Is Unistrut Permitted for Sprinkler Pipe Trapeze?

7/22/2021

Is Unistrut allowed to be used for trapeze braces on sprinkler systems?

There are two issues at hand - one is making sure the section modulus allows for Unistrut to be used. In this case we have a 16-inch span with only a 2-1/2" main on the trapeze, so we're good there.

The other issue is the listing and approval of components. Any tips here?

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23 Comments

Pete

7/22/2021 06:22:18 am

I think the MeyerFire Trapeze sizer tool can specifically help you with sizing Unistrut as trapeze as well as having items from the Unistrut Catalog noted.

https://www.meyerfire.com/blog/building-the-trapeze-sizer-week-2-of-3

Bob Kluck

7/22/2021 08:32:47 am

It can, however if you don't have that package, the data sheets for Unistrut or Approved Equal Competitors specify the section modulus. If you look that up for the one you plan to use, you should be able to determine if it meets the requirement for NFPA 13.

Glenn Berger

7/22/2021 09:18:02 am

Using Unistrut or similar products are fine. Just provide the catalog data and calculations for the intended use and you will be fine.

Craig Hanson

7/22/2021 09:24:51 am

Although Unistrut can be used, Unistrut pipe clamps are not listed for use for fire sprinkler systems [NFPA 13 2016 : 9.1.1.5.1, see handbook notes]. additionally unistrut spring nuts cannot be used for trapeze hangers [NFPA 13: 9.1.1.7.8] double nuts and washers need to be used. I hope this helps.

Darrel Jacobs

7/22/2021 12:05:46 pm

Craig -I don't understand the comment about the use of unitstrut spring nuts. Are you referring to the hangers from the structure to the unistrut? I would think if you are referring to the pipe attachment to the unistrut via a short pipe strap with bolts and unitrut nuts that it would be acceptable since NFPA 13 2016 sec. A.9.1.1.5.2 says generic items used with hanger rods and fasteners are not required to be listed -such as bolts, nuts etc...

Bob Kluck

7/23/2021 08:26:10 am

I come from a Control Systems background, so I understand the comment. Most Electricians hang electrical boxes off Unistrut with a small piece of metal backed up/held in place by a spring. These are not really meant to hold an enormous amount of weight, and in a Sprinkler Pipe support application would likely bend and fail over time. However there are ways to have rods go straight through the Unistrut, and this would be WAY more structurally supportive.

Michael Wayne Sims

4/9/2024 12:55:23 pm

The Spring Nut is a specific component provided through Uni-Strut manufacturer. And they cannot be used for Trapeze style hangers.

CJ Bonczyk

7/22/2021 09:26:36 am

Yes unistrut is an acceptable means of support. You may want to verify from the E.O.R. that is allowed for your project though first. The manufacturers can provide modulus data in their catalogs or from their technical engineers. Also you can have a structural PE provide calculations as well although that will cost.

Dan Wilder

7/22/2021 09:50:42 am

Yes - However, I have not found many instances that Unistrut provides enough of a section modulus or weight allowance to be useful.

So you need to watch both making the section modulus work and the total weight.

P1000 in Axis 2-2 has a section modulus of 0.29
Concentrated load allowance with T-Series slots reduce the load by 15% (or multiply by .85)
Concentrated midspan loads reduce by an additional 50% (or multiply by .5)

So a 1 5/8" Slotted Strut that has a uniform allowable load of 1,690lbs can only support 718.25 lbs (1,690 x .5 x .85).

Now, with a 15' section of water filled pipe (5.893 for Sch 10 + 15% for fittings) only equaling out to ~101.6lbs, you've met both the section modulus and weight bearing BUT you don't meet the 5x + 250 requirement. You'll need to reduce the spacing accordingly.

Page 25, bottom right corner for all of this.

https://www.unistrutohio.com/wp-content/uploads/unistrut-general-engineering-catalog-17A.pdf

Casey Milhorn

7/22/2021 09:56:21 am

As others have said, yes. BUT, you will find some local or state AHJs, engineers, etc... that will fight you on it. You just have to decide if it's worth the fight, or just use pipe or angle instead....

Jessica Lutz

7/22/2021 10:18:16 am

Check your AHJ. In California, unistrut is not allowed for hanging.

Craig Hanson

7/22/2021 10:31:19 am

I have the CFC with NFPA 13 amendments but I cannot find where unistrut is not allowed. Oddly lag screws are not allowed per CFC chapter 80 NFPA 13 amendment 9.3.5.12.5 (which I have no idea why lag screws cannot be used) but I do not see anything that restricts unistrut beyond the normal exceptions in NFPA 13. Do you have a source document for the unistrut restriction.

Dave

7/22/2021 10:34:56 am

Good comments above ^^. And ditto Craig on the pipe clamps. One thing often ignored is the means of fastening said strut to the structure above, especially if the joist/truss manufacturer has particular guidelines. And don't forget NFPA #13 now requires all components of the trapeze to be sized for the pipe being supported - I still frequently see this overlooked, or special trapeze rings shown, but not installed. Lastly, while you may still pull that same old Unistrut or Powerstrut cut-sheet off the shelf or off the internets, if you list "strut" you know you may just get whatever brand your supplier or purchaser bought at the time.

Dave

7/22/2021 11:20:46 am

Dan (and I ask this with great respect for your expertise), Doesn't NFPA consider the trapeze member to be an extension of the building structure, thus the 5x does not apply? I can't find a formal or informal interpretation, but I have read this in at least one source, and the NFPA #13 trapeze tables are designed for only water-filled pipe +250. One isn't designing/certifying a hanger.

Greg

7/22/2021 12:14:31 pm

I may be wrong, but I believe the 5x plus 250 comes from the listing requirements. NFPA 13 Section 9.1.1.5 requires the hanger assemblies be listed between the building and the pipe. The requirements to be listed are 5x the weight of the pipe+250.

Dave L

7/22/2021 02:21:25 pm

For those with a NFPA #13 handbook, the commentary under 9.1.1.7 (2016) p.448, or 17.3.1 (2019) p.504, states not to include the five-times. "The requirement for trapeze hangers matches the requirement for the building structure."

Dan Wilder

7/22/2021 02:15:45 pm

Dave - Na...call me out, I'm gonna get it wrong (and I did so nice catch) and I'm fine with it (wish I could edit the statement above so others don't read and apply things incorrectly - ahem....Joe? wish list for the forum). I'm thinking my last go around with this was a shared support situation (see 9.1.1.3.1.1 13-16 Ed.), but in a sprinkler only assembly I probably lost an argument.

Attachments from structure to Unistrut & the Unistrut to the pipe does require the 5x per 9.1.1.7.5 but the actual Unistrut only needs to comply with 9.2.1.3.1

Handbook Section Commentary:

"A trapeze hanger, as shown in Exhibit 9.2, must be capable of supporting the weight of the water-filled piping plus 250lbs. Other types of hangers are generally required to be able to support five times the weight of the water-filled piping plus 250lb. The safety factor of five is used in the traditional listing requirements for hangers and is also found in the performance-based criteria of 9.1.1.2(1). The requirement for trapeze hangers matches the requirement for the building structure found in 9.2.1.3.1."

Dave

7/22/2021 02:33:13 pm

Ah, you just beat me to posting the commentary, Dan, probably the only time I will ever catch you on something (marking it on my calendar before Joe removes the evidence.) It reminded me I do need to re-educate myself on the difference with shared-support requirements. I was ready and willing to be proven wrong, the older I get, the more I forget and have to look things up. Honestly I never looked into why the 5x doesn't apply to trapezes. Surprised I couldn't immediately find an informal interpretation on it or an article by Victoria.

Michael

7/23/2021 08:00:38 am

What about the holes for bolts or rods requirements? How does UniStrut meet these requirements?

9.1.1.7.7 Holes for bolts or rods shall not exceed 1⁄16 in.
(1.6 mm) greater than the diameter of the bolt or rod.

9.1.1.7.9 Where angles are used for trapeze hangers and slot-ted holes are used, the slotted holes shall meet all of the following:
(1) The length of each slotted hole shall not exceed 3 in.(80 mm)
(2) The width of the slotted hole shall not exceed 1⁄16 in.(1.6 mm) greater than the bolt or rod diameter.
(3) The minimum distance between slotted holes shall be 3 in. (80 mm) edge to edge.
(4) The minimum distance from the end of the angle to the edge of the slotted hole shall be 3 in. (80 mm)
(5) The number of slots shall be limited to three per section of angle.
(6) The washer(s) required by 9.1.1.7.8 shall have a minimum thickness of one-half the thickness of the angle.
(7) Washers and nuts required by 9.1.1.7.8 shall be provided on both the top and bottom of the angle.

Dave

7/28/2021 11:49:44 am

Michael, according to a 2/17/2016 Sprinkler Age article by Tom Wellen of AFSA's Tech Services Dept. (available on-line),

"Do these requirements apply to manufactured slotted material such as Unistrut? The answer to that is also no. These requirements only apply when slots are provided in angles. Slotted materials by manufacturers are structures that have published section modulus or design loads. The key is to install the assembly per the manufacturer’s instructions. We see in Section 9.1.1.7.2 that any other sizes or shapes giving equal or greater section modulus shall be acceptable. As such, other structural materials other than pipe or angles can be used. The argument against using the slotted materials is that the slots are 1⁄16 in. greater than the bolt or rod diameter. That could be true but it is not a violation of the standard. When using a manufactured slotted material, their instructions should include how it is to be installed. The instructions commonly refer to the use of thicker square washers with the slotted product. These assemblies have been proven through testing of the allowable loads or section modulus. As such, the new requirements for slots in angles do not apply to the slotted material assemblies by manufacturers."

James Lewis

9/18/2024 06:00:32 pm

Dan,

Your explanation and calculations are spot on. However, you're applying the most conservative approach possible. When it comes to Unistrut trapeze hangers, they are most effective when the load is positioned near the end of the span. In such cases, NFPA 13 (section A.9.1.1.7, 2016 edition) allows for the use of the harmonic mean formula to calculate an equivalent length. The closer the load is to the end of the span, the shorter the equivalent length becomes.

For example, in a 9'-0" span with the load applied 6" from one end, the equivalent length would be reduced to 22.67". When determining allowable loads, you can then use the row in the Unistrut manufacturer’s table that corresponds to this equivalent length. This allows you to more accurately account for the non-centered load, treating it as if it were applied over a shorter, uniformed span.

Since manufacturers base their tables on span lengths and allowable loads, using the equivalent length gives a more precise estimation of the Unistrut’s capacity, especially when dealing with off-center loads.

In practical terms, applying the load near the end of the span results in a shorter equivalent length, effectively increasing the allowable load. Using my example above, where the max allowable uniform load for a 24" span is 1,740#, your calculation would look like this:

(1,740 × 0.5 × 0.85) = 739.5

After subtracting the mandatory 250 pounds, you end up with 489.5 lbs of allowable load. This means that a P1000 series OS Unistrut could support up to a 6" schedule 40 pipe with spanners 15'-0" on center, spanning 9'-0", provided the load is applied 6" from one end.

In this case, Unistrut becomes a much more resonable option.

James Lewis

9/18/2024 09:07:36 pm

One aspect that still confuses me is the sizing of trapezes according to the code. There appear to be two methods:
1. Engineered Method (Section 9.1.1.2): This method considers the weight load as the only factor for sizing.
2. Modulus Method (Section 9.1.1.7): This method only takes the modulus of the spanner material into consideration. However, hanger components that attach to the spanner and pipe must still comply with the 5x + 250 requirements (see Section A.9.1.1.7.1a).
In my previous example, I combine these rules. This is really a half-baked approach. While I believe my calculations are correct, they don’t fully comply with the code.
If using the engineered method, the spanner and all assembly components must meet the load requirement of 5x + 250. The modulus may be considered in your calculations, but it’s not a primary factor for compliance under this section.
When using the modulus method, the modulus of the x-x axis must match or exceed the values in Table 9.1.1.7.1(a). The Unistrut product data table need only be consulted to ensure that the maximum allowable load at the slot face and all components between the spanner and pipe comply with the 5x + 250 rule. Beyond this, the table values are not directly relevant.
I understand that this explanation might sound a bit confusing, but it aligns with what the code specifies. Personally, I believe a Unistrut spanner should only be required to meet the weight of the water-filled pipe plus 250, as Unistrut’s safety factors have already been applied. Adding extra safety factors in this case is excessive. Maybe I’ll write a PR…

James Lewis

9/18/2024 09:10:00 pm

One aspect that still confuses me is the sizing of trapezes according to the code. There appear to be two methods:

1. Engineered Method (Section 9.1.1.2): This method considers the weight load as the only factor for sizing.

2. Modulus Method (Section 9.1.1.7): This method only takes the modulus of the spanner material into consideration. However, hanger components that attach to the spanner and pipe must still comply with the 5x + 250 requirements (see Section A.9.1.1.7.1a).

In my previous example, I combine these rules. This is really a half-baked approach. While I believe my calculations are correct, they don’t fully comply with the code.

If using the engineered method, the spanner and all assembly components must meet the load requirement of 5x + 250. The modulus may be considered in your calculations, but it’s not a primary factor for compliance under this section.

When using the modulus method, the modulus of the x-x axis must match or exceed the values in Table 9.1.1.7.1(a). The Unistrut product data table need only be consulted to ensure that the maximum allowable load at the slot face and all components between the spanner and pipe comply with the 5x + 250 rule. Beyond this, the table values are not directly relevant.

I understand that this explanation might sound a bit confusing, but it aligns with what the code specifies. Personally, I believe a Unistrut spanner should only be required to meet the weight of the water-filled pipe plus 250, as Unistrut’s safety factors have already been applied. Adding extra safety factors in this case is excessive. Maybe I’ll write a PR…

Is Unistrut Permitted for Sprinkler Pipe Trapeze?

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