' of water filled pipe. Not sure if anyone else installs main hangers on 15' centers but we use 10'-6" spacing and 5'-3" from each end (typical 21' lengths. 2 hanges per pipe.

Is there a formula available (maybe I missed it in 13) or another preengineered table for Section modulus based on 10'-6" centers?

Seems like I should be able to get the trapeze pipe member size down if I didn't have to use 15' centers.

TIA, ShawnO

 

[firepe]

Shawn,

You may need to get help from a qualified (competent) engineer on this one. If you do not use the tables and parameters outlined within NFPA 13 (which many have argued contains all the engineering so you don't have to), then (as others have argued previously), you are interpretting, or worse, doing a calculation that by rights is "engineering", which is outside the scope of a NICET technician.

Fire Sprinklers Save Lives.
Can You Live Without Them?

 

[SprinklerDesigner2]

'you are interpretting, or worse, doing a calculation that by rights is "engineering", which is outside the scope of a NICET technician.'

But not all is lost, there are easy and correct workarounds.

For a 10'-6 trapeze simply keep the pipe away from the center using A9.1.1.6 to find the equivalent length.

If the span is 11'-0" simply keep the pipe within 3'-9" of the left or right side for an equivalent length of 9.89'.

If the job is so big, so complex you can't offset pipe in the trapeze then pay the few hundred it will take to have a PE look at it.

 

[TravisMack]

His trapeze span is not necessarily exceeding the tables in NFPA 13, but he is just questioning why he should have to use the spacing in NFPA 13 which is based on 15' hanger spacing when they are using 10'-6" hanger spacing. It seems to make sense that you could reduce the section modulus for hangers at 10'-6" on center and could mean changing your trapeze member from 3" to 2" Sch 40 or something similar. It could be a huge savings in today's market.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[AZsprink]

Remember also that you can delete alternating trapeze hangers on cross mains as long as you hang these branch lines at first available joist "excluding side feed systems".Also I think you are required to have a hanger between each branch line, and this should determine your hanger spacing, not the length of pipes.

We see a lot of companies shift cross mains to the side of the nearest truss and adjust starter pieces accordingly to maintain plan head locations. This eliminates trapezes, but I would careful if it is a large shift, so as not to affect pipe lengths too much.

 

[KDLLC]

You might be able to hire a PE to create a new table for you (similar to Table 9.1.1.6.1(a)) based on 10'-6" of pipe. If he/she stamped it and wrote a supporting letter, I'd think you could use it on any future project.

Ed Kramer
Kramer Design, LLC

 

[firepe]

I agree with Ed's suggestion.

Fire Sprinklers Save Lives.
Can You Live Without Them?

 

[ForestW]

I recall this being addressed in a previous Tech Alert from the NFSA years ago.
The technician was told to make the necessary adjustments.
You dont need to hire a PE to perform basic math for you.

Forest Wilson
Cherokee Fire

 

[ShawnO1]

Helpful links...

https://www.wheatland.com/SpecSheet...A_13_2010_Sway-Seismic_Bracing_Sch10 pipe.pdf

https://www.wheatland.com/SpecSheet...A_13_2010_Sway-Seismic_Bracing_Sch40 pipe.pdf

Thanks for all of your advice. I have an engineer buddy that's helping research this. I'll update as I get more information. He'll explain to me how he got there. Then we'll see if it is actually basic math or not.

 

[SprinklerDesigner2]

"we'll see if it is actually basic math or not."

I tried and while the math was basic and easy I messed up by not knowing 1)the correct terminology and 2)where to apply the correct numbers to arrive at a correct answer.

While I would like to know more, I would like to be able to do it, I recognize it's engineering so I throw the responsibility to someone else.

It's like knowing when a 13R system becomes a full 13 through the building code. I pretty much understand it, most of the time I can get the right answer but why would I want to accept the responsibility when it's clearly the practice of engineering and architecture? I have enough responsibility without volunteering.

 

[TravisMack]

Shawn:

Shoot me an email. I have a spreadsheet that a PE buddy of mine created a couple of years ago that calculates the section modulus for various hanger spacings and distance from side of the structural member. The table in 13 is for center loading and 15' on center.

Mine is not signed/sealed by the PE that created it. Your guy may want to use it as a starting point. He may find it is correct or needs to be changed.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com