How to achieve 1-hour rating at existing glulam/decking floor/ceiling?

[Jason Land]

We have a project that involves an A-2 banquet hall tenant improvement at the 3rd floor of an existing 4 story Type III-B structure with an NFPA-13 sprinkler system throughout. We have a separated occupancy condition between these two floors (B-office occupancy at the 4th floor), so we are required to provide a 1-hour fire-resistance rated floor/ceiling assembly at the 3rd floor ceiling/4th floor per Table 508.4.

The existing floor/ceiling assembly between the 3rd and 4th floors is comprised of 1/2” plywood over 2x T&G car decking, supported by 3-1/8” x 13-1/2” glulam purlins @ 4’-0” on center, which are in turn hung off of 6-3/4” x 19-1/2” glulam girders.

Our desired approach is to install 2x furring @ 16” on center to the underside of the purlins, and then attach (2) layers of 1/2" gypsum wallboard to the furring. Our structural engineer has determined that the existing 6-3/4” x 19-1/2” glulam girders maintain adequate structural capacity after 1 hour of charring depth, so we are showing only (1) layer of 1/2" gypsum wallboard around the bottom edge of these girders to finish them out.

I have been unable to determine any prescriptive fire-resistance minimum protections for this existing condition in Section 721, and likewise unable to determine how to calculate the fire-resistance of the existing condition coupled with our desired approach using Section 722 (or more correctly, I don't know *how* to do this).

Will the addition of the furring and (2) layers of 1/2" gypsum wallboard, together with the thickness of the existing floor assembly members, be adequate to create a 1-hour fire-resistance rated at this location? If not, what is the minimum amount of gypsum wallboard (thicknesses and types) that we can install using an approach similar to what we have detailed? And what about sprinklers? Will the 4' wide x 24' long x 13.5" deep voids created by the GWB ceiling need to be separately sprinklered?

 

[Jason Land]

Here is a detail of our desired approach. Note that the purlins and girder beams are not noted with the correct sizes in this detail.

 

[Jason Land]

Let's try that again. Inserting an image didn't work, so trying a hyperlink instead:
https://www.dropbox.com/s/vb0soy2x1n44yc0/rated floor-ceiling assembly.jpg?dl=0

 

[RLGA]

Is an occupancy separation necessary? Can the nonseparated occupancies method be used?

 

[Jason Land]

Unfortunately no. A-2 occupancies are limited to 3 stories above grade plane in Type III-B sprinklered buildings, while B occupancies can go to 4 stories. Using non-separated occupancies would require that we use the most restrictive case for the building (A-2), but since this is an existing 4-story building, non-separated occupancies are only allowed from the 3rd floor and lower (i.e. an occupancy separation is still required between the 3rd floor A-2 and 4th floor B occupancies.

 

[RLGA]

Does the interior construction conform to the requirements of Type IV construction? If it does, then I would reclassify the building, which would allow four stories for Group A-2. If the building cannot conform to Type IV construction, does the floor assembly conform to the heavy timber requirements of Type IV construction (See Sections 602.4.4 and 602.4.6)? If so, then that is considered 1-hour construction without any additional treatment.

If none of the above applies, then the gypsum board layering could be considered but may require a code modification as an alternative method per Section 104.11 since there is no tested assembly that would match your specific conditions (that I know of).

 

[cda]

Welcome

Glad you do an easy question your first go around

 

[Jason Land]

No, everything except the floors conforms to Type IV construction. The existing floors are only 2x T&G decking laid flat, with 1/2" plywood sheathing over that. Type IV requires floors to be of 4 inch nominal thickness. It is not feasible to add any wood thickness to the floor on the upper or lower sides, so reclassifying at Type IV is not an option for us.

I am attempting to contact the building official about this, but as I like to do, I'd really like to have a solution that I am fairly confident will fly before taking to the City about it.

 

[mtlogcabin]

I am a little confused. What is the occupancy of each floor and is there a change of occupancy on any of the floors or is it all tenant improvement.
Depending on the answers the existing building code may be a better way to go

 

[JPohling]

Does this rating need protection from only the underside?
I do not see how you will achieve a one hour assembly with only gyp on the underside and combustible wood floor assembly above without protection. Rated lay in ceiling grid and tile below an option? still no protection from above.

 

[RLGA]

Does this rating need protection from only the underside?
I do not see how you will achieve a one hour assembly with only gyp on the underside and combustible wood floor assembly above without protection. Rated lay in ceiling grid and tile below an option? still no protection from above.

Fire ratings for floor and roof assemblies are only tested from the underside.

From ASTM E119-18:

"8.6.1 Application—This procedure is applicable to floor and roof assemblies with or without attached, furred, or suspended ceilings and requires the application of the fire exposure to the underside of the test specimen."​