Sprinkler Main OK to Hang from Wood Trapeze?

2/2/2024

We have a new construction building that's wood frame (Type III-B), and we have an 8-inch horizontal standpipe. It's hung from wooden trapeze (two 2x4's) spanning wood floor trusses.

Per NFPA 13-2019 Section 17.1.7.1, hangers and their components shall be ferrous metal.

Is a ferrous metal trapeze (steel angle or pipe span) permitted to span wood floor trusses?

If so, must the trapeze avoid attaching to the web components of the truss?

Thanks in advance.

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14 Comments

Anthony

2/2/2024 07:20:51 am

First off obvious answer that'll to just hang of that I beam in that photo and move on.

Wood is not given in the prescriptive methods for constructing a trapeze in NFPA CH 9 so that's out.

The next argument is that all hangers must go to structure. Can you hang to a wood structure? Yes you can. Is adding (2) 2x4's spanning the bottom cord of 2 open web wood joists considered a structural member? Ehh, probably not.

This will go back to NFPA 13-16 9.2.1.3.1:

Sprinkler piping shall be substantially supported from the building structure, which must support the added load of the water-filled pipe plus a minimum of 250 lb (115 kg) applied at the point of hanging, except where permitted by 9.2.1.1.2, 9.2.1.3.3, and 9.2.1.4.1.

The annex goes one to say:

A.9.2.1.3 The method used to attach the hanger to the structure and the load placed on the hanger should take into account any limits imposed by the structure.

The building structure is only required to handle the
weight of the water-filled pipe and components, while the
hangers are required to handle 5 times the weight of the
water-filled pipe. In addition, a safety factor load of 250 lb
(114 kg) is added in both cases. The difference in requirements has to do with the different ways that loads are calculated and safety factors are applied.

In summery if the structure can't support your pipe you need to hang somewhere it can or reenforce the building. It is implied that only a structural engineer can approve changes and alterations to a structure to allow for such loads. So yeah you're going to need to run by the structural engineer how often you need place your trapezes and where.

Dan Wilder

2/2/2024 07:39:57 am

The AWC provides section modulus values for sawn lumber but it's based on a distributed load not single point (that I can find readily). Unfortunately, not only did they use a 2x in the worst orientation, even if it is doubled up, with shorter spans, and it looks like they did a through connection with washers & nuts, this really needs to be signed off by the truss structural engineer and the building structural engineer.

The lower plate connector needs to be reinforced to avoid it from ripping apart, the truss hanger needs to be evaluated for the additional load, the upper connection from the truss to the beam also needs to be reviewed for its weight capacity and the overall dead load for the floor above should be recalculated for this span. I would have rather seen pipe vs lumber in this case and using that beefy steel beam lower flange (or notch the upper plate to get a good TBC bite into the upper flange.

17.3.5 only says the components of each trapeze member that attach to the trapeze shall be listed, the trapeze is part of the structure so it can be of any material once confirmed for weight capacity.

Jesse

2/2/2024 08:15:52 am

I'd love to know more about why they ran dimensional lumber that way...

Good question to think about, but I admit I'm having difficulty getting beyond the lumber.13 directs us to ensure hat all components of the trapeze be listed material.

Mike F

2/2/2024 08:17:44 am

Dan W. Is right on with the answer. Question with the steel I Beam so close, why did they not hang right from the steel shown in the picture

Chad

2/2/2024 08:18:56 am

100% argree.. you need a PE ok that added load to an engineered truss, there is no truss manufacturer that ok’s significant loads like that on their spec sheets…. Especially to the bottom chord.

Or move it to the steel and it’s good.

Josh

2/2/2024 08:59:58 am

The ultimate irony of the decision is the beam not only appearing in the same picture, but being right next to it.

DAN

2/2/2024 09:34:52 am

Plus- the traps should be supported from the top cord pf the truss...not the bottom

Rlujan

2/2/2024 09:42:58 am

My guess is they offset from the beam to allow the pipe, which looks to be six in., to pass the column without adding extra fittings.

Jack G

2/2/2024 09:53:24 am

#1- are the floor joists rated for mechanical load.
Probably not. The engineer might spec the right joist but the contractor/ truss manufacturer provides a cheaper less supportive truss.
Never hang from the bottom. Truss manufacturer will provide details and weight loads.
And like everyone says -- hang to the beam .

Dave

2/2/2024 10:43:11 am

There’s some relevant commentary in the NFPA #13 handbook, but it basically says, “where materials other than steel are proposed for trapeze members, an engineering analysis must be completed to determine the material’s capability.” You can’t go by gut-feel; the building designer has the option to modify the structure. As Jack says, consult the truss manufacturer’s guidelines and details (in our neck of the woods it is usually Red-Built / old TJI). If suspending the top chord, remember one must have enough meat below the attachment point, plus per NFPA 13 each individual component of the trap must be sized for the 8-in. pipe being supported.

Pete H

2/2/2024 01:09:25 pm

The beam is right there. Put a beam clamp on the rod and stop hanging from the wood trapeze.

sean

2/4/2024 09:28:03 am

this photo is so wrong. hanging from the bottom of a truss/joist shouldn't be done unless prescribed by the engineer.

Janakkumar B. Patel link

2/7/2024 02:40:02 pm

The ferrous hanger components (Listed pipe clamp and ferrous threaded rod meet NFPA 13 requirements. The wooden beam needs to be calculated and certified by a Registered Professional Engineer.

NFPA 13 (2019), Section 17.1.2(4) states: "Hanger components shall be ferrous."
NFPA 13 (2019), Section 17.1.7.2 states: "Nonferrous components that have been proven by fire tests to be adequate for the hazard application, that are listed for this purpose, and that are in compliance with the other requirements of this section shall be acceptable.

So, the wooden beam used for supporting the hanger shall comply with not only the design load of 'w+250' lb., but also NFPA 13 (2019), Section 17.1.7.2. Also, see NFPA 13, Sections 17.5.2.2, 18.5.3.2.

janak patel
NFPA 13.

Chris

2/8/2024 08:58:22 am

You are allowed to hang from wood structure but technically it is only considered wood structure if the framers were the ones that put it there IIRC, you are not to add wood yourself to hang from. If you cant hang off the H beam in the photo I would say you will need to install a steel member for the trapeze to the top cord of the truss, a piece of angle iron or unistrut may be easiest.

Sprinkler Main OK to Hang from Wood Trapeze?

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