rated shaft in 2 story bldg??

This is a continuation of the thread below but it was locked.


I have gone back and forth over the years on whether a mech shaft going through the 2nd floor to get ducts from the roof to the first floor needs to be rated or not and here I am again except this time with a new twist.

First off, I always get hung up on CBC 708.7.3 which makes me think that this one story shaft needs to be a rated shaft since it is concealed within the construction of walls. But yet the Code does say WALL (singular) and not within the construction of wallS. I do understand that 7.1 states a shaft enclosure is NOT required in this situation but what if you install one anyway, then does 7.3 kick you into a rated shaft?

The definitions of shafts and shaft enclosures don't help

SHAFT ENCLOSURE. The walls or construction forming the boundaries of a shaft.

SHAFT. An enclosed space extending through one or more stories of a building, connecting vertical openings in successive floors, or floors and roof.

They sure sound the same to me. If I under the Code correctly, the definition of shaft enclosure should read,

FIRE RATED walls or construction forming the boundaries of a shaft.

If it read that way, I think it would be clear that this is a shaft since it is not required to be rated.

Now for the twist, there is a corporate architect who represents my client and he wants to use the shaft as a return pleum and leave the bottom open. Now the thread linked above would indicate that that is OK per 708.11.1 since this duct chase is a shaft and NOT a shaft enclosure.

What are your thoughts

Wayne Calif-Architect

Original Post
What is the "intent" of the shaft. If the intent is return air plenum transfer, and supply vents from the roof, then I see nothing to prevent it to be used as a plenum. You will need vent dampers. Proper shaft enclosures must include all sides as well as the top and bottom, unless the bottom of the shaft is at the bottom of the structure. Where the bottom of the shaft is not at the bottom of the structure, the fire-resistance rating for the bottom of the shaft must be the same as that rating required for the shaft enclosure.

But if the "intent" is just supply vents from the roof, then no because per CBC 707.7.1 Prohibited openings. Openings other than those necessary for the purpose of the shaft shall not be permitted in shaft enclosures.
I'm also not entirely clear on the specifics of the situation, but let me offer this:

It sounds like you are dealing with a structure in which you have only two floors, and if I understand it correctly you have determined that no rating is required for the duct passing from first floor, through the floor structure, and the roof plane. In such buildings, where not otherwise prohibited you can have unrestricted and unrated openings between the two floors for stairs, etc., as well as simply open second floor areas with guardrails that look down into the first floor.

When you have a situation where there is no requirement to enclose and rate a component, the act of surrounding the component with a wall does not create a new requirement (i.e. to make those walls rated, or to add a bottom closure or dampers. The essential function of rated shafts is to prevent passage of fire and gasses from area to area. If you have, in essence, an 'open' plan with free passage of fire and air from first to second floor, then the ducts and/or enclosing walls do nothing to alter this basic parameter. The walls you build become nothing more than cosmetic improvements (to hide the ducts), and do not increase any hazard that might otherwise exist.

Again, I'm not sure I'm understanding your query or the building you are describing, but perhaps this will be of some help.


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