Size Rod to Trapeze Separately than Rod to Pipe?

1/4/2024

In regards to a trapeze pipe support, isn't there an option to size the rod that connects structure to the trapeze pipe separately from the rod from the trapeze pipe to sprinkler pipe?

Couldn't that be two separate calculations considering there are two rods for the trapeze and only one to the sprinkler main?

Thanks in advance.

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8 Comments

Alex

1/4/2024 08:10:13 am

I'm not sure if I understand your question fully, but to size the rods, NFPA 13 (2013) Table 9.1.2.1 should be used.

Pete H

1/4/2024 09:07:22 am

Table 17.2.1.1 in 2022 edition, but other than section it looks virtually unchanged

Dan Wilder

1/4/2024 08:34:28 am

NFPA 13 - 2019

It's possible, but not viable as it typically will cost more to prove (and likely field test) the smaller sized components via a structural engineer per 17.1.2.

17.2.1.1 For Hanger Rods
17.2.1.2 allows for smaller rod if tested AND LISTED for the HANGER ASSEMBLY (not just an individual component) ...just because the supported weight indicates a number below the total weight (15' of water filled pipe, x5, +250), there are typically additional safety factors required that are not met.

17.3.5* All components of each hanger assembly that attach to a trapeze member shall conform to 17.1.6 and be sized to support the suspended sprinkler pipe.

The below section is important because it requires the ring hanger to be sized per the pipe it's attached to but still comply with the support of the actual water filled pipe noted in 17.3.5 (this is where using the "Heavy Duty" rings like the Tolco Fig 200H using a 1/2" knurl nut but to attach to a pipe that is <4" instead of the standard 3/8")

17.3.6 The ring, strap, or clevis installed on a pipe trapeze shall be manufactured to fit the pipe size of the trapeze member.

There are some exceptions for some fasteners to be a lesser diameter but are then required, or already manufactured, to have the increaser to the correct rod size (The Hilti HUS-EZ-I is a good example with a 1/4" shank but a 3/8" coupler for the ATR). Section 17.2.3.3 has this language.

Anthony

1/5/2024 07:27:35 am

I agree with Dan here, especially with the use of the 200H. Those seem to be forgotten every time with trap hangers.

Casey Milhorn

1/4/2024 11:02:45 am

As Dan said, it is possible if you involve a structural engineer. If the project is large enough, and you have a line on an economical structural engineer, it might pay off, but unlikely.
If this is more of a practical issue (for example the roof structural engineer isn't allowing certain size penetrations, attachments, etc.) then you might have to go that direction. Or even ask them to prove it for you (at a small cost to you).

Dave

1/4/2024 11:10:54 am

Incorrectly-sized trapeze hangers components is one the most common review comments I make. As Dan pasted above, “All components of each hanger assembly that attach to a trapeze member shall conform to 17.1.6 and be sized to support the suspended sprinkler pipe.” By that I interpret that as EACH component. The NFPA #13 Handbook has a long commentary on this including, “One of the most common mistakes with pipe trapezes is to use an incorrect ring, rod, and/or attachment size.” Violations include ring-to-ring hangers that do not utilize special trapeze rings, trapezes whose ends are suspended by say, 3/8-in rod yet supporting a 6-in diameter pipe. Short of a specifically-engineered hangar assembly, about the only part you can get creative with on trapeze hangers is where along the trapeze the pipe is being suspended.

Glenn Berger

1/4/2024 02:16:27 pm

Dan nailed it. When reviewing submittals I prefer to see standard and approved designs that apply to the specific project.

Janak Patel link

1/10/2024 12:52:41 pm

A sprinkler pipe may not always be at the center of the trapeze structural member. In the worst-case scenario, the rod (very) close to the sprinkler pipe will carry almost all the load (i.e., as if it alone is supporting the pipe). Therefore, an option of sizing the rod size by special structural calculation which must be signed by a PE, is not provided.

Please note that a (ferrous) rod is not required to be 'Listed' per NFPA 13 (2019), Section 17.1.6.2.

The method,15' of water filled pipe x 5, + 250 has factor of safety already included in it, and it is applicable to the hanger component in tension only. In fact, this load is equal to the 'test load' per UL 203. For the trapeze member, (beam), the design load is (wt of 15' or 12' water filled pipe+250 lb.) acting at the center of trapeze, bending stresses not to exceed 15 ksi.

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Size Rod to Trapeze Separately than Rod to Pipe?

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