Calculate 3,000 sqft for TJI w/ Attached Gypsum?

9/28/2021

We have a hotel that is constructed with 12-inch TJI joists (composite engineered wood joists which have a continuous web, not an open web). There are gypsum ceilings attached directly to the bottom of the TJI joists. There is also at least 3.5-inches of insulation above the gypsum ceilings.

Are we required to calculate 3,000 sqft or 8 residential sprinklers, or do we meet the noted code exception below?

NFPA 13-2019 19.3.3.1.5.2(4) Item (10): Light or ordinary hazard occupancies where noncombustible or limited-combustible ceilings are attached to the bottom of composite wood joists either directly... , provided the adjacent joist channels are firestopped into volumes not exceeding 160 cubic ft, using materials equivalent to 1/2-inch gypsum board, and at least 3-1/2 inches of batt insulation is installed at the bottom of the joist channels when the ceiling is attached utilizing metal channels.

What are the "adjacent joist channels"?

And does the construction we have equal the firestopping as noted?

Does the gypsum ceiling attached to the bottom equal the firestopping, or is the firestopping something additional?

Roland Huggins wrote a good article on this issue, but I'm still not sure.
https://sprinklerage.com/sprinkler-protection-3000-ft2-modifier/

I have another senior tech convinced that this section is referring to an "unprotected" or non-firestopped combustible concealed space.

Thanks in advance.

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10 Comments

Mike

9/28/2021 08:13:40 am

Fire stopping is blocking perpendicular to the joists. This is the fire stopping to limit the volume.

Each joist bay needs this.

What did the estimator who sold the job anticipate?

Alex

9/28/2021 09:48:25 am

Hi Anonymous,

Awesome post! Shout out to Meyer fire for providing some great details to go along side the question as well. This is an ongoing discussion and I believe Roland did a good job within his article. With that said, I tried my best to explain my thought process.

To answer your first question, I do not believe that you require a minimum area of sprinkler operation of 3000 Square Feet. Based on NFPA 13 (2013) 11.2.3.1.4 4(j) (Same code section you listed within the question from 2019), as long as the adjacent joist channels are fire stopped into volumes not exceeding 160 Cubic Feet, you do not need to increase your design area. This leads into your second question:

Its my understanding of the code that a joist channel is what is shown in the second diagram/picture. The adjacent joist channel would be the joist channel to the left and right of the diagram. From the code, if your channels are greater than 160 cubic feet, you need to use materials equivalent of ½ in. gypsum board to block off each channel. (Example: if your joists are 16 inches on center and 12 inches deep, every foot would be slightly under 1.33 Cubic Feet. Meaning, every 120 feet, you would need to provide firestopping.

The construction you have shown matches the code as I interpret it. As far as firestopping goes, I believe the 3.5 inches of insulation is only required when you are utilizing metal channels to hang the ceiling. If you are attaching the gypsum directly to the joists, batt insulation is not required as long as the gypsum provides the same fire resistance as the insulation would.

The above is simply my understanding and interpretation of the code. I look forward to what others have to say!

Best of luck with your design,
Alex

Anonymoose

9/28/2021 01:41:07 pm

Reading that article it seems that you would need to put gypsum parallel to the joists to divide the spaces into 160 CF or less.

Question for y'all, If that's the case, would you be able to exclude the 3.5 inches in calculating the space, meaning for 12" joists you'd get about 160 SF / (8.5/12) ≈ 226 SF in area between firestopped volunes?

Just in case it hadn't been brought up, if your pipe is above the insulation, is anyone signing off on guaranteeing at least 40°F above the ceilings?

David

9/28/2021 04:48:49 pm

According to an NFSA interpretation (May 2007 #6) the volume must be calculated without taking the insulation into account. The NFSA expert of the day references intent sourced from previous editions of NFPA 13.

Side Note: I have found the Expert of the Day 2004-2018 volumes to be very helpful when interpreting NFPA 13. Thanks MeyerFire for working with NFSA on getting those published.

9/28/2021 05:32:52 pm

Good to know, thanks!

David Kulbacki

9/29/2021 09:08:03 am

Looks like the 2019 edition cleared this up, and you can discount the volume taken up by the insulation. My mistake.

9/28/2021 04:52:11 pm

One thing to watch out for are the manner by which the joists are hung. I have seen quite a few joist hangers used that don't allow the joist to but hard up against the beams they are hung from. The result is that there is a gap where the joist meets the beam that will allow fire spread from joist channel to joist channel.

Mr. Quixote

9/29/2021 02:01:54 am

There is lots of confusion regarding this topic, but lets start at are sprinklers required within the TJI (Composite wood joists), assuming 13 code application of course.

Generally no, and according to 13 2019 9.2.16, as long as the JOIST CHANNELS as measured from top of insulation (note this, as it's come up in previous replies) are separated into volumes not exceeding 160 cubic ft using materials equivalent to web construction, then the areas within the joists shall not require protection. I think this is obvious, but JOIST CHANNEL is the space between each individual joist as shown in the image.

So what are "materials equivalent to web construction"? Well, TJI webs are generally just a type of OSB or thin wood substrate, and NFPA seems to imply that insulation may meet this requirement. In reality, the intent here is not "firestopping", as I typically see, but more like draft stopping. It's simply there to keep a draft from spreading throughout the structure and spreading a fire. You could typically see 2x wood, but plywood, osb, drywall, insulation seem to all be acceptable as "equivalent to web construction". .

For a 16" deep x 16" on center TJI, it would take 90' approximately to exceed the 160 cubic ft. require to sprinkler between joists. And, if that situation occurred, it may be as simply as adding a 2x block of wood to separate that area within the JOIST CHANNEL. And note that this does not include insulation depth, which can be subtracted from the area as allowed by 9.2.1.6 2019.

Now to to the more important point, do these areas as described above require an increase in design area size to 3000 sqft or 8 sprinkler heads for the residential special design requirements (residential or room design by 2022) as required by 19.3.3.1.5 2019? The answer is YES generally. However, if you look at 19.3.3.1.5 (B) (below), you will see that if there are no "adjacent areas" to extend the design area too, then the design area would not be required to be extended.

(B) The term adjacent shall apply to any sprinkler system protecting a space above, below, or
next to the qualifying concealed space except where a barrier with a fire resistance rating at
least equivalent to the water supply duration completely separates the concealed space from
the sprinklered area.

In most cases, hotels are highly compartmentalized, meaning that all of the separating walls between all areas are at least a 1 hr rated assembly, and these assemblies extend from the base floor to the floor level above. And the floor assembly is a rated 1 or 2 hour rated assembly.

In conclusion, if you end up with a hotel, light hazard (except a small ordinary hazard mech room), with TJI combustible spaces that require an increase to design area, but in reality, if all of the spaces are divided by at least a 30 min fire rating (2 layers 1/2" gypsum as defined by IBC), then you should NOT have to increase the design area.

It's not a simple question that you ask....are we required to calculate 3,000 sqft or 8 residential sprinklers, or do we meet the noted code exception below?

The answer is generally yes you must increase the area, but given that most hotels are highly compartmentalized with fire rated assemblies, you probably won't have to because all of your fire ratings will qualify as a separation of at least 30 mins.

9/29/2021 09:06:56 am

Looks like 19.3.3.1.5.2 (10) in 2019 edition would allow design area to not be increased.

RM Ballam

2/10/2022 10:55:51 am

The section requires 1/2" of gyp or equivalent. Most volumes created by composite wood joist are much smaller than the 160 cu stated in standard which means that the gyp bd. would only be required every 4th or 5th joist in most situations.
Example: 1x1.33x18=23.94, 160/23.94=6.68
The IBC, Table 722.6.2 (1) state that 1/2" gyp equals 15 minutes, It also states that 3/8" OSB (product used for TJI webs) equals 5 minutes.
Would the 3 or 4 layers of 3/8" OSB meet the equivalent of the 1/2" gyp bd. requirement?
Thanks,
Rob

Calculate 3,000 sqft for TJI w/ Attached Gypsum?

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