NFPA 13 and Openweb Wood Trusses

[khsmith55]

As an Architect, for over 30 years I have generally avoided, and directed my Engineers to avoid using open web wood trusses in Type V Construction with a NFPA 13 sprinkler system. This philosophy has been based on the challenges posed by the NFPA 13 requirements for concealed combustible spaces. I have always maintained an open web wood truss is similar to "Double joist Obstructions" (2013-NFPA 13 Section 10.2.6.1.5.1and Figure 10.2.6.1.5) and if the distance between the chords was more than 6" sprinklers were required. Some might say 2013-NFPA 13 Section 9.2.1.6 applies, not requiring sprinklers, I say NO, this section references "joists" NOT trusses. Can one of the "sprinkler guru's" confirm my Philosophy has been correct for 30 years.

Thanks in advance,

Ken

 

[khsmith55]

Just a follow up. I am aware I can fill the truss space with non-combustible insulation to avoid sprinklers (have done this in extreme cases) but this causes problems with listed fire rated assemblies. Both GA and UL require an additional layer of gypsum board if the insulation thickness exceeds the thickness in the listed assemblies.

Thanks,

Ken

 

[cda]

Not a guru


You are correct

If a combustible concealed space cannot meet, what used to be the exceptions, but still in NFPA 13, than sprinklers are required.

Will look at the sections you mentioned, just not near the book right now.

 

[cda]

“””Both GA and UL require an additional layer of gypsum board if the insulation thickness exceeds the thickness in the listed assemblies.””

Can you restate this please,


Are you saying if the listed assembly shows insulation in it already,,

If more is added, another layer of rock is sometimes required??

Have not heard that, but maybe in the fine print.

If so will you post a link to one of those assemblies.

 

[cda]

Which edition of 13 are you using??

Ok first chapter 10 of 2013 deals with underground pipe::

"(2013-NFPA 13 Section 10.2.6.1.5.1and Figure 10.2.6.1.5"


If using 2013 than look at

8.15.1 Concealed Spaces.
8.15.1.1 Concealed Spaces Requiring Sprinkler Protection.

8.15.1.2 through 8.15.1.2.18.4

 

[cda]

what i found for double joist:::
8.6.4.1.5 Double Joist Obstructions.
8.6.4.1.5.1
Unless the requirements of 8.6.4.1.5.2 are met, where two sets of joists are under a roof or ceiling, and no flooring is over the lower set, sprinklers shall be installed above and below the lower set of joists where a clearance of 6 in. (152 mm) or more is between the top of the lower joist and the bottom of the upper joist as indicated in Figure 8.6.4.1.5.1.

8.6.4.1.5.2
Sprinklers shall be permitted to be omitted from below the lower set of joists where at least 18 in. (457 mm) is maintained between the sprinkler deflector and the top of the lower joist.

FAQ: Does the requirement of 8.6.4.1.5 apply to wood truss construction?

Paragraph 8.6.4.1.5 applies only to joist construction, such as solid sawn wood joists or composite wood joists, and does not apply to wood truss construction. When spacing sprinklers within wood truss construction, sprinklers should be spaced from the end of the truss.

 

[cda]

ok I think I see your question now::

"""I have always maintained an open web wood truss is similar to "Double joist Obstructions""""

I don't think so.

Maybe similar, but appears NFPA 13 calls out double joist obstructions.

For open wood truss, I would go back to 8.15.1 Concealed Spaces,, to see if sprinkler protection was required.

 

[cda]

Not sure if this helps also:::

3.7.1 * Obstructed Construction.
Panel construction and other construction where beams, trusses, or other members impede heat flow or water distribution in a manner that materially affects the ability of sprinklers to control or suppress a fire.

3.7.2 * Unobstructed Construction.
Construction where beams, trusses, or other members do not impede heat flow or water distribution in a manner that materially affects the ability of sprinklers to control or suppress a fire. Unobstructed construction has horizontal structural members that are not solid, where the openings are at least 70 percent of the cross-section area and the depth of the member does not exceed the least dimension of the openings, or all construction types where the spacing of structural members exceeds 71∕ 2 ft (2.3 m) on center.


A.3.7.2 Unobstructed Construction.
The following examples of unobstructed construction are provided to assist the user in determining the type of construction feature:
If you look in the appendix it gives examples,,,

But would say more applies to exposed, and not concealed.

 

[khsmith55]

“””Both GA and UL require an additional layer of gypsum board if the insulation thickness exceeds the thickness in the listed assemblies.””

Can you restate this please,


Are you saying if the listed assembly shows insulation in it already,,

If more is added, another layer of rock is sometimes required??

Have not heard that, but maybe in the fine print.

If so will you post a link to one of those assemblies.

I was going from memory (fading at times) but you got me to go look it up again. My memory wasn't totally correct.....what I was thinking of is in the GA Design Manual 2006 Section I, Item 11, also checked my 2018 Edition Section II, Item12 is the same. These sections are in the front of the Manual. No longer have my old hard copy of the UL Directory but as I remember there were similar requirements in the front of the Directory too. After re-reading the section I may have been too conservative on my projects since my last reading. Maybe I need to read the up front notes more then every 10 years. Thanks for flagging me on this.

 

[cda]

I was going from memory (fading at times) but you got me to go look it up again. My memory wasn't totally correct.....what I was thinking of is in the GA Design Manual 2006 Section I, Item 11, also checked my 2018 Edition Section II, Item12 is the same. These sections are in the front of the Manual. No longer have my old hard copy of the UL Directory but as I remember there were similar requirements in the front of the Directory too. After re-reading the section I may have been too conservative on my projects since my last reading. Maybe I need to read the up front notes more then every 10 years. Thanks for flagging me on this.

I am on the fire inspection side, so the last few years, the apartments have been using fill to negate fire sprinklers.

I have not looked at the assembly listing to see if extras were needed, if filled.

If so everyone involved in our projects has missed it, or does want to mention it??

 

[khsmith55]

Which edition of 13 are you using??

Ok first chapter 10 of 2013 deals with underground pipe::

"(2013-NFPA 13 Section 10.2.6.1.5.1and Figure 10.2.6.1.5"


If using 2013 than look at

8.15.1 Concealed Spaces.
8.15.1.1 Concealed Spaces Requiring Sprinkler Protection.

8.15.1.2 through 8.15.1.2.18.4

WORD OF CAUTION! I have been using NFPA's "free access", Earlier I selected the 2013 Edition, that is where I got the section numbers. I just went back to the "free access" and selected the 2013 Edition again and got Section 8.6.4.1.5.1 and Figure 8.6.4.1.5...........the kicker.........I had SELECTED 2013 but after looking at the footnote it said 2016 Edition even though I selected the 2013 Edition (as referenced in the 2015 IBC). Sorry for the confusion but I got jumbled in the NFPA free access. Thanks for the replies, I got the general jest now and it appears my philosophy has been correct over the years.

 

[Paul Sweet]

I always recommended wood I-joists instead of floor trusses to avoid having to sprinkle the concealed space. It's been a long time since I've done one, but if I remember right there was also a volume or area (between joists) restriction.

 

[khsmith55]

I am on the fire inspection side, so the last few years, the apartments have been using fill to negate fire sprinklers.

I have not looked at the assembly listing to see if extras were needed, if filled.

If so everyone involved in our projects has missed it, or does want to mention it??

CDA, I replied with the GA Note in an earlier reply. Since then I was able to find the UL Directory Note online;

From UL Fire Resistance Directory

II. General

17. Blanket Insulation

Unless specifically described in the individual designs, the addition of insulation in the concealed space between the ceiling membrane and the floor or roof structure may reduce the hourly rating of an assembly by causing premature disruption of the ceiling membrane and/or higher temperatures on structural components under fire-exposure conditions.

– For 1-hour-rated G500, L500, M500, and P500 Series assemblies, fiberglass insulation, loose-fill, batts, or blankets may be added to the plenum or joist space above the gypsum wallboard, provided an additional layer of gypsum wallboard is added to the assembly.

https://iq.ulprospector.com/en/profile?e=206790#SectionIIIitem17

After reading the above UL Note and the GA Note referenced earlier could you give me and the Forum your take and interpretation?

 

[cda]

CDA, I replied with the GA Note in an earlier reply. Since then I was able to find the UL Directory Note online;

From UL Fire Resistance Directory

II. General

17. Blanket Insulation

Unless specifically described in the individual designs, the addition of insulation in the concealed space between the ceiling membrane and the floor or roof structure may reduce the hourly rating of an assembly by causing premature disruption of the ceiling membrane and/or higher temperatures on structural components under fire-exposure conditions.

– For 1-hour-rated G500, L500, M500, and P500 Series assemblies, fiberglass insulation, loose-fill, batts, or blankets may be added to the plenum or joist space above the gypsum wallboard, provided an additional layer of gypsum wallboard is added to the assembly.

https://iq.ulprospector.com/en/profile?e=206790#SectionIIIitem17

After reading the above UL Note and the GA Note referenced earlier could you give me and the Forum your take and interpretation?

Well the game of codes is an interactive contact sport, where you learn something all the time!!


Anyway,
I am thinking now need to read and look at every assembly submitted,,,

Especially “””For 1-hour-rated G500, L500, M500, and P500 Series assemblies””


If that is what UL says, that it must be so.


Suggest wait a few days for others to respond.

 

[khsmith55]

Well the game of codes is an interactive contact sport, where you learn something all the time!!


Anyway,
I am thinking now need to read and look at every assembly submitted,,,

Especially “””For 1-hour-rated G500, L500, M500, and P500 Series assemblies””


If that is what UL says, that it must be so.


Suggest wait a few days for others to respond.

CDA, thanks for your input, hope to hear from others. Here's another "wild card" to consider. As I've always understood there are basically three paths when selecting fire resistive assemblies; 1) prescriptive method like 2015 IBC Tables 721.1 (1-3) (usually least restrictive), 2) GA Fire Resistance Manual or 3) UL Fire Resistance Manual (usually most restrictive), all three paths being code compliant. The "wild card"......both GA and UL have the note for adding insulation, however 2015 IBC Table 721.1(3) has no similar footnote I can find. For an example......could you use Table 721.1(3), Item 21-1.1 with the trusses (or floor joists, for sound) filled full depth with non-combustible insulation be used (less restrictive than GA or UL), in lieu of a GA or UL listed assembly? Food for thought, huh. Your feedback would be appreciated.

Thanks in advance,

Ken

 

[cda]

Hum as said now I need to look at assemblies submitted.



https://www.woodworks.org/experttip/december-2018/



How about pick assemblies tested with insulation ??