Fire Dampers for Wood Floor Assembly

[dturn]

I have a building that will have ductwork in the crawlspace. The architect is going to put gyp board on the underside of the existing 2x10 floor joists to make it 1 hour fire rated. The ducts will penetrate the bottom of the gyp board and terminate in a grille at the floor. Looking at the California Building Code, I see this as a through penetration that requires a fire damper. Problem is, all the fire dampers able to be horizontally installed only mention concrete or masonry floors, not a wood floor assembly. Are there fire dampers for a through penetration in a wood floor assembly? Does it matter that the floor is wood as long as it can be properly framed and secured?

 

[classicT]

Why is the floor being rated?

 

[dturn]

Why is the floor being rated?

The building is a dormitory. The architect thinks that the dwelling units need to be separated by a fire partition. Originally, this meant that the partitions needed to extend all the way to the bottom of the crawlspace. Now, instead of that, they want to put gyp on the bottom of the floor joists so they don't have to extend the partitions all the way to the bottom of the crawlspace.

 

[classicT]

2022 CBC​

717.6 Horizontal Assemblies​

Penetrations by ducts and air transfer openings of a floor, floor/ceiling assembly or the ceiling membrane of a roof/ceiling assembly shall be protected by a shaft enclosure that complies with Section 713 or shall comply with Sections 717.6.1 through 717.6.3.

717.6.1 Through Penetrations​

In occupancies other than Groups I-2, I-2.1 and I-3, a duct constructed of approved materials in accordance with the California Mechanical Code that penetrates a fire-resistance-rated floor/ceiling assembly that connects not more than two stories is permitted without shaft enclosure protection, provided that a listed fire damper is installed at the floor line or the duct is protected in accordance with Section 714.5. For air transfer openings, see Section 712.1.9.​

Exception: A duct is permitted to penetrate three floors or less without a fire damper at each floor, provided that such duct meets all of the following requirements:​

1. The duct shall be contained and located within the cavity of a wall and shall be constructed of steel having a minimum wall thickness of 0.0187 inches (0.4712 mm) (No. 26 gage).
2. The duct shall open into only one dwelling unit or sleeping unit and the duct system shall be continuous from the unit to the exterior of the building.
3. The duct shall not exceed 4-inch (102 mm) nominal diameter and the total area of such ducts shall not exceed 100 square inches (0.065 m2) in any 100 square feet (9.3 m2) of floor area.
4. The annular space around the duct is protected with materials that prevent the passage of flame and hot gases sufficient to ignite cotton waste where subjected to ASTM E119 or UL 263 time-temperature conditions under a minimum positive pressure differential of 0.01 inch (2.49 Pa) of water at the location of the penetration for the time period equivalent to the fire-resistance rating of the construction penetrated.
5. Grille openings located in a ceiling of a fire-resistance-rated floor/ceiling or roof/ceiling assembly shall be protected with a listed ceiling radiation damper installed in accordance with Section 717.6.2.1.

 

[classicT]

The building is a dormitory. The architect thinks that the dwelling units need to be separated by a fire partition. Originally, this meant that the partitions needed to extend all the way to the bottom of the crawlspace. Now, instead of that, they want to put gyp on the bottom of the floor joists so they don't have to extend the partitions all the way to the bottom of the crawlspace.

Why does the architect think that they need to do that?

420.3 only required a horizontal assembly to provide separation from the dwelling/sleeping unit from other units or occupancies. If it is a crawlspace, it is unoccupied and unclassified for occupancy. The fire partitions required by 420.2 can terminate at the floor.

420.2 Separation Walls​

Walls separating dwelling units in the same building, walls separating sleeping units in the same building and walls separating dwelling or sleeping units from other occupancies contiguous to them in the same building shall be constructed as fire partitions in accordance with Section 708.

420.3 Horizontal Separation​

Floor assemblies separating dwelling units in the same buildings, floor assemblies separating sleeping units in the same building and floor assemblies separating dwelling or sleeping units from other occupancies contiguous to them in the same building shall be constructed as horizontal assemblies in accordance with Section 711.

708.4 Continuity​

Fire partitions shall extend from the top of the foundation or floor/ceiling assembly below and be securely attached to one of the following:

1. The underside of the floor or roof sheathing, deck or slab above.
2. The underside of a floor/ceiling or roof/ceiling assembly having a fire-resistance rating that is not less than the fire-resistance rating of the fire partition.

Exceptions: {omitted for clarity}

 

[classicT]

I've recently posted this sentiment, but I will reiterate here again. It is important to understand that fire resistant construction serves two purposes. Fire resiliency (ability for structure to remain standing during a fire event) and fire compartmentalization (preventing the spread of fire).

In the case of 420.2/420.3, the intent of requiring the fire partitions and horizontal assemblies is purely fire compartmentalization. Understanding this, and that fire generally spreads either laterally or upwards (it seldom travels downward), the compartmentalization is required on the walls and ceiling. Extending the fire partitions below the floor does not serve to keep the fire contained to the unit where the fire starts (as the fire does not travel downward). Therefore, it is not required for the fire-resistant construction to be continuous through the crawlspace.

 

[dturn]

Hmm... It seems that you are correct. Thank you for the guidance. I'll look into it and talk to the architect.

 

[steveray]

I've recently posted this sentiment, but I will reiterate here again. It is important to understand that fire resistant construction serves two purposes. Fire resiliency (ability for structure to remain standing during a fire event) and fire compartmentalization (preventing the spread of fire).

It's a tough one for people to grasp....

 

[dturn]

One additional question. Looking at the exception for 708.4, it seems that a fire partition is usually required to extend into a crawl space, unless the floor is rated, which seems to be the aim of the architect. I understand that 420.3 doesn't require the floor to be rated, but the wording of this exception gives me worry that the floor does need to be rated in order to not need to extend the walls.

708.4 Continuity​

Fire partitions shall extend from the top of the foundation or floor/ceiling assembly below and be securely attached to one of the following:

1. The underside of the floor or roof sheathing, deck or slab above.
2. The underside of a floor/ceiling or roof/ceiling assembly having a fire-resistance rating that is not less than the fire-resistance rating of the fire partition.

Exceptions:

1. Fire partitions shall not be required to extend into a crawl space below where the floor above the crawl space has a minimum 1-hour fire-resistance rating. (Other exceptions not relevant)

 

[classicT]

One additional question. Looking at the exception for 708.4, it seems that a fire partition is usually required to extend into a crawl space, unless the floor is rated, which seems to be the aim of the architect. I understand that 420.3 doesn't require the floor to be rated, but the wording of this exception gives me worry that the floor does need to be rated in order to not need to extend the walls.

708.4 Continuity​

Fire partitions shall extend from the top of the foundation or floor/ceiling assembly below and be securely attached to one of the following:

1. The underside of the floor or roof sheathing, deck or slab above.
2. The underside of a floor/ceiling or roof/ceiling assembly having a fire-resistance rating that is not less than the fire-resistance rating of the fire partition.

Exceptions:

1. Fire partitions shall not be required to extend into a crawl space below where the floor above the crawl space has a minimum 1-hour fire-resistance rating. (Other exceptions not relevant)

Because a fire partition must (where not meeting an exception) still be supported by 1-hr construction under 708.4.1. However, in the case provided it should meet the exception, or it would be far more simplistic to protect the supporting ponywalls (supposing that they are aligned with the fire partitions above) than the entire floor.

708.4.1 Supporting Construction​

The supporting construction for a fire partition shall have a fire-resistance rating that is equal to or greater than the required fire-resistance rating of the supported fire partition.
Exception: In buildings of Types IIB, IIIB and VB construction, the supporting construction requirement shall not apply to fire partitions separating tenant spaces in covered and open mall buildings, fire partitions separating dwelling units, fire partitions separating sleeping units, fire partitions serving as corridor walls, fire partitions separating ambulatory care facilities from adjacent spaces or corridors, fire partitions separating dwelling and sleeping units from Group R-1 and R-2 occupancies and fire partitions separating vestibules from the level of exit discharge.

 

[Kevin]

I worked on a project that had a ground floor w/ commercial space and 5 floors above it that were R-2. We also added onto the back with 2A construction the full height of the building. IBC section 510 has a few different scenarios for this situation. In our situation we had to rate the floor between the commercial level and residential levels for 3 hours. The building was originally 3B construction and we had to bring it up to 3A construction, so we rated all the residential floors 1 hour. We also used the joist space for ductwork and provided ceiling radiation dampers when the ducts penetrated the gwb membrane. The majority of the duct work was to serve that floor/condo.

There were a few common mechanical shafts too for restrooms or kitchen exhaust. I think for the kitchen exhausts we did sub ducts and for the bathrooms we dis combination fire/smoke dampers connected to the fire alarm system and a duct smoke detector. IIRC correctly, the subducts that penetrate the common duct and turn up 20" are so kitchen smoke can escape and not be blocked by a damper.

 

[steveray]

If it is "B" construction it just gets to terminate at the unrated floor in theory....And there is also a gimmie in there somewhere that allows you to eliminate the drywall in the crawlspace "ceiling"....I think if it is sprinklered